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String Theory Demystified

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Final Exam<br />

1<br />

1. Consider the lagrangian for a classical string L = X 2 µ 2 µ 2<br />

X′ − ( X X<br />

µ<br />

µ ′ ) .<br />

Write down the canonical momentum.<br />

2πα<br />

′<br />

2. Consider a classical string which is in a confi guration described by a rigid<br />

rod rotating about the origin with angular velocity ω . Find the energy of the<br />

1 l 1<br />

string from E = d<br />

′ ∫ σ<br />

2 −l<br />

2 2<br />

πα 1−<br />

ωσ .<br />

3. Consider the action for a p-brane with cosmological constant<br />

T p+ 1 αβ<br />

p+<br />

1<br />

S =− ∫d σ −hh ∂ X⋅∂ X + Λ ∫d<br />

σ −h.<br />

α β Find the classical equations<br />

2<br />

of motion for the metric.<br />

4. Using the action of the previous problem, fi nd a constraint on the<br />

µν<br />

cosmological constant Λ using h h = p+1.<br />

5. Consider the classical string, describing its dynamics using the Polyakov<br />

action. What form does the action take if the worldsheet metric is taken to<br />

be fl at?<br />

6. Using the action of Prob. 5, what is the canonical momentum?<br />

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µν

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