speed travel in vacuum in mph

What is the speed of light?

The speed of light is the speed limit of the universe. Or is it?

What is a light-year?

• Speed of light FAQs
• Special relativity
• Faster than light
• Slowing down light
• Faster-than-light travel

Bibliography

The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed. 

According to physicist Albert Einstein 's theory of special relativity , on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter's mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe . The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology , it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin .

But despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend time contemplating faster-than-light travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.

Related: Special relativity holds up to a high-energy test

A l ight-year is the distance that light can travel in one year — about 6 trillion miles (10 trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe.

Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight takes about 8 minutes to reach our eyes, so the sun is about 8 light minutes away. Light from Alpha Centauri , which is the nearest star system to our own, requires roughly 4.3 years to get here, so Alpha Centauri is 4.3 light-years away.

"To obtain an idea of the size of a light-year, take the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end," NASA's Glenn Research Center says on its website . "The resulting distance is almost 6 trillion (6,000,000,000,000) miles!"

Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. And everything astronomers "see" in the distant universe is literally history. When astronomers study objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them. 

This principle allows astronomers to see the universe as it looked after the Big Bang , which took place about 13.8 billion years ago. Objects that are 10 billion light-years away from us appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.

Related: Why the universe is all history

Speed of light FAQs answered by an expert

We asked Rob Zellem, exoplanet-hunter and staff scientist at NASA's Jet Propulsion Lab, a few frequently asked questions about the speed of light. 

Dr. Rob Zellem is a staff scientist at NASA's Jet Propulsion Laboratory, a federally funded research and development center operated by the California Institute of Technology. Rob is the project lead for Exoplanet Watch, a citizen science project to observe exoplanets, planets outside of our own solar system, with small telescopes. He is also the Science Calibration lead for the Nancy Grace Roman Space Telescope's Coronagraph Instrument, which will directly image exoplanets. 

What is faster than the speed of light?

Nothing! Light is a "universal speed limit" and, according to Einstein's theory of relativity, is the fastest speed in the universe: 300,000 kilometers per second (186,000 miles per second). 

Is the speed of light constant?

The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light *can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second). 

Who discovered the speed of light?

One of the first measurements of the speed of light was by Rømer in 1676 by observing the moons of Jupiter . The speed of light was first measured to high precision in 1879 by the Michelson-Morley Experiment. 

How do we know the speed of light?

Rømer was able to measure the speed of light by observing eclipses of Jupiter's moon Io. When Jupiter was closer to Earth, Rømer noted that eclipses of Io occurred slightly earlier than when Jupiter was farther away. Rømer attributed this effect due the time it takes for light to travel over the longer distance when Jupiter was farther from the Earth. 

How did we learn the speed of light?

As early as the 5th century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of light speed. Empedocles proposed that light, whatever it was made of, must travel and therefore, must have a rate of travel. Aristotle wrote a rebuttal of Empedocles' view in his own treatise, On Sense and the Sensible , arguing that light, unlike sound and smell, must be instantaneous. Aristotle was wrong, of course, but it would take hundreds of years for anyone to prove it. 

In the mid 1600s, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart. Each person held a shielded lantern. One uncovered his lantern; when the other person saw the flash, he uncovered his too. But Galileo's experimental distance wasn't far enough for his participants to record the speed of light. He could only conclude that light traveled at least 10 times faster than sound.

In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA , accidentally came up with a new best estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter's moon , Io, from Earth . Over time, Rømer observed that Io's eclipses often differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from one another, showed up ahead of time when the planets were approaching and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know as the Doppler effect, the change in frequency of light or sound emitted by a moving object that in the astronomical world manifests as the so-called redshift , the shift towards "redder", longer wavelengths in objects speeding away from us. In a leap of intuition, Rømer determined that light was taking measurable time to travel from Io to Earth. 

Rømer used his observations to estimate the speed of light. Since the size of the solar system and Earth's orbit wasn't yet accurately known, argued a 1998 paper in the American Journal of Physics , he was a bit off. But at last, scientists had a number to work with. Rømer's calculation put the speed of light at about 124,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley based a new set of calculations on the change in the apparent position of stars caused by Earth's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — accurate to within about 1% of the real value, according to the American Physical Society .

Two new attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly rotating toothed wheel, with a mirror set up 5 miles (8 km) away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel to perform essentially the same experiment. The two independent methods each came within about 1,000 miles per second (1,609 km/s) of the speed of light.

Another scientist who tackled the speed of light mystery was Poland-born Albert A. Michelson, who grew up in California during the state's gold rush period, and honed his interest in physics while attending the U.S. Naval Academy, according to the University of Virginia . In 1879, he attempted to replicate Foucault's method of determining the speed of light, but Michelson increased the distance between mirrors and used extremely high-quality mirrors and lenses. Michelson's result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, until Michelson re-measured it himself. In his second round of experiments, Michelson flashed lights between two mountain tops with carefully measured distances to get a more precise estimate. And in his third attempt just before his death in 1931, according to the Smithsonian's Air and Space magazine, he built a mile-long depressurized tube of corrugated steel pipe. The pipe simulated a near-vacuum that would remove any effect of air on light speed for an even finer measurement, which in the end was just slightly lower than the accepted value of the speed of light today. 

Michelson also studied the nature of light itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang . The best minds in physics at the time of Michelson's experiments were divided: Was light a wave or a particle? 

Michelson, along with his colleague Edward Morley, worked under the assumption that light moved as a wave, just like sound. And just as sound needs particles to move, Michelson and Morley and other physicists of the time reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (also known as "ether"). 

Though Michelson and Morley built a sophisticated interferometer (a very basic version of the instrument used today in LIGO facilities), Michelson could not find evidence of any kind of luminiferous aether whatsoever. Light, he determined, can and does travel through a vacuum.

"The experiment — and Michelson's body of work — was so revolutionary that he became the only person in history to have won a Nobel Prize for a very precise non-discovery of anything," Siegal wrote. "The experiment itself may have been a complete failure, but what we learned from it was a greater boon to humanity and our understanding of the universe than any success would have been!"

Special relativity and the speed of light

Einstein's theory of special relativity unified energy, matter and the speed of light in a famous equation: E = mc^2. The equation describes the relationship between mass and energy — small amounts of mass (m) contain, or are made up of, an inherently enormous amount of energy (E). (That's what makes nuclear bombs so powerful: They're converting mass into blasts of energy.) Because energy is equal to mass times the speed of light squared, the speed of light serves as a conversion factor, explaining exactly how much energy must be within matter. And because the speed of light is such a huge number, even small amounts of mass must equate to vast quantities of energy.

In order to accurately describe the universe, Einstein's elegant equation requires the speed of light to be an immutable constant. Einstein asserted that light moved through a vacuum, not any kind of luminiferous aether, and in such a way that it moved at the same speed no matter the speed of the observer. 

Think of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative movement to themselves as zero. But observers moving nearly the speed of light would still perceive light as moving away from them at more than 670 million mph. (That's because moving really, really fast is one of the only confirmed methods of time travel — time actually slows down for those observers, who will age slower and perceive fewer moments than an observer moving slowly.)

In other words, Einstein proposed that the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 

Therefore, objects with mass cannot ever reach the speed of light. If an object ever did reach the speed of light, its mass would become infinite. And as a result, the energy required to move the object would also become infinite: an impossibility.

That means if we base our understanding of physics on special relativity (which most modern physicists do), the speed of light is the immutable speed limit of our universe — the fastest that anything can travel. 

What goes faster than the speed of light?

Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. The universe expands at a little more than 42 miles (68 kilometers) per second for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous article for Space.com . (A megaparsec is 3.26 million light-years — a really long way.) 

In other words, a galaxy 1 megaparsec away appears to be traveling away from the Milky Way at a speed of 42 miles per second (68 km/s), while a galaxy two megaparsecs away recedes at nearly 86 miles per second (136 km/s), and so on. 

"At some point, at some obscene distance, the speed tips over the scales and exceeds the speed of light, all from the natural, regular expansion of space," Sutter explained. "It seems like it should be illegal, doesn't it?"

Special relativity provides an absolute speed limit within the universe, according to Sutter, but Einstein's 1915 theory regarding general relativity allows different behavior when the physics you're examining are no longer "local."

"A galaxy on the far side of the universe? That's the domain of general relativity, and general relativity says: Who cares! That galaxy can have any speed it wants, as long as it stays way far away, and not up next to your face," Sutter wrote. "Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."

Does light ever slow down?

Light in a vacuum is generally held to travel at an absolute speed, but light traveling through any material can be slowed down. The amount that a material slows down light is called its refractive index. Light bends when coming into contact with particles, which results in a decrease in speed.

For example, light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, slowing down by just three ten-thousandths of the speed of light. But light passing through a diamond slows to less than half its typical speed, PBS NOVA reported. Even so, it travels through the gem at over 277 million mph (almost 124,000 km/s) — enough to make a difference, but still incredibly fast.

Light can be trapped — and even stopped — inside ultra-cold clouds of atoms, according to a 2001 study published in the journal Nature . More recently, a 2018 study published in the journal Physical Review Letters proposed a new way to stop light in its tracks at "exceptional points," or places where two separate light emissions intersect and merge into one.

Researchers have also tried to slow down light even when it's traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, even as it moved through a vacuum, as described in their 2015 study published in the journal Science . In their measurements, the difference between the slowed photon and a "regular" photon was just a few millionths of a meter, but it demonstrated that light in a vacuum can be slower than the official speed of light. 

Can we travel faster than light?

— Spaceship could fly faster than light

— Here's what the speed of light looks like in slow motion

— Why is the speed of light the way it is?

Science fiction loves the idea of "warp speed." Faster-than-light travel makes countless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop back and forth between star systems with ease. 

But while faster-than-light travel isn't guaranteed impossible, we'd need to harness some pretty exotic physics to make it work. Luckily for sci-fi enthusiasts and theoretical physicists alike, there are lots of avenues to explore.

All we have to do is figure out how to not move ourselves — since special relativity would ensure we'd be long destroyed before we reached high enough speed — but instead, move the space around us. Easy, right? 

One proposed idea involves a spaceship that could fold a space-time bubble around itself. Sounds great, both in theory and in fiction.

"If Captain Kirk were constrained to move at the speed of our fastest rockets, it would take him a hundred thousand years just to get to the next star system," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in a 2010 interview with Space.com's sister site LiveScience . "So science fiction has long postulated a way to beat the speed of light barrier so the story can move a little more quickly."

Without faster-than-light travel, any "Star Trek" (or "Star War," for that matter) would be impossible. If humanity is ever to reach the farthest — and constantly expanding — corners of our universe, it will be up to future physicists to boldly go where no one has gone before.

Additional resources

For more on the speed of light, check out this fun tool from Academo that lets you visualize how fast light can travel from any place on Earth to any other. If you’re more interested in other important numbers, get familiar with the universal constants that define standard systems of measurement around the world with the National Institute of Standards and Technology . And if you’d like more on the history of the speed of light, check out the book " Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light " (Oxford, 2019) by John C. H. Spence.

Aristotle. “On Sense and the Sensible.” The Internet Classics Archive, 350AD. http://classics.mit.edu/Aristotle/sense.2.2.html .

D’Alto, Nick. “The Pipeline That Measured the Speed of Light.” Smithsonian Magazine, January 2017. https://www.smithsonianmag.com/air-space-magazine/18_fm2017-oo-180961669/ .

Fowler, Michael. “Speed of Light.” Modern Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson .

Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, and Miles J. Padgett. “Spatially Structured Photons That Travel in Free Space Slower than the Speed of Light.” Science, February 20, 2015. https://www.science.org/doi/abs/10.1126/science.aaa3035 .

Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. “Light Stops at Exceptional Points.” Physical Review Letters 120, no. 1 (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901 . 

Hazen, Robert. “What Makes Diamond Sparkle?” PBS NOVA, January 31, 2000. https://www.pbs.org/wgbh/nova/article/diamond-science/ . 

“How Long Is a Light-Year?” Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm . 

American Physical Society News. “July 1849: Fizeau Publishes Results of Speed of Light Experiment,” July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm . 

Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses.” Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017 . 

NIST. “Meet the Constants.” October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants . 

Ouellette, Jennifer. “A Brief History of the Speed of Light.” PBS NOVA, February 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/ . 

Shea, James H. “Ole Ro/Mer, the Speed of Light, the Apparent Period of Io, the Doppler Effect, and the Dynamics of Earth and Jupiter.” American Journal of Physics 66, no. 7 (July 1, 1998): 561–69. https://doi.org/10.1119/1.19020 . 

Siegel, Ethan. “The Failed Experiment That Changed The World.” Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/ . 

Stern, David. “Rømer and the Speed of Light,” October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm . 

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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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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What Is the Speed of Light?

The speed of light is the rate at which light travels. The speed of light in a vacuum is a constant value that is denoted by the letter c and is defined as exactly 299,792,458 meters per second. Visible light , other electromagnetic radiation, gravity waves, and other massless particles travel at c. Matter , which has mass, can approach the speed of light, but never reach it.

