# What is Light speed: Definition and 319 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. ### B Time Dilation

Just joined because I have been unable to find an explanation: please avoid comments about FTL speed since they don't pertain to the question. The question: A planet is 30 light years away, so when viewing it from earth we are seeing it from "30" years previous. Now, we travel 30 years at light...
2. ### B Light Speed In Forward and Backward Directions

In the illustration below, a laser beam is emitted, and the time recorded, from the rear of a frame which is 150,000 km long and is moving at velocity 0.5 c to the right. When the beam reaches a half silvered mirror at the front of the frame, the frame being shown as a rectangle and using...
3. ### I Speed faster than light speed in a medium

hello everyone I read this text in physics book: "Another example is the so-called Cerenkov radiation, which consists of light waves emitted by charged particles that move through a medium with a speed greater than the phase speed of light in that medium. The blue glow of the water that often...
4. ### B Problem about postulate of the invariance of the speed of light

Hello, I have a problem with the postulate of the invariance of the speed of light. When we move away from a light source it is redshift, it is the sign that the relative velocity between us and the light source has changed. If a stationary observer observes the phenomenon, he will measure that...
5. ### I Do objects gain mass while approaching light speed?

I watched a fermilab video claiming objects don't actually gain mass as they approach light speed. Is that true? What keeps things of mass from reaching the speed of light or beyond? I assume matter doesn't accumulate higgs-bosons while in motion?
6. ### A Calculate the group velocity in EIT (famous paper: light speed 17m/s)

hello everyone! Recently,i'm reading a paper about slow light,that's really a famous work published in Nature.[Light speed reduction to 17 metrespersecond in an ultracold atomicgas]. But I'm trouble with some calculation about the velocity of slow light.here are below: i try to use the...
7. ### I Light speed independent of source

Please bear with me I am trying to get a grip with underlying principles. Starting to try and understand Einstein’s second postulate and distinguish that the speed of light is independent of the speed of the source – v - of objects, other than light with travel initiated independent of the...
8. ### B Can You Become a Black Hole from Approaching Light Speed?

If you're in a relativistic starship, approaching the speed of light, then if you get too close to it, do you end up becoming a black hole? Relativistic length decreases as you get closer to light speed. Relativistic mass increases as you do the same. Will your relativistic mass and relativistic...
9. ### B Measuring Light Velocity Correctly: Thought Experiment

Let there be a track 450,000 km long and a rocket 300,000 km long with a laser attached to the bottom of it's back end with a clock beside it, and a second synchronized clock attached to bottom of its front end. Both clocks were also synchronized with a track clock while the rocket was parked...
10. ### I Cosmological Positioning - How does one determine?

Thought experiment I'm wondering if anyone has explored yet. Imagine by whatever means you like, that your spaceship plucks you out of our solar system and drops you at another random point in the observable universe. Due to light speed limitation, where ever you end up will be looking at our...
11. ### B Distance Length Contraction at Light Speed

I know it's not possible to travel at light speed so this is just theoretical. As I understand it at relativistic speeds the distance you need to cover to travel to a destination are length contracted. If you were traveling at light speed is the distance between you and any object ever zero?
12. ### B Does this experiment prove that light speed is actually faster?

Some people thought light speed is actually instantaneous which can be demonstrated by doing an experiment involving a taut clothesline and two clothespins. On each ends of the taut clothesline, put one of the clothespins on both ends and touch one of the clothespin will make the other one at...
13. ### B Do Gravitational Forces Increase for Objects Moving at Light Speed?

Since an object's apparent mass increases as it approaches the speed of light, does it's gravitational forces also increases? (From a stationary observer's point of view)
14. ### B Explanation of light speed limit

To an average person with high school math knowledge how would you explain in a few words why no object could travel faster than the speed of light ? Well it's because...
15. ### B Light Speed Invariance: Experiments, Difficulties & Clarification

Let me clarify my question, is there any experiment directly proved the invariance of light speed to observers? Let's not get to the argument of equivalence between source and observer. SR was based on the postulate that the light speed is constant and independent of both the motions of source...
16. ### I Understanding Relativity: How Moving Objects Experience Near Light Speed Travel

Light speed is impossible for anything with mass as more and more energy is required with increasing velocity. But this is only to an observer in a different reference frame. To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?
17. ### Light speed and refractive index

Refractive index of a medium is defined as : n = c / v; v is speed of light in medium. I believe n is never measured directly as here is no way to directly verify c / v. So what I guess is that all refractive index values are experimentally measured using n = sin α₁ / sin α₂. But then there is...
18. ### I Measuring One-Way Anisotropy of Light Speed

