Relativity of Simultaneity and Length Contraction in Observing Lightning Strikes

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Discussion Overview

The discussion revolves around the relativity of simultaneity and length contraction as observed in a thought experiment involving two observers, one at rest and the other in motion, witnessing lightning strikes at both ends of a moving reference frame. The focus is on how each observer perceives the timing of the lightning strikes and the implications of their relative motion on their observations.

Discussion Character

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

Main Points Raised

  • Observer (A) proposes that they will see the lightning strikes occur at different times depending on the direction of observer (B)'s motion relative to them.
  • Another participant clarifies that while both observers will see the flashes at the same time in their respective frames, they will not agree on the simultaneity of the events.
  • It is noted that there is a distinction between what observers "see" (the arrival of light) and what they measure as the time-coordinates of the events in their own frames.
  • Some participants emphasize the importance of local versus remote simultaneity and the need for synchronization of clocks when measurements are taken remotely.
  • There is a consensus that observer (B) will see the two flashes simultaneously from their frame, but the interpretation of when the events occurred will differ between observers.
  • A later post introduces the idea that length contraction may affect the understanding of the situation, suggesting a reconsideration of the initial assumptions.

Areas of Agreement / Disagreement

Participants generally agree on the distinction between what is seen and what is measured regarding simultaneity. However, there remains disagreement on the implications of length contraction and how it affects the observers' interpretations of the events.

Contextual Notes

There are unresolved assumptions regarding the synchronization of clocks and the implications of length contraction on the observations made by the two observers.

FossilFew
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I read the post entitled "Thought experiment in relativity of simultaneity". It's pretty close to my question but not necessarily needed to answer my questions.

Situation: On observer (A) is "at rest" and another observer (B) is in a reference frame moving with respect to observer (A). Assume that when an observer (B) in the moving reference frame is directly across from the "at rest" observer (A), he sees lightning strike at each end of the car in which he is riding, flash at the same time. What will the observer (A) who is "at rest" see? (assume that the distance between the lightning on the left side of observer (B) and observer (B) is equivalent to the distance between the lightning on the right side of observer (B) and observer (B) )

Based on simultaneity I would say that the observer (A) who is at rest would see one of two scenarios:

1. From the reference frame of observer (A): If observer (B) is moving left of observer (A), observer (A) will see lightning to the right of observer (B) strike first followed by the striking of the lightning to the left of observer (B).

2. From the reference frame of observer (A): If observer (B) is moving right of observer (A), observer (A) will see lightning to the left of observer (B) strike first followed by the striking of the lightning to the right of observer (B).

Does this reasoning seem correct? Both observers will also suspect that the other's clocks are incorrect too.

TIA!
 
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If I understand your setup, A is coincident with B at the instant that the two flashes reach them. They will therefore each see the two flashes at the same time in their own frames. But A will not agree that the flashes occurred simultaneously.
 
Yes, you have to be careful about distinguishing what they "see" (in terms of when the light from each flash actually reaches them) and what they measure the time-coordinates of the flashes to be in their own rest frames (often the words 'observe' or 'measure' is often used as a shorthand for this). In terms of their measurements, your reasoning in 1) and 2) is correct.
 
Right, the measurements could be peformed anywhere in either moving or rest frames. And when the observers "see" the flashes, those are also measurements (they could have used photodetectors).
 
country boy said:
Right, the measurements could be peformed anywhere in either moving or rest frames. And when the observers "see" the flashes, those are also measurements (they could have used photodetectors).
Sure, but I meant something more like "the measurement of the time-coordinate of the event in their own rest frames". I think if a physicist says something like "when did this observer measure this event occurring" she'd usually be talking about the retroactively assigned time-coordinate, not the moment the light from the event actually reaches her.
 
This goes to the heart of the problem, the difference between remote simultaneity and local simultaneity. When I read that the observer "sees the lightning strike," I assume that light has arrived at her or his eye. That is local, in that the observer and the measurement are at the same place and time. That there is a coincidence does not depend on when the light was emitted. If the measurements take place remotely, a sychronization of clocks would have to be established. When I re-read FossilFew's question, I am pretty sure it means that both observers and the light from both flashes are all at the same location at the same time. Maybe FossilFew will chime in here.
 
Yes "light from both flashes are all at the same location at the same time". We know that observer (B) will see the two light flashes at the same time (from (B)'s frame of reference).

TIA
 
FossilFew said:
Yes "light from both flashes are all at the same location at the same time". We know that observer (B) will see the two light flashes at the same time (from (B)'s frame of reference).

TIA
If you're talking about what they see, then they'll both see the two light signals reach their eyes at the same moment, if I understand your scenario right. But when they subtract out the travel time of the light to figure out when each flash "really" happened in their own rest frame, one will conclude both happened at the same time and one will conclude they both happened at different times.
 
Weeks later I'm thinking I am incorrect on my assumption. It seems that length contraction is an issue I didn't think about when viewing the lightning flashes

(I search on lightning in this link and read the example which is the same as the one I was attempting to answer:

http://www.upscale.utoronto.ca/GeneralInterest/Logan/Poetry/CHAPTER11-12.html

)
 
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