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Can an Event be in one frame and not be in another?

  1. Apr 11, 2005 #1
    Whether simultenously or not, I think the two events must occur for both frame of reference. Then I thought about this situation that there seems to be something wrong but I can't find what's wrong with this. I hope one of you guyz can clear me up on this thing.

    I devise two special swtichs and put them on the ground. The two switches will turn on a green light only and only if the two ends of spear coincides (at least vertically) simultenously with the points where the two switches are located. Now, because I know that the moving spear will contract it's leangth I speicifically calculated it's contracted length and made sure that the two switches are aparted in equal distances as of that contraced spear. Now, I ask my friend to hold the spear and run as fast he can (near the speed of light) through the track where the two switches are laid on. To my reference frame, I have no doubt that the switches will turn the green light on. But for my friend who's moving along with the spear will measure the distance between the two switches are contracted such that it will be shorter than the length of the spear and therefore concludes that it is just not possible for the two ends of the spear will coincides with the two switches simultaneously so he concludes that the green light will not be turned on.

    Whether the green light turns on or not can't hardly be just opinion from different frame of reference (at least it seems to me that way) and I think if one observer sees green light so the other one have to see that same green light (may not be at the same instant of time). Can somebody explain to me what went wrong here???? :cry:
  2. jcsd
  3. Apr 11, 2005 #2


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    Sometimes observers can't see events, but when observers can see them, everyone agrees what happesns. Your particular problem appears to be a variant of the pole vaaulter in the barn "paradox". Like many SR "paradoxes", it can be resolved by the fact that events that are simultaneous in one frame are not simultaneous in another.
  4. Apr 11, 2005 #3


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  5. Apr 11, 2005 #4


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    The probelm is that you assume that the light will only turn on in your friend's frame if, from his frame, the ends of the spears touch the switches simultaneously. This is only a condition in your frame. In his frame, the requirements to turn on the light will be different, how different depends exactly on the scheme you used to turn the green light on in your frame.
  6. Apr 11, 2005 #5

    Another way of saying what Janus told you (I think) is that it's impossible to construct a physical system with an observable property (in your case the observable property is "the light turning on or not") that depends on the simultaneity of two events at different locations.
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