Value for the Speed of Light in Different Units

Here are values for the speed of light in various units:

• 299,792,458 meters per second ( exact number )
• 299,792 kilometers per second (rounded)
• 3×10 8 m/s (rounded)
• 186,000 miles per second (rounded)
• 671,000,000 miles per hour (rounded)
• 1,080,000,000 kilometers per hour (rounded)

Is the Speed of Light Really Constant?

The speed of light in a vacuum is a constant. However, scientists are exploring whether the speed of light has changed over time.

Also, the rate at which light travels changes as it passes through a medium. The index of refraction describes this change. For example, the index of refraction of water is 1.333, which means light travels 1.333 times slower in water than in a vacuum. The index of refraction of a diamond is 2.417. A diamond slows the speed of light by more than half its speed in a vacuum.

How to Measure the Speed of Light

One way of measuring the speed of light uses great distances, such as distant points on the Earth or known distances between the Earth and astronomical objects. For example, you can measure the speed of light by measuring the time it takes for light to travel from a light source to a distant mirror and back again. The other way of measuring the speed of light is solving for c in equations. Now that the speed of light is defined, it is fixed rather than measured. Measuring the speed of light today indirectly measures the length of the meter, rather than c .

In 1676, Danish astronomer Ole Rømer discovered light travels at a speed by studying the movement of Jupiter’s moon Io. Prior to this, it seemed light propagated instantaneously. For example, you see a lightning strike immediately, but don’t hear thunder until after the event . So, Rømer’s finding showed light takes time to travel, but scientists did not know the speed of light or whether it was constant. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave that travelled at a speed c . Albert Einstein suggested c was a constant and that it did not change according to the frame of reference of the observer or any motion of a light source. In other words, Einstein suggested the speed of light is invariant . Since then, numerous experiments have verified the invariance of c .

Is It Possible to Go Faster Than Light?

The upper speed limit for massless particles is c . Objects that have mass cannot travel at the speed of light or exceed it. Among other reasons, traveling at c gives an object a length of zero and infinite mass. Accelerating a mass to the speed of light requires infinite energy. Furthermore, energy, signals, and individual photos cannot travel faster than c . At first glance, quantum entanglement appears to transmit information faster than c . When two particles are entangled, changing the state of one particle instantaneously determines the state of the other particle, regardless of the distance between them. But, information cannot be transmitted instantaneously (faster than c ) because it isn’t possible to control the initial quantum state of the particle when it is observed.

However, faster-than-light speeds appear in physics. For example, the phase velocity of x-rays through glass often exceeds c. However, the information isn’t conveyed by the waves faster than the speed of light. Distant galaxies appear to move away from Earth faster than the speed of light (outside a distance called the Hubble sphere), but the motion isn’t due to the galaxies traveling through space. Instead, space itself it expanding. So again, no actual movement faster than c occurs.

While it isn’t possible to go faster than the speed of light, it doesn’t necessarily mean warp drive or other faster-than-light travel is impossible. The key to going faster than the speed of light is to change space-time. Ways this might happen include tunneling using wormholes or stretching space-time into a “warp bubble” around a spacecraft. But, so far these theories don’t have practical applications.

• Brillouin, L. (1960). Wave Propagation and Group Velocity. Academic Press.
• Ellis, G.F.R.; Uzan, J.-P. (2005). “‘c’ is the speed of light, isn’t it?”. American Journal of Physics . 73 (3): 240–27. doi: 10.1119/1.1819929
• Helmcke, J.; Riehle, F. (2001). “Physics behind the definition of the meter”. In Quinn, T.J.; Leschiutta, S.; Tavella, P. (eds.). Recent advances in metrology and fundamental constants . IOS Press. p. 453. ISBN 978-1-58603-167-1.
• Newcomb, S. (1886). “The Velocity of Light”. Nature . 34 (863): 29–32. doi: 10.1038/034029c0
• Uzan, J.-P. (2003). “The fundamental constants and their variation: observational status and theoretical motivations”. Reviews of Modern Physics . 75 (2): 403. doi: 10.1103/RevModPhys.75.403

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How Fast is the Speed of Light?

With our current understanding of motion, it seems that the speed of light is the highest of all, being 874,030 times faster than the speed of sound.

The speed of sound travels at around 343 m/s, while the speed of light travels at 299,792,458 m/s. In miles per hour/mph, the speed of light is at around 670,616,629, while in kilometers per hour, light travels at 1,079,252,848.

In terms of seconds, light travels at around 300,000 kilometers per second or 186,000 miles per second in a vacuum.

In water, the speed of light is slower, at 225,000 km / 139,808 mi per second, and 200,000 km / 124,274 mi per second in glass. It seems that nothing can be faster than the speed of light.

If you want an example of how fast the speed of light is, think about this, if we were to launch an imaginary spacecraft from Earth that would travel at around 153,454 mi / 246,960 km per hour constantly, it would reach the Sun in 606 hours, or 25 days. 

However, if our spacecraft would be traveling at the speed of light, we would reach the Sun in only 8.3 minutes. If you traveled around the Earth with the speed of light, you would make a complete tour of our planet 7.5 times in just one second.

In theory, it seems that nothing is faster than the speed of light, or is there? Let’s find out.

Is There Anything Faster Than the Speed of Light?

It appears that nothing is faster than the speed of light, but the Universe , as always, eludes our perception once again. Scientists have demonstrated that the Universe is expanding, and this expansion is even faster than the speed of light.

Since space is theoretically “nothing,” it isn’t susceptible to the laws of physics. If you were to hold a torch and run with it, the speed of its light would still travel at the same rate.

Some galaxies are moving away from our Milky Way faster than the speed of light, and this is happening because space itself is moving along with them.

If there were something more efficient than traveling with the speed of light, it would be traveling through wormholes. Wormholes are hypothetical, but their mechanism is quite intriguing, and in a way, if it were possible, they are supposedly faster than the speed of light.

This is because a wormhole connects two distant points, and, in theory, if you were to travel from point a to b, regardless of its distance, you would reach your destination extremely fast.

How Fast is the Speed of Dark?

Many consider that the speed of darkness is simply a poetic metaphor and wouldn’t have any legitimate scientific basis, since dark is simply the absence of light.

However, this may seem a bit more complicated. If we were to put a dark spot in a beam of light, darkness would theoretically move at the same speed as light.

The same holds true if we would illuminate a dark corner. It is uncertain if darkness itself has a speed, but when it comes to dark matter, things start to unfold.

Dark matter is hypothetical energy, which makes up more than 80% of our Universe. In some studies, scientists estimated that this mysterious element might travel at around 54 m/s, to equate for its existence, but this is quite slow when compared to the speed of light.

Things get complicated if we look at black holes as part of the definition of darkness. Black holes are devoid of light, and if anything gets near their event horizon, not even light can escape from them.

Some black holes are fast-spinner as well, with some of them being recorded with having a spinning speed of around 84% of the speed of light. Darkness or the speed of dark is quite a fascinating subject, but it remains elusive to our current understanding.

What is the Fastest Thing in the Universe?

The fastest thing in the Universe, based on our current knowledge, is light. If you want to play dirty, you could say that the Universe/space is the fastest thing in existence, since it expands with a speed even faster than the speed of light.

If, in the future, we will understand how black holes can capture even light, maybe some of their mechanisms are the fastest thing in the Universe.

What Would Happen if You Would Travel Faster Than the Speed of Light?

The theory of special relativity states that nothing should travel faster than the speed of light, and if something does so, it will move backward in time.

Traveling faster than the speed of light might simply mean time travel. However, is this were true, in some ways, you might as well achieve immortality, as no cause could affect you, not even time, especially if, hypothetically speaking, you wouldn’t even be subjected to the impacts of the objects you would travel through.

Our current understanding of light speed is minimal, and even more so when it comes to surpassing it. We, as a species, with our current technology, have only just reached small percentages of the speed of light. We aren’t even halfway there.

What is the 2 nd Fastest Thing in the Universe?

Blobs of hot gas embedded in streams of material ejected from blazars, which are highly active galaxies , travel at around 99.9% of the speed of light.

Thus, the physical processes that occur at the cores of blazars are so energetic that they can propel matter quite close to light speed, and as such, they are probably the second fastest thing in the Universe. 

Did you know?

The fastest speed reached by a land vehicle is the ThrustSSC supersonic car. This vehicle reached 1,227 km / 772 mi/h, and it maintains its title as the most rapid land vehicle since 1997.

The fastest plane/aircraft in the world is the Lockheed SR-71 Black Bird. It achieved this title in 1976, and it reached a speed of 3,529.6 km/ 2,192 mi per hour.

The Parker Solar Probe is currently the fastest spacecraft ever designed by man. It reached 153,454 miles / 246,960 kilometers per hour.

Image Sources:

• https://images.immediate.co.uk/production/volatile/sites/4/2018/08/GettyImages-524396835-bca79f7.jpg?quality=90&resize=960%2C408
• https://cdn.britannica.com/s:800×450,c:crop/83/179683-138-D3C80B7C/Scientists-speed-of-light.jpg
• https://cdn.hswstatic.com/gif/speed-of-darkness-orig.jpg
• https://4.bp.blogspot.com/-7qK2eGoouKI/TprUhQT6hcI/AAAAAAAAEww/BHe3uF_S-rU/s1600/Time-travel-through-a-wormhole-thumb-550xauto-38205.jpg
• https://i.insider.com/5b47524e744a9820008b4838?width=1100&format=jpeg&auto=webp

Learn About the True Speed of Light and How It's Used

Roberto Moiola/Sysaworld/Getty Images 

• An Introduction to Astronomy
• Important Astronomers
• Solar System
• Stars, Planets, and Galaxies
• Space Exploration
• Weather & Climate
• Ph.D., Physics and Astronomy, Purdue University
• B.S., Physics, Purdue University

Light moves through the universe at the fastest speed astronomers can measure. In fact, the speed of light is a cosmic speed limit, and nothing is known to move faster. How fast does light move? This limit can be measured and it also helps define our understanding of the universe's size and age.

What Is Light: Wave or Particle?

Light travels fast, at a velocity of 299, 792, 458 meters per second. How can it do this? To understand that, it's helpful to know what light actually is and that's largely a 20th-century discovery.

The nature of light was a great mystery for centuries. Scientists had trouble grasping the concept of its wave and particle nature. If it was a wave what did it propagate through? Why did it appear to travel at the same speed in all directions? And, what can the speed of light tell us about the cosmos? It wasn't until Albert Einstein described this theory of special relativity in 1905 it all came into focus. Einstein argued that space and time were relative and that the speed of light was the constant that connected the two.

What Is the Speed of Light?

It is often stated that the speed of light is constant and that nothing can travel faster than the speed of light. This isn't entirely accurate. The value of 299,792,458 meters per second (186,282 miles per second) is the speed of light in a vacuum. However, light actually slows down as it passes through different media. For instance, when it moves through glass, it slows down to about two-thirds of its speed in a vacuum. Even in air, which is nearly a vacuum, light slows down slightly. As it moves through space, it encounters clouds of gas and dust, as well as gravitational fields, and those can change the speed a tiny bit. The clouds of gas and dust also absorb some of the light as it passes through.

This phenomenon has to do with the nature of light, which is an electromagnetic wave. As it propagates through a material its electric and magnetic fields "disturb" the charged particles that it comes in contact with. These disturbances then cause the particles to radiate light at the same frequency, but with a phase shift. The sum of all these waves produced by the "disturbances" will lead to an electromagnetic wave with the same frequency as the original light, but with a shorter wavelength and, hence a slower speed.

Interesting, as fast as light moves, its path can be bent as it passes by regions in space with intense gravitational fields. This is fairly easily seen in galaxy clusters, which contain a lot of matter (including dark matter), which warps the path of light from more distant objects, such as quasars.

Lightspeed and Gravitational Waves

Current theories of physics predict that gravitational waves also travel at the speed of light, but this is still being confirmed as scientists study the phenomenon of gravitational waves from colliding black holes and neutron stars. Otherwise, there are no other objects that travel that fast. Theoretically, they can get close to the speed of light, but not faster.

One exception to this may be space-time itself. It appears that distant galaxies are moving away from us faster than the speed of light. This is a "problem" that scientists are still trying to understand. However, one interesting consequence of this is that a travel system based on the idea of a warp drive . In such a technology, a spacecraft is at rest relative to space and it's actually space that moves, like a surfer riding a wave on the ocean. Theoretically, this might allow for superluminal travel. Of course, there are other practical and technological limitations that stand in the way, but it's an interesting science-fiction idea that is getting some scientific interest. 

