But that left one problem, vast in its own right: what event could possibly generate such incredible energies?
KABOOM!
No matter how you slice it, GRBs are, for a short time, the most luminous objects in the Universe, the best bangs since the Big One.
This is no small problem. Imagine a source of light in space: the light it emits will expand as a sphere with the source at the center. As the sphere grows, the light gets spread out, and will appear dimmer to an observer (that’s why lights get fainter with distance). When the distance to the object doubles, the area over which the light spreads out goes up by four times,32 so the brightness will dim by four times. If you increase your distance to 10 times farther away, the light will be only one-hundredth (1 percent) as bright, and so on. The brightness of an object therefore decreases very rapidly with distance. This presented a serious problem for GRB researchers: from a distance of billions of light-years, the explosion that formed the GRB must be huge to be able to be detected at all from Earth. When the numbers were crunched, it didn’t make sense. Even converting an entire star into energy using Einstein’s E = mc 2 (see chapter 2) wouldn’t provide enough energy to fuel the burst, and that is literally the most energy you can get from a star (ignoring the inconvenient fact that there’s no known way to convert an entire star into energy, and certainly not in the span of a few seconds).
But there was still an out. What if the blast wasn’t symmetric, expanding equally in all directions? What if it was beamed?
If you take a small lightbulb and turn it on, it emits light in all directions, and its apparent brightness fades rapidly with distance. But if you put it in a flashlight, which collects its light and focuses it into a beam, the light appears brighter from farther away.
Astronomers could almost taste the answer to this piece of the GRB puzzle. Instead of a nearly impossibly energetic blast at a colossal distance expanding spherically and fading rapidly, maybe the explosion was less energetic, but focused into beams. Beaming would mean only a tiny fraction of the energy would be needed compared to a spherical blast.
The energy of the detonation would still have to be frighteningly huge to be seen clear across the Universe, but not impossibly so. In fact, the energy involved would be similar to that of a supernova. This gave astronomers hope that they could find the Holy Grail of GRB science: the engine that drove this phenomenon.
And of all the objects in the cosmic zoo that astronomers knew of, only one could possibly generate those kinds of forces.
THE GRAVITY OF THE SITUATION
Black holes are notorious for sucking down matter and energy, not spewing them out, so it might seem paradoxical that they could be at the heart of gamma-ray bursts, the brightest objects in the Universe.
But the key to this is gravity. And the key to that is how black holes form, so let’s take a step back (a good idea when dealing with black holes) and take a look at this singular event.
In chapter 3, we saw that massive stars explode when they run out of fuel to fuse in their core. The incredibly strong gravity of the core makes it collapse, which sets off a series of events that blows up the star. That description focused mostly on what happens to the outer layers in a supernova, but not what happens to the core itself. But it’s there that the power of the GRB lies.
As the iron core of the incipient supernova collapses, the electrons are rammed into the protons, making neutrons (and emitting neutrinos, the major trigger of the supernova explosion). In a flash the entire core of the star becomes a sea of neutrons with almost no normal matter left. What was once a ball of iron thousands of miles across is now an ultradense neutron star, perhaps ten miles across. It has a mass equal to the Sun, but a density magnified beyond belief: a spoonful of neutron star matter would weigh a billion tons! That is somewhat more than the combined mass of every single car in the United States—imagine 200 million cars crushed down to the size of a sugarcube and you’ll start to get an idea of how extreme neutron star matter is.
The neutron star’s incredible mass is supported by a weird quantum mechanical effect called degeneracy (see chapter 3). It is similar to electrostatic repulsion—the idea that like charges repel—but instead it’s a property of certain subatomic particles where they resist being squeezed too tightly together. Degeneracy will occur if you try to pack too many electrons together, but it also affects neutral particles like neutrons. It’s an astonishingly strong force, and is able to keep the vast bulk of the core from collapsing further. The collapsing core of the star slams to a halt, and a neutron star is born . . .
. . . most of the time. It turns out that if the mass of the collapsing stellar core exceeds about 2.8 times the Sun’s mass, even neutron degeneracy cannot hold it up. The core’s gravity is too strong, and the core collapse continues. This time there is no force in the Universe strong enough to stop it.
What happens next is so bizarre that it stretches the human mind to its limit to understand. As an object gets smaller, but retains its mass, its gravity gets stronger. As an easy example, if you were to somehow shrink the Earth to half its current diameter while still keeping its mass, the gravity you feel (and therefore your weight) would increase. The smaller the Earth gets, the stronger its gravity.
If you wanted to launch a rocket to the Moon from this newly shrunken globe, you’d have to give it a lot more power to overcome the Earth’s gravity. If you shrank the Earth more, the rocket would need even more power, and so on. Eventually, the Earth would shrink so much that its gravity would be literally impossible to overcome.
You might think you just need to add more thrust to the rocket, but when matter gets this dense, Einstein has something to say about the situation. He postulated that gravity is really just a manifestation of bent space. What you feel as a force downward, toward the center of the Earth, is actually a bending of space, like the way the surface of a mattress would bend if you plunked a bowling ball down on it. Roll a marble across the bed, and the path of the marble bends, just the same way an asteroid’s path bends because of gravity when it passes near the Earth.
