# Speed of light Definition and 222 Discussions

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any signal carrying information can travel through space.
Though this speed is most commonly associated with light, it is also the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation (of which light is a small range in the frequency spectrum) and gravitational waves. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonzero rest mass can approach c, but can never actually reach it, regardless of the frame of reference in which their speed is measured. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence, E = mc2. In some cases objects or waves may appear to travel faster than light (e.g. phase velocities of waves, the appearance of certain high-speed astronomical objects, and particular quantum effects). The expansion of the universe is understood to exceed the speed of light beyond a certain boundary.
The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200000 km/s (124000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.
For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light also ultimately limits the data transfer between the CPU and memory chips in computers. The speed of light can be used with time of flight measurements to measure large distances to high precision.
Ole Rømer first demonstrated in 1676 that light travels at a finite speed (non-instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism.
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (983571056 ft/s; 186282.397 mi/s) with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1 / 299792458 of a second.

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1. ### I Michelson-Morley experiment - errors in calculations?

I would like to know the opinions on the paper that I saw about Michelson-Morley experiment. Michelson-Morley experiment was done in 1887 and had an impact on the future direction of physics. It is taught in schools as an experiment that proves the non-existence of the aether and proves the...
2. ### I How to measure the speed of light, and is it a constant?

The question constantly arises how the speed of light is measured and what does it mean that the speed is constant, including at remote points for the observer, including at points beyond the local frame of reference, as you understand it in general relativity (GR). First of all, it should be...
3. ### B Speed of Push vs Speed of Light

Been thinking about this since I was 12. Suppose you had a 1 lightyear long pole, and on the other end was a button 1 inch away that activates a light. The light is powerful enough for you to see it 1 lightyear away. You push the pole 1 inch forward. How long does it take for the light to reach...
4. ### B Speed of light -- What's the experience of travel like for a photon?

We know that as speed increases, time slows, and at the speed of light time apparently ceases. Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are...
5. ### I Black hole waterfall analogy and speed of light

Recently I have seen a number of General Relativity visualisations that show spacetime flowing towards any mass, similar to water flowing into a sink hole. ScienceClic's video is an example. That model is also used in the "waterfall model" to explain the event horizon of a black hole, as the...
6. ### I Time and Gravity if rotating faster than a critical frequency

If a person was rotating on a verticle axis from head to toe like the earth or quasar. If nothing can go faster than light, from the person's perspective looking at the stars traveling across the night sky, if you increase the rotation of the earth, stars further than a certain critical distance...
7. ### I How do forces change with speed?

Hey dear physics community :) I ask myself what exactly happens to forces between materia when the materia hits near lightspeed. I know, that for an objective bystander watching lets say elon in his rocket with 99,99999% the speed of light, that the time goes slower, the mass of the rocket...
8. ### Measuring the speed of light in a straight line

So recently I watched a video detailing how it is impossible to measure the speed of light in a straight line because it's not possible to synchronize two-time measuring devices without first knowing the speed of light. But I was thinking if light can orbit a black hole in the photon sphere...
9. ### Achieving the speed of light at the end of a whip

Is there a equation for the speed at the end of the whip. is there a consistent way to calculate how fast the tip of the whip will be including constant velocity and trajectory or is it chaotic. Im new to physics and had a thought that with enough legnth and power a wiphs end colud reach the...
10. ### I Speed of light not an invariant in GR

Hi all, I need help understanding the light ray bending in the original GR 1916 paper, Die Grundlagen.... First of all, Einstein states the ##c## is not an invariant in GR. In fact, from (70) and (73), it stems that $$\gamma = \sqrt{ -\frac {g_{44}}{g_{22}} },$$ where ##\gamma## is ##|c| <= 1##...
11. ### Optical Compact Device For Measuring Speed of Light

Hi. This is an idea which I just happened to think of, and I was curious if it would be at all feasible. Here's a quick sketch I drew: The two curved mirrors should have a laser attached on one end and a video camera attached on the other. The laser would be tilted very slightly above...
12. ### B Can the relative speed of an object meet/exceed the speed of light?

I was curious if the relative speed of an object can exceed the speed of light. Specifically, I am curious about the following thought experiment. I am not a physicists (and if I were asking the following would make me a poor one) and it has been 20 years since college physics. If a vessel is...
13. ### B Decay in c caused by Universe expansion

I'm currently writing a research paper about the speed of light. I have researched universe expansion, specifically, the quantised redshift spectral index fluctuations of distant galaxies and other structures over time, however, I need to suggest why universe expansion possibly causes a recorded...
14. ### B Is there a thought experiment to show that the speed of light is constant?

