Black holes are one of the few natural phenomena that were mathematically predicted first and then
observed later. They were first predicted in 1780s by natural philosopher Jon Michell who
suggested that the gravity of a sufficiently dense star could be so great that even light
emitted by it would be unable to escape its pull. The discovery in Cygnus relieved the astronomers
because otherwise they would’ve been labelled crazy to believe
in an object so massive that it’s invisible! Because Michell’s star doesn’t emit any light,
it wouldn’t look like this but perhaps something like this?. This is the reason why they are
called black holes, because that’s what they are: a black void in space.
To understand why a star would grow so withdrawn, we must take a look at its rough childhood.
A star is born when gravity forces a huge volume of mostly hydrogen gas to collapse in on itself.
This compression increases the gas’ temperature causing its atoms to violently collide with each
other. These collisions further heat the gas until the hydrogen atoms don’t collide and
ricochet but coalesce to form helium atoms! The mass of a helium atom is less than the
combined mass of two hydrogen atoms. The remaining mass is released as energy whose magnitude is
given by Einstein’s E equals MC squared. The energy released might be small for two
coalescing atoms, but for billions and billions of them, the cumulative release is tremendous.
This same principle that makes a star shine is replicated inside a devastating hydrogen bomb,
albeit in a controlled manner. To survive, the star’s expansion
driven by the explosion must neutralize the compression driven by its gravity.
But eventually, the star runs out of fuel. All the hydrogen has fused to form helium,
all of which has fused to form carbon and so on until finally iron is synthesized. Iron refuses
to fuse any further, so the star is now jam-packed with heavy elements. With no more fuel to burn,
the star begins to cool, and with no heat to combat the compression, it begins to contract.
In 1928, during his voyage from India to England, Chandrashekhar realized that a star could survive
if the gravity’s contraction is counteracted by the repulsive forces between its clustered matter.
We now call such stars -- white dwarfs. These are hundreds of tons per cubic-inch dense as
all the mass is packed in a sphere of just 1000 of miles in diameter.
However, if the star is any denser, its gravity would overcome even the repulsion
forces between the electrons. He calculated that a cold star 1.5 times the mass of the Sun
would undergo further contraction. Yet, it could hold on to life by neutralizing
gravity’s pressure now with the repulsive forces between its neutrons and protons.
Such a star is thus known as a neutron star. These are millions of tons per
cubic-inch dense as all the mass is packed in a sphere merely 20 miles in diameter!
But what would happen if a star were to be even denser?
After arriving in England, when Chandrashekhar showed his results to Arthur Eddington,
the astronomer couldn’t believe that a star could become infinitely dense.
He refused to believe that a star such as the Sun could collapse to a point!
In the 60s, Stephen Hawking and Roger Penrose predicted that an even denser star
would contract into a point of infinite density and hence spacetime curvature,
a point where all known laws of physics break down. The point is called a singularity.
The star becomes a “black hole” because a singularity distorts the spacetime around
it so severely, that any light which falls into the pit is held captive forever. Nothing
escapes it. The boundary where the pit begins is known as the black hole’s event horizon.
It is imperative to know that black holes don’t suck everything like a galactic vacuum cleaner.
If a massive or supermassive black hole the mass of the Sun was to replace it,
we’d still be revolving undisturbed. Survival though would not be guaranteed.
So, we developed a mathematically rigorous model of a black hole, but how is one supposed to find
evidence for its existence? How is one supposed to find, as Hawking asks, a black cat in a dark room?
Then we pointed our telescope towards the constellation Cygnus.
Cygnus X-1 is one of the strongest sources of X-rays that is visible from Earth.
Astronomers realized that matter from the rotating star was blown off into orbit around
its invisible companion. The rotation caused it to heat so greatly that it gave off X-rays.
The unseen thing, however, didn’t necessarily have to be a black hole,
it was equally likely to be a massive star just too faint to be visible. But with the
knowledge of the dynamics of the star’s orbit, astronomers determined the object’s mass to be
six times that of the Sun. The object was too massive to be a white dwarf or a neutron star.
While we lack any explicit evidence due to its withdrawn nature, we have a multitude
of indirect evidence that suggests that Cygnus X-1 is definitely a black hole.
Hawking believed that the Universe is replete with such black holes. He audaciously speculated that
the number of black holes is greater than the visible stars. Surely, millions of stars have
exhausted their fuel during the billions of years this Universe has existed? Supermassive
black holes, billions of times the mass of the Sun, are believed to exist at the center of our
own and probably every galaxy there is. What’s more astonishing is that Hawking,
who went on to occupy the prestigious Lucasian Chair at Cambridge once held by Newton,
showed that black holes aren’t so black after all. He found that they emit very tiny amounts of
radiation we now call Hawking radiation! A black hole radiates particles and eventually vanishes,
but it takes one, billions and billions of years to completely evaporate.
Lastly, a singularity is the most notorious phenomenon in the Universe. Nobody knows what
mystery lies at the bottom of the pit, but it seems to combine the two final pieces of physics.
Its large-scale properties concern the classical General Relativity while its point size concerns
the microscopic Quantum Mechanics. Together they combine to form the Theory of Everything.
For over 60 years, no one, including Einstein, has been able to get hold of it.
It would combine every little discovery man has made throughout his quest for certitude.
The Theory of Everything will surely be the greatest triumph of human reason.