There is nothing in the Universe more awe
inspiring or mysterious than a black hole.
Because of their massive gravity and ability
to absorb even light, they defy our attempts
to understand them.
All their secrets hide behind the veil of
the event horizon.
What do they look like?
We don't know.
They absorb all the radiation they emit.
How big are they?
Do they have a size, or could they be infinitely
dense?
We just don't know.
But there are a few things we can know.
Like how massive they are, and how fast they're
spinning.
Wait, what?
Spinning?
Consider the massive star that came before
the black hole.
It was formed from a solar nebula, gaining
its rotation by averaging out the momentum
of all the individual particles in the cloud.
As mutual gravity pulled the star together,
through the conservation of angular momentum
it rotated more rapidly.
When a star becomes a black hole, it still
has all that mass, but now compressed down
into an infinitessimally smaller space.
And to conserve that angular momentum, the
black hole's rate of rotation speeds up...
a lot.
The entire history of everything the black
hole ever consumed, averaged down to a single
number: the spin rate.
If the black hole could shrink down to an
infinitely small size, you would think that
the spin rate might increase to infinity too.
But black holes have a speed limit.
There is a speed limit to the spin of a black
hole.
It's sort of set by the faster a black hole
spins, the smaller is its event horizon.
That's Dr. Mark Morris, a professor of astronomy
at UCLA.
He has devoted much of his time to researching
the mysteries of black holes.
There is this region, called the ergosphere
between the event horizon and another boundary,
outside.
The ergosphere is a very interesting region
outside the event horizon in which a variety
of interesting effects can occur.
Imagine the event horizon of a black hole
as a sphere in space, and then surrounding
this black hole is the ergosphere.
The faster the black hole spins, the more
this ergosphere flattens out.
The speed limit is set by the event horizon,
eventually, at a high enough spin, reaches
the singularity.
You can't have what's called a naked singularity.
You can't have a singularity exposed to the
rest of the Universe.
That would mean that the singularity itself
could emit energy or light and somebody outside
could actually see it.
And that can't happen.
That's the physical limitation of how fast
it can spin.
Physicists use units for angular momentum
that are cast in terms of mass, which is a
curious thing, and the speed limit can be
described as the angular momentum equals the
mass of the black hole, and that sets the
speed limit.
Just imagine.
The black hole spins up to the point that
it's just about to reveal itself.
But that's impossible.
The laws of physics won't let it spin any
faster.
And here's the amazing part.
Astronomers have actually detected supermassive
black holes spinning at the limits predicted
by these theories.
One black hole, at the heart of galaxy NGC
1365 is turning at 84% the speed of light.
It has reached the cosmic speed limit, and
can't spin any faster without revealing its
singularity.
The Universe is a crazy place.
