A black hole is formed when a sufficiently large star runs out of fuel and then collapses
under its own gravity, into a single point containing all of its mass.
This point is called a gravitational singularity and the mathematics we use to explain the
physical world around us, namely Einstein’s theory of relativity and quantum mechanics,
both break down at this point and so we do not know what happens at the singularity.
General relativity describes a spacetime continuum as the fabric of the cosmos.
Spacetime incorporates the three dimensions of space (up down, left right and back and
forth) and the fourth dimension of time. Matter and energy can warp space time and this gives
rise to the phenomena of gravity.
There are three other fundamental forces in the Universe; electromagnetism, the strong
nuclear force and the weak nuclear force and it is quantum mechanics that successfully
describes how these forces work as well as how elementary particles behave.
Both general relativity and quantum mechanics fail us when it comes to describing what happens
at the gravitational singularity of a black hole.
The gravity of a black hole is so powerful that nothing can escape it, not even light.
The boundary around a black hole beyond which no light or other radiation can escape is
known as the event horizon.
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Now back to black holes …
A black hole is essentially an invisible star that moves through space.
Black holes also come in different sizes. Stellar black holes are formed when the collapsed
core of the star is at least about three times the mass of our sun.
Black holes can grow by taking in more matter and merging with other black holes.
Supermassive black holes are found in the centre of galaxies. The supermassive black
hole at the centre of our own Milky Way galaxy is an impressive 4.3 million times the Sun’s mass.
It’s a myth that black holes suck up everything in its vicinity like a
destructive cosmic vacuum cleaner.
If the Sun were somehow compressed enough to become a theoretical black hole the Earth
would continue to orbit around it in the same way, although the planet would freeze without
the heat and light from the sun.
Once an object crosses the event horizon it’s only possible future is to go to the centre
of the black hole.
So what happens if you fall into a black hole?
If you fall toward a stellar black hole say 10 times the size of the Sun then the force
of gravity on your feet would be millions of times the force of gravity on your head
and the result would be that you would be ripped apart to become a stream of matter.
This process is known as spaghettification.
If you fall toward and into a supermassive black hole the difference in the gravitational
force between your feet and head would not be as pronounced
and you would remain intact for longer.
However there is the theory that at or in the vicinity of the event horizon is a massive
firewall which will incinerate you as soon as you encounter it.
For an outside observer they would never see you actually cross the event horizon into
the black hole.
Einstein’s theory of relativity describes time as being relative -
it is not the same for everyone.
Incredible though it sounds, time passes slower in a stronger gravitational field relative
to a weaker gravitational field.
So when you are at the event horizon, an observer from a distance will actually never see you
cross the event horizon as your time will appear to stop altogether, to them you remain
at the event horizon forever.
For you, time would flow normally and you’d just fall in.
For the observer you would also get redder and redder and dimmer and dimmer until you
effectively disappeared.
The red shifting comes from light you emit having to expend energy to escape
the gravitational pull of the black hole.
As particles of light must always travel at the speed of light, this energy must be lost
through a change of frequency rather than a change in speed.
If the energy of the photon decreases, the frequency also decreases. This corresponds
to an increase in the wavelength of the photon, or a shift to the red end
of the electromagnetic spectrum.
You will become invisible to the observer when your light becomes infinitely red shifted
at the event horizon.
For you, as you approached the event horizon, you’d conversely see the Universe speed
up and at the event horizon all of time would pass in an instant.
Blackholes will eventually evaporate through Hawking radiation so called after
Stephen Hawking, who predicted it.
Quantum fluctuations in empty space result in the creation of virtual particles.
Virtual particles are particle and anti-particle pairs that are created out of nothing and
disappear back into nothingness.
At the event horizon one of the particles will be snatched up by the black hole while
the other will radiate away. The escape of one of the particles lowers the mass of the
black hole and given enough time, the black hole will radiate away its mass, and vanish.
And that in a nutshell is black holes explained simply.
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As always thanks so much for watching and I'll see guys next time. Bye!