- #1
Jimmy Snyder
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Homework Statement
Page 154, equation (9.19) is
[tex]n\cdot\mathcal{P}^{\sigma} = 0[/tex]
Homework Equations
The text says:
Indeed, [itex]n\cdot\mathcal{P}^{\sigma}[/itex] is required to vanish at the string endpoints to guarantee the conservation of
[tex]n\cdot p = \int d\sigma n\cdot \mathcal{P}^{\sigma}[/tex]
(consider equation (8.38) dotted with n.)
Here is equation (8.38) on page 138
[tex]\frac{dp_{\mu}}{d\tau} = \mathcal{P}_{\mu}^{\sigma}(\sigma = 0) - \mathcal{P}_{\mu}^{\sigma}(\sigma = \sigma_1)[/tex]
The Attempt at a Solution
So conservation of [itex]n\cdot p[/itex] gives (using (8.38))
[tex]0 = \frac{d(n\cdot p)}{d\tau} = n\cdot \mathcal{P}^{\sigma}(\sigma = 0) - n\cdot\mathcal{P}^{\sigma}(\sigma = \sigma_1)[/tex]
But we already knew that from equation (9.18):
[tex]\frac{\partial}{\partial \sigma}n\cdot\mathcal{P}^{\sigma} = 0[/tex]
It seems that in fact, Zwiebach means to apply equation (6.56) on page 103 which is the free endpoint condition and says (slightly edited)
[tex]\mathcal{P}_{\mu}^{\sigma}(0) = \mathcal{P}_{\mu}^{\sigma}(\sigma_1) = 0[/tex]
Am I correct? Actually, I doubt it because I think he means to imply that it is true regardless of the boundary conditions. Besides, if that was what he meant, then he could have applied it directly to eqn (9.19) without reference to (8.38).
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