Zwiebach Equation 9.80: Why M^2 = -p^2?

  • Thread starter Thread starter ehrenfest
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ehrenfest
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Homework Statement


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.


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The Attempt at a Solution

 
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Equation (9.80) is just equation (2.72) page 25, with c set to 1.
 
I see. Thanks.
 
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