Death From the Skies!: These Are the Ways the World Will End...

by Philip C. Plait

  Solar astronomers detect that the Sun’s position is off as well. That doesn’t make any sense. What could move an entire star . . . ? But they quickly realize the trouble is not with the Sun, but with the Earth. Like the other planets, the Earth is no longer circling the Sun as usual, but is moving off its prescribed orbit.

  Panic spreads. Scientists come to the obvious conclusion: some massive object is approaching the Earth, and its gravity is pulling us off course. They use the data on the other planets’ motions to determine where this object must be, but find nothing at that location of the sky.

  Ironically, seeing nothing confirms their worst fears: it’s a black hole. Backtracking its position reveals it’s headed almost straight at us at the incredible speed of 500 miles per second. Astronomers calculate its mass as a terrifying ten times that of the Sun’s—easily enough to spell doom for us on Earth. The gravitational effects are subtle at first, but accelerate.

  Just a few weeks after the first trouble began—and its position still 300 million miles away—the black hole’s gravity as felt on Earth is equal to that of the Sun. Earth no longer orbits one star: it is enthralled by two: one living, one dead. Within a few more days, the black hole’s influence is far stronger than the Sun’s. Grasping the Earth with invisible fingers, it tears us away from the Sun, bringing us closer to the collapsed star.

  As we approach, the gravitational tides from the black hole begin to stretch the Earth. Tides from the Moon cause the oceans to ebb and flow, but the black hole has 200 million times the mass of the Moon. Even from millions of miles away, the tidal force is causing enormous floods, gigantic earthquakes, tsunamis.

  The coup de grâce quickly arrives. When the black hole reaches a distance of just seven million miles, the force of its gravity as felt by objects on the Earth’s surface is equal to the gravity of the Earth itself. The few survivors of the past few days’ events suddenly find themselves weightless as they are pulled both up and down with equal force.

  Within minutes, as the black hole draws ever closer, the force upward dominates. A rising hurricane of air now blows weightless people up, along with rocks, cars, the oceans . . .

  An hour later, it’s all over. The immense gravity of the dead star rips the Earth to pieces, shredding it into vapor. The material that once constituted our home world falls toward the voracious maw of the hole, swirling around it ever faster, forming a disk of million-degree plasma before taking the final plunge.

  Without a hiccup, without a stumble, the black hole sails on, down and out of the solar system, leaving behind chaos, scattered planets, and death.

  THE HOLE TRUTH

  What is it about black holes? The mind-boggling physics, the sheer destructive power, the weird way they twist our notions of reality, space, and time?

  Maybe they fascinate us simply because they’re cool.

  Born in the hellish heart of a supernova, announcing their presence with twin beams of unstoppable fury, and devouring (almost) all that is in their path, black holes are firmly fixed in the public’s mind. Movies, television shows, books, countless articles, and endless discussion have revolved around them. Yet with all this excitement and interest, most people really have only a vague idea of just what black holes are, and what they can and cannot do.

  But never forget, they’re dangerous. There are many ways a black hole can kill you. Some are simple, and some are truly bizarre. Unless you’re looking for trouble, they’re all unlikely in the extreme, but if you want rampant destruction on a large scale, then a black hole is a good place to start.

  I’VE FALLEN AND I CAN’T GET UP

  As pointed out in chapter 4, a black hole, by definition, is an object whose escape velocity is equal to or greater than the speed of light. That means that anything that falls in cannot get out, because as far as we know nothing can exceed the speed of light.

  Therefore, the first and most obvious danger from a black hole is, simply, falling in. If that happens, well, that’s that. It’s a one-way trip. You’re done. End of discussion.

  As a way a black hole can kill you, that’s not terribly exciting—no death rays, no vast and terrible wreaking of havoc. Just bloop! And you’re gone.

  This lack of drama is a bit unsatisfying from a storytelling stance. But it also defies our common sense.36 If you’re in a rocket plunging into a black hole, can’t you just turn the rocket around and thrust really, really hard and get out?

  No, you can’t. The extremely strong gravity near a black hole forces us to change the way we think about space, time, and motion.

  Mathematically, the gravitational pull you feel from an object drops as the square of your distance from that object; double your distance from an object and the gravity you feel from it drops by a factor of 2 × 2 = 4. Get ten times farther away and the force drops by 10 × 10 = 100. Make the distance as big as you please; gravity goes on forever, and the force never actually drops to zero.37

  So imagine you are on the surface of the Earth (which should be easy enough to do) and you have a ball in your hand. You throw it straight up into the air. As it goes up, gravity pulls on it, slowing its velocity. Eventually, the ball stops (velocity = 0) and then starts to fall back to Earth, accelerating the whole way down until you catch it.

  Now imagine you throw the ball very high, like several miles high. Gravity pulls it downward as it goes up, slowing it, but as it gets higher up, the force of gravity is getting weaker because it’s farther from the Earth. So it’s slowing down, but as it gets higher, the rate at which it’s slowing is itself slowing, because gravity is getting weaker with height.

  This means that if you can throw the ball at just the right speed, gravity will slow it down at the same rate that gravity itself is getting weaker. The ball will always slow down, but never actually reach zero. It will always move away from the Earth, but ever more slowly.

  That’s the definition of escape velocity—the initial velocity you have to give a projectile such that it will always move away from an object (like the Earth), always slowing down, but never stopping, and never falling back.

  If you throw a ball up with slightly less than escape velocity, it will go a long way, but it will eventually come back. If you throw it harder, it’ll just go away. At escape velocity—seven miles per second for the surface of the Earth—the ball is just able to escape from the Earth.

  However, since gravity gets weaker with distance, the escape velocity gets smaller with distance too. If you were on top of a very tall mountain, the velocity at which you have to throw a ball is slightly less than the velocity you’d have to give it down at sea level. Also, escape velocity is an impulse; that is, it’s the velocity you have to give an object all at once to get it to escape. If you can somehow continue to add velocity to a projectile as it heads up, then the concept of escape velocity gets a little trickier.

  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.

  IMPORTANT NOTE: Inevitably, when someone explains the idea behind black holes bending space, they use the analogy of a flat surface being bent by a heavy object, like the mattress and bowling ball. Unfortunately, this leads to a misconception that black holes are circles in space, surrounded by a funnel-shaped distortion of space. But that’s not really the case: the reality is three-dimensional, and the analogy uses only two (the surface of the mattress can be considered two-dimensional but then is bent into the third dimension by the bowling ball). Black holes are spherical,39 and the bending of space is not shaped like a funnel. It’s actually incredibly difficult to describe the shape of the space being bent, because we live in those dimensions, and describing them is like trying to describe the color red to someone blind from birth. We can describe it mathematically, make predictions about it, and possibly even use it to understand other aspects of physics, but picturing it in our heads is almost if not totally impossible.

  So all the following descriptions of waterfalls, cliffs, and all that—those are analogies, two-dimensional representations of a warped three-dimensional reality. That may not make you feel any better, but the Universe has a way of making us uncomfortable. If that weren’t true, this book would have no topic at all.

  We now return you to the regularly scheduled death and destruction by black holes.

  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:

  A good way to understand what happens is to think of a black hole as like a waterfall. Except that what is falling into the black hole is not water, but space itself. Outside the horizon, space is falling at less than the speed of light. At the horizon, space falls at the speed of light. And inside the horizon, space falls faster than light, carrying everything with it, including light. This picture of a black hole as a region of space-time where space falls faster than light is not only a good conceptual picture . . . it has a sound mathematical basis [emphasis added].

  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.

  TIME OUT

  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?

  PASTA-TA

  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 × 1027 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!