Learn About: Black Holes
First question any people ask.
What is a black hole:
Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its
gravitational pull. Since our best theory of gravity at the moment is Einstein's general theory of relativity, we have to delve into some results of
this theory to understand black holes in detail, but let's start of slow, by thinking about gravity under fairly simple circumstances.
Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don't throw it too hard, it will
rise for a while, but eventually the acceleration due to the planet's gravity will make it start to fall down again. If you threw the rock hard
enough, though, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the
rock in order that it just barely escapes the planet's gravity is called the "escape velocity." you would expect, the escape velocity depends on
the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would
have a smaller escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the
escape velocity. The Earth's escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second
(about 5300 m.p.h.).
Now imagine an object with such an enormous concentration of mass in such a small radius that its escape velocity was greater than the velocity of
light. Then, since nothing can go faster than light, nothing can escape the object's gravitational field. Even a beam of light would be pulled back
by gravity and would be unable to escape.
In general relativity, gravity is a manifestation of the curvature of spacetime. Massive objects distort space and time, so that the usual rules of
geometry don't apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very strange
properties. In particular, a black hole has something called an 'event horizon.' This is a spherical surface that marks the boundary of the black
hole. You can pass in through the horizon, but you can't get back out. In fact, once you've crossed the horizon, you're doomed to move inexorably
closer and closer to the center of the black hole. You can think of the horizon as the place where the escape velocity equals the velocity of light.
Outside of the horizon, the escape velocity is less than the speed of light, so if you fire your rockets hard enough, you can give yourself enough
energy to get away. But if you find yourself inside the horizon, then no matter how powerful your rockets are, you can't escape.
The horizon has some very strange geometrical properties. To an observer who is sitting still somewhere far away from the black hole, the horizon
seems to be a nice, static, unmoving spherical surface. But once you get close to the horizon, you realize that it has a very large velocity. In fact,
it is moving outward at the speed of light! That explains why it is easy to cross the horizon in the inward direction, but impossible to get back out.
Since the horizon is moving out at the speed of light, in order to escape back across it, you would have to travel faster than light. You can't go
faster than light, and so you can't escape from the black hole.
How big is a black hole?
There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up.
There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a
black hole if you compress it to a high enough density. We suspect that most of the black holes that are actually out there were produced in the
deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole
would be about 10 times the mass of the Sun, or about 10^ kilograms. (Here I'm using scientific notation: 10^ means a 1 with 31 zeroes after
it, or 10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers.
These are thought to weigh about a million times as much as the Sun, or 10^ kilograms.
What would happen to me if I fell into a black hole?
Let's suppose that you get into your spaceship and point it straight towards the black hole in the center of our galaxy. (Actually, there's some
debate about whether our galaxy contains a central black hole, but let's assume it does for the moment.) Starting from a long way away from the black
hole, you just turn off your rockets and coast in. What happens?
At first, you don't feel any gravitational forces at all. Since you're in free fall, every part of your body and your spaceship is being pulled in
the same way, and so you feel weightless. (This is exactly the same thing that happens to astronauts in Earth orbit: even though both astronauts and
space shuttle are being pulled by the Earth's gravity, they don't feel any gravitational force because everything is being pulled in exactly the
same way.) As you get closer and closer to the center of the hole, though, you start to feel "tidal" gravitational forces. Imagine that your feet
are closer to the center than your head. The gravitational pull gets stronger as you get closer to the center of the hole, so your feet feel a
stronger pull than your head does. As a result you feel "stretched." (This force is called a tidal force because it is exactly like the forces that
cause tides on earth.) These tidal forces get more and more intense as you get closer to the center, and eventually they will rip you apart.
For a very large black hole like the one you're falling into, the tidal forces are not really noticeable until you get within about 600,000
kilometers of the center. Note that this is after you've crossed the horizon. If you were falling into a smaller black hole, say one that weighed as
much as the Sun, tidal forces would start to make you quite uncomfortable when you were about 6000 kilometers away from the center, and you would have
been torn apart by them long before you crossed the horizon. (That's why we decided to let you jump into a big black hole instead of a small one: we
wanted you to survive at least until you got inside.)
What do you see as you are falling in? Surprisingly, you don't necessarily see anything particularly interesting. Images of faraway objects may be
distorted in strange ways, since the black hole's gravity bends light, but that's about it. In particular, nothing special happens at the moment
when you cross the horizon. Even after you've crossed the horizon, you can still see things on the outside: after all, the light from the things on
the outside can still reach you. No one on the outside can see you, of course, since the light from you can't escape past the horizon.
Thank you for reading.