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?
BLACK HOLETTES
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, si
nce 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.
MAN HOLE
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?
Death From the Skies!: These Are the Ways the World Will End... Page 16