Armageddon Science (4 page)

Thanks to Newton’s work on the force of gravity, Michell knew that this is because the ball can’t escape our planet’s pull. Before it gets high enough, the downward acceleration from the Earth’s gravity slows it to a stop and sends it falling back. If Superman had been dreamed up in Michell’s day, however, he could have thrown a ball faster than 11.2 kilometers per second (twenty-five thousand miles per hour), which means it would have escaped before gravity dragged it back.

It might seem that this minimum speed limit would make it impossible to send a rocket into space. Anyone who has seen a launch from Cape Canaveral will know that space probes take off much slower than twenty-five thousand miles an hour—to begin with, they appear to crawl their way into the sky. But escaping from the Earth’s pull is far easier than it sounds.

First, we can cheat a little by sending something off into space eastward near the equator, moving against the Earth’s spin, which means we have to achieve only around 10.7 kilometers per second (twenty-four thousand miles an hour) because the rotation of the Earth gives the rocket a boost. But more important, the farther away from the Earth our rocket gets, the lower the escape velocity becomes. Because the rocket is constantly under power it can take off slowly, and as long as it keeps moving up, it will escape. As it rises, the escape velocity becomes lower and lower. If Superman throws a ball into space it has to have that escape velocity at the moment it leaves his hand, because nothing else can push the ball upward. The only force the ball experiences after being launched is the downward force of gravity, which is why it needs to start off with such a high velocity.

Michell imagined how escape velocity would vary if he were on a much bigger planet, or even a body as enormous as the Sun. Newton had shown that the force of gravity goes up with the mass of the planet or star—as the body you are standing on gets bigger, then the escape velocity increases too. What would happen, Michell wondered, if the mass was so great that the escape velocity was faster than the speed of light? Under those circumstances the light would never make it away from the star—no light would get out. It would appear to be a dark star even though it blazed furiously away on its surface. (Michell didn’t call his concept a black hole—the name was dreamed up by American physicist John Wheeler as recently as 1969.)

No one took much notice of Michell’s idea, published in the
Philosophical Transactions of the Royal Society
in 1783. It was a hypothetical concept, not much different in its philosophical abstraction from concerns about how many angels could dance on the head of a pin. (Apparently this was never a true medieval philosophical subject; it was dreamed up in modern times as an example of the kind of thing it was thought medieval scholars worried about.)

It wasn’t until the early part of the twentieth century that anyone would come up with a way to envisage black holes with mathematical precision. Einstein’s newly developed general theory of relativity predicted that light would be influenced by gravity. General relativity predicted that massive bodies, like stars, distorted the very fabric of space. As light traveled through the warped space, its path would be bent, sending the straight light beam around a corner.

This idea inspired German physicist Karl Schwarzschild to consider how larger and larger stars would warp the path of the light they emitted. It was 1916, and remarkably, Schwarzschild managed to come up with this concept while fighting in the First World War, using Einstein’s equations to describe the action of a star on light using math. Of itself this was no surprise (except for Schwarzschild’s amazing ability to undertake serious mathematical work on the battlefield)—but a strange possibility dropped out of the numbers. Just as Michell had found with his basic assumptions on escape velocity, Schwarzschild showed that a massive enough star would bend space so far that light would never get away. Instead it would turn in on itself and return into the star.

Schwarzschild thought this was nothing more than a mathematical nicety without a real application, because the ability to bend space was dependent on both the mass of the star and its size. It wasn’t enough to have a supermassive star; it would also have to be much tinier than any star that had so far been observed. To get our Sun, which at 1.4 million kilometers (865,000 miles) in diameter is on the small side for a star (technically it’s a yellow dwarf), into a state where its mass was concentrated enough to turn it into a black hole, you would have to compress it until it was just two miles across.

However, when Indian physicist Subrahmanyan Chandrasekhar and American Robert Oppenheimer, who would later head the Manhattan Project, looked into the practicalities of Schwarzschild’s strange compact stars, they realized that there was a way for such compression to occur. Any star has a huge amount of mass—the Sun, for instance, is over 300,000 times as massive as the Earth. All that material in the star is pulling together with a vast gravitational attraction that is constantly trying to compress it like we would scrunch up a paper ball. While the star is very active, the outward pressure from the nuclear reactions that power it keeps the star “fluffed up,” but as the nuclear fuel runs low, that pressure drops and the star begins to collapse.

Now another physical force comes into play—a quantum feature called the Pauli exclusion principle, which means that similar particles of matter that are close in distance must have different velocities. This requirement will counter the gravitational collapse as a star cools—unless the star is so massive that gravity overwhelms the Pauli effect. The mass required for this to happen is around one and a half times that of the Sun (so our star is not on its way to being a black hole).

Sometimes such a star explodes as a supernova, seeding the universe with heavy atoms. But if this fails to happen, the theory goes, the star should contract, getting smaller and smaller until the gravitational intensity is such that light never escapes—it has become a black hole. In theory, though, there is nothing to stop the contraction from continuing indefinitely until a singularity is formed, a point of infinite density, at the center of the black hole.

The outer surface of the black hole that we would see (or rather that we wouldn’t see) isn’t an actual material surface, like the outside of a normal star. The remnants of the star are much smaller than this boundary, known as the the event horizon. The surface of the black hole is just the radius at which the gravitational force becomes so strong that light cannot get out.

Cosmologists think that there are black holes out in space ranging in size from a couple of times the mass of the Sun to thousands of times that size for the supermassive black holes thought to be at the center of most galaxies. And in the normal way of forming a black hole, there is no way to make one from a star if it’s smaller than that minimum radius of around one and a half times the Sun. But there is, in theory, another way that a black hole could be produced.