Travel Times for Light

One of the questions that astronomers get from members of the public is: "how long would it take light to go from object X to Object Y?" Light gives them a very accurate way to measure the size of the universe by defining distances. Here are a few of the common ones distance measurements:

• The Earth to the Moon : 1.255 seconds
• The Sun to Earth : 8.3 minutes
• Our Sun to the next closest star : 4.24 years
• Across our Milky Way  galaxy : 100,000 years
• To the closest  spiral galaxy (Andromeda) : 2.5 million years
• Limit of the observable universe to Earth : 13.8 billion years

Interestingly, there are objects that are beyond our ability to see simply because the universe IS expanding, and some are "over the horizon" beyond which we cannot see. They will never come into our view, no matter how fast their light travels. This is one of the fascinating effects of living in an expanding universe. 

Edited by Carolyn Collins Petersen

• Learn about the Doppler Effect
• Can Anything Move Faster Than the Speed of Light?
• Is Warp Drive From 'Star Trek' Possible?
• Why Is the Water Blue in a Nuclear Reactor? Cherenkov Radiation
• Einstein's Theory of Relativity
• Amazing Astronomy Facts
• An Introduction to Black Holes
• Mathematical Properties of Waves
• Fundamental Physical Constants
• Can a Planet Make a Sound in Space?
• Can Humans Hear Sound in Space?
• Time Travel: Dream or Possible Reality?
• What Is Quantum Optics?
• How to Define Acceleration
• Wave-Particle Duality - Definition
• How Redshift Shows the Universe is Expanding

How we found the speed of light: it's not infinite!

"Nothing in the universe can travel at the speed of light, they say, forgetful of the shadow's speed." - Howard Nemerov

I know many of you are still mad at the night sky because of the full Moon preventing you from seeing the recent, close-by supernova in the Pinwheel Galaxy, at least until the end of the week.

Image credit: Original source unknown, retrieved from the Rose City Astronomers.

With the full Moon brightly illuminating your sky (and filling it with light pollution ), only the brightest, most compact objects are visible at most locations on Earth.

But there is one object -- rising at about 9:30 PM each night, these days -- that's not only visible to the naked eye, but one of the night sky's grandest sights through any telescope, large or small.

Image credit: David Richards.

The planet Jupiter! In fact, those of you with the extraordinary patience to track our Solar System's largest planet over the course of a few hours will not only have the chance to see Jupiter and its four Galilean moons , you'll get to see them in action!

In particular, the innermost of Jupiter's Moons, Io, completes one revolution around Jupiter in an incredibly precise 42 hours, 27 minutes, and 33.5047 seconds. (Yes, we really do know it to that accuracy!) Every time Io passes in front of Jupiter -- and it does this about four times every week -- it casts a shadow onto Jupiter's surface, creating a solar eclipse.

Image credit: John Spencer (Lowell Observatory) and NASA.

But what's more interesting for our purposes is that every time Io heads behind Jupiter, the gas giant's mightly shadow plunges its satellites into darkness, causing a lunar eclipse.

This can result, depending on where Earth in is its orbit relative to Jupiter and the Sun, in one of Jupiter's moon's seeming to eerily "appear" out of nowhere, some distance away from the edge of Jupiter itself.

Animation credit: Robert J. Modic, over a mere 10-minute timespan! Image was processed down from 3.2 MB to a mere 274 kb by Cleon Teunissen.

This is just geometry at work here, of course. By time the 1670s came around, it was universally recognized (among astronomers, at any rate) that the planets of our Solar System all orbited the Sun, with the Earth being an inner (and hence faster) planet as compared to Jupiter.

Image credit: Ole Rømer's notebook, sketch from 1676!

During six months out of the year, the Earth would be approaching Jupiter (at point B), for example, at the points F and G. When this happens, whenever one of Jupiter's moons reaches point C, above, it disappears into Jupiter's shadow.

During the other six months, the Earth recedes away from Jupiter, for example, at points L and K. During this time, whenever one of Jupiter's moons reaches point D, it appears to emerge from Jupiter's shadow.

However -- and here's the interesting part -- it appears to take longer for a moon, like Io, to emerge from Jupiter's shadow when you're at point K as compared to point L! And by the same token, it appears to take longer for a moon to plunge into Jupiter's shadow when you're at point F as compared to point G! What gives?

In the early 1600s, it was known that the speed of light was very, very fast , but it wasn't known whether it was infinitely fast or not. The only experiment done to measure it was Galileo's experiment in 1638, which I call the Beacon of Gondor experiment.

Image credit: Flickr user fallen petals.

One night, Galileo sent his assistant out far across the fields, and ordered him to stand at the crest of a hill. Galileo would be atop a distant hill. Galileo would unveil his lantern, so that his light would shine brightly atop the hill, and as soon as the assistant saw the light, he would then unveil his lantern. Galileo reasoned that he would be able to measure the distance between his assistant and himself, as well as the time it took for the light to travel round-trip, and hence he'd be able to figure out the speed of light.

Well, considering that the speed of light is around 300,000 kilometers per second , you can imagine what the results of this experiment, conducted from two hills across a field, were.

Very, very fast , was Galileo's conclusion. But was it infinitely fast, or was it simply too fast to measure between two humans on Earth?

For a couple of generations, the question was unsettled. But in the 1670s, Danish astronomer Ole Rømer not only settled it, he became the first person to measure what the speed of light actually was. All you need to know is where the Earth, Jupiter, and the Sun are positioned, which we knew in great detail thanks to the work of Kepler and Brahe nearly a century prior. Here's what you do.

Above is where the Earth, Jupiter, and the Sun were positioned back in June of this year; this corresponds to point "F" in Rømer's diagram from his sketchbook. What you can do is measure when Io appears to complete its transit of Jupiter (when it finishes passing in front of it), and accurately time how long it takes before it plunges into Jupiter's shadow.

Image credit: Jim Ferreira.

Then, wait a few months, until the Earth is at a closer position to Jupiter, but still makes the same angle it did all those months ago when you made your earlier observation. Today, for example, could correspond to point "G" in Rømer's diagram.

Above image made from a screen capture using the orrery application at dynamicdiagrams.com.

Do the exact same thing; time how long from the end of Io's transit until it plunges into Jupiter's shadow. If you've done your measurements accurately and correctly, you will find -- this time -- that it was several minutes shorter than it was a few months ago!

Why would this happen? Let's go back to Rømer's sketch.

The light emitted by Io, at point C, is the last bit of light we'll be able to see from it before the lunar eclipse begins. If that light traveled infinitely fast , then someone at point G would get the light at the same time as someone at point F, and there would be no difference .

But if the speed of light were finite , that last bit of emitted light would arrive at point G earlier than at point F! If you can determine the distance between point F and point G, and you can measure the time difference between when the moon plunges into shadow (at point C), you can measure the speed of light !

(Of course, you can do it just as easily with the moon re-emerging from Jupiter's shadow -- at point D -- during the other six months of the year.)

In the centuries since, of course, we've refined our measurements of the speed of light, and we've even measured the speed of gravity , which turns out to be the same. It turns out that the original, 1676 measurement was low by about 25%, mostly due to errors in the Earth-Sun distance.

Image credit: Steve Hamilton.

You can do this yourself, with any of Jupiter's four Galilean satellites, with simply a telescope and a clock, and some careful observations over the course of a year. If you'd like to get started, tomorrow night, September 13th, at about 11:24 PM Pacific Daylight Time (sorry folks, it's my time zone), you can see Jupiter's moon Europa plunge into shadow off of its eastern limb.

And that's how we found that the speed of light is not only finite , but were able to measure what it actually was! Not bad for 300 years ago!

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"Light pollution" seems to me to be an inappropriate and pejorative term for the magical radiance emitted by our beautiful, mysterious, unique moon, placing it in the same category as that emitted by vulgar street lamps and billboards.

I have always enjoyed the story of how Galileo attempted to measure the speed of light and enjoyed having it reiterated here.

Hi Ethan, very good post! I always knew that Römer measured lightspeed via the moons of Jupiter, but never, how he did that... Thanks a lot.

It turns out that the original, 1676 measurement was low by about 25%, mostly due to errors in the Earth-Sun distance.

Stay tuned... Captain Cook visiting Tahiti...

Love your posts in general. But I think your a bit unclear here. No matter if the earth is in position F or G, isn't the time difference between the completion of the transit and the lunar eclipse going to be the same. (The light at the end of the transit had to travel just as far as the light beginning the eclipse.)

What changes as the earth gets nearer or farther is the apparent timing between between each orbit. So if you watch a transit at F and then count in 42 hour 27 minute chunks, when you get to G you are surprised to find the transits happening earlier than you expected. But the time between transit and eclipse should remain (almost) constant.

Anyway, keep up the good work. Love the blog.

Rømer had it all sussed but I never understood what he had against "I" and "J". Actually, although Rømer knew how long it took light to cross the Earth's orbit and the size of the orbit he never, so far as we know, divided one by the other to get a velocity.

There are some young-earth creationists who try to claim that the speed of light was much faster in the past. To back that up, they use Romer's measurement, which they say gave a result somewhat more than the present value. But you are saying that his measurement gave a result less than the present value. Can anybody explain this discrepancy?

@csrster (5): Did you ever tried in "Arial" to identify "l" (el) and "I"? Moreover in the past everything was written by hand, and very often in LATIN. In handwritings "1", "I", "J" and "l" often are not unified and thus they were avoided.

I have to agree with Dhnice, the explaination is somewhat unclear. The Wikipedia article http://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_… is confusing too. It says: "During those 42½ hours, the Earth has moved further away from Jupiter by the distance LK"

One easy way to understand it, is to think of the Doppler-Effect. As the Earth moves towards Jupiter, the orbital period of Io decreases, and increases as it moves away.

This Wolframalpha demonstration includes an illuminative Formula: http://demonstrations.wolfram.com/RomersMeasurementOfTheSpeedOfLight/

Dhnice and CHP,

It is almost constant; the difference is mere seconds. What you really need, if you want to be accurate, is a table of many values of when one of the Jovian moons plunges into (or exits from) Jupiter's shadow. (Such a table, in astronomy, is known as an ephemeris .)

Since you know the period of the satellite very accurately, you can determine when it actually is plunging into/out of the planet's shadow, and compare that with when you observe it entering/exiting the shadow. Over the course of a year, you'll find variations of up to sixteen minutes or so, where that difference corresponds to the light-travel time of the diameter of the Earth-Sun distance.

This is what Rømer actually did, but it is asking a bit much of any amateur astronomer.

Hi Ethan, As always, I like your explanations and I thank you for letting us know about the Rømer experiment. I hadn't heard about the story before and thought the speed of light was first measured by Michelson & Morley. I admire Rømer for his achievement, but I have to ask you this: weren't the points F and K on his drawing somewhat closer to Jupiter than the drawing shows? Otherwise, the Sun would still be visible to the observer, as your picture above the caption "Above image made from a screen capture using this orrery application" clearly shows - am I right? Tihomir

You have to be on the "night" side of the Earth in all of those cases, which thankfully you get to do every time the Earth rotates at nearly all latitudes.

Very very nice science.

I did not know.

the next to the last sentence of the article makes no sense as it seems to conclude the the speed of light IS infinite.

Lovely post Ethan. I remember reading a beautiful essay by Isaac Asimov (probably called 'clock in the sky') on this very subject, and he had mentioned the same thing Dhnice did.. IIRC: that it was the absolute time of lunar eclipses that seemed off. i.e If we notice Io's first lunar eclipse at T, then the Nth one should be approx' T+N*42.xx (or some such thing that astronomers of those days had already worked out). But they werent. Since accurate clocks were also invented maybe a few decades earlier than that, it seems they started wondering if their clocks were not working properly. And then Romer came up with the explanation. Newton thought that it was pretty cool, but another famous astronomer Cassini didnt agree. I didnt realize that the moons would disappear (in the shadow) like that until now. Your visuals made it so clear.

I think Ole Rømer's diagram would make an awesome tattoo. Maybe I'll place it next to Darwin's Tree of Life, once I get it that is.

@11: Ethan, Exactly that is my point: on the drawing, the night side of the Earth on the points F and K never look towards Jupiter, but away from it. In order for the night side of the Earth to look at least a bit in the direction of Jupiter, the Earth would have to reside somewhere between the 9 o'clock position on the circle and 3 o'clock. The drawing shows the points F and K underneath the 9->3 middle line.

@DHnice: the time taken by the moon to disappear from the edge of Jupiter and make contact with the shadow is exactly the same (provided the viewing angles are the same as explained). However, the time of actual contact with the shadow to the time you observe the light to disappear depends on your distance from Jupiter. SO, when you measure the time between the moon leaving the disc of Jupiter to the total disappearance of the moon, that time will vary depending on your distance from Jupiter. If you were right at Jupiter this may appear instantaneous but as you move further from Jupiter it takes longer. So what is being observed as described in this experiment is (time to leave disc + time to contact shadow + time to notice disappearance of light) and what we see is the apparent variation in time to notice the disappearance of light which is due to our distance from Jupiter. The (time to leave disc + time to contact shadow) can be left as one term - we know from mechanics that it is a constant (for any particular geometry) and we are really only interested in the difference between the two observations since this gives us the time difference based on our difference in distance. Now you simply calculate the difference in distance between the two observing positions of the earth (easier said than done as Romer's result shows) and divide it by the distance in total time for each observation.

