The Extremely Long Odds Against the Destruction of Earth

ESA / V. Beckmann (NASA-GSFC)

No object in space is more mysterious—and more psychologically menacing—than a black hole. Once known as a frozen star, a black hole is formed when a massive star burns out and collapses upon itself, ultimately producing gravitational energy so powerful that not even light can escape from it. Although physicists can infer the existence of black holes in space, they cannot directly observe them. Yet making mini black holes may be possible when the world’s largest particle accelerator—the Large Hadron Collider (LHC)—goes online outside Geneva, Switzerland. At the heart of the new machine is a phenomenal 17-mile circular tunnel where particles will smash together at nearly the speed of light, producing temperatures 100,000 times hotter than the core of the sun. Physicists will observe the collisions not only for clues to fundamental constituents of matter, hidden dimensions, and the elusive Higgs boson—the hypothetical particle that gives matter its heft—but also for tiny black holes winking in and out of existence.

But a couple of Jeremiahs would halt the fireworks before they begin. A lawsuit filed in U.S. district court in Honolulu seeks to halt the opening of the accelerator, which is funded in part by the Department of Energy and the National Science Foundation. A similar suit was filed in 2000 against the Brookhaven National Laboratory to prevent the operation of the Relativistic Heavy Ion Collider, an accelerator that started up that year. The charge, then as now, is that microscopic black holes produced at the collider might coalesce and engulf the earth, ending all life as we know it. LHC scientists have publicly dismissed the lawsuit as bunkum while quietly double-checking their math just to be sure. DISCOVER asked Brown University physicist Greg Landsberg, who is involved in experiments at the LHC, if we should lose any sleep over the matter.

First off, how might microscopic black holes be produced at the LHC?
When too much matter is put into too small a space, it collapses under its own gravity and forms a black hole. That’s what is happening when astronomical black holes are formed. Now, the LHC doesn’t really create much matter, but it does put a lot of energy in a very small volume, and Einstein showed that for a moving particle, the energy, not the mass, governs gravitational attraction. You might create black holes at the LHC when two particles pass very close to each other, if the gravitational interaction between them is strong enough. But this is possible only in certain models that predict the existence of extra dimensions.

What is the connection between extra dimensions and black holes?
Black hole production requires a strong gravitational attraction. But gravity is much weaker than other forces, such as electromagnetism. One way of remedying this problem is to assume the existence of extra dimensions in space accessible to the carrier of gravitational force—called the graviton—but not accessible to other particles, such as quarks, electrons, and photons. If this is the case, gravity may be fundamentally strong but still appear weak to us, as the gravitons spend most of their time in the extra space and rarely cross into our world.

Imagine a very long and thin straw. If you are observing it from far away, you don’t really resolve the fact that the straw has the second curled-up dimension, its circumference. The straw appears to you as a line—that is, one-dimensional. However, if you approach the straw at a distance comparable to its radius, you would start resolving its second dimension and see that it is truly two-dimensional. Pretty much the same way, when two particles are close to one another, they start feeling gravity from extra dimensions and thus feel the true, undiluted gravitational pull. That’s basically the framework in which it turns out that black hole production at the LHC is a possibility. But one should understand that this is just one model. Whether it’s true or not is anybody’s guess.

How would microscopic black holes be observed?
They would emit light that is much, much hotter than, say, light coming from the stars or sun, because their temperature is many orders of magnitude greater. They would emit high-energy gamma rays, and they could emit all sorts of species of particles, such as electrons and muons, that we could detect.

Can we be sure that a black hole created at the LHC wouldn’t expand and swallow the earth?
I think the honest answer to this question is yes. The black holes that would be produced at the LHC must also be produced by the hundreds every day due to energetic cosmic rays bombarding our earth. When cosmic rays smash into particles of earth material, it’s the same type of collision that happens in the LHC. So the very fact that we exist here on earth to talk about these things tells us that even if black holes are produced, pretty much everything is very safe. Either black holes are not produced at all, or they decay very, very quickly due to Hawking radiation or an equivalent mechanism.

What exactly is Hawking radiation?
Stephen Hawking showed in the early 1970s that black holes are not completely black. They have a slight tint of gray, if you will. That means black holes do not just suck everything in—or accrete, as they call it scientifically—but in fact they must radiate some energy out. This process is known as Hawking radiation.

The intensity of Hawking radiation is determined by the temperature of the black hole. The higher it is, the more intense the radiation is, just like a hot bar of metal emits much more heat than a cold one. Now it turns out that the temperature of the black hole is inversely proportional to its mass. The more massive a black hole, the cooler it is. Thus small black holes are very hot and radiate a lot, while large, astronomical black holes are extremely cold and barely radiate at all. The black holes found in the universe are so cold that it takes forever for them to evaporate, many orders of magnitude longer than the age of the universe.

By contrast, black holes at the LHC would live only a fraction of a second before they radiate their mass away. This is not long enough for them to accrete anything before they disappear in a blast of radiation. These black holes would evaporate almost instantaneously, without moving by more than the size of the atomic nucleus.

Is it possible to quantify the chance of something catastrophic occurring at the LHC?
The probability is never equal to zero in quantum mechanics, but you don’t worry about it if the probability is very small. There is some probability that all the air molecules in your room will suddenly cross over and end up on one half of the room and you won’t be able to breathe. But we are talking about risk management here, and I think people should be worried about probabilities that are large.

If black holes are detected at the LHC, what would it mean for physics?
Above all, it would probably help us build a quantum theory of gravity, the one force that hasn’t really been explained by quantum mechanics. We have very little understanding of what the quantum theory of gravity looks like, and producing these black holes at the LHC would probably be as close as you could get to approaching the answer to this question.

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