This is more than just a model, more than mere speculation. Its consequences are very real: if too much matter is packed into too small a volume, the bending of space can become so severe that it literally becomes an infinitely deep pit. You can fall in, but you can never climb back out.
An object like this is like a hole in space. Nothing can escape it, not even light. Since it cannot emit light, this hole would be black. What would you name such a thing?
And so it goes in the core of the exploding star. If the core is too massive to form a stable neutron star, it collapses. All the way down. It shrinks to a mathematical point, space gets bent to the breaking point, and a black hole is born.
The gravity of the hole is intense. Any matter close by will be drawn inexorably into it. But there’s a hitch. Stars spin, and so do their cores. As the core collapses on its way to forming a black hole, that rotation increases, the same way an ice skater can increase her spin by drawing her arms in. Once the black hole is created, it will be spinning very rapidly, and any matter falling in will also revolve around it, like water going down a drain. The closer to the black hole it gets, the faster that matter will swirl around it.
So matter falling into a black hole doesn’t just fall straight in—plonk!—and disappear; it spirals in. The matter just outside the black hole begins to pile up, and it forms a flattened disk called an accretion disk (accretion is the process of accumulating matter). This will happen for any star that is spinning before it collapses, but models have shown that GRB progenitors may be spinning even faster than normal. These rapid rotators form an accretion disk much more readily than a slowly rotating star. And once the disk forms, the ferocious gravity of the black hole will get the inner part of the disk moving very close to the speed of light, and even matter farther out from The Edge will still be moving incredibly rapidly.
When a black hole forms, spin and gravity are not th
e only things to get amplified. Stars also have magnetic fields, like giant bar magnets (see chapter 2). Just as gravity increases as the star shrinks, so does the magnetic field. A typical star may have a magnetic field not much stronger than the Earth’s: just enough to make a needle in a compass move. But if you take a star a few million miles across and squeeze it into a ball just a few miles across, the magnetism increases hugely as well, getting billions and even trillions of times stronger.
Any matter trying to fall into the black hole is therefore under the influence of a witches’ brew of forces. Gravity tries to draw it in, but its angular momentum counteracts that, forming the disk. The magnetic fields also get twisted up like a tornado as the matter spins around the disk. And on top of it all, there’s just plain old heat, created, oddly, by something familiar amid all these exotic forces: friction. As the matter in the disk swirls madly around under the force of the black hole’s gravity, the particles in it slam together at incredibly high speeds, which generates immense quantities of friction. This heats the disk to millions of degrees.
The sheer heat tends to drive particles away from the black hole. If a particle tries to move outward in the plane of the disk, it slams into other particles and cannot escape. But if it goes up, out, it’s free to travel—there’s less material in that direction. Moreover, the monstrously amplified magnetic fields can also accelerate the particles up and out. The heat and magnetism combine to focus a pair of tight beams, like two ultra-mega-superflashlights glued together at their bases. These twin beams shoot out from directly above and below the black hole, firing outward, away from the black hole in directions perpendicular to the disk.
What happens next is a vision of hell so apocalyptic that it’s difficult to exaggerate. Moments after the black hole is created and the disk forms around it, all that energy—a billion billion times the Sun’s output—is focused into twin beams of unmitigated fury. So much energy is packed so tightly into the beams that they blast outward in opposite directions, eating their way through the star at the speed of light. Within seconds, the beams have chewed their way out to the surface and are free. Any matter in their way is torn apart, heated to billions of degrees, rendered into its constituent subatomic particles, and accelerated to within a hairbreadth of the speed of light. Ironically, by the time they punch their way through the star, perhaps only a few hundred Earth-masses of matter are in the beam, which is huge on a human scale, but tiny on a cosmic one. But that also is a key to their power: since the total amount of matter in the beams is relatively small, it can be accelerated to incredible speeds.
Clouds of gas still surround the doomed star, echoes of past eruptions before the final explosion. The beams of energy and matter slam into this material, creating vast shock waves, sonic booms in the material, but on a mind-numbing scale.
There are also shock waves generated inside the jet itself as parts of it move faster than others. When these collide, the awesome energy of the jet churns up the matter inside them, creating unimaginable turbulence, which in turn adds greatly to the energy emitted. The ensuing conflagration emits gamma rays, huge amounts of them, as the magnetic fields and raw energy of the beams bombard the matter.
When a very massive star’s core collapses, twin beams of matter and energy can be focused by the incredible forces in the star’s center. The beams may last only a few seconds, but contain as much energy as the Sun will emit in its entire lifetime, or more.
DANA BERRY, SKYWORKS DIGITAL INC.
A gamma-ray burst is born.
The beams continue on. Behind them, the rest of the star finishes its collapse, forming what would otherwise be a normal supernova. Before the discovery of GRBs, a supernova was considered the most violent, the most energetic single event in the Universe. But a decent GRB can dwarf the energy of even a supernova. Because of this, astronomers coined a new word to describe the event: hypernova.