These days we can calculate our relative positions so much better than Romer could, so we can get far more accurate estimates of the speed of light via his method. However, there are quicker and far more accurate methods of measuring the speed of light thanks to modern technology.

About a year ago, I replicated Romer's experiment using a small telescope. I computed a value for the speed of light that was a bit more accurate than Romer's.

The experiment is described in this blog post: http://brightstartutors.com/blog/2010/10/25/speedoflight/

An excellent example of the practical use of geometry and astronomy to determine the speed of light.

@MadScientist

However, the time of actual contact with the shadow to the time you observe the light to disappear depends on your distance from Jupiter

The time of actual ending of transit to the the time you observe transit to end will also vary similarly. So, as Dhnice said, the difference between the two, as observed for a single orbit of Io, should be the same.

woww.... amazing.. speed of light now debated...

The light rings around SN1987a also demonstrate that light doesn't go at infinite speed. They appear because light takes slightly longer to travel to the dust that reflects some light towards us from the supernova flash than it took to go the direct route.

TomS, Variable speed of light theory was proposed by Barry Setterfield primarily based on observations of the speed of light by the US Naval observatory. Setterfield adjusted the Ramer numbers for light speed by correcting with modern values for planetary distances (as he should). With this adjustment, the Ramer data falls within error limits for speed of light in either constant C or Setterfield theories. The Setterfield assumption is exponential decay to the currently observed value. Extrapolating backwards in time and arriving at at exponential decay based on his data is not justifiable even though a statistically valid fit of the data, as there is no theoretical basis for such a decay and the data is far from being conclusive to justify such as decision. BTW, there are big-bang cosmologists who propose much faster light speed short after the big bang (60 orders of magnitude faster)

I have seen other expositions of Ole Rømer's idea, which present a different reasoning.

My suspicion is that many stories are embellished versions . The general outline of the story is that as seen from the Earth there are anomalies in motion of one of the Jupiter moons, anomalies that are resolved on the assumption that light propagates at a finite speed.

But I see different stories as to which timings are actually used.

Hi Ethan, you present diagrams from Rømer's notebooks. That's very interesting! Original source!

Looking at those diagrams I realize I don't understand them. I've read other versions of the Ole Rømer story, I did understand those stories.

I wonder what Ole Rømer's actual reasoning was. As I understand it, Rømer did not actually perform any calculation. As I understood it from other sources Rømer pointed out the possibility of assuming the Jupiter Moon anomaly arises from finite speed of light propagation.

Interestingly, in the Principia Newton refers to Rømer's hypothesis. In Newton's time it was unknown whether light transmits instantaneously. Newton's favored Rømer's hypothesis.

MadScientist,

Actually, today there are no longer ANY methods for measuring the speed of light. That's because the speed of light is no longer a measured quantity. Effectively, it's a defined quantity.

The definition of a meter is the distance travelled by light in a vacuum in a given fraction of a second. Once we have a definition of a second, the measurement of speed of light really amounts to measurement of a distance.

This occurred because our methods for measuring the speed of light became progressively more accurate. They became so accurate that the main limitation on their accuracy was the definition of the standard meter. Therefore, it makes sense to DEFINE the meter in terms of the speed of light.

Tihomir @16,

Look at the orrery image that's fifth from the end. That corresponds, roughly, to point F on the diagram: the one you're worried about.

It's true that as night falls, i.e., when you're at roughly the 3 o'clock position on the Earth, you cannot see Jupiter. However, as the night goes on, the Earth continues to rotate counterclockwise, and by time you reach the 11 o'clock position, Jupiter rises above the horizon. It will continue to ascend until roughly the 9 o'clock position, when dawn approaches. Remember that even though at any particular instant during the night, you can only see 180 degrees worth of the sky, over the course of the entire night, you can view something more akin to 270-300 degrees. With the exception of when the Earth is very close to position E in Romer's diagram, you will always have a window of time during the night when Jupiter is visible.

@26: Ethan, Thanks for the clarification. Of course, you're right. I made a more detailed drawing and came to the same conclusion, wondering why I wasn't able to see it sooner myself. My admirements to Rømer - and you! Tihomir

"The definition of a meter is the distance travelled by light in a vacuum in a given fraction of a second."

The definition of a second, however, doesn't depend on light except insofar as quantum tunnelling and metastable state radiative relaxation depend on the speed of light.

"The definition of a second, however, doesn't depend on light except insofar..."

It's true that the definition of a second does not depend on the speed of light. (It does depend on the frequency of light emitted by a particular electronic state transition of a cesium atom, though). It cannot depend on the speed of light simply BECAUSE that's how a meter is defined. Consider, if we defined a meter as the distance travelled by light in a given fraction of a second, and then defined a second as the time it takes light to travel a given number of meters, we haven't really defined either of those units.

"It does depend on the frequency of light emitted by a particular electronic state transition of a cesium atom, though"

Which doesn't depend on the speed of it. A 200 Hz signal going at 300m/s is still a 200Hz signal at 400m/s.

"It cannot depend on the speed of light simply BECAUSE that's how a meter is defined."

"It" being the length of a meter. Not the length of a second. The original conversation was: "That's because the speed of light is no longer a measured quantity. Effectively, it's a defined quantity."

Since that statement was then followed with:

"The definition of a meter is the distance travelled by light in a vacuum in a given fraction of a second. Once we have a definition of a second, the measurement of speed of light really amounts to measurement of a distance."

The only way light speed becomes a defined quantity is if the definition of a second is defined by the speed of light.

"Consider, if we defined a meter as the distance travelled by light in a given fraction of a second, and then defined a second as the time it takes light to travel a given number of meters, we haven't really defined either of those units."

Indeed, that would be silly.

Good job that isn't what they do, isn't it.

I think we may be talking past each other, but essentially agreeing. Assume a thought experiment in which you can watch a beam of light travel in a vacuum for precisely 1 second. Define "1 second" any way you wish. You then measure the distance travelled by that light in that second.

Remember, we are allowed to define a second in any manner we wish. Let's first define 1 second as some number, x of vibrations of light emitted during such and such a hyperfine transition of a cesium atom, ie. the modern standard definition. We have already defined the meter as the distance travelled by light in 1/300,000,000 seconds, so, by definition, the light travelled 300,000,000 meters. From this data, we would calculate the speed of light to be roughly 300 million meters per second. (I am aware that the real value is slightly less than this, and is more precisely defined)

Let's now define a second in such a way that it is 10 times longer than a standard second. The actual distance travelled by the light is now 10 times greater, but based on our current definitions, we would still state that the light still travelled roughly 300 million meters. That's because we define the term "meter" to mean the distance travelled by light in 1/300,000,000 of a second (again, I am aware that the real definition contains a more precise value). From our data, we again calculate that the speed of light is roughly 300 million meters per second.

This result holds true no matter how we define the second. I am glad you agree with me that it's ridiculous to use the speed of light to define both the meter and the second. But all the same, defining the meter in terms of light speed certainly defines the speed of light all the same. We will get the same numerical value for the speed of light in units of meters per second no matter what definition we use for the second.

To put it more simply, the speed of anything is the ratio of the distance travelled by it to the time needed to travel that distance. So the speed of light is the distance travelled by light divided by the time needed to travel that distance. The modern definition of a meter is that a meter is the distance travelled by light in a given amount of time. Therefore, the modern definition of a meter also give a defined value to the speed of light.

@SpinozaB: "The time of actual ending of transit to the the time you observe transit to end will also vary similarly. So, as Dhnice said, the difference between the two, as observed for a single orbit of Io, should be the same."

No, that is incorrect. Read what I wrote very carefully. Regardless of where you're at, it will take the exact same amount of time for the moon to move away from Jupiter's disc and around and into the shadow. (Seeing the shadow from a distant location such as earth is not the same thing as the object physically plunging into the shadow). From earth you are actually observing an event quite a few minutes in the past and when the earth is at a different position then you are observing with a different time delay. However, the fact remains that it actually takes exactly the same amount of time to move around and into the shadow. And yet if you actually time how long it took for the moon to disappear, you can see that the time appears to vary - that variation is due to the variation in your distance from Jupiter and the transit time of the light.

Look at it this way - imagine you can stand right near Jupiter. To get around the parallax problem at Jupiter, we observe the center of the moon as it passes the edge of the planet and goes around into the shadow. What you observe is an event that takes, say 4 hours. Let's say that it takes exactly 4 hours for the moon's center to cross the disc then plunge into the shadow and from our vantage point near Jupiter we have a delay of only milliseconds between the event and the observation. Now we move far away so that we have a delay of, say, 1 minute in the observations. You now see the center of the moon cross the disc a full minute after the event happened and you expect the center of the disk to plunge into shadow 4 hours after that observed (and already 1 minute delayed) crossing. However, the center of the moon doesn't appear to go into shadow until 4 hours and 1 minute later (a full 2 minutes after the actual physical event). It might sound counterintuitive, but if you write down the equations it's correct.

Oops... I was mistaken in my earlier post. (Dang, only 20 years and my maths is no good.) It is the time on earth at which we expect to observe the occultation which will vary. So SpinozaB is correct - everything is observed with a different time delay but the time between observing the moon cross the disc and observing it going into shadow is the same.

"To put it more simply, the speed of anything is the ratio of the distance travelled by it to the time needed to travel that distance."

Indeed it is.

Have a gold star.

"So the speed of light is the distance travelled by light divided by the time needed to travel that distance."

Yes, this is true.

Two gold stars!

"The modern definition of a meter is that a meter is the distance travelled by light in a given amount of time."

True yet again. Three gold stars!

"Therefore, the modern definition of a meter also give a defined value to the speed of light."

Aaaw. Failed. You were doing so well.

No. You see to get speed you not only need a meter length accurately measured, you also need a second accurately measured.

The second is not defined by the speed of light.

"since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom."

From the wiki entry.

"Let's now define a second in such a way that it is 10 times longer than a standard second."

Why? It isn't a second then, it's a decasecond.

"From this data, we would calculate the speed of light to be roughly 300 million meters per second."

However, since the second is not defined as "How long it takes light to travel 300km", we have not circular reasoning going on.

A cycle of transition takes time, not distance to happen.

Additionally, you'd have 10x more Cs transition cycles. So light would have travelled 10x longer. Your light speed would then be 3,000 kps as opposed to 300kps. Hence not a constant defined by the definition of second and meter.

You're missing the point. The "second" is an arbitrary unit of time. We could theoretically define it any way we wish. If we define what you are calling a decasecond to be a second, then it's a second. A meter is also an arbitrary unit of length. If the SI tomorrow changed the definition of a second to become a duration that is 10x longer than the current definition, then the meter would automatically become a distance that is 10x greater than the distance that is currently defined to be a meter.

The definition of the meter inherently includes the definition of the numerical value for c. c = distance/time. The distance, is certainly 1 meter; that's what the definition of the meter defines. The definition of 1 meter states that 1 meter is the distance travelled by light in a given time, namely 1/300000000 seconds. Therefore, the time is 1/300000000 by definition. 1 meter divided by 1/300000000 seconds is the speed of light and is equal to 300,000,000 meters per second. The fact is that if we change the definition of second, this is still true. The length of a meter, by the modern definition is no longer independent of our definition of a second. Making the second a longer time interval will automatically increase the actual distance that we call a meter in such a way that the numerical value we obtain for the speed of light remains constant.

Re: your post 36,

Given a definition of a second that is 10x longer than its current duration, the light would indeed travel 10x further in terms of actual distance. However, the way we define the unit of distance would have automatically changed at the same time so that the unit we call "meter" would become what we currently would refer to as a decameter. Therefore, even though, in current usage, we would say that the light travels 3,000,000,000 meters, we would have to change our definition of the meter as a result of our new definition of second. Under the new definition of meter, we would have to measure this distance as 300,000,000 meters. That gives a speed of 300,000,000 m/sec just as it was before we changed the definition of second, not the value of 3,000,000,000 m/sec that you state.

I realize that it's tough to grasp that our units are of arbitrary magnitude. However, that's because of common usage for an extended period of time, not because of any inherent physical principal. If we defined the meter in some other way, such as the distance between two marks on a metal bar, for instance, then the speed of light would no longer be a defined value. It would make sense to measure it. If we defined the meter in another way and then defined the second as the time needed for light to travel 300,000,000 meters, we would end up in the same situation we are currently in. That would define c just as the current definition of meter does. The time would then be 1 second, the distance would be 300,000,000 meters, the ratio of the two would be 300,000,000 meters per second.

I'm not sure if I'm being unclear with all of this. Maybe think of it this way. Before the current definition of the meter was adopted, the definition of the meter was independent of the definition of the second. At one time, for instance, there was an actual metal bar with two marks on it and the meter was defined as the distance between these marks. That definition was obviously not dependent on the definition of the second. As long as that was true, the speed of light was not a defined value.

