light travel infinite distance

Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.   

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

Follow-up on this answer

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January 19, 1998

Why isn't the speed of light infinite?

Stephen Reucroft and John Swain, professors of physics at Northeastern University in Boston, Mass, provided the following explanation:

The common experience of turning on a light switch certainly shows that light travels very quickly. But careful experiments reveal that it travels at a finite speed. This speed, which we call "c," is measured to be 300,000,000 meters per second.

The speed of light is strange in that it has the same value independent of the relative velocity between the source and the observer. This fact is an experimental one that can only make sense if relative motion changes the relationship between space and time intervals to keep the distance covered by light per unit time the same for all observers.

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The fact that space and time must get mixed up to keep the speed of light constant implies that, in some sense, space and time must be the same, despite our habit of measuring space in meters and time in seconds. But if time and space are similar to the extent that they can be converted one into the other, then one needs some quantity to convert the units--namely, something measured in meters per second that can be used to multiply seconds of time to get meters of space. That something, the universal conversion factor, is the speed of light. The reason that it is limited is simply the fact that a finite amount of space is equivalent to a finite amount of time.

Another explanation of light's finite nature can be obtained from thinking about what we mean by light itself. Light, by definition, is an electromagnetic wave, a propagating disturbance in space and time that carries information about the acceleration of charges.

Were there an infinite value for the speed of light, light itself would not exist at all. Mathematically, the wave equation that describes light as an electromagnetic wave would lose its time-dependence.

In physical terms, an electromagnetic wave arises due to the finite time it takes for news of the change of location of an accelerated charge to arrive at a distant point. Think of an electric charge as being like a hedgehog with flexible rubber spikes going out to infinity in all directions. These spikes represent the electric field lines, the lines along which a test charge would move.

If the charge is jerked, the segments of the spikes close to the charge will move, but those farther out will still point in their original directions. The result is that each spike will get a kink that moves out to infinity. This kink relays the news that the charge has moved to the distant parts of the spikes and corresponds to an electromagnetic wave. If the wave moves infinitely fast, it is as if it were not there at all; the spikes are infinitely stiff and the news gets out to everywhere without any seeming kinks. In other words, there would be no electromagnetic wave, and thus no light.

The previous two arguments are two slightly different ways to say that if you think light is a wave, then it has to be something that propagates and takes time to go from one point to another. In other words, it has to travel at a finite speed. Infinite speed of propagation is an instantaneous magical change in things everywhere all at once, and not a wave at all!

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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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Why is the speed of light the way it is?

It's just plain weird.

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of " How to Die in Space ." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights . 

We all know and love the speed of light — 299,792,458 meters per second — but why does it have the value that it does? Why isn't it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics? 

Well, it's because the speed of light is just plain weird.

Related: Constant speed of light: Einstein's special relativity survives a high-energy test

Putting light to the test

The first person to realize that light does indeed have a speed at all was an astronomer by the name of Ole Romer. In the late 1600s, he was obsessed with some strange motions of the moon Io around Jupiter. Every once in a while, the great planet would block our view of its little moon, causing an eclipse, but the timing between eclipses seemed to change over the course of the year. Either something funky was happening with the orbit of Io — which seemed suspicious — or something else was afoot.

After a couple years of observations, Romer made the connection. When we see Io get eclipsed, we're in a certain position in our own orbit around the sun. But by the next time we see another eclipse, a few days later, we're in a slightly different position, maybe closer or farther away from Jupiter than the last time. If we are farther away than the last time we saw an eclipse, then that means we have to wait a little bit of extra time to see the next one because it takes that much longer for the light to reach us, and the reverse is true if we happen to be a little bit closer to Jupiter.

The only way to explain the variations in the timing of eclipses of Io is if light has a finite speed.

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Making it mean something

Continued measurements over the course of the next few centuries solidified the measurement of the speed of light, but it wasn't until the mid-1800s when things really started to come together. That's when the physicist James Clerk Maxwell accidentally invented light.

Maxwell had been playing around with the then-poorly-understood phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Laying the groundwork for what we now understand to be the electromagnetic force , in those equations he discovered that changing electric fields can create magnetic fields, and vice versa. This allows waves of electricity to create waves of magnetism, which go on to make waves of electricity and back and forth and back and forth, leapfrogging over each other, capable of traveling through space.

And when he went to calculate the speed of these so-called electromagnetic waves, Maxwell got the same number that scientists had been measuring as the speed of light for centuries. Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space.

And this was all well and good until Einstein came along a few decades later and realized that the speed of light had nothing to do with light at all. With his special theory of relativity , Einstein realized the true connection between time and space, a unified fabric known as space-time. But as we all know, space is very different than time. A meter or a foot is very different than a second or a year. They appear to be two completely different things.

So how could they possibly be on the same footing?

