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Homework Help: Zwiebach equation 9.80

  1. Oct 4, 2007 #1
    1. The problem statement, all variables and given/known data
    Can someone explain to me why in the world Zwiebach sets M^2 = -p^2 in this equation? Even in natural units, that doesn't seem right.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 5, 2007 #2
    Equation (9.80) is just equation (2.72) page 25, with c set to 1.
  4. Oct 5, 2007 #3
    I see. Thanks.
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