The current definition of the meter is that it is the distance travelled by light during a time interval of 1/300,000,000 seconds. That definition is OBVIOUSLY not independent of the definition of the second. Since the length of a meter and the duration of a second are now intertwined, and the factor that intertwines them is the speed of light, the speed of light has a defined value.

Consider an alternate definition of the meter that is dependent on the second but does not utilize the speed of light. Define the meter as the distance travelled by a sound wave of 1 kHz frequency in still air at 760 mm Hg pressure, 0% relative humidity and a temperature of 20C during a time of 1 second. We have just defined the speed of sound in air. The speed of sound in air would then be 1 meter per second. This definition of meter would yield a meter stick that is much larger than what we normally call a meter stick. In fact it would be probably not fit within the borders of some small towns. However, it's a perfectly valid definition.

Let me give it one more try:

The Apollo astronauts left a "cat's eye" reflector on the surface of the moon. That reflector allows earth-based scientists to send a laser beam to the moon and have it reflect directly back to their detector on earth. It's possible to measure the time it takes to do so. Dividing twice the distance between the detector and the reflector by the time needed for the laser beam to travel that distance would yield a value for the speed of light.

That begs the question, though: how do you know what the distance between the reflector and the detector is? According to the modern definition of the meter, the distance is determined by multiplying the time needed for this beam to travel to the reflector and back by 300,000,000. Therefore, the distance is 300,000,000t and the speed of light is 300,000,000t divided by t, or simply 300,000,000.

No, the second is not defined as the time needed for light to travel 300,000,000 meters. That's true. A distance of 300,000,000 meters IS defined to be the distance travelled by light in one second. Assume that you can measure 1 second to any arbitrary degree of accuracy you wish. The light then has, by definition, travelled a distance of 300,000,000 meters. There's no point in measuring that distance independently of the time measurement. The measurement of the time has established the distance by definition. If you think you have designed an experiment to measure the speed of light, what you are REALLY doing is a very accurate distance measurement using the primary standard for the meter. That's really what you have done in my prior post, for instance. You have done an accurate measurement of the distance between your laser beam detector and the reflector on the moon.

To measure any arbitrary speed, you are correct; an accurate measurement of both distance and time is needed. However, that's not true for the speed of LIGHT. Light is different because the speed of light is used to define one of the quantities that we are measuring.

"No, the second is not defined as the time needed for light to travel 300,000,000 meters"

Good. Will you keep saying true things, though?

"Assume that you can measure 1 second to any arbitrary degree of accuracy you wish."

Nope. Assume you can measure the time taken for the requisite cycles of the metastable exited state of 133Cs to any accuracy you wish.

Bugger. It didn't take you long to get it all wrong again, did it.

"The light then has, by definition, travelled a distance of 300,000,000 meters."

If you've waited the requisite time for 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom, yes.

"There's no point in measuring that distance independently of the time measurement."

There is if you want to find out how long light took to travel any given distance. Light won't have traveled 300 km in 891,453,184 cycles.

"If you think you have designed an experiment to measure the speed of light, what you are REALLY doing is a very accurate distance measurement using the primary standard for the meter."

See, still completely arsed up.

If I've very very accurately measured a distance of 825 furlongs in SI meters, I have found a very accurate measure of distance.

But the time taken for light to cross that distance is required too.

For which you would have to count the number of Cs cycles and divide by the number of those in one second.

You can then deduce that light goes at some speed.

If all you know is the distance light went, then you haven't worked out the speed of light.

"an accurate measurement of both distance and time is needed. However, that's not true for the speed of LIGHT."

You measure a very accurate distance.

Then you need the time to travel that distance for light.

In a refractive medium this will not be the same as light speed in vacuuo.

Or is your contention that light moves at 300kps in water, say?

"Light is different because the speed of light is used to define one of the quantities that we are measuring."

Yes. But it isn't used to define the other.

Therefore if you decide the unit of time (the second) will be a rounder 1,000,000,000 cycles you will find that your speed of light is (approximately) 32.6kps.

So your speed of light figure is different if the interval of one second changes.

"That begs the question, though: how do you know what the distance between the reflector and the detector is?"

Nope, it doesn't. "It's possible to measure the time it takes to do so." isn't why they put that mirror up there.

And that is answered anyway. How many cycles of Cs occur between you sending a light pulse and you getting that light pulse back.

This, however, is the definition of the meter.

Not the definition of time. And not the definition of the speed of light.

Light speed depends on the definition of the second.

Since that second depends on something that isn't based on the speed of light, we don't have, as you perspicaciously pointed out, a case where:

"if we defined a meter as the distance travelled by light in a given fraction of a second, and then defined a second as the time it takes light to travel a given number of meters, we haven't really defined either of those units."

"The length of a meter, by the modern definition is no longer independent of our definition of a second."

One depends on light traveling, one depends on an electric field oscillation at a single point.

Your problem here is that you've got your knickers COMPLETELY in a twist with:

"1 meter divided by 1/300000000 seconds is the speed of light and is equal to 300,000,000 meters per second."

The fact is that that was because it gave a meter that was very close to the one defined earlier which was a bar of metal.

If we used a time interval of 1/10,000 a sidereal day we would have a different divisor defining the meter.

"However, the way we define the unit of distance would have automatically changed at the same time so that the unit we call "meter" would become what we currently would refer to as a decameter."

Nope, we'd use a divisor that was 10x larger.

Our meter would still be around 39 inches long. The speed of light would be 10x higher too.

I'm not sure what else I can do to explain this, so I'll take one last shot and then I'm done. You can have the last word if you want.

What you are saying is true for an arbitrary measurement of velocity. It's not true when what you are measuring the velocity of is light.

It IS pointless to measure both the time taken by a beam of light to travel a certain distance and independently measure that distance. Let's say you do so anyway. Let's say you measure the time as 1 second and then you independently measure the distance as 350,000,000 meters. You have not proven that the speed of light is 350,000,000 meters. All you have proven is that your independent measurement is wrong. How do I know it's wrong. Simple. The distance travelled by light in 1 second is DEFINED to be 300,000,000 meters. Any measurement that gives another number is wrong.

Sean, you're still completely wrong.

What happens when another significant digit is put on end of the speed of light figure? Do we redo all our meters and recalculate all our clocks?

We change the speed of light.

"Simple. The distance travelled by light in 1 second is DEFINED to be 300,000,000 meters."

Yes. This defines a meter.

It doesn't define a second.

You see velocity is written down in meters per second.

You need both.

http://en.wikipedia.org/wiki/Speed_of_light#Increased_accuracy_of_c_and…

"Because the previous definition was deemed inadequate for the needs of various experiments, the 17th CGPM in 1983 decided to redefine the metre."

So it wasn't to keep the speed of light absolutely in the median.

"Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realization of the metre."

But not of the second.

If we'd defined a second differently, we'd have defined the meter differently, just like we redefined the inch to be 2.51cm. That doesn't mean we changed the duration of a second.

To be absolutely clear:

ME: What happens when another significant digit is put on end of the speed of light figure? Do we redo all our meters and recalculate all our clocks?

YOU: They redefined the meter!!!!

If, for example, they find an actual reason why their determination of the speed of light is incorrect (maybe finding that the mechanism they took to define the light speed in vacuuo was incorrect, they'd pick a different figure. Just like the speed of light went from

299,792,456.2±1.1 m/s.

299,792,458 m/s

which is outside the error bars of the MEASUREMENT.

Why? To make a better accord with what measurements they could manage and had already made on other SI units. I.e.. they picked a WHOLE number of Cs cycles rather than fractional to fit.

So we changed the speed of light so that we could define the meter.

We didn't change the speed of light to refit the definition of the second.

If you don't believe me or the wiki site, then maybe some other sites will convince you:

"The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. This defines the speed of light in vacuum to be exactly 299,792,458 m/s." From http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_…

or from http://physics.nist.gov/cgi-bin/cuu/Value?c

speed of light in vacuum

Value 299 792 458 m s-1

Standard uncertainty (exact)

Relative standard uncertainty (exact)

Concise form 299 792 458 m s-1

I'm done. take it up with the NIST (The US government agency charged with maintaining standards of measurements)

from http://physics.nist.gov/cgi-bin/cuu/Value?c

In case you don't get the significance of the NIST information, the only values that are exact are values that are DEFINED. Measured values always have some uncertainty. NIST certainly thinks that the speed of light has been defined. I'm inclined to trust their thoughts.

Sean, the problem here may well be that you haven't thought through your concerns properly.

1) If the second had been significantly different, they would have used a different definition of the meter. Therefore your assertion is 100% absolutely and definitely WRONG on large differences in the prior definitions of both the meter and the second.

2) Metrology is about the PRACTICAL measurement of quantities. Therefore if something better than "the distance light goes over a second" comes up to measure linear distance, THIS WILL BE USED.

Additionally, currently the most accurate measure we have involved here is time. The meter was less well defined, basically being "the length of this stick here". Weight (being dependent on volume) increases the accuracy needed for linear distance and may have been the reason for the change of the "speed of light" that you insist would never be done.

Measuring the distance light goes in a set time is more accurate than "the length of this stick here", so they used it. But if the tolerances they need mean they can't change the length of a meter when they get higher accuracy on that distance light travelled means that they will, if needs be, change the definition of the meter by the length light travels in a second to a different figure.

Remember: they changed the speed of light once. They can do so again.

They haven't just said "the speed of light is this AND WILL NEVER CHANGE". They've defined the second accurately and can measure it to great precision. They've measured the distance light travels in a set time to great precision. The tolerances needed for the meter allowed them to pick a whole number of meters in the definition of distance by that traveled by light in a second.

But you see as far as metrology is concerned, both time and distance are continuous.

That means that at least one of the trifecta is incorrect. Either time needs to be a non-even number of Cs cycles, in which case they could define it as a different transition or stress the Cs to change the frequency of the cycles produced, or change the speed of light, or change the definition of a meter.

They did that once. It was easier to redefine the speed of light the first time to set the length of a meter. In a subsequent change, it was easier to change the length of a meter and keep the speed of light as defined. And to date, it's not acceptable to change the speed of light because we have measuring devices to whom that change would be noticed.

But there's no reason why they'd change the meter again next time.

Your assertion in that light speed is defined and the meter depends on that is correct. Your assertion that this means that the speed of light CANNOT change is wrong.

You just knocked down a straw man. I NEVER said that light speed could not be changed. All I said was that the CURRENT definition of the meter also defines the CURRENT value of light speed. Since the particular fraction of a second used in the definition is an arbitrary one, certainly if it made practical sense to do so, it could be changed and light speed would have a different value if that happened. Maybe I wasn't clear in my discussion of this. In all of my posts, I was simply stating that defining the meter in the way that it is currently defined also at the same time defines the speed of light. I never intended to suggest that this definition is immutable for all time.

I think we are in full agreement then. If you measure the time taken for a beam of light to travel from point A to point B, then absent a redefinition of the meter, all you have really done is measured the distance between points A and B, because (again, absent a change in the definition of a meter) this distance is DEFINED in terms of the travel time of the light beam.

"I NEVER said that light speed could not be changed."

Actually, today there are no longer ANY methods for measuring the speed of light. That's because the speed of light is no longer a measured quantity. Effectively, it's a defined quantity."

That's your first post.

In your scenario, the speed of light is still defined. When I say the speed of light is defined, I mean it's value is set by the unit definitions as opposed to experimentally determined. Even if the numerical value for the fraction of a second that is specified in the definition of the meter is changed (which is equivalent to changing the numerical value of the speed of light), light speed is still defined and not an experimentally determined quantity.

I'm not sure why you think that to say something is "defined" means that it cannot change. For example, the meter has always been a defined distance. Whether it's been defined as a fraction of the circumference of the earth, the distance between two scratches on a metal bar, a multiple of the wavelength of the light emitted during a specific electronic transition of a particular atom, or as a distance travelled by light during a specified fraction of a second, it's still a defined quantity. Similarly, the speed of light is defined. If it's found to be useful in the future to redefine the meter in some way other than as the distance travelled by light in some specified time period, then the speed of light will no longer be defined.

I am going to stop arguing with you here (and this time I REALLY mean it). I am doing so because I think our differences on the matter at this point are more a result of misunderstandings due to imperfections of language and/or our respective inability to properly express ourselves in it. (Probably more so on my part than yours). Thanks for the lively discussion.

> In your scenario, the speed of light is still defined

If, for example, they find a better method of defining length (say the length of X carbon atoms in a nanotube regular solid), then that will be used because it doesn't rely on another measure (time), like I said. You need time defined as well as measure distance light traveled.

"I'm not sure why you think that to say something is "defined" means that it cannot change."

I'm not sure why you think that's what you said:

"Actually, today there are no longer ANY methods for measuring the speed of light."

"That's because the speed of light is no longer a measured quantity."

