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Beginner question about special relativity

  1. Oct 21, 2011 #1
    Saw this in a book last night. I hope I read it right and am remembering it right.

    If two rectangular coordinate systems share the same x axis and one is moving at a constant speed towards positive x and a beam of light is travelling along their x axes going towards the positive, then at the beam is at x=ct and x'=ct', one for each system.

    The book says then x-ct=M(x'-ct') for some constant M.
    What I don't get is doesn't x-ct=0=x'-ct' mean M is 1?

    What am I not understanding about this?
  2. jcsd
  3. Oct 21, 2011 #2
    You can't add a variable to one side of an equation without adding it to both sides. A variable can end up being something other than one later on so that is why it is not done.
  4. Oct 21, 2011 #3


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    Why do you limit the solution to M=1?

    M can be any value and the equation is still true, isn't it?

    0 = M(0) is true for any value of M, correct?
  5. Oct 21, 2011 #4
    Thank you both. x-ct=x'-ct'=0 so x-ct=anything*(x'-ct').
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