Three Roads To Quantum Gravity

HOW TO WEAVE A STRING
189
in Einstein's theory space and time are described as continuous,
and no mention is made of the discrete, atomic
structure that may exist on the Planck scale.
Given this general picture, it is natural to ask whether the
black hole's entropy is a measure of the missing information
that could be obtained from an exact quantum description of
the geometry of space and time around a black hole. The fact
that the entropy of a black hole is proportional to the area of
its horizon should be a huge clue to its meaning. String theory
and loop quantum gravity have each found a way to use this
clue to construct a description of a quantum black hole.
In string theory, good progress has been made by conjecturing
that the missing information measured by the black hole's
entropy is a description of how the black hole was formed. A
black hole is a very simple object. Once formed, it is featureless.
From the outside one can measure only a few of its properties:
its mass, electric charge and angular momentum. This means
that a particular black hole might have been formed in many
different ways: for example, from a collapsing star, or ± in
theory at least ± by compressing, say, a pile of science-®ction
magazines to an enormous density. Once the black hole has
formed there is no way to look inside and see how it was
formed. It emits radiation, but that radiation is completely
random, and offers no clue to the black hole's origin. The
information about how the black hole formed is trapped inside
it. So one may hypothesize that it is exactly this missing
information that is measured by the black hole's entropy.
Over the last few years string theorists have discovered that
string theory is not just a theory of strings. They have found
that the quantum gravity world must be full of new kinds of
object that are like higher-dimensional versions of strings in
that they extend in several dimensions. Whatever their
dimension, these objects are called branes. This is shortened
from `membranes', the term used for objects with two spatial
dimensions. The branes emerged when new ways to test the
consistency of string theory were discovered, and it was
found that the theory can be made mathematically consistent
only by including a whole set of new objects of different
dimensions.