Since these refute the option of the speed of light being anything other than a set figure like "2".

You definitely CAN measure the speed of light. And if a more acceptable method to measure a set invariant distance is possible, then this will be done.

As it was done once already.

WoW is 100% correct. The distance covered per unit time. You can use arbitrarily define the meter and the second, but the fact remains that the total distance covered in a certain time is the same. If you define the meter as x Hz and the second y Hz, and you take a piece of string and make it as long as the distance it travels in a given interval of time, it doesn't matter what the f*****g units are, the length of the string is the same, whether the second is defined by the interval between x number of oscillations of a butterfly's wings or a quantum logic clock, as long as the time interval is the same. Changing the definitions of meter or second, no matter where you get them - even from light itself - does not impact the absolute distance covered per absolute interval of time. The accuracies can be refined, the string length is the same whether you call it 300,000,000m/s, or 186,000M/s, or 3cutirbs/whufflesnuff. You can define your meter any way you want, you can say your length of string is 3.01x10^8 meters, or 2.89x10^8 meters, it is still the same length.

While on the subject of Rømer, maybe a post on thermometers next?

Or on /why/ he was working on observations of Jupiter's moons? (He did it in order to find out where Tycho Brahe's observatory on Ven was in relation to the observatory in Paris. The latitudes were not that hard to measure but the longitudes were pretty hard, given the state of the art of their clocks. The observations had something to do with that.)

earth- sun roughly 150.000.000 km. which makes circumference about 2 x pi x earth-sun. . It takes 42.5h for Io orbit Jupiter. Earth travels at most 2.677.493km a day around the sun and approximately x (42.5/24h) = 4.741.395 km away from Jupiter-Io before the next immersion or emergence. Speed of light 300.000km/s makes that light takes 15seconds longer before observed on earth. I canât see other reasonable explanations for Io's observed time difference from earth.

15 seconds delay could easily be detected in the course of 42.5/24 days because pendulum clocks were widely available in western europe at the time of Rømers experiment.

But if predictions of Ioâs emergence /immersion time were calculated in advance and written on a calendar we easily get to a difference in minutes over the timespan of a year. Taking into account that pendulum clocks were accurate enough at that point in history.

Speed of light in a vacuum (c)

Explore the speed of light in a vacuum (c), its historical context, implications on modern physics, and a sample calculation.

The Speed of Light in a Vacuum (c)

The speed of light in a vacuum, commonly denoted as c , is a fundamental constant of nature and has played a crucial role in shaping our understanding of the universe. This article delves into the significance of this constant, its relation to the famous equation E=mc 2 , and its implications on modern physics.

Defining the Speed of Light

The speed of light in a vacuum is defined as the constant speed at which electromagnetic waves, including light, propagate through empty space. This constant value is approximately 299,792 kilometers per second (km/s) or 186,282 miles per second (mi/s). As a result, it is considered the ultimate speed limit in the universe, as no known object or information can travel faster than the speed of light.

Historical Context

Over the centuries, scientists have attempted to measure the speed of light with increasing accuracy. In the late 17th century, Danish astronomer Ole Rømer was among the first to provide a reasonably accurate estimate of this constant by observing the motion of Jupiter’s moon Io. Later, in the 19th century, French physicist Hippolyte Fizeau and English physicist James Clerk Maxwell further refined the measurements using various experimental methods.

Implications on Modern Physics

The significance of the speed of light in a vacuum extends beyond its role as a fundamental constant. It serves as a foundation for various theories and principles in modern physics, including Einstein’s Theory of Relativity.

• Special Relativity: Introduced by Albert Einstein in 1905, the Theory of Special Relativity states that the laws of physics are the same for all observers moving at a constant velocity. As a direct consequence of this theory, time dilation and length contraction occur when approaching the speed of light. These relativistic effects have been experimentally verified through numerous tests and observations.
• General Relativity: Expanding upon Special Relativity, General Relativity provides a geometric description of gravity. The speed of light in a vacuum plays a critical role in this theory, as it dictates how gravity influences the curvature of spacetime.
• Energy-Mass Equivalence: One of the most famous equations in physics, E=mc 2 , demonstrates the relationship between energy (E), mass (m), and the speed of light squared (c 2 ). This equation shows that energy and mass are interchangeable, and that a small amount of mass can be converted into a large amount of energy.

In conclusion, the speed of light in a vacuum is a fundamental constant with far-reaching implications in modern physics. From shaping our understanding of time and space to providing the foundation for relativity and energy-mass equivalence, the speed of light remains an essential cornerstone in the study of the universe.

Example of Calculation Involving the Speed of Light

One common calculation involving the speed of light is determining the distance light travels within a specific time frame. The formula for this calculation is:

Distance = Speed × Time

Given that the speed of light in a vacuum is approximately 299,792 km/s, we can calculate the distance light travels within a certain period. For instance, let’s determine the distance light covers in one minute:

• Convert the time frame to seconds: 1 minute = 60 seconds
• Use the formula with the speed of light and the given time: Distance = 299,792 km/s × 60 s
• Calculate the result: Distance ≈ 17,987,520 km

Thus, light travels approximately 17,987,520 kilometers in one minute. This example demonstrates the incredible speed of light and highlights its significance in various calculations within physics and astronomy.

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Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

In 1676, by studying the motion of Jupiter's moon Io, Danish astronomer Ole Rømer calculated that light travels at a finite speed. Two years later, building on data gathered by Rømer, Dutch mathematician and scientist Christiaan Huygens became the first person to attempt to determine the actual speed of light, according to the American Museum of Natural History in New York City. Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum" of empty space is about 186,282 miles per second (299,792 km per second) — but his assessment showcased that light travels at an incredible speed.

According to Albert Einstein 's theory of special relativity , light travels so fast that, in a vacuum, nothing in the universe is capable of moving faster. 

"We cannot move through the vacuum of space faster than the speed of light," confirmed Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama in Huntsville.

Question answered, right? Maybe not. When light is not in a vacuum, does the rule still apply?

Related: How many atoms are in the observable universe?

"Technically, the statement 'nothing can travel faster than the speed of light' isn't quite correct by itself," at least in a non-vacuum setting, Claudia de Rham, a theoretical physicist at Imperial College London, told Live Science in an email. But there are certain caveats to consider, she said. Light exhibits both particle-like and wave-like characteristics, and can therefore be regarded as both a particle (a photon ) and a wave. This is known as wave-particle duality.

If we look at light as a wave, then there are "multiple reasons" why certain waves can travel faster than white (or colorless) light in a medium, de Rham said. One such reason, she said, is that "as light travels through a medium — for instance, glass or water droplets — the different frequencies or colors of light travel at different speeds." The most obvious visual example of this occurs in rainbows, which typically have the long, faster red wavelengths at the top and the short, slower violet wavelengths at the bottom, according to a post by the University of Wisconsin-Madison . 

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When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second). This means both radio waves and gamma rays have the same speed," Rhett Allain, a physics professor at Southeastern Louisiana University, told Live Science in an email.

So, according to de Rham, the only thing capable of traveling faster than the speed of light is, somewhat paradoxically, light itself, though only when not in the vacuum of space. Of note, regardless of the medium, light will never exceed its maximum speed of 186,282 miles per second.

Universal look

According to Cassibry, however, there is something else to consider when discussing things moving faster than the speed of light.

"There are parts of the universe that are expanding away from us faster than the speed of light, because space-time is expanding," he said. For example, the Hubble Space Telescope recently spotted 12.9 billion year-old light from a distant star known as Earendel. But, because the universe is expanding at every point, Earendel is moving away from Earth and has been since its formation, so the galaxy is now 28 billion light years away from Earth.

In this case, space-time is expanding, but the material in space-time is still traveling within the bounds of light speed.

Related: Why is space a vacuum?

So, it's clear that nothing travels faster than light that we know of, but is there any situation where it might be possible? Einstein's theory of special relativity, and his subsequent theory of general relativity, is "built under the principle that the notions of space and time are relative," de Rham said. But what does this mean? "If someone [were] able to travel faster than light and carry information with them, their notion of time would be twisted as compared to ours," de Rham said. "There could be situations where the future could affect our past, and then the whole structure of reality would stop making sense."

This would indicate that it would probably not be desirable to make a human travel faster than the speed of light. But could it ever be possible? Will there ever be a time when we are capable of creating craft that could propel materials — and ultimately humans — through space at a pace that outstrips light speed? "Theorists have proposed various types of warp bubbles that could enable faster-than-light travel," Cassibry said.

But is de Rham convinced?

"We can imagine being able to communicate at the speed of light with systems outside our solar system ," de Rham said. "But sending actual physical humans at the speed of light is simply impossible, because we cannot accelerate ourselves to such speed.

"Even in a very idealistic situation where we imagine we could keep accelerating ourselves at a constant rate — ignoring how we could even reach a technology that could keep accelerating us continuously — we would never actually reach the speed of light," she added. "We could get close, but never quite reach it."

Related: How long is a galactic year?

This is a point confirmed by Cassibry. "Neglecting relativity, if you were to accelerate with a rate of 1G [Earth gravity], it would take you a year to reach the speed of light. However, you would never really reach that velocity because as you start to approach lightspeed, your mass energy increases, approaching infinite. "One of the few known possible 'cheat codes' for this limitation is to expand and contract spacetime, thereby pulling your destination closer to you. There seems to be no fundamental limit on the rate at which spacetime can expand or contract, meaning we might be able to get around this velocity limit someday."

— What would happen if the speed of light were much lower?

— What if the speed of sound were as fast as the speed of light?

— How does the rubber pencil illusion work?

Allain is similarly confident that going faster than light is far from likely, but, like Cassibry, noted that if humans want to explore distant planets, it may not actually be necessary to reach such speeds. "The only way we could understand going faster than light would be to use some type of wormhole in space," Allain said. "This wouldn't actually make us go faster than light, but instead give us a shortcut to some other location in space."

Cassibry, however, is unsure if wormholes will ever be a realistic option.

"Wormholes are theorized to be possible based on a special solution to Einstein's field equations," he said. "Basically, wormholes, if possible, would give you a shortcut from one destination to another. I have no idea if it's possible to construct one, or how we would even go about doing it." Originally published on Live Science.

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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Light Year Calculator

What is light year, how to calculate light years.

With this light year calculator, we aim to help you calculate the distance that light can travel in a certain amount of time . You can also check out our speed of light calculator to understand more about this topic.

We have written this article to help you understand what a light year is and how to calculate a light year using the light year formula . We will also demonstrate some examples to help you understand the light year calculation.

A light year is a unit of measurement used in astronomy to describe the distance that light travels in one year . Since light travels at a speed of approximately 186,282 miles per second (299,792,458 meters per second), a light year is a significant distance — about 5.88 trillion miles (9.46 trillion km) . Please check out our distance calculator to understand more about this topic.

The concept of a light year is important for understanding the distances involved in space exploration. Since the universe is so vast, it's often difficult to conceptualize the distances involved in astronomical measurements. However, by using a light year as a unit of measurement, scientists and astronomers can more easily compare distances between objects in space.

As the light year is a unit of measure for the distance light can travel in a year , this concept can help us to calculate 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: 2 years

You can perform the calculation in three steps:

Determine the speed of light.

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 , which is 9.46×10¹² km/year .

Compute the time that the light has traveled.

The subsequent stage involves determining the duration of time taken by the light to travel. Since we are interested in light years, we will be measuring the time in years.

To facilitate this calculation, you may use our time lapse calculator . In this specific scenario, the light has traveled for a duration of 2 years.

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 9.46×10¹² km/year × 2 years = 1.892×10¹³ km

How do I calculate the distance that light travels?

You can calculate the distance light travels in three steps:

Determine the light speed .

Determine the time the light has traveled.

Apply the light year formula :

distance = light speed × time

How far light can travel in 1 second?

The light can travel 186,282 miles, or 299,792,458 meters, in 1 second . That means light can go around the Earth just over 7 times in 1 second.

Why is the concept of a light year important in astronomy?

The concept of a light year is important in astronomy because it helps scientists and astronomers more easily compare distances between objects in space and understand the vastness of the universe .

Can light years be used to measure time?

No , despite the name, you cannot use light years to measure time. They only measure distance .

Alien civilization

Helium balloons, internal resistance, young's modulus.

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Warp drives: Physicists give chances of faster-than -light space travel a boost

Associate Professor of Physics, Oklahoma State University

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The closest star to Earth is Proxima Centauri. It is about 4.25 light-years away, or about 25 trillion miles (40 trillion km). The fastest ever spacecraft, the now- in-space Parker Solar Probe will reach a top speed of 450,000 mph. It would take just 20 seconds to go from Los Angeles to New York City at that speed, but it would take the solar probe about 6,633 years to reach Earth’s nearest neighboring solar system.

If humanity ever wants to travel easily between stars, people will need to go faster than light. But so far, faster-than-light travel is possible only in science fiction.

