Simultaneity in IB Exam Q: Person A & B's Perspective

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Discussion Overview

The discussion revolves around the concept of simultaneity in the context of special relativity, specifically examining a scenario presented in an IB exam question. Participants explore how events perceived as simultaneous in one reference frame may not be perceived as such in another, using the example of two matches being lit in a moving carriage and the perspective of observers in different frames of reference.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that Person A sees the lighting of the matches as simultaneous due to being equidistant from both events in their reference frame.
  • Others argue that this simultaneity is frame-dependent, and Person B, standing on the platform, would not perceive the events as simultaneous due to the relative motion of Person A.
  • A participant questions the validity of using Person A's reference frame to draw conclusions about Person B's observations, suggesting that it may not be appropriate to apply A's perspective to B's frame.
  • Another participant highlights that while simultaneity is frame-dependent, all frames will agree on the timing of light reaching Person A from the two matches, as they are equidistant.
  • One participant draws a parallel to lightning strikes occurring outside the carriage, questioning how this scenario differs from the matches if the motion of the source is considered irrelevant.
  • Another participant responds that the key factor is which frame measures the events as simultaneous, emphasizing that if one frame sees the events as simultaneous, the other will not.

Areas of Agreement / Disagreement

Participants generally agree that simultaneity is dependent on the observer's frame of reference. However, there is disagreement regarding the implications of using one frame's observations to infer conclusions about another frame, and the discussion remains unresolved on how to reconcile these perspectives.

Contextual Notes

Participants express uncertainty about the application of simultaneity across different frames and the implications of relative motion on the perception of events. There are unresolved questions about the validity of certain premises used in the IB exam question.

theelusivecamel
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This question is from an IB exam in may, 2003 and I'm having a little trouble understanding how they got their answer.

The question is, Person A is in a carriage that is traveling in a straight line with uniform speed relative to person B who is standing on the platform. Person A is halfway between two people, X and Y, who are at either end of the carriage. At the moment person A is directly opposite person B as they pass each other at the station, X and Y both light a match, assume it's instantaneous. According to person A the events are simultaneous.
Discuss whether the two events will appear to be simultaneous to person B.

The answer is that they will not appear to be simultaneous. And the IB awards marks for thinking along these lines.
1) B sees A move away from the signal from X and since A receives them at the same time;
2) and since c is independent of the motion of the source;
3) B will see the light from X first / light from Y will reach B after light from X

The problem I've got is their first point, that A receives them at the same time. This is only his reference frame and they're using this 'fact' to prove a point in another reference frame. I wouldn't have thought it's possible to do that. Also wouldn't person B believe that person A doesn't seem them simultaneously? How would you answer the question without using the IB's first premise? or is there some other mistake?
 
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theelusivecamel said:
The problem I've got is their first point, that A receives them at the same time. This is only his reference frame and they're using this 'fact' to prove a point in another reference frame. I wouldn't have thought it's possible to do that. Also wouldn't person B believe that person A doesn't seem them simultaneously? How would you answer the question without using the IB's first premise? or is there some other mistake?
Careful. While the two matches being lit at the same time is only true from A's reference frame, all frames will agree that the light from the two matches will reach A at the same time. (If the light didn't reach A at the same time, then A would conclude that they couldn't have been lit at the same time, according to his frame, since they are equally distant from A.)

Simultaneity of spatially separated events (like the lighting of the two matches) is frame dependent. But when things happen at the same time and at the same place (like the receipt of the light from match X and match Y at A), all frames will agree.
 
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Alright now for a more basic question. I've also seen examples where lightning strikes outside the carriage on either side and since A moves between the lightning strike and the light reaching him they don't seem simultaneous. how is that different from the match example if the motion of the source doesn't matter?
 
It's not different at all. The only thing that matters is: What frame measures the lightning strikes (or match lightings) to be simultaneous? We know that if one frame measures the strikes as occurring simultaneously, the other frame will not.
 

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