Memories of a Theoretical Physicist by Joseph Polchinski

Author:Joseph Polchinski
Language: eng
Format: epub
Tags: multiverse; theory of everything; D-branes; string theory; black holes; theoretical physics; Stephen Hawking; holographic universe; universe; string theorist; string theory; black holes; autobiography; theoretical physics; scientist; Joseph Polchinski; life of a theoretical physicist
Publisher: MIT Press

9.2   REVOLUTIONS THREE AND FOUR

Almost immediately after my D-brane paper, Strominger came to me excited that he would be able to calculate the microscopic density of states of black holes. Having learned GR from Weinberg, I had not given this question much thought, but Strominger, a more gravitational physicist, told me that this was just as important as the information problem. His calculation was just off by a constant, and he was looking for help. This was all too new to me, and I had nothing to contribute. But he found Vafa, who had the right tools, and they got the first precise counting of black hole states. They had connected string theory to a new aspect of quantum gravity.

Gary Horowitz also had a long-standing interest in the black hole entropy. He kept coming back to the question, how do we count the states for ordinary Schwarzschild black holes, not just the highly supersymmetric Strominger-Vafa black holes? We could not get as sharp an answer as SV, but we did get a crude but useful result, extending an idea of Susskind. Imagine turning down the string coupling for a black hole. The black hole gets smaller, and eventually reaches the string length. At that point, one should match the black hole density of states to that of the weakly coupled D-branes and strings. This gave a correspondence principle, matching the approximate counting for various black holes. In a follow-up we studied the transitions of long strings to black holes.

BLACK HOLE MICROSTATE COUNTING

One of the most mysterious aspects of a black hole is its similarity with a hot box of gas. It has an associated temperature and thermodynamic entropy—a notion quantifying the ignorance of the particular state of a system—that are encoded geometrically in the shape of the spacetime around it. Its entropy is given by the Bekenstein-Hawking (BH) formula as the area of its event horizon as measured in Planck units. For a black hole of a given mass, charge, and angular momentum, this suggests that the number of distinct configurations or microstates of a black hole is equal to the exponential of the BH entropy. However, a black hole spacetime does not have any obvious structure able to account for this many microstates.

Figure 9.1a

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