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String Theory Demystified

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

i<br />

4. Let α−1 0,<br />

k be the state of an open string and suppose that the normal<br />

ordering constant a is undetermined. What is the mass of this state?<br />

i j ij i j ij<br />

5. In the light-cone gauge, use [ x , p ] = iδ ,[ X ( σ), P ( σ′ )] = iδ<br />

δ( σ − σ ′ )<br />

i j<br />

to fi nd a commutation relation for the transverse modes, [ α , α ] .<br />

6. Consider light-cone quantization for the closed string case. What additional<br />

commutation relation do you think should be imposed for the modes?<br />

m<br />

n

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