20 April 2019 | NewScientist | 31AL
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The laws of thermodynamics are ruthless.
Even a black hole can’t escape their
judgement. According to the second law,
the amount of entropy or disorder in the
universe can never decrease. So when
your disordered body falls in, the entropy
it contains can’t just be wiped away. It has
to be accounted for by an increase in the
entropy of the black hole itself. But if a
black hole has entropy, it must have a
temperature, so – like any object with
a temperature – it has to radiate heat.
How can radiation escape an
inescapable object? In 1974, Stephen
Hawking came up with a workaround,
inventing a type of radiation that now
bears his name.
Rather than emerging from the black
hole’s heart, this Hawking radiation
is made up of particles and their
antiparticle twins that pop into existence
near the event horizon. Such pairs of
particles and anti-particles are always
springing into existence all over the
place. These subatomic twins are
intrinsically linked, with any change
to one immediately affecting the other.
In the language of quantum mechanics,
they are said to be entangled.
By virtue of this subatomic conspiracy,
any anti-particle that falls into a black
hole will have a partner particle that
survives, radiating away and reducing
the black hole’s energy. But this leaves
us with an alarming consequence:
at some point, the black hole willradiate away entirely, taking you with it.
That disappearing act violates another
fundamental tenet of theoretical physics:
that information can’t just vanish. Even if
information is destroyed in one form, like
a book burning when it gets thrown on a
fire, that information is still retained in
the particles of smoke and ash that
survive it – albeit in a form much harder
to read. Physicists rely on this continuity
to access information about the past and
make predictions about the future. If it
could be conjured in and out of thin air,
all the physics we know of would be moot.Stay where you are
If a black hole can destroy information,
it can destroy all trace of you. Game over
(or perhaps head to ∞, page 33)Some physicists have suggested that
you could cling on inside a black hole
as it evaporates via Hawking radiation
(see “A”, left), before getting belched
out in its dying breath. That way at
least some of your information
wouldn’t be lost forever, but held in
captivity for aeons as the black hole
slowly leaks away.
At some time in the distant future,
depending on the black hole’s initial size,
the event horizon would become so small
that even a single wavelength of light
could no longer squeeze inside. At that
point, radiation associated with your
remains would be burped out, leaving
nothing behind but the empty space
where the black hole used to be.
But there is a fatal flaw to this
prolonged game of hide and seek: the
black hole doesn’t get to decide how
quickly it pumps out radiation; the laws
of thermodynamics decide. The more
radiation something emits, the smaller it
should get, and the smaller it gets the less
radiation it can emit. By the time the
black hole gets small enough to vomit out
its last meal, there wouldn’t be anything
left to expel. The black hole would go out
with a whimper, not a bang, and any
residual traces of your existence would
be long gone.B
You bet on a burp – and lost. Goodbye (or there
is always the ∞ option, page 33)Wait for a black hole burp
A