Yesterday, a giant in physics died. Stephen
Hawking was the contemporary face of physics, much like Einstein was in his
time, and Isaac Newton before him. There are many obituaries that cover his
life, detailing how he was diagnosed with motor neurone disease (or ALS) at the
age of 21, and was expected to only live anther two years (he lived another
55), or how he was a difficult student at Oxford, and threatened to do his PhD
there if they gave him a second class degree, with the understanding he’d move
to Cambridge if he got a first. This led to him eventually occupying the Lucasian
Chair of Mathematics at Cambridge. But instead of covering all that, let’s
rather take a look at a small part of his life’s work – the radiation that’s
named for him.
After Eintstein’s publication of the
General Theory of Relativity, it became rather well known that light speed was
the speed limit of the universe, and that gravity affected light. Karl
Schwarzschild took Einstein’s field equations, and found a solution that
described the peculiar behaviour that occurred when that gravity of an object
was so massive that its escape velocity (or the speed at which an object would
have to move to escape its gravitational pull) was so large that it exceeded
the speed of light. At this border where this happened, some terms in
Einstein’s equations became infinite, resulting in a singular point in space
with no properties able to be observed from the outside – a singularity.
At first, it was thought this was but an
aberration, but by 1931, Subrahmanyan Chandrasekhar showed that for stars
above a certain size, it was inevitable. By 1939, even Robert Oppenheimer
conceded that such objects might occur, but called them ‘frozen stars’ as light
at the surface would just stop. By 1958, David Finkelstein identified the Schwarzschild
radius, at which no light could escape, an event horizon – a perfect
unidirectional membrane from which nothing could escape. As such, these objects
were now called ‘black holes’ and became subject to research. By the late
1960’s, Roger Penrose and Stephen Hawking had proved that black holes could, in
fact, exist in nature. But then Hawking noticed something else. Something that
occurred when he applied quantum field theory to black holes…
Now, to explain this part, we first have to
examine the underlying structure of the universe. We need to look at quantum
mechanics. In essence, this is the fundamental theory in physics that describes
nature at its smallest scales – below atoms, and into subatomic particles. It
is so named because it examines the quanta (singular: quantum) or smallest
individual pieces of particles of the universe. At this level, everything
exists as both small particles and waves – the wave being the frequency dependent
on the energy of the particle.
To wit, this means that at the smallest
scales of the universe, particles don’t exist all the time – they instead pop
in and out of existence in a ‘probability wave.’ As these waves overlap, we get
areas where there is a greater probability that a particle exists, and that’s
where we’ll find it most likely. By the time all these subatomic particles have
collapsed their wave functions to the atomic level, these larger particles are
a lot more predictable, and we get to atomic theory – with protons, neutrons
and electrons making up atoms of matter.
It’s at this point I’d like to note that
I’m not a physicist, and I’m simplifying heavily to attempt to make it easier
to understand.
To continue, matter also has its ‘inverse,’
antimatter. Antimatter is indistinguishable from normal matter except for its
atomic charge, which is opposite. Hydrogen, for example, is normally composed
of a proton (positive charge) and electron (negative charge), whereas
antihydrogen has an antiproton (negative charge) and a positron (positive
charge). When matter and antimatter particles meet, they annihilate each other
and release energy. In fact, in early days of the universe, it was filled with
both matter and antimatter, but as they interacted, more than 98% of the
matter/antimatter in the universe was annihilated. It’s just due to a
statistical chance that a bit more matter was present than antimatter, and all
that we’re made of is the residual matter left in the universe. No wonder space
is mostly empty, eh?
So what Stephen Hawking noticed is that,
due to the quantum nature of subatomic particles, their waveforms in a vacuum will
inevitably build up and for a brief moment form particles even when there was
none before. Normally, this is not normally noticed, as both a particle and an
antiparticle will be created, and they’ll annihilate each other instantly.
However, at the edge of a black hole, there exist an event horizon, and if one
particle emerges on its outside, while the other passed through the event
horizon from which nothing can return, these ‘virtual’ particles are not
annihilated – one is left free in the universe, while the other eliminates
particles on the other side of the event horizon. The black hole would emit
particles and lose mass, in what is now termed ‘Hawking radiation.’
In other words, Stephen Hawking showed that
black holes… evaporate! Or they would, if they were small enough. The radiation
emitted, he showed, is inversely proportional to the mass of the black hole, and
cosmic background radiation would inevitably overpower the small effect of
evaporation with larger black holes. For a black hole to evaporate, it would
need to have a mass of about the same as the moon. And, like Stephen Hawking’s
life, if a black hole had about the mass of a car, it would be only about a
yoctometer in size – but after a nanosecond, would evaporate and shine briefly
brighter than the light of 200 suns.
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