32 | NewScientist | 20 April 2019
Just as black holes permit no object to
escape their pull, hypothetical objects
known as white holes can’t hold
anything together. One idea is that
every black hole is connected to a white
hole via an interdimensional tunnel
known as a wormhole. Fall into one,
and you will, eventually, get thrown
out the other.
For Carlo Rovelli at Aix-Marseille
University in France, however, there is
a way to make this happen without even
needing a wormhole. Instead, he suggests
that every white hole used to be a black
hole, meaning that at some point in
the far future, your remains could be
vomited up as the black hole flips back
into a white hole. But how does this
transition happen?
Once a star collapses to form a
black hole, its constituent atoms get
so close together that they start being
subject to the laws of quantum physics.
And this can lead to some strange and
counter-intuitive phenomena. The most
important of these is called quantum
tunnelling, which says that particles
have a small but non-zero chance of
travelling straight through an otherwise
impenetrable barrier. For particles falling
into a black hole, says Rovelli, this means
they could theoretically pass straight
through the “singularity” of infinite
density at its heart, and bounce back out.
“What falls inside a black hole gets to the
centre, and it’s like it hits a wall, because
according to Einstein’s theory, it cannot
come back,” says Rovelli. “But quantum
theory allows it to ‘tunnel’ through
and re-emerge.”
This creates a scenario in which space-
time bounces back outwards, creating
a white hole. To an outside observer,
this process would take billions of years,
but inside, owing to the black hole’s
enormous gravitational pull, time would
be speeded up. “The time it would take
you to fall down the centre, go through
and bounce up the other side would be
milliseconds,” says Rovelli. “If we could
look inside, you would look frozen.” And
there’s just a chance that this frozen you –
or frozen particles of you – might make it
out of the white hole.
C
Seek a white hole exit
Congratulations, you found a way out
of the black hole. Possibly – and you
do look rather a mess
As Stephen Hawking identified, black
holes are constantly emitting radiation.
So what if, instead of being destroyed or
hidden away for eternity, your identity was
leaking out in the form of this Hawking
radiation? Sad to say, this actually makes
things a lot more complicated.
Hawking radiation is made up of virtual
particles that blip into existence near the
event horizon with one entangled partner
particle sucked in while the other escapes
(see “A”, page 31). But if that is the case,
quantum mechanics presents us with an
unpalatable paradox. We already know
that particles of Hawking radiation are in
an entangled state with their partners that
just fell in. But because Hawking radiation
contains information on every single
particle that has already fallen in (and
those particles were entangled with radiated
particles of their own), any outgoing
particle must also be entangled with the
Hawking radiation that preceded it.
Quantum mechanics doesn’t allow
this kind of polyamory. A fundamental
principle known as the monogamy of
entanglement says that particles can’t be
entangled with two things at once. If your
information wants to exit the black hole
via Hawking radiation, something else
needs to happen to it.D
Radiate yourself away
One way out of your quantum dilemma is
to conjure up a blazing wall of fire. If this
firewall existed just inside the event
horizon, it would break the entanglement
between any infalling particles and their
partners on the outside of the black hole.
Trouble is, this combustible barricade
is completely incompatible with Albert
Einstein’s general theory of relativity,
which predicts that a black hole’s event
horizon shouldn’t feel any different from
the space around it. According to Einstein,
the pull of gravity should always be
indistinguishable from the consequences
of acceleration: if you were in a rocket
accelerating quickly through space, you
would feel as if gravity were pushing you
back in your seat. This equivalence
principle is supposed to hold on either
side of the event horizon, which means
you shouldn’t notice anything out of the
ordinary as you cross it.
But even if we do allow the existence
of such a barrier, it’s not a pretty way out.
If you were to cross it, you would be fried
to a crisp, and so would any other
infalling matter.X
Brave the firewallYou braved the firewall – and fried. You might
want to try ∞ (right) as your get-out clauseYou are attempting to violate the
laws of quantum physics. Choose
immediately between these
three options
To brave the firewall, go to X
To take a space-time leak, go to Y
To split the universe, go to Z