# Light speed Definition and 26 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 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...
2. ### B Thought Experiment Showing Correct Way to Measure Light Velocity

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...
3. ### 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...
4. ### 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...
5. ### I Light's speed and relativity

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?
6. ### 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?
7. 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?
8. ### B Question about the light

what makes the light moving and why it moves with this speed 299792458 mps?
9. ### 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...
10. ### 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...
11. ### 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...
12. ### 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...

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'...
14. ### I Communication at a near-light speed

So, I've been wondering: how would time dilation affect communications? For the sake of visualisation, imagine the Flash is running at 99% the speed of light in a circle around a fixed position. There's a building in this position, and inside this building are his friends. Due to time dilation...
15. ### Does light always travel at light speed?

I've been told contradicting ideas about this. I've been told that light doesn't travel at a constant speed everywhere (i.e. light slowing down in speed after entering a more dense medium). However, I've also read that light speed is constant everywhere (i.e. if you could travel close to the...
16. ### B Speed of light lowered in vacuum

Hello ! A few days ago, I came across this article http://www.bbc.com/news/uk-scotland-glasgow-west-30944584 And I was wondering if it has any consequences with the relativity ? I mean, in my mind, light velocity is supposed to be constant in vaccum, c0 = 3e08 [m/s] approximatively... What...
17. ### Could we "catch" the light from prehistoric Earth and see dinosaurs?

My first question on your forum. I just found you last week and have spent a whole lot of time reading. My question: Not actually a question, but a supposition seeking confirmation. Were it possible to travel faster than light, could we "catch" the light from prehistoric Earth and see...
18. ### Proof of Lorentz transformation

The thought experiment used to prove Lorentz transform uses a light signal as an assumption. What if there was something other than the light signal then Lorentz transformation would have totally different term in place of 'c'(speed of light).
19. ### Are all massless particles "born" at light speed?

(I think) I know that massless particles can only exist traveling at c, but I find it somehow counter-intuitive (like many other real things... :D ) Would anyone please be so kind to confirm that, for instance, a gamma photon generated by the radioactive decay of a stationary isotope is already...
20. ### How can the observable universe be 46 billion lyrs in size?

I watched a BBC documentary that said that the observable universe is about 46 billion light years in size. How can this be if the age of the universe is 13.7 billion years (and nothing travels faster than the speed of light)?
21. ### Speed of light bothering me

Dear PF Forum, There's one thing still bothering me. The speed of light. Supposed two observer, A and B A is from the west, B from the east. Separated by 100 lys. B sends a signal, say, B1 to A. So B1 will be received by A in 100 years, right? Now, supposed B travels at 0.8c and at the distance...
22. ### Is comunication faster that light possible: yes( maybe )

i think it is possible but its too complicated to write down in english so i think comunication faster than light is possible without anything acctualy moving faster than light. If you had a big stick between Earth and the sun and you push the stick towards the sun for 1 inch it will instantly...
23. ### Light speed is not constant or accurate

Hi all, this my first forum ever. I'm a self taught physicist. So pardon my ignorance. I pose this question. If speed :s = distance :d ÷ by time :t Then how can we possibly measure our cosmos accurately with a measure of speed aka "Light Speed". The problem I see with this is TIME is a variable...
24. ### A new problem with starships

While the main problems of a functioning starship are commonly known such as energy requirements and radiation I have realized another problem when near the speed of light. The dynamic Casimir effect occurs when a reflecting object reaches relativistic velocities. Therefore a starship with a...
25. ### What do you mean, approaching the speed of light?

If velocity is relative and dependent on an observer then how does an isolated object "approach the speed of light"? Approaching the speed of light relative to what? Does the ubiquitous constant velocity/closed compartment analogy break down at relativistic speeds? If one were traveling "near...
26. ### Is the speed of light wrong?

Hi there. Just wondering if the speed of light is wrong for a 3 dimensional universe. As each photon of light traverses space its velocity is 299 792 458 m/s. And since each photon has a wavelength and amplitude, then the actual distance that each photon travels, depending on its wavelength and...