appears exactly the same as the world to
an observer in empty space. Polchinski’s
firewall implies that falling through the
horizon is dramatically different to being in
empty space.
It is to sidestep this ‘firewall paradox’
that Hawking has stepped into the fray.
The collapse of an object such as a star
to form a black hole is violently chaotic.
Rather than a horizon, all that forms,
claims Hawking, is a boundary of extreme
space-time turbulence. Information can
leak out through such an ‘apparent horizon’,
so there is no need to worry about pesky
entanglements and destroying them with a
firewall. Since the firewall is nothing more
than a fiction, there is no contradiction with
Einstein’s theory of gravity.
Hawking’s conclusion is dramatic. “The
absence of event horizons mean that there
are no black holes – in the sense of regimes
from which light can’t escape to infinity,”
he explains. “There are, however, apparent
horizons which persist for a period of time.”
Of course, Polchinski’s firewall had to
gain its energy from somewhere and that
could only be the violently convulsing
space-time within the horizon. So isn’t the
idea very similar to Hawking’s? “If I just
read the words in his paper, it sounds like
he is replacing a firewall with a chaos-wall,
yes,” says Polchinski. “But I doubt that
this is what he means.” The trouble, he
says, is “Hawking’s paper is short and does
not have a lot of detail, so it is not clear
what his precise picture is, or what the
justification is.”
So is the horizon around a black hole
the point of no-return everyone thought it
was? Or is it merely an apparent horizon,
as Hawking maintains, leaking stuff from
inside the hole? The answer may come from
radio astronomers who are trying to image
Sagittarius A*, the black hole at the heart
of our Galaxy. They need merely to zoom
in by another factor of three and they will
see the horizon itself. Currently, nature is
hiding its ultimate secret. But it may not be
able to do so for much longer.MARCUS CHOWN is the author of What A Wonderful
World: One Man’s Attempt to Explain The Big Stuff,
published by Faber and FaberAn object approaches a
black hole’s event horizon
in this artist’s impression,
but is this really the point
of no return?