the time travel definition

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

If you liked this, you may like:

Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science 

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna. 

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

Sign up for the Live Science daily newsletter now

Get the world’s most fascinating discoveries delivered straight to your inbox.

Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida . 

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London. 

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine . 

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said. 

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean . 

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery. 

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid." 

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us." 

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings. 

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

• If There Were a Time Warp, How Would Physicists Find It?
• Can Animals Tell Time?
• Why Does Time Sometimes Fly When You're NOT Having Fun?

Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

Why do babies rub their eyes when they're tired?

Why do people dissociate during traumatic events?

Early humans lived on 'Persian plateau' for 20,000 years after leaving Africa, study suggests

Most Popular

By Joe Rao April 03, 2024

By Lloyd Coombes April 03, 2024

By Sharmila Kuthunur April 03, 2024

By Rahul Rao April 03, 2024

By Ben Turner April 02, 2024

By Josh Dinner April 02, 2024

By Sascha Pare April 02, 2024

By Jennifer Nalewicki April 02, 2024

By Emily Cooke April 02, 2024

By Marlowe Starling April 02, 2024

• 2 'You could almost see and smell their world': Remnants of 'Britain's Pompeii' reveal details of life in Bronze Age village
• 3 Hidden chunk of Earth's crust that seeded birth of 'Scandinavia' discovered through ancient river crystals
• 4 Do animals really have instincts?
• 5 Orcas aren't all the same species, study of North Pacific killer whales reveals
• 2 Explosive green 'Mother of Dragons' comet now visible in the Northern Hemisphere
• 3 Nuclear fusion reactor in South Korea runs at 100 million degrees C for a record-breaking 48 seconds
• 4 'It's had 1.1 billion years to accumulate': Helium reservoir in Minnesota has 'mind-bogglingly large' concentrations

• Cambridge Dictionary +Plus

Meaning of time travel in English

Your browser doesn't support HTML5 audio

• around Robin Hood's barn idiom
• baggage drop
• communication
• first class
• peripatetically
• public transportation
• super-commuting

Examples of time travel

Translations of time travel.

Get a quick, free translation!

Word of the Day

the birds and the bees

the basic facts about sex and how babies are produced

Shoots, blooms and blossom: talking about plants

Learn more with +Plus

• Recent and Recommended {{#preferredDictionaries}} {{name}} {{/preferredDictionaries}}
• Definitions Clear explanations of natural written and spoken English English Learner’s Dictionary Essential British English Essential American English
• Grammar and thesaurus Usage explanations of natural written and spoken English Grammar Thesaurus
• Pronunciation British and American pronunciations with audio English Pronunciation
• English–Chinese (Simplified) Chinese (Simplified)–English
• English–Chinese (Traditional) Chinese (Traditional)–English
• English–Dutch Dutch–English
• English–French French–English
• English–German German–English
• English–Indonesian Indonesian–English
• English–Italian Italian–English
• English–Japanese Japanese–English
• English–Norwegian Norwegian–English
• English–Polish Polish–English
• English–Portuguese Portuguese–English
• English–Spanish Spanish–English
• English–Swedish Swedish–English
• Dictionary +Plus Word Lists
• English    Noun
• Translations
• All translations

Add time travel to one of your lists below, or create a new one.

{{message}}

Something went wrong.

There was a problem sending your report.

• Table of Contents
• Random Entry
• Chronological
• Editorial Information
• About the SEP
• Editorial Board
• How to Cite the SEP
• Special Characters
• Advanced Tools
• Support the SEP
• PDFs for SEP Friends
• Make a Donation
• SEPIA for Libraries
• Entry Contents

Bibliography

Academic tools.

• Friends PDF Preview
• Author and Citation Info
• Back to Top

Discussions of the nature of time, and of various issues related to time, have always featured prominently in philosophy, but they have been especially important since the beginning of the twentieth century. This article contains a brief overview of some of the main topics in the philosophy of time—(1) fatalism; (2) reductionism and Platonism with respect to time; (3) the topology of time; (4) McTaggart’s argument; (5) the A-theory and the B-theory; (6) presentism, eternalism, and the growing block theory; (7) the 3D/4D debate about persistence; (8) the dynamic and the static theory; (9) the moving spotlight theory; (10) time travel; (11) time and physics and (12) time and rationality. We include some suggestions for further reading on each topic and a bibliography.

Note: This entry does not discuss the consciousness, perception, experience, or phenomenology of time. A historical overview and general presentation of the various views is available in the entry on temporal consciousness . Further coverage can be found in the SEP entry on the experience and perception of time . For those interested specifically in phenomenological views, see the entries on Husserl (Section 6), and Heidegger (Section 2: Being and Time).

1. Fatalism

2. reductionism and platonism with respect to time, 3. the topology of time, 4. mctaggart’s argument, 5. the a-theory and the b-theory, 6. presentism, eternalism, and the growing block theory, 7. three-dimensionalism and four-dimensionalism, 8. the dynamic and the static theory, 9. the moving spotlight theory, 10. time travel, 11. time and physics, 12. time and rationality, other internet resources, related entries.

Many logical questions about time historically arose from questions about freedom and determinism—in particular worries about fatalism. Fatalism can be understood as the doctrine that whatever will happen in the future is already unavoidable (where to say that an event is unavoidable is to say that no agent is able to prevent it from occurring). Here is a typical argument for fatalism:

The conclusion appears shocking. Future moral catastrophes are unavoidable. Every weighty decision that now feels up to you is already determined.

The argument for fatalism makes some significant metaphysical assumptions that raise more general questions about logic, time, and agency.

For example, Premise (1) assumes that propositions describing the future do not come into or go out of existence. It assumes that there are propositions now that can accurately represent every future way things might go. This is a non-trivial logical assumption. You might, for instance, think that different times becoming present and actual (like perhaps possible worlds) have different associated sets of propositions that become present and actual.

Premise (2) appears to be a fundamental principle of semantics, sometimes referred to as the Principle of Bivalence.

The rationale for premise (4) is that it appears no one is able to make a true prediction turn out false. (4) assumes that one and the same proposition does not change its truth value over time. The shockingness of the conclusion also depends on identifying meaningful agency with the capacity to make propositions come out true or false.

A proper discussion of fatalism would include a lengthy consideration of premises (1) and (4), which make important assumptions about the nature of propositional content and the nature of agency. That would take us beyond the scope of this article. For our purposes, it is important to note that many writers have been motivated by this kind of fatalist argument to deny (2), the Principle of Bivalence. According to this line, there are many propositions—namely, propositions about events that are both in the future and contingent—that are neither true nor false right now. Consider the proposition that you will have lunch tomorrow. Perhaps that proposition either has no truth value right now, or else has a third truth value: indeterminate. When the relevant time comes, and you either have lunch or don’t, then the proposition will come to be either true or false, and from then on that proposition will forever retain that determinate truth value.

This strategy for rejecting fatalism is sometimes referred to as the “Open Future” response. The Open Future response presupposes that a proposition can have a truth value, but only temporarily—truth values for complete propositions can change as time passes and the world itself changes. This raises further questions about the correct way to link up propositions, temporal passage and truth values. For example, which of the following formulas expresses a genuine proposition about the present?

Tensed Proposition: “Sullivan is eating a burrito”.

Tenseless Proposition: “Sullivan eats a burrito at <insert present time stamp>”.

The tensed proposition will no longer be true when Sullivan finishes her lunch. So it has, at best, a temporary truth value. The tenseless proposition expresses something like “Sullivan eats a burrito at 3pm on July 20th 2019”. That proposition is always true.

Some philosophers argue that only the latter, eternally true kind of proposition could make sense of how we use propositions to reason over time. We need propositions to have stable truth values if we are to use them as the contents of thoughts and communication. Other philosophers—particularly those who believe that reality itself changes over time—think that tensed propositions are needed to accurately reason about the world. We’ll return to these issues in Section 4 and Section 5 .

Suggestions for Further Reading: Aristotle, De Interpretatione , Ch. 9; Barnes and Cameron 2009; Boethius, The Consolation of Philosophy , Book V; Crisp 2007; Evans 1985; Lewis 1986; Markosian 1995; McCall 1994; Miller 2005; Richard 1981; Sullivan 2014; Taylor 1992; Torre 2011; Van Inwagen 1983.

What if one day things everywhere ground to a halt? What if birds froze in mid-flight, people froze in mid-sentence, and planets and subatomic particles alike froze in mid-orbit? What if all change, throughout the entire universe, completely ceased for a period of, say, one year? Is such a thing possible?

If the answer to this last question is “yes”—if it is possible for there to be time without change—then time is in some important sense independent of the events within time. Other ways of investigating whether time is independent of the events within time include asking whether all of the physical processes that happen in time could happen at a faster or slower rate, and asking whether all events could have happened slightly earlier or later in time. After all, if every physical process could suddenly happen twice as fast, or if every event could take place slightly earlier or later in time, then it follows that in some important sense time can remain the same even if the way that events are distributed in time changes wholesale.

Aristotle and Leibniz, among others, have argued that time is not independent of the events that occur in time. This view is typically called either “reductionism with respect to time” or “relationism with respect to time”, since according to this view, all talk that appears to be about time can somehow be reduced to talk about temporal relations among things and events. The opposing view, normally referred to either as “Platonism with respect to time” or “substantivalism with respect to time” or “absolutism with respect to time”, has been defended by Plato, Newton, and others. On this view, time is like an empty container into which things and events may be placed; but it is a container that is independent of what (if anything) is placed in it.

Another way to present this distinction is to say that those like Plato and Newton who think that time is independent of the events that occur in time believe in “absolute time”. Those like Aristotle and Leibniz, who think that time is not independent of the events that occur in time, deny the existence of absolute time, though they still endorse “relative time”, where relative time is nothing over and above the temporal relations between events.

These views about time are closely connected to views about space and about motion. Most obviously, these views about time have straightforward spatial analogues—one may be a substantivalist about space (and thus endorse the existence of absolute space in addition to spatial relations between things), or one may be a relationist about space (and thus deny the existence of absolute space). Substantivalism and relationism about time have traditionally been taken to stand or fall with their spatial counterparts. In addition, the choice between substantivalism and relationism about space and time has consequences for your theory of motion. If you are a relationist about space and time then you must also be a relationist about motion: all motion is motion relative to something. If you are a substantivalist about space and time, you will endorse, in addition to relative motion, the notion of absolute motion, where absolute motion is motion relative to absolute space and time. If you are a substantivalist, in addition to facts about whether and how fast a train car is moving relative to the track, whether and how fast it is moving relative to the cars, and so on, there will also be a fact about whether and how fast the train car is really moving—whether and how fast it is moving relative to absolute space and time.

Why would someone endorse the existence of absolute time? One reason is that the empty container metaphor has a lot of intuitive appeal. Another reason is that some philosophers have thought that there must be such a thing as absolute motion—as opposed to merely relative motion—in order to explain certain physical phenomena, like the forces felt during acceleration. Newton had an especially famous argument along these lines involving a spinning bucket of water—the entry on Newton’s views on space, time , and motion has a careful discussion of this argument.

Why would someone deny the existence of absolute time? Some relationists have put forward arguments that are supposed to show that absolute space and time are philosophically problematic in some important way. Perhaps most famously, Leibniz argued that the existence of absolute space or time would lead to violations of the principle of sufficient reason and violations of the identity of indiscernibles.

In order to see why, consider two ways of describing the way things could be. On the one hand, everything is as it actually is. On the other, every event happens one second later than it actually does, but is otherwise exactly the same. If there is such a thing as absolute time then these two descriptions would pick out distinct possible worlds. But this, Leibniz claimed, would violate the principle of sufficient reason. For given that the actual world and the one-second-late world are exactly the same except for where things are located in absolute time, there could not (at least according to Leibniz) be any reason why one exists rather than the other. Moreover, Leibniz claimed, the actual world and the one-second-late world are indistinguishable; so if they were in fact distinct possible worlds, that would violate the principle that if two things are indistinguishable, then they are identical.

Leibniz’s arguments are examples of arguments that attempt to identify something philosophically problematic with absolute time and space. Perhaps more generally, many philosophers have been moved by the idea that even if absolute time and space are not problematic in a way that makes them unacceptable, they are still the kinds of things that we should do without if we can. This kind of attitude can be motivated by a straightforward kind of parsimony—we should always make do with the fewest types of entities possible. Or it can be motivated by a more specific worry about the nature of absolute space and time. You might, for instance, be especially loath to admit unobservable entities into your ontology—you are willing to admit them if you must, but you would rather eliminate them wherever possible. As absolute space and time are unobservable, someone who endorses this attitude will be inclined to think there are no such things.

Suggestions for Further Reading : Alexander 1956; Ariew 2000; Arntzenius 2012; Coope 2001; Mitchell 1993; Newton, Philosophical Writings ; Newton-Smith 1980; Shoemaker 1969.

It’s natural to think that time can be represented by a line. But a line has a shape. What shape should we give to the line that represents time? This is a question about the topology, or structure, of time.

One natural way to answer our question is to say that time should be represented by a single, straight, non-branching, continuous line that extends without end in each of its two directions. This is the “standard topology” for time. But for each of the features attributed to time in the standard topology, two interesting questions arise: (a) does time in fact have that feature? and (b) if time does have the feature in question, is this a necessary or a contingent fact about time?

Questions about the topology of time appear to be closely connected to the issue of Platonism versus relationism with respect to time. For if relationism is true, then it seems likely that time’s topological features will depend on contingent facts about the relations among things and events in the world, whereas if Platonism is true, so that time exists independently of whatever is in time, then time will presumably have its topological properties as a matter of necessity. But even if we assume that Platonism is true, it’s not clear exactly what topological properties should be attributed to time.

Consider the question of whether time should be represented by a line without a beginning (so a line, rather than a line segment). Aristotle has argued (roughly) that time cannot have a beginning on the grounds that in order for time to have a beginning, there must be a first moment of time, but that in order to count as a moment of time, that allegedly first moment would have to come between an earlier period of time and a later period of time, which is inconsistent with its being the first moment of time. (Aristotle argues in the same way that time cannot have an end.)

Aristotle’s argument may or may not be a good one, but even if it is unsound, many people will feel, purely on intuitive grounds, that the idea of time having a beginning (or an end) just does not make sense. And here we have an excellent illustration of what is at stake in the controversy over whether time has its topological properties as a contingent matter or as a matter of necessity. For suppose we come to have excellent evidence that the universe itself had a beginning in time. (This seems like the kind of thing that could be supported by empirical evidence in cosmology.) This would still leave open the question of whether the beginning of the universe occurred after an infinitely long period of “empty” time, or, instead, coincided with the beginning of time itself. There are interesting and plausible arguments for each of these positions.

It is also worth asking whether time must be represented by a single line. Perhaps we should take seriously the possibility of time’s consisting of multiple time streams, each one of which is isolated from each other, so that every moment of time stands in temporal relations to other moments in its own time stream, but does not bear any temporal relations to any moment from another time stream. Likewise we can ask whether time could correspond to a branching line (perhaps to allow for the possibility of time travel or to model an open future), or to a closed loop, or to a discontinuous line. And we can also wonder whether one of the two directions of time is in some way privileged, in a way that makes time itself asymmetrical. (We say more about this last option in particular in the section on time and physics.)

Suggestions for Further Reading: (1) On the beginning and end of time: Aristotle, Physics , Bk. VIII; Kant, The Critique of Pure Reason (especially pp. 75ff); Newton-Smith 1980, Ch. V. (2) On the linearity of time: Newton-Smith 1980, Ch. III; Swinburne 1966, 1968. (3) On the direction of time: Price 1994, 1996; Savitt 1995; and Sklar 1974. (4) On all of these topics: Newton-Smith 1980.

In a famous paper published in 1908, J.M.E. McTaggart argued that there is in fact no such thing as time, and that the appearance of a temporal order to the world is a mere appearance. Other philosophers before and since (including, especially, F.H. Bradley) have argued for the same conclusion. We will focus here only on McTaggart’s argument against the reality of time, which has been by far the most influential.

McTaggart begins his argument by distinguishing two ways in which positions in time can be ordered. First, he says, positions in time can be ordered according to their possession of properties like being two days future, being one day future, being present, being one day past, etc. These properties are often referred to now as “ A properties” because McTaggart calls the series of times ordered by these properties “the A series”. But he says that positions in time can also be ordered by two-place relations like two days earlier than, one day earlier than, simultaneous with, etc. These relations are now often called “ B relations” because McTaggart calls the series of times ordered by these relations “the B series”.

McTaggart argues that the B series alone does not constitute a proper time series; the A series is essential to time. His reason for this is that he assumes change is essential to time, and the B series without the A series does not involve genuine change (since B series positions are forever “fixed”, whereas A series positions are constantly changing).

McTaggart also argues that the A series is inherently contradictory. For, he says, the different A properties are incompatible with one another. No time can be both future and past, for example. Nevertheless, he insists, each time in the A series must possess all of the different A properties, since a time that is future will be present and then will be past. McTaggart concludes that, since neither the A-series nor the B-series can order the time series, time is unreal.

One response to this argument that McTaggart anticipates involves claiming that it’s not true of any time, t , that t is both future and past. Rather, the objection goes, we must say that it was future at some moment of past time and will be past at some moment of future time. But this objection fails, according to McTaggart, because the additional times that are invoked in order to explain t ’s possession of the incompatible A properties must themselves possess all of the same A properties (as must any further times invoked on account of these additional times, and so on ad infinitum ). Thus, according to McTaggart, we never resolve the original contradiction inherent in the A series, but, instead, merely generate an infinite regress of more and more contradictions.

McTaggart’s argument has had staying power because it organizes crucial debates about the metaphysics of temporal passage, because it hints at how those debates connect to further debates about where evidence for the time series and the nature of change come from, and because the difference between A-theoretic and B-theoretic approaches to the debate has continued in the intervening century.

Suggestions for Further Reading: Bradley 1893; Dyke 2002; McTaggart 1908; Mellor 1998; Prior 1967, 1968.

In Section 1 , we introduced the distinction between a tensed proposition and a tenseless proposition. Tensed propositions can fully and accurately describe the world, but nevertheless change truth value over time. Tenseless propositions, on the other hand, are always true or always false—they reference a particular time in the proposition and never change. Propositions represent ways reality could be. So, which view of propositions we adopt depends on what we think it means for reality itself to undergo change.

In section 4 , we discussed McTaggart’s distinction between time conceived of as a B-series (events ordered by which come before and which come after) and time conceived of as an A-series (events ordered by which are present, which are past, and which are future). Though not particularly creative as names, the A/B distinction has stuck around as a way of classifying theories of change.

B-theorists think all change can be described in before-after terms. They typically portray spacetime as a spread-out manifold with events occurring at different locations in the manifold (often assuming a substantivalist picture). Living in a world of change means living in a world with variation in this manifold. To say that a certain autumn leaf changed color is just to say that the leaf is green in an earlier location of the manifold and red in a later location. The locations, in these cases, are specific times in the manifold. And all of the metaphysically important facts about change can be captured by tenseless propositions like “The leaf is red at October 7, 2019”. “The leaf is not red at September 7, 2019”.

A-theorists, on the other hand, believe that at least some important forms of change require classifying events as past, present or future. And accurately describing this kind of change requires some tensed propositions—there is a way reality is (now, presently) which is complete but was different in the past and also will be different in the future. These tensed propositions also explain why we tend to attribute significance to the past-present-future distinction. For example, you might think the A-theorist is in a better position to explain why we care whether a horrible event is already in the past versus still in the future. Some A-theorists will argue that we aren’t concerned with location—we care that the event is over with in reality.

Note, also, there is a significant range of views within the A-theory camp about whether there is a spacetime manifold (Moving Spotlighters think there is), or whether only present events are real (the presentist view), or whether only present and past events are real (the Growing Block view). We say more about all of these views below. A-theorists also debate whether objects themselves undergo A-theoretic change or whether it is only entire regions of spacetime that change this way.

A-theorists and B-theorists appeal to different sources of evidence for their different views of passage. A-theorists typically emphasize how psychologically we seem to perceive a world of robust passage or “flow” of time. In physics, the laws of thermodynamics seem to imply a strong past-to-future direction to time. And quantum mechanics seems to identify an important sense of simultaneity, which could be identified with presentness (see section 11 below). Finally many commonsense ways of thinking of change seem to rely on A-theory descriptions of passage. For instance, they will use the fact that we care so much about whether bad events are past as evidence that there are ineliminable tensed propositions and those propositions represent ineliminable A-properties.

B-theorists typically emphasize how special relativity eliminates the past/present/future distinction from physical models of space and time. Thus what seems like an awkward way to express facts about time in ordinary English is actually much closer to the way we express facts about time in physics. Moreover, thinking of change in tenseless terms makes it easier to describe in a logically consistent way how objects survive change—objects have properties only relative to particular times, so there is no worry about attributing absolutely inconsistent properties to anything. We’ll consider some of these arguments in more detail in the remaining sections of this entry, as we consider more specific variations on A-theories and B-theories of time.

Suggestions for Further Reading: For general discussion of The A theory and The B theory: Emery 2017; Le Poidevin 1998; Le Poidevin and McBeath 1993; Markosian 1993; Maudlin 2007 (especially Chapter 4); Mellor 1998; Paul 2010; Prior 1959 [1976], 1962 [1968], 1967, 1968, 1970, 1996; Sider 2001; Skow 2009; Smart 1963, 1949; Smith 1993; Sullivan 2012a; Williams 1951; Zimmerman 2005; Zwart 1976.

A further question that you might ask about time is an ontological question. Does whether something is past, present, or future make a difference to whether it exists? And how do these ontological theses connect to debates about the A-theory and the B-theory?

According to presentism, only present objects exist. More precisely, presentism is the view that, necessarily, it is always true that only present objects exist. Even more precisely, no objects exist in time without being present (abstract objects might exist outside of time). (Note that some writers have used the name differently, and unless otherwise indicated, what is meant here by “present” is temporally present, as opposed to spatially present.) According to presentism, if we were to make an accurate list of all the things that exist—i.e., a list of all the things that our most unrestricted quantifiers range over—there would be not a single merely past or merely future object on the list. Thus, you and the Taj Mahal would be on the list, but neither Socrates nor any future Martian outposts would be included. (Assuming, that is, both (i) that each person is identical to his or her body, and (ii) that Socrates’s body ceased to be present—thereby going out of existence, according to presentism—shortly after he died. Those who reject the first of these assumptions should simply replace the examples in this article involving allegedly non-present people with appropriate examples involving the non-present bodies of those people.) And it is not just Socrates and future Martian outposts, either—the same goes for any other putative object that lacks the property of being present. No such objects exist, according to presentism.

There are different ways to oppose presentism—that is, to defend the view that at least some non-present objects exist. One version of non-presentism is eternalism, which says that objects from both the past and the future exist. According to eternalism, non-present objects like Socrates and future Martian outposts exist now, even though they are not currently present. We may not be able to see them at the moment, on this view, and they may not be in the same space-time vicinity that we find ourselves in right now, but they should nevertheless be on the list of all existing things.

