Evaporating Black Holes
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| Credit - ParallelVision: https://pixabay.com/illustrations/black-hole-nebula-space-eye-5868615/ |
Black holes are usually defined as regions of space-time
that are so dense that nothing, not even light can escape from it. However,
this is not quite true. It turns out that black holes could be evaporating, losing
mass, and shrinking in the process. This phenomenon, proposed by Stephen
Hawking in 1974, is called Hawking radiation.
“Black holes ain’t as black as they are painted” –
Stephen Hawking, August 2015
In empty space, pair production and annihilation occur spontaneously.
Pair production describes the formation of a particle and its corresponding
antiparticle, normally from the interaction of a high energy photon with a
heavier particle. For example, an electron and a positron (antiparticle of the
electron) can be formed when a high energy photon interacts with an atomic
nucleus. Pair annihilation is essentially the reverse of this process: a particle
colliding with its antiparticle will result in their annihilation and the
production of two high energy photons. When this occurs at the event horizon of
a black hole, sometimes, one of the particle-antiparticle pair disappears into
the event horizon (due to the strong gravitational pull) leaving the other free
to escape.[1] Antimatter-matter pair creation requires energy –
usually energy is temporarily borrowed from the vacuum itself. When the pair annihilate
each other, this energy is ‘returned.’ However, if, at the event horizon, a
particle is able to escape, it takes energy away from the black hole. This loss
of energy results in the black hole losing mass.[2] Eventually, this
could cause black holes to seemingly disappear.
Searching for Hawking radiation
Although most scientists agree that Hawking radiation is a
real phenomenon, it has yet to be observed. The Fermi Gamma-ray Space telescope
was launched in June of 2008. Researchers hoped that it would be able to detect
gamma rays emitted by primordial black holes (formed during the early universe soon
after the Big Bang) which may have reached their final stages (over the course
of the lifetime of the universe) which would have caused an increase in the number
of particles emitted. [3] Unfortunately, the telescope has been
unable to provide evidence for Hawking radiation and so the search continues. An
alternative approach to provide experimental evidence is to examine analogue
black hole which aim to model the behaviour of real black holes. For instance, some
researchers have used sonic black holes: this is a phenomenon where phonons are
unable to escape a fluid (with special conditions) which emulates a black hole.
[4]
Black hole information paradox
The concept of Hawking radiation has sparked a lot of
questions among the scientific community. Hawking’s theory suggests that the ‘radiation
emitted from a black hole does not depend on how the object was created.’ [5]
This indicates that two different black holes can ultimately end up as the same
emitted radiation pointing to the idea that black holes destroy information.
This violates unitarity – a key principle in quantum mechanics. Unitarity
states that ‘the present always preserves information about the past.’ [6]
This problem has troubled physicists for decades with many offering potential
solutions. Overall, this paradox has not been conclusively solved.
Sources:
1. Britannica: https://www.britannica.com/science/Hawking-radiation
2. PBS Spacetime: https://www.youtube.com/watch?v=qPKj0YnKANw
3. Amber Jorgenson, Astronomy.com: https://astronomy.com/news/2018/02/the-search-for-primordial-black-holes-continues
4. Meredith Fore, Live Science: https://www.livescience.com/65683-sonic-black-hole-spews-hawking-radiation.html
5. Yasunori Nomura,
Scientific American: https://blogs.scientificamerican.com/observations/have-we-solved-the-black-hole-information-paradox/
6. Quanta Magazine: https://www.youtube.com/watch?v=D0-JbxX209g&t=14s
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