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String Theory Demystified

String Theory Demystified

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CHAPTER 14 Black Holes 253<br />

Now, the area in fi ve dimensions is<br />

The entropy is<br />

g<br />

A rrr<br />

RV QQn<br />

2 8<br />

2<br />

2 s s<br />

= 2π = 2π<br />

1 2 3<br />

1 5<br />

S<br />

A<br />

=<br />

4G5 (14.51)<br />

5<br />

5 6 2 8<br />

where G = ( 2π) G RV = ( 2π) 8π<br />

g . Putting everything together we obtain<br />

5<br />

10<br />

s s<br />

the result:<br />

S = 2π QQ 1 5n<br />

(14.52)<br />

One of the recent successes of string theory has been its ability to count up the<br />

microscopic states of a black hole to calculate its entropy. The result obtained in<br />

this manner agrees with the semiclassical expressions, providing strong support for<br />

string theory as a quantum theory of gravity.<br />

Quiz<br />

1. Find the temperature of a D = 4 charged black hole.<br />

2. The Hagedorn temperature is the temperature above which multiple strings<br />

would coalesce into a single string. Take the density of string states to be<br />

n= exp( 4πm α ′ ) and write down the partition function. What condition<br />

is necessary for the partition function to be fi nite? This gives the Hagedorn<br />

temperature.<br />

3. Suppose that the sun were to collapse to a black hole. What would be its<br />

temperature?<br />

4. Estimate the lifetime of a six solar mass black hole that is evaporating from<br />

the Hawking process.<br />

Summary

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