There needed to be some sort of glue, some connection that allowed us to translate between movement in space and movement in time. In other words, we need to know how much one meter of space, for example, is worth in time. What's the exchange rate? Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa.

Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell and found that this conversion rate is exactly the speed of light.

Of course, this conversion rate, this fundamental constant that unifies space and time, doesn't know what an electromagnetic wave is, and it doesn't even really care. It's just some number, but it turns out that Maxwell had already calculated this number and discovered it without even knowing it. That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of modern physics.

But still, why that number, with that value, and not some other random number? Why did nature pick that one and no other? What's going on?

Related: The genius of Albert Einstein: his life, theories and impact on science

Making it meaningless

Well, the number doesn't really matter. It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is. And so the definition of the speed of light is tied up with the definitions of length and time.

In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers. These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers.

One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space. Its value is approximately 0.007. 0.007 what? Just 0.007. Like I said, it's just a number.

So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is.

So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know.

Learn more by listening to the episode "Why is the speed of light the way it is?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert H, Michael E., @DesRon94, Evan W., Harry A., @twdixon, Hein P., Colin E., and Lothian53 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

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].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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• voidpotentialenergy This is just my opinion but i think L speed is it's speed because the particle part of it is the fastest it can interact with the quanta distance in quantum fluctuation. Light is particle and wave so the wave happens in the void between quanta. Gravity probably travels in that void and why gravity seems instant. Reply
• rod The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*. Reply

Admin said: We all know and love the speed of light, but why does it have the value that it does? Why isn't it some other number? And why did it become such a cornerstone of physics? Why is the speed of light the way it is? : Read more

rod said: The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*.

• rod FYI. When someone says *the universe has chosen*, I am reminded of these five lessons from a 1982 Fed. court trial. The essential characteristics of science are: It is guided by natural law; It has to be explanatory by reference to natural law; It is testable against the empirical world; Its conclusions are tentative, i.e., are not necessarily the final word; and It is falsifiable. Five important points about science. Reply
• Gary If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ? Reply
• Gary Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ? Reply

Gary said: Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ?

Gary said: If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ?

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• Plate Tectonics
• Geological Processes
• Impact Craters
• The Geological Timescale
• Mass Extinctions
• Evolution and the Cosmic Environment
• Earth's Atmosphere and Oceans
• Weather Circulation
• Environmental Change on Earth
• The Earth-Moon System
• Geological History of the Moon
• Tidal Forces
• Effects of Tidal Forces
• Historical Studies of the Moon
• Lunar Surface
• Ice on the Moon
• Origin of the Moon
• Humans on the Moon

Chapter 6 The Terrestrial Planets

• Studying Other Planets
• The Planets
• The Terrestrial Planets
• Mercury's Orbit
• Mercury's Surface
• Volcanism on Venus
• Venus and the Greenhouse Effect
• Tectonics on Venus
• Exploring Venus
• Mars in Myth and Legend
• Early Studies of Mars
• Mars Close-Up
• Modern Views of Mars
• Missions to Mars
• Geology of Mars
• Water on Mars
• Polar Caps of Mars
• Climate Change on Mars
• Terraforming Mars
• Life on Mars
• The Moons of Mars
• Martian Meteorites
• Comparative Planetology
• Incidence of Craters
• Counting Craters
• Counting Statistics
• Internal Heat and Geological Activity
• Magnetic Fields of the Terrestrial Planets
• Mountains and Rifts
• Radar Studies of Planetary Surfaces
• Laser Ranging and Altimetry
• Gravity and Atmospheres
• Normal Atmospheric Composition
• The Significance of Oxygen

Chapter 7 The Giant Planets and Their Moons

• The Gas Giant Planets
• Atmospheres of the Gas Giant Planets
• Clouds and Weather on Gas Giant Planets
• Internal Structure of the Gas Giant Planets
• Thermal Radiation from Gas Giant Planets
• Life on Gas Giant Planets?
• Why Giant Planets are Giant
• Ring Systems of the Giant Planets
• Structure Within Ring Systems
• The Origin of Ring Particles
• The Roche Limit
• Resonance and Harmonics
• Tidal Forces in the Solar System
• Moons of Gas Giant Planets
• Geology of Large Moons
• The Voyager Missions
• Jupiter's Galilean Moons
• Jupiter's Ganymede
• Jupiter's Europa
• Jupiter's Callisto
• Jupiter's Io
• Volcanoes on Io
• Cassini Mission to Saturn
• Saturn's Titan
• Saturn's Enceladus
• Discovery of Uranus and Neptune
• Uranus' Miranda
• Neptune's Triton
• The Discovery of Pluto
• Pluto as a Dwarf Planet
• Dwarf Planets