It might be objected that there is something odd about attributing to a non-presentist the claim that Socrates exists now, since there is a sense in which that claim is clearly false. In order to forestall this objection, let us distinguish between two senses of “ x exists now”. In one sense, which we can call the temporal location sense, this expression is synonymous with “ x is present”. The non-presentist will admit that, in the temporal location sense of “ x exists now”, it is true that no non-present objects exist now. But in the other sense of “ x exists now”, which we can call the ontological sense, to say that “ x exists now” is just to say that x is now in the domain of our most unrestricted quantifiers. Using the ontological sense of “exists”, we can talk about something existing in a perfectly general sense, without presupposing anything about its temporal location. When we attribute to non-presentists the claim that non-present objects like Socrates exist right now, we commit non-presentists only to the claim that these non-present objects exist now in the ontological sense (the one involving the most unrestricted quantifiers).

According to the eternalist, temporal location does not affect ontology. But according to a somewhat less popular version of non-presentism, temporal location does matter when it comes to ontology, because only objects that are either past or present exist. On this view, which is often called the growing block theory, the correct ontology is always increasing in size, as more and more things are added on to the leading “present” edge (temporally speaking). (Note, however, that the growing block theory does not involve any commitment to four-dimensionalism as discussed in section 7 . In this way, the name “growing block” is somewhat misleading and the view is probably better described as the growing universe theory.) Both presentism and the growing block theory are versions of the A-theory.

Despite the claim by some presentists that theirs is the commonsense view, it is pretty clear that there are some major problems facing presentism (and, to a lesser extent, the growing block theory; but in what follows we will focus on the problems facing presentism). One problem has to do with what appears to be perfectly meaningful talk about non-present objects, such as Socrates and the year 3000. If there really are no non-present objects, then it is hard to see what we are referring to when we use expressions such as “Socrates” and “the year 3000”.

Another problem for the presentist has to do with relations involving non-present objects. It is natural to say, for example, that Abraham Lincoln was taller than Napoleon Bonaparte, and that World War II was a cause of the end of The Depression. But how can we make sense of such talk, if there are no non-present objects to be the relata of those relations?

A third problem for the presentist has to do with the very plausible principle that for every truth, there is a truth-maker—something whose existence suffices for the truth of the proposition or statement. If you are a presentist, it is hard to see what the truth-makers could be for truths such as that there were dinosaurs and that there will be Martian outposts.

Finally, the presentist, in virtue of being an A-theorist, must deal with the arguments against the A-theory that were mentioned above, including especially the worry that the A-theory is incompatible with special relativity. We will discuss these physics-based objections below.

Suggestions for Further Reading: Adams 1986; Bourne 2006; Bigelow 1996; Emery 2020; Hinchliff 1996; Ingram 2016; Keller and Nelson 2001; Markosian 2004, 2013; McCall 1994; Rini and Cresswell 2012; Sider 1999, 2001; Sullivan 2012b; Tooley 1997; Zimmerman 1996, 1998.

In Section 4 and Section 5 we saw that there have been two main theories developed in response to McTaggart’s Argument: The A-theory and The B-theory. Then, in Section 6 we saw that there are two main ways of thinking about the relation between ontology and time: presentism and eternalism. (There was also a third way, The Growing Block Theory, which we will mainly set aside for the sake of simplicity in this section.) Two main ways of thinking about time emerge from these discussions. On the one hand, A-theorists and presentists think that our pre-theoretical idea of time as flowing or passing, and thus being very different from the dimensions of space, corresponds to something objective and real. B-theorists and eternalists, on the other hand, reject the idea of time’s passage and instead embrace the idea of time as being a dimension like space. There is another important way in which philosophers in the second camp (the B-theory/eternalist camp) think time is like space, and it has to do with how objects and events persist over time. The debate typically centers around the doctrine of “temporal parts”, which those in the B-theory/eternalist camp tend to accept while those in the A-theory/presentist camp tend to reject.

To get an intuitive idea of what temporal parts are supposed to be, think of a film strip depicting you as you walk across a room. It is made up of many frames, and each frame shows you at a moment of time. Now picture cutting the frames, and stacking them, one on top of another. Finally, imagine turning the stack sideways, so that the two-dimensional images of you are all right-side-up. Each image of you in one of these frames represents a temporal part of you, in a specific position, at a particular location in space, at a single moment of time. And what you are, on this way of thinking, is the fusion of all these temporal parts. You are a “spacetime worm” that curves through the four-dimensional manifold known as spacetime . Moreover, on this view, what it is to have a momentary property at a time is to have a temporal part at the time that has the property in question. So you are sitting right now in virtue of the fact that your current temporal part is sitting.

The doctrine of temporal parts that B-theorists and eternalists tend to like can be stated like this:

Four-Dimensionalism: Any physical object that is located at different times has a different temporal part for each moment at which it is located.

On this view you have a temporal part right now, which is a three-dimensional “time slice” of you. And you have a different temporal part at noon yesterday, but no temporal parts in the year 1900 (since you are not located at any time in 1900). Also on this view, the physical object that is you is a fusion of all of your many temporal parts. (Note: there is a variation on the standard four-dimensional view, which is sometimes called “the worm view”. The variation, known as “the stage view”, holds that names and personal pronouns normally refer, not to entire fusions of temporal parts but, rather, to the individual person-stages, each of which is located at just an instant of time, and each of which counts as a person, rather than a mere part of a person).

The opposing view is three-dimensionalism, which is just the denial of the claim that temporally extended physical objects must have temporal parts. Here is a formulation of the view:

Three-Dimensionalism: Any physical object that is located at different times is wholly present at each moment at which it is located.

According to three-dimensionalism, the thing that was doing whatever you were doing at noon yesterday was you. It was you who was doing that, and now you are doing something different (namely, reading this sentence). So the relation between “you then” and “you now” is identity . According to four-dimensionalism, on the other hand, the thing that was doing whatever you were doing at noon yesterday was an earlier temporal part of the thing that is you, and the thing that is doing what you are doing now is the present temporal part of you. The relation between “you then” and “you now” is the temporal counterpart relation. (This is similar to the relation between your left hand and your right hand, which is the spatial counterpart relation. Your two hands are distinct parts of a bigger thing that contains them both.)

David Lewis, one of the main proponents of four-dimensionalism, suggests that the principal reason to accept the view is to solve what he calls “the problem of temporary intrinsics”. How can a single thing—Lewis, for example—have different intrinsic properties—like being straight, while he is standing, and then being bent, while seated—at different times? Not by standing in different relations—the being straight at and being bent at relations—to different times, he argues. (Since, he says, being straight and being bent are genuine properties rather than disguised relations.) And not in virtue of there being only one reality—such as the time when Lewis is bent—so that reality consists of Lewis, and every other thing, being the way it is now and not any other way. (For Lewis points out that we all believe we have a past and a future, in addition to a present.) So Lewis suggests that the best answer to the question about how a single thing can have different intrinsic properties at different times is that such an object has different temporal parts which themselves have the different intrinsic properties.

There is, however, a natural three-dimensionalist response to this argument. It involves appealing to a certain way of thinking about time, truth, and propositions that we touched on briefly in Section 1 , namely, the idea that propositions are in some way “tensed” as opposed to “tenseless”. Here is a way to formulate the relevant semantic thesis:

The Tensed Conception of Semantics

• Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time.
• We cannot eliminate verbal tenses like is , was , and will be from an ideal language.

On this view, a sentence like “Sullivan is eating a burrito” expresses a proposition that used to be true, but is false now.

The alternative to the tensed conception of semantics is the tenseless conception of semantics . On the latter view, an utterance of a sentence like “Sullivan is eating a burrito” expresses a proposition about a B-relation between events—it says that Sullivan’s eating a burrito is simultaneous with the utterance itself (or perhaps with the time of the utterance). Here is a way of stating this view:

The Tenseless Conception of Semantics

• Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time.
• We can in principle eliminate verbal tenses like is , was , and will be from an ideal language.

Consideration of Lewis’s argument from temporary intrinsics has shown that a three-dimensionalist should probably endorse the tensed conception of semantics, in order to account for changing truths about the world and its objects. And once we have seen this, it also becomes clear that A-theorists, presentists, and proponents of the growing block theory all have similar reasons for adopting the tensed conception of semantics. For the A-theorist is committed to there being changing truths about which times and events are future, which are present, and which are past; and presentists and growing block theorists are both committed to there being changing truths about what exists.

Suggestions for Further Reading: Hawley 2004 [2020]; Lewis 1986; Sider 2001; Thomson 1983; van Inwagen 1990

Many of the above considerations—especially those about McTaggart’s Argument; the A-theory and the B-theory; presentism, eternalism, and the growing block theory; and the dispute between three-dimensionalism and four-dimensionalism—suggest that there are, generally speaking, two very distinct ways of thinking about the nature of time. The first is the Static Theory of Time, according to which time is like space, and there is no such thing as the passage of time; and the second is the Dynamic Theory of Time, according to which time is very different from space, and the passage of time is a real phenomenon. These two ways of thinking about time are not the only such ways, but they correspond to the two most popular combinations of views about time to be found in the literature, which are arguably the most natural combinations of views on these issues. In this section we will spell out these two popular combinations, mainly as a way to synthesize much of the preceding material, and also to allow the reader to appreciate in a big-picture way how the different disputes about the nature of time are normally taken to be interrelated.

The guiding thought behind the Static Theory of Time is that time is like space. Here are six ways in which this thought is typically spelled out. (Note: The particular combination of these six theses is a natural and popular combination of related claims. But it is not inevitable. It is also possible to mix and match from among the tenets of the Static Theory and its rival, the Dynamic Theory.)

The Static Theory of Time

• The universe is spread out in four similar dimensions, which together make up a unified, four-dimensional manifold, appropriately called spacetime .
• Any physical object that is located at different times has a different temporal part for each moment at which it is located.
• There are no genuine and irreducible A-properties; all talk that appears to be about A-properties can be correctly analyzed in terms of B-relations. Likewise, the temporal facts about the world include facts about B-relations, but they do not include any facts about A-properties.
• The correct ontology does not change over time, and it always includes objects from every region of spacetime.
• Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time. Also, we can in principle eliminate verbal tenses like is , was , and will be from an ideal language.
• There is no dynamic aspect to time; time does not pass.

Static Theorists of course admit that time seems special to us, and that it seems to pass. But they insist that this is just a feature of consciousness—of how we perceive the world—and not a feature of reality that is independent of us.

The second of the main ways of thinking about time is the Dynamic Theory of Time. The guiding thought behind this way of thinking is that time is very different from space. Here are six ways in which this thought is typically spelled out. (Note: The particular combination of these six theses is a natural and popular combination of related claims. But, like the Static Theory, it is not inevitable. It is also possible to mix and match from among the tenets of the Dynamic Theory and the Static Theory.)

The Dynamic Theory of Time

• The universe is spread out in the three dimensions of physical space, and time, like modality, is a completely different kind of dimension from the spatial dimensions.
• Any physical object that is located at different times is wholly present at each moment at which it is located.
• There are genuine and irreducible A-properties, which cannot be correctly analyzed in terms of B-relations. The temporal facts about the world include ever-changing facts involving A-properties, including facts about which times are past, which time is present, and which times are future.
• The correct ontology changes over time, and it is always true that only present objects exist.
• Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time. Also, we cannot eliminate verbal tenses like is , was , and will be from an ideal language.
• The passage of time is a real and mind-independent phenomenon.

Opponents of the Dynamic Theory (and sometimes proponents as well) like to characterize the theory using the metaphor of a moving spotlight that slides along the temporal dimension, brightly illuminating just one moment of time, the present, while the future is a foggy region of potential and the past is a shadowy realm of what has been. The moving spotlight is an intuitively appealing way to capture the central idea behind the Dynamic Theory, but in the end, it is just a metaphor. What the metaphor represents is the idea that A-properties like being future , being present , and being past are objective and metaphysically significant properties of times, events, and things. Also, the metaphor of the moving spotlight represents the fact that, according to the Dynamic Theory, each time undergoes a somewhat peculiar but inexorable process, sometimes called temporal becoming . It goes from being in the distant future to the near future, has a brief moment of glory in the present, and then recedes forever further and further into the past.

Despite its being intuitively appealing (especially for Static Theorists, who see it as a caricature of the Dynamic Theory), the moving spotlight metaphor has a major drawback, according to some proponents of the Dynamic Theory: it encourages us to think of time as a fourth dimension, akin to the dimensions of space. For many proponents of the Dynamic Theory, this way of thinking—“spatializing time”—is a mistake. Instead, we should take seriously the ways that time seems completely different from the dimensions of space—for instance, time’s apparent directionality, and the distinctive ways that time governs experience.

Suggestions for Further Reading: Hawley 2001; Lewis 1986; Markosian 1993; Markosian 2004; Markosian (forthcoming); Moss 2012; Price 1977; Prior 1967; Prior 1968; Sider 2001; Smart 1949; Sullivan 2012a; Thomson 1983; and Williams 1951.

Above we mentioned that a metaphor sometimes used to characterize the Dynamic Theory is that of a moving spotlight that slides along the temporal dimension and that is such that only objects within the spotlight exist. A similar sort of metaphor can also be used to characterize the Moving Spotlight Theory, which is an interesting hybrid of the Static Theory and the Dynamic Theory. Like the Static Theory, the Moving Spotlight Theory incorporates the idea of spacetime as a unified manifold, with objects spread out along the temporal dimension in virtue of having different temporal parts at different times, and with past, present, and future parts of the manifold all equally real. But like the Dynamic Theory, it incorporates the thesis that A-properties are objective and irreducible properties, as well as the idea that time genuinely passes. The metaphor that characterizes the Moving Spotlight Theory is one on which there is a moving spotlight that slides along the temporal dimension and that is such that only things that are within the spotlight are present (but things that are outside the spotlight still exist).

Thus the Moving Spotlight Theory is an example of an eternalist A-theory that subscribes to the dynamic thesis. Unlike presentist or growing block theories, spotlighters deny that any objects come into or out of existence. Unlike the B-theories, however, spotlighters think that there is an important kind of change that cannot be described just as mere variation in a spacetime manifold. Spotlighters think instead that there is a spacetime manifold, but one particular region of the manifold is objectively distinguished—the present. And this distinction is only temporary—facts about which region of spacetime count as the present change over time. For example, right now a region of 2019 is distinguished as present. But in a year, a region of 2020 will enjoy this honor. The term “moving spotlight theory” was coined by C.D. Broad—himself a growing blocker—because he thought this view of time treated passage on the metaphor of a policeman’s “bull’s eye” scanning regions in sequence and focusing attention on their contents.

Just as there are different understandings of presentism and eternalism, there are different versions of the moving spotlight theory. Some versions think that even though the present is distinguished, there is still an important sense in which the past and future are concrete. Other versions (like Cameron 2015) treat the spotlight theory more like a variant of presentism—past and future objects still exist, but their intrinsic properties are radically unlike those of present objects. Fragmentalists (see Fine 2005) think that there is a spacetime manifold but that every point in the manifold has its own type of objective presentness, which defines a past and future relative to the point.

Why be a spotlighter? Advocates think it combines some of the best features of eternalism while still making sense of how we seem to perceive a world of substantive passage. It also inherits some of the counterintuitive consequences of eternalism (i.e., believing dinosaurs still exist) and the more complicated logic of the A-theories (i.e., it requires rules for reasoning about tensed propositions involving the spotlight).

Suggestions for Further Reading: Broad 1923; Cameron 2015; Fine 2005; Hawley 2004 [2020]; Lewis 1986 (especially Chapter 4.2); Sider 2001; Skow 2015; Thomson 1983; Van Inwagen 1990; Zimmerman 1998.

We are all familiar with time travel stories, and there are few among us who have not imagined traveling back in time to experience some particular period or meet some notable person from the past. But is time travel even possible?

One question that is relevant here is whether time travel is permitted by the prevailing laws of nature. This is presumably a matter of empirical science (or perhaps the correct philosophical interpretation of our best theories from the empirical sciences). But a further question, and one that falls squarely under the heading of philosophy, is whether time travel is permitted by the laws of logic and metaphysics. For it has been argued that various absurdities follow from the supposition that time travel is (logically and metaphysically) possible. Here is an example of such an argument:

Another argument that might be raised against the possibility of time travel depends on the claim that presentism is true. For if presentism is true, then neither past nor future objects exist. And in that case, it is hard to see how anyone could travel to the past or the future.

A third argument, against the possibility of time travel to the past, has to do with the claim that backward causation is impossible. For if there can be no backward causation, then it is not possible that, for example, your pushing the button in your time machine in 2020 can cause your appearance, seemingly out of nowhere, in, say, 1900. And yet it seems that any story about time travel to the past would have to include such backward causation, or else it would not really be a story about time travel.

Despite the existence of these and other arguments against the possibility of time travel, there may also be problems associated with the claim that time travel is not possible. For one thing, many scientists and philosophers believe that the actual laws of physics are in fact compatible with time travel. And for another thing, as we mentioned at the beginning of this section, we often think about time travel stories; but when we do so, those thoughts do not have the characteristic, glitchy feeling that is normally associated with considering an impossible story. To get a sense of the relevant glitchy feeling, consider this story: Once upon a time there was a young girl, and two plus two was equal to five . When you try to consider that literary gem, you mainly have a feeling that something has gone wrong (you immediately want to respond, “No, it wasn’t”), and the source of that feeling seems to be the metaphysical impossibility of the story being told. But nothing like this happens when you consider a story about time travel (especially if it is one of the logically consistent stories about time travel, such as the one depicted in the movie Los Cronocrímenes (Timecrimes) ). One task facing the philosopher who claims that time travel is impossible, then, is to explain the existence of a large number of well-known stories that appear to be specifically about time travel, and that do not cause any particular cognitive dissonance.

Suggestions for Further Reading: Bernstein 2015, 2017; Dyke 2005; Earman 1995; Markosian (forthcoming); Meiland 1974; Miller 2017; Sider 2001; Thorne 1994; Vihvelin 1996; Yourgrau 1999.

Our best physical theories have often had implications for the nature of time, and by and large, it is assumed that philosophers working on time need to be sensitive to the claims of contemporary physics. One example of the interaction between physics and philosophy of time that was mentioned in Section 2 was Newton’s bucket argument, which used the observed effects of acceleration to argue for absolute motion (and thus absolute space and time). Another example mentioned above was the worry that the A-theory conflicted with special relativity. The latter has proved especially influential in contemporary metaphysics of time and so deserves some further discussion.

According to standard presentations of special relativity, there is no fact of the matter as to whether two spatially separated events happen at the same time. This principle, which is known as the relativity of simultaneity , creates serious difficulty for the A-theory in general and for presentism in particular. After all, it follows from the relativity of simultaneity that there is no fact of the matter as to what is present, and according to any A-theory there is an important distinction between what is present and what is merely past or future. According to presentism, that distinction is one of existence—only what is present exists.

A different way of describing the relativity of simultaneity involves the combination of two claims:

• the claim that whether two spatially separated events happen at the same time depends on the reference frame you use to describe them, and
• the claim that no reference frame is privileged.

This way of putting the relativity of simultaneity requires a new bit of technical jargon: the notion of a reference frame. For our purposes, a reference frame is nothing more than a coordinate system that is used to identify the same point in space at different times. Someone on a steadily moving train, for instance, will naturally use a reference frame that is different from someone who is standing on the station platform, since it is natural for the person on the train to think of themselves as stationary, while for the person on the platform it seems obvious that they are moving.

The reason why it is worth introducing this bit of jargon is that once you present the relativity of simultaneity as the combination of claims (i) and (ii), you can also note that the motivation for claim (i) is importantly different from the motivation for claim (ii). The motivation for (i) is a series of empirical results at the end of the nineteenth and beginning of the twentieth century, including, most famously, the Michelson-Morley experiment. No one should deny this part of the relativity of simultaneity. The motivation for (ii), by contrast, is less often explicitly discussed, and seems to involve the commitment to some sort of general extra-empirical principle like “eliminate unobservable entities whenever possible”, or “eliminate excess spacetime structure whenever possible”. This means that presentists and other A-theorists have a way of avoiding the worry from relativity without any conflict with empirical results—they can reject whatever extra-empirical principle motivates (ii). Whether you think the costs associated with this move are worth paying will depend on your degree of commitment to the A-theory, what exactly you think of the relevant extra-empirical principle supporting (ii), and whether that principle plays an important role elsewhere in physics.

It is often said that philosophers should defer to physics with respect to what the latter says about time. But the interaction between the A-theory and special relativity illustrates one way in which that claim is more complicated than it first appears. Must philosophers respect both the empirical and the extra-empirical aspects of our best physical theories? Or is it sufficient that they respect the former?

Another way in which this assumption is complicated is that different physical theories often seem to imply different things about the nature of time. Consider, for instance, the fact that in general relativity there is sometimes (though not always!) a preferred way of “foliating” spacetime into instants of time and thus reintroducing a notion of absolute simultaneity, or the fact that on some interpretations of quantum mechanics, the dynamical laws seem to require a notion of absolute simultaneity.

Two additional questions about the nature of time that have been especially influenced by contemporary physics have to do with the arrow of time and the extent to which time itself might be emergent.

To motivate the first question, start from the observation that the order in which events happen in time seems to matter a great deal. There seems to be an important difference, for instance between a train traveling from Boston to Providence and a train traveling from Providence to Boston. This is because, even though both of these sequences may be constituted by the very same events, those events are in a different order in each sequence. In the former sequence the train being in Boston happens earlier than the train being in Providence. In the latter, the train being in Boston happens later.

These straightforward observations show that we experience time as having a direction. This is what philosophers call “the arrow of time”. But is the arrow of time a fundamental feature of the world? Or can it be reduced to some other feature, thus simplifying our metaphysics as a whole?

One way to try to eliminate the arrow of time at the fundamental level is to make use of certain interpretations of statistical mechanics inspired by Ludwig Boltzmann’s work. Imagine the history of the universe as a long timeline, but with no indication of which end of the time line represents the first moment of time and which end represents the last moment. It follows from certain interpretations of statistical mechanics that there is a physical quantity, the entropy of the universe, that will be relatively low at one end of the timeline and relatively high at the other end and will always increase as you move from the former end of the timeline to the latter. (More carefully, the entropy will almost always increase or at least stay constant.) The thought, then, is that we might be able to reduce the arrow of time to this entropy gradient. Earlier moments of time are just moments of time when the entropy of the universe is lower.

This way of eliminating the arrow of time from the fundamental level is promising, but has at least some unintuitive consequences. For instance, it seems natural to think that entropy could have decreased over time, instead of increasing over time as it actually does. But given the reduction described above, it is not in fact possible for entropy to decrease over time.

The second question mentioned above is a question about whether time itself—as opposed to just some particular feature of time, like time’s arrow—might merely be an emergent feature of the world. This question has become especially pressing as philosophers of physics have turned their attention to theories of quantum gravity in which there does not seem to be anything like temporal structure at the fundamental level. Work in this area is nascent, but some of the questions of interest include: Does quantum gravity eliminate time entirely or does it merely make time a non-fundamental feature of the world? What would it mean for something temporal to be grounded in something atemporal and what sort of grounding relation would be involved? What is the distinction between causal structure (especially the causal structure in causal set theory—one approach to quantum gravity) and temporal structure? And how can a theory that eliminates time entirely be empirically confirmed or disconfirmed?

