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Most people think of a black hole
as a voracious whirlpool in space, sucking down everything around it. But that’s not
really true! A black hole is a place where gravity
has gotten so strong that the escape velocity is faster
than light. But what does that mean, exactly?
Gravity is what keeps us on the Earth, but it can be
overcome. If you toss a rock up in the
air, it will only go up a little ways before
the Earth’s gravity slows it and pulls it
back down. If you throw it a little harder,
it goes faster and higher before coming
back down. If you could throw the rock
hard enough, it would have enough velocity that the Earth’s gravity could not
slow it down enough to stop it. The
rock would have enough velocity to
escape the Earth.
For the Earth, that velocity is about 11
kilometers per second (7 miles/second). But
an object’s escape velocity depends on its gravity:
more gravity means a higher escape velocity, because
the gravity will “hold onto” things more strongly. The Sun
has far more gravity than the Earth, so its escape velocity is
much higher—more than 600 km/s (380 miles/s).
That’s 3000 times faster than a jet plane!
If you take an object and squeeze it down in
size, or take an object and pile mass onto it,
its gravity (and escape velocity) will go up. At
some point, if you keep doing that, you’ll have
an object with so much gravity that the escape
velocity is faster than light. Since that’s the ultimate speed limit of the Universe, anything
too close would get trapped forever. No light
can escape, and it’s like a bottomless pit: a
black hole.
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