In Issac Asimov’s Foundation series , humanity can travel from planet to planet, star to star or across the universe using jump drives. As a kid, I read as many of those stories as I could get my hands on. I am now a theoretical physicist and study nanotechnology, but I am still fascinated by the ways humanity could one day travel in space.

Some characters – like the astronauts in the movies “Interstellar” and “Thor” – use wormholes to travel between solar systems in seconds. Another approach – familiar to “Star Trek” fans – is warp drive technology. Warp drives are theoretically possible if still far-fetched technology. Two recent papers made headlines in March when researchers claimed to have overcome one of the many challenges that stand between the theory of warp drives and reality.

But how do these theoretical warp drives really work? And will humans be making the jump to warp speed anytime soon?

Compression and expansion

Physicists’ current understanding of spacetime comes from Albert Einstein’s theory of General Relativity . General Relativity states that space and time are fused and that nothing can travel faster than the speed of light. General relativity also describes how mass and energy warp spacetime – hefty objects like stars and black holes curve spacetime around them. This curvature is what you feel as gravity and why many spacefaring heroes worry about “getting stuck in” or “falling into” a gravity well. Early science fiction writers John Campbell and Asimov saw this warping as a way to skirt the speed limit.

What if a starship could compress space in front of it while expanding spacetime behind it? “Star Trek” took this idea and named it the warp drive.

In 1994, Miguel Alcubierre, a Mexican theoretical physicist, showed that compressing spacetime in front of the spaceship while expanding it behind was mathematically possible within the laws of General Relativity . So, what does that mean? Imagine the distance between two points is 10 meters (33 feet). If you are standing at point A and can travel one meter per second, it would take 10 seconds to get to point B. However, let’s say you could somehow compress the space between you and point B so that the interval is now just one meter. Then, moving through spacetime at your maximum speed of one meter per second, you would be able to reach point B in about one second. In theory, this approach does not contradict the laws of relativity since you are not moving faster than light in the space around you. Alcubierre showed that the warp drive from “Star Trek” was in fact theoretically possible.

Proxima Centauri here we come, right? Unfortunately, Alcubierre’s method of compressing spacetime had one problem: it requires negative energy or negative mass.

A negative energy problem

Alcubierre’s warp drive would work by creating a bubble of flat spacetime around the spaceship and curving spacetime around that bubble to reduce distances. The warp drive would require either negative mass – a theorized type of matter – or a ring of negative energy density to work. Physicists have never observed negative mass, so that leaves negative energy as the only option.

To create negative energy, a warp drive would use a huge amount of mass to create an imbalance between particles and antiparticles. For example, if an electron and an antielectron appear near the warp drive, one of the particles would get trapped by the mass and this results in an imbalance. This imbalance results in negative energy density. Alcubierre’s warp drive would use this negative energy to create the spacetime bubble.

But for a warp drive to generate enough negative energy, you would need a lot of matter. Alcubierre estimated that a warp drive with a 100-meter bubble would require the mass of the entire visible universe .

In 1999, physicist Chris Van Den Broeck showed that expanding the volume inside the bubble but keeping the surface area constant would reduce the energy requirements significantly , to just about the mass of the sun. A significant improvement, but still far beyond all practical possibilities.

A sci-fi future?

Two recent papers – one by Alexey Bobrick and Gianni Martire and another by Erik Lentz – provide solutions that seem to bring warp drives closer to reality.

Bobrick and Martire realized that by modifying spacetime within the bubble in a certain way, they could remove the need to use negative energy. This solution, though, does not produce a warp drive that can go faster than light.

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Independently, Lentz also proposed a solution that does not require negative energy. He used a different geometric approach to solve the equations of General Relativity, and by doing so, he found that a warp drive wouldn’t need to use negative energy. Lentz’s solution would allow the bubble to travel faster than the speed of light.

It is essential to point out that these exciting developments are mathematical models. As a physicist, I won’t fully trust models until we have experimental proof. Yet, the science of warp drives is coming into view. As a science fiction fan, I welcome all this innovative thinking. In the words of Captain Picard , things are only impossible until they are not.

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Speed of Sound in Vacuum: Debunking the Myths and Misconceptions

The speed of sound is a fascinating concept that refers to the rate at which sound waves travel through a medium. However, when it comes to a vacuum, the speed of sound is quite different. In a vacuum, where there is no air or any other medium to transmit sound waves, the speed of sound is zero. This is because sound waves require a medium to propagate, and in the absence of one, they cannot travel. It is important to note that a vacuum is a space devoid of matter, making it impossible for sound to travel.

Key Takeaways

Understanding the concept of sound in vacuum, definition of sound in vacuum.

When we think of sound, we often associate it with vibrations traveling through the air or other mediums . However, in the context of vacuum physics, the concept of sound takes on a whole new meaning . In a vacuum, sound propagation is quite different due to the absence of a medium for sound waves to travel through.

In simple terms , sound in a vacuum refers to the absence of sound waves or any form of audible vibrations . In this unique environment , the physics principles that govern the transmission of sound are fundamentally altered. The absence of a medium means that there are no particles to vibrate and transmit sound energy . As a result, the concept of sound in a vacuum challenges our traditional understanding of how sound travels.

Theoretical Background of Sound in Vacuum

To understand the theoretical background of sound in a vacuum, we need to delve into the physics of wave propagation and the speed of light. In a vacuum, such as outer space , sound waves cannot travel because there are no particles to transmit the vibration s. Unlike in a medium, where particles can vibrate and transfer energy, the absence of particles in a vacuum prevents sound transmission.

In the absence of a medium, the speed of sound is no longer limited by the acoustic velocity of the medium. Instead, the speed of light becomes the ultimate speed limit . In vacuum conditions , the speed of light is approximately 299,792,458 meters per second. This means that any sound waves generated in a vacuum would be unable to surpass the speed of light.

The silence in vacuum is a result of the absence of particles to vibrate and transmit sound. Without a medium for sound to travel through, the vacuum environment remains devoid of any audible vibrations . This phenomenon highlights the unique nature of sound in a vacuum and the limitations imposed by the physics of wave propagation.

The Speed of Sound in Vacuum

The speed of sound refers to the rate at which sound waves propagate through a medium. However, when it comes to the vacuum, where there is an absence of any medium, sound propagation behaves differently. In vacuum physics, the speed of sound is a fascinating concept that is influenced by various factors .

Speed of Sound in Vacuum at Different Temperatures

The speed of sound in a vacuum is not affected by temperature variations since there are no particles to transmit the sound waves. In a vacuum, sound waves cannot travel as they rely on the vibration of particles to propagate. Therefore, the concept of temperature does not apply to the speed of sound in a vacuum.

Speed of Sound in Vacuum in Different Units

When discussing the speed of sound in a vacuum, it is essential to consider the units used to measure it. In the field of physics, the speed of light is often used as a reference point for comparison. The speed of light in a vacuum is approximately 299,792,458 meters per second (m/s). However, sound waves travel at a much slower speed compared to light.

To provide a better understanding , let’s compare the speed of sound in a vacuum to the speed of light in different units :

As seen in the table above, the speed of sound in a vacuum is zero since sound waves cannot propagate without a medium. On the other hand , the speed of light remains constant regardless of the absence of a medium.

Understanding the physics principles behind the speed of sound in a vacuum is crucial for comprehending the limitations and possibilities of sound transmission in different environments . While sound waves cannot travel through a vacuum, they play a significant role in our everyday lives when there is a medium for sound to travel through.

So, next time you ponder the silence in vacuum or the physics of sound, remember that sound waves require a medium to travel, and in the absence of such a medium, sound propagation is impossible.

Comparing the Speed of Sound in Vacuum and Other Mediums

When it comes to sound propagation, the speed of sound varies depending on the medium through which it travels. In this article , we will compare the speed of sound in a vacuum with other common mediums such as air and water. We’ll explore the fascinating physics principles behind sound transmission and how it behaves in the absence of a medium.

Speed of Sound in Vacuum vs Air

In a vacuum, sound waves cannot propagate as there are no particles to vibrate and transmit the sound energy . Unlike light, which can travel through the vacuum of space, sound requires a medium to propagate. Therefore, in a vacuum, there is no speed of sound. This is due to the absence of particles that can transmit the vibration s necessary for sound wave propagation .

On the other hand , in air, sound waves can travel at a speed of approximately 343 meters per second at room temperature and normal atmospheric pressure . The speed of sound in air can vary depending on factors such as temperature, humidity, and altitude. It is important to note that the speed of sound in air is significantly slower than the speed of light, which travels at approximately 299,792,458 meters per second.

Speed of Sound in Vacuum vs Water

Similar to a vacuum, sound waves cannot propagate in water due to the absence of a medium for sound transmission. In a vacuum, the absence of particles prevents sound waves from traveling, while in water, the density and arrangement of water molecules do not allow for efficient sound propagation . Therefore, in both a vacuum and water, there is no speed of sound.

In contrast, sound waves can travel through water at a speed of approximately 1,482 meters per second. This is significantly faster than the speed of sound in air. The higher density of water compared to air allows sound waves to propagate more efficiently, resulting in a higher speed of sound.

Is Sound Faster in a Vacuum?

No, sound is not faster in a vacuum. As mentioned earlier, sound waves cannot propagate in a vacuum due to the absence of a medium for sound transmission. In a vacuum, there are no particles to vibrate and transmit the sound energy , resulting in the absence of sound propagation. Therefore, the concept of speed of sound does not apply in a vacuum.

The Velocity of Sound in Vacuum

Understanding the velocity of sound in vacuum.

When we think of sound, we often associate it with the vibration s that travel through the air or other mediums . However, in a vacuum, where there is an absence of any medium, sound behaves quite differently. In this article , we will explore the fascinating concept of the velocity of sound in a vacuum and the factors that affect it.

In order to understand the velocity of sound in a vacuum, we need to delve into the realm of vacuum physics. Sound propagation relies on the interaction of particles in a medium, such as air or water. These particles transmit sound waves by vibrating and transferring energy from one particle to another. However, in a vacuum, there are no particles to vibrate or transmit these waves . This absence of a medium poses a unique challenge for sound transmission.

In vacuum conditions , sound waves cannot propagate as they do in a medium. The physics principles that govern the speed of sound in air or water do not apply in a vacuum. Instead, we must turn to the speed of light, which is the fastest speed possible in the universe . The speed of light in a vacuum is approximately 299,792,458 meters per second. This speed sets the upper limit for the velocity of sound in a vacuum.

Factors Affecting the Velocity of Sound in Vacuum

While the speed of light provides a theoretical limit for the velocity of sound in a vacuum, there are other factors that can affect this velocity . Let’s take a look at some of these factors :

Frequency of the Sound Waves : The frequency of sound waves refers to the number of vibrations per second. In a vacuum, the frequency of sound waves does not change, but it does affect the perception of sound. Higher frequencies are perceived as higher-pitched sounds , while lower frequencies are perceived as lower-pitched sounds .

Energy of the Sound Waves : The energy of sound waves is directly related to their amplitude . In a vacuum, the energy of sound waves remains constant, but it is not transmitted or perceived in the same way as in a medium. The absence of particles to vibrate means that the energy of sound waves cannot be transferred or detected in a vacuum.

Vacuum Environment : The conditions of the vacuum environment can also impact the velocity of sound. Factors such as temperature and pressure can affect the behavior of sound waves in a vacuum. However, since sound waves cannot propagate in a vacuum, these environmental factors do not have the same influence as they would in a medium.

Sonic Vibrations : While sound waves cannot propagate in a vacuum, there can still be sonic vibrations caused by other sources . For example, spacecraft or satellites moving through space can create vibrations that can be detected by instruments. These vibrations, however, are not considered sound waves in the traditional sense .

The Travel of Sound in Vacuum

Sound propagation is a fascinating phenomenon that occurs when waves of pressure travel through a medium, such as air or water. However, have you ever wondered what happens when sound encounters a vacuum? In the absence of any medium, the behavior of sound waves changes significantly. Let’s explore the intriguing world of sound in a vacuum.

How Fast Does Sound Travel in a Vacuum?

When we think about the speed of sound, we often associate it with its velocity in air, which is approximately 343 meters per second at room temperature . However, in a vacuum, the story is quite different. In the absence of any particles to transmit the sound waves, there is no medium for sound to travel through. As a result, sound cannot propagate in a vacuum.

Factors Influencing the Speed of Sound Travel in Vacuum

The speed of sound in a vacuum is essentially zero, as there are no particles to carry the vibration s that create sound waves. However, it is important to note that the speed of sound in a vacuum is not the same as the speed of light. While light travels at a staggering speed of approximately 299,792 kilometers per second in a vacuum, sound waves cannot travel at all in the absence of a medium.

In a vacuum, the absence of a medium for sound transmission is the primary factor influencing the speed of sound. Without particles to vibrate and propagate the sound waves, there is no mechanism for sound to travel. This fundamental principle of physics highlights the importance of a medium for sound wave propagation .