Suggestions for Further Reading: Albert 2000; Emery 2019 & forthcoming; Godfrey-Smith 1979; Healey 2002; Huggett and Wüthrich 2013; Knox 2013; Markosian 2004; Maxwell 1985; Monton 2006; Price 1996; Putnam 1967; Rovelli 2017; Savitt 2000; Stein 1968, 1970; Weingard 1972; Wüthrich and Callender 2017.

A final important question concerns how considerations about the nature of time ought to impact the ways that we reason about time. For example, if it turns out that a B-theory is true, and there is no metaphysically important difference between the past and future, then should we adopt a more neutral attitude about events in our personal past and future? Epicurean philosopher Lucretius famously suggested that if there is no substantive difference between the times in the past before we came to exist and the times in the future after we die, we should care much less about the deprivation that death will bring. But we may think that even if the B-theory can describe everything that is metaphysically important without positing an important difference between the past and future, there is still an indispensable psychological importance to the past/future distinction that rational agents honor. Still other A-theorists argue that while there is an important metaphysical distinction between the past and future, the distinction has no normative importance.

If we deny three-dimensionalism and instead view ourselves as objects that persist through time by having temporal parts, then does that justify caring less about temporal parts in the distant future that are less strongly linked with our present part? Derek Parfit famously argued that a proper understanding of what we care about when we care about our own future persistence should motivate us to be less self-interested and more interested in redistributing resources to others. Endurantists have argued that facts about how we persist through time underwrite a strong distinction between moral principles (which concern what we owe to others now) and prudential rationality (which concerns what we owe to our future selves).

Another interesting line of research uses empirical work in psychology to better understand what is happening cognitively when we judge time as passing. This is especially pressing for B-theorists, who must explain why time seems to pass in psychologically or rationally significant ways, even though all passage is really just variation in an eternal manifold. Some B-theorists explain the apparent passage of time as an illusion of flow caused by perceptual processes that attribute apparent motion to events that happen in sequence. Another, compatible approach considers the way that evolutionary pressures might have shaped emotions and cognitive heuristics to give us a strong past/future distinction in our reasoning.

Suggestions for Further Reading: Brink 2003; Suhler and Callender 2012; Parfit 1971; Paul 2010; Prosser 2016; Sullivan 2018.

• Adams, Robert Merrihew, 1986, “Time and Thisness”, Midwest Studies in Philosophy , 11: 315–329. doi:10.1111/j.1475-4975.1986.tb00501.x
• Albert, David, 2000, Time and Chance , Cambridge, MA: Harvard University Press.
• Alexander, H.G. (ed. and trans.), 1956, The Leibniz-Clarke Correspondence , Manchester: Manchester University Press.
• Ariew, Roger (ed), 2000, Leibniz and Clarke: Correspondence , Indianapolis, IN: Hackett Publishing.
• Aristotle, De Interpretatione , in Aristotle, The Complete Works of Aristotle , Princeton, NJ: Princeton University Press, 1984, Chapter 9.
• –––, Physics , in Aristotle, The Complete Works of Aristotle , Princeton, NJ: Princeton University Press, 1984, Book VIII.
• Arntzenius, Frank, 2012, Space, Time, and Stuff , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780199696604.001.0001
• Baker, Alan, 2004 [2016], “Simplicity”, The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/win2016/entries/simplicity/ >.
• Barnes, Elizabeth and Ross Cameron, 2009, “The Open Future: Bivalence, Determinism and Ontology”, Philosophical Studies , 146(2): 291–309. doi:10.1007/s11098-008-9257-6
• Bernstein, Sara, 2015, “Nowhere Man: Time Travel and Spatial Location: Nowhere Man”, Midwest Studies In Philosophy , 39: 158–168. doi:10.1111/misp.12041
• –––, 2017, “Time Travel and the Movable Present”, in Being, Freedom, and Method , John A. Keller (ed.), Oxford: Oxford University Press, 80–92. doi:10.1093/acprof:oso/9780198715702.003.0005
• Beyer, Christian, 2003 [2018] “Edmund Husserl”, The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/sum2018/entries/husserl/ >.
• Bigelow, John, 1996, “Presentism and Properties”, Philosophical Perspectives , 10: 35–52. doi:10.2307/2216235
• Boethius, The Consolation of Philosophy , Book V, Prose vi.
• Bourne, Craig, 2006, A Future for Presentism , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780199212804.001.0001
• Bradley, F.H., 1893, Appearance and Reality , London: Swan Sonnenschein; second edition, with an appendix, 1897; ninth impression, corrected, Oxford: Clarendon Press, 1930.
• Brink, David O., 2003, “Prudence and Authenticity: Intrapersonal Conflicts of Value”, Philosophical Review , 112(2): 215–245. doi:10.1215/00318108-112-2-215
• Broad, C.D., 1923, Scientific Thought , New York: Harcourt, Brace and Co.
• Cameron, Ross P., 2015, The Moving Spotlight: An Essay on Time and Ontology , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780198713296.001.0001
• Coope, Ursula, 2001, “Why Does Aristotle Say That There Is No Time Without Change?”, Proceedings of the Aristotelian Society , 101(1): 359–367. doi:10.1111/j.0066-7372.2003.00036.x
• Crisp, Thomas M., 2007, “Presentism and The Grounding Objection”, Noûs , 41(1): 90–109. doi:10.1111/j.1468-0068.2007.00639.x
• Dainton, Barry, 2010 [2018], “Temporal Consciousness”, The Stanford Encyclopedia of Philosophy (Winter 2018 Edition), Edward N. Zalta (ed.), URL=< https://plato.stanford.edu/archives/win2018/entries/consciousness-temporal/ >.
• Dyke, Heather, 2002, “McTaggart and the Truth about Time”, in Time, Reality & Experience , Craig Callender (ed.), Cambridge: Cambridge University Press, 137–152. doi:10.1017/CBO9780511550263.008
• –––, 2005, “The Metaphysics and Epistemology of Time Travel”, Think , 3: 43–52.
• Earman, John, 1995, “Recent Work on Time Travel”, in Savitt 1995: 268–310. doi:10.1017/CBO9780511622861.014
• Emery, Nina, 2017, “Temporal Ersatzism”, Philosophy Compass , 12(9): e12441. doi:10.1111/phc3.12441
• –––, 2019, “Actualism without Presentism? Not by Way of the Relativity Objection”, Noûs , 53(4): 963–986. doi:10.1111/nous.12247
• –––, 2020, “Actualism, Presentism and the Grounding Objection”, Erkenntnis , 85(1): 23–43. doi:10.1007/s10670-018-0016-6
• –––, forthcoming, “Temporal Ersatzism and Relativity”, Australasian Journal of Philosophy , first online: 7 July 2020. doi:10.1080/00048402.2020.1780621
• Evans, Gareth, 1985, “Does Tense Logic Rest on a Mistake?” in Collected Papers: Gareth Evans , Oxford: Clarendon Press, pp. 343–363.
• Fine, Kit, 2005, Modality and Tense: Philosophical Papers , Oxford: Clarendon Press. doi:10.1093/0199278709.001.0001
• Godfrey-Smith, William, 1979, “Special Relativity and the Present”, Philosophical Studies , 36(3): 233–244. doi:10.1007/BF00372628
• Hawley, Katherine, 2001, How Things Persist , Oxford: Oxford University Press.
• –––, 2004 [2020], “Temporal Parts”, The Stanford Encyclopedia of Philosophy (Summer 2020 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/sum2020/entries/temporal-parts/ >.
• Healey, Richard, 2002, “Can Physics Coherently Deny the Reality of Time?”, Royal Institute of Philosophy Supplement , 50: 293–316. doi:10.1017/S1358246100010614
• Hinchliff, Mark, 1996, “The Puzzle of Change”, Philosophical Perspectives , 10: 119–136. doi:10.2307/2216239
• Huggett, Nick and Christian Wüthrich, 2013, “Emergent Spacetime and Empirical (in)Coherence”, Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics , 44(3): 276–285. doi:10.1016/j.shpsb.2012.11.003
• Ingram, David, 2016, “The Virtues of Thisness Presentism”, Philosophical Studies , 173(11): 2867–2888. doi:10.1007/s11098-016-0641-3
• Kant, Immanuel, 1771/87, The Critique of Pure Reason , Norman Kemp Smith (trans.), London: Macmillan, 1963, pp. 75ff.
• Keller, Simoon and Michael Nelson, 2001, “Presentists Should Believe in Time-Travel”, Australasian Journal of Philosophy , 79(3): 333–345. doi:10.1080/713931204
• Knox, Eleanor, 2013, “Effective Spacetime Geometry”, Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics , 44(3): 346–356. doi:10.1016/j.shpsb.2013.04.002
• Le Poidevin, Robin (ed.), 1998, Questions of Time and Tense , Oxford: Oxford University Press.
• –––, 2000 [2019], “The Experience and Perception of Time”, The Stanford Encyclopedia of Philosophy (Summer 2019 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/sum2019/entries/time-experience/ >.
• Le Poidevin, Robin and Murray McBeath (eds.), 1993, The Philosophy of Time , Oxford: Oxford University Press.
• Lewis, David, 1986, “The Paradoxes of Time Travel”, in his Philosophical Papers , Volume 2, Oxford: Oxford University Press, pp. 67–80.
• Markosian, Ned, 1993, “How Fast Does Time Pass?”, Philosophy and Phenomenological Research , 53(4): 829–844. doi:10.2307/2108255
• –––, 1995, “The Open Past”, Philosophical Studies , 79(1): 95–105. doi:10.1007/BF00989786
• –––, 2004, “A Defense of Presentism”, in Oxford Studies in Metaphysics , volume 1, Dean W. Zimmerman (ed.), Oxford: Oxford University Press, pp. 47–82.
• –––, 2013, “The Truth About the Past and the Future”, in Around the Tree: Semantic and Metaphysical Issues Concerning Branching Time and the Open Future , Fabrice Correia and Andrea Iacona (eds.), Dordrecht: Springer Netherlands, 127–141. doi:10.1007/978-94-007-5167-5_8
• –––, forthcoming, “The Dynamic Theory of Time and Time Travel to the Past”, Disputatio .
• Maudlin, Tim, 2007, “On the Passing of Time”, in his The Metaphysics Within Physics , Oxford: Oxford University Press, Chapter 4.
• Maxwell, Nicholas, 1985, “Are Probabilism and Special Relativity Incompatible?”, Philosophy of Science , 52(1): 23–43.
• McCall, Storrs, 1994, A Model of the Universe: Space-Time, Probability, and Decision , Oxford: Clarendon Press. doi:10.1093/acprof:oso/9780198236221.001.0001
• McTaggart, J. M. Ellis, 1908, “The Unreality of Time”, Mind , 17(4): 457–474. Reprinted in Le Poidevin and McBeath 1993: 23–34. doi:10.1093/mind/XVII.4.457
• Mellor, D.H., 1998, Real Time II , London: Routledge.
• Meiland, Jack W., 1974, “A Two-Dimensional Passage Model of Time for Time Travel”, Philosophical Studies , 26(3–4): 153–173. doi:10.1007/BF00398876
• Miller, Kristie, 2005, “Time Travel and the Open Future”, Disputatio , 1(19): 223–232. doi:10.2478/disp-2005-0009
• –––, 2017, “Is Some Backwards Time Travel Inexplicable?” American Philosophical Quarterly , 54(2): 131–141.
• Mitchell, Sam, 1993, “Mach’s Mechanics and Absolute Space and Time”, Studies in History and Philosophy of Science Part A , 24(4): 565–583. doi:10.1016/0039-3681(93)90053-M
• Monton, Bradley, 2006, “Presentism and Quantum Gravity”, in The Ontology of Spacetime , Dennis Dieks (ed.), (Philosophy and Foundations of Physics 1), Dordrecht: Elsevier, 263–280. doi:10.1016/S1871-1774(06)01014-X
• Moss, Sarah, 2012, “Four-Dimensionalist Theories of Persistence”, Australasian Journal of Philosophy , 90: 671–686.
• Newton, Isaac, 2004, Isaac Newton: Philosophical Writings , Andrew Janiak (ed.), Cambridge: Cambridge University Press. doi:10.1017/CBO9780511809293
• Newton-Smith, W.H., 1980, The Structure of Time , London: Routledge & Kegan Paul. doi:10.4324/9780429020506
• Parfit, Derek, 1971, “Personal Identity”, The Philosophical Review , 80(1): 3–27. doi:10.2307/2184309
• Paul, L. A., 2010, “Temporal Experience”, Journal of Philosophy , 107(7): 333–359. doi:10.5840/jphil2010107727
• Price, Huw, 1994, “A Neglected Route to Realism about Quantum Mechanics”, Mind , 103(411): 303–336. doi:10.1093/mind/103.411.303
• –––, 1996, Time’s Arrow and Archimedes’ Point: New Directions for the Physics of Time , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780195117981.001.0001
• Price, Marjorie, 1977, “Identity Through Time”, The Journal of Philosophy , 74: 201–217.
• Prior, Arthur N., 1959 [1976], “Thank Goodness That’s Over”, Philosophy , 34(128): 12–17. Reprinted in his Papers in Logic and Ethics , P. T. Geach and A. J. P. Kenny (eds), London: Duckworth, 1976, pp. 78–84. doi:10.1017/S0031819100029685
• –––, 1962 [1968], Changes in Events and Changes in Things (Lindley Lecture Series), Lawrence, KS: University of Kansas. Reprinted in Prior 1968b: 1–14. [ Prior 1962 available online ]
• –––, 1967, Past, Present, and Future , Oxford: Oxford University Press.
• –––, 1968, Papers on Time and Tense , Oxford: Oxford University Press.
• –––, 1970, “The Notion of the Present”, Studium Generale , 23: 245–248. Reprinted in The Study of Time , J. T. Fraser, F. C. Haber, and G. H. Müller (eds), Berlin: Springer-Verlag, 1972, 320–323.
• –––, 1996, “Some Free Thinking About Time”, an undated manuscript first published after his death in Logic and Reality: Essays on the Legacy of Arthur Prior , Jack Copeland (ed.), Oxford: Clarendon Press, pp. 47–51.
• Prosser, Simon, 2016, Experiencing Time , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780198748946.001.0001
• Putnam, Hilary, 1967, “Time and Physical Geometry”:, Journal of Philosophy , 64(8): 240–247. doi:10.2307/2024493
• Rini, Adriane A. and Max J. Cresswell, 2012, The World–Time Parallel: Tense and Modality in Logic and Metaphysics , Cambridge: Cambridge University Press. doi:10.1017/CBO9781139084215
• Richard, Mark, 1981, “Temporalism and Eternalism”, Philosophical Studies , 39(1): 1–13. doi:10.1007/BF00354808
• Rovelli, Carlo, 2017, Reality is Not What It Seems: The Journey to Quantum Gravity , New York: Riverhead Books.
• Rynasiewicz, Robert, 2004 [2014], “Newton’s Views on Space, Time, and Motion”, The Stanford Encyclopedia of Philosophy (Summer 2014 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/sum2014/entries/newton-stm/ >.
• Savitt, Steven F. (ed.), 1995, Time’s Arrows Today: Recent Physical and Philosophical Work on the Direction of Time , Cambridge: Cambridge University Press. doi:10.1017/CBO9780511622861
• –––, 2000, “There’s No Time like the Present (In Minkowski Spacetime)”, Philosophy of Science , 67(supplement: Proceedings of the 1998 Biennial Meetings of the Philosophy of Science Association): S563–S574. doi:10.1086/392846
• Shoemaker, Sydney, 1969, “Time Without Change”, The Journal of Philosophy , 66(12): 363–381. doi:10.2307/2023892
• Sider, Theodore, 1999, “Presentism and Ontological Commitment”, The Journal of Philosophy , 96(7): 325. doi:10.2307/2564601
• –––, 2001, Four-Dimensionalism: An Ontology of Persistence and Time , Oxford: Oxford University Press. doi:10.1093/019924443X.001.0001
• Sklar, Lawrence, 1974, Space, Time, and Spacetime , Berkeley, CA: University of California Press.
• Skow, Bradford, 2009, “Relativity and the Moving Spotlight”:, Journal of Philosophy , 106(12): 666–678. doi:10.5840/jphil20091061224
• –––, 2015, Objective Becoming , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780198713272.001.0001
• Smart, J. J. C., 1949, “The River of Time”, Mind , 58(232): 483–494. Reprinted in Antony Flew (ed.), Essays in Conceptual Analysis , New York: St. Martin’s Press, 1966, pp. 213–227. doi:10.1093/mind/LVIII.232.483
• –––, 1963, Philosophy and Scientific Realism , London: Routledge & Kegan Paul.
• Smith, Quentin, 1993, Language and Time , Oxford: Oxford University Press.
• Stein, Howard, 1968, “On Einstein-Minkowski Space-Time”:, Journal of Philosophy , 65(1): 5–23. doi:10.2307/2024512
• –––, 1970, “A Note on Time and Relativity Theory”:, Journal of Philosophy , 67(9): 289–294. doi:10.2307/2024388
• Suhler, Christopher and Craig Callender, 2012, “Thank Goodness That Argument Is Over: Explaining the Temporal Value Asymmetry”, Philosopher’s Imprint , 12: art. 15. [ Suhler and Callender 2012 available online ]
• Sullivan, Meghan, 2012a, “The Minimal A-Theory”, Philosophical Studies , 158(2): 149–174. doi:10.1007/s11098-012-9888-5
• –––, 2012b, “Problems for Temporary Existence in Tense Logic”, Philosophy Compass , 7(1): 43–57. doi:10.1111/j.1747-9991.2011.00457.x
• –––, 2014, “Change We Can Believe In (and Assert): Change We Can Believe In (and Assert)”, Noûs , 48(3): 474–495. doi:10.1111/j.1468-0068.2012.00874.x
• –––, 2018, Time Biases: A Theory of Rational Planning and Personal Persistence , Oxford: Oxford University Press. doi:10.1093/oso/9780198812845.001.0001
• Swinburne, R. G., 1966, “The Beginning of the Universe”, Aristotelian Society Supplementary Volume , 40: 125–138. doi:10.1093/aristoteliansupp/40.1.125
• –––, 1968, Space and Time , London: Macmillan.
• Taylor, Richard, 1992, “Fate”, in his Metaphysics , fourth Edition, Englewood Cliffs, NJ: Prentice-Hall, Chapter 6.
• Thomson, Judith Jarvis, 1983, “Parthood and Identity Across Time”, The Journal of Philosophy , 80(4): 201–220. doi:10.2307/2026004
• Thorne, Kip S., 1994, Black Holes and Time Warps , New York: W.W. Norton.
• Tooley, Michael, 1997, Time, Tense, and Causation , Oxford: Oxford University Press. doi:10.1093/0198250746.001.0001
• Torre, Stephan, 2011, “The Open Future”, Philosophy Compass , 6(5): 360–373. doi:10.1111/j.1747-9991.2011.00395.x
• Van Inwagen, Peter, 1983, An Essay on Free Will , Oxford: Clarendon Press.
• –––, 1990, “Symposia Papers: Four-Dimensional Objects”, Noûs , 24(2): 245–255. doi:10.2307/2215526
• Vihvelin, Kadri, 1996, “What Time Travelers Cannot Do”, Philosophical Studies , 81(2–3): 315–330. doi:10.1007/BF00372789
• Weingard, Robert, 1972, “Relativity and the Reality of Past and Future Events”, The British Journal for the Philosophy of Science , 23(2): 119–121. doi:10.1093/bjps/23.2.119
• Wheeler, Michael, 2011 [2018], “Martin Heidegger”, The Stanford Encyclopedia of Philosophy (Winter 2018 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/win2018/entries/heidegger/ >.
• Williams, Donald C., 1951, “The Myth of Passage”:, Journal of Philosophy , 48(15): 457–472. doi:10.2307/2021694
• Wüthrich, Christian and Craig Callender, 2017, “What Becomes of a Causal Set?”, The British Journal for the Philosophy of Science , 68(3): 907–925. doi:10.1093/bjps/axv040
• Yourgrau, Palle, 1999, Gödel Meets Einstein: Time Travel in the Göodel Universe , La Salle: Open Court.
• Zimmerman, Dean W., 1996, “Persistence and Presentism”, Philosophical Papers , 25(2): 115–126. doi:10.1080/05568649609506542
• –––, 1998, “Temporary Intrinsics and Presentism”, in Peter van Inwagen and Dean W. Zimmerman (eds.), Metaphysics: The Big Questions , Oxford: Basil Blackwell, pp. 206–209.
• –––, 2005, “The A-Theory of Time, The B-Theory of Time, and ‘Taking Tense Seriously’”, Dialectica , 59(4): 401–457. doi:10.1111/j.1746-8361.2005.01041.x
• Zwart, P.J., 1976, About Time , Amsterdam: North-Holland Publishing Co.

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• A.N. Prior: The Founding Father of Temporal Logic , a web site devoted to the study of Arthur Norman Prior (the founder of tense logic), at the Danish Centre for Philosophy and Science Studies
• The Centre for Time at the Department of Philosophy, University of Sydney.
• Eternalism , entry in Wikipedia
• Philosophy of space and time , entry in Wikipedia
• Philosophical presentism , entry in Wikipedia
• Time , entry in the Internet Encyclopedia of Philosophy
• Time , entry in Wikipedia
• Time Travel , entry in the Internet Encyclopedia of Philosophy

causation: backward | consciousness: temporal | fatalism | future contingents: medieval theories of | Heidegger, Martin | Husserl, Edmund | indexicals | logic: temporal | McTaggart, John M. E. | Newton, Isaac: views on space, time, and motion | presentism | Prior, Arthur | propositions: singular | quantum theory: quantum gravity | space and time: absolute and relational space and motion, post-Newtonian theories | space and time: being and becoming in modern physics | space and time: conventionality of simultaneity | temporal parts | time: the experience and perception of | time: thermodynamic asymmetry in | time travel | time travel: and modern physics | Zeno of Elea: Zeno’s paradoxes

Copyright © 2020 by Nina Emery < emery @ mtholyoke . edu > Ned Markosian < markosian @ umass . edu > Meghan Sullivan < sullivan . meghan @ gmail . com >

• Accessibility

Support SEP

Mirror sites.

View this site from another server:

• Info about mirror sites

The Stanford Encyclopedia of Philosophy is copyright © 2023 by The Metaphysics Research Lab , Department of Philosophy, Stanford University

Library of Congress Catalog Data: ISSN 1095-5054

• Subject List
• Take a Tour
• For Authors
• Subscriber Services
• Publications
• African American Studies
• African Studies
• American Literature
• Anthropology
• Architecture Planning and Preservation
• Art History
• Atlantic History
• Biblical Studies
• British and Irish Literature
• Childhood Studies
• Chinese Studies
• Cinema and Media Studies
• Communication
• Criminology
• Environmental Science
• Evolutionary Biology
• International Law
• International Relations
• Islamic Studies
• Jewish Studies
• Latin American Studies
• Latino Studies
• Linguistics
• Literary and Critical Theory
• Medieval Studies
• Military History
• Political Science
• Public Health
• Renaissance and Reformation
• Social Work
• Urban Studies
• Victorian Literature
• Browse All Subjects

How to Subscribe

• Free Trials

In This Article Expand or collapse the "in this article" section Time Travel

Introduction, general overviews.

