# I Does light always travel at light speed?

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1. Apr 29, 2016

### Zahidur

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 speed of light then you would experience warped space-time so light would still travel at light speed relative to you). So which is it or are both these ideas not the whole story?

2. Apr 29, 2016

### sophiecentaur

The speed of light in a vacuum is c. It is reckoned to be the same wherever that region of vacuum is. It travels slower everywhere else. I don't think that is a pair of contradictory statements.

3. Apr 29, 2016

### DrStupid

That applies to plane light waves in vacuum.

4. Apr 29, 2016

### Zahidur

Oh right, I just thought they contradicted because if light slows down in other objects then it is no longer travelling at light speed (c) but at some lower speed. So light isn't the same speed everywhere (I now get that it's only the same in a vacuum). I know that change in direction in the more dense medium occurs due to the speed change, but why does light slow down in the more dense material. Is it because the object is more dense and therefore space-time is more warped and so it takes longer for light to travel throughout that object or because of some other reason?

5. Apr 29, 2016

### George Jones

Staff Emeritus
6. Apr 29, 2016

### sophiecentaur

No. It isn't a Gravitational /GR effect; it's an electromagnetic effect. Dense materials have more densely packed charges which interact with an EM wave going through.

7. Apr 29, 2016

### Zahidur

8. Apr 29, 2016

### Zahidur

So, is the gravitational effect on the photon too insignificant to be considered (relative to the effect of the electromagnetic force)?

9. Apr 29, 2016

### sophiecentaur

Of course. How would a low mass piece of glass hope to slow light down to 0.6c by relativistic effects?
The Refractive Index of a material is to do with the arrangement of charges. This was explained long before GR came on the scene.

10. Apr 29, 2016

### FactChecker

A more precise statement would be "light isn't the same speed through all materials". It is the same in any inertial space through a vacuum. And I believe it would be the same through the same material in any inertial reference space.

Last edited: Apr 29, 2016
11. Apr 29, 2016

### Zahidur

Aight sfe.

12. Apr 30, 2016

### sophiecentaur

I had to look that one up. turns out it probably wasn't a typo.

13. Apr 30, 2016

### Redbelly98

Staff Emeritus
I don't think that's right. Wouldn't it follow the usual formulas for transforming velocity between different reference frames?

14. Apr 30, 2016

### FactChecker

I was thinking that there should be no way for any inertial frame to detect an effect of its motion. So measuring the speed of light through any material would be the same as if it was stationary. That is what I meant to say. I think that must be right.

15. Apr 30, 2016

### DrGreg

Yes, as experimentally confirmed by Fizeau in 1851 (approximately, for low speeds).

16. Apr 30, 2016

### DrGreg

If you mean the speed of light through a given material relative to an inertial frame in which the material is at rest, then, yes, that will be constant.

17. Apr 30, 2016

### DrStupid

Only if the medium is homogeneous and isotropic.

18. Apr 30, 2016

### DrGreg

Yes, I was assuming that, too.

19. Apr 30, 2016

### FactChecker

Yes. That is what I meant: relative to an inertial frame in which the material is at rest

20. Apr 30, 2016

### Svein

Light always travels at light speed. But light speed is given by $c=\frac{1}{\sqrt{\epsilon\mu}}$ and thus varies with the medium i travels through. In vacuum, with $\epsilon =\epsilon_{0}$ and $\mu =\mu_{0}$, you get the often-cited value of c (or should we say c0) = 299792458 m/s.