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Clocks and boxcars

  1. Apr 22, 2009 #1
    Can someone look at the attachment and tell me where I went wrong thanx
     

    Attached Files:

  2. jcsd
  3. Apr 22, 2009 #2

    JesseM

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    Let's start with fig. 1 in those attachments. The text file starts out saying:
    For those who don't want to download the diagram, G and D are on either side of the lightning rod at A at equal distances from it, and H and E are on either side of the lightning rod at B at equal distances from it.
    So, clock X is to the right of lightning rod B at point E, and clock Y is to the right of lightning rod A at point D.

    Dash then goes and stands at position J. Length JA = length JB.

    A lightening bolt then strikes the two lightening rods at time T0. [/quote]
    "At time T0" is too vague--do you understand that because of the relativity of simultaneity, if the two strikes happen at the same time in Dash's frame then they happen at different times in Still Bill's frame, and vice versa? If so, in whose frame do they both strike simultaneously at T0? You go on to say:
    The light will only reach his eyes simultaneously if the strikes occurred simultaneously in Still Bill's frame, i.e. if T0 referred to Still Bill's time coordinate. If the strikes occurred simultaneously in Dash's frame, then the light from the strikes will reach Still Bill's eyes at different moments.
    But here you are assuming something incompatible with the above. If the strikes occurred simultaneously in Still Bill's frame, then they occurred non simultaneously in Dash's frame, therefore the time on the clocks X and Y will be different.

    Do you understand that the relativity of simultaneity means if the strikes were simultaneous in one frame, they were non-simultaneous in the other? If so, you should be able to understand why only one of your two assumptions could be true--either Still Bill sees both strikes at the same time (because the strikes occurred simultaneously in Still Bill's frame), or Dash's clocks X and Y read the same time when the light hits them (because the strikes occurred simultaneously in Dash's frame), but both these assumptions cannot possibly be true.
     
  4. Apr 22, 2009 #3
    I agree T0 is too vague replace it with lightening bolts strike at points A and B. Dash and Still Bill want to know if they were simultaneous or not.

    I dont assume anything different from any explanation you will find for simultenaiety.
     
  5. Apr 22, 2009 #4
    you say "Do you understand that the relativity of simultaneity means if the strikes were simultaneous in one frame, they were non-simultaneous in the other?"

    Yes I understand that is the theory I mean that is very basic. If you dont know that then I think you shouldnt be posting here in the first place.

    The problem I have is the results I get appear to be paradoxical with this concept
     
  6. Apr 22, 2009 #5
    Even if you ignore what Still Bill sees all together and concentrate on just what Dash observes I seems to get paradoxical results
     
  7. Apr 22, 2009 #6

    Dale

    Staff: Mentor

    Ignore Dash for a moment. Write down the t and x coordinates of the following events: the two lightning strikes and when Bill sees them.
     
  8. Apr 22, 2009 #7
    sorry there was an error in fig 1 I didnt have Dash positioned at point J the correct pic is attached
     

    Attached Files:

  9. Apr 22, 2009 #8
    FYI the round circles under the boxcar are its wheels they are a little confusing
     
  10. Apr 22, 2009 #9

    JesseM

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    In a standard explanation of simultaneity using train cars and lightning strikes, they always specifically state which frame the strikes are simultaneous in (usually the Earth's rest frame). You can't just say two strikes happen at A and B because it isn't enough information to answer your questions, it might be they were simultaneous in Still Bill's frame (Still Bill sees light from both strikes at same moment, Dash's clocks show different readings when light reaches them), it might be that they were simultaneous in Dash's frame (Still Bill sees light from both strikes at different moments, Dash's clocks show same reading when light reaches them), or it might be they weren't simultaneous in either frame (Still Bill sees light at different moments and Dash's clocks show different readings). These are all physically different scenarios, and they are all physically possible.
     
  11. Apr 22, 2009 #10
    referring to fig 1 Just a question. If Still Bill percieves the bolts as simultaneous will the photons from A strike clock Y simultaneously as the photons from B to clock X?
     
  12. Apr 22, 2009 #11
    OK lets say the strikes are simultaneous in Still Bills frame
     
  13. Apr 22, 2009 #12
    referring to fig 1 Just a question. If Still Bill percieves the bolts as simultaneous will the photons from A strike clock Y simultaneously as the photons from B to clock X? from Still Bills perspective
     
  14. Apr 22, 2009 #13

    JesseM

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    Yes, in Still Bill's frame. However, in Still Bill's frame these two clocks are not synchronized, so they will show different times when the light hits them despite the fact that the light hits them simultaneously in this frame.
     
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