• David Lewis’s Analysis, Its Forerunners and Critics
• Gödel and the Ideality of Time
• Models and Issues from Relativity
• Models and Issues from Quantum Theory
• Causal Loops and Probability
• Time Travel in Many Worlds and the Autonomy Principle
• Travel in Dynamic Time and Multi-Dimensional Time
• General Metaphysical Issues

Related Articles Expand or collapse the "related articles" section about

About related articles close popup.

Lorem Ipsum Sit Dolor Amet

Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Aliquam ligula odio, euismod ut aliquam et, vestibulum nec risus. Nulla viverra, arcu et iaculis consequat, justo diam ornare tellus, semper ultrices tellus nunc eu tellus.

• Contemporary Metaphysics
• Foreknowledge
• Laws of Nature
• Persistence
• Philosophy of Cosmology
• Space and Time
• Time and Tense

Other Subject Areas

Forthcoming articles expand or collapse the "forthcoming articles" section.

• Alfred North Whitehead
• Feminist Aesthetics
• Find more forthcoming articles...
• Export Citations
• Share This Facebook LinkedIn Twitter

Time Travel by Alasdair Richmond LAST REVIEWED: 28 May 2019 LAST MODIFIED: 26 October 2015 DOI: 10.1093/obo/9780195396577-0295

Time travel is a philosophical growth industry, with many issues in metaphysics and elsewhere recently transformed by consideration of time travel possibilities. The debate has gradually shifted from focusing on time travel’s logical possibility (which possibility is now generally although not universally granted) to sundry topics including persistence, causation, personal identity, freedom, composition, and natural laws, to name but a few. Besides metaphysical discussions, some time travel works draw on the philosophies of science, spacetime, and computation. Some interesting forerunners notwithstanding, serious physical interest in time travel begins with Gödel’s 1949a demonstration that general relativity permits space-times that are riddled with closed timelike curves (“CTCs” henceforth). A key philosophical text on time travel is Lewis 1976 and its argument for the logical possibility of certain backward time travel journeys and even for the possibility of casual loops. Lewis concludes that time travel could occur in a possible world, albeit perhaps a strange world that would feature (or seem to feature) strange restrictions on actions. In Lewis’s analysis, a traveler can arrive in the past of the same history they come from provided that the traveler’s actions on arrival are consistent with the history that they come from. So other worlds or multiple temporal dimensions are not necessary to make time travel consistent. Granted, the physics, persistence conditions, agency, and epistemology of agents in such worlds might look weird indeed. Since Lewis, philosophical time travel questions include the following: given that a traveler into the past cannot create any paradoxical outcomes on arrival, what then would stay their hand? Are the constraints on a traveler’s actions admissible within our ordinary understanding of physical law or human agency? Is time travel compatible with dynamic time or even with the existence of time itself? Can backward time travel be physically possible within a single history? If a time traveler meets another stage of him- or herself, is the traveler in two places at once, and what theory of persistence can cope with this puzzling multiplication? Can time-travel spacetimes resolve otherwise intractable computational problems?

Despite several hundred philosophical and scientific articles, book chapters, and Internet resources devoted to philosophical problems posed by time travel, there is currently no full-length monograph or anthology on the subject. The best introduction to the topic in general so far is chapter 8 of Dainton (second edition 2010), Dainton 2010 being the best general philosophical resource available on time and space. The key work is Lewis 1976 , a defense of the logical possibility of backward time travel, from which a large number of subsequent treatments take their cue. A useful overview, albeit largely from a physical science perspective, is Nahin 1999 . Also largely physical in emphasis but comprehensive and thorough is Earman 1995 . Richmond 2003 surveys philosophical work on time travel to date. Arntzenius 2006 details the problems of free action and nomological constraint posed by backward time travel. Arntzenius and Maudlin 2005 is helpful on (especially) problems of physical law. Carroll 2008 is perhaps the best single online resource available on any aspect of time travel. Le Poidevin 2003 is a highly commendable introduction to the philosophy of time in general but especially good on problems of time travel. Bourne 2006 offers some useful arguments and clarifications centered on Gödel’s arguments about time travel and the relations between time travel and the status of times themselves. Earman and Wüthrich 2006 offers scientifically well informed but approachable and philosophically cogent discussions of what physics might, and might not, allow by way of time travel.

Arntzenius, Frank. “Time Travel: Double Your Fun.” Philosophy Compass 6 (2006): 599–616.

DOI: 10.1111/j.1747-9991.2006.00045.x

Entertaining survey of the philosophical terrain around time travel that concentrates particularly on the constraints on action likely to be suffered by travelers in the past. An excellent introduction to the nomological contrivance problem and more. Available online for purchase or by subscription.

Arntzenius, Frank, and Tim Maudlin. “ Time Travel and Modern Physics .” In Stanford Encyclopedia of Philosophy . Edited by Edward N. Zalta. 2005.

Notably acute survey of physical possibilities for time travel, including detailed arguments that backward time travel threatens to create correlations that conflict with standard quantum predictions.

Bourne, C. A Future for Presentism . Oxford: Oxford University Press, 2006.

DOI: 10.1093/acprof:oso/9780199212804.001.0001

Although primarily devoted to defending presentism, chapter 8 offers one of the best treatments of Gödel’s ideality argument around and pp. 132–134 offer some interesting sidelights on the possible compatability of time travel and presentism.

Carroll, John W. A Time Travel Website . 2008–.

Extremely thorough, engagingly-written, well-designed, and continually evolving online resource that offers helpful discussions, well-chosen readings, and helpful animations to boot.

Dainton, Barry. Time and Space . 2d ed. Durham, NC: Acumen, 2010.

Revised and expanded edition of Dainton’s classic 2001 introduction to the philosophy of space and time. Can be highly recommended but notable here for its extensive, essential treatments of time travel, relativity, and Gödel’s “ideality” argument.

Earman, John. “Recent Work on Time Travel.” In Time’s Arrows Today . Edited by Steven F. Savitt, 268–310. Cambridge, UK: Cambridge University Press, 1995.

DOI: 10.1017/CBO9780511622861

Thorough discussion of the then-current state of play in the philosophical and physical literature on time travel. This is still a valuable resource.

Earman, John, and Christian Wüthrich. “ Time Machines .” In Stanford Encyclopedia of Philosophy . Edited by Edward N. Zalta. 2006.

Comprehensive discussion of physical resources for time travel, among other intriguing suggestions, develops the view that physically realistic time machines might be uncontrollable even if they become a possiblility.

Le Poidevin, Robin. Travels in Four Dimensions: The Enigmas of Space and Time . Oxford: Oxford University Press, 2003.

Engaging and clearly written introduction to the philosophy of space and time. Often offers problems and discussions that lend themselves to time travel interpretation. An excellent introductory and pedagogical resource.

Lewis, David. “ The Paradoxes of Time Travel .” American Philosophical Quarterly 13 (1976): 145–152.

The philosophical time travel work. Includes Lewis’s discrepancy definition of time travel: the most useful by far. Invokes the notion of compossibility to disambiguate “Grandfather paradox” arguments and argues that backward time travel and causal loops can occur in (nonbranching) possible worlds. Usefully distinguishes between replacement change and counterfactual change. (This is often cited and sometimes rebutted but never refuted.)

Nahin, Paul. Time Machines: Time Travel in Physics, Metaphysics and Science Fiction . 1st ed. New York: American Institute of Physics, 1999.

DOI: 10.1007/978-1-4757-3088-3

Engaging and comprehensive attempt at surveying all the scientific, philosophical, and fictional literature on time travel. Perhaps slightly more at ease with physics and fiction than with philosophy, but this is a detailed and thorough treatment.

Richmond, Alasdair. “Recent Work: Time Travel.” Philosophical Books 44 (2003): 297–309.

DOI: 10.1111/1468-0149.00308

Survey of the time travel debate from Lewis 1976 onward, sketching links with debates in persistence, philosophy of spacetime and temporal topology. Available online by subscription.

back to top

Users without a subscription are not able to see the full content on this page. Please subscribe or login .

Oxford Bibliographies Online is available by subscription and perpetual access to institutions. For more information or to contact an Oxford Sales Representative click here .

• About Philosophy »
• Meet the Editorial Board »
• A Priori Knowledge
• Abduction and Explanatory Reasoning
• Abstract Objects
• Addams, Jane
• Adorno, Theodor
• Aesthetic Hedonism
• Aesthetics, Analytic Approaches to
• Aesthetics, Continental
• Aesthetics, Environmental
• Aesthetics, History of
• African Philosophy, Contemporary
• Alexander, Samuel
• Analytic/Synthetic Distinction
• Anarchism, Philosophical
• Animal Rights
• Anscombe, G. E. M.
• Anthropic Principle, The
• Anti-Natalism
• Applied Ethics
• Aquinas, Thomas
• Argument Mapping
• Art and Emotion
• Art and Knowledge
• Art and Morality
• Astell, Mary
• Aurelius, Marcus
• Austin, J. L.
• Bacon, Francis
• Bayesianism
• Bergson, Henri
• Berkeley, George
• Biology, Philosophy of
• Bolzano, Bernard
• Boredom, Philosophy of
• British Idealism
• Buber, Martin
• Buddhist Philosophy
• Burge, Tyler
• Business Ethics
• Camus, Albert
• Canterbury, Anselm of
• Carnap, Rudolf
• Cavendish, Margaret
• Chemistry, Philosophy of
• Childhood, Philosophy of
• Chinese Philosophy
• Cognitive Ability
• Cognitive Phenomenology
• Cognitive Science, Philosophy of
• Coherentism
• Communitarianism
• Computational Science
• Computer Science, Philosophy of
• Computer Simulations
• Comte, Auguste
• Conceptual Role Semantics
• Conditionals
• Confirmation
• Connectionism
• Consciousness
• Constructive Empiricism
• Contemporary Hylomorphism
• Contextualism
• Contrastivism
• Cook Wilson, John
• Cosmology, Philosophy of
• Critical Theory
• Culture and Cognition
• Daoism and Philosophy
• Davidson, Donald
• de Beauvoir, Simone
• de Montaigne, Michel
• Decision Theory
• Deleuze, Gilles
• Derrida, Jacques
• Descartes, René
• Descartes, René: Sensory Representations
• Descriptions
• Dewey, John
• Dialetheism
• Disagreement, Epistemology of
• Disjunctivism
• Dispositions
• Divine Command Theory
• Doing and Allowing
• du Châtelet, Emilie
• Dummett, Michael
• Dutch Book Arguments
• Early Modern Philosophy, 1600-1750
• Eastern Orthodox Philosophical Thought
• Education, Philosophy of
• Engineering, Philosophy and Ethics of
• Environmental Philosophy
• Epistemic Basing Relation
• Epistemic Defeat
• Epistemic Injustice
• Epistemic Justification
• Epistemic Philosophy of Logic
• Epistemology
• Epistemology and Active Externalism
• Epistemology, Bayesian
• Epistemology, Feminist
• Epistemology, Internalism and Externalism in
• Epistemology, Moral
• Epistemology of Education
• Ethical Consequentialism
• Ethical Deontology
• Ethical Intuitionism
• Eugenics and Philosophy
• Events, The Philosophy of
• Evidence-Based Medicine, Philosophy of
• Evidential Support Relation In Epistemology, The
• Evolutionary Debunking Arguments in Ethics
• Evolutionary Epistemology
• Experimental Philosophy
• Explanations of Religion
• Extended Mind Thesis, The
• Externalism and Internalism in the Philosophy of Mind
• Faith, Conceptions of
• Feminist Philosophy
• Feyerabend, Paul
• Fichte, Johann Gottlieb
• Fictionalism
• Fictionalism in the Philosophy of Mathematics
• Film, Philosophy of
• Foot, Philippa
• Forgiveness
• Formal Epistemology
• Foucault, Michel
• Frege, Gottlob
• Gadamer, Hans-Georg
• Geometry, Epistemology of
• God and Possible Worlds
• God, Arguments for the Existence of
• God, The Existence and Attributes of
• Grice, Paul
• Habermas, Jürgen
• Hart, H. L. A.
• Heaven and Hell
• Hegel, Georg Wilhelm Friedrich: Aesthetics
• Hegel, Georg Wilhelm Friedrich: Metaphysics
• Hegel, Georg Wilhelm Friedrich: Philosophy of History
• Hegel, Georg Wilhelm Friedrich: Philosophy of Politics
• Heidegger, Martin: Early Works
• Hermeneutics
• Higher Education, Philosophy of
• History, Philosophy of
• Hobbes, Thomas
• Horkheimer, Max
• Human Rights
• Hume, David: Aesthetics
• Hume, David: Moral and Political Philosophy
• Husserl, Edmund
• Idealizations in Science
• Identity in Physics
• Imagination
• Imagination and Belief
• Immanuel Kant: Political and Legal Philosophy
• Impossible Worlds
• Incommensurability in Science
• Indian Philosophy
• Indispensability of Mathematics
• Inductive Reasoning
• Instruments in Science
• Intellectual Humility
• Intentionality, Collective
• James, William
• Japanese Philosophy
• Kant and the Laws of Nature
• Kant, Immanuel: Aesthetics and Teleology
• Kant, Immanuel: Ethics
• Kant, Immanuel: Theoretical Philosophy
• Kierkegaard, Søren
• Knowledge-first Epistemology
• Knowledge-How
• Kristeva, Julia
• Kuhn, Thomas S.
• Lacan, Jacques
• Lakatos, Imre
• Langer, Susanne
• Language of Thought
• Language, Philosophy of
• Latin American Philosophy
• Legal Epistemology
• Legal Philosophy
• Legal Positivism
• Leibniz, Gottfried Wilhelm
• Levinas, Emmanuel
• Lewis, C. I.
• Literature, Philosophy of
• Locke, John
• Locke, John: Identity, Persons, and Personal Identity
• Lottery and Preface Paradoxes, The
• Machiavelli, Niccolò
• Martin Heidegger: Later Works
• Martin Heidegger: Middle Works
• Material Constitution
• Mathematical Explanation
• Mathematical Pluralism
• Mathematical Structuralism
• Mathematics, Ontology of
• Mathematics, Philosophy of
• Mathematics, Visual Thinking in
• McDowell, John
• McTaggart, John
• Meaning of Life, The
• Mechanisms in Science
• Medically Assisted Dying
• Medicine, Contemporary Philosophy of
• Medieval Logic
• Medieval Philosophy
• Mental Causation
• Merleau-Ponty, Maurice
• Meta-epistemological Skepticism
• Metaepistemology
• Metametaphysics
• Metaphilosophy
• Metaphysical Grounding
• Metaphysics, Contemporary
• Metaphysics, Feminist
• Midgley, Mary
• Mill, John Stuart
• Mind, Metaphysics of
• Modal Epistemology
• Models and Theories in Science
• Montesquieu
• Moore, G. E.
• Moral Contractualism
• Moral Naturalism and Nonnaturalism
• Moral Responsibility
• Multiculturalism
• Murdoch, Iris
• Music, Analytic Philosophy of
• Nationalism
• Natural Kinds
• Naturalism in the Philosophy of Mathematics
• Naïve Realism
• Neo-Confucianism
• Neuroscience, Philosophy of
• Nietzsche, Friedrich
• Nonexistent Objects
• Normative Ethics
• Normative Foundations, Philosophy of Law:
• Normativity and Social Explanation
• Objectivity
• Occasionalism
• Ontological Dependence
• Ontology of Art
• Ordinary Objects
• Other Minds
• Panpsychism
• Particularism in Ethics
• Pascal, Blaise
• Paternalism
• Peirce, Charles Sanders
• Perception, Cognition, Action
• Perception, The Problem of
• Perfectionism
• Personal Identity
• Phenomenal Concepts
• Phenomenal Conservatism
• Phenomenology
• Philosophy for Children
• Photography, Analytic Philosophy of
• Physicalism
• Physicalism and Metaphysical Naturalism
• Physics, Experiments in
• Political Epistemology
• Political Obligation
• Political Philosophy
• Popper, Karl
• Pornography and Objectification, Analytic Approaches to
• Practical Knowledge
• Practical Moral Skepticism
• Practical Reason
• Probabilistic Representations of Belief
• Probability, Interpretations of
• Problem of Divine Hiddenness, The
• Problem of Evil, The
• Propositions
• Psychology, Philosophy of
• Quine, W. V. O.
• Racist Jokes
• Rationalism
• Rationality
• Rawls, John: Moral and Political Philosophy
• Realism and Anti-Realism
• Realization
• Reasons in Epistemology
• Reductionism in Biology
• Reference, Theory of
• Reid, Thomas
• Reliabilism
• Religion, Philosophy of
• Religious Belief, Epistemology of
• Religious Experience
• Religious Pluralism
• Ricoeur, Paul
• Risk, Philosophy of
• Rorty, Richard
• Rousseau, Jean-Jacques
• Rule-Following
• Russell, Bertrand
• Ryle, Gilbert
• Sartre, Jean-Paul
• Schopenhauer, Arthur
• Science and Religion
• Science, Theoretical Virtues in
• Scientific Explanation
• Scientific Progress
• Scientific Realism
• Scientific Representation
• Scientific Revolutions
• Scotus, Duns
• Self-Knowledge
• Sellars, Wilfrid
• Semantic Externalism
• Semantic Minimalism
• Senses, The
• Sensitivity Principle in Epistemology
• Shepherd, Mary
• Singular Thought
• Situated Cognition
• Situationism and Virtue Theory
• Skepticism, Contemporary
• Skepticism, History of
• Slurs, Pejoratives, and Hate Speech
• Smith, Adam: Moral and Political Philosophy
• Social Aspects of Scientific Knowledge
• Social Epistemology
• Social Identity
• Sounds and Auditory Perception
• Speech Acts
• Spinoza, Baruch
• Stebbing, Susan
• Strawson, P. F.
• Structural Realism
• Supererogation
• Supervenience
• Tarski, Alfred
• Technology, Philosophy of
• Testimony, Epistemology of
• Theoretical Terms in Science
• Thomas Aquinas' Philosophy of Religion
• Thought Experiments
• Time Travel
• Transcendental Arguments
• Truth and the Aim of Belief
• Truthmaking
• Turing Test
• Two-Dimensional Semantics
• Understanding
• Uniqueness and Permissiveness in Epistemology
• Utilitarianism
• Value of Knowledge
• Vienna Circle
• Virtue Epistemology
• Virtue Ethics
• Virtues, Epistemic
• Virtues, Intellectual
• Voluntarism, Doxastic
• Weakness of Will
• Weil, Simone
• William of Ockham
• Williams, Bernard
• Wittgenstein, Ludwig: Early Works
• Wittgenstein, Ludwig: Later Works
• Wittgenstein, Ludwig: Middle Works
• Wollstonecraft, Mary
• Privacy Policy
• Cookie Policy
• Legal Notice
• Accessibility

Powered by:

• [66.249.64.20|81.177.180.204]
• 81.177.180.204

Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

Disclosure statement

Peter Watson received funding from NSERC. He is affiliated with Carleton University and a member of the Canadian Association of Physicists.

Carleton University provides funding as a member of The Conversation CA.

Carleton University provides funding as a member of The Conversation CA-FR.

View all partners

• Bahasa Indonesia

Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

• Time travel
• Stephen Hawking
• Albert Einstein
• Listen to this article
• Time travel paradox

Audience Development Coordinator (fixed-term maternity cover)

Data and Reporting Analyst

Lecturer (Hindi-Urdu)

Director, Defence and Security

Opportunities with the new CIEHF

Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second. 

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town. 

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines). 

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth. 

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more. 

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line. 

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space. 

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference. 

Read more: Can we stop time?

The Global Positioning System , or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles (20,100 kilometers) away, moving at 8,700 mph (14,000 km/h). 

According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to the American Physical Society publication Physics Central .  

Read more: Could Star Trek's faster-than-light warp drive actually work?

Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds. 

This means that in order to maintain the accuracy needed to pinpoint your car or phone — or, since the system is run by the U.S. Department of Defense, a military drone — engineers must account for an extra 38 microseconds in each satellite's day. The atomic clocks onboard don’t tick over to the next day until they have run 38 microseconds longer than comparable clocks on Earth.

Given those numbers, it would take more than seven years for the atomic clock in a GPS satellite to un-sync itself from an Earth clock by more than a blink of an eye. (We did the math: If you estimate a blink to last at least 100,000 microseconds, as the Harvard Database of Useful Biological Numbers does, it would take thousands of days for those 38 microsecond shifts to add up.) 

This kind of time travel may seem as negligible as the Kelly brothers' age gap, but given the hyper-accuracy of modern GPS technology, it actually does matter. If it can communicate with the satellites whizzing overhead, your phone can nail down your location in space and time with incredible accuracy. 

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake. 

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life. 

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported . 

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute. 

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .  

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements. 

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades. 

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff." 

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over. 

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

• Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
• A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
• The Time Machine (William Heinemann, 1895) by H. G. Wells
• A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
• The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
• A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
• The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
• Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
• Thief of Time (Doubleday, 2001) by Terry Pratchett
• The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
• All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

• Planet of the Apes (1968)
• Superman (1978)
• Time Bandits (1981)
• The Terminator (1984)
• Back to the Future series (1985, 1989, 1990)
• Star Trek IV: The Voyage Home (1986)
• Bill & Ted's Excellent Adventure (1989)
• Groundhog Day (1993)
• Galaxy Quest (1999)
• The Butterfly Effect (2004)
• 13 Going on 30 (2004)
• The Lake House (2006)
• Meet the Robinsons (2007)
• Hot Tub Time Machine (2010)
• Midnight in Paris (2011)
• Looper (2012)
• X-Men: Days of Future Past (2014)
• Edge of Tomorrow (2014)
• Interstellar (2014)
• Doctor Strange (2016)
• A Wrinkle in Time (2018)
• The Last Sharknado: It's About Time (2018)
• Avengers: Endgame (2019)
• Tenet (2020)
• Palm Springs (2020)
• Zach Snyder's Justice League (2021)
• The Tomorrow War (2021)

Television about time travel:

• Doctor Who (1963-present)
• The Twilight Zone (1959-1964) (multiple episodes)
• Star Trek (multiple series, multiple episodes)
• Samurai Jack (2001-2004)
• Lost (2004-2010)
• Phil of the Future (2004-2006)
• Steins;Gate (2011)
• Outlander (2014-2023)
• Loki (2021-present)

Games about time travel:

• Chrono Trigger (1995)
• TimeSplitters (2000-2005)
• Kingdom Hearts (2002-2019)
• Prince of Persia: Sands of Time (2003)
• God of War II (2007)
• Ratchet and Clank Future: A Crack In Time (2009)
• Sly Cooper: Thieves in Time (2013)
• Dishonored 2 (2016)
• Titanfall 2 (2016)
• Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Get the Space.com Newsletter

Breaking space news, the latest updates on rocket launches, skywatching events and more!

Ailsa is a staff writer for How It Works magazine, where she writes science, technology, space, history and environment features. Based in the U.K., she graduated from the University of Stirling with a BA (Hons) journalism degree. Previously, Ailsa has written for Cardiff Times magazine, Psychology Now and numerous science bookazines. 

