Can a black hole be destroyed? – Fabio Pacucci

Black holes are among the most
destructive objects in the universe. Anything that gets too close to the
central singularity of a black hole, be it an asteroid, planet, or star, risks being torn apart by its
extreme gravitational field. And if the approaching object happens
to cross the black hole’s event horizon, it’ll disappear and never re-emerge, adding to the black hole’s mass and
expanding its radius in the process. There is nothing we could throw
at a black hole that would do the least bit of
damage to it. Even another black hole won’t destroy it– the two will simply merge into a larger
black hole, releasing a bit of energy as gravitational
waves in the process. By some accounts, it’s possible that the universe may
eventually consist entirely of black holes in a very distant future. And yet, there may be a way to destroy,
or “evaporate,” these objects after all. If the theory is true, all we need to do is to wait. In 1974, Stephen Hawking theorized a process that could lead a black hole
to gradually lose mass. Hawking radiation, as it came to be known, is based on a well-established phenomenon
called quantum fluctuations of the vacuum. According to quantum mechanics, a given point in spacetime fluctuates
between multiple possible energy states. These fluctuations are driven by the
continuous creation and destruction of virtual particle pairs, which consist of a particle and its
oppositely charged antiparticle. Normally, the two collide and annihilate
each other shortly after appearing, preserving the total energy. But what happens when they appear just at
the edge of a black hole’s event horizon? If they’re positioned just right, one of the particles could escape the
black hole’s pull while its counterpart falls in. It would then annihilate another
oppositely charged particle within the event horizon
of the black hole, reducing the black hole’s mass. Meanwhile, to an outside observer, it would look like the black hole
had emitted the escaped particle. Thus, unless a black hole continues
to absorb additional matter and energy, it’ll evaporate particle by particle,
at an excruciatingly slow rate. How slow? A branch of physics, called black hole
thermodynamics, gives us an answer. When everyday objects or celestial bodies
release energy to their environment, we perceive that as heat, and can use their energy emission to
measure their temperature. Black hole thermodynamics suggests that we can similarly define the
“temperature” of a black hole. It theorizes that the more massive the
black hole, the lower its temperature. The universe’s largest black holes would give off temperatures of the
order of 10 to the -17th power Kelvin, very close to absolute zero. Meanwhile, one with the
mass of the asteroid Vesta would have a temperature close to 200
degrees Celsius, thus releasing a lot of energy
in the form of Hawking Radiation to the cold outside environment. The smaller the black hole, the hotter it seems to be burning– and the sooner it’ll burn out completely. Just how soon? Well, don’t hold your breath. First of all, most black holes accrete,
or absorb matter and energy, more quickly than they emit
Hawking radiation. But even if a black hole with the
mass of our Sun stopped accreting, it would take 10 to the 67th power years– many many magnitudes longer than the
current age of the Universe— to fully evaporate. When a black hole reaches
about 230 metric tons, it’ll have only one more second to live. In that final second, its event horizon becomes
increasingly tiny, until finally releasing all of its energy
back into the universe. And while Hawking radiation has never
been directly observed, some scientists believe that certain gamma
ray flashes detected in the sky are actually traces of the last moments of small, primordial black holes formed
at the dawn of time. Eventually, in an almost inconceivably
distant future, the universe may be left
as a cold and dark place. But if Stephen Hawking was right, before that happens, the normally terrifying and otherwise
impervious black holes will end their existence in a final
blaze of glory.

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