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

by Philip C. Plait

  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 bl
ack 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.

  NASA/CXC/SAO

  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 ha
ppens 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.