NASA gets $25.4 billion in White House's 2025 budget request

'Interstellar meteor' vibrations actually caused by a truck, study suggests

New moon phase on April 8 will bring on the 2024 total solar eclipse

Most Popular

By Fran Ruiz January 29, 2024

By Fran Ruiz January 26, 2024

By Conor Feehly January 05, 2024

By Keith Cooper December 22, 2023

By Fran Ruiz December 20, 2023

By Fran Ruiz December 19, 2023

By Fran Ruiz December 18, 2023

By Tantse Walter December 18, 2023

By Robert Lea December 05, 2023

By Robert Lea December 04, 2023

By Robert Lea December 01, 2023

• 2 Total solar eclipse 2024: Live updates
• 3 Solar eclipse sights might vary on the edge of totality: report
• 4 Chinese space junk falls to Earth over Southern California, creating spectacular fireball (photos, video)
• 5 ISS astronauts ready to watch the solar eclipse from space on April 8

Time Travel

Arguably, we are always travelling though time, as we move from the past into the future. But time travel usually refers to the possibility of changing the rate at which we travel into the future, or completely reversing it so that we travel into the past. Although a plot device in fiction since the 19 th Century (see the section on Time in Literature ), time travel has never been practically demonstrated or verified, and may still be impossible.

Time travel is not possible in Newtonian absolute time (we move deterministically and linearly forward into the future). Neither is it possible according to special relativity (we are constrained by our light cones). But general relativity does raises the prospect (at least theoretically) of travel through time, i.e. the possibility of movement backwards and/or forwards in time, independently of the normal flow of time we observe on Earth, in much the same way as we can move between different points in space.

Time travel is usually taken to mean that a person’s mind and body remain unchanged, with their memories intact, while their location in time is changed. If the traveller’s body and mind reverted its condition at the destination time, then no time travel would be perceptible.

Time Travel Scenarios

Although, in the main, differing fundamentally from the H.G. Wells concept of a physical machine with levers and dials, many different speculative time travel solutions and scenarios have been put forward over the years. However, the actual physical plausibility of these solutions in the real world remains uncertain.

At its simplest, as we have seen in the section on Relativistic Time , if one were to travel from the Earth at relativistic speeds and then return, then more time would have passed on Earth than for the traveller, so the traveller would, from his perspective, effectively have “travelled into the future”. This is not to say that the traveller suddenly jumped into the Earth’s future, in the way that time travel is often envisioned, but that, as judged by the Earth’s external time, the traveller has experienced less passage of time than his twin who remained on Earth. This is not real time travel, though, but more in the nature of “fast-forwarding” through time: it is a one-way journey forwards with no way back.

There does, however, appear to be some scientific basis within the Theory of Relativity for the possibility of real time travel in certain scenarios. Kurt Gödel showed, back in the early days of relativity, that there are some solutions to the field equations of general relativity that describe space-times so warped that they contain “ closed time-like curves ”, where an individual time-cone twists and closes in on itself, allowing a path from the present to the distant future or the past. Gödel’s solution was the first challenge in centuries to the dominant idea of linear time on which most of physics rests. Although a special case solution, based on an infinite, rotating universe (not the finite, non-rotating universe we actually find ourselves in), other time travel solution have been identified since then that do not require an infinite, rotating universe, but they remain contentious.

In the 1970s, controversial physicist Frank Tipler published his ideas for a “time machine”, using an infinitely long cylinder which spins along its longitudinal axis, which he claimed would allow time travel both forwards and backwards in time without violating the laws of physics, although Stephen Hawking later disproved Tipler’s ideas.

In 1994, Miguel Alcubierre proposed a hypothetical system whereby a spacecraft would contract space in front of it and expand space behind it, resulting in effective faster-than-light travel and therefore (potentially) time travel, but again the practicalities of constructing this kind of a “ warp drive ” remain prohibitive.

Other theoretical physicists like Kip Thorne and Paul Davies have shown how a wormhole in space-time could theoretically provide an instantaneous gateway to different time periods, in much the same way as general relativity allows the theoretical possibility of instantaneous spatial travel through wormholes. Wormholes are tubes or conduits or short-cuts through space-time, where space-time is so warped that it bends back on itself, another science fiction concept made potential reality by the Theory of Relativity. The drawback is that unimaginable amounts of energy would be required to bring about such a wormhole, although experiments looking into the possibility of creating mini-wormholes and mini-black holes are being carried out at the particle accelerator at CERN in Switzerland. It also seems likely that such a wormhole would collapse instantly into a black hole unless some method of holding it open were devised (possibly so-called “negative energy”, which is known to be theoretically possible, but which is not yet practically feasible). Stephen Hawking has suggested that radiation feedback, analogous to feedback in sound, would destroy the wormhole, which would therefore not last long enough to be used as a time machine. Actually controlling where (and when) a wormhole exits is another pitfall.

Another potential time travel possibility, although admittedly something of a long shot, relates to cosmic strings (or quantum strings ), long shreds of energy left over after the Big Bang, thinner than an atom but incredibly dense, that weave through the entire universe. Richard Gott has suggested that if two such cosmic strings were to pass close to each other, or even close to a black hole, the resulting warpage of space-time could well be so severe as to create a closed-time-like curve. However, cosmic strings remain speculative and the chances of finding such a phenomenon are vanishingly small (and, even if it were possible, such a loop may well find itself trapped inside a rotating black hole).

Physicist Ron Mallet has been looking into the possibility of using lasers to control extreme levels of gravity, which could then potentially be used to control time. According to Mallet, circulating beams of laser-controlled light could create similar conditions to a rotating black hole, with its frame-dragging and potential time travel properties.

Others are looking to quantum mechanics for a solution to time travel. In quantum physics, proven concepts such as superposition and entanglement effectively mean that a particle can be in two (or more) places at once. One interpretation of this (see the section on Quantum Time ) is the “ many worlds ” view in which all the different quantum states exist simultaneously in multiple parallel universes within an overall multiverse. If we could gain access to these alternative parallel universes, a form of time travel might then be possible.

At the sub-sub-microscopic level – at the level of so-called quantum foam , tiny bubbles of matter a billion-trillion-trillionths of a centimetre in length, perpetually popping into and out of existence – it is speculated that tiny tunnels or short-cuts through space-time are constantly forming, disappearing and reforming. Some scientists believe that it may be possible to capture such a quantum tunnel and enlarge it many trillions of times to the human scale. However, the idea is still at a very speculative stage,

It should be noted that, with all of these schemes and ideas, it does not look to be possible to travel any further back in time that the time at which the travel technology was devised.

Faster-Than-Light Particles

The equations of relativity imply that faster-than-light ( superluminal ) particles, if they existed, would theoretically travel backwards in time. Therefore, they could, again theoretically, be used to build a kind of “ antitelephone ” to send signals faster than light, and thus communicate backwards in time. Although the Theory of Relativity disallows particles from accelerating from sub-light speed to the speed of light (among other effects, time would slow right down and effectively stop for such a particle, and its mass would increase to infinity), it does not preclude the possibility of particles that ALWAYS travel faster than light. Therefore, the possibility does still exist in theory for faster-than-light travel in the case of a particle with such properties.

There is a rather strange theoretical particle in physics called the tachyon that routinely travels faster than light, with the corollary that such a particle would naturally travel backwards in time as we know it. So, in theory, one could never see such a particle approaching, only leaving, and the particle could even violate the normal order of cause and effect. For a tachyon, the speed of light is the lower speed limit, while the upper speed limit is infinity, and its speed increases as its energy decreases. Even stranger, the mass of a tachyon would technically be an imaginary number (i.e. the number squared is negative), whatever that might actually mean in practice.

It should be stressed that there is no experimental evidence to suggests that tachyons actually exist, and many physicists deny even the possibility. A tachyon has never been observed or recorded (although the search continues, particularly through analysis of cosmic rays and in particle accelerators), and neither has one ever been created, so they remain hypothetical, although theory strongly supports their existence.

Research using MINOS and OPERA detectors has suggested that tiny particles called neutrinos may travel faster than light. Other more recent research from CERN, however, has put the findings into dispute, and the matter remains inconclusive. Neutrinos are not merely hypothetical particles like tachyons, but a well-known part of modern particle physics. But they are tiny, almost-massless, invisible, electrically neutral, weakly-interacting particles that pass right through normal matter, and consequently are very difficult to measure and deal with (even their mass has never been measured accurately).

Time Travel Paradoxes

The possibility of travel backwards in time is generally considered by scientists to be much more unlikely than travel into the future. The idea of time travel to the past is rife with problems, not least the possibility of temporal paradoxes resulting from the violation of causality (i.e. the possibility that an effect could somehow precede its cause). This is most famously exemplified by the grandfather paradox : if a hypothetical time traveller goes back in time and kills his grandfather, the time traveller himself would never be born when he was meant to be; if he is never born, though, he is unable to travel through time and kill his grandfather, which means that he WOULD be born; etc, etc.

Some have sought to justify the possibility of time travel to the past by the very fact that such paradoxes never actually arise in practice. For example, the simple fact that the time traveller DOES exist at the start of his journey is itself proof that he could not kill his grandfather or change the past in any way, either because free will ceases to exist in the past, or because the outcomes of such decisions are predetermined. Or, alternatively, it is argued, any changes made by a hypothetical future time traveller must already have happened in the traveller’s past, resulting in the same reality that the traveller moves from.

Theoretical physicist Stephen Hawking has suggested that the fundamental laws of nature themselves – particularly the idea that causes always precede effects – may prevent time travel in some way. The apparent absence of “tourists from the future” here in our present is another argument, albeit not a rigorous one, that has been put forward against the possibility of time travel, even in a technologically advanced future (the assumption here is that future civilizations, millions of years more technologically advanced than us, should be capable of travel).

Some interpretations of time travel, though, have tried to resolve such potential paradoxes by accepting the possibility of travel between “ branch points ”, parallel realities or parallel universes , so that any new events caused by a time traveller’s visit to the past take place in a different reality and so do not impact on the original time stream. The idea of parallel universes, first put forward by Hugh Everett III in his “ many worlds ” interpretation of quantum theory in the 1950s, is now quite mainstream and accepted by many (although by no means all) physicists.

>> Quantum Time

Physics of Time

Random pages.

• April 4, 2024 | New Research Reveals Keto Diet’s Potential To Combat Early Alzheimer’s
• April 4, 2024 | MIT’s AI Breakthrough: Pioneering New Antibiotics To Combat MRSA
• April 4, 2024 | Effective Anger Management: Chilling Out vs. Blowing Off Steam
• April 4, 2024 | Chilling Revelations: Ice Shells Expose Alien Ocean Temperatures
• April 3, 2024 | Could Modified RNA Be the Key to Healing Neurodegeneration Caused by Brain Disorders Like Alzheimer’s?

Exploring the Reality of Time Travel: Science Fact vs. Science Fiction

By Adi Foord, University of Maryland, Baltimore County November 16, 2023

Time travel, a longstanding fascination in science fiction, remains a complex and unresolved concept in science. The second law of thermodynamics suggests time can only move forward, while Einstein’s theory of relativity shows time’s relativity to speed. Theoretical ideas like wormholes offer potential methods, but practical challenges and paradoxes, such as the “grandfather paradox,” complicate the feasibility of actual time travel.

Will it ever be possible for time travel to occur?

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The Science Behind Time Travel

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time Is Relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Theoretical Possibilities and Challenges

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes, or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and Failed Dinner Parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time, and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Artist’s rendering of the James Webb Space Telescope. Credit: Northrop Grumman

Telescopes Are Time Machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel . As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope , the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang , about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies, and dreams.

Written by Adi Foord, Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County.

More on SciTechDaily

A Significant Surge: How Tropical Cyclones Increase the Cost of Carbon

Voyager’s New Horizon: NASA Engineers Tackle Thruster Buildup & Software Glitches

New ScienceCast Video Previews the ‘Opposition of Mars’

“Our Solar System May Be Unusual”: Rogue Planets Unveiled With NASA’s Roman Space Telescope

AI-Powered Lasers: A Modern Solution to Space Debris

IBS Relief: Common Drug Improves Symptoms of Irritable Bowel Syndrome

Scientists Discover Chinmo – “The Youth Gene”

Jupiter’s Moon Europa Could Be Pulling Oxygen Down Below the Ice To Feed Life

11 comments on "exploring the reality of time travel: science fact vs. science fiction".

Until the problem of the second law of thermodynamics(entropy) is solved, the concept of time travel will remain the subject of science fiction. Since this is a basic law of our universe, there is no conceivable way that we know of to do this. The great thing about our knowledge of the universe is that it continues to grow and with that our view of what is possible continues to change. After all, at one time it was believed we could never leave earth!

The 7 planets are soul pollen in the space @ life has been the world has been prepared @ The pollen of the universe is hidden @ Around the axis of the galaxy, the universe is hidden@@ The pollen of the galaxies is a hidden cluster Itis made that we thought about what wisdom is in God’s work and how it is arranged in the form of words.The verse that is made is as follows @@ John, you are in time @ worlds, planets around the axis of the branches of galaxies @@ 8 Prophets, divine prophets, God-aware witnesses @ God’s words of revelation, they are aware @@ 48 What the words of revelation that every prophet has about @ sometimes Sometimes the message of God has become a verbal cliché in the head @@ 62 The message of God was given to every prophet @ The message was made around the power of God @@ 46 The truth of the religions of the cradle of time is said in the world @ Prophets always came for justice in time @@ 62 A warrior became brave in time @ Delaver Ghahrmani Boud Taarani@@ 43 Omar Noah never died, who knows!@ Imransan Is there an unseen world, immortality!?@@ 49 Men’s rights in the sign @ Human rights, the observance of world justice @@ 40 We have God’s love @ A love that is not patched, separation!!!: @@ 39 Take one word from the end of the first and second stanzas to the bottom of these eight verbal verses, and this sentence is made @@ God-aware, the world is born, you know the sign of God @ Agah,the beginning of time, the world, eternity, the world of separation @@ The meaning of this sentence is That God, who before us humans lived on the earth, formed the earth’s crusts with full knowledge, and we humans know the sign of God, which is on the earth, on the continents and countries, and some names have been shown by God for our knowledge since the end of time.In a later video, if I have a lifetime left, I will explain exactly about these poems, God willing @@ The word of the Prophet 17 is the 17th and Muhammad (PBUH) said that the Bedouin Arabs should pray 17 rakats so that theyperform ablution and be clean and not kill and loot.From the caravans, these were all God’s will, and he is good everywhere, in every nation, God does not like evildoers, sinners, and oppressors.Muhammad (PBUH) was God’s last messenger to the Arab people, he was God’s best prophet, and the third verse is because they do not accept Muhammad (PBUH).Some Iranians who were in contact with me, that’s why in the third verse of this surah, he said that his message was given to every prophet, the message is based on divine power, and the word truth, the first two letters of which istruth, is truth, and truth is the 43rd and forty-third word, and the word is time.It is exactly 46, and this song of the Prophets was revealed at the age of 46 ببخشید اگر قبلآ مطالبی فرستادم که به مذاهب مذاهم ارتباط داد شاید به درد شما نمیخورد من در کامنت های بعدی سعی میکنم از نجوم و حیات زمین سخن بگویم این چند بیت را به خارجی انگلیسی که تبدیل کردم معنی آن حیرت انگیز تعغیر کرد گفتم برای شما بفرستم و بداند که این کلام من نبوده کلام خداوند بوده شما نظرتان در مورد خداوند چیست من میگویم خداوند که پدر ومادر انسان بودند قادر به ساختن ما بودند اما آیا آنها قادر به ساختن ستاره ها هم بودند

Your comment has validity to God. But it surely has no place here, it is only fare if the hole comment were in english and has less of a convincing push in a belief a person either believes or not.

I thank Johnson for saying that my opinion is valid before God.I read Johnson’s other comments about time.Johnson said that time is forward in physics.Time is determined by us.Previously, Einstein said that if an object moves faster than light, time It becomes zero, and it was said that there is a fourth dimension, I should say that they move a thousand times the speed of physical light, time does not become zero,and the world has three dimensions, length, width, height, but we determined and interpreted this length, width, and height, and we must determine the fourth dimension in At the moment, they did not define the fourth dimension and it does not exist.For now, in the case of Johnson, who said that we should see strange changes faster than light, maybe Johnson was right, but how can we see changes ahead of the speed of light, and what will be the use of Johnson’s opinion?The events that are going to happen in the future must be known.I understood from the Lords who were raised by humans that the earth was previously inhabited by the same humans who lived on the earth, continents and seas in the form of four-legged mammals and humans and birds by humans who lived in the millions.They had years of history, continents and seas were formed on the earth, and those who inhabited the earth before us in the past had advanced so much in the science of astronomy, physics and chemistry that they simulated the souls of humans and raised many godsand lords.By the same Lords, I got very, very important information and secrets that I explained in my comments, but because I have been familiar with this very interesting website for a few weeks, I have not yet provided many comments, but soon more information aboutastronomy.and the structure of the solar system and the earth, the issues of belief and the events that are going to happen in the solar system in a few thousand years, I will report to you, those who are interested, the sun is currently located inthe big arm of the galaxy and it is hitting the stars of this arm, the stars that are close to Neptune Uranus, Saturn, Jupiter and eventually hit the sun.There were more than hundreds of stars, all of which were a hundred times smaller than the sun and their mass was several hundred thousand times smaller than the sun, and most of the planets that came with these small stars collided with the objects of the solarsystem.About 240 planets turned into moons and rocky planets of the solar system.I have already said that the sun is slower than the stars of the two arms of the galaxy due to the very heavy mass and weight that it has attached to the stars of the two large arms of the galaxy, but it may not be just becauseof this issue.The objects of the solar system collide with the stars and planets of the arm of the galaxy.Maybe it is because of the very strong gravitational force of several large planets, Jupiter, Saturn, Uranus and Neptune, that the star systems of the arm of the galaxy are pulled towards the solar system.I know for a fact that those who live on earth They had advanced so much in astronomy, chemistry and simulation that they knew about the sad incident of the collision of the stars of the galaxy’s arm with the solar system, that’s why they drew America asa bird that is flying in the sky.God bless you in all the languages ​​of the earth, hoping to meet you dear ones

If the big bang happened, why aren’t there galaxies that are a billion times the size of the Milky Way?I say this theory that when an explosion occurs, the order that galaxies have now could not exist in an explosion in the core of a super black hole, regular materials and elements.They do not spread.I understood from the distance of the stars that the age of the sun is at least a few hundred billion years, and life on earth arose in the last hours and minutes of the sun’s life.It is possible that the earth revolved around the stars of the arm of the galaxy tens of billions of years ago.The galaxy returned to the earth, the moons of the four planets Neptune, Uranus, Saturn, Jupiter, which are about 231 moons, they were separated from the stars that came from the arm of the galaxy towards the sun, from those stars that collidedwith the gas planets, the cause of the hydrogen gas in the gas planets was due to the collision of the stars that I said.Their size is a hundred times smaller than the sun.If those stars had a large size, the earth would never have a temperate climate and it would be so hot that life would never exist. I translate the content that I send to those interested in astronomy and technology on this very interesting website using Google.I retranslate them from Latin to Farsi.Problems occur during translation in dictation, composition and spelling, but because I am very I have to take time to correct them.I sent them to you, dear readers, without any mistakes.I hope you understand the content.If you don’t understand something, call my phone number via text message.Thank you, sorry, and apologies to you, dear doctor.and chosen by the Lords of the world, Mehrdad Kathiri Kasiri time in tehran 10:46 pm

I have to explain a point that there were mistakes in the translation.In the comment at 22:46 that I sent a few minutes ago, I did not say that the stars collided and I did not say that I understood from the distance of the stars how many hundred million years have passed, I said from the distancebetween the stars The arm of the galaxy, I realized that the sun lives not a few years from the age of the sun, I said that since the sun was in the form of a huge cloud and very big and like a nebula, from theage of the formation of the sun to this time, at least a few hundred billion years have passed.

It’s too bad physics can’t come to a complete consensus about time, I would like to add some thought about discoveries it has been proven that time travels in only one direction forward, the experiment dealt with light thru glass and how it reacts in the middle and what change happens after light exits the other side, a simple explanation of this experiment. Brings me to theorize and start that time existed before the big bang and is outside of our universes influence, when time is acted on by gravity the ( Form ) of time is changed until the influence no longer has effect, this could go hand in hand with light photons the photon has a influence in the Form that time has. This can not be a observed difference unless we were to see beyond the speed of light. We do agree that physics changes at a subatomic state and also does some strange changing once the speed of light has been exceeded.

The experiment I referred to was posted on IFL in October 2023 headlined ( Solution to complex light problem shows that time can only go Forward ).

One of the problem with travel time is the one people keep forever. And, that is that the earth is always move through space. Matter of fact, the earth is not in the same place that it was 50 years ago. So you will have to move through space as well as time.

Ironically, the only science fiction that seems to handle this is the original story “The Time Machine”.

Time travel is happening now. It has been done since the 1950s. The method satisfies all the requirements. Traveler can’t change the past, but only observe. You can’t go farther back than when the machine was first invented (1950s). There’s one more limitation, you can only observe what the machine was directed. The time machine, the common video camera, and video tape recorder. Now it’s the camera, and file capture computer. Yes, viewing a video tape is effectively going back in time. It’s more than the video, it’s the sound too. There are working versions of smell, and touch which can be recorded too. If you record, and replicate all the senses, you have effectively complete time travel. The most primitive form is the picture. This technology has been around for thousands of years, and is manually intensive. Later many pictures were strung together to make a film. Using a camera to record film was the first example of complete visual time travel (back to when the film was made). Later sound was added, and we have the movies. A way of going back in time that included sight, and sound. Now we have video systems (YouTub) that can play back past events selected by you. Yes, video systems are virtual time travel machines we have now.

Tom Mariner | December 17, 2023 at 5:26 am | Reply The folks “out there” are just answering the SETI message sent by the Arecibo radar station in Puerto Rico in 1974.

Our neighbors light years away are waiting for another message from that most power radar transmitter ever, but the bright lights have turned the facility into a STEM training facility after other organizations stole the money from Puerto Rico.

dr mehrdad kesiri | December 18, 2023 at 9:06 am | Reply Steve | December 9, 2023 at 6:20 pm | Reply

Would you be able to calculate if the whole multiverse Is spinning and depending on the distance. You are from the center you would be going faster than in another position?

Fixed gravity for you. | December 10, 2023 at 1:20 am | Reply

Kroupa is allowed out there with high profile exposure because he’s flacking for an idea by a new hero of famously top-dog self-victimizing self-absorbed insect opportunists, as Einstein’s gravity theory is becoming known as a closet failure over-celebrated by the generation that bred said famously top-dog self-victimizing self-absorbed insect opportunists.

Fixed gravity for you. | December 10, 2023 at 1:48 am | Reply

The overexposed cat icon seems to be a free-range reddit group run by AI frat boys.

