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

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In summary: And we know that the frame in which the strikes occur simultaneously will also measure the arrival times of the light to be simultaneous.In summary, the conversation discusses a question from an IB exam about whether two events will appear simultaneous to two different people in different frames of reference. The answer is that the events will not appear simultaneous, and the IB awards marks for thinking along these lines. The conversation also touches on the concept of simultaneity in different frames of reference and how it applies to examples such as lightning strikes and the lighting of matches. Ultimately, all frames will agree that events occurring at the same time and place are simultaneous.
  • #1
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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  • #2
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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  • #3
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?
 
  • #4
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.
 

What is simultaneity in the context of an IB exam question?

Simultaneity in an IB exam question refers to the occurrence of two events at the same time, as perceived by two different individuals, usually Person A and Person B.

Why is simultaneity important in IB exams?

Simultaneity is important in IB exams because it tests a student's understanding of relativity and perspective. It also allows students to demonstrate their ability to synthesize information from different points of view.

How can I determine simultaneity in an IB exam question?

To determine simultaneity in an IB exam question, you need to carefully read and analyze the given scenario and identify the events and the perspectives of Person A and Person B. Then, consider the relative positions and movements of the two individuals to determine if the events are simultaneous or not.

What are some common mistakes students make when dealing with simultaneity in IB exams?

Some common mistakes students make include not considering the relative positions and movements of Person A and Person B, not taking into account the time delay caused by the speed of light, and not understanding the concept of relativity and perspective.

How can I improve my understanding of simultaneity for IB exams?

To improve your understanding of simultaneity for IB exams, it is important to practice solving questions that involve simultaneity and to familiarize yourself with the concepts of relativity and perspective. You can also seek help from your teacher or a tutor if you are struggling to understand the concept.

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