# What are the characteristics of light entering a disk spinning at near $c$?

If I were to spin a translucent disk so that the edge is spinning at .9c and shoot a laser beam at it perpendicular to the edge, what would happen to the light as it travels in one end of the disc and out the other?

I would expect the light would become strongly redshifted as it enters the medium and then when it passes the center it would blue shift and escape the disk at the same wavelength it entered in.

Would the wave lengths change due to gravitational blue/red shift (due to the artificial gravity nature of a spinning disc), or would this all be tied to a doppler shift? Or both?

• Please add a diagram. In particular, two points to make clear are (1) is the laser is at rest relative to the narrator (who lives in a flat Minkowski space) and (2) is the laser directed radially, or orthogonal to the plane of the disk? – Selene Routley Aug 31 '15 at 6:14

Light is an emergent phenomenon from the confluence of innumerable individual photons with the energy $$h\nu$$, where $$\nu$$ is the frequency of the emergent macroscopically beam.

The question then reduces imo to "does a photon see/interact with a rotating "transparent" disk the same way it does with a gravitational field".

Checking on the special relativity part first: What does transparency mean to a single photon?

A disk rotating at such speeds will no longer be transparent to the photon, imo, as the probability of finding an available quantum state will be very high (depending on the radius of the disk) but the quantum states of the molecules composing the disc will also be stressed to new quantum mechanical solutions.

I agree with this exposition about the behavior/transformation of solid bodies at relativistic speeds.

So the disk will be non transparent to light, as it is for photons, just from the effects of special relativity.

As far as general relativity goes the main question is, "does the accelerated motion of the disk distort space time within it in a graduated manner"?.

The answer should be yes, in a graduated manner, as all predictions of the mathematics of General Relativity have been validated up to now. It can not be tested with photons though even in gedanken experiments. One could try to think of one with neutrinos, but not in the set up described.

I'm going to quote bits of the question out of sequence.

Would the wave lengths change due to gravitational blue/red shift (due to the artificial gravity nature of a spinning disc)?

The frequency of a light beam moving down or up in a gravitational field doesn't change. Imagine you have a laser at the top of a tower, shining down. You measure the frequency at the top, and you measure the frequency at the bottom. The latter is greater than the former, but that's because your clocks are running slower when you're lower, because of gravitational time dilation. The light hasn't changed, you have. You can check this out for yourself by imagining you send a 511keV photon into a black hole. Conservation of energy applies. The black hole mass increase is 511keV/c². The descending photon didn't gain any energy. Since E=hf it didn't increase in frequency either.

or would this all be tied to a doppler shift?

Imagine we're in gravity-free space and I shine the laser at you. You measure the frequency, then you move towards me at some rapid speed, and measure the frequency again. You now measure a greater frequency. But again the light hasn't changed, instead you have.

If I were to spin a translucent disk so that the edge is spinning at .9c and shoot a laser beam at it perpendicular to the edge, what would happen to the light as it travels in one end of the disc and out the other?

We can probably simplify this by asking what happens to light if it's shone perpendicularly at a long glass block moving at 0.9c. And then we could liken that to you crossing a road. If the cars are stationary you cross the road no problem. But if the cars are moving at 90mph, you don't. So like anna v, I think the disk will be non transparent to light.

I would expect the light would become strongly redshifted as it enters the medium and then when it passes the center it would blue shift and escape the disk at the same wavelength it entered in.

I think you're looking at this wrong I'm afraid. Let's forget the above and say the light can go through the spinning glass disk. When it enters the medium the atoms/electrons are sweeping past it perpendicularly from left to right at 0.9c. As it approaches the centre this sweep diminishes to zero. As it departs the centre the sweep increases from zero, but going the other way. Then as the light exits the other edge the atoms/electrons are sweeping past it perpendicularly from right to left at 0.9c. But apologies if I've misunderstood the scenario here.