Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)
Chapter 5. The Bottomless Pits of Black Holes
EVER VIGILANT, IT’S AN AMATEUR ASTRONOMER WHO catches the first whiff of trouble.
He had hopes of doing some imaging of Uranus using his automated telescope, but the computer consistently points the telescope in the wrong direction. After going to manual, he eventually finds the giant planet several arc minutes from its calculated position. Puzzled, he calls a friend who quickly confirms that he has the same problem. An Internet search on a few astronomy bulletin boards reveals many such events from astronomers all over the world.
As days go by, things get worse. Jupiter seems to be off-kilter as well. Saturn, however, located on the other side of the Sun, appears unaffected. Rumors start to spread.
Then the situation gets really weird. The Solar and Heliospheric Observatory, parked in an orbit where the Earth’s gravity and the Sun’s gravity are in balance, starts to drift. Engineers are puzzled, but soon have other problems with which they must contend. Now Mars is in the wrong place. NASA has a probe on its way to the Red Planet; will it miss? But soon that point becomes moot, as the spacecraft is drifting too. After a few days it’s clear the probe is lost . . . and that the probe is the least of our worries.
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 upfaster 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 × 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!
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.
This happens because the Moon pulls harder on the center of the Earth than it does on the far side—the center of the Earth is closer to the Moon. So, in effect, the Moon is pulling the center of the Earth away from the far side; the result is a bulge on the far side of the Earth from the Moon. To an object suffering under tidal forces, it’s as if it’s being stretched—like taking one end of a rubber band in one hand and the other end in your other hand, and moving your hands apart.
Tidal force is similar to the force of gravity, but while gravity gets stronger with the inverse square of the distance, tides get stronger with the inverse cube. Halve your distance to an object and the gravity goes up by four times, but the tidal force goes up by eight times. Get ten times closer and the gravity goes up by a hundred times, but tides go up by a thousand times.
Obviously, this is going to be a problem.
Let’s say you are an astronaut in a space suit, hovering over a typical black hole of, say, five times the mass of the Sun, which would have an event horizon about 18 miles across. Astronomers call this kind a stellar mass black hole, because its mass is about the same as a star’s.46 Let’s also say you’re a long way off, like 10,000 miles out. If you start your fall from this distance, the entire journey to the event horizon, even starting from a standstill, will last only a couple of seconds! From that distance, the hole is pulling on you at an incredible 270,000 times the Earth’s gravity. But oddly, you wouldn’t feel it. Since you would be in free fall, with nothing to resist the force of gravity, you would actually feel weightless, just as skydivers do for the first few seconds of their fall, or astronauts as they orbit the Earth.
From this distance, the tidal force due to the six feet between your head and your feet isn’t noticeable.
A second or so into your fall and you’d be accelerated even more. At 5,000 miles away, you have about one second left before you hit the event horizon, even from this distance. If you could speed up your reflexes, speed up your awareness (because you have only one second left to live, and we want you to be aware of what horrifying things are happening to you), you might notice an odd sensation, a feeling as if you’re being pulled in two directions, toward and away from the hole simultaneously, as if people were playing tug-of-war, with you as the rope. The overall force on your body is still enormous, but the tides from the black hole generate a slight extra force on your feet of about a quarter of the Earth’s gravity toward the black hole, and there will be an extra force on your head, up and away from the black hole, of the same amount. If you weigh 160 pounds, it would feel like a 40-pound weight hanging off your feet, and the same pulling your head up. It’s uncomfortable, though not fatal. It will, literally though, make your hair stand up. Unfortunately, this changes a fraction of a second later.
At 1,500 miles away, the sensation is far stronger. It’s as if you’re being pulled apart like taffy—the force downward on your feet is now 10 Earth gravities, 1,600 pounds of weight. So is the force up on your head! The blood pools in your head, and you pass out (what fighter pilots call a “redout,” the opposite of a “black out” when the blood leaves your brain). This, it turns out, is a blessing. You don’t really want to be awake for the next few milliseconds.