The concept of sound in a vacuum also has implications for our understanding of space. In the vast emptiness of outer space , where a vacuum environment prevails, there is no sound . Despite the presence of celestial bodies and cosmic events , the silence in vacuum is absolute. The absence of sound waves in space is a stark reminder of the unique physics principles that govern our universe .

Debunking Myths about the Speed of Sound in Vacuum

Is the speed of sound maximum in vacuum.

When it comes to the speed of sound, there is a common misconception that it is maximum in a vacuum. However, this is not true. In fact, sound cannot propagate in a vacuum at all.

To understand why, let’s delve into some vacuum physics . Sound waves are essentially vibrations that travel through a medium, such as air, water, or solids. These vibrations create a disturbance in the particles of the medium, causing them to collide and transfer energy, which we perceive as sound.

In a vacuum, there is an absence of any medium for sound to travel through. Without particles to vibrate and transmit the sound energy , there can be no sound propagation. Therefore, the speed of sound in a vacuum is effectively zero.

It is important to note that the speed of light, not sound , is the fastest possible speed in the universe . In a vacuum, light travels at a constant speed of approximately 299,792 kilometers per second. This speed is determined by the fundamental physics principles governing electromagnetic waves , not sound waves.

Can a Vacuum Cleaner Affect the Speed of Sound?

Another myth surrounding the speed of sound in a vacuum is the idea that a vacuum cleaner can somehow affect it. This misconception likely stems from the association of a vacuum cleaner with the word “vacuum.” However, the function of a vacuum cleaner is entirely different from the vacuum we are discussing here.

A vacuum cleaner works by creating a partial vacuum , which is a region of space with lower air pressure compared to its surroundings . This pressure difference creates suction, allowing the vacuum cleaner to remove dust and debris from surfaces. However, it does not alter the fundamental properties of sound or affect the speed of sound in any way .

Remember, sound waves need a medium to travel, and a vacuum is devoid of any medium. So, in a vacuum, silence prevails, and sound cannot exist.

How Does the Speed of Sound in Water Compare to the Speed of Sound in a Vacuum?

The speed of sound in water is significantly slower compared to the speed of sound in a vacuum. In water, sound travels at approximately 1,480 meters per second, whereas in a vacuum, sound can’t propagate as there is no medium for it to travel through. This disparity in speed is due to the difference in density and compressibility of water and the absence of any medium in a vacuum.

Frequently Asked Questions

Q1: what is the speed of sound in a vacuum.

The speed of sound in a vacuum is essentially zero. Sound waves require a medium such as air, water, or solids to propagate. In a vacuum, where there are no atoms or molecules to vibrate, sound cannot travel.

Q2: Is there a speed of sound in a vacuum?

No, there is no speed of sound in a vacuum. Sound requires a medium to propagate, and a vacuum is defined by the absence of matter. Therefore, sound cannot travel in a vacuum.

Q3: How fast is the speed of sound in a vacuum?

The speed of sound in a vacuum is zero. Sound is a mechanical wave that requires a medium to travel. In a vacuum, there are no particles to carry the wave , so sound cannot propagate.

Q4: What is the velocity of sound in a vacuum?

The velocity of sound in a vacuum is zero. Sound waves require a medium to propagate, and in a vacuum, there are no particles to transmit the sound .

Q5: Is the speed of sound maximum in a vacuum?

No, the speed of sound is not maximum in a vacuum. In fact, sound cannot travel in a vacuum because it requires a medium to propagate.

Q6: What is the speed of sound in vacuum at 25 degrees Celsius?

The temperature does not affect the speed of sound in a vacuum because sound cannot travel in a vacuum. Regardless of the temperature , the speed of sound in a vacuum is zero.

Q7: What is the speed of sound in vacuum at 0 degrees Celsius?

Just like at any other temperature , the speed of sound in a vacuum at 0 degrees Celsius is zero. Sound cannot propagate in a vacuum, regardless of the temperature .

Q8: Can sound waves travel in a vacuum?

No, sound waves cannot travel in a vacuum. Sound is a mechanical wave that requires a medium to propagate. In a vacuum, there are no particles to carry the wave , so sound cannot propagate.

Q9: Is sound faster in a vacuum?

No, sound is not faster in a vacuum. In fact, sound cannot travel at all in a vacuum because there is no medium for the sound waves to travel through.

Q10: What is the speed of sound in vacuum in meters per second or mph?

The speed of sound in a vacuum, whether measured in meters per second or miles per hour, is zero. Sound requires a medium to propagate, and a vacuum is defined by the absence of matter. Therefore, sound cannot travel in a vacuum.

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2 passenger planes surpassed 800 mph on recent flights with strong winds. That's over 200 mph faster than a typical plane ride.

• On Saturday, a Virgin Atlantic plane and a United Airlines plane reached speeds of more than 800 mph.
• These are some of the highest-known recorded speeds for passenger flights. 
• Near record-breaking winds are to blame.

On Saturday, two flights arrived at their destinations early.

A Virgin Atlantic flight from Washington, DC, to London landed 45 minutes ahead of schedule . Meanwhile, a United Airlines flight from Newark, New Jersey, arrived in Lisbon 20 minutes early .

The passengers have strong winds to thank for that.

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High-altitude winds over the mid-Atlantic helped the planes surpass speeds of 800 mph, The Washington Post reported.

According to Simple Flying , passenger planes typically have a cruising speed of about 600 mph.

The planes flew faster than the speed of sound

The National Weather Service in the DC area reported that the winds reached speeds of 265 mph.

This evening's weather balloon launch detected the 2nd strongest upper-level wind recorded in local history going back to the mid 20th century! Around 34,000-35,000 ft, winds peaked around 230 kt (265 mph!). For those flying eastbound in this jet, there will be quite a tail wind. pic.twitter.com/0MXK0HvsCV — NWS Baltimore-Washington (@NWS_BaltWash) February 18, 2024

NPR reported that the highest recorded wind speed in the area was 267 mph in 2002. This would make Saturday's winds the second highest in the region.

Contrasting temperatures often cause strong winds, which seems to be what happened on Saturday. The Washington Post reported that cold air from the Northeast and warm air from the Southeast led to the winds.

These winds pushed Virgin's Boeing 787 to a ground speed of 802 mph and United's Boeing 787 to 838 mph .

The speed of sound is 767 mph. While they were flying faster than the speed of sound, the planes didn't break the sound barrier, The Washington Post reported.

That's because the planes were still flying at their typical cruising speeds — they were just flying in unusually fast air.

While there isn't an official record of top ground speeds, The Washington Post reported that the two Saturday flights were among the highest known recorded speeds.

Some of the other top recorded speeds include a China Airlines flight that reached 826 mph over the Pacific Ocean last month and a British Airways flight that reached 825 mph in 2020.

Watch: Video shows the moment a Chinese passenger jet crashed into mountains

• Main content

First Drive: The 2024 McLaren 750S Is a Balanced Speed Machine

S inewy mountain roads on the outskirts of Estoril, Portugal , are an ideal test locale for driving the new 2024 McLaren 750S. Unadulterated asphalt, nominal traffic, decent sight lines, and exemplary views abound. Unless it’s raining. Combine custom-hewn Pirelli P-Zeros, a soupçon of wetness, and a twin-turbo 4.0-liter V-8, churning out 740 horsepower and 590 lb-ft of torque, and you’re left with wheel spin through fourth gear in the McLaren 750S—Alcantara-shrouded instrument panel aglow with warnings.

Nerve-wracking? A bit. This $307,438 coupe’s comely gray hue, dubbed Saros, won’t look great wrinkled. Until spritzing clouds dissipate, light throttle is best.

McLaren spent hours improving the mid-mounted M840T engine (used in its 720) to increase oomph for the new McLaren 750S—increasing turbocharger pressure, improving the fuel pumps, and revamping the head gasket—and the sum of those efforts is felt when putting the hammer down. It’s been a minute since I drove the 720S , but my brain is forever seared with that overwhelming sensation of speed.

Quicker and More Visceral

Some theater comes from a shorter final drive, inspired by the 765LT’s gearbox, along with recalibrated shift mapping, imbuing greater urgency when accelerating. A new, 5-pound lighter exhaust, tweaked for optimal snarling and barking, and upgraded engine mounts, reward you with a satisfying thump in the back during gear changes—and a crescendoing roar when ripping through the rev range. New ignition cuts on downshifts result in cracks so loud, it makes some Esotril locals jump. (Thumbs up and smiles followed.)

Related: The 13 Best Dive Watches of 2024

McLaren enhances livability too. Apple CarPlay is standard. Lifting the nose takes four seconds, shaving six seconds off the 720S's time. You’re no longer stuck at a speed hump, nervously monitoring your rearview as you wait. The biggest upgrade is mounting the instrument display on the steering column. It moves with the wheel, handling, and powertrain controls relocated to the sides of the display. Clicking through the rocker switches for various drive configurations is as easy as reaching your fingertips forward. It’s nice and helpful, though I miss the 720’s folding display.

Taking to the Track

The rain backs off, allowing for track stints on Estoril’s circuit. Here, the updates to the chassis, aero kit, and suspension are evident. The 750’s front track widens by 6mm, and the springs are softer in the front, stiffer in the rear. McLaren rejiggers custom dual-valve dampers, too. A honking rear wing enlarges the surface area by 20 percent over the 720. Combined with a longer front splitter, there’s more downforce. And, since the rear wing doubles as an air brake, stopping distances are shorter.

The 750 feels glued to the track, still a bit greasy from the morning shower. And the 750 gulps up laps with aplomb, ready to spring out of any corner. Learning a tight uphill exchange on the backside of the track, I correct the steering mid-corner, to get on the right line. A faster steering ratio helps the coupe nimbly comply. It’s lighter steering than the 720, but gives greater feedback.

“We tried to increase the feeling of agility,” says Sandy Holford, the 750’s chief engineer, during a break. “You don’t want it super light, but you don’t want such a high-turning effort, in terms of feedback, that you won’t be able to turn in straight away.” Nailed it.

The 750 inspires confidence, even for track neophytes. Credit, in part, a new brake booster and vacuum pump for that. Barrel down Estoril’s front straight, cresting 170 mph, and it’s surprising how deep into the braking zone you can go before drilling the stoppers for the upcoming right-hander. It’s consistent, in terms of pedal travel before brake bite, and modulates well as you trail off. The 720S likes to tail wiggle under hard stopping, but the 750S sloughs speed without niggling shaking. (There’s an optional track brake upgrade, which uprates the discs and adds cooling to the monoblock front caliper, but unless you’re Lando Norris, you won’t need that.)

Seasoned hot shoes will find the 750S equally splendid. The rain resumes, turning Estoril’s racing line into a slick and sketchy mess. Still, when riding shotgun with a McLaren driving coach, the 750 has no problem hooking up for tight corners and chicanes, all while off-line and on cooling tires. More steering input is needed to keep us hammering forward, but it always maintains balance and finds traction.

Related: Best Travel Accessories of 2024 You Should Never Leave Home Without

Final Thoughts: This Machine Is Unrelenting

Most indelible for those lucky enough to wheel a 750 is the otherworldly speed. It's intoxicating—and unrelenting. Benchmarked against Ferrari’s 296 GTB, the 750S is 440 pounds lighter (at 3,241 pounds) meaning it has class-leading power-to-weight. You feel every ounce of that power.

Given the option, go for the 750S Spider. The weight penalty from the drop-top is 108 pounds, but you won’t notice. Instead, tap the button, watch the roof recede, and smile as that prodigious engine howls inches behind your head.

2024 McLaren 750S Specs

• ENGINE: 4 liter, twin-turbocharged V8, 740 horsepower, 590 lb-ft torque
• TRANSMISSION: 7-speed twin-clutch automatic
• 0 to 60: 2.7 seconds
• TOP SPEED: 206 mph
• PRICE: From $331,470

California's high-speed rail project moves step closer to becoming reality

LOS ANGELES (KABC) -- Traveling from Los Angeles to the Bay Area in under three hours is one step closer to becoming a reality.

The board of directors for the California High-Speed Rail Authority is releasing a request for proposals to build the nation's first 220 mph electrified trains.

They hope to award a contract by the end of this year and have already requested proposals from two pre-qualified firms, Alstom Transportation, Inc. and Siemens Mobility, Inc.

This is possible, in part, due to the record $3.1 billion federal grant the U.S. Department of Transportation awarded in December 2023, which included funding for new electric trains.

By the end of the decade, officials behind the project hope there can be electrified high-speed trains in service .

The project's development has created more than 13,000 construction jobs, mostly in the Central Valley, and has environmentally cleared 422 miles for construction of the high-speed rail program from the Bay Area to the Los Angeles Basin, according to officials.

The California High-Speed Rail Authority says the project will eventually extend into rail connections with Sacramento and San Diego.

Related Topics

• LOS ANGELES
• NORTHERN CALIFORNIA
• SOUTHERN CALIFORNIA
• PUBLIC TRANSPORTATION
• HIGH SPEED RAIL

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