There's a sci-fi concept of slow light that I find very amazing:

Imagine a glass material that has index of refraction $n$ say, $3,000,000,000$ which means:

$$v_{glass} = \frac{c_{vacuum}}{n} = 0.0001 ms^{-1} = 0.1 mm\ s^{-1}$$

Simply a glass material that reduces the light propagation speed to $0.1\ mm/s$. A glass that shows what happened 10 seconds ago if it's 1 centimetre thick.

I was thinking about this and I happened to imagine a cube made of the same material, $1\times 1$ meter. Now imagine pointing a laser pointer in the cube. It's going to take $00:16:40$ for the beam to pass.

Now the question: If you move the cube around or rotate it, will you alter the beam rotation?
My guess is yes, but quantum physics doesn't always follow the rules of common sense.

I want to point out that no matter what the answer is, it's not only matter of fictional cubes but the real world too - flowing water in the river, blowing air...

  • $\begingroup$ I believe so, yes. I won't make an answer to this since I could very well be wrong, but I believe it would. I appreciate that you used "light propagation speed" as opposed to "speed of light" since, of course, the photons themselves are not moving any more quickly or slowly. At any rate, I believe, yes, it would, indeed, continue in its original path relative to the cube. i.e. from top view: (1) shoot laser into left side of cube, (2)rotate it 1/2 turn, (3) the laser is now shooting out of the right side. It is important to note that once it exits the medium, it moves at $c$. $\endgroup$
    – Goodies
    Nov 19 '14 at 23:12
  • $\begingroup$ Imagine this as a cool sci-fi weapon... :) $\endgroup$ Nov 19 '14 at 23:13

Yes, you will alter the beam direction, but not by the same amount as you rotated the cube.

The reason I think this is what happens is as follows. Light can interact with phonons ('quanta of movement in the material'). For example, acousto-optic modulators make explicit use of this coupling to change the direction of an incoming laser beam. They are often used in the lab.

So, movement of the material is coupled to the movement of the photon. (This coupling might be strong or weak depending on the material). Since there is limited time for the movement of the material to be transfered to the change in direction of the photon, it will not rotate the beam by the same angle as you rotated the cube. There will be a certain rate at which rotation of the cube will be transfered to rotation of the photons (depending on the strength of the coupling). When you have very strong coupling and enough time for the rotation of the material to be transfered to the rotation of the photons, the rotation of the photons might approach the rotation of the cube.

  • $\begingroup$ This sounds reliable. What if you move cube around. Is it any different from rotating? $\endgroup$ Nov 20 '14 at 0:05
  • 1
    $\begingroup$ Acousto optical modulators work due to the fact, that the acoustic wave induces changes in the refractive index. So we have waves of different refractive index moving through the crystal. There is nothing corresponding to this in the original question. There refractive index is homogeneous and constant. $\endgroup$
    – Andreas H.
    Nov 20 '14 at 0:14
  • $\begingroup$ Movement of the material will induce a lot of incoherent phonons in the material. Imagine having a very very long rod consisting of millions of tiny spheres attached to each other by springs. If one suddenly moves one end of the rod, this movement will travel trough the material as a wave (more precisely a lot of waves with different wavelength), thus changing the refractive index from place to place. The same thing happens to the cube, but we now have a rotation. $\endgroup$
    – Georg
    Nov 20 '14 at 0:23
  • $\begingroup$ But will there be a net effect? With rotation one part moves to the right and the other to the left. $\endgroup$
    – Andreas H.
    Nov 20 '14 at 0:28
  • $\begingroup$ I unaccepted this answer because there is no backing to the proof. Regarding acousto optical modulators, @AndreasH. has correctly pointed out that they are at least partially irrelevant. $\endgroup$ Oct 3 '16 at 21:32

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