That’s when you get into real trouble. At 500 miles from the black hole, the opposite head-to-feet tidal forces are pulling you apart with a horrifying 550 Earth gravities, over 40 tons of weight. The human body isn’t capable of withstanding that kind of stress. Soft tissue pulls apart, and your head and feet burst open from the pressure due to hundreds of pounds of blood pooling in them.
At 50 miles from the black hole surface, the tides are now over 700,000 times the Earth’s gravity. It’s like being suspended over an abyss with a cruise ship strapped to your feet. Your bones snap in half, and then again and again, pulled into tiny pieces.
But wait! There’s more: you’re not just getting stretched along your length, you’re getting compressed across your width. Your left side falls toward the center of the black hole along a slightly different path from the one your right side wants to take. Both are trying to fall straight into the center of the hole, so your right side feels a force to the left, and your left side to the right. This squeezes you, and the force is also incredibly strong, about the same as the stretching force. You’re being stretched out and squeezed in.
You’re like a tube of toothpaste, and the black hole has a fist of steel. You’re turning into a thin noodlelike tube of human goo.
When your feet—well, when what used to be your feet—are right above the black hole’s event horizon, you’re not even recognizable as a human being. You’re stretched into an incredibly thin line, miles long, like a strand of pasta. Scientists call this process spaghettification.47
And the black hole, as if in appreciation of the analogy, slurps you down.
So you see, simply falling into a black hole isn’t the only way it can kill you. The journey there is half the fun.
FRIED BY THE LIGHT
As we saw in chapter 4, the birth of a black hole can wreak damage on an unimaginable scale, blasting out beams of radiation that will burn twin holes through the galaxy. The beams are generated when matter from the collapsing star forms an accretion disk around the black hole, channeling and funneling the matter into the hole. Coupled with the incredible magnetic fields involved, the beams are formed along the spin axis of the disk.
It turns out that this situation is not unique to the birth of a black hole. Anytime matter falls into a black hole, it can form such a disk, and beams can be generated as well. For example, if a black hole is orbiting a normal star (they form a binary pair, with one originally a very high-mass star that explodes and forms a black hole), then the hole can “siphon” matter off the other star. Usually this happens when the normal star nears the end of its life and becomes a giant star (see chapter 8); the outer layers of the giant star can be drawn into the black hole.
As matter falls into a black hole, it can pile up outside the event horizon, forming a flattened disk. Friction and other forces heat it to millions of degrees and can focus jets of energy and matter, as seen in this artist’s illustration.
The incoming matter forms an accretion disk just like when the black hole itself was born, and that disk gets incredibly hot. Surprisingly, the bulk of this heating is due to a rather mundane and everyday force: friction! When the matter gets near the black hole, it orbits faster and faster. Because of the ferocious gravity, a particle just slightly closer to the black hole can be moving substantially faster than one slightly farther out. They rub against each other, and friction heats them up.
As usual with black holes, they do nothing by halves. The friction can heat the disk to literally millions of degrees. Matter that hot generates radiation across the electromagnetic spectrum, from radio waves up to X-rays.48 In fact, so much light is generated that, ironically, black holes (really, their accretion disks) can be among the brightest objects in the Universe.
Actually, this was how the first black hole was discovered. When the first X-ray satellites were launched, they found many such sources of high-energy light in the sky. One was traced to a giant star in the constellation of Cygnus. While this star, called HDE 226868, is a real bruiser—it has 30 times the Sun’s mass—it just doesn’t have the oomph needed to make X-rays on the scale observed. Sure enough, spectra49 taken of the star indicated that it was being orbited by another object with about 7 times the Sun’s mass, yet nothing was seen in the images. That meant it had to be a black hole; a 7-solar-mass star would have been be easy to detect. Plus, this naturally explained the X-rays blasting out of this system; the black hole (dubbed Cygnus X-1) was accreting matter from the giant star, and blasting out X-rays as the material swirled to its doom.