Chapter 8 Interplanetary Bodies

• Interplanetary Bodies
• Early Observations of Comets
• Structure of the Comet Nucleus
• Comet Chemistry
• Oort Cloud and Kuiper Belt
• Kuiper Belt
• Comet Orbits
• Life Story of Comets
• The Largest Kuiper Belt Objects
• Meteors and Meteor Showers
• Gravitational Perturbations
• Surveys for Earth Crossing Asteroids
• Asteroid Shapes
• Composition of Asteroids
• Introduction to Meteorites
• Origin of Meteorites
• Types of Meteorites
• The Tunguska Event
• The Threat from Space
• Probability and Impacts
• Impact on Jupiter
• Interplanetary Opportunity

Chapter 9 Planet Formation and Exoplanets

• Formation of the Solar System
• Early History of the Solar System
• Conservation of Angular Momentum
• Angular Momentum in a Collapsing Cloud
• Helmholtz Contraction
• Safronov and Planet Formation
• Collapse of the Solar Nebula
• Why the Solar System Collapsed
• From Planetesimals to Planets
• Accretion and Solar System Bodies
• Differentiation
• Planetary Magnetic Fields
• The Origin of Satellites
• Solar System Debris and Formation
• Gradual Evolution and a Few Catastrophies
• Chaos and Determinism
• Extrasolar Planets
• Discoveries of Exoplanets
• Doppler Detection of Exoplanets
• Transit Detection of Exoplanets
• The Kepler Mission
• Direct Detection of Exoplanets
• Properties of Exoplanets
• Implications of Exoplanet Surveys
• Future Detection of Exoplanets

Chapter 10 Detecting Radiation from Space

• Observing the Universe
• Radiation and the Universe
• The Nature of Light
• The Electromagnetic Spectrum
• Properties of Waves
• Waves and Particles
• How Radiation Travels
• Properties of Electromagnetic Radiation
• The Doppler Effect
• Invisible Radiation
• Thermal Spectra
• The Quantum Theory
• The Uncertainty Principle
• Spectral Lines
• Emission Lines and Bands
• Absorption and Emission Spectra
• Kirchoff's Laws
• Astronomical Detection of Radiation
• The Telescope
• Optical Telescopes
• Optical Detectors
• Adaptive Optics
• Image Processing
• Digital Information
• Radio Telescopes
• Telescopes in Space
• Hubble Space Telescope
• Interferometry
• Collecting Area and Resolution
• Frontier Observatories

Chapter 11 Our Sun: The Nearest Star

• The Nearest Star
• Properties of the Sun
• Kelvin and the Sun's Age
• The Sun's Composition
• Energy From Atomic Nuclei
• Mass-Energy Conversion
• Examples of Mass-Energy Conversion
• Energy From Nuclear Fission
• Energy From Nuclear Fusion
• Nuclear Reactions in the Sun
• The Sun's Interior
• Energy Flow in the Sun
• Collisions and Opacity
• Solar Neutrinos
• Solar Oscillations
• The Sun's Atmosphere
• Solar Chromosphere and Corona
• The Solar Cycle
• The Solar Wind
• Effects of the Sun on the Earth
• Cosmic Energy Sources

Chapter 12 Properties of Stars

• Star Properties
• The Distance to Stars
• Apparent Brightness
• Absolute Brightness
• Measuring Star Distances
• Stellar Parallax
• Spectra of Stars
• Spectral Classification
• Temperature and Spectral Class
• Stellar Composition
• Stellar Motion
• Stellar Luminosity
• The Size of Stars
• Stefan-Boltzmann Law
• Stellar Mass
• Hydrostatic Equilibrium
• Stellar Classification
• The Hertzsprung-Russell Diagram
• Volume and Brightness Selected Samples
• Stars of Different Sizes
• Understanding the Main Sequence
• Stellar Structure
• Stellar Evolution

Chapter 13 Star Birth and Death

• Star Birth and Death
• Understanding Star Birth and Death
• Cosmic Abundance of Elements
• Star Formation
• Molecular Clouds
• Young Stars
• T Tauri Stars
• Mass Limits for Stars
• Brown Dwarfs
• Young Star Clusters
• Cauldron of the Elements
• Main Sequence Stars
• Nuclear Reactions in Main Sequence Stars
• Main Sequence Lifetimes
• Evolved Stars
• Cycles of Star Life and Death
• The Creation of Heavy Elements
• Horizontal Branch and Asymptotic Giant Branch Stars
• Variable Stars
• Magnetic Stars
• Stellar Mass Loss
• White Dwarfs
• Seeing the Death of a Star
• Supernova 1987A
• Neutron Stars and Pulsars
• Special Theory of Relativity
• General Theory of Relativity
• Black Holes
• Properties of Black Holes