Fixed gravity for you. | December 10, 2023 at 8:35 am | Reply

Replacing “expansion of the universe” with “dilation of the photon” would not be respecting “an establishment of religion” (establishment of a religion-based theory) but science has no bill of rights and gravity experts are prone to shunning reason anyway.

dr mehrdad kesiri | December 17, 2023 at 7:36 am | Reply

I understand to a certain extent, but religion and religions always interfere with progress and old religions should be put aside.I am the representative of your Lords and I am the master of time with hundreds of signs.The Lords of the world tried for hundreds of years until they came to the conclusion that they had to remove the disturbing religions by bringing the new religion and law of God in the Middle East because many people abused religion and came to power.Unfortunately, that person who should appear with his companions I am, but this work will be done because all the signs that the Lords of the world have placed on me in the earth and time are the numbers that are connected to time in the date ofmy birth and the surahs that have been revealed to me.The surahs and the words of the surahs, which are numbered, connect us to the words of signs and time.What we mean is that when the words are counted, they are connected to the time of the events of the words.This issue is very important.I will give you an example.I was injured in the history of 365 and this number belongs to time.This is the kind of owner of time, and God has revealed books, treatises, and surahs to me.This is a religious debate.It has something to do with astronomy, and the destruction of life on Earth, which God said earlier, is the end of time.The sun is hitting the big arm of the galaxy, and it is possible.It is possible that life on earth will be destroyed.This happened more or less a hundred million years ago and humans have raised gods.It is possible that in a few hundred years we will be able to raise gods with the help of artificial intelligence, physics and chemistry.A thousand system stars have joined our solar bodies and collided with Mars and Earth.No stars and planets from the arm of the galaxy that came towards the Sun did not collide with Earth and Mars, but when they collided with gas planets and the Sun, the system The stars of the arm of the galaxy after hitting the sun, theelements of the said planets and stars, like meteorites, their elements hit the earth and the life of the earth was destroyed about a hundred million years ago.This is the true story of Dr. Mehrdad Kathiri. thank you gods by time in in tehran 7:07

I thank Johnson for saying that my opinion is valid before God.I read Johnson’s other comments about time.Johnson said that time is forward in physics.Time is determined by us.Previously, Einstein said that if an object moves faster than light, time It becomes zero, and it was said that there is a fourth dimension, I should say that they move a thousand times the speed of physical light, time does not become zero,and the world has three dimensions, length, width, height, but we determined and interpreted this length, width, and height, and we must determine the fourth dimension in At the moment, they did not define the fourth dimension and it does not exist.For now, in the case of Johnson, who said that we should see strange changes faster than light, maybe Johnson was right, but how can we see changes ahead of the speed of light, and what will be the use of Johnson’s opinion?The events that are going to happen in the future must be known.I understood from the Lords who were raised by humans that the earth was previously inhabited by the same humans who lived on the earth, continents and seas in the form of four-legged mammals and humans and birds by humans who lived in the millions.They had years of history, continents and seas were formed on the earth, and those who inhabited the earth before us in the past had advanced so much in the science of astronomy, physics and chemistry that they simulated the souls of humans and raised many godsand lords.By the same Lords, I got very, very important information and secrets that I explained in my comments, but because I have been familiar with this very interesting website for a few weeks, I have not yet provided many comments, but soon more information aboutastronomy.and the structure of the solar system and the earth, the issues of belief and the events that are going to happen in the solar system in a few thousand years, I will report to you, those who are interested, the sun is currently located inthe big arm of the galaxy and it is hitting the stars of this arm, the stars that are close to Neptune Uranus, Saturn, Jupiter and eventually hit the sun.There were more than hundreds of stars, all of which were a hundred times smaller than the sun and their mass was several hundred thousand times smaller than the sun, and most of the planets that came with these small stars collided with the objects of the solarsystem.About 240 planets turned into moons and rocky planets of the solar system.I have already said that the sun is slower than the stars of the two arms of the galaxy due to the very heavy mass and weight that it has attached to the stars of the two large arms of the galaxy, but it may not be just becauseof this issue.The objects of the solar system collide with the stars and planets of the arm of the galaxy.Maybe it is because of the very strong gravitational force of several large planets, Jupiter, Saturn, Uranus and Neptune, that the star systems of the arm of the galaxy are pulled towards the solar system.I know for a fact that those who live on earth They had advanced so much in astronomy, chemistry and simulation that they knew about the sad incident of the collision of the stars of the galaxy’s arm with the solar system, that’s why they drew America asa bird that is flying in the sky.God bless you in all the languages ​​of the earth, hoping to meet you dear ones

Should America, Britain and its allies be silent, why did they allow Imam Khomeini (RA) to brutally shoot those great chosen ones of the Gods of the Earth?Then God struck the parliament in the year 60 and seventy-two people were killed from those who signed the order to shoot the soldiers.Was it the will of the lords of the gods of the soldiers to avenge the blood of the soldiers that Khomeini and the Ayatollah approved in the Islamic Council of the Islamic Republic?Did they follow the order that Imam Khomeini had given for the execution and shooting of the soldiers?Why do the Gods of Iran send Al-Qaeda to shoot the American passenger planes inside the twin towers on September 11?Maybe these things were done by the Gods of the Earth.It is not known.Before al-Qaeda carried out that brutal suicide operation, in the same way as the United States in 1967, they fired two missiles at the Iranian passenger plane from the launcher, and the missiles proved to the Iranian passenger plane that the two F-14fighters Iran was escorting a passenger plane to reach the destination of Dubai, 290 people were killed in the skies of the Persian Gulf of Iranzmin, 290 innocent people were killed.I don’t know if this was the will of the gods of the earth, that Imam Khomeini should sign Resolution 598, or it was the will of American politicians, why didn’t the big powers stop that horrible and brutal crime of the evilImam Khomeini and Iranian mullahs in 1957?that the gods of Iran bring war in Iran, where the mullahs forbid a morsel, a morsel, a morsel that is not halal, or the mullahs take a morsel from the Islamic Qur’an, God and the people are not pleased that the mullahseat that morsel, the mullahs do not work, most of them and Hafiz The great Iranian poet in 900 years ago, when the Arab mullahs had dominated Iran and the Arabs had attacked the Iranian soil, Muhammad Hafez said in Surah Vaizan that the sheikhs andmullahs eat a morsel of ghosts and hypocrisy, and the mullahs are heartless in the work of God, which means interference.In the case of Iran, the mullahs act as judges and the god of Iran.The mullahs are the judges in Hafez’s poem, which means God.Why did they start the war between Iran and Iraq?The gods of Iran, after the mullahs slaughtered the soldiers unjustly, say that they are stupid and ignorant anklets.In one night, 57 generals and officers of the imperial army Iran was executed and shot, I say Khalkhali is crazy, he didn’t know that the imperial government had spent millions of dollars for a general.Until that general, Khalkhali became a general because of the enemy he made to the Iranian army and destroyed Reza Shah’s grave.People should curse him the worst.But The messenger of the gods of the earth tells me, Mehrdad, you are the representative of the gods of the world of Iran, don’t call anyone a bastard, or if you say a bastard, say that he himself was a bastard,neither his family nor his ancestor, the messenger of the gods of the earth tells me, Mehrdad, because you are the most accurate person, the kindest person, and the smartest.And you are the greatest poet of Iran.Perhaps the gods and lords of the earth chose you, and perhaps they raised you from Nadfa, who entrusted your justice to you.May the gods bring forth the truth and truth of religions until the world is no longer at war over the old religions.As for religion and religions, because religions are a collection of religions, they disturb peoples and nations, like Islam, which is not a religion, there are religions that disturb people.Is it possible to follow all the religions of Islam?Also, know why the word disturbing is the same family as religions.Why is the word disturbing similar to religions?

I am the owner of time, with dozens of signs in the date of my birth, the gods and lords of Iran, the God of the world, made me the owner of time, that is, the owner of time means the chosen ones.Gods who are connected to time, those who own time on earth in this age, what There may be many people like me on earth, people in Iran should own their time in a different way, because they say that the Imam of the Time doesnot appear except with his companions, but I do not say that I am the Imam of the Time, because of Imam Khomeini who destroyed Iran.I use the word Imam.Before and after 1957, there was Khomeini, whose father was a rebel and was executed by Reza Shah, and Khomeini took revenge from the army for his father Seyyed Mustafa Khomeini, and in 1958, when he came to Iran, he killed hundreds of armytroops.I hate the word imam, but the people of Iran tell me that you are the imam of the time, but I say that I am not the imam of the time and I am the owner of the time with dozens of signs.The earth and in the eight surahs that were revealed to me.I am the owner of time and a human being.I am like all Iranians, but the difference between me and others is that I know what is happening in the unseen world and I will inform you about the future of the earth in time.My date of birth and age with street numbers Tehran and the squares of Tehran are related to time in history when numbers are connected to the year.These things I want to claim have a reason because I am the owner of time and the only representative of the gods of the earth.I know all the secrets of the unseen world and dozens of signs.In the places where I said the street numbers of Tehran and Tehran squares, I was connected to time, for example, how in 1960, when I was 17 years old, our house was on 117th Street, or I was 54 years old.Our house was exactly on 54 Square.At the age of 55. We bought a house at 55 Narmak Square.I was injured in Qasr Shirin war zone in 365, and 365 is the middle of the year, which is the leap year of 366, and I finished my service in the 88th army division in 66 and I did three jobs.88 Zahedan, I did three things.I brought the ammunition to the front line.I brought food to the front line for three companies from the 196th battalion of the 88th army and led the warriors and soldiers to the front line.That is, I was… in the middle of my military career, I went to the army I helped the 88th Armored Division of Zahedan, that is, I helped the army of the 88th Armored Division of Zahedan.I was an IFA driver and I played my role like this, and five times at a distance of a few meters and tens of meters, a Russian IFA truck that was I was driving and was hit by bullets and mortars from a French tank.Our car hit the ground, but maybe the gods of the earth saved me from all the attacks and fire that Saddam Hussein’s military party threw at us soldiers, many fires on the war front.They gave me, but I was lucky to survive in high school.When I was in my first year, in 1957, the guide of the revolution became available, and I was in the seventh grade, and it was the year 1957, and I was connected to time.I belonged to the imperial government and Mohammad Reza Shah was 11 years old.He fed millions of students all over Iran every day, but for several years I was unable to guide people in religion, which direction to go, and I was 17 years old.I should not raise these issues

But there are many issues that I am connected to time, but you should say, well, what is the use of these things that God has placed me as the owner of time on earth, who has chosen me and chosen me as the savior andleader of the world, to the people of Iran.guide and let the elders of the world accept my proxy to express the orders of the Lord according to the order of the new religion and religion that God has revealed to me so that justice is done and we understand that Muslims especially in the Middle East willnot fight over the old religions because Islam It has been everywhere, the Muslim men of those Muslim countries were at war, why should the people of the earth still fight over religious and Islamic issues?Prejudice, prejudices?Why did Israel, Palestine and Iran go to war with Iraq?How am I the owner of time?There is a number in the date of my birth.When I multiply my date of birth from the beginning and the end, it is equal to the speed of the earth.It is obtained in one second and one hour.The speed of the earth and the speed of light are obtained in one second and one hour.And these are all related to time.You can multiply the date, month, year, and decade of my birthday.Then comes the number 90, and the number ninety is related to the right angle, and Qam has several meanings.Qam means a ninety-degree angle.Qam means firm.Unfortunately, Mehrdad is the ruler of time and being the representative of God is not interesting to me.Now how am I the owner of time when ninety multiplied by 40, which is forty, is ten Kan. On my date of birth, ninety multiplied by forty equals three thousand six hundred three thousand six hundred seconds in one hour, and This is howI am the ruler of time in the combination of my father’s birthday, which is Muhammad, and he was born on the 17th, and I was born on the 43rd, together with my father’s birthday, which is the 17th, and my birthday, whichis the 43rd.60 and sixty seconds in one minute and sixty minutes in One hour should be 360 ​​and I was 17 years old in the year 360. And from a school in Vahidiya called Hefdeh Azar, that school was called when I was 17 years old andwe went to 17th Street in Tehran, Pars.You should know that there is a very strange issue that you You must know that I know for sure that God has chosen me in such a way that England and America, who are the most versatile people in the world, should know the secret of whoI am, Mehrdad, the owner of time, and why God has entrusted all the secrets of the earth to me.See, words are not like that.pass by them, there is a secret in the words of the Lord of the Earth, that not everyone is privy to the secrets of God, that God will reveal this secret to a human being, if the capacity and tolerance of that person is notperfect and he does not have the capacity to hear it, with whom should the secret be kept explain the world of the unseen and how they can help the justice of God to be implemented by his savior who unfortunately this very heavy responsibility is institutionalized on Mehrdad’sshoulders.

This very, very heavy responsibility on Mehrdad is institutionalized in me.This issue is so complex, sensitive and precise that it has no limits.Maybe I know the entire secret of the creation of the earth and the creation of the universe, but this is not an interesting thing for me, a person who knows everything.He can’t have a peaceful life.These issues in the date of my son’s birth become more complicated by combining my own date of birth, my wife’s date of birth with birth certificate number and my date.My birthday with the date and the time I was in my mother’s womb with the addition of the day I was born will be the number of 114 surahs of the Qur’an, the number of 30 parts of the Qur’an and the number of60 parts of the Qur’an, all in the combination of the dates that I said at the time of my birth with the date of birth I and the number 17 of the Muslim prayer and the number 72, which is related to the seventy-two people who were killed, are related to the story of the Qur’an.By combining the dates of birth of my paternal family members with mine and dozens of these more complicated issues, it made me wonder how they could do God’s work.So, compared to the work of the gods and lords of the world, it is very strange and amazing and immeasurable.It is not possible for anyone to learn from me all the experiences that I have and have experienced about the unseen world, and now I have learned all my knowledge in one night, unless you have experienced what I have experienced in practice, the same problemthat I have said, but I I may not be a happy person.I have not saved anything from the world and I suffered very, very hard.I was forced to go to camps for 11 years and sat in 900 sessions in Narcotics Anonymous and collected the experiences of others.If he has worked the twelve steps and sat for ninety sessions and listened to the words, he will become a saint.A saint means, for example, he will become a professor and perhaps a philosopher, but I sat for 900 sessions and listened to the words, and later I understood that all of this was a divine test.I will tell you later.I am sending a message that I have a book and a treatise, and the great powers of the world should help me bring my treatise to the fore, so that the old religions become new and the disturbing religions are abandoned, and the law of thegods and the masters of the world gradually take the place of religion and religions.He revealed and said that the time has come for the Muslims of the world to follow the law instead of religion, and the treatise that I bring from the Lords and Masters of the world separated religion and religion from politics and did not make the choice ofreligion mandatory, our Lord God said that time It has come that the law works instead of religion, and God and our reason said that mankind has progressed so much in terms of establishing security in countries that there is no longer a need for the clergy toforbid people from sinning and history has shown that in order to implement the orders of the Qur’an The old religions are the most abuses and killings of Muslims because of disturbing religions and the time has come for God to bring a new religion and religion thatrepresents God from among the Shiites who should be from those who agree with the Islamic principles of the Qur’an and the correct verses of the Qur’an and the perspective of the Qur’an.I have to reveal the book of God’s law that I have written

to bring to light so that old religions become new and disturbing religions are abandoned and the law of God and the masters of the world gradually take the place of religion and religions.God revealed to me and said that the time has come for the Muslims of the world to practice law instead of religion which I bring from the Gods and the masters of the world, they separated religion from politics and did not force the choice of religion.The idea of ​​establishing security in countries where there is no longer a need for clerics to forbid people from sinning and history has shown that due to the implementation of the false orders of the old books of religions, the most abuses and killings of Muslimsare due to disturbing religions and it is time that God By bringing a new religion and religion that is the representative of God from among the Shiites, he must be one of those who agree with the Islamic principles of the Qur’an and the correct versesof the Qur’an and the view of the Qur’an.He set me up with the Quran and other times when the numbers of the times are related to the verses of the holy books and to the times when I am connected in the numbers of the squares of Tehran to the numbers of the verses of the Quranin the times when I was residing in Mehrdad and I was living in the numbers The fields that are related to the numbers of the verses of the Quran, none of the Muslims, especially the clergy of Iran, can not accept Mehrdad’s being chosen and theowner of time, who is the vicegerent of the time, the vicegerent, the savior, and the savior of the human world, but I have nothing to do with ordinary people.I have no difference from a human point of view, only my difference is that the Gods and the masters of the world chose me to put aside the old religions and wars and conflicts because of religious, ethnic, racial differences and other issues, etc., which caused war, and no religion over another There is no superiority.All religions lived in order to guide mankind to the right and true path.They came so that people should have good deeds and good behavior.Whether these principles came from God or from earthly people, we must be righteous and righteous.I was 17 years old.We went to Haq Azar Middle School, Tehran, Pars Khazarin, 117. I shouldn’t bring up these issues, but there are many issues that I am tied to time, but you say, well, what is the use of these things thatGod has given me time on earth to to be chosen by the world as a savior and executor to guide the people of Iran and let the elders of the world accept my proxy so that I can put forward the divine orders through the orders of the religionand the new religion that God has revealed to me.Lord, so that justice is done and realized, and Muslims, especially in the Middle East, do not fight over old religions, because wherever Islam was, the Muslim men of those Muslim countries were fighting, why should people still fight over religious issues andIslamic prejudices, Israel and Palestine.And why did Iran and Iraq go to war?How do I own time?Now there are signs in my birth date.When I multiply my date of birth from beginning to end, it is equal to the speed of the earth.It is obtained in one second and one hour.The speed of the earth and the speed of light are obtained in one second and one hour

And these are all related to time.From the day, month, year, to the decade of my birth, multiply it and get the time ruler.In the combination of the birth of my father who is Muhammad and he was born on the 17th and I was born on the 43rd with the birth My father, who is 17 years old, and my birthday, which is 43, are combined.60 and sixty seconds in a minute and sixty minutes in an hour.It may be 360 ​​and I was 17 in 360. These issues in the date of my son’s birth are combined with my own date of birth., my wife’s date of birth is complicated with birth certificate number and my date.Birthday with my wife and son, combining the date of birth of my paternal family members with me and dozens of such things.He thought how to do God’s work.Work May the gods of the world be so precise that all this is shown to me, compared to the work of the gods and lords of the world, it is very strange and amazing and immeasurable, this is how Iran is connected to the earth.It was connected and has a strange meaning.The old word Iran-earth I remember 52 years.Before the start of the nationwide program of the Iranian television network, which started working, the logo of the program was Iranzmin, the name of the program of the first channel of Iranzmin and Iran was a message to all the countries of the earth.In terms of the language of the gods and lords of the earth, there are many words.I have to say because I have to tell and tell the truths of the gods of the earth.Why am I the owner of time, because I know from the future what will happen in the world and what will come to the earth and what will be the order of the gods of the earth for the superpowers of the world and my religions.I am saying important things, but those who were from the Iranian army were shot in 1958 and 1957 by Imam Khomeini (RA) and the leadership of the Assembly of Experts.The execution and shooting of generals were done by the Ayatollahs, but why America and England?did not stop them from being executed, know that the structure of the gods made by humans is so detailed that I believe there are forty of your gods.It still appears in various forms.I could not understand the structure of your gods and my gods, but you The superpowers of the world, with all that progress, you still haven’t been able to understand the secrets of God’s creation, how many million years of history and civilization didhumans have?When did the earth get destroyed?It has been burning for trillions of years and the earth is a planet that has been formed for twenty billion years, but it was not habitable because it has thousands of active volcanoes.Scientists said that the life of the universe is about thirteen billion years.A year has passed, but now that the James Webb telescope The most distant galaxies arrived, those galaxies said that they are ninety-seven billion light-years away from us earthlings.Now they should say that ninety-seven billion years have passed since the age of the universe, but this is not the case.The distance of the galaxies has nothing to do with the age of the universe.No bang.Consider that two hundred billion galaxies are placed in a black hole, in that big black hole there will be no healthy hydrogen atoms left and after the explosion the stars will be reborn and in a very, very big explosion, it’s great.Materials Chemicals, proteins and vitamins, which are the necessary ingredients for life.They are not healthy enough to create life on a planet next to the stars after the big bang.I said thirty years ago that Herschel and Kepler’s theory could not be true.

Of course, I read the book Asterism and the Galaxy, which the two American scientists used a radio telescope to emit microwave waves from the galaxies, and it took billions of years for them to reach the earth.They were able to imagine the world with that very large radio telescope and I saw the images of billions of galaxies opened my mind and I read that astronomy book when we were in Tehran Pars Street 117 and I was 17 years old, that is forty-two years ago I had read that book and at that time I also believed in the Big Bang theory I had my doubts about the two great American scientists, but I would not reject it.However, I accept America, which is very supportive.Its astronaut scientists are the first to speak in the world, but the general policy of America is in the hands of the big American capitalists, and they, unfortunately, determine the final decisions of the Parliament and the Assembly.They are the enemies of America, and this has caused the United States to implement policies for the countries of the Middle East region in which the economic interests of the United States are considered, and this policy has caused a lot of damage to Iranians, whichincludes many other social and cultural issues.The clerics have been left open in Iran and this policy of American politicians has caused the power in Iran to be in the hands of a handful of religious sheikhs, mullahs and extreme Shia ayatollahs in Iran and the situation of the unfortunate people of Iran and theMuslim people of the Middle East since the time when Christians The priest and the pope held power in Europe and committed heinous crimes against the people of Europe.The situation of the people of Iran is no better than during the Middle Ages and the Renaissance, it is even worse.And the institution, headquarters, and courts that the clerics had at their disposal have already been shown in history that they oppressed the people through both Islamic and Christian religions.And they came to power according to the Qur’an, they were the cruelest religious government and they committed so much cruelty and murder.Just refer to the history books of Tabari and Sana’i.It is not necessary for me to explain anymore.I only ask America and England, which are the great powers of the world, to Help me so that I can reveal the instructions of our Gods, whose language is the Gods and masters of the world, most of them are Farsi, perhaps all theGods of the earth, those who previously had millions of years of history on the earth came to the conclusion that the alphabets of the Gods who create should be It should be Farsi because the most accurate alphabet and words are in Farsi and the signs I sawon the countries and seas are related to Farsi.This issue is so important in the work of the authors of the earth and you should not easily pass it by.Regarding this issue, it is very, very important.I can’t explain it in a few pages, but over time, I will send you a message in this regard, the great servers of the world, and let me also say that when I translate Persian into English, you know the text,dictation, and spelling will change, but you will understand the meaning of the text in general.May you understand Mehrdad, chosen by God, the land of Iran, may he protect you in all stages of your life

Sorry thank you dr aghakesiri ghaeem sahebzaman in iran and universal

How Stellar Cannibalism Illuminates Cosmic Evolution

جزایر فیلیپین دایناسوری که توسط انسان خلق شده در بیش ده ها میلیون سال و بخاطر ریخته شدن دوهزار متر خاک غرق شده بخاطر بالا آمدن آب اقیانوس اما جزایر فیلیپین شبیه دایناسوریست

The address of the above comment on the site about a thief who was trained by a dinosaur bird who was trained by humans tens of millions of years ago and who arrested murderers and robbers.The police were arrested by big birds.don’t the scientists of the world think about this?They were buried in the bed of important cities, they were buried with all the tools and machines, the traces of humans tens of millions of years ago, they had a civilization and a history of hundreds of thousands of years, they built a base underthe earth, from the meteorites that explode from the planets when they hit the sun, and they knew that several thousand meters of soil is poured on the surface of the seas and islands, and they knew that the shapes they made of the islands in thecountry of Papua may go under the ocean, of course, the Philippine islands.The picture is of a baby dinosaur that went under the ocean, but the northern island of Australia, which is Papua, is quite clear.It is a big dinosaur whose tail is towards the east and its mouth is open towards the west.There is the Philippines, but it was more difficult to take the soil to the Philippine islands to create a dinosaur than the island of Papua, that is why the height of the soil in the Philippines is lower, and when the meteorites fell a few kilometerson the surface of the ocean, the image created by humans under the ocean in the shape of a dinosaur is hidden in the American continent The picture is of a bird in the shape of a dinosaur that is flying, and this bird was made to flyby the Indians of the tribe, that bird was talking to people, but its spirit might have heard something from the police because a thief while in the bird’s mouth He was handcuffed by the police and the weapon, which is a machine gun, is fromthe east of the American continent on the coast

The country of Florida is a machine gun.When you continue to New York City, you will reach the mouth of the dinosaur, where a thief is trapped in the mouth of a bird, and the little finger of the police handcuffed the thief’s hand, and a small colt is in the hand ofthe thief, who the police caught in the mouth of the dinosaur.put in the mouth of the bird dinosaur, you can clearly see that the thief fell on the ground and was shot in the head, and it is clear that his forehead was pierced, the bird’s mouth is open in flight, the head of the birdis from the east of the American continent, and a fish is placed in the bird’s mouth in the water of the ocean.The stretched glove of the police, which is in the form of a fist, with a handcuff attached to the left hand of the thief who fell on the ground in the sea and the mouth of the bird, the head of the thief and the killer, is located towards the southwest and west coast of America.All these images were created from the American continent and islands by Humans were created, but they didn’t have enough fuel and time to create more accurate images and meteorites ruined the beauty of the images, but it is clear that all these changes were createdby humans, but you have to consider that two thousand meters of soil from meteorites are fish.And they buried the whales under the beaches, and after a very long time, the bodies of the whales turned into oil under the two thousand meters of soil on the beaches, and on the other hand, the presence of two thousand meters of soil onthe surface of the seas and droughts could not make the created images disappear.

