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String Theory Demystified

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226 <strong>String</strong> <strong>Theory</strong> Demystifi ed<br />

So, the presence of a momentum term in the modal expansion would mean that<br />

Dirichlet boundary conditions could not be satisfi ed. Putting everything together,<br />

the modal expansion for the DD coordinates is<br />

i<br />

a a αn<br />

−inτ<br />

X ( στ , ) = x + i 2α′<br />

∑ e sin( nσ)<br />

n≠0<br />

n<br />

Quantization will involve imposing the usual commutators:<br />

a b ab<br />

⎡<br />

⎣X<br />

( στ , ), X ( σ′ , τ) ⎤<br />

⎦ = iδδσ<br />

( − σ′<br />

)<br />

a b ⎡<br />

⎣α<br />

, α ⎤ m n ⎦ = m +<br />

ab<br />

δ δ m n,0<br />

(13.11)<br />

(13.12)<br />

Now, for a moment we consider the general light-cone expansion of the string (so<br />

+ for a moment we let i = 2, ..., 25).<br />

We gauge fi x by choosingα n = 0 for all n ≠ 0<br />

and so<br />

The momentum p +<br />

is defi ned as<br />

The light-cone gauge condition is<br />

Now, X −<br />

It follows that<br />

is an NN coordinate, so<br />

+ + +<br />

X ( στ , ) = x + 2α′<br />

p τ<br />

p<br />

0<br />

1<br />

= α (13.13)<br />

2α<br />

′<br />

+ +<br />

0<br />

p + 1<br />

=<br />

2α ′<br />

(13.14)<br />

−<br />

− − −<br />

αn<br />

−inτ<br />

X ( στ , ) = x + 2α′ p τ+ i 2α′∑e<br />

cos( nσ)<br />

n<br />

n≠0<br />

X<br />

− − − −<br />

± X′ = 2α′ p + 2α′∑αne<br />

n≠0<br />

− in(<br />

τ± σ)<br />

(13.15)

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