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Causality question

  1. Jul 27, 2008 #1


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    I didn't know where to ask this question, so excuse me if I posted it in the wrong place.
    Recently I've been listening to a video in which a physicist asked a question to the public (in a high school I believe). I've had my own idea about the answer of the question, but the physicist (Étienne Klein) claimed an unexpected answer.
    I'm talking about the causality in our universe.

    The problem was stated more or less like that :
    Say we have 2 incandescent lamps connected with a wire. I call a lamp the A lamp and the other the B one.
    If someone turn on the A lamp then the B lamp will light just after (the information that the A lamp has been turned on travels by the wire between the 2 lamps).
    Now suppose the disposition of the 2 lamps is like that : B-------------A. And that I am there : (.)-----B-------A. (I am situated over the point).
    The question was "If the A lamp is turned on, which lamp will you see that light first?". Few people answered it was the A lamp that would be seen first as lighted, because they were just told the causality concept. But for my part I don't understand why wouldn't I see the 2 lamps be turned on on the same time. The information that travels in the wire is almost the speed of light, so I would see the 2 lamps light in the same time... or am I really wrong? If yes, why? I know that the causality cannot be violated but in such an example, the only way for me to understand that I'd see the A lamp light first is that the information that the A lamp doesn't travel at the speed of light in the wire.
    In that case if the B lamp light when it sees that the A lamp light, then I'd see the 2 lamps light at the same time, right? Is the causality violated in such case?
    Hope you could understoood my question.
  2. jcsd
  3. Jul 28, 2008 #2


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    Your answer is pretty much correct. But the fact that light traveling through air is a bit faster than signal travelinmg through wire does make a difference, so you would see light from lamp A just a little earlier than from B. For most physysists, there is a big difference between "at the same time" and "almost at the same time."
  4. Jul 28, 2008 #3


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    Thank you LURCH. Now I understand it better at least. Maybe the causality concept could be explained as "nothing can travel faster than light" and then all the conclusions would follow. But as I never studied it yet, I better not to make such a guess!
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