Death from the Skies! (20 page)

For example, you can in fact escape from the Earth by going more slowly than the escape velocity—at least, the escape velocity at the surface. Suppose you had a rocket with an inexhaustible fuel supply. You launch it at, say, 60 miles per hour, and keep the engines throttled so that it maintains that exact velocity, never slowing or accelerating. Eventually, it will be so far from Earth that the gravity is much weaker and the escape velocity has dropped to 60 mph.
³⁸
At that point, you’ll have escaped, despite never having gone anywhere near seven miles per second, the escape velocity from the
surface
of the Earth.

So, you might say, we can extrapolate this to black holes, right? If I fell into a black hole and had a big enough rocket, I could just thrust away, getting far enough away from the hole to where the escape velocity is something reasonable. Then I’m free!

Sadly, this won’t work. If black holes were just another massive object then you’d be fine, just like the example above. But black holes are
not
just any old objects!

One of Albert Einstein’s big breakthroughs in science was his idea that space is a
thing.
It’s not empty; it’s like a fabric in which massive objects sit. An object with mass has gravity, and that gravity bends space (the example in the last chapter was of a bowling ball sitting on the surface of a mattress, creating a dip in the middle). Any object going past a more massive one will have its path bent by that dip in space, by gravity.

 

 

But a black hole doesn’t just make a dip in space; it carves out a bottomless pit, an infinitely deep hole with vertical sides. Once you’re inside, no velocity will ever get you out again. You fall in, and nothing can prevent it. For a black hole, the escape velocity at its “surface”—called the
event horizon
—is the speed of light.
⁴⁰

A more accurate way to think of this is using Einstein’s mathematics and physics of relativity. Andrew Hamilton, an astrophysicist at the Department of Astrophysical and Planetary Sciences at the University of Colorado, Boulder, has studied black holes for quite some time, and has an interesting analogy:

 

This may seem like it breaks another of Einstein’s laws—nothing can go faster than light—but that only applies to physical objects with mass (and light itself). Space itself is different than matter and light (another one of Einstein’s Big Ideas) and so it can do whatever it wants, including moving faster than light.

If you are inside the event horizon, space is flowing down faster than light speed . . . and if you fall in, it’s carrying you with it. If you try to paddle up a waterfall, you’ll fail, because you cannot possibly get your boat moving
up
faster than the water coming
down.
So it is inside a black hole: with space flowing toward the center at transluminal speed, you can’t paddle your rocket fast enough. You’re doomed.

There is another way to think of this as well, but it’s even weirder (if that’s possible). If you look at the (fiendishly complex) equations that govern how space and time work near a black hole, you find that inside the event horizon, the variables representing space are constricted. Outside a black hole—like where you are now—you can move freely in space: up and down, front and back, left and right. However, inside a black hole, that freedom is removed. There is only one direction in which you can move: down.

Black holes are funny: even such a simple act as moving around turns out to be complicated. But the basic lesson is: if you fall in, no matter what, you’re dead.

Or are you?

Another one of Einstein’s Big Ideas was that time and space are inextricably entwined, so much so that we actually refer to them together as space-time. When he formulated his theory of relativity, he realized that both space and time look different to someone who is moving relative to someone else. You may have heard of this already: imagine two people, each one in a separate spaceship, and each holding a clock. If one spaceship is moving very rapidly relative to the other, each of them will see the other’s clock running at a slower pace, but their own will tick normally.

This is not a mechanical issue in the clocks; it’s a physical manifestation woven into the fabric of space-time itself. And it’s not just a guess: there have been countless experiments that show that Einstein was exactly right. Because space and time are two sides of the same coin, relative motion through
space
affects the way we perceive
time.

Not only that, but gravity warps the way time flows as well. The closer you are to an object with strong gravity, the slower your clock will run—the slower time will appear to flow—as seen from someone farther away from the massive object. To you, your clock appears to be keeping time perfectly. Again, this has been confirmed via experiment. If you want to live longer, find the lowest spot you can! You’ll experience more gravity, and others will perceive your biological clock as running more slowly. Of course, the effect is small for the Earth because its gravity is so weak. You might live a microsecond or two longer at sea level than if you lived your life out on a mountaintop, but that’s about it. And worse, you yourself won’t notice the difference, since you see your clock as running fine no matter where you are.
⁴¹

But black holes have
lots
of gravity (and time to kill). Time dilation is very strong near a black hole. Imagine you are an astronaut near a black hole. You leave your copilot behind and let yourself drop in. As you approach, your friend, safe and snug in the capsule above, sees your time as flowing more slowly than his. The closer you get to the black hole’s event horizon, the slower your time flows. You can try to talk to him, but your sentences get ssstttrrreeetttccchhheeeddd oooouuuuutttttttt . . .

