# Train Experiment in Rotation

1. Mar 13, 2009

### jkg0

Suppose one constructs a thought experiment similar to the famous light beam bouncing around a train car. In this experiment a laser and mirror are attached to the surface of a rotating disc such that the beam is sent through the center of the disc. From my understanding of the train experiment, to an observer on the disc the light would appear to pass through the center of the disc.

But to an observer outside of the disc the light would have to move to hit the appropriate mirrors and would appear to avoid the center of the disc.

Is this a reasonable understanding? Or do the reference frames vary by radius from the center of the disc such that on the edge of the disc the beam would appear to move through the center but as one approached the center the beam would appear to avoid it?

2. Mar 13, 2009

### JesseM

Light is only constrained to travel in straight lines at constant speed c in inertial frames (the rest frames of observers moving at constant speed and direction--any change in speed or direction is a form of acceleration, and the observer will know he's accelerating because he experiences G-forces, like the 'centrifugal force' felt by rotating observers). In a rotating frame this is no longer the case, so your inference here is not correct:

3. Mar 14, 2009

### cragar

k I don’t know this might be a little off topic But isn't there a thought experiment by Einstein He imagines and elevator going up really fast and a hole is drilled in the side of the
elevator and the light coming in hits lower on the other side or appears to bend.

4. Mar 14, 2009

### JesseM

Yes, this is a thought-experiment showing the equivalence between being at constant height in a gravitational field and accelerating in deep space (or equivalently, between freefall in a gravitational field and inertial motion in deep space). See http://www.einstein-online.info/en/spotlights/equivalence_principle/index.html:

http://www.phy.syr.edu/courses/modules/LIGHTCONE/anim/equv-m.gif [Broken]

Last edited by a moderator: May 4, 2017
5. Mar 14, 2009

### cragar

i see ya that clears it up.