Black holes are strange objects. They are accumulations of extremely compact matter, which exerts such huge gravity that at less than a certain distance (the event horizon) nothing can escape their attraction, not even light. Hence their name.
The existence of black holes had been predicted in the 18th century by the English geologist John Michell and the French astronomer Laplace. At that time nobody paid attention, but from 1915, when Einstein formulated the theory of General Relativity, the interest in these mysterious objects grew. It was soon concluded that when a massive star exhausted its ability to produce nuclear fusion reactions, no force of nature would be able to overcome the gravitational pull of the remaining matter, resulting in a black hole. But for a long time there were doubts about their real existence, for the theory seemed to predict that the matter located inside a black hole would occupy a zero volume and therefore would have an infinite density. As physicists usually suspect that infinity is a mathematical concept that cannot happen in real life, there were two possibilities: either black holes do not exist, or Einstein's theory would have to be modified so that they wouldn’t have an infinite density.
Another related problem is the fact that General Relativity and Quantum Mechanics are incompatible.
However, in the case of a black hole, both should be applied: since it is a
large concentration of matter acting through gravitation, General Relativity
should be applied, but as it is a very small object, whose radius tends to
zero, it should be studied using Quantum Mechanics.
During the 1960s, great interest was
aroused in the theoretical study of black holes. One of the results was the no-hair theorem, which states that the
physical information about the object that gave rise to a black hole can be expressed,
after the collapse, by just three variables: its mass, its electric
charge, and its angular momentum. This means that all
black holes that came from objects with these three values equal would be
indistinguishable, even if other variables were very different. Among the unreachable
information would be the baryon number (number of protons and
neutrons in the initial object); the lepton number (number of
electrons in said object); whether the object was made of matter or of
antimatter; and more. According to the no-hair theorem, this information would
be inside the black hole, but it would not be possible to measure it from the
outside. This is what is meant by the metaphorical phrase that black holes have
no hair that may distinguish them.
In 1974, Stephen Hawking observed that, in
theory, a black hole could disintegrate over time. If a pair of virtual particles
were to arise spontaneously, exactly on the event horizon of a black hole, one
of the two particles could go inside the black hole and the other outside. Then
the two particles won’t be able to mutually annihilate, so the one left out
will automatically become a real particle (Hawking
radiation). According to the principle of conservation of energy,
the mass of the black hole must decrease by an amount equal to the mass of that
particle. If this process is repeated over billions of years, a black hole
would eventually evaporate. The smaller the mass of the black hole, the faster
it would evaporate.
Notice, by the way, that the Hawking
radiation is a purely theoretical construct. It has not been detected, nor is
it likely to be detected in the near future. On the other hand, virtual
particles, to which I dedicated another
post, have not been detected either. All these theories move further and
further away from experimental physics, and become mere speculations.
The possibility of a black hole
disintegrating, combined with the no-hair theorem, gave rise to Hawking's paradox. In principle, it seems
clear that virtual particles escaping from a black hole could not carry
information about it. If this were true, when the entire black hole
disintegrates, what happens to the information it contained about the object it
came from, which was supposed to be inside, even though we can't access it?
Does it disappear? But this contradicts a fundamental statement of Quantum
Mechanics: that this type of information cannot disappear, for it is always
preserved. So we have a paradox.
One possible solution to the paradox is the
following: that the information inside the black hole somehow escapes through
the virtual particles into which it disintegrates. Stephen Hawking began by
denying that this could happen, but after the black
hole war, Leonard Susskind and other physicists managed to
convince him. The main argument they provided was the holographic
principle, according to which the information contained in the black
hole would be encoded in its two-dimensional boundary (the event horizon). But remember
that the holographic principle is a consequence of string theory,
which has not been confirmed, and of a possible theory of quantum gravity
that would make General Relativity and Quantum Mechanics compatible, which does
not yet exist.
One can see that physicists have a great
time developing increasingly complex theories, further and further from
reality, because they can’t be verified. I wonder whether by studying these
things they are doing science, or just playing. Of course, from time to time
they are given fame by the media that don’t understand what they are talking
about, but make appealing
headlines.
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