When you fall into a black hole you are essentially riding along with space as it falls in. As you get closer, it falls in faster and faster. At the event horizon, space is falling into the hole at the speed of light. To a crewmate above, observing you through the light you emit, you never actually appear to cross the event horizon because the light you are emitting is going
upward
at the same speed space is traveling
downward.
It’s basically treading water. As far as your crewmate can tell, you will remain suspended for an eternity at the event horizon, never falling in.

However, as a ticket to immortality, this is a bum ride. Because
this is only how your friend perceives it.
To your perception, you simply fall in. Plop! The event horizon, to you, is not a special place or time, and to you your clock takes that licking and keeps on ticking. You fall all the way to the center (to the singularity where all the matter is compressed to a dot), and you’re dead.

Some people argue that because of this time-stretching, you can never fall into a black hole, but that’s a misconception. You sure can, and when you do, you’re gone. Your friends may not see it that way, but then they are sitting someplace safe while you’re falling into a black hole, so who cares what they think?

In some ways, a black hole isn’t all that different from any other object.

Anything that has mass has gravity. You do. I do. A bag of hammers does, the Earth does, the Sun does, and so does a black hole. The gravity you feel from an object depends on just two things. One is the mass of that object: double an object’s mass, and the gravity you feel from it doubles as well.
⁴²

The other factor gravity depends on is your distance from the object—or actually, your distance from its
center of mass.
Remember, as described above, the force of gravity drops as the square of the distance, and that means the force
increases
at the same rate as you
approach
that object.

Let’s take a look at the Sun. It’s very massive—2 × 10
²⁷
tons (a 2 followed by 27 zeros), which is pretty impressive—and it’s pretty big, about 860,000 miles across. If you could stand on the surface of the Sun without being vaporized, you’d feel a gravitational force about 28 times what you feel here on Earth.

But that’s really the most gravity you could feel from the Sun. If you backed off (which is a good idea), the gravity you feel from it would drop, because you are farther away. And if you stand on its surface, you can’t get any closer. If you did, you’d be
inside
the Sun. That would put you closer to its center, but now there is mass outside of your position, above your head. You can think of that mass as pulling you up, canceling a little bit of the gravity pulling you down.
⁴³
As you get closer to the center of the Sun, the gravity you feel gets smaller. At the very center, you’d feel no gravity at all.

But now let’s change the situation a bit. Let’s compress the Sun so that the mass stays exactly the same, but it now has a diameter of, say, 3.6 miles. Since all that mass is now packed into a sphere only 1/240,000th as wide, the gravity at the surface will scream up . . . but the gravity you would feel 430,000 miles away (the original solar radius)
would be exactly the same!

Think about it: the mass is the same, and your distance (from the center of mass of the compacted Sun) is the same. Since gravity only depends on these two things, the force from gravity that you feel is the same as it was when the Sun was normal-sized.

The difference is, if you get closer,
the gravity goes up.
Before, it went
down
because you were inside the Sun. But now the Sun is small, so you can keep getting closer, and as you do, the force of gravity increases. It would go up and
up
and UP until you got 1.8 miles from the center (half the diameter), and at that point you’d be in real trouble.

Why? Because I didn’t pull that “3.6 miles” number out of thin air. At that size, the Sun’s gravity would be so strong that not even light could escape (you were wondering where this was going, I bet). That’s right—if we could compress the Sun to that size, it would become a black hole.

The important point here is that from a long way off, the gravity from a black hole is exactly the same as from an object that’s far larger but has the same mass. From a zillion miles away, the gravity from a black hole with ten times the Sun’s mass would feel exactly the same as the gravity from a normal star with ten times the Sun’s mass.
⁴⁴

Black holes are dangerous because you can get closer to them.
That’s where their real power lies. They are not necessarily more massive than other objects—many stars are far more massive than black holes. Their strength is in their
size.
Or their lack of it: they’re
small.
They’re so small that you can get really close, and their gravity increases enormously as you get closer.

This would have a very surprising consequence if you were brave—or foolhardy—enough to approach a black hole. A ripping good one, in fact.

If you fall into a black hole feet first, your head will be roughly six feet farther from the black hole than your feet (depending on your height, of course). Since gravity depends on distance from the center, the black hole will pull on your feet harder than it will on your head.

From far away this difference in the force of gravity between your head and feet is small, but as you get closer it will increase.

This difference in force is called a
tidal force.
⁴⁵
The Earth experiences a tidal force from the Moon: the side of the Earth nearer the Moon is pulled slightly harder by the Moon’s gravity than the far side of the Earth. This raises a bulge on the Earth under the Moon. But counter-intuitively, it actually raises two bulges: the one
under
the Moon, and another one on the opposite side of the Earth,
away
from the Moon.