100 Cosmological Models
However, there may be reasons to doubt the properties of classical event horizons.
The Schwarzschild metric in Equation (5.72) is perfectly classical, the radial coordi-
nate and the time are exact. But this ignores the fact that coordinates in quantum
mechanics are uncertain, merely statistical quantities subject to fluctuations. If an
object meets the event horizon exactly at radius푟, the time for this occurrence is
completely undetermined according to Heisenberg’s uncertainty relation. Thus the
left hand of the infalling pilot may disappear from the view of the outside observer a
week before his right hand which is still seen waving goodbye.
Quantum theory appears to dictate that the event horizon actually is a highly ener-
getic region, or ‘firewall’, that would burn the astronaut to a crisp, in contradiction
with the property endowed black holes by general relativity, that they ‘have no hair’.
To really understand thisblack-hole firewall paradoxwould require a theory encom-
passing quantum mechanics and general relativity, still missing.
Instead of a firewall, Hawking has recently speculated in a brief paper without
quantitative calculations [6], that quantum mechanics and general relativity remain
intact, but black holes are not surrounded by a classical event horizon or a firewall
hindering radiation and information to escape to infinity. Rather they are surrounded
by a much more benign ‘apparent horizon’ which would enable the release of some
energy and information temporarily.
Such apparent horizons would persist only for a period of time. This suggests that
black holes are collapsed objects which should be redefined as metastable bound
states. Thus black holes would not be black except for a period of time. Inside the
event horizon, the metric and matter fields would be classical and deterministic but
chaotic. The key to his claim is that quantum effects around the black hole cause
space-time to fluctuate too wildly for a sharp boundary surface to exist [6].
In general relativity, for an unchanging black hole, these two horizons are identical,
because light trying to escape from inside a black hole can reach only as far as the
event horizon and will be held there. However, the two horizons can, in principle, be
distinguished. If more matter gets swallowed by the black hole, its event horizon will
swell and grow larger than the apparent horizon. Conversely, in the 1970s, Hawking
also showed that black holes can slowly shrink, spewing outHawking radiation(see
below). In that case, the event horizon would, in theory, become smaller than the
apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real
boundary. Unlike the event horizon, the apparent horizon can eventually dissolve.
Anything in principle could then get out of a black hole. The new idea that there are
no inside points from which you cannot escape a black hole is in some ways an even
more radical and problematic suggestion than the existence of firewalls [6].
If Hawking is correct, there could even be no singularity at the core of the black
hole. Instead, matter would be only temporarily held behind the apparent horizon,
which would gradually move inward owing to the pull of the black hole, but would
never quite crunch down to the center. Information about this matter would not be
totally destroyed, but would be released in a vastly different form, making it almost
impossible to work out what the swallowed objects once were like [6].
Theinformation loss paradoxcan be stated neatly as follows: if one throws a book
into a black hole, all that comes out is blackbody radiation or chaotic radiation, the