# I Why does relativity not affect the speed of light?

1. May 10, 2017

### parshyaa

Why speed of light is independent of frame of reference,why is it constant everywhere, speed of an object is different from different FOR then why this is not follwed by light, In deep space there is nothing to measure the speed of light relatively, then how it got its speed(299 792 458 m/s)
Please try to explain it in layman's term, becuause i haven't read special relativity or general relativity.

2. May 10, 2017

### Staff: Mentor

The invariance of the speed of light is the cause of relativity, not an effect.

Why it is everywhere the same? Because the universe has evolved this way.
How can we know? Because we measured it in various situations. I cannot list them, but a google search will probably do.

3. May 10, 2017

### Staff: Mentor

Physics doesn't answer such "why" questions. It appeared from experiments that all observers measure the same speed of light, whatever they speed or the speed of the light source. Using this as a given (a postulate), Einstein derived special relativity, and found that this leads to time dilation and length contraction. As @fresh_42 just wrote, these are consequences of the theory, the constant speed of light being the "cause."

4. May 10, 2017

### Staff: Mentor

It is possible to derive from some basic assumptions (homogeneity, isotropy, etc) that there are only two possibilities: either the invariant speed is finite (Einsteins relativity) or it is infinite (Galileos relativity). It is just a matter of experiment to determine which is correct.

5. May 10, 2017

### davidge

It's worth remembering that invariance of the speed of light is observed in inertial frames, namely those in which the system acceleration vanishes. So if you don't have a intertial frame, there's nothing wrong with varying light speed.

6. May 10, 2017

### DrGreg

The relativistically correct formula for "adding" velocities (when you measure in a different frame) is$$\frac{u+v}{1+ \frac{uv}{c^2}}$$When $v$ is $\pm c$, the answer is still $\pm c$

7. May 11, 2017

### strangerep

Although that's the historical "2-postulate" approach, the modern "1-postulate" approach has it the other way around.
See @Dale's post #4.

8. May 13, 2017

### parshyaa

Okk,We measure speed of anything with respect to something(ex: book on a table is at rest with respect to earth but it is in motion with respect to moon), soo when light is in deep space(nothing to compare speed with) how can we measure its speed.
As you said speed of light is invariant, but above definition of speed is getting violated,we are not measuring it w.r.t something.

9. May 13, 2017

### Staff: Mentor

If you have a device to measure the speed of light then you can always use the device's frame if you wish, even in deep space.

If you do not have a device to measure the speed then you cannot measure the speed regardless of if it is in deep space or not.

10. May 13, 2017

### jbriggs444

A violation of theory would be an experiment producing a result that contradicts the theory. The lack of an experiment can never result in a violation.

11. May 17, 2017

### ImStein

...is essentially a vacuum and a vacuum has properties! James Clerk Maxwell was able to calculate the speed of light from the properties of a vacuum in manifesting electric and magnetic fields. From this, he deduced (at Michael Faraday's prior suggestion) that light is an electromagnetic wave propagating at speed c. This matches the best measurements of light speed.

It is reasonable to surmise that if speed limit c (in a vacuum), is "universal" (i.e. the same everywhere in the universe) that it derives from the underlying structure of the universe. So far, from observations of distant objects, there is every reason to believe speed limit c is universal.

12. May 17, 2017

### Staff: Mentor

But again, all of that depends on your choice of units. The universality of c is tautological in SI units.

13. May 17, 2017

### Mister T

When we say the speed of light is the same for all observers what we mean is that if you measure it relative to something, you'll always get the same value.

14. May 17, 2017

### parshyaa

Yes and my question was how do we know that it is invariant(ie: its speed is constant relative to every FOR)

15. May 17, 2017

### Staff: Mentor

It's been measured a whole bunch of times in a whole bunch of different frames of reference (both directly and indirectly). Obviously it is inherently impossible to do any experiment everywhere, but "everywhere we have tried it" is a good enough reason to believe it is actually constant everywhere.

16. May 17, 2017

### parshyaa

You mean Just like law of conservation of mass is followed by many chemicals therefore we accept it as a law, there's no proof for that, we tested it with so many chemicals and we always got same results.

17. May 17, 2017

### Staff: Mentor

I'm not sure how you are using the word "proof", but otherwise yeah, if we do a bunch of experiments and always get the same results, we conclude the theory is valid.

18. May 17, 2017

### Staff: Mentor

Also:
I'm not sure what significance you place on "deep space" or why (all it means is we aren't there, which is kinda self-defeating), but I suspect there has been more testing of Relativity in space (deep or otherwise) than you realize. Perhaps the "deepest" is gravitational lensing, which has been observed over billions of light years (as well as within our own galaxy).

19. May 17, 2017

### Staff: Mentor

You may want to read about Bayes' theorem and how it applies to making inductive inferences based on prior knowledge and new observations. It explains why we don't include unnecessary parameters in a model.

20. May 17, 2017

### Thecla

Because of Galilean(or Newtonian ) Relativity, if you are in a train moving at constant velocity, there is no experiment that you can perform totally within the train that will tell you if the train is moving. With Maxwell and his equations in the middle of the nineteenth century(and the supposed existence of the ether) there was a means to measure the motion of the train. Einstein restored the old 17th century fact: You can not tell if the train is moving by an experiment inside the train. The constant measurement of the speed of light in all inertial frames is what saves the "you can't tell if the train is moving" phenomenon.