Chapter 14 The Milky Way

• The Distribution of Stars in Space
• Stellar Companions
• Binary Star Systems
• Binary and Multiple Stars
• Mass Transfer in Binaries
• Binaries and Stellar Mass
• Nova and Supernova
• Exotic Binary Systems
• Gamma Ray Bursts
• How Multiple Stars Form
• Environments of Stars
• The Interstellar Medium
• Effects of Interstellar Material on Starlight
• Structure of the Interstellar Medium
• Dust Extinction and Reddening
• Groups of Stars
• Open Star Clusters
• Globular Star Clusters
• Distances to Groups of Stars
• Ages of Groups of Stars
• Layout of the Milky Way
• William Herschel
• Isotropy and Anisotropy
• Mapping the Milky Way

Chapter 15 Galaxies

• The Milky Way Galaxy
• Mapping the Galaxy Disk
• Spiral Structure in Galaxies
• Mass of the Milky Way
• Dark Matter in the Milky Way
• Galaxy Mass
• The Galactic Center
• Black Hole in the Galactic Center
• Stellar Populations
• Formation of the Milky Way
• The Shapley-Curtis Debate
• Edwin Hubble
• Distances to Galaxies
• Classifying Galaxies
• Spiral Galaxies
• Elliptical Galaxies
• Lenticular Galaxies
• Dwarf and Irregular Galaxies
• Overview of Galaxy Structures
• The Local Group

Light Travel Time

• Galaxy Size and Luminosity
• Mass to Light Ratios
• Dark Matter in Galaxies
• Gravity of Many Bodies
• Galaxy Evolution
• Galaxy Interactions
• Galaxy Formation

Chapter 16 The Expanding Universe

• Galaxy Redshifts
• The Expanding Universe
• Cosmological Redshifts
• The Hubble Relation
• Relating Redshift and Distance
• Galaxy Distance Indicators
• Size and Age of the Universe
• The Hubble Constant
• Large Scale Structure
• Galaxy Clustering
• Clusters of Galaxies
• Overview of Large Scale Structure
• Dark Matter on the Largest Scales
• The Most Distant Galaxies
• Black Holes in Nearby Galaxies
• Active Galaxies
• Radio Galaxies
• The Discovery of Quasars
• Types of Gravitational Lensing
• Properties of Quasars
• The Quasar Power Source
• Quasars as Probes of the Universe
• Star Formation History of the Universe
• Expansion History of the Universe

Chapter 17 Cosmology

• Early Cosmologies
• Relativity and Cosmology
• The Big Bang Model
• The Cosmological Principle
• Universal Expansion
• Cosmic Nucleosynthesis
• Cosmic Microwave Background Radiation
• Discovery of the Microwave Background Radiation
• Measuring Space Curvature
• Cosmic Evolution
• Evolution of Structure
• Mean Cosmic Density
• Critical Density
• Dark Matter and Dark Energy
• Age of the Universe
• Precision Cosmology
• The Future of the Contents of the Universe
• Fate of the Universe
• Alternatives to the Big Bang Model
• Particles and Radiation
• The Very Early Universe
• Mass and Energy in the Early Universe
• Matter and Antimatter
• The Forces of Nature
• Fine-Tuning in Cosmology
• The Anthropic Principle in Cosmology
• String Theory and Cosmology
• The Multiverse
• The Limits of Knowledge

Chapter 18 Life On Earth

• Nature of Life
• Chemistry of Life
• Molecules of Life
• The Origin of Life on Earth
• Origin of Complex Molecules
• Miller-Urey Experiment
• Pre-RNA World
• From Molecules to Cells
• Extremophiles
• Thermophiles
• Psychrophiles
• Acidophiles
• Alkaliphiles
• Radiation Resistant Biology
• Importance of Water for Life
• Hydrothermal Systems
• Silicon Versus Carbon
• DNA and Heredity
• Life as Digital Information
• Synthetic Biology
• Life in a Computer
• Natural Selection
• Tree Of Life
• Evolution and Intelligence
• Culture and Technology
• The Gaia Hypothesis
• Life and the Cosmic Environment

Chapter 19 Life in the Universe

• Life in the Universe
• Astrobiology
• Life Beyond Earth
• Sites for Life
• Complex Molecules in Space
• Life in the Solar System
• Lowell and Canals on Mars
• Implications of Life on Mars
• Extreme Environments in the Solar System
• Rare Earth Hypothesis
• Are We Alone?
• Unidentified Flying Objects or UFOs
• The Search for Extraterrestrial Intelligence
• The Drake Equation
• The History of SETI
• Recent SETI Projects
• Recognizing a Message
• The Best Way to Communicate
• The Fermi Question
• The Anthropic Principle
• Where Are They?