Leave a comment Cancel reply

Email address is optional. If provided, your email will not be published or shared.

Save my name, email, and website in this browser for the next time I comment.

• Red Light, Green Light
• Engineering Healthy Silence: Using Noise-Cancelling Headphones to Block Harmful Sound
• Crushing the Sound Barrier
• The Powers of Melanin
• E-Ink Technology: the Secret Behind Kindle

USC Viterbi School of Engineering

The Science of Time Travel

About the Author: Mark Villanueva

At the time of publication, Mark Villanueva was a student at the University of Southern California in his third year towards his BS in Computer Science. Back to the Future is one of his favorite movies.

Introduction

Definition of time travel.

Traversing in the fourth dimension is vastly different than moving in the previously mentioned three. We simply cannot will ourselves to move forward five minutes or back ten days. In other words, time only moves forward, and we are stuck moving in that direction like corks bobbing helplessly in a river [1]. Thus, the end goal of time travel is to enable us to control where we go in this fourth dimension. Much like moving back and forth, time travel involves moving to either the past or the future. However, our actions in the fourth dimension are cause for concern. What we do now in the present affects our future. In the same manner, our actions in the past should have affected our present lives. Changing the outcomes of past events leads to what physicists and philosophers refer to as the “Grandfather Paradox,” an issue that needs to be addressed in any serious discussion of time travel.

The Grandfather Paradox

“building” a time machine.

Unfortunately​, no black hole has yet been positively identified. Black holes, if they exist, could come in an extreme range of sizes. The English physicist Stephen Hawking has speculated that tiny black holes with masses no larger than that of a large mountain are possible. Such black holes, in the size range of elementary particles, would have been formed only under the extreme conditions that cosmology theories indicate existed in the very first moments of the universe. On the other hand, gigantic black holes may lie at the center of galaxies [7]. Einstein envisioned a situation in which the ends of two different black holes could be connected. This is known as a wormhole.

The wormhole is one basis for time travel into the past. Physicists liken wormholes to quick paths through the universe, much like the hole a worm burrows through an apple [8]. Instead of apples, these wormholes are theoretical tunnel shortcuts through space (Fig. 3).The trick in this case would be flying a spaceship into the one mouth of the wormhole and coming out the other side in a different time and place [9]. This involves moving one end of the wormhole close to the speed light and keeping the other end stationary near earth in outer space. Like the example with the spaceship traveling near the speed of light in space, the moving end of the wormhole in space will “age” slower than the stationary one; the “younger” end is a quick shortcut that connects to an earlier time on the fixed end [1], so the moving end of the wormhole will bring the traveler back to the past. The time travel process will probably involve a team of advanced scientists that can create wormholes and move them while the time traveler goes through the wormhole in outer space via a spaceship.

The Reality of Time Travel

• [1] J. R. Gott. “Will We Travel Back (Or Forward) In Time?”  Time , Apr. 10, 2000: pp. 68.
• [2] M. Kaku.  Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the Tenth Dimension. New York: Anchor Books, 1994.
• [3] P. J. Riggs. “The Principal Paradox of Time Travel.”  Ratio X , Apr. 1, 1997: pp. 49-64.
• [4] S. Mowbray. “Let’s do the time warp again.”  Popular Science]/i], Mar. 2002: pp. 46-51.
• [5] “How to murder your grandfather and still get born.”  The Economist , Jan. 20, 1996: pp. 81.
• [6] J. M. Zavisa. “How Special Relativity Works” Internet: http://www.howstuffw​orks.com, May 2, 2003. [Oct. 18, 2002].
• [7] L. Smarr. “Black Hole”.  The New Grolier Multimedia Encyclopedia . CD-ROM. 1993 Grolier Electronic Publishing, Inc.
• [8] A. Ramirez. “Clockwork: time travel isn’t what it used to be.”  The New York Times  Jul. 28 2002, natl. ed.: pp. WK3.
• [9] S. W. Hawking. “Protecting the Past: Is Time Travel Possible?”  Astronomy , Apr. 2002: pp. 46.
• [10] M. Moyer. “The Physics of Time Travel.”  Popular Science , Mar. 2002: pp. 52-53.
• [11] C. J. Wheeler. “Of wormholes, time machines and paradoxes.”  Astronomy , Feb 1996: pp. 52-58.
• ← The Quest for the Perfect Racket: Advances in Tennis Racket Design
• Improving the Bicycle →

Similar Posts

The Beauty of Science: New Technologies in Art Restoration

To Boldly Go Where No Man Has Gone Before: Faster-than-Light Travel in the 21st Century

Another atomic age, leave a reply cancel reply.

Your email address will not be published. Required fields are marked *

• Daily Crossword
• Word Puzzle
• Word Finder
• Word of the Day
• Synonym of the Day
• Word of the Year
• Language stories
• All featured
• Gender and sexuality
• All pop culture
• Grammar Coach ™
• Writing hub
• Grammar essentials
• Commonly confused
• All writing tips
• Pop culture
• Writing tips
• time travel

hypothetical transport through time into the past or the future.

Words Nearby time travel

• Time's wingéd chariot
• time switch
• time-tested
• time will tell

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2024

How to use time travel in a sentence

Whatever variant they meet will likely be very interested in time travel through the Quantum Realm, as the title Quantumania suggests.

It feels like every day, we get one step closer to figuring out the science behind time travel .

Considering that Loki will involve lots of time travel , it’s probable that we’ll get a glimpse of the multiverse in this show.

Then we would better understand space and time and perhaps finally decide if time travel is a realistic possibility, and if so, how to achieve it.

Physicists are far from agreeing over whether time travel of this sort is possible.

Underneath its comic-book action and time-travel shenanigans, X-Men: Days of Future Past questions the use of military robots.

The title of his forthcoming book is Rush Revere and the Brave Pilgrims: time travel Adventures with Exceptional Americans.

But somehow in the long run, truth and time travel the same road.

As soon as I entered West 100th Street, I understood that this experience was going to involve time travel .

Nine years ago he dazzled audiences with his $7,000 time-travel flick ‘Primer.’

His story is plausible, logical, once you grant the basic premise that time travel is an actuality.

It seems absurd that parts of the same train can at any time travel in opposite directions, but such is the case.

During this journey we recovered something of the conditions of old- time travel .

Even for younger Destinyworkers, time travel at best was an exhausting business.

We may dimly perceive something of the trials and hardships of old- time travel in that expression harbouring.

Words and phrases

Personal account.

• Access or purchase personal subscriptions
• Get our newsletter
• Save searches
• Set display preferences

Institutional access

Sign in with library card

Sign in with username / password

Recommend to your librarian

Institutional account management

Sign in as administrator on Oxford Academic

time travel noun

• Hide all quotations

What does the noun time travel mean?

There is one meaning in OED's entry for the noun time travel . See ‘Meaning & use’ for definition, usage, and quotation evidence.

How common is the noun time travel ?

How is the noun time travel pronounced, british english, u.s. english, where does the noun time travel come from.

Earliest known use

The earliest known use of the noun time travel is in the 1910s.

OED's earliest evidence for time travel is from 1914, in Journal of Philosophy, Psychology & Scientific Methods .

time travel is formed within English, by compounding.

Etymons: time n. , travel n.

Nearby entries

• timetabling, n. 1957–
• time-taker, n. 1576–
• time-taking, adj. 1839–
• time-taper, n. 1810–18
• time-taught, adj. 1758–1859
• time term, n. 1878–
• time-tested, adj. 1821–
• time thrust, n. 1771–
• time ticket, n. 1840–
• time train, n. 1853–
• time travel, n. 1914–
• time-travel, v. 1937–
• time traveller | time traveler, n. 1894–
• time travelling | time traveling, n. 1894–
• time-travelling | time-traveling, adj. 1871–
• time trial, n. 1857–
• time trialist, n. 1939–
• time trialling, n. 1953–
• time triangle, n. 1920–
• time-tried, adj. 1780–
• time value, n. 1848–

Thank you for visiting Oxford English Dictionary

To continue reading, please sign in below or purchase a subscription. After purchasing, please sign in below to access the content.

Meaning & use

Pronunciation, entry history for time travel, n..

Originally published as part of the entry for time, n., int., & conj.

time travel, n. was revised in March 2012.

time travel, n. was last modified in July 2023.

oed.com is a living text, updated every three months. Modifications may include:

• further revisions to definitions, pronunciation, etymology, headwords, variant spellings, quotations, and dates;
• new senses, phrases, and quotations.

Revisions and additions of this kind were last incorporated into time travel, n. in July 2023.

Earlier versions of this entry were published in:

A Supplement to the OED, Volume IV (1986)

• Find out more

OED Second Edition (1989)

• View time, n. in OED Second Edition

Please submit your feedback for time travel, n.

Please include your email address if you are happy to be contacted about your feedback. OUP will not use this email address for any other purpose.

Citation details

Factsheet for time travel, n., browse entry.

University of Notre Dame

Notre Dame Philosophical Reviews

• Home ›
• Reviews ›

Paradoxes of Time Travel

Ryan Wasserman, Paradoxes of Time Travel , Oxford University Press, 2018, 240pp., $60.00, ISBN 9780198793335.

Reviewed by John W. Carroll, North Carolina State

Wasserman's book fills a gap in the academic literature on time travel. The gap was hidden among the journal articles on time travel written by physicists for physicists, the popular books on time travel by physicists for the curious folk, the books on the history of time travel in science fiction intended for a range of scholarly audiences, and the journal articles on time travel written for and by metaphysicians and philosophers of science. There are metaphysics books on time that give some attention to time travel, but, as far as I know, this is the first book length work devoted to the topic of time travel by a metaphysician homed in on the most important metaphysical issues. Wasserman addresses these issues while still managing to include pertinent scientific discussion and enjoyable time-travel snippets from science fiction. The book is well organized and is suitable for good undergraduate metaphysics students, for philosophy graduate students, and for professional philosophers. It reads like a sophisticated and excellent textbook even though it includes many novel ideas.

The research Wasserman has done is impressive. It reminds the reader that time travel as a topic of metaphysics did not start with David Lewis (1976). Wasserman (p. 2 n 4) identifies Walter B. Pitkin's 1914 journal article as (probably) the first academic discussion of time travel. The article includes a description of what has come to be called the double-occupancy problem, a puzzle about spatial location and time machines that trace a continuous path through space. The same note also includes a lovely passage, which anticipates paradoxes about changing the past, from Enrique Gaspar's 1887 book:

We may unwrap time but we don't know how to nullify it. If today is a consequence of yesterday and we are living examples of the present, we cannot unless we destroy ourselves, wipe out a cause of which we are the actual effects.

These are just two of the many useful bits of Wasserman's research.

Chapter 1 usefully introduces examples of time travel and some examples one might think would involve time travel, but do not (e.g., changing time zones). There is good discussion of Lewis's definition of time travel as a discrepancy between personal and external time, including a brief passage (p. 13) from a previously unpublished letter from Lewis to Jonathan Bennett on whether freezing and thawing is time travel. I had often wonder what Lewis would have said; now I know what he did say!

Chapter 2 dives into temporal paradoxes deriving from discussions of the status of tense and the ontology of time (presentism vs. eternalism vs. growing block vs. . . . ). Here, Wasserman also includes the double-occupancy problem as a problem for eternalism -- though it is not clear that it is only a problem for eternalism. Then he turns to the question of the compatibility of presentism and time travel, the compatibility of time travel and a version of growing block that accepts that there are no future-tensed truths, and finally to a section on relativity and time travel. The section on relativity is solid and seems to me to pull the rug out from under some earlier discussions. For example, Lewis's definition of time travel is shown not to work. It also becomes clear that presentism and the growing block are consistent with both time-dilation-style forward time travel and traveling-in-a-curved-spacetime "backwards" time travel.

Chapters 3 and 4 cover the granddaddies of all the time-travel paradoxes: the freedom paradoxes that include the grandfather paradox, the possibility of changing the past, and the prospects of such changes given models of branching time, models that invoke parallel worlds, and hyper time models. Chapter 4 gets serious about Lewis's treatment of the grandfather paradox and Kadri Vihvelin's treatment of the autoinfanticide paradox (about which I will have more to say).

Chapter 4 also includes discussion of "mechanical" paradoxes that, as stated, do not require modal premises about what something can and cannot do, and no notion of freedom or free will. (See Earman's bilking argument on p. 139 and the Polchinski paradox on p. 141.) Wasserman introduces modality to these paradoxes, but I would have liked them to be addressed on their own terms. As I see it, these paradoxes are introduced to show that backwards time travel or backwards causation in a certain situation validly lead to a contradiction. On their own terms, for these arguments to be valid, the premises of the arguments themselves must be inconsistent. How can one make trouble for backwards time travel if the argument is thus bound to be unsound?

Chapter 5 takes on the paradoxes generated by causal loops or more generally backwards causation including bilking arguments, the boot-strapping paradox (based on a presumption that self-causation is impossible), and the ex nihilo paradox with causal loops and object loops (i.e., jinn) that seem to have no cause or explanation.

Chapter 6 deals with paradoxes that arise from considerations regarding identity, with a focus on the self-visitation paradox from both perdurantist and endurantist perspectives. I was surprised to learn that Wasserman had defended an endurantist-friendly property compatibilism -- similar to my own -- to resolve the self-visitation paradox. I was then delighted to find out that he cleverly extends this sort of compatibilism to the time-travel-free problem of change (i.e., the so-called, temporary-intrinsics argument).

The outstanding scientific issue regarding backwards time travel is whether it is physically possible. There is no question that forwards time travel is actual, or even whether it is ubiquitous. There is also not much question that backwards time travel is consistent with general relativity. Still, we await more scientific progress before we will know whether backwards time travel really is consistent with the actual laws of nature. In the meantime, there is still much to be said about Lewis's treatment of the grandfather paradox and Vihvelin's stated challenge to that treatment in terms of the autoinfanticide paradox.

I will start by being somewhat critical of Lewis's approach. For his part (pp. 108-114), Wasserman does a terrific job of laying out Lewis's position as a metatheoretic discussion of the context sensitivity of 'can' and 'can't'. My concern is that not enough attention is given to the 'can' and 'can't' sentences that turn out true on the semantics. The semantics works only by a contextual restriction of possible worlds based on relevant facts -- the modal base -- associated with a conversational context. In meager contexts, false 'can' sentences will turn out true too easily. For example, suppose two people are having a conversation about Roger. Maybe all the two know about Roger is his name and that he is moving into the neighborhood. So, the proposition that Roger doesn't play the piano is not in the modal base. So, according to Lewis's semantics applied to 'can', 'Roger can play the piano' is true in this context. That seems wrong. This would be an unwarranted assertion for either of the participants in the conversation to make. Notice it is also true relative to the same meager context that Roger can play the harpsichord, the sousaphone, and the nyatiti. Quite a musician that Roger! [1]

Interestingly, though this problem arises for 'can', it does not arise for other "possibility" modals. For example, notice that, with the meager context described above, there is a big difference regarding the assertability of 'Roger could play the piano' and of 'Roger can play the piano'. Similarly, there is also no serious issue with regard to 'Roger might play the piano'. 'Could' and 'might' add tentativeness to the assertion that seems called for. There also seems to be no problem for the semantics insofar as it applies to 'is possible'. 'It is possible that Roger plays the piano' rings true relative to the context. But 'Roger can play the piano'? That shouldn't turn out true, especially if Roger is physically or psychologically unsuited for piano playing.

This issue has been frustrating for me, but Wasserman's book has me leaning toward the idea that what is needed is a contextual semantics that includes a distinguishing conditional treatment of 'can' of the sort Wasserman suggests:

(P1**) Necessarily, if someone would fail to do something no matter what she tried, then she cannot do it (p. 122).

This is a suggestion made by Wasserman on behalf of Vihvelin. I find (P1**) as a promising place to start in terms of the conditional treatment.

Speaking of Vihvelin, her thesis is "that no time traveler can kill the baby that in fact is her younger self, given what we ordinarily mean by 'can'" (1996, pp. 316-317). Vihvelin cites Paul Horwich as a defender of a can-kill solution, what she calls the standard reply :

The standard reply . . . goes something like this: Of course the time traveler . . . will not kill the baby who is her younger self . . . But that doesn't mean she can't . (Vihvelin 1996, p. 315)

Vihvelin's doing so is appropriate given what Horwich says about Charles attending the Battle of Hastings: "From the fact that someone did not do something it does not follow that he was not free to do it" (1975, 435). In contrast, it strikes me as odd that Vihvelin (1996, p. 329, fn. 1) also attributes the standard reply to Lewis. I presume that she does so based on some comments by Lewis. He says, "By any ordinary standards of ability , Tim can kill Grandfather," (1976, p. 150, my emphasis) and especially "what, in an ordinary sense , I can do" (1976, p. 151, my emphasis). So, admittedly, Vihvelin fairly highlights an aspect of Lewis's view as holding that, in the ordinary sense of 'can', Tim can kill Gramps. And I can see how this is a useful presentation of Lewis's position for her argumentative purposes.

Nevertheless, I take Lewis's talk of ordinary standards or an ordinary sense to just be a way to identify the ordinary contexts that arise with uses of 'can' in day-to-day dealings, where the possibility of time travel is not even on the table. Simple stuff like:

Hey, can you reach the pencil that fell on the floor?

Sure I can; here it is.

More importantly, we have to keep in mind that the basic semantics only has consequences about the truth of 'can' sentences once a modal base is in place. To me, the fact that Baby Suzy grows up to be Suzy is exactly the kind of fact that we do not ordinarily hold fixed. Lewis's commitment to the semantics does not make him either a can-kill guy or a can't-kill guy.

What is the upshot of this? There is a bit of underappreciation of Lewis's approach in Wasserman's discussion of Vihvelin's views. The pinching case on p. 119 provides a way to make the point. Consider:

(a) If Suzy were to try to kill Baby Suzy, then she would fail.

(b) If Suzy were to try to pinch Baby Suzy, then she would fail.

According to Wasserman, Vihvelin thinks that even in ordinary contexts (a) and (b) come apart (p. 119, note 32) -- (a) is true and (b) is false. As I see it, a natural context for (a) includes the fact that Baby Suzy grows up normally to be Suzy. That is a supposition that is crucial to the description of the scenario and so is likely to be part of the modal base. No canonical story or suppositions are tied to (b), though Vihvelin stipulates that Suzy travels back in time in both cases. We are not, however, told a story of Baby Suzy living a pinch-free life all the way to adulthood. We are not told whether Suzy decided go back in time because Baby Suzy deserved a pinch for some past transgression. My point is that the stories affect the context. So, with parallel background stories, (a) and (b) need not come apart.

I am not sure whether Wasserman was speaking for himself or for Vihvelin when he says about (a) and (b), "Self-defeating acts are paradoxical in a way other past-altering acts are not" (p. 120). Either way, I disagree. Lewis gives a more general way to resolve the past-alteration paradoxes that is not obviously in any serious conflict with Vihvelin's many utterances that turn out true relative to the contexts in which she asserts them. Wasserman also says, "The only disagreement between Lewis and Vihvelin is over whether Suzy's killing Baby Suzy is compatible with the kinds of facts we normally take as relevant in determining what someone can do" (p. 117). That is an odd thing for him to say. Lewis sketches a semantic theory that provides a framework for the truth conditions of 'can' and 'can't' sentences. He is not in disagreement with Vihvelin. For Lewis, there is one specification of truth conditions for 'can' that gives rise to both 'can kill' and 'can't kill' sentences turning out true relative to different contexts. Indeed, it is tempting to think that Vihvelin takes the fact that Baby Suzy grows up to be Adult Suzy as part of the modal base of the contexts from which she asserts the compelling 'can't-kill' sentences.

That all said, Wasserman's book is a significant contribution. There are those of us who focus a good chunk of our research on the paradoxes of time travel for their intrinsic interest, and especially because they are fun to teach. That is contribution enough for me. But, ultimately, from this somewhat esoteric, fun puzzle solving, we also learn more about the rest of metaphysics. The traditional issues of metaphysics: identity-over-time, freedom and determinism, causation, time and space, counterfactuals, personhood, mereology, and so on, all take on a new look when framed by the questions of whether time travel is possible and what time travel is or would be like. Wasserman's book is a wonderful source that spotlights these connections between the paradoxes of time travel and more traditional metaphysical issues.

Cargile, J., 1996. "Some Comments on Fatalism" The Philosophical Quarterly 46, No. 182 January 1996, 1-11.

Gaspar, E., 1887/2012. The Time-Ship: A Chronological Journey . Wesleyan University Press.

Horwich, P., 1975. "On Some Alleged Paradoxes of Time Travel" The Journal of Philosophy 72, 432-444.

Lewis, D., 1976 "The Paradoxes of Time Travel" American Philosophical Quarterly 13, 145-152.

Pitkin, W., 1914. "Time and Pure Activity" Journal of Philosophy, Psychology and Scientific Methods 11, 521-526.

Vihvelin, K., 1996. "What a Time Traveler Cannot Do" Philosophical Studies 81, 315-330.

