# Does light always travel at light speed?

• Zahidur

#### 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?

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 traveling 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?

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)?

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.

Oh right, I just thought they contradicted because if light slows down in other objects then it is no longer traveling 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.

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

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?

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.

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).

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.

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

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

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.

That applies to plane waves but light waves don't need to be plane.
Actually, I was stating a tautology (of course light is traveling 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 traveling 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.

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.

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?)

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?)
See the link in post #5.

See the link in post #5.
I have already read that. However, it doesn't say anything about that. Let's consider the following: Suppose that you have a "transmitter" that emits a photon and a "receiver" that receives this photon, and that the "transmitter" and "receiver" are both located in the vacuum and very close to each other (at a distance similar to the distance between the atoms of a medium). (This "transmitter" and "receiver" could be two atoms of a gas -one transmitting a photon and the other receiving this photon- which happens to be so close to each other.) What is the speed of light in this case? As the photon travels inside the vacuum, it's speed should be c (no matter how short its travel is). If this is correct, then what is the difference between this case and the travel of light inside the "vacuum areas" (i.e. between the atoms) of a medium?

what is the difference between this case and the travel of light inside the "vacuum areas" (i.e. between the atoms) of a medium?
I would say that you are mixing your models up. Photons don't travel from molecule to molecule. The propagation is in the form of waves and individual photons cannot be regarded as 'being' anywhere at any given time. It has been said many times before but you cannot treat photons as little bullets. They are essentially only 'there' when they interact. If the whole wave is being involved than where can you say the individual photons are? Stick a tiny light sensor in the middle of the medium and you can say that, when it 'sees' light, it has interacted with a particular few photons - defining where they are at the time.
It is not a good idea to try to bend the accepted terms to your own model.

I would say that you are mixing your models up. Photons don't travel from molecule to molecule. The propagation is in the form of waves and individual photons cannot be regarded as 'being' anywhere at any given time. It has been said many times before but you cannot treat photons as little bullets. They are essentially only 'there' when they interact. If the whole wave is being involved than where can you say the individual photons are? Stick a tiny light sensor in the middle of the medium and you can say that, when it 'sees' light, it has interacted with a particular few photons - defining where they are at the time.
It is not a good idea to try to bend the accepted terms to your own model.
Ok, I agree that I should not use the term photon. But actually the question remains. In my example, one atom of the gas emits light (ok, as a wave) and another nearby atom of the gas receives this light (yes, as a wave). What is the speed of light in this case? I suppose that the speed must be c, no matter how close to each other these atoms are (and that's because there is only vacuum between them). Am I right?

Ok, I agree that I should not use the term photon. But actually the question remains. In my example, one atom of the gas emits light (ok, as a wave) and another nearby atom of the gas receives this light (yes, as a wave). What is the speed of light in this case? I suppose that the speed must be c, no matter how close to each other these atoms are (and that's because there is only vacuum between them). Am I right?
I'm going to throw something else in, here. You are talking about "light" but any model you come up with should also be applicable to 1500m long waves and the shortest of gamma waves.
In a 'condensed' substance, the spacing between atoms / molecules will probably be less than one wavelength of visible light. So it is not realistic to talk in terms of launching a light wave (or photon, if you must) and then it arriving at another atom. (That would be like what happens in a low density gas, where the effect is random scattering from individual atoms.) The fields around each atom (and all the other nearby atoms) will all be involved in setting up the energy levels and the transitions. The model to use is much more classical - coupled oscillators or a transmission line with masses on springs. The 'space' in between (where you could say that c applies) is only part of it; it's the interaction between the charge systems along the line that counts. The contribution to the delay along each step the journey is, of course, affected by the 'c' delay of the fields but each atom is also contributing a significant delay as it reacts with the fields around it and the fields of its neighbouring atoms. This 'loads' the line.
RF model:
If you take a line of parallel dipoles (say each one is 1m long) and you feed the first one with a 150MHz RF signal, that signal will propagate along the line. Some of the power will couple with the each dipole- they each have an equivalent cross section- but slower than c because each dipole introduces a phase shift as the currents flowing will cause a re-radiated signal that lags behind the incoming signal. The signal that emerges from the other end will be the net sum of the incident signal and each of the re-radiated signals. But there's something else here. Each element will be interacting with its neighbours (depending on the spacing). Appropriate spacing can produce a Null in the emerging RF wave in the direction of the line.
In a solid, the system is three dimensional but the same principle is at work and the majority of the power will travel in a straight line.

In a 'condensed' substance, the spacing between atoms / molecules will probably be less than one wavelength of visible light.

It is definitely so.
The typical spacing in a solid is of the order of angstroms or at most tens of angstroms.
The wavelength of visible light is of the order of a micron. 3 to 4 orders of magnitude larger than the spacing.
So even the notion of propagation between two neighboring atoms is not so clear, at this scale.

I think the image of the solid being composed of mostly empty space is not such a good idea. It gives some image about the smallness of the atomic nucleus, at some elementary level. But this space (in solids) is filled by electrons which may have no volume but have strong electric fields. So propagating thorough this "vast" empty space is nothing like going through vacuum. Most of the time, the spacing in the lattice is very close to the ionic or atomic diameter, no matter how you define these.