In the everyday world, as perceived by the human senses, light seems to travel instantaneously from one place to another. In fact, the speed of light is not infinite, and light doesn't instantly jump from your ceiling light to your desk and then to your eye. We perceive light as moving instantly because its actual velocity is almost unimaginably high; light travels at 300,000 km/s, denoted c. Using the equation Rate × Time = Distance, you can divide any distance by this number to figure out the time it would take light to cross that distance. In this way, we can see that light takes 1.5 × 10 8 / 3 × 10 5 = 500 seconds to reach Earth from the Sun, or just over 8 minutes. It takes light about 40 times longer ( Pluto at a distance of 39.4 A.U.) to leave the Solar System or about 5 hours.

The speed of light is a built-in quality of our universe . All evidence to date indicates that light has always traveled at this speed, that the speed is exact, and that the same speed is observed for all observers. The vast size of the universe, coupled with the finite (albeit large) speed of light, means that as we look out in space, we look back in time. Distant light is old light.

The 5 hours it takes light to travel across our Solar System may seem like a short period to cross such a large distance, but we have to think about scale. While distances within the Solar System are large to us, they are dwarfed by the distances between the stars. Considering larger regions of the Milky Way, a natural distance unit is the distance light travels in one year. This is called a light year. We can easily calculate the size of this unit by remembering that distance has the units of velocity times time. So:

D ly = vt = c x 1 year = 3 × 10 5 x (3600 × 24 × 365) = 9.5 × 10 12 km

A light year is the typical distance between stars in the neighborhood of the Sun. It is nearly 10 trillion kilometers or 6 trillion miles! The fundamental unit of distance defined by geometry is the 13 km; defined as the distance corresponding to a parallax of 1 second of arc.">parsec , equal to 3.1 × 10 13 km. This is described in more detail in the article on parallax . Geometrically, one parsec is the height of a right triangle with an angle of 1 arcsec describing its apex , and a distance of 1 AU describing its base. The units are related by a small numerical constant D ly = 3.26 D pc . So to roughly convert from parsecs to light years, multiply by 3.3.

The following list gives the distance to various points within the Milky Way and beyond, both in terms of parsecs and the light travel time in years (which is also the distance in light years or 3.3 times the distance in parsecs). To fully appreciate how isolated we are in space, remember that light is the fastest thing we know of. The fastest spacecraft can not reach 1% of the speed of light. So you would have to multiply the numbers on the right-hand side of the table by at least 100 to estimate how long it would take to send a probe through the Milky Way and into the Local Group with current technology.

• Nearest star (α Centauri) - 1.3 pc, 4.2 years • Sirius - 2.7 pc, 8.8 years • Vega - 8.1 pc, 26 years • Hyades cluster - 42 pc, 134 years • Pleiades cluster - 125 pc, 411 years • Orion nebula - 460 pc, 1500 years • Nearest spiral arm - 1200 pc, 3900 years • Center of the 8 to 10 13 solar masses.">galaxy - 8500 pc, 29,000 years • Far edge of the galaxy - 24,000 pc, 78,000 years • Large Magellanic Cloud - 50,000 pc, 163,000 years • Andromeda galaxy (M31) - 670,000 pc, 2.2 million years

What does Andromeda look like now? Nobody knows. Since nothing travels faster than light (and this applies to all the colors of light across the electromagnetic spectrum ), there is no quicker way to send information from one place to another. We are stuck with collecting and measuring "old" light. While this seems like a limitation, scientists actually find that it turns out that light travel time is a wonderful tool. By looking further out in space we look further back in time. In this way, astronomers get to explore the earlier stages of the universe seeing firsthand (with a delay) what the early universe looked like.

Universe Today

Space and astronomy news

How Does Light Travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century’s BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern era, the 20th century led to breakthroughs that showed us that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein’s Theory of Relativity . However, there remains many unanswered questions about light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light to the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle’s theory of light, which viewed it as being a disturbance in the air (one of his four “elements” that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or “corpuscles”). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton .

Newton’s corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise “ Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light “. According to Newton, the principles of light could be summed as follows:

• Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
• These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to “wave theory”, which had been advocated by 17th century Dutch astronomer Christiaan Huygens . . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his “ Traité de la lumière “ (“ Treatise on Light “). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment , which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young’s version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation, would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter’s moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century, James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell’s equations ) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c ).

In 1905, Albert Einstein published “ On the Electrodynamics of Moving Bodies ”, in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity , which reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect , Einstein based his idea on Planck’s earlier work with “black bodies” – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein’s photoelectric effect was attempt to explain the “black body problem”, in which a black body emits electromagnetic radiation due to the object’s heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes ).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein’s explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named “photons”. For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg’s “uncertainty principle” (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger’s paradox that claimed that all particles have a “wave function”.

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function , a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly “collapse”, or rather “decohere”, to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the “ Schrödinger Cat” paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation ). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly divert it, or arrest it, is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light? , How Far is a Light Year? , What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel .

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56 Replies to “How Does Light Travel?”

“HOW DOES LIGHT TRAVEL?”

it travels lightly. 😀

Light doesn’t exist. This is an observation from light’s point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it’s source and reaching it’s destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy…..forever…….

I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

Join the discussion

Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we’re living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

Brad,…So You’re suggesting that “life” as we know and call it “is some kind of retro-virus” or “bio-intelligent format” heaped upon a perceived “set of accepted data sets” that are not in sync with each other in most cases with exception to Math 94% of the time….Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,…right?……

Brad has watched the Matrix too many times.

Correction: Even gravity doesn’t slow light down. Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame. It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer. You might even say, “One man’s radio wave is another man’s gamma ray.”

Light actually “slows down” every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it’s fastest path through any medium, and it slows down in that medium.

if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

What are you smoking?

The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not “propose” the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein’s paper on the photoelectric effect was published in his “miracle” year of 1905. The photoelectric effect has nothing to do with black body radiation.

Einstein did not coin the name “photons” for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

I have to say that I do not know what the author of the article means when he says ” calculating the wavelength at which light functioned” in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

I gave up on the article after seeing these errors. I’m afraid I have a low tolerance for sloppy writing.

Oh, it’s BCE now, “Before the Common Era” BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

mecheng1, you must be very young. BCE has been in used in academia for decades. It’s nothing “new”, just out of your circle of knowledge.

Decades??? Really?? How does that compare to 2000 years?

Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a “0” year or a “year 1” from which to reckon the passage of time, largely based on their own religions or celestial observations.

Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a “common era” insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted “year 1” established by that calendar (which is supposed to be the year of Jesus’s birth, but it probably isn’t according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar – even the names of the months come from the Romans.

In short, it is more accurate, as well as respectful, to go with BCE in these global times.

Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

That’s the mystery, isn’t it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80’s by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these “neutrino” sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

i would like to know the temperature in a black hole…maybe absolute zero? is absolute zero the moment that time stop?

I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein’s equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn’t be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

Light doesn’t travel, it just IS. It is we, the condensed matter, that travels, through time.

Oh really? Is this just your imagination/illusion or you have published a paper on it?

So you don’t believe you travel through time?

I wish I understood just a portion of I just read, love sicence so bad BUT, sighs

It would be easier to understand if it wasn’t pure gibberish written by someone with no science background.

I have two “mind-bending relativity side effects” to share. At least they are mind-bending to me.

1) Light travels the same speed relative to all particles of mass, regardless of how those particles move relative to each other:

I can conceptualize this if we are only talking about two mass-particles/observers and the examples I’ve seen always involve only two observers. But if you have many mass-particles/observers, how does the space-time seem to know to adjust differently for all of them. I am sure i am understanding this correctly as it is a basic concept of special relativity and nobody seems to bring this issue up. But it “bends my mind” when i try to include more than two observers. Maybe you can help.

2) General Relativity’s (“GR”) prediction that the big bang started with “Infinite” energy and now the universe appears to have finite mass energy and Regarding the first effect: How can something infinite turn into something finite? Is the answer that at that early in the universe, quantum takes over and GR’s prediction of infinite mass-energy at the start of the universe is just wrong?

I need to correct a typo in my previous comment. Where i say “i am sure am understanding this correctly” I meant to include the word NOT. so it should read “i am sure am NOT understanding this correctly” Mark L.

Mark,….I think you’re understanding it just fine from the standpoint of multiple observers, The point might be that in space, the density of “emptiness” or “lack of emptiness” might be impacted from one area of observation to another by an observer who’s perceptions are not equal but not being taken into consideration by each observer. ( an example if I may?) If you were to use a Clear medium which is oil based beginning with 5 gallons of mineral spirits in a large barrel and keep adding 5 gallons of thicker clear oil and then heavy grease and stop with using a clear heavy wax,…what happens is you end up with a barrel of clear fluid that begins with a floating substrate but the liquid begins to keep floating and the heaviest stuff goes to the bottom,…You end up with a sort of solid tube of clear fluids which if you could keep them in shape here on the earth, “you could observe them” from several positions, #1. the fluid end #2, the less fluid part, #3, the semi solid part #4. the seemingly solid part #5. the almost solid part & #6. the solid part……all of which would be transparent….You could then shine a laser through all of it and perhaps do that again from different places and see what happens at different angles…..I think what happens as a result would be, an observer would end up be influenced as per his or her ideas thusly because of the quasi-nature of what the density of space is at the point of space is where the observation is made. just a guess.

All Special Relativity really says about light is that it appears to move at the same rate from any observer POV. There are other more advanced rules relating to light speeds. One of them is the implication of infinite energy in a photon because of the fact it’s mass-less, therefore it can move at the maximum rate a mass-less particle or wave can (not necessarily that it does) Later when the electron was discovered (also mass-less particle or wave), it was also found to conform to the rules of special relativity.

