Death from the Skies! (22 page)

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

 

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 waves
⁵³
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 10
⁶⁰
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.