I've always thought that light could possibly be transmitted faster in one direction than the other depending on the velocity of its container - despite what the current understanding of physics says! The problem is that its very hard to prove, and to this end I've read through all the...
19. ### B Train traveling near light speed

A train is traveling around the Earth at just under light speed. Light would circle the Earth around 7 times per second so let's say this train cricles the Earth 6 times per second. There is a physical ticker on the track of the train that records revolutions. Each time the train makes one...
20. ### Thought Experiment about light speed

If we’re looking through a telescope at a craft we launched from Earth that is now passing Mars and send a radio signal to our craft telling it to turn on one of its lights on and it takes thirteen minutes for the radio waves to get from Earth to our craft how long will it take before we see the...
21. ### I Observing a rotating body when approaching it at near light speed

Summary: Will the motion around us become faster when we travel faster? When we approach a body rotating on its axis with certain speed v, will we see the body rotating in speed slightly more than the v during our motion ?And what happens assuming that we are approaching the same body in speed...
22. ### B Train platform -- light speed and time quandry

Summary: how can a train traveling at light speed, travel at two different speeds being the same train, depending upon whether you are on the train platform or in the train You say good bye to your friend at the train station. You get into train that will travel at just under light speed...
23. ### What Would Happen if the Sun Suddenly Disappeared?

I have a question I have been wondering about lately ,somewhat abstract . Hopefully my description will be adequate . Here it is … The Earth revolves around the sun because of the suns gravitational pull and Earth's inertia . It takes about 8 minutes for sunlight to reach the Earth . What would...
24. ### 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...
25. ### I Question about the limits of space contraction near light speed

Hi all. I have a question about something Nima Arkani-Hamed said in his lecture on space-time about space contraction near light speed. I included a link to the lecture at the point where he refers to contraction of two space ships with a 'cable' between them, they are accelerating towards the...
26. ### B Near Light Speed Runner: Will We See the Light?

hello, i am a near light runner at 3000000 km from us there is a mirror to start the run , my friend turns on a very powerful lamp and i start running toward the mirror me and my friend will see both the light coming back after 20 seconds?
27. ### Light Speed: Distance & Time Explained

Hello forums, I'm kind of new nice to meet you guys and girls, Could someone please explain this to me? D=S * TTherefore Distance = Speed of light * TimeAnd then Distance = Speed of light + 7 * TimeWhat is Distance and what is time?
28. ### I Traveling at Light Speed: Is a Photon Stationary?

If you were to travel alongside a train, as fast the train, to you the train would seem stationary. I read that if you were to travel along a photon of light, as fast as the speed of light, that photon would not seem stationary. Is this true? If so, why?
29. ### If light speed (not in a vacuum) was about human walking speed ....

If light speed (not in a vacuum) was about human walking speed ... Did any sci fi writer try that?
30. ### B Transformation of velocity exceeding light speed

In discussion with my friend, we reached a conclusion that transformation formula of velosity v to another IFR moving V, i.e. v'=\frac{v+V}{1+vV/c^2} is valid even if v is hypothetical velocity,i,e, v=\frac{x_2-x_1}{t_2-t_1} v'=\frac{x'_2-x'_1}{t'_2-t'_1} where interval of ##(t_1,x_1)\rightarrow...
31. ### Elasticity and light speed vibrations

Special relativity requires any substance to be compressible. Indeed, if an item were made of a perfectly rigid substance, then move one end of it, and the other end must move at the same moment - the movement must be transmitted instantly, faster than light. Thus, the special relativity sets...
32. ### Light speed travel (now "understanding the difference between weight and mass")

So an object unaffected by any gravity free floating in space has zero mass. Now could this object reach light speed. Or am I confused?
33. ### Mass, Light Speed, and the Mystery of Photons Explained

A particle without mass is supposed to travel at speed of light. But isn't it that a photon has no mass but has relativistic energy.. therefore why does it travel at speed of light.. or perhaps if a photon had no relativistic energy.. it would travel a bit faster than the current lightspeed?
34. ### I Measuring Speed of Light Relative to Earth

The question: If the max speed I can go in the universe is the speed of light measured from? Background: If there are three rocks in a vacuum along a line (the x axis), one is stationary (rock 1), one is moving 5 m/s in the x direction (rock 2) and one is move -5 m/s in the x...
35. ### I Measuring Light Speed Anisotropy