And by “blasting,” I mean blasting. If you took all the energy emitted by the Sun and added it up, the black hole would be 10,000 times brighter just in X-rays. It’s one of the brightest sources of X-rays in the sky, even at its distance of 6,500 light-years. If it were instead just a few light-years away, the X-rays could pose a threat to our satellites and manned space program (see chapter 2).
So you don’t even have to be particularly close to a black hole for it to be dangerous.
Cygnus X-1 is the closest known black hole to the Earth, but by estimating how many stars are born capable of turning into black holes over the lifetime of the Milky Way, scientists have extrapolated that there may be millions more black holes in our galaxy alone.
I know what you’re thinking: “Millions? Millions of black holes lurking throughout our galaxy? AIIIEEEE!”
Well, yeah. That sounds bad, so maybe we should take a moment and talk about that too . . .
JUST PASSING THROUGH
Our galaxy is lousy with black holes. They’re everywhere! But what if one of them comes knocking on our door? Will that affect the planets, and even Earth?
Let’s get this out of the way right now: this is an incredibly unlikely event. Space is big, and there’s lots of room to knock around.
The Milky Way Galaxy is a collection of gas, dust, and something like 200 billion stars held together by their mutual gravity. It’s a spiral galaxy, which means its major feature is a flat disk 100,000 light-years across punctuated by vast and beautiful spiral arms, like a pinwheel. To give you a sense of how big that is, the Sun, which is about halfway from the center to the edge of the disk, orbits the center of the galaxy at 160 miles per second, yet it still takes well over 200 million years to complete one orbit.
All the stars you see in the sky are relatively nearby; most are less than 100 light-years distant, a tiny fraction of the galaxy’s size. The nearest known star is the triple system Alpha Centauri located a little over four light-years away. In English, that’s about 26 trillion miles, so we’re not exactly crowded out here in the galactic suburbs.
Over the Sun’s lifetime there have certainly been stars closer to us than Alpha Cen, but that depends on what you mean by “close.” Space is big, and stars are small. One study showed that a star passes about a parsec (3.26 light-years, or 20 trillion miles) from the Sun only once every 100,000 years, and that distance is still way too great for the star to affect us through its gravity. Closer encounters are even less common, and it would be unlikely in the extreme for a star to pass close enough for its gravity to significantly affect the Earth.
And that’s for stars in general. There are thousands of stars for every black hole in the galaxy. I hope you’re getting a sense that a close encounter with a black hole has pretty low odds. The closest one known is the aforementioned Cygnus X-1, which is a pretty distant 1,600 light-years away. That’s a bit of a bummer for a book with a title like this one’s, but we must face reality, even if it means we’re safe from the black hole menace.
Still, as with the other topics in these chapters, it’s fun to think about. What would happen if a black hole came to pay us a call?
There’s a good chance we’d never see it. If it was traveling solo through space, it would just be, well, a black hole in space, invisible, emitting no light. A stellar mass black hole is typically only a few miles across, making it far too small to spot until far too late.50
Although nearby stars are orbiting the center of the Milky Way in pretty much the same direction and with roughly the same speed as the Sun, there is some variation. Like cars around a racetrack, a small difference in speed means some cars pass each other. Even though the cars may be traveling at 200 mph, they pass each other relatively slowly, at a few miles per hour. The same is true for stars. The Sun is orbiting the galaxy at 160 miles per second, but so are other nearby stars. The typical speed at which we see them move relative to the Sun is far less, just a few dozen miles per second. At those speeds, it would take years for a star to get from the orbit of Pluto to the orbit of the Earth.
But, it turns out, there are exceptions. Some stars are real speed demons, and interestingly, we see some compact objects like neutron stars moving across the galaxy at amazingly high speeds, hundreds of miles per second faster than you’d expect.
