# 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 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?

sophiecentaur
Gold Member
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.

However, I've also read that light speed is constant everywhere

That applies to plane light waves in vacuum.

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?

George Jones
Staff Emeritus
Gold Member
sophiecentaur
Gold Member
Is it because the object is more dense and therefore space-time is more warped and so it takes longer for light to travel
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.

It isn't a Gravitational /GR effect

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

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

FactChecker
Gold Member
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).
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.

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Aight sfe.

sophiecentaur
Gold Member
Aight sfe.
I had to look that one up. turns out it probably wasn't a typo.

Redbelly98
Staff Emeritus
Homework Helper
And I believe it would be the same through the same material in any inertial reference space.
I don't think that's right. Wouldn't it follow the usual formulas for transforming velocity between different reference frames?

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

DrGreg
Gold Member
And I believe it would be the same through the same material in any inertial reference space.
I don't think that's right. Wouldn't it follow the usual formulas for transforming velocity between different reference frames?
Yes, as experimentally confirmed by Fizeau in 1851 (approximately, for low speeds).

DrGreg
Gold Member
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.
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.

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.

Only if the medium is homogeneous and isotropic.

DrGreg
Gold Member
Only if the medium is homogeneous and isotropic.
Yes, I was assuming that, too.

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

Svein
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?
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.

Light always travels at light speed.

That applies to plane waves but light waves don't need to be plane.

Svein
That applies to plane waves but light waves don't need to be plane.
Actually, I was stating a tautology (of course light is travelling at light speed - by definition). The problem is to relate "light speed" to other speeds and measurement systems.

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?

D. Giovannini, J. Romero, V. Potoček, G. Ferenczi, F. Speirits, S.M. Barnett, D. Faccio, M.J. Padgett,
Spatially structured photons that travel in free space slower than the speed of light,
Science 347 (2015) 857-860).

Actually, I was stating a tautology (of course light is travelling at light speed - by definition).

Actually, reality is not that simple.

• Buckleymanor
Actually, reality is not that simple.
Neither is a medium,show me a medium that is both homogeneous and isotropic and it will probably turn out to be a felt hat.

rude man
Homework Helper
Gold Member
D. Giovannini, J. Romero, V. Potoček, G. Ferenczi, F. Speirits, S.M. Barnett, D. Faccio, M.J. Padgett,
Spatially structured photons that travel in free space slower than the speed of light,
Science 347 (2015) 857-860).
At Harvard in 1998, the speed of light was slowed down to 38 miles per hour!

At Harvard in 1998, the speed of light was slowed down to 38 miles per hour!

But that was in an Bose-Einstein condensate and not in free space.

rude man
Homework Helper
Gold Member
But that was in an Bose-Einstein condensate and not in free space.
So?

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.
Every medium consists of 99,99999..... % vacuum. So, what happens when the light travels inside these "vast areas" of vacuum (as it travels inside this material)??? Is its speed slower than c??? (And if yes then how is this possible?)

DrGreg