Relative Simultaneity: Flashlight in Moving Ship

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    Relative Simultaneity
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Discussion Overview

The discussion revolves around the concept of relative simultaneity as it pertains to a scenario involving a moving ship with a flashlight and reflectors at each end. Participants explore how observers in different inertial frames perceive the timing of light reflections from the ship's ends, addressing both theoretical implications and observational outcomes.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that an observer on the ship will see the reflected light from both ends hit them simultaneously, as they are at rest relative to the light source.
  • Others argue that an observer on Earth will perceive the reflections as occurring at different times due to the ship's motion, leading to a disagreement about simultaneity.
  • It is noted that while both observers agree on the total round-trip distance covered by each flash, they disagree on the distances traveled by the flashes as perceived in their respective frames.
  • Some participants emphasize that the reflections from the rear and front mirrors occur at different times in the Earth observer's frame, but both paths return to the flashlight simultaneously.
  • A later reply questions the consistency of simultaneity across different frames, suggesting that if two flashes arrive at the same time in one frame, they must do so in all frames.
  • There is a discussion about the implications of the speed of light being constant in all inertial frames and how this affects the perception of events.

Areas of Agreement / Disagreement

Participants generally disagree on the simultaneity of the reflections as perceived by observers in different frames. While there is some consensus on the round-trip distances, the timing of the reflections remains contested.

Contextual Notes

Participants express various assumptions about the nature of light propagation and the effects of relative motion on perception, but these assumptions are not universally accepted or resolved within the discussion.

  • #31
gmalcolm77 said:
Yes, it is a what if question, but wouldn't an outside observer see a light? ?:)
What color is that sheep again?
 
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  • #32
jbriggs444 said:
What color is that sheep again?

Well, try to think of it this way: It is impossible for me to run a hundred miles an hour, but if I could, would I get to that town 100 miles away in one hour? :cry:
 
  • #33
gmalcolm77 said:
Well, try to think of it this way: It is impossible for me to run a hundred miles an hour, but if I could, would I get to that town 100 miles away in one hour? :cry:
Not really a helpful way of thinking about it. Your inability to reach one hundred miles per hour is a mere technical challenge, not a logical impossibility.
 
  • #34
gmalcolm77 said:
Well, try to think of it this way: It is impossible for me to run a hundred miles an hour, but if I could, would I get to that town 100 miles away in one hour? :cry:
That isn't the question you are asking. You are asking how much energy you would use in the run. But a realistic model of you running at 100mph would involve you tearing ligaments and collapsing - so there's no way to answer that question.

Your original question involves asking what happens if you are in a situation where light is both stationary and traveling at 3×108m/s. There can be no coherent answer to a question that doesn't make sense. I realize it isn't obvious that your question isn't coherent. I've explained (post #22) why it isn't, and I'm happy to help you understand that answer. But at some point you need to accept that there is no answer in the terms you want.
 
  • #35
gmalcolm77 said:
Well, try to think of it this way: It is impossible for me to run a hundred miles an hour, but if I could, would I get to that town 100 miles away in one hour? :cry:
The answer is: it depends relative to what frame your speed is measured and relative to what frame the time to get there is measured.
 
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  • #36
gmalcolm77 said:
Yes, it is a what if question, but wouldn't an outside observer see a light?

OK, so let's get rid of the spaceship since we don't need it anyway. A photon traveling at c undergoes spontaneous parametric down-conversion when it runs into a crystal(not moving) and splits into two new photons. Would the two new photons be seen?
 
  • #37
gmalcolm77 said:
An electron traveling at c
Nothing with non-zero mass can travel at c. Rocket, electron, whatever.
 
  • #38
Ibix said:
Nothing with non-zero mass can travel at c. Rocket, electron, whatever

Sorry, typo corrected.
 
  • #39
gmalcolm77 said:
Sorry, typo corrected.
OK.
gmalcolm77 said:
OK, so let's get rid of the spaceship since we don't need it anyway. A photon traveling at c undergoes spontaneous parametric down-conversion when it runs into a crystal(not moving) and splits into two new photons. Would the two new photons be seen?
Of course (assuming there's something there to see them).

Edit: The problem with your earlier scenario is two-fold. First that it has a massive object traveling at the speed of light. Second that you are trying to imagine riding along with the object at the speed of light. The second one is impossible, directly from the invariance of the speed of light. The first one is impossible because mass, in relativity, is the modulus of the energy-momentum four-vector. That is zero for things traveling at the speed of light and non-zero for all other things. In other words "has zero mass" and "travels at the speed of light" turn out to be two ways of saying the same thing. Parametric down conversion has neither of these issues, unless you try to imagine riding beside one of the photons.
 
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  • #40
OK. Great answers, thanks.
 

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