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Special Relativity & Light

  1. Sep 27, 2007 #1
    Hello, I'm sorry to ask a question that has probably been asked many times before, but I currently have limited time, and am very anxious to find an answer. Anyways, here goes:

    Hypothetically: If two people start running from point A, one running at 10000 km/s, and the other running at .99c. Two seconds after they begin, a light source is shined in their direction. (Lets say that the area on which they were running was all perfectly flatland and stretched on for billions of miles.) Will the photons emitted from the light source reach them both at the same time? Or will they reach the slower runner first?

    I know that this is a pretty simple question, and I'm pretty sure that I know the answer, but I heard someone talking about this, and what they were saying was opposite to what I'd believed.

    Any help is greatly appreciated.
  2. jcsd
  3. Sep 27, 2007 #2


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    The speeds are relative to A, presumably?
    When you say a light source is shined in their direction, do you mean it's shined from the position of A, two seconds after they left A as measured by a clock at rest with respect to A? If so, the light will reach the runner whose speed is lower relative to A first.
  4. Sep 27, 2007 #3
    That's exactly what I meant. Thanks for the confirmation.
  5. Sep 27, 2007 #4
    admd, you can only learn so much from someone who agrees with you. Go find the person whom you antisocially overheard saying the opposite, ask them to explain why, tell them the explanation for your version in return, and you should both have a higher understanding by the time you reach agreement.

    By the way, by "hypothetically" I presume you mean "according to my homework problem"?
    Last edited: Sep 27, 2007
  6. Sep 27, 2007 #5
    By Hypothetically, I meant "suppose that." That as of now, noone can run anywhere near 10000km/h, let alone .9c. I'm in the midst of reading Brian Greene's The Elegant Universe, and what my philosophy teacher mentioned in class when trying to explain self-evident truths differed from what I thought I knew was true.

    Should your response lead me to presume that you think it's possible for one to run at these speeds? =)
    Also, it's a matter of opinion, but I dont think that paying attention in class should be labeled as "antisocial."

    You're right though, I probably should bring it up with him to double check.
    Last edited: Sep 27, 2007
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