These runaway stars were pretty mysterious at first, but it’s now thought that their high velocities are the product of the supernova explosion in which the stars themselves were born. If the supernova event itself is slightly off-center, exploding more to one side than the other, the material and energy blown out of the star will act like a rocket, pushing it in the other direction. Incredibly, even a slightly off-kilter explosion can impart vast energies to the neutron star remnant, accelerating it to high speeds. Also, if the supernova progenitor is in a close binary system, orbiting another star, the orbital speeds can be several hundred miles per second. When the progenitor explodes, both stars get flung away in opposite directions at large velocities.
Either way, it’s physically possible, even likely, that a neutron star or black hole can be slicing across the galaxy at a pretty good clip.
What if it’s aimed at us? Will we survive a drive-by of a 10-solar-mass black hole, moving at, say, 500 miles per second (a large but reasonable velocity)?51
A black hole getting near the Earth would be bad enough, but if it was actively “feeding,” gulping down material, the outpouring of X- and gamma rays would cook our planet to a crisp.
DANA BERRY, SKYWORKS DIGITAL INC.
The scenario at the start of this chapter should give you a taste of what’s to come. But it depends, of course, on how close it gets to the Earth. Let’s run through what happens on approach and see.
As the marauding black hole approaches the solar system, a planet will feel its gravity as well as the gravity from the Sun. As the hole gets nearer, the planet feels its gravity getting stronger. Like a toy being pulled on by two greedy kids, the planet’s orbit will start to distort. If the passage is distant enough (say, it’s on the opposite side of the Sun), the planet may be relatively unaffected—its orbit may become a bit more elliptical, but that’s about it. But if the hole gets close enough, its gravity will dominate over the Sun’s, especially for more distant planets like Uranus or Neptune, where the Sun’s gravity is relatively weak. If that happens, the planet may start to orbit the black hole, or, more likely, the hole’s gravity will simply slingshot it out of the solar system. In general, in an encounter where you have two massive bodies (like a star and a black hole), and a smaller one (a planet) gets involved, the smaller one is very likely to be ejected from the system.
Such is the scale of disaster of which we are talking: whole planets are literally flicked away.
As the black hole approaches the Earth, we surface dwellers won’t really notice any change in gravity at first, but the Earth as a whole will. Its orbit around the Sun is perturbed more and more as the black hole nears. When the hole is about three times farther away from the Earth as the Sun, or roughly 300 million miles, its gravity will equal that of the Sun.52 When that happens, the Earth is no longer “bound” to the Sun. It could fall into the Sun, or fall into the black hole, or be ejected from the solar system.
Which do you prefer? Hmmm . . . no happy endings here.
Not that we’d have much of a choice. And things are about to get a lot worse.
The tidal force from the black hole, responsible for the spaghettification of our unfortunate astronaut earlier, will begin to affect the Earth as well. At a distance of 300 million miles, where its gravity is equal to that of the Sun, the tidal force is about a third of the Sun’s. That’s not much (much less than the tides from the Moon), and unlikely to cause any damage.
But the black hole is nearing by 500 miles every second, 40 million miles every day. At that speed, it can cover those 300 million miles in about a week, so just a day or so later its tides start to dominate. By the time it’s the same distance as the Sun from the Earth, its tides will be five times stronger than the Moon’s. Water will flood coastal communities, and small earthquakes may be felt.
A day later, it’s half as far as the Sun. Its tides are now 40 times that of the Moon. Tidal waves53 many yards high inundate the coastlines, killing millions of people. And every minute the force gets stronger.
Just a few hours later, when the black hole is a mere 7 million miles away (30 times farther away than the Moon), someone standing on the surface of the Earth will feel the same force from the black hole as from the Earth itself. For just a few moments, you’d be weightless, and a small jump would send you flying upward.
