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String Theory Demystified

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

90. Consider an open bosonic string under a T-duality transformation. What<br />

happens to the momentum of the dual string along the compactifi ed dimension?<br />

91. The string momenta along a certain spatial direction are found to be<br />

fractional. How are the fi elds described in the dual theory?<br />

µ<br />

92. What type of boundary condition is ∂ X 0 = 0?<br />

σ<br />

σ= , π<br />

93. In type II A superstring theory, what dimensions are allowed for Dp-branes?<br />

94. If the value of the fi elds X µ<br />

are specifi ed on the boundary, what type of<br />

problem is being posed?<br />

In Probs. 95–96 let D = 10 and suppose that there is a pure electric background<br />

fi eld F = E.<br />

0 i i<br />

95. What is the form of the Born-Infeld action?<br />

96. What is the maximum value of the electric fi eld?<br />

97. How is the dilaton fi eld φ related to string coupling gs? The fi nal problems will be very challenging for many readers. For problems<br />

98, 99, and 100, suppose that you have closed strings with periodic boundary<br />

25 25<br />

conditions for µ = 01 , ,..., 24but<br />

X ( στ , ) =− X ( σ+ , τ)<br />

s . Use the light-cone<br />

gauge (based on Polchinski 1.9).<br />

98. How does the modal expansion change for X 25<br />

as opposed to the usual<br />

modal expansion for closed bosonic strings?<br />

99. Using Π µ<br />

+<br />

∞<br />

p µ<br />

1 1<br />

2<br />

= ∂ X and ( n − ) = − ( − )<br />

τ ∑ θ 2θ 1 fi nd the hamiltonian<br />

<br />

24 8<br />

s<br />

<br />

n=<br />

1<br />

s s<br />

i i i i<br />

from H = d ′ + X X<br />

′ p<br />

′ ∂ ∂<br />

⎛<br />

1 ⎞<br />

+ ∫ σ<br />

⎝<br />

⎜ 2παΠΠ<br />

4πα<br />

0<br />

σ σ<br />

2πα<br />

⎠<br />

⎟ .<br />

100. What is the mass spectrum? Note that the level-matching condition is still<br />

satisfi ed.

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