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Things that happens simultaneously

  1. Sep 30, 2013 #1
    I am studying a book about special theory and I was reading about things that happened simultaneously. For example lets say that a two lights come at position A and B.
    After couting we find the middle of AB that is M. There we put a human. This human is going to see two lights happens simultaneously.

    Now lets say that this human is into a train. The second human will see the light of B comes faster because he runs with a velocity W at the position of B. So comes more near at B and more away from A. So will see light speed of B will happenes first.


    I undestand that logic,but that what I can not undestand is "What is the true?".
    I know that you will tell me that both are true,but if we put two mini humans into the A and into the B,both will feel the same time both lights?? If answer is yea,then I think tthat the first paragraph with standing guy is the correct.


    Sorry for my bad english. Thank you!!!
     
  2. jcsd
  3. Sep 30, 2013 #2

    ghwellsjr

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    Is your question: do the lights think they came on at the same time?
     
  4. Sep 30, 2013 #3

    Nugatory

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    How would they know that they both experienced the light on at the same time? True, it they are not moving with respect to their respective lights they'll be able to say something like "My light flashed; two seconds later the light from the other guy hit my eyes; the distance between us is two light-seconds so I know the light took two seconds to get to me; therefore the other light flashed at the same time as mine". But that's just a long-winded way of saying that because they're moving at the same speed as the center guy on the train, they agree with him about times, distances, and simultaneity. It doesn't make their truth any "better" than anyone else's. They do not agree with the guy who is standing on the ground outside the track, in the middle.
     
  5. Sep 30, 2013 #4

    Dale

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    Neither is "true" in the sense that it is a frame invariant fact of nature.

    What is the "true" speed? Is it "truly" at rest or "truly" moving? Hopefully you understand that there is no "true" speed, there is only the speed wrt a given reference frame.

    Similarly there is no "true" simultaneity, there is only simultaneity wrt a given reference frame.
     
  6. Sep 30, 2013 #5
    The guy on the ground will observe that the two flashes arrive at his location at the same time. His conclusion is that the two flashes occurred at the same time according to the synchronized clocks in his particular frame of reference.

    The guy on the train will observe that the two flashes arrive at his location at different times. His conclusion is that the two flashes occurred at different times, according to the synchronized clocks in his particular frame of reference.

    If a second guy on the train was physically present at the location where the forward flash occurred, and, if he recorded on a piece of paper the time on his clock that it occurred, and a third guy on the train was physically present at the location where the rear flash occurred, and, if he recorded on a piece of paper the time on his clock that the rear flash occurred, when the second and third observers got together later and compared notes, they would find that they had written down different times on their pieces of paper (even though all the clocks on the train were previously synchronized with one another).
     
  7. Oct 1, 2013 #6
    Loss of Simultaneity

    I too have a problem understanding how Relativity explains Loss of Simultaneity.

    Consider two points ‘A’ and ‘B’ fixed in space in an Inertial Frame of Reference. There is a flash bulb at each of those points. When a flash bulb fires, a spherical bubble-surface of light radiates out from the bulb at the speed of light, ( c ).

    Mid-way between A and B is point M, where sits a controller, able to fire the two flash bulbs simultaneously, causing two bubble-surfaces of light to expand into space. These surfaces will intersect on an imaginary flat plane perpendicular to line AB, and passing through M.

    Any observer in a different Inertial Frame of Reference, who is around when the bulbs are fired, must necessarily pass through each of the bubble-surfaces, from the outside to the inside. (The observer cannot go faster than light, so he cannot out-run the expanding surfaces). As he passes through each surface, he will see a flash of light coming from A or B.

    When he passes through the bubble-surfaces, if he is on the side of the Plane of Intersection which is nearest to point A, the observer will see ‘A’ flash first. If he is on the side nearest to ‘B’, he will see ‘B’ flash first. If he is actually on the Plane of Intersection, he will see both ‘A’ and ‘B’ flash simultaneously. This happens, (it seems to me), irrespective of the speed and direction of the observer relative to points ‘A’ and ‘B’.

    For an observer travelling towards 'A' and 'B', the faster that the observer travels relative to those points, the shorter will be the time interval he sees between the two flashes.

    This is quite contrary to the usual conclusions about Loss of Simultaneity. These assert that if the observer is travelling at high speed along a path parallel to the line A-B, and in the direction A => B, the observer will see bulb ‘B’ flash first; and the faster the relative speed of the observer, the greater will be the time interval between the two flashes.

    Where have I gone wrong in my thinking?
     
    Last edited: Oct 1, 2013
  8. Oct 1, 2013 #7

    A.T.

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    It doesn't explain it, but simply postulates it. Special Relativty replaces Galilean transformation with Lorentz transformation, in order to keep light speed invariant under the transformation. Lorentz transformation keeps light speed invariant, but not simultaneity.

    https://www.youtube.com/watch?v=C2VMO7pcWhg

    You can apply the Lorentz transformation to any scenario that is confusing you, to clear it up.
     
    Last edited: Oct 1, 2013
  9. Oct 1, 2013 #8
    Here is where you have gone wrong. According to the synchronized clocks in the traveling observer's frame of reference, the two flashes did not occur at the same time. If a second observer in the traveling frame had been physically present at the location where one of the flashes occurred, and a third observer in the traveling frame had been physically present at the location where the other flash occurred, the times on their respective synchronized clocks at which the flashes were observed would be different.

    Chet
     
  10. Oct 1, 2013 #9
    I think I understand the reply from Chestermiller. I depends what you mean by "Simultaneity". If I look up at the night sky and see two flashes of light occuring at exactly the same time, but I then discover that one is from a star 10 light years away, and the other from a star 100 light years away, then were the flashes simultaneous?
    On the other hand, if those two stars were stationary in a common inertial frame of reference, (possibly different from mine), and both flashed at the same time, I would see the nearer star flash many years before I saw the other. From my point of view, would they be simultaneous?
     
  11. Oct 1, 2013 #10

    A.T.

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    Relativity of simultaneity has nothing to do with what you visually see a simultaneous. It is what is left, after you have already accounted for the observational delays due to finite signal speed.
     
  12. Oct 1, 2013 #11

    Nugatory

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    There are four events here, not two. They are:
    1) Flash leaves star 1
    2) Flash leaves star 2
    3) Flash from star 1 arrives at your eyes
    4) Flash from star 2 arrives at your eyes

    When you say you see the flashes "at exactly the same time", you're talking about events #3 and #4, and all observers everywhere regardless of speed and frame will agree that they are simultaneous - because they happened at the same location. If you had a light detector wired to explode a bomb if an only if it was hit by two light signals simultaneously, everyone would have to agree about the fact that bomb exploded.

    Because events #1 and #2 were spatially separated, different observers moving at different speeds relative to each other will not agree about the simultaneity or the ordering of these two events. They will all define "the time that the event happened" as what they get by subtracting the light travel time from the time that the light reached them (as A.T. says above, relativity of simultaneity is what's left over after you allow for light travel time) but this procedure will not lead them to the same conclusions about the simultaneity of these events; they're working with different distances and times.
     
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