Enjoy it while it lasts. At that distance, the tides from the black hole are a staggering 20,000 times that of the Moon (well, what used to be from the Moon—it would have already been ejected from orbiting the Earth by the black hole’s mighty gravity). The Earth is under colossal strain, and earthquakes would be larger than any ever measured. Whole continents would begin to tear apart, and volcanic eruptions would be constant.
Finally, the tides are more than the Earth itself can handle. It gets torn apart, spaghettified on a planetary scale. What’s left of our once lush planet is shredded and heated to millions of degrees, finally spiraling into the maw of the black hole.
And that, once again, is pretty much that.
Amazingly, all this time, the black hole itself is so small—just under 40 miles across—that even if it weren’t totally black, it would still appear as nothing more than a dot in the sky. Only the most powerful telescopes would see it as anything else . . . but again, it’s black. There’s nothing to see.
As for the prognosis for the rest of the solar system, it depends on the trajectory of the black hole. The Sun itself may escape relatively unharmed if the hole doesn’t get too close to it—otherwise it’ll get torn up pretty well. If the black hole misses by a sufficient margin, the Sun’s path around the galaxy might be only slightly affected, and the Sun itself may survive.
Isn’t that comforting?
The smallest black hole that can form in a supernova is about twelve miles across, and that’s pretty scary. Picture it this way: it’s about twice the size of Mount Everest, and three quadrillion times the mass.
That’s terrifying! But if big is scary, is small cute?
When it comes to black holes, no. They’re all pretty frightening. But can smaller black holes even exist?
Theoretically, they might. Called primordial black holes (or mini black holes, or sometimes even quantum black holes), these would be very small, with masses much less than those of their stellar mass cousins, and maybe even less than the Earth’s. They’ve never been observed, but there may be countless examples of them floating in the depths of space, and they’re called primordial because they’d be as old as the cosmos itself.
In the very early Universe, just moments after the Big Bang, vast energies and densities were being tossed around like snowflakes in a blizzard. Space itself was folded like origami, and for the briefest of instants, just a razor’s edge of time after the initial Bang, conditions were such that a relatively small amount of matter could find itself squeezed by immense forces. If the density of the matter shot high enough quickly enough, it would actually form an event horizon and become a black hole. These mini black holes could have had very modest masses, on the scale of the mass of mountains, a few billion or trillion tons.
Such a tiny black hole would be weird, even for a black hole. The event horizon would be teeny-tiny: a black hole with the mass of the Earth would be only about half an inch across—the size of a marble. One with the mass of an asteroid or a mountain would be far smaller than an atom!
Obviously, such a black hole would be even harder to detect than the normal flavor, which may be why they’ve never been seen (although, to be honest, they may not exist at all; they’re still theoretical). Even if they were to accrete matter, the flow onto a mini black hole would be so small that they’d be invisible even from relatively small distances.
But mini black holes have a secret. You might think that black holes always grow, eternally eating matter and energy, getting larger in the process. But black holes, it turns out, may not be forever. They may evaporate.
In the 1970s, the scientist Stephen Hawking had an idea. It was pretty crazy, but when you’re dealing with black holes, ideas reach the “crazy” category pretty quickly. By applying the laws of quantum mechanics and thermodynamics to black holes, he realized that in some sense, black holes have a temperature. They can actually radiate away energy, just as normal matter does. That energy has to come from someplace, and as he conjectured, it comes from the black hole mass itself.
Here’s how it works. In quantum mechanics, the rules by which the Universe plays get truly bizarre. Energy and mass are interchangeable, with energy easily able to be converted to mass and vice versa.54 But another odd aspect is that space itself can belch out small amounts of energy out of nowhere, ex nihilo if you will. In fact, the fabric of space is positively bubbling with energy that can pop out into the real world.
This may seem to violate one of the most basic properties of the Universe: you cannot create or destroy energy or matter. Normally that’s true. But this energy created out of nothing can exist for only very brief amounts of time, as long as it goes away, back into the nothingness whence it came, very quickly.