As far as the big bang, there are a lot of cracks in that theory, and many different ones are beginning to dispute some of the common ideas behind the “Big Bang” as well as “Inflationary Cosmology”. Honestly though, both standard and quantum physics applied, and yet both went out the window at the same time at some point. That’s what all the theories really say. At some point, everything we know or think we know was bunk, because the math just breaks down, and doesn’t work right anymore.

i think until there is an understanding of the actual “fabric” of space itself, the wave vs particle confusion will continue. another interesting article recently was the half integer values of rotating light. planck’s constant was broken? gravity? a bump in the data? lol these are interesting times.

There’s no fabric.

Tesla insists there is an aether, Einstein says not. Tesla enjoyed far less trial and error than Einstein. The vast majority of Tesla’s projects worked the first time around and required no development or experimentation. I’ll go with Tesla; there is an aether as a fabric of space.

http://weinsteinsletter.weebly.com/aether.html

Maybe Special Relativity is not correct? 🙂

Feynman said unequivocally that QED is NOT a wave theory. In fact, the math only looks like Maxwell’s wave function when you are looking at a single particle at a time, but the analogy breaks down as soon as you start looking at the interactions of more than one, which is the real case. There’s no light acting alone, but always an interaction between a photon and some other particle, an electron, another photon, or whatever. He said “light is particles.” So the question re: how can light travel through a vacuum if it’s waves is a nonsensical question. There are no collapsing wave functions in light. There’s only probabilities of position that look like waves on a freaking piece of paper. Even calling light properties as “wavelengths” is nonsensical. Light comes in frequencies, i.e., the number of particles traveling tightly together. Higher frequency is more energy because it’s more particles (E=MC[squared]). “Wavicles” is pure bullshit.

I don’t agree with the John Wheeler theory that there is only one electron since the computer I am using was built by ion implantation and uses a very large number of them simultaneously to function.

Black holes don’t stop or slow light, if they even exist. A black hole could phase shift light, which is why we see things emitting xrays and call them black holes….but they could be something else too.

Photons have no mass but they do have energy. Energy and mass are transformable into each other. Gravity works on energy as well as mass. As massive particles approach the speed of light their measurable mass increases to infinity. But since energy is equivalent to mass, why doesn’t the photon, which has energy, not seem to have infinite mass?

NO other wave travels thru a vacuum? what about radio?

Radio waves are a specific frequency range of light.

Technically speaking, radio waves are emitted at various frequencies that share the same space time as light. They are not however light. They’re modulated electrons. Modulated photons certainly can be used to carry a vast amount of information a great distance. It cannot do it any faster or better than a radio wave though. Both electrons and photons are mass-less, therefore they both conform to the rules of Special Relativity in the same way. Both travel at the speed of light.

I just don’t understand is it a particle of a wave? It seems like it behaves like wave and sometimes like particle and in some situations is like a what ever you are going to call it.

So, the logical idea would to have formula Photon_influence * weight_for_particle + Wave_influence * weight_for_wave

Make it more compact.

This article is good but the title is bad as by the end we still weren’t told how light travels through space. Also, there are some historical mistakes as already pointed out. Now for my contribution: I think that light and Gravity have a lot in common; for one – an atom’s electrons transmit light and an atom contains the tiny heavy place that knows everything there is to know about gravity, that is, the nucleus. Light and Gravity are both related to the same entity, the atom. Unfortunately, we, still cannot grasp how what’s heavy brings about gravitation. For those of you with a creed for new ideas go to: https://www.academia.edu/10785615/Gravity_is_emergent It’s a hypothesis…

Gravity and light are infinite, like space and time… Mind the concept that there are waves within waves, motions within motion, vibrations within vibration, endless overtones and universal harmony…

From this article, I have “And in the end, the only thing that can truly slow down or arrest the speed of light is gravity”

Doesn’t light slow down in water and glass and other mediums. I was only a Physics minor, but I do remember coivering this though way back in the early 80’s. And in my quick checking online, I found the following.

“Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index = 1.3; see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5).”

Were they saying something else here. I did like the article.

Photons are not massless, but their mass is incredibly small even compared to a proton or neutron. So, by Einstein’s E=MC^2, the energy required for a photon to move is greatly reduced, but photons do have mass and are affected by gravity. If photons had no mass at all, then gravity would have no affect on them, but gravity does. Gravity bends light and can change it’s course through space. We see that in the actual test first performed to prove Einstein’s theory buy observing the distorted placement of stars as their light passes near the sun observed during an eclipse. We can also see it through gravitational lensing when viewing deeps space objects. And the fact that there are black holes that are black because light cannot escape it’s gravity. So photons do have mass, be it miniscule, and with that their propagation with light waves through space will eventually run out of energy and stop. but this would probably require distances greater to several widths of our universe to accomplish. Light from the furthest reaches of the universe are not as bright, or as energetic, as they are at anyplace between here and their origins. That reduction in their energy is also attributed to Einstein’s equation and the inverse square law, where the intensity of light is in relation to the inverse square of the distance. That proves that light looses energy the further it travels, but it still moves at the speed of light. As light looses energy, it doesn’t slow the light wave.

