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!
