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Homework Help: Relativity-Finding the time of an event

  1. Dec 9, 2013 #1
    For the problem below, I understand how to solve it using numbers, but I am having trouble understanding the physics/logic behind the solution. More specifically, I do not understand how the time of seeing the light flash is different compared to the actual arrival of light. Any assistance would be very helpful. Thanks.

    Experimenter A in reference frame S stands at the origin looking in the positive x-direction. Experimenter B stands at x = 900 m looking in the negative x-direction. A firecracker explodes somewhere between them. Experimenter B sees the light flash at t = 3.0 ms. Experimenter A sees the light flash at t = 4.0 ms. What are the spacetime coordinates of the explosion?
  2. jcsd
  3. Dec 9, 2013 #2


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    hi tmlfan_17! :wink:
    they are the same: you do see the light when it arrives at your eye :smile:

    (but what does this have to do with relativity? the observers are at rest relative to each other :confused:)
  4. Dec 9, 2013 #3
    I think that this problem relates to relativity because frame of references are possibly important to the understanding of relativity. From my understanding, light takes 1.0 μs to reach experimenter B (as light travels at 300 m/μs). However, he/she sees it at 3.0 μs. I don't understand why they are not seeing it at 1.0 μs.
  5. Dec 10, 2013 #4


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    i think i don't understand the original question :confused:

    what is S ?
  6. Dec 10, 2013 #5
    S is the standing reference frame for which both Experimenter A and B are within. I will post the solution to the problem below in (.jpg format). Hopefully this will help clarify my concerns.

    Attached Files:

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  7. Dec 10, 2013 #6


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    ah, that clears up what was puzzling me about what you originally posted …
    … that should have been 3.0 µs and 4.0 µs :redface:

    ok, so light would take 3 µs to go from A to B

    so if the flash takes a µs to reach A and b µs to reach B, then a + b = 3

    and also a - b = 1 (because when the flash reaches B, it stil has 1 µs to go before it reaches A)

    so a = 2, b = 1 :smile:
    yes, b = 1 µs

    so since the question tells you that the time on the clock when it reaches B is 3 µs (and for A, a = 2 µs, and the arrival time is 4µs), that means it must have started at 2 µs

    (and yes, there's no relativity in all of that! :wink:)
  8. Dec 10, 2013 #7
    I apologize for the copy and paste mishap. From my understanding the time on the clock that the light reaches Experimenter B is 3 µs, but the actual time of travel for the light is 1 µs (making the time of the incident at t=2 µs). The 3 µs is almost like a "false time". Does that look correct?
  9. Dec 11, 2013 #8


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    hi tmlfan_17! :smile:

    (just got up :zzz:)
    yes that's correct :smile:

    the flash happens at 2, the light takes 1 to reach B at 3, and 2 to reach A at 4

    but i don't understand your worry about "false time" …

    what do you think "real time" is??

    time is what we want it to be, i may set my watch at zero at a totally different time to when you set yours at zero

    if we live in different time zones, that's obvious

    but even the same person may use different clocks: if you're timing an experiment, you're very likely to start a stop-watch at zero, to make calculation easier (ie to avoid having to subtract the "real start time" from every reading)
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