It’s like borrowing money from the bank. Eventually, you have to return it. And the more you borrow, the faster you’d better pay it back.
If the Universe decides to belch out a tiny bit of energy, that’s okay, as long as it quickly goes back into the fabric of space. All laws of nature are conserved if this happens quickly enough.
But if it happens near the event horizon of a black hole, things get sticky. The gravity of the black hole can cause this bundle of energy to fragment, creating matter. This happens in the bigger Universe all the time; gamma rays, a form of energy (light), can convert into matter if they collide with each other or interact with matter. Because of the way things must balance, two particles are created: one is normal matter, like a regular old electron, say, and the other is antimatter. Antimatter is exactly like matter, but it has an opposite charge, so an antielectron (called a positron) has a positive charge. That counteracts the negative charge of the electron, and the cosmic ledger books remain balanced.
But if this happens right at the very edge of the event horizon, it’s possible that one particle can fall in while the other remains free. It can escape, and to a distant observer it looks as if the black hole has emitted a particle. This mass (or, equivalently, energy) balance must be repaid, and it comes out of the mass of the black hole. In effect, the black hole has lost a tiny amount of mass.55
Another way to look at it is using tidal force. The particles appear—poof—near the black hole event horizon. The tidal force from the black hole pulls the two particles apart. One falls in, and the other gets out. It takes energy to separate the particles, which has to come from somewhere. It comes from the black hole itself—energy and mass are equivalent, remember, so the black hole loses a tiny bit of mass when this happens.
This process is very slow, and depends on the mass of the black hole. The lower the black hole’s mass, the smaller the event horizon, and the easier it is for this process to happen (or, equivalently, the lower the mass the stronger the tides are near the event horizon). Since the black hole is radiating away mass and energy, this whole process acts as if the black hole has a temperature—it’s warm, and it emits energy to cool off. The smaller the black hole, the higher the temperature, since it loses mass and energy more rapidly. This means, in turn, that massive black holes will last longer than smaller ones, since they radiate away their mass more slowly. A stellar mass black hole will have a temperature of only about 60 billionths of a degree!
But a smaller black hole will be “hotter,” radiating away particles more rapidly. As it loses mass, its temperature goes up, and that means it radiates away matter even faster . . . it’s a runaway process, accelerating all the time. Once it gets below a certain mass—about a thousand tons—it releases all the remaining energy in less than a second. Kaboom! You get an explosion. A big explosion: energy and matter would scream out of the black hole, releasing the equivalent of the detonation of a million one-megaton nuclear bombs.
A mini black hole created in the formation of the Universe with a mass of about a billion tons would be just about at that stage now. Any with smaller masses would have evaporated long ago, and more massive ones are still stable. A stellar mass black hole can tool along for incredible lengths of time before worrying about evaporation; the projected life span of such a hole is more than 1060 years, which is far, far longer than the current age of the Universe (but see chapter 9 to find out what happens when that time finally arrives).
No quantum black hole explosion has ever been seen (though for a while, some people conjectured it might explain gamma-ray bursts), but even that amount of energy would be difficult to detect from light-years away. Could quantum black holes wander the galaxy? What would happen if one got too close; would it be as dangerous as a stellar mass black hole?
Imagine a black hole with a mass of 10 billion tons—roughly the same as a small mountain—heading toward Earth. It is far too small to detect through its distortion of background stars—it’s less than a trillionth of an inch across, smaller than an atom. The gravity from it wouldn’t be enough to affect the planets, the Moon, or the Earth, which are far, far more massive. However, we’d certainly notice it long in advance: because of Hawking radiation, it would burn fiercely at a temperature of billions of degrees! Because it’s so small, it would actually be fainter than the faintest star you can see with your unaided eye, but satellites like NASA’s Swift observatory might detect the gamma rays it emits as it approaches.