[1] This criticism was first presented to me by Natalja Deng in the question-and-answer period for a presentation at the 2014 Philosophy of Time Society Conference. Later on, I found a parallel challenge in work by James Cargile (1996, 10-11) about Lewis's iconic, 'The ape can't speak Finnish, but I can'.

the eight types of time travel

Time travel is a stable in science fiction. Countless books, comics, movies, and TV shows have used it as their main plot device. Even more have incorporated it into a key moment of the story. Over the years, eight major types of time travel logic emerged. Recently, YouTubers Eric Voss and Héctor Navarro examined all eight types, and looked at which one gets it most correct in term of the real world science behind science fiction.

Type 1 Anything goes

Definition: Characters travel back and forth within their historical timeline.

This approach frees you to have fun and not get lost in the minutiae of how time travel works. Usually, there’s a magical Maguffin that to quote the great Dr. Ememett Brown, “makes time travel possible”. Writers have used a car, a phone booth, and a hot tub, among other options. This approach leads to inconsistent limits on the logic of the time travel, but this doesn’t mean the story is poorly plotted, won’t be enjoyable or won’t be an enormous hit. This approach is more science fantasy than science fiction with no basis in real-world science.

Examples: Back to the Future , Bill and Ted’s Excellent Adventure , Hot Tube Time Machine , Frequency , Austin Powers , Men In Black 3 , Deadpool 2 , The Simpsons , Galaxy Quest , Star Trek TOS , Doctor Who , 11/22/63 by Stephen King.

Type 2 Branch Reality

Definition: Changes to the past don’t rewrite history. They split the timeline into an alternate branch timeline. This action does not change or erase the original timeline.

As authors got more familiar with the science behind time travel in theoretical physics, this type, based upon the many worlds theory in quantum mechanics, emerged. When the character travels back into the past and changes events, they create a new reality. Their original reality is unchanged. Branches themselves can branch leading to a multiverse of possibilities.

Examples: The Disney Plus series, Loki , used this extensively. See also: Back to the Future Part II , Avenger’s Endgame , the DC Comics multiverse, the Marvel Comics multiverse, Rick and Morty , Star Trek (2009), A Wrinkle in Time by Madeleine L’Engle.

Type 3 Time Dilation

Definition: Characters traveling off-world experience time moving more slowly than elsewhere in the universe, allowing them to move forward in time (but not backward).

This type is the based upon our scientific understanding of how time slows down as you approach the speed of the light. This is a forward-only type of time travel. There’s no going backwards.

Examples: Planet of the Apes , Ender’s Game , Flight of the Navigator , Interstellar , Buck Rodgers .

Type 4 This Always Happened

Definition: All of time is fixed on a predestined loop in which the very act of time travel itself sets the events of the story into motion.

This one can confuse and delves closer to the realm of theology than science. It feels gimmicky, and has become something of a trope making it hard to pull this off in a satisfying way for your audience. This type also invites the audience to question if your protagonist ever had free will or agency in the story.

Examples: Terminator , Terminator 2 , Harry Potter and the Prisoner of Azkaban , Game of Thrones -Season 6, Twelve Monkeys , Interstellar , Kate and Leopold , The Butterfly Effect , Predestination , Ricky and Morty -Season 5, Looper .

Type 5 Seeing the Future

Definition: After seeing a vision of their fate, characters choose to change their destiny or embrace their lot.

We’re stretching to call this time travel, but it provides your story with built-in conflict and stakes. Will the hero choose to walk the path knowing how it will end, or will they choose a different path?

Examples: Oedipus Rex , A Christmas Carol , Minority Report , Arrival , Next (Nicolas Cage), Rick and Morty -Season Four. Star Trek:Discovery -Season 2, Avenger’s EndGame with Dr. Strange and the Mind Stone.

Type 6 Time Loop / Groundhog Day

Definition: Characters relive the same day over and over, resetting back to a respawn point once they die or become incapacitated.

This type gained popularity after the movie, Groundhog Day , became a tremendous hit. Most of the other examples take the Groundhog Day idea and put a slight twist on it. Like Type 4 “This Always Happened”, the popularity of this type can make it harder to pull off in a fresh and innovative way.

Examples: Obviously, Groundhog Day with Bill Murray. Edge of Tomorrow , Doctor Strange in the ending battle with Dormammu, Russian Dolls (Netflix), Palm Springs , Star Trek TNG .

Type 7 Unstuck Mind

Definition: Characters consciousness transport through time within his body to his life at different ages.

Nostalgia for the past and dreaming of the future are core parts of the human experience. This type runs more metaphorically than scientific.

Examples: Slaughterhouse 5 by Kurt Vonnegut, X-Men: Days of Future Past , Desmond in the series Lost .

Type 8 Unstuck Body

Definition: A character’s body or object becomes physically detached from the flow of time within the surrounding universe, becoming inverted or younger. Only certain objects or bodies are unstuck from time. Also called Inverted Entropy.

This one will blow your mind if you think about it for too long. Like Type 2 “Branch Reality”, this one comes from the realm of quantum mechanics and theoretical physics. Scientists and mathematicians have all the formulas worked out to make this de-aging a reality, but currently lack the technology to control all the variables in the ways needed. It would like scientists working out than an object could break the speed of the sound in 1890. It would look inconceivable, given the technology of the day, but I wouldn’t put limits on human ingenuity.

Examples: Dr. Strange (the Hong Kong battle). Tenet , briefly in Endgame with Scott Lang and Bruce, Primer .

If you’re writing a time travel story, you’ll need to decide which one of these types you want to deploy. They all have their advantages and disadvantages. In many ways, its similar to designing your magic system, especially if you go with a Type 1 time travel story. The most important thing remains to have relatable characters and to tell a great story while being internally consistent with the rules and logic of your story world.

Ted Atchley  is a freelance writer and professional computer programmer. Whether it’s words or code, he’s always writing. Ted’s love for speculative fiction started early on with Lewis’  Chronicles of Narnia,  and the Star Wars movies. This led to reading Marvel comics and eventually losing himself in Asimov’s Apprentice Adept and the world of Krynn ( Dragonlance Chronicles ). 

After blogging on his own for several years, Blizzard Watch ( blizzardwatch.com ) hired Ted to be a regular columnist in 2016. When the site dropped many of its columns two years later, they retained Ted as a staff writer. 

He lives in beautiful Charleston, SC with his wife and children. When not writing, you’ll find him spending time with his family, and cheering on his beloved Carolina Panthers. He’s currently revising his work-in-progress portal fantasy novel before preparing to query. 

Ted has a quarterly newsletter which  you can join here . You’ll get the latest on his writing and publishing as well as links about writing, Star Wars, and/or Marvel.

You Might Also Like

The Perfect Blend of History and Fantasy

World-building From the Eyes of a Character

Your protagonist: the eyes to your story, a writer’s guide to crafting the perfect resume, how i spent the mesozoic era.

Follow Us Elsewhere

Latest tweets, recent posts.

Award-Winning Author Interview with Theresa Parker Pierce

An astrophysicist claims he finally figured out time travel

If you buy through a BGR link, we may earn an affiliate commission, helping support our expert product labs.

Time travel has been one of the biggest tropes in science fiction for years. But what if you could actually go back in time and visit a loved one before their death? There’s, obviously, a lot we don’t know about what kind of consequences time travel might bring to the table , but that hasn’t stopped physicist Ron Mallet from a lifelong obsession with trying to figure out the time travel equation.

What’s even more impressive about this lifelong endeavor, though, is that Mallet now claims to have solved the equation and figured out how to build an actual time machine. Mallet’s inspiration and obsession with time travel originally began when he was much younger, Earth.com notes . Following the death of his beloved father, Mallet lost himself in novels, including The Time Machine by H.G. Wells.

It is certainly a respectable goal, especially for anyone who has lost someone they loved dearly. Mallet says that his idea of a time machine centers around an “intense and continuous rotating beam of light” that can manipulate gravity. A device built by him, following his equation, would use a ring of lasers to mimic the effects of a black hole, which appears to distort space and time around them.

Tech. Entertainment. Science. Your inbox.

Sign up for the most interesting tech & entertainment news out there.

By signing up, I agree to the Terms of Use and have reviewed the Privacy Notice.

Of course, learning the time travel equation and building a working time machine are two different things altogether. Sure, scientists have simulated black holes in a lab once or twice, but never anything with the kind of power or reality-effecting pull that Mallet seems to think would make time travel possible. That isn’t to say that he’s got things wrong, though.

His equation for time travel may be exactly what is needed to break through this lifelong obsession and actually travel back in time. But building something capable of testing it is going to be a whole separate endeavor in and of itself.

This article talks about:

Josh Hawkins has been writing for over a decade, covering science, gaming, and tech culture. He also is a top-rated product reviewer with experience in extensively researched product comparisons, headphones, and gaming devices.

Whenever he isn’t busy writing about tech or gadgets, he can usually be found enjoying a new world in a video game, or tinkering with something on his computer.

• This free app claims it can tell if someone is lying
• Construction on the first EV charging highway will start this month
• Revolutionary new power tech can transmit electricity through glass to your outdoor devices

More Science

New tech will let wind turbines build themselves

How to make sure your eclipse glasses are actually safe to use

Our landfills are destroying the Earth, study shows

Scientists made implantable batteries that are powered by your body’s oxygen

Latest news.

YouTube CEO to OpenAI: Don’t you dare use our videos to train Sora

Roku wants to show you ads whenever you press pause

How to watch coachella 2024 on youtube.

Google’s Find My Device network is finally working for some people

Sign up for the most interesting tech & entertainment news out there.

When is the 2024 total solar eclipse? Your guide to glasses, forecast, where to watch.

Editor's note: An updated cloud forecast for the April 2024 total solar eclipse is in. Read the latest eclipse forecast and news as of Wednesday, April 3 .

We're less than two weeks away from the astronomical event of the decade: A total eclipse of the sun , which will grace the nation's skies from Texas to Maine on the afternoon of Monday, April 8 .

Millions of people are expected to travel to see the spectacle, which will also attract scientists from across the country to study its unique effects on the Earth and its atmosphere . (Meanwhile, brands such as Burger King, Pizza Hut, Applebee's and Sonic are getting in on the eclipse excitement too.)

The full total solar eclipse experience will plunge people along a narrow path into darkness midday, but people outside the path of totality could still use eclipse glasses to see the moon pass in front of the sun. It's an awesome and confusing sight on the ground and in the sky.

It should go without saying: Don't drive while wearing eclipse glasses

One task to do now is to get yourself a pair of special eclipse glasses (and luckily there's still time to score a free pair of eclipse glasses ). They're important eyewear during an eclipse because they will block out the sun's harmful rays, which could damage your eyes when you're looking at the sun — even if it's partially blocked.

Solar eclipse glasses are still readily available from plenty of vendors across the internet as of Thursday, March 28. But be wary of counterfeit or fake glasses , experts warned.

The other big factor everyone's anxiously awaiting is the weather forecast, which most experts say is still a bit far away for a specific forecast. But more detailed, realistic forecasts will start to roll in over the next few days.

Of particular interest will be the cloud forecast , as a deck of thick, low clouds would likely ruin the entire event. But if the clouds are higher up in the atmosphere, such as thin, wispy cirrus , they'd act to diffuse but not ruin the eclipse.

USA TODAY 10Best: 10 solar eclipse fun facts to share with your friends

There's plenty to know before the big day, which won't happen again for about two decades . Read on for answers for all your eclipse questions, including how to watch the eclipse , understand the eclipse and use eclipse glasses to stay safe .

WATCHING THE APRIL 2024 ECLIPSE

How do i watch the 2024 solar eclipse.

There are a few ways of watching the eclipse:

• Get the full experience in person : If you're in a narrow band of U.S. land that spans from Texas to Maine, you could see the moon block the sun and its shadow cast a night-like darkness over Earth for a few minutes. You'll briefly be able to look up without eye protection and see the moon block the sun.
• Watch from outside the path of totality : Much of the U.S. is set to get a partial view of the eclipse that isn't nearly as impressive as being in the path of totality. Earth won't be plunged into complete darkness and you'll have to wear protective eyewear to see the moon partially block the sun.
• Watch a livestream : Check back on April 8 for a video feed from the path of totality. It's not the same as being there in person, but hey, at least you won't have to sit in traffic .

Totality explained: The real April 2024 total solar eclipse happens inside the path of totality

What time is the solar eclipse on April 8?

The eclipse will begin in Texas at 1:27 p.m. CDT and end in Maine at 3:35 p.m. EDT, but the exact time of the eclipse varies by where you are in its path.

You can search by zip code to find the exact time for your location.

Where will the April eclipse be visible?

All of the lower 48 U.S. states will see the moon at least clip the sun, but that sight is a trifle compared to being in the path of totality.

Because the narrow path includes or is near some of the nation's largest cities, expect millions of people to crowd into a band of land a little over 100 miles wide that spans from the Texas/Mexico border to the Maine/Canada border.

Here are the major cities in each state where you can expect to experience totality (note that the included times do not account for when the partial eclipse begins and ends):

• Dallas, Texas: 1:40-1:44 p.m. CDT
• Idabel, Oklahoma: 1:45-1:49 p.m. CDT
• Little Rock, Arkansas: 1:51-1:54 p.m. CDT
• Poplar Bluff, Missouri: 1:56-2:00 p.m. CDT
• Paducah, Kentucky: 2-2:02 p.m. CDT
• Carbondale, Illinois: 1:59-2:03 p.m. CDT
• Evansville, Indiana: 2:02-2:05 p.m. CDT
• Cleveland, Ohio: 3:13-3:17 p.m. EDT
• Erie, Pennsylvania: 3:16-3:20 p.m. EDT
• Buffalo, New York: 3:18-3:22 p.m. EDT
• Burlington, Vermont: 3:26-3:29 p.m. EDT
• Lancaster, New Hampshire: 3:27-3:30 p.m. EDT
• Caribou, Maine: 3:32-3:34 p.m. EDT

Will clouds block the April 2024 eclipse?

It's too soon to say for sure, but history offers some clues.

Chances of cloudy skies are pretty high along much of the eclipse's northern path, and some areas such as Buffalo have about a 1-in-3 chance of clear skies in early April.

Skies are usually more clear to the south. Right along the Texas/Mexico border, chances of a clear sky can be nearly 75%.

However, early forecasts tell a different story — although forecaster caution it's too early for truly accurate forecasts.

Keep in mind that clouds don't always ruin an eclipse . High, wispy clouds won't spoil the show in the same way that low thick clouds would. In that case, you wouldn't be able to see the moon pass in front of the sun, but you would still notice a sudden darkness in the path of totality.

What dangers are associated with the eclipse?

The  eclipse , as exciting and fun as it promises to be, comes with a growing number of safety warnings — both for what will happen in the sky and what will happen on the ground.

In addition to the dangers of looking at the eclipse without proper eyewear, we've heard warnings about potentially dangerous  fake eclipse glasses , the possibility for increased  car  c rashes  around the time of the eclipse — and possible  air travel headaches  due to delayed or cancelled flights.

Where is the best place to watch the eclipse?

In the weeks leading up to the eclipse, Texas was considered the best state for eclipse viewing . Historically, there's a good chance the skies will be clear and its location along the southern path of the eclipse means totality will last a little longer. That could change as the forecast evolves .

A few lucky travelers will also have a front-row seat with unobstructed views — from a plane .

Will traffic be bad?

Most likely, yes — especially after the eclipse ends . Millions of people will crowd into the narrow path of totality, then many will attempt to leave all at once as soon as the eclipse is over.

When a total solar eclipse passed over the U.S. in 2017, reports say some traffic jams didn't fully clear for more than 12 hours. Even worse, a recent study reported that the 2017 eclipse "was associated with increased risks of a fatal traffic crash" – potentially as much as a 31% increase, the study said .

Also worth noting: The eclipse is expected to have impacts for air travel as well.

Cities across the eclipse's path of totality are also bracing for an influx of visitors who are already booking up hotels and short-term vacation rentals , officials have told USA TODAY. "Hotels are almost sold out," said Shalissa Perry, the chief marketing officer for Downtown Dallas.

Why are schools closing for the eclipse?

Primarily out of a concern for students' safety, schools across the country have given students  the day of the eclipse, April 8,  as a day off, a half day or a chance for e-learning. In Tennessee, for example, officials also say it gives students a chance  to experience the eclipse with their family and friends .

What's it like to watch a solar eclipse from space?

Ever wondered what it is like see the  solar eclipse  from space? NASA scientist and veteran astronaut Terry Virts has witnessed the spectacle in space .

"When I was in space in 2015, we saw an eclipse over the North Atlantic," Virts told USA TODAY this week. "It was an eclipse that very few humans saw I think, but it was a really unique experience to look down and just see this big black circle kind of moving across the planet."

"It was unlike anything I've ever seen," Virts adds. "I joke I'm glad they told us there was an eclipse because it would have been unsettling to look down at Earth and see this black spot moving across. It was really cool."

Virts, who is the face of  Sonic's eclipse-themed Blackout Slush Float , said that though he's seen one eclipse from space and many from Earth, he is super excited about the upcoming one on April 8.

UNDERSTANDING THE 2024 ECLIPSE

What is a solar eclipse definition explained..

A total solar eclipse happens when three celestial spheres — the sun, moon and Earth — line up in a specific way in space.

According to NASA, a solar eclipse happens when the moon passes between the sun and Earth. That alignment casts a moving shadow on Earth that either fully or partially blocks the sun's light in some areas. This leads to a period of partial or full darkness on a narrow stretch of Earth. 

The path is so narrow because of the huge distance and size of the sun — as well as the moon's distance from Earth. That focuses the moon's shadow on an area of land much smaller than the moon itself. The movement of the shadow across the land happens as the Earth's rotation interacts with the orbit of the moon.

A total eclipse only happens occasionally because the moon doesn't orbit in the exact same plane as the sun and Earth do. In addition, a solar eclipse can only happen during a new moon.

What's special about a total solar eclipse?

The total solar eclipse on April 8  is causing such a stir because the rare event is an astronomical experience like no other that will be unusually accessible to millions of people.

April's total solar eclipse will fall over more places in the U.S. than the total eclipse before and after it. And the broad length of the path of totality – where Americans have the best shot of getting a clear view – is "much wider"  than it was for the eclipse in 2017 , according to the National Aeronautics and Space Administration.

A total solar eclipse is also far more impressive  than a lunar or an annular solar eclipse. During an annular eclipse, the moon covers the Sun but leaves an outside ring some call a "ring of fire" — it darkens the sky instead of plunging Earth into a night-like darkness, which is what happens during a total solar eclipse. And a lunar eclipse – the appearance of a red moon – happens when the moon passes into the Earth's shadow, according to NASA.

Total solar eclipses can have spiritual significance, too . Ancient cultures viewed as a sign of the gods' anger or impending departure. Some religions today are hosting eclipse viewings and services.

Historically, eclipses have left major marks on religious and spiritual civilizations. In Christianity, an eclipse has been associated with the darkness that accompanied Jesus' crucifixion and in Islam, the passing of the Prophet Muhammad's son Ibrahim.

When was the last solar eclipse?

The USA's most recent total solar eclipse was on August 21, 2017, and stretched from Oregon to South Carolina.

When will the next solar eclipse happen?

The  next visible total solar eclipse  to cross over the U.S. after April will come in more than two decades on Aug. 23, 2044, according to NASA.

And that eclipse won't be as accessible as the 2024 one: The path of totality in 2044 will only touch the states of Montana, North Dakota and South Dakota, according to the Planetary Society, a nonprofit involved in research, public outreach and political space advocacy. Another total eclipse  will pass over the U.S. in 2045  that will be more accessible to Americans, including for people who live in California, Florida and Nevada.

ECLIPSE GLASSES AND SAFETY

What are eclipse glasses and why are they needed for the eclipse.

Gazing at the bright rays from the eclipse without  protective eyewear  can seriously damage your eye, so wearing a pair of protective glasses is important.

There's a technical standard for eclipse glasses, which are designed to block out most light and let you safely see the moon pass in front of the sun: It's called ISO 12312-2 after the International Organization of Standardization.

While there's concerns that not all glasses marketed as eclipse glasses live up to that standard, experts say in the past, the shortcomings haven't been significant .

But as the 2024 eclipse approaches, the American Astronomical Society  has warned that there are some counterfeit and fake eclipse glasses being sold from unverified vendors that would be unsafe to use during the eclipse. They recommend buying from a vetted vendor and testing the glasses before the eclipse.

How do I get solar eclipse glasses?

You should buy from a reputable source who can ship the glasses to you in time for April 8. A guide to last-minute eclipse glasses explains more.

Heads up: The online marketplace is flooded with retailers selling solar eclipse glasses they claim have the NASA seal of approval . Consider that a red flag to look elsewhere.

How can you test eclipse glasses?

NASA shared an easy method to check eclipse glasses at home.

Buyers should put on their glasses and look at a bright light, like a flashlight. If the light is "extremely dim," or doesn't appear at all, the glasses are safe, Susannah Darling, of NASA's Goddard Space Flight Center, said in an instructional video. Viewers should be able to see the filament of the lightbulb, not the glow surrounding the bulb.

Can you really go blind watching a solar eclipse?

You could severely damage your eyes . Directly staring at the sun before and after the total eclipse, or watching a partial eclipse outside the path of totality without proper eye protection, can result in permanent damage including blurred and altered vision.

While rare, eye damage from watching a partial eclipse happens in part because a person's natural response to squint when looking at sunlight does not get triggered. In the lead-up to the  April 8 solar eclipse ,  doctors and a rare set of eclipse watchers are warning about  watching this planetary event without adequate eclipse glasses or with the naked eye.

It’s hard for experts to know or even estimate how many people experience eye damage from solar eclipses. Since looking at an eclipse does not cause complete blindness, people with permanent damage may not know they have it or report it to a doctor. The 2017 eclipse , which passed from Oregon to South Carolina, is thought to have caused about 100 cases, according to the  American Astronomical Society .

How can I watch the eclipse without glasses?

If you don't have access to eclipse glasses do not use regular sunglasses — You need a more creative solution for safe viewing, like a pinhole projector .

Welding glasses are not recommended for eclipse viewing .

Should I take dogs or cats to see the eclipse? Is it safe for pets?

An eclipse itself isn't dangerous for domestic animals such as dogs and cats, but experts say it's probably best to not bring pets .

Experts' biggest concern is not what’s happening in the sky but on the ground as crowds of excited and anxious people gather, said Dr. Rena Carlson, president of the American Veterinary Medical Association.

“Rather than the effects of the eclipse, I would be more worried about the excitement and all of the people,” she said.

Another fun way to experience the eclipse: disco balls

Don't just reach for the solar glasses; a disco ball might be a fun and safe way to enhance the total solar eclipse experience .

Inspired by a research paper from European scientists, the Round Rock Public Library in Round Rock, Texas, near Austin, used disco balls during the annular eclipse in October and recommends people do the same for the April 8 eclipse. Officials said the use of a disco ball creates a "party-like atmosphere" to make a solar eclipse event more fun.

The mirrored ball can be placed outside where it can catch the sunlight and reflect it on a shaded wall at least several feet away, or it can be inside near a window to cast reflections of the sun around the room. Youth Services Librarian Andrea Warkentin recommends people get disco balls that have smaller mirrors on them as they will create bigger and better images.

"It's a way to make it really memorable and fun for little kids who may not really understand what's going on in the sky," Warkentin said.

-Fernanda Figueroa, Austin American-Statesman

Contributing: Ramon Padilla, Karina Zaiets and Janet Loehrke