How much do you know about black holes?
What  would happen if you fell into one?
Say one day you were exploring space looking
for a new planet for humans to inhabit,
but came across a black hole
and decided - why not check it out?
This is WHAT IF,
And here's what would happen
if you fell into a black hole.
Despite being "black" and a "hole",
a black hole is not a dark, empty space.
In his theory of relativity, Einstein
predicted how black holes are formed.
When a massive star dies, it leaves
a smaller, remnant core behind.
If the core's mass is at least three
times bigger than the mass of our Sun,
gravity overwhelms all other forces
and turns the core into a black hole.
But don't let its name fool you,
but rather a huge amount of matter
packed into a very small space.
Think of the Sun with its gravitational
field 28 times stronger than Earth's.
If you were to walk on the Sun,
you'd be 28 times heavier than you are on Earth.
Now imagine squeezing four suns
into something that is just
15 miles (24 km) in diameter,
the distance you can cover in a 30-minute drive.
What would the gravity be like there?
A black hole's gravity is so
strong, that even light can't get out.
That's why you'll never see one.
But you can detect it from the
gamma-ray bursts that the hole emits.
These bursts, discovered by
Stephen Hawking, now carry his name. 
Stephen Hawking himself believed
that black holes can be
passages to another universe.
So if you were to fall into one,
would you find yourself in an alternate dimension?
Every black hole has an event horizon,
the point at which the gravitational
pull becomes so strong
that you can't escape from it.
If you found yourself outside of that point,
you'd see that stars are twisted
around a perfect circle of darkness.
As you start being pulled toward a black hole,
you'd be moving faster and faster,
accelerated by the force of gravity.
That's the first bad news for
you, brave space traveler.
The gravitational force of a
black hole is very, very strong.
If your fell feet-first,
your legs would be feeling a stronger
gravitational pull than your head.
Your body would be stretched apart.
The most common black holes are called "stellar".
They can stretch to about 9 miles
(15 km) across and be as heavy as 20 suns.
If you happened to be pulled
towards a stellar black hole,
you'd be completely torn apart
before you even reach the event horizon.
So make sure to choose a supermassive one.
The one that is a million
times heavier than our Sun.
In this case,
your body would remain unharmed
as you cross the event horizon,
as the gravity would be pulling both your feet
and your head with almost the same strength. 
And if you're wondering where to find
one - you don't have to look too far.
There's one right at the center
of our Milky Way galaxy.
Luckily, it's 165 quadrillion miles
(or 265 quadrillion km) away from us
and will suck neither the Sun nor the planets. 
But don't pack your suitcase just yet -
crossing the event horizon is
just the beginning of the challenge.
There's a gravitational singularity
at the center of the black hole,
where density becomes infinite.
You'd be squashed into that center
and become one with the black hole.
You wouldn't be able to tell
anyone about your experience.
However, a person observing
you outside of the event horizon
would see a very different picture.
As you were falling into a black hole,
they'd see you slowing down,
getting dimmer and redder.
In the end, you'd just freeze,
never crossing the event horizon.
This is because space and time
in a black hole swap their roles.
At the event horizon, time comes to a standstill,
while space, on the other hand, moves forward.
You wouldn't be able to turn around
and escape the black hole,
any more than you can travel back in time.
Even when the black hole eventually died,
emitting all the particles it had
absorbed (including your body),
it would be impossible to tell
whether those particles were you.
Stephen Hawking, however, found a way,
in which the information about
your body wouldn't be lost.
He theorized that there are alternate
universes with different histories.
This means, in one reality
you fell into a black hole.
In the second one, there was no black hole.
It's impossible to be certain
from outside of the event horizon
whether there's a black hole
or not until you fall in there. 
If you crossed the event horizon
and there was a black hole...
Sayonara...
But if you happened to be in a reality
where the black hole didn't exist,
you'd still be alive.
Just in a different universe.
There would be no way
for you to get back to ours.
Would you dare explore the possibility?
Are you drawn to black holes and the universe?
Very soon we'll take you on your
next hypothetical adventure.
