# Slowing down light

## Main Question or Discussion Point

When light is greatly slowed via a medium in laboratories, is it still constant among reference frames?

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Gold Member
When light is greatly slowed via a medium in laboratories
What kind of mediums? Glass?

Nugatory
Mentor
When light is greatly slowed via a medium in laboratories, is it still constant among reference frames?
No.
There's no violation of the principle of relativity here, because the slowing is caused by the light interacting with the atoms of the medium (for example, it's being absorbed and reemitted by each atom, with a frame-dependent delay between the absorption and reemission) which are moving at different speeds in the different frames. In particular, Maxwell's equations do not predict the same speed for light in a moving medium and a stationary medium. This is very different from the prediction for the speed of light in a vacuum, and is vaguely analogous to the speed of sound in air.

No.
There's no violation of the principle of relativity here, because the slowing is caused by the light interacting with the atoms of the medium (for example, it's being absorbed and reemitted by each atom, with a frame-dependent delay between the absorption and reemission) which are moving at different speeds in the different frames. In particular, Maxwell's equations do not predict the same speed for light in a moving medium and a stationary medium. This is very different from the prediction for the speed of light in a vacuum, and is vaguely analogous to the speed of sound in air.
Alright, so would it be correct to say that the constant speed of light in a vacuum is as much an inherent property of the speed itself as a property of a massless particle? Or is such a statement not-needed because a particle will always travel at the SoL in a vacuum if and only if it is a massless particle, thus making the two synonymous in ways?

I suspect that in this medium, light is only "slowed" in that it is made to "travel more distance" through the absorption and reemission. Thus the "speed" that this light beam is perceived to go is not literally a speed of light in this medium, just the speed of the average collective motion of light as it is constantly being redirected and thrown around through this medium. Is that true?

Bill_K
I suspect that in this medium, light is only "slowed" in that it is made to "travel more distance" through the absorption and reemission. Thus the "speed" that this light beam is perceived to go is not literally a speed of light in this medium, just the speed of the average collective motion of light as it is constantly being redirected and thrown around through this medium. Is that true?
No. And Nugatory's claim that it is due to "absorption and re-emission with a delay" is not true either. Light propagation in a medium is a complex many-body interaction between the electromagnetic field and the electrons in the medium. It is a classical effect, and cannot be easily described in terms of the behavior of individual photons.

For example, at the leading edge of a light pulse there is a transient, a precursor. Initially the propagation is into a quiescent medium and travels at the speed of light in vacuum, c. Only after the oscillation of the electrons has reached a steady state does the main body of the wave travel at the speed indicated by the index of refraction.

Nugatory
Mentor
No. And Nugatory's claim that it is due to "absorption and re-emission with a delay" is not true either. Light propagation in a medium is a complex many-body interaction between the electromagnetic field and the electrons in the medium. It is a classical effect, and cannot be easily described in terms of the behavior of individual photons.
I did say "for example" - that's just one of many interactions that make this situation very different than light propagation in a vacuum.

DrGreg
Gold Member
We have an FAQ on this: https://www.physicsforums.com/showthread.php?t=511177 [Broken]

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We have an FAQ on this: https://www.physicsforums.com/showthread.php?t=511177 [Broken]
One would note that my question in the OP is not about that, though.

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DrGreg
The point is that individual photons don't slow down; they still travel at c, the speed of light in vacuum. But the overall progress of the light does slow down, and its speed varies according to the velocity addition law$$\frac{u+v}{1+uv/c^2}$$where $u=c/n$ is the velocity of light relative to the medium and $v$ is the velocity of the medium relative to the observer (assuming both velocities are parallel to each other; there's a more complicated formula if they're not).
The point is that individual photons don't slow down; they still travel at c, the speed of light in vacuum. But the overall progress of the light does slow down, and its speed varies according to the velocity addition law$$\frac{u+v}{1+uv/c^2}$$where $u=c/n$ is the velocity of light relative to the medium and $v$ is the velocity of the medium relative to the observer (assuming both velocities are parallel to each other; there's a more complicated formula if they're not).