I know the amazing thought experiment by Albert Einstein with the two light clocks. (The observer at the train station has a light clock and the person in the train.) It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes. But this experiment...
15. ### B How much in the past do we see these planets?

The HR 8799 star is 129 light-years away from us, in this image we can see thermal emissions coming from these planets. Do these emissions move at the speed of light? If so does it mean that what we are seeing in this image is how the planets looked like in infrared 129 years in the past?
16. ### I The vacuum speed of light (invariant speed) in a non-inertial frame

Hi, I read various threads in PF about the concept of invariant speed and the speed of light in vacuum that in our universe happens to be the same as the 'invariant speed'. My doubt is about the speed of the light in vacuum as measured from a non-inertial frame (basically in the context of SR...
17. ### B Energy, mass, and the speed of light

Hello everyone! Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship...
18. ### I Why is the speed of light absolute?

To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?
19. ### B Is red-shifted light traveling at a speed less than c?

The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source. If so then the original quantity of...
20. ### Why does the speed of light change in different media?

In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down? Macroscopically speaking, the speed of light does slow down. What about in the...
21. ### I Another mirror question...

If you’re travelling at the speed of light TOWARDS a mirror, will you be able to see yourself?
22. ### An experiment to measure the Speed of Light

So, pretty much I want to make an experiment in order to get the speed of light. What I plan to do is to have a lantern in the dark(initially off) perpendicular to a wall, two sensors(one closest to the lantern and the other closest to the wall), then turn on the light making sensor 1 go off as...
23. ### I Can an object fall into a black hole at faster that the speed of light?

I am wondering if an object can fall into a blackhole at faster that the speed of light. I have heard that the expansion of the universe can make distant galaxies appear to recede from one another at velocities faster than the speed of light.   Intuitively, this makes sense to me. I am...
24. ### Speed of Light when not in a vacuum?

In a vacuum the speed of light is a constant. What if its not in a vacuum? Okay, as always I'm confused. I'm very interested in physics but am a layman. Alright, so we know that time is relative to one's distance from a mass, so that as we go further from the Earth, let's say, time moves...
25. ### B What does the constancy of speed of light mean?

The second postulate says the speed of light is constant c independent of all inertial observers. Does it mean the speed of the wave front relative to the observer , that is, the relative speed between the wave front and the observer?
26. ### Maxwell's calculations on electromagnetism's speed = light speed

I'm studying history of electromagnetism. Here is my question regarding Maxwell's classical brilliant work. I understand these constants (for now) And I understand this: And I understand this: But I need to understand how this math below gets us to the speed of light. I don't understand...
27. ### B Speed of light during acceleration

I'm a bit confused as to why the speed of light changes in an accelerating ship relative to an onboard observer. In other words, on a ship with a clock at the nose and a clock and observer at the tail, in an accelerating ship, the clock at the nose will tic faster. The reason (according to a...
28. ### B Can an observer perceive he is traveling faster than light?

I have seen thought problems with an observer on a train or in a station, etc., but I have not seen ones with the observer travelling at relativistic speeds. It seems to me that at sufficient speed he would observe himself exceeding the speed of light due to the slowing of time. This seems...
29. ### B Why is the speed of light a constant?

Why is the speed of light a constant?
30. ### B How can any physical body truly be at rest?

Can we truly have a rest frame or should it be a close to rest frame? Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity. The earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph. I'm never in a single...
31. ### B Does the size of the Universe change with motion?

This is a question I was looking at based on Relativity and John Wheeler's one-electron universe theory. https://en.wikipedia.org/wiki/One-electron_universe My question is this. The faster you move towards the speed of light, wouldn't everything in the universe contract to a single particle...
32. ### B What would happen if the speed of light were different?

if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
33. ### B Speed of light circling around a black hole

Let us imagine a photon circling around a black hole, as the picture shows. The gravity of the black hole curves the movement path of the photon into the shape of a circle. From point 0, geometric points A and B appear simultaneously with the photon, each in its own direction. The points travel...
34. ### I Consequences of light moving in a medium

I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM...
35. ### A General relativity: time dilation and speed of light

Hello everyone, I'll go straight to the question. The gravitational time dilation is equal to tearth = tspace*sqrt(1 - rs/r), with rs = 2GM/c2. However, the formula for speed of light in gravitational field is equal to v = c(1 - rs/r). My intuition tells me that these two formulas must be the...
36. ### B How can the speed of light be constant for all observers?