Finally, it dives through our atmosphere. It wouldn’t draw in much matter as it fell through the air; a 10-billion-ton black hole would hardly be noticeable gravitationally even from a few yards away. But up close, at distances less than an inch, the gravity would be hundreds of times that of the Earth. Any air within that distance would get sucked right in. This might form a small and temporary accretion disk, but at typical collision speeds of several miles per second there would hardly be time for it to do much before plunging into and beneath the Earth’s surface.
To such a black hole, the solid matter of the Earth might as well be a high-grade vacuum. Far smaller than an atom, it would pass right through the Earth, and at supersonic speeds it wouldn’t get much of a chance to eat much matter. It would almost certainly be traveling faster than Earth’s escape velocity too, so it would blow right through us and move on, perhaps just the teeniest bit heavier, and then continue on its merry way.
Well, that’s not very dangerous. And not much fun either. Let’s try a bigger one.
Suppose instead we have a black hole with a mass equal to the Earth itself, and, through an unfortunate series of circumstances, it was headed right for us. Moreover, just to make sure we get some fun results, let’s also assume it’s moving very slowly relative to the Earth, only a few miles per second. This is incredibly unlikely—it probably wouldn’t happen once even if the Universe were a thousand times older—so it is really just a “what if” scenario, and you needn’t let it keep you up at night.
Getting such a slow approach would be hard, but not impossible. For example, if it was moving slowly enough to start with, and it swung by a planet or two and the Moon on its way in, the primordial black hole’s orbit could be changed sufficiently that it would be able to collide with the Earth and not keep traveling out into space. This would be quite the gravitational dance, and less likely than, say, sinking every pool ball on the opening break ten racks in a row. But we’re looking for some action here, so let’s see what this gets us.
Things would be . . . interesting. First off, we’d never directly detect its approach. Hawking radiation from it would be very weak; its temperature would be similar to that of space itself, far below zero, so it would not be emitting any observable light. However, we’d certainly see it indirectly. As it approached, we would experience vast tidal forces. The black hole is very small—about half an inch across, the size of a marble—but has the mass of the entire Earth. From far away, remember, the force of gravity is the same as the Earth’s. The Moon would be affected profoundly; most likely it would be ejected from the Earth’s orbit forcibly. It’s possible, if things were just right, that the Moon’s velocity relative to the Earth would be slowed enough that it would plummet toward us like the giant stone that it is. If it impacted, the least of our worries would be the black hole. The energy released in the impact would vaporize the surface of the Earth and kill every living thing on it down to the base of the crust.
While that’s quite the apocalyptic scene, we want the black hole to do the deed in this fantasy scenario, so let’s assume that the Moon gets ejected. What happens as the black hole approaches the Earth?
When it is still 240,000 miles away, the same distance from the Earth as the Moon, its tides would be huge, 80 times the strength of the Moon’s. As it gets closer the tidal force strengthens, prompting earthquakes and floods.
Eventually, it falls into our atmosphere. At that point, while it is, say, 100 miles above the Earth’s surface, the destruction would be beyond comprehension. Just the gravity alone would be awesome: you’d feel a force upward, toward the black hole, 1,600 times stronger than Earth’s gravity! Anyone within sight of the black hole’s approach would be picked up and flung away like a leaf in a tornado.
As it plunged through our atmosphere it would suck down quite a bit of gas, possibly creating an accretion disk and emitting high-energy radiation. There would be an enormous shock wave, similar to a nuclear detonation, which would wreak all sorts of havoc—if there were anything left to be wreaked upon.
When the black hole reaches one mile above the ground, anyone still standing (not that there could be) would feel a tidal force of 40,000 times Earth’s gravity trying to rip him apart. Spaghettification would be inevitable. Everything on the Earth’s surface would be literally torn apart.
When the black hole hits solid ground a moment later, the accretion rate would increase, heating it up considerably. There might even be enough energy emitted quickly enough to act like an explosion . . . but at this point that’s fairly moot.