Once the beams pass through the gas, they continue on, leaving behind superheated matter that begins to cool. As it does, it emits light for some time after the beams have moved on. This is the source of the afterglow sought so dearly by scientists on Earth. The matter can get extremely bright—one GRB in 2008 was nearly 8 billion light-years away, but was visible to the naked eye! But the afterglows fade rapidly, dropping in brightness by factors of thousands in just a few minutes. That’s why the optical afterglow was initially so difficult to detect. Even the titanic energy of a GRB is mitigated by raw distance.
But we now know that GRBs are created in a hypernova, when a massive star explodes . . . and we see massive stars in our own galaxy. Sure, all the GRBs we have ever seen have been at terribly remote distances, billions of light-years removed from Earth.
But what happens if one goes off that’s not far away? What if a nearby star becomes a GRB?
BEAM ME OUT
An object that finds itself in the path of the beams of a nearby GRB will have bad things happen to it.
Very bad things.
But before I scare the pants off you, remember that if you are far enough away they are no danger at all. The only reason we can see GRBs at all is because we are in the path of the beams: since all the light of the GRB is focused into those beams, if they miss us we don’t see anything. So if they are far enough away you just see a faint blip, and it’s gone. But if you’re too close . . .
The effects from a GRB are very similar to those of a supernova, which isn’t surprising. They are related phenomena, with GRBs being produced in supernovae, and they both emit huge amounts of energy in the form of gamma rays, X-rays, and optical light.
Where they differ is how well they sow their destruction over different distances. With a supernova, which emits radiation and matter in all directions, the effects die down rapidly with distance. As we saw in chapter 3, they appear to be mostly harmless from a distance of more than 25 to 50 light-years or so.
But GRBs are beamed. Their luminosity does not decrease as rapidly with distance, and this makes them dangerous from farther away. Much farther away.
Every GRB is different, making prognostication difficult. But enough have been observed to do a little averaging and get the effects from a typical GRB, whatever “typical” means when you’re dealing with Armageddon focused into a death ray.
Let’s set the scene.
Why fool around? Let’s say a GRB went off really close: 100 light-years away. Even from that very short distance, the beam of a GRB would be huge, 50 trillion miles across. This means that the whole Earth, the whole solar system, would be engulfed in the beam’s maw like a sand flea in a tsunami.
GRBs, mercifully, are relatively short-lived, so the beam would impact us for anywhere from less than a second to a few minutes. The average burst lasts for about ten seconds.
This is short compared with the rotation of the Earth, so only one hemisphere would get slammed by the beam. The other hemisphere would be relatively unaffected . . . for a while, at least. The effects would be worst for locations directly under the GRB (where the burst would appear to be straight up, at the sky’s zenith), and minimized where the burst was on the horizon. Still, as we’ll see, no place on Earth would be entirely safe.
The raw energy that would be dumped onto the Earth is staggering, well beyond the sweatiest of cold war nightmares: it would be like blowing up a one-megaton nuclear bomb over every square mile of the planet facing the GRB. It’s (probably) not enough to boil the oceans or strip away the Earth’s atmosphere, but the devastation would be beyond comprehension.
Mind you, this is all from an object that is 600 trillion miles away.
Anyone looking up at the sky at the moment of the burst might be blinded, although it would probably take several seconds to reach peak optical brightness, enough time to flinch and look away. Not that that would help much.
Those caught outside at that moment would be in a lot of trouble. If the heat didn’t roast them—and it would—the huge influx of ultraviolet radiation would instantly give them a le
thal sunburn. The ozone layer would be destroyed literally in a flash, allowing all the UV from both the GRB and the Sun down to the Earth’s surface unimpeded. This would sterilize the surface of the Earth and even the oceans down to a depth of several yards.
And that’s just from the UV and the heat. It seems cruel to even mention the far, far worse effects of gamma rays and X-rays.
Instead, let’s take a step back. GRBs are incredibly rare phenomena. Although they probably happen several times a day somewhere in the Universe, it’s a really big Universe. The odds of one happening 100 light-years away are currently zero. Zip. Nada. There are no stars close to us that are anywhere near the capability of becoming a burst. The nearest supernova candidate is farther away than that, and GRBs are far rarer than supernovae.
Feel better? Good. So let’s try to be more realistic. What is the nearest GRB candidate?
In the southern sky is a star that looks unremarkable to the naked eye. Called Eta Carinae,33 or just Eta for short, it’s a faint star in a crowded field of brighter ones. However, its faintness belies its fury. It’s actually about 7,500 light-years away, and is in fact the most distant star that can be seen with the unaided eye.
The star itself34 is a monster: it may have 100 or more times the mass of the Sun, and it emits 5 million times the energy of the Sun—in one second, it gives off as much light as the Sun does in two months. Eta suffers periodic spasms, blowing off huge amounts of matter. In 1843, it underwent such a violent episode that it became the second brightest star in the sky, even at its vast distance. It expelled huge quantities of matter, more than 10 times the mass of the Sun, at over a million miles per hour. Today, we see the aftereffects of that explosion as two huge lobes of expanding matter, each looking like the blast from a cosmic cannon. The energy of the event was almost as powerful as a supernova itself.
Death From the Skies!: These Are the Ways the World Will End... Page 12