Three Roads To Quantum Gravity

BLACK HOLES ARE HOT
89
The other moves away from the
black hole. It is correlated with the
one lost beyond the horizon. Because
of this its properties are random.
The result is that heat is generated
One falls in and
disappears behind
the horizon.
All information
about it is
apparently lost
to outside
observers
A pair of photons are
created just outside the
horizon, in a correlated
state as in the EPR
experiment
time
space
Horizon
Singularity
FIGURE 18
Radiation from black holes, as discovered by Stephen Hawking. The photon
that travels away from the black hole has random properties and motion
because it is correlated, as in one of the photons in Figure 17, with the one
lost behind the horizon. Because observers outside the horizon cannot
recover the information that the infalling photon carries, the outmoving
photon appears to have a thermal motion, like a molecule in a hot gas. The
result is that the radiation leaving the black hole has a non-zero temperature.
It also has an entropy, which is a measure of the missing information.
consistent with the fact that there is no hidden region, so no
horizon. But with the black hole there are an in®nite number
of observers who agree that there is a horizon, beyond which
they cannot see. And this is not just a consequence of their
motion, for all observers who do not fall through the horizon
will agree that the black hole and its horizon are there. This
means that all observers who are far from a black hole will
agree that it has a temperature and an entropy.