How Does Light Behave on a Rotating Disc in Different Reference Frames?

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Discussion Overview

The discussion revolves around the behavior of light on a rotating disc as perceived from different reference frames. Participants explore the implications of special relativity and the effects of rotation on light's path, drawing parallels to thought experiments involving inertial and non-inertial frames.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes a thought experiment where a laser beam is sent through the center of a rotating disc, suggesting that to an observer on the disc, the light appears to pass through the center, while an external observer sees it avoiding the center.
  • Another participant argues that light travels in straight lines at constant speed only in inertial frames, indicating that the initial inference about the light's path from the disc's perspective is incorrect due to the effects of rotation.
  • A participant introduces a separate thought experiment by Einstein involving an elevator accelerating upwards, questioning its relevance to the discussion on light behavior in rotating frames.
  • The same participant later clarifies that the elevator thought experiment illustrates the equivalence principle, linking acceleration in deep space to gravitational effects.
  • One participant expresses understanding after the clarification, indicating that the discussion has provided some clarity on the topic.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of light in rotating frames, with no consensus reached on the implications of the thought experiments discussed. The discussion remains unresolved regarding the exact nature of light's path as perceived from different reference frames.

Contextual Notes

Participants acknowledge the complexity of reference frames and the effects of acceleration, but do not resolve the mathematical or conceptual intricacies involved in the discussion.

jkg0
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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?
 
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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:
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.
 
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.
 
cragar said:
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.
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
 
Last edited by a moderator:
i see you that clears it up.
 

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