To the black hole, which is incredibly dense, the Earth is essentially a vacuum. It would fall pretty much freely through the Earth. Its ferocious tides would tear the planet’s surface apart as it fell, most likely destroying everything above.
In a sense, that’s too bad. We’d miss the really scary part.
The black hole is so dense that it would essentially be orbiting the center of the Earth inside the Earth itself. As it passed through the Earth’s matter, even a microscopic chunk of rock would feel a tremendous change in the force of gravity if it got too close to the black hole, easily equaling millions of gravities. This tidal effect would tear the rock to bits, heating it up hugely, vaporizing it. Because of this, the black hole deep inside the Earth would be surrounded by a sphere of intensely hot and incredibly compressed gas, similar to what you might find in the core of the Sun. At the center of this cloud, the black hole would be greedily swallowing down the matter. As the black hole moved through the Earth it would be like a blowtorch, heating the material around it and feeding on it.
Even though the black hole is small, this vaporous halo is big enough that it would rub against the solid or liquid rock around it, creating friction. This friction drags on the black hole, which over time slows its speed through the Earth. It would spiral in, falling to the Earth’s core. There, the pressure of the overlying matter would give it a continuous source of food . . . and it would eventually eat the Earth.
The whole planet.
Nothing would be left . . . except the black hole.
We’d be long gone by the time that happened, of course. But to an observer off planet, those last few moments—only a few decades after the black hole first approached the Earth—would be spectacular. The shrunken and distorted planet would be only a few meters across, and white-hot. Finally, in a millisecond’s time, the last piece would fall into the black hole’s accretion disk. Heated to millions of degrees, the remaining bits of what was once our planet would probably explode outward as they absorbed the tremendous energy emitted near the black hole’s event horizon. When the debris cleared, there would be nothing left to see, just a slightly larger black hole, now a whole inch across after its gorging, calmly orbiting the Sun.
While those last scenarios are certainly apocalyptic—they’re the first ones we’ve run across where the Earth is quite literally destroyed—they’re also by far the least likely to occur. We don’t even know if primordial black holes exist, for example, or in what numbers if they do. And even if they are out there, and in huge numbers, the odds of one getting close enough to the Earth are incredibly low. And even though we are very sure that stellar mass black holes lurk in the galaxy, the odds of one of those getting close enough to ruin our day are microscopic as well. Space is vast, and the Earth is tiny, so we’re pretty easy to miss. The very fact that the Earth has existed for about 4.6 billion years is rock-solid proof of that.
But what if one doesn’t start in the depths of space? What if one were to start off right here, on Earth?
The new generation of particle colliders—what used to be called atom smashers—can actually slam subatomic particles into each other so hard that it’s theoretically possible that they will create extremely tiny mini black holes. A few years back, this news made some headlines when it was revealed that the Relativistic Heavy Ion Collider (RHIC) in New York might be able to do just that. Would the Earth get eaten by an artificial black hole?
Many newspaper articles speculated it might, but there are two reasons why that can’t and won’t happen. One is that, as we saw, tiny black holes will evaporate through Hawking radiation extremely rapidly. A black hole made by the types of collisions done at RHIC would last the tiniest fraction of a second. They’d never get a chance to accrete any mass before evaporating (and their mass would be so small that the explosion would be really tiny too).
Second, the energies created at RHIC are actually much smaller than what naturally occur at the top of the Earth’s atmosphere billions of times a day! Cosmic rays—subatomic particles accelerated to fiendish energies in supernova explosions—slam into the air all the time at far higher energies than we can hope to create here on Earth. These are more than enough to create extremely tiny black holes, yet here we are. Over the billions of years that these particles have been raining down on us, not once has the Earth been eaten by a subsequently created black hole.
Newspapers, magazines, and TV like to inflate such stories because they know they will sell. But when you look at the actual science, you see that we’re in no danger of being gobbled up by a black hole, whether by nature’s hand or our own.
And that is the hole truth.56