It has been proven that more energetic light does in fact travel slightly faster. You can find the experiments done with light that has traveled billions of light years, the more energetic is in fact faster over a number of seconds, around 10 -15 or so. As people encounter this information, they see that many accepted theories can now be debunked.

The point of the article is nothing new; light acts like a particle AND a beam. So when you sit behind a closed door and someone shines a light on the door, the light will engulf the door and wave through and around the edges, the particle does not just bounce straight back. You can focus a beam of light on an object, but it will sneak though the corners and underneath the door, through any opening,. And yes, light travels forever. It is a constant, that cannot be sped up. We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe: 13,8B light years away *the time that it takes for light to travel in one year, is one light year. So, it has taken 13,8B light years for the light of other galaxies to get here, so those galaxies could be gone by now, since it took so long to reach us, We are truly looking back in time as we see the light emitted from those galaxies and stars.

It propagates through the quantum mish-mash know as the aether . . .

If light is a particle and particles have mass why does not the mas increase with it speed?

Wow…there are errors in the article, yes…the enthusiasm demonstrated by all the comments is encouraging…but when I read these comments, I am a bit dismayed at the lack of understanding that is evident in most of them…confusing energy and intensity and wavelength…confusing rest mass and inertial mass…not to mention some off-the-wall hypotheses with no experimental evidence to support them. There are some great primers out there…books, documentaries, podcasts (like Astronomy Cast). Good luck!

Precisely correct. Sci-fi rules basic physics, which reflects on the poor education system. Pity.

First time I heard about A. A. and his theory about light I really didn’t like him. Why? Because light was the the fastest thing in the universe and there is no other thing faster than the light. Later, when I have red about angular speed I have asked my self if you have linear and angular speed and both of them are speeds how that will result in the maximum speed. Since then, I have not had a chance to get right answer.

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Speed of Light Calculator

Table of contents

With this speed of light calculator, we aim to help you calculate the distance light can travel in a fixed time . As the speed of light is the fastest speed in the universe, it would be fascinating to know just how far it can travel in a short amount of time.

We have written this article to help you understand what the speed of light is , how fast the speed of light is , and how to calculate the speed of light . We will also demonstrate some examples to help you understand the computation of the speed of light.

What is the speed of light? How fast is the speed of light?

The speed of light is scientifically proven to be the universe's maximum speed. This means no matter how hard you try, you can never exceed this speed in this universe. Hence, there are also some theories on getting into another universe by breaking this limit. You can understand this more using our speed calculator and distance calculator .

So, how fast is the speed of light? The speed of light is 299,792,458 m/s in a vacuum. The speed of light in mph is 670,616,629 mph . With this speed, one can go around the globe more than 400,000 times in a minute!

One thing to note is that the speed of light slows down when it goes through different mediums. Light travels faster in air than in water, for instance. This phenomenon causes the refraction of light.

Now, let's look at how to calculate the speed of light.

How to calculate the speed of light?

As the speed of light is constant, calculating the speed of light usually falls on calculating the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

• Source: Light
• Speed of light: 299,792,458 m/s
• Time traveled: 100 seconds

You can perform the calculation in three steps:

Determine the speed of light.

As mentioned, the speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s .

Determine the time that the light has traveled.

The next step is to know how much time the light has traveled. Unlike looking at the speed of a sports car or a train, the speed of light is extremely fast, so the time interval that we look at is usually measured in seconds instead of minutes and hours. You can use our time lapse calculator to help you with this calculation.

For this example, the time that the light has traveled is 100 seconds .

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m

What is the speed of light in mph when it is in a vacuum?

The speed of light in a vacuum is 670,616,629 mph . This is equivalent to 299,792,458 m/s or 1,079,252,849 km/h. This is the fastest speed in the universe.

Is the speed of light always constant?

Yes , the speed of light is always constant for a given medium. The speed of light changes when going through different mediums. For example, light travels slower in water than in air.

How can I calculate the speed of light?

You can calculate the speed of light in three steps:

Determine the distance the light has traveled.

Apply the speed of light formula :

speed of light = distance / time

How far can the speed of light travel in 1 minute?

Light can travel 17,987,547,480 m in 1 minute . This means that light can travel around the earth more than 448 times in a minute.

Speed of light

The speed of light in the medium. In a vacuum, the speed of light is 299,792,458 m/s.