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?
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
CHAPTER 6
Alien Attack!
MINDLESSLY—AT LEAST, LACKING WHAT WE WOULD
call a mind as we know it—it examined the bright light of the star ahead of it. Employing a highly sophisticated complex of observational instrumentation, it patiently took data, examining each bit of information as it came in. After weeks of steadily staring at its target, the results were in.
The star was orbited by several gas giants. Each of these had icy moons with possible water under the surface. The star also had not just one but three smaller planets with the potential for liquid water as well. And on the second one of these out from the star were unmistakable signs of biotic life—free O
2
in the atmosphere at large levels of disequilibrium. If it had been equipped with emotions it would have whooped with joy. Instead, it silently and efficiently began preparing for the next phase of its mission.
Using sophisticated engineering and technology the probe began to slow its approach to the solar system. Its fantastic speed—nearly that of light itself—gradually bled away over the course of nearly a year. Course corrections were made, angling the probe this way and that. All the while, it took observations, scouting for the target it needed. Finally, the target was acquired: a metallic asteroid over a mile wide. As the probe passed the
asteroid, aiming carefully, it released a small package just a few meters across.
The package was a probe in its own right, and it used its onboard rocket to decelerate further and land on the surface of the asteroid. It immediately sent an “all clear” signal to the mothership, which did not respond, but instead sharply accelerated away, heading off to the next star on its list, a star it would not reach for decades.
On the asteroid’s surface, a hatch opened on the probe, and a small spider emerged . . . then another, and another. In all, a dozen such robots started crawling over the landscape. Composed of a sophisticated amalgam of metal, ceramics, and spun carbon fiber, they went to work: digging, smelting, manufacturing. They worked without fatigue, without emotion, tirelessly day and night (such as there was on the slowly rotating asteroid). After a month they were ready.
Like a fungus expelling spores, the asteroid erupted in thousands of tiny explosions. Each puff imparted velocity to a ball of metal a meter across, each of which headed toward one of the planets and moons initially targeted by the interstellar probe. Inside each ball were over a hundred of the spiders. The robotic arachnoids were possessed of sophisticated programming, but in the end the goal was simple: convert any and all available materials into more spiders. When enough were manufactured, build more mothership probes. Launch them, and repeat the cycle.
They needed metal of almost any sort, and what they couldn’t find they could create. Their programming was very sophisticated, honed over millions of years of such missions. And they weren’t picky about materials; almost anything would do. Each spider could create the components needed to replicate itself in just a few hours, and these would then move on to replicate themselves as well. Once the first spiders touched down, they could cover a planet in just a few days, converting everything—
everything
—into more spiders and more probes.
Being smaller and closer, the surface of Mars was destroyed first. The rocks were rich in iron, which made things easier. Within hours, that many more spiders went off in search of raw material.
Food.
Earth fell within days. The first spiders landed in Australia and consumed everything in their sight. Rock, metal, gas; all could be converted if needed. Water, plants, flesh—these would do as well. Humans never had a chance. Though the intense light of the interstellar probe’s engine had been tracked by Earthbound telescopes for months, it didn’t answer any hails, and there wasn’t enough time for any of the governments to react anyway. By the time the spiders landed it was already far too late. They swept over the planet, and after less than two weeks there were in essence no living creatures left on Earth. The entire surface of the globe had been converted into robotic factories. Within a year, bright flashes blossomed over the planet as more interstellar probes were launched, each an exact replica of that first one—which itself was generations removed from the first probe launched so many eons before. That first probe was long since dead, having expended all its packages. It was no longer useful. But its progeny “lived” on, sweeping across the galaxy.