Question about the relative motion of an eye relative to light source

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Discussion Overview

The discussion revolves around the implications of the second postulate of relativity in the context of relative motion between an eye and a light source. Participants explore how the motion of the eye affects the time it takes for light to reach it, considering different scenarios of movement towards and away from the light source.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant proposes that if an eye moves towards a light source at a velocity v, the time for light to reach the eye is calculated as ##\frac {D} {c + v}##, suggesting the gap closes at a rate of ##c + v##.
  • Another participant agrees with the initial calculations but questions the relevance of these results to the broader implications of relativity.
  • There is a reiteration that the calculations provided are valid in a single frame of reference and do not contradict the postulates of relativity.
  • One participant notes that more complex physics, such as conservation laws or frame changes, would reveal differences in predictions compared to Newtonian physics.

Areas of Agreement / Disagreement

Participants generally agree on the validity of the calculations presented for the specific scenarios discussed. However, there is a lack of consensus on the implications of these results for the framework of relativity, with some participants questioning the relevance of the calculations to broader relativistic principles.

Contextual Notes

The discussion is limited to a specific scenario and does not address potential complexities that may arise in different frames of reference or under more complex conditions.

Chenkel
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Hello everyone,

I've been thinking about 2nd postulate of relativity, it seems that and the Michaelson-Morley experiment seems to imply that there is no ether, but I was thinking about a special situation that doesn't seem to go against that postulate.

I think my question is basic so hopefully it makes sense and I don't make any major blunders.

Suppose there is an initial light source at distance D away from an eye and the light turns on at the start of the problem, and the eye moves with a relative velocity of v towards the light source, how long does it take light to reach the eye?

I was thinking about it and the gap seems to be closing at a rate of ##c + v## So I would expect the light to reach the eye at the time ##\frac {D} {c + v}##

If the eye is moving away from the light source at the start of the problem when it turns on at a distance D I would imagine the gap to be closing at a rate of ##{c - v}## so the time to cover the distance D should be ##\frac {D} {c - v}##

Hopefully I'm not making any major mistakes in my analysis, let me know what you think.

Thanks.
 
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In the rest frame of the light source where the eye is moving, yes, these are the expressions. It is unclear why you think this affects the results predicted by relativity.
 
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Orodruin said:
In the rest frame of the light source where the eye is moving, yes, these are the expressions. It is unclear why you think this affects the results predicted by relativity.
I don't think it affects the results predicted by relativity, I'm just trying to make sure I'm not making any mistakes regarding the postulates of relativity and the framework of it.
 
Chenkel said:
I don't think it affects the results predicted by relativity, I'm just trying to make sure I'm not making any mistakes regarding the postulates of relativity and the framework of it.
You have specified everything in one frame here, so the same intercept calculations you do in Newtonian physics apply. It's only when you want to do more complex physics (conservation laws or frame changes, for example) that you will find differences.
 
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