Black holes are the gravitational
monsters of the Universe.
They are so powerful that nothing,
not even light, can escape their grasp.
One black hole is bad enough, but...
if you took two black holes
and smashed them into each other,
they'd be capable of changing
the shape of space itself.
Of course, we have no means
to capture or control black holes yet.
So, imagine you're observing one from
a giant telescope here on Earth.
And suddenly, you notice something strange.
Is that another black hole?
It sure looks like it.
And it also looks like these two
are about to collide.
It turns out that even gravitational
giants like black holes
occasionally run into each other
in the seemingly vast Universe.
Are you about to see a
nuclear bomb-like explosion
of supersize proportions?
The reality might be a little disappointing.
But you still wouldn't want that galactic bomb
to detonate anywhere near your planet.
Bear with me here, because
to understand how black holes collide,
you need to arm yourself with a bit of theory.
Black holes come in all shapes and sizes.
And, of course, the magnitude of the collision
would depend on which black holes
are slammed together.
Stellar-mass black holes
are the most common type.
They can have up to 20 solar masses,
but could fit in a ball
much smaller in diameter than our Sun. 
Now, let's go back to you
observing this gravitational show firsthand.
First, you'd see the two black
holes getting a little closer,
until they started to orbit each other.
They would begin pulling matter and gas
into a vortex between them.
Get comfortable, because it could take
billions of years for their centers to merge.
But as the black holes merged into one,
they'd wobble a little,
and settle into a new, bigger black hole. 
I hope you're not disappointed
with such an explosion-free outcome.
But wait, there is something
I haven't mentioned yet.
The excess energy from the collision
would be expelled back into the Universe
as gravitational waves.
As I mentioned before, colliding black holes
can change the shape of
the space around them.
They do it by rippling this space
with gravitational waves.
Albert Einstein predicted gravitational waves
over a hundred years ago.
But scientists didn't observe them until 2015,
when the advanced LIGO
gravitational-wave observatory
finally detected gravitational waves
after 8 years of operation.
The cool thing about these waves
is that they affect the distance
between the Sun and the Earth.
They only change it by the
diameter of a hydrogen atom.
You'd need a whole lot of colliding black holes
to really see a difference.
These black holes have a mass
of at least hundreds of thousands of times
the mass of our Sun.
But the thing is, they might
never even collide at all.
Two supermassive giants would get
close to each other until they reached
a gravitational balance, and
froze in their respective orbits.
But add another supermassive
black hole to the equation,
and the three of them could
tip over the gravitational
scale, and, finally, collide.
Not in a massive cosmic explosion,
but a silent merger, sending
powerful gravitational waves
throughout the Universe.
Despite the fact that black holes
are an unstoppable, insatiable force
from which nothing can escape,
we have nothing to fear from their collision.
Okay, maybe if that collision happened
somewhere in the neighborhood
of our Solar System.
Then the merging black holes would rip
all the planets in their vicinity to shreds,
including Earth.
If you really want to look at a black hole,
even a single black hole,
and not turn into a
human-flavored spaghetti noodle,
you need make sure you
stay a safe distance away.
But that's a story for another WHAT IF.