If you could cram together
any
amount of material with enough force—this would require much more energy than the gravitational pull experienced on a star—then you could, in theory, produce a black hole. Such a black hole could be formed not from a stellar mass, but from just a spoonful of matter—it could be as small as you liked. This is where the Large Hadron Collider at CERN comes in—and in some people’s minds where it could put the world at risk. In theory, the LHC could come close to battering particles together with enough energy to make microscopic black holes pop into being.

To be precise, it’s not so much the Large Hadron Collider that would be making the micro black holes, as the LHC with the addition of a little help from another universe. According to the best-supported current theory, even the vast energy levels the LHC can generate are not enough to bring tiny black holes into existence. But if, as some theories suggest, there are many parallel universes, and gravity can leak from one universe to another, it is thought that extra compressing force would be enough to enable the LHC to spawn micro black holes.

If they do appear, they will generate a lot of interest in the existence of other universes in near reach of our own. Apart from producing a fascinating subject of study in the black hole itself, the LHC would be providing fundamental data on the nature of the universe—this is part of the reason scientists get so excited about the collider, and believe that the immense amount of money spent on it is worthwhile.

So let’s imagine the LHC really did make one of those microscopic black holes. We’re talking about a body compressed to a size much smaller than an atom, not directly visible in any way. But in the Hollywood model of the black hole, this would be attracted toward the center of the Earth and would start to act something like the central character in the Pac-Man game, eating its way through the surrounding matter, absorbing it and growing ever bigger until it eventually swallowed the whole world. Here, surely, is good reason to worry about the Large Hadron Collider. If a micro black hole is formed, we’ll see the world eaten away from under our feet.

Luckily, this picture misses one of the key points of the modern theory of black holes—something called Hawking radiation. Quantum theory tells us that in space, pairs of particles are constantly winking into existence. Usually these particles immediately annihilate each other and disappear again. It’s a form of constant quantum interchange between energy and mass. But at the event horizon of a black hole, the tendency would be for one particle to be sucked into the black hole while the other shoots off into space, forming the Hawking radiation. It would give a black hole a faint glow around its perimeter.

About the most fundamental concept in all of physics is the conservation of energy. This now comes into play. The result of this interaction between the black hole and the particle pair is a net decrease in energy of the black hole. Every time it absorbs half a pair and gives off Hawking radiation, it loses energy. Effectively such a small black hole will fizzle out of existence long before it can eat anything up.

If micro black holes were formed (and remember they could come into being only if something like the gravitational field of a parallel universe gave a boost to the LHC’s energy), they would disappear before they could be observed. All we would see is a little spatter of particles as the holes were transformed into Hawking radiation. Most physicists at CERN would love it if micro black holes were formed—but they would only ever be seen indirectly, lasting for a minuscule period of time. They would not provide any threat to our survival.

Though black holes are unlikely to do any damage, another worry of those who feel that the LHC could destroy us all is the formation of strangelets. We’ll come back to what these are in moment. Although their existence involves the piling of hypothesis on hypothesis, if strangelets really were formed and behaved as some predict they would, the LHC could destroy the entire universe. Strangelets are hypothetical tiny chunks of strange matter, a form of matter different from the stuff we normally experience. To understand what the threat from strangelets would be, we need to take a quick diversion into the nature of matter itself.

Most of us will remember from high school science that all matter is made up of three types of particle: protons, neutrons, and electrons. The protons and neutrons sit in a vibrating bundle in the tiny nucleus of the atom, while the electrons are distributed in a fuzzy cloud around the outside. These three types of particles used to be considered fundamental particles—ones that have no subcomponents—and electrons still are seen that way, but we now believe that neutrons and protons are made up of smaller particles called quarks.

Quarks were named by American physicist Murray Gell-Mann. He intended the word to be pronounced to rhyme with “dork,” hearing the sound in his head before he worked out how to spell it. Soon after dreaming up the name, he came across the line “three quarks for Muster Mark!” in James Joyce’s novel
Ulysses
. This sounded apt, as quarks often come in groups of three, but Gell-Mann wanted to keep his original pronunciation. Mostly he is ignored, and the word is pronounced to rhyme with “bark.”

Quarks (however you pronounce them) come in different types, whimsically called “flavors,” such as “up,” “down,” “charm,” and “strange.” These names are just labels—there is nothing inherently strange about a strange quark or charming about a charm quark (nor is anything pointing up or down in the flavors with those names). A proton is made up of two up quarks and one down, while the neutron is two downs and one up. But what if different quarks were used to build matter—what would be the result?

We have already met one alternative kind of matter based on a different assembly of quarks: antimatter. Every quark has an equivalent antiquark—and it’s equally possible to make a kind of proton from two antiup quarks and an antidown quark. The result is an antiproton—just like a proton but with a negative charge instead of positive. As we have seen, it is also possible to have antiatoms, made up of antiprotons and antielectrons (also called positrons).

A more speculative construct is strange matter. Instead of being built from the familiar building-brick particles like protons and electrons, this would consist of a collection of quarks that aren’t bound up in triplets, but would form a stable material that has roughly equal numbers of up, down, and strange quarks in it.

In principle, strange matter should be more stable than a normal atomic nucleus, and atomic nuclei should decay irreversibly to form strangelets—strange-matter particles—but the probabilities involved for any particular nucleus are so small that the chances are that none has yet had time to form in the lifetime of the universe.

If a strangelet did form and collided with a normal atomic nucleus, it could act as a catalyst, converting that nucleus to strange matter too. This process would give off energy, which could produce another strangelet, and so forth. The result would be a chain reaction, like the one powering a nuclear power station, but happening with ordinary matter at room temperature, resulting in an increasingly rapid breakdown of matter into strangelets.