Sorry, me again. In another thread #47 I gave a experimental setup to detect an anisotropy in light speed of the form ##c(\hat{n}) = c_o + \alpha (\hat{n}\cdot\hat{v})##. It was pointed out quite correctly that the error incurred in the long arm to the sensor would cancel the anisotropy making...
36. ### Distortions in time when going near light speed

So I was wondering that why does time move slower the faster you get ? And does it happen at all or is this is just fake ?
37. K

### I Does the wave function shorten when approaching light speed?

Relative to the observer, objects shorten when approaching the speed of light exponentially. Does this rule also apply to the wave function? Does this rule also apply to massless particles like Photons? Or am I just simply forgetting something?
38. ### B Einstein's Epiphany of Constant Light Speed

How did Einstein first contemplate the idea that the speed of light was constant in all frames or reference? Did he say "I wonder what would happen if we considered light speed to be constant" in some kind of thought experiment. Did the concept fall serendipitously from the results of...
39. ### B Question: What Makes Light Move & Why Does It Travel at 299792458 mps?

what makes the light moving and why it moves with this speed 299792458 mps?
40. ### I LIGO and light speed constancy

From LIGO website re how gravity waves are measured: "The arrival times change because when the arms of the interferometer change lengths, so too do the distances the light waves travel before exiting the interferometer. What gravitational waves do not change, however, is the speed of light...
41. ### Faster than light communication

Imagine a small cylinder with a dot at the top. Rotating the cylinder 1 degree clockwise of 0 degrees could signify a 1; and 1 degree anticlockwise of 0 degrees a zero. 0 degrees would be a space. Someone rotating one end of the cylinder would cause the other end of the cylinder to rotate...
42. ### Faster than Light... Superluminal Group Velocity

If general relativity in the formal sense constrains all velocities to the speed of light as a maximum, how would superluminal group velocities exceeding speeds of light (at their superpositions) be evaluated in mainstream physics? Would this be a case of General Relativity and Physics...
43. ### Is light speed limit an artifact of the math of measurement

A delightful video here A lecturer derives special relativity in a world of bats and echo location using a bat (sound) clock. All communication is limited by the speed of sound. (so a parallel of our world and light speed communication) He ends up observing that in no cases for any observer...
44. ### I Black Hole Creation at Near Light Speed? - Prof. Andrea Ghez

Yesterday, I attended the following presentation by Prof. Andrea Ghez. https://www.meetup.com/physicists/events/236886090/?gj=co2&rv=co2 In the Q&A session towards the end, there was a question she fumbled to give a proper answer. She mentioned that as long as the mass is compressed into...
45. ### Does mass increase as velocity increases?

First let me see if I understand what mass is ...it's the measure of an objects ability to attract other masses , and also resist acceleration ... the two always come together and define the term "mass" ... there are no subdivisions in the term 'mass' ... no different kinds of mass . I was...
46. ### B Could gravity be used to send light speed signals?

So there's a spaceship 5 light years away from Earth and they want to send a signal to Earth but a cloud of interestellar dust don't let them use light or radio signals so they decide to send a gravity signal. So they produce a violent huge thermonuclear explosion that will send some particles...
47. ### B Is a Rocket at Light Speed Truly Infinite Mass?

I read some time ago a resume of the book the Crack In The Cosmic Egg and this is more or less what it told: When a rocket acquires light speed its mass becomes infinite which creates a gravity force field that crunches all the universe making it restart. Its argued that to propel a rocket to...
48. ### What propagates changes of Coulomb force?

Hi. Say a positive charge is at (x,y,z)=(0,0,0) at t<0 then (1,0,0) at t>0. Electric field at (x,y,z)=(l,0,0) is (a,0,0) at t<l/c then (b,0,0) at t>l/c thus a<b. With time delay of l/c, what propagates from (x,y,z)=(0,0,0) to (l,0,0) ? I do not think it is electromagnetic wave because it is...
49. ### Light Speed Q&A: Can Cameras Capture Light?

Hi, Are anyone here familiar with this? It shows a camera that can capture 1 trillion frames per second, and the camera can be used to record the speed of light. The other day, I saw an article when I was searching something on Google and it said a 4 trillion camera were created by Japanese...
50. ### I Time Dilation Between Astronaut A and Person B

Definitions: Astronaut is A Person on Earth is B A travels to a star far away at near light speed, A would see B's time dilate. B would also see A's time dilate Twin paradox revived: What would happen if A returns to B at a very slow speed? Then both frames of reference would see each others'...