A rocket is in constant velocity. The velocity of the rocket is 150Mm/s (or 0.5 of the speed light, or 150 million meters per second) relative to us (we as observer). We observe two lights, one moving in parallell with the rocket, another is moving in the opposite direction. Below I have made...
37. ### B Mass of photon and FTL travel

Really silly question, but if we assume that our current science is correct, is it plausible that we can move faster than light in a vacuum? Say, for example, can we make the mass of something less than a photon so it then can it move faster than light in a vacuum. I know this sounds like a...
38. ### I Speed of gravitational waves vs Universe's expansion rate

Is there any relationship between the Speed of gravitational waves and the Universe's "local" expansion rate? Speed of gravitational waves is supposed to be equal to the speed of light. Gravitational waves don't travel faster than light. But we can observe far galaxies moving away from us with...
39. ### I Speed of Light and magnetic fields

If the speed of light is ~300 m/s in a vacuum, can the electric and magnetic fields of the wave be condensed such that it travels faster? The idea being that an outside force condenses these fields and lenghtens the wavelenght with no loss of energy thus increasing the directional speed. The...
40. ### B How does vibration affect the Higgs field interaction?

I have been thinking about the Higgs field and how things interacts with it. I thought about why photons for example does not interact with it while quarks does. It could be because of the size and if when a particle is a certain size it will interact with the Higgs bosons and then slow down and...
41. ### I How does the speed of light affect expansion acceleration?

I may have a fundamental misunderstanding of the concept, but I was wondering, how does the accelerating expansion of the universe calculate for the time dilation in light travel? From my understanding, we know that the universe expansion is accelerating because the farthest galaxies that we...
42. A

### B Telescope and Stopwatch for the Mass of a Planet

I was wondering if it is possible to use only a stopwatch and a telescope to find the mass of a planet, such as Saturn. I've experimented with a couple of things but I keep running into problems. I previously asked this question in the homework section, but it does not involve numbers, is not...
43. A

### Finding the Mass of Saturn

Homework Statement Using only a telescope and a stopwatch, find the mass of Saturn.[/B] (This question may or may not make any sense at all, it was a theoretical lab that my professor said without giving us a chance to copy it down and I am trying to recall the question from memory) If it is...
44. J

### I Speed of light on a moving platform

1. We have a moving platform traveling 700 miles per hour to the Right-Direction. 2. In the middle of the platform, we have a gun which fires a laser both at the front and back of the platform at the same time. 3. We have at both the front and back of the platform, a mechanical device. These...
45. ### I Why do people keep saying photons are timeless?

If you search for "does a photon experience time", almost every other link says that they travel at the speed of light and so STR tells us that its clock doesn't tick at all. However why do they use the arguments for special relativity which was developed for massive particles moving close to...
46. T

### I Really parabolic light ray in an accelerated elevator?

In an elevator "vertically" accelerated at g in outer space, the equivalence principle says a "horizontal" light ray in the elevator looks like a parabola. I completely understand that the light ray is curved but don't understand why the deflected light ray is an exactly parabola. Almost all...
47. ### I Gravity as geometry vs gravity traveling at c

Hi, I am seeking to understand better how this well accepted idea: "....according to general relativity, gravity is a manifestation of the geometry of spacetime." (https://en.wikipedia.org/wiki/Loop_quantum_gravity) is compatible with the equally well accepted idea that gravity travels at the...
48. M

### Effect of electrical permittivity on the speed of light?

Why does an increased electrical permittivity reduce the phase velocity of light in a medium? Furthermore, what interactions do we see on an atomic level? I am aware of the equation that defines the speed of light in terms of the electrical permittivity and magnetic permeability, but I do not...
49. A

### B Time Travel is Impossible

I just wanted to share this idea with other people who may be interested: I watched a PBS Space Time episode about the speed of light more accurately described as the speed of causality. And I Submit! That "time" is simply a interaction between atoms. And though an interaction can be...
50. ### B Photon producing Electron-Positron pair's effect on c.

Hello all, disclaimer here, I haven't taken quantum mechanics yet, all I know of it comes form books and PBS Space Time Series. Since a photon can, at any given instant, "split" into an Electron-Positron pair, does it mean that if this occurs between point a and b, the measured speed o that...