The black hole next door; mighty particle collisions may bring black holes to venues near you.

Think of a black hole. No one has ever observed one directly, but chances are that you envision some gargantuan jet-black entity that's far, far away and insatiably consuming any matter or light that comes near it. Some physicists whose job description includes thinking about black holes have conjured up another possibility. They're suggesting that extremely tiny, lightweight versions of these exotic objects could be forming right over our heads when ultra-high-energy particles, called cosmic rays, from space strike atoms or molecules in the atmosphere. Those newly created black holes would then quickly decay, harmlessly raining subatomic particles down onto our planet and ourselves.

"Black holes popping up in the sky is a very spectacular possibility," says theorist Jonathan L. Feng of the University of California, Irvine. If it turns out to be true, some scientists say they might be able to mass-produce black holes in particle colliders.

Should either prospect be fulfilled, the world of physics would be turned on its ear, many scientists say. That's because the discovery of tiny, local black holes would confirm one of the more outlandish ideas circulating in the physics community these days--that we live in a universe with detectable dimensions beyond the three of space and one of time to which we're accustomed (SN: 2/19/00, p. 122).

For that reason and others, the chances that ideas about small, local black holes will prove correct are very small, says Frank Wilczek of the Massachusetts Institute of Technology (MIT). He derides the predictions as "wildly speculative."

Undaunted by the long-shot status of their ideas, some researchers are now gearing up to record signs of tiny black holes, initially by searching for the distinctive particle showers that they presume any miniature black holes in the atmosphere would trigger.

If those searches fail, however, physicists at a powerful new collider expected to begin operating in 2007 would be next in line to claim the prize for being first to observe a black hole. In this case, tiny black holes would form as an aftermath of super-high-energy, head-on collisions between protons. Some theorists think that black holes might even show up on the first day of operation of the so-called Large Hadron Collider now under construction near Geneva. But if the exotic objects don't appear then or later, even their absence may teach scientists important lessons about the nature of the universe.

ELBOW ROOM It may sound like science fiction, but the notion that we are living in a universe with extra, unseen dimensions has a long history in theoretical physics. Nearly 90 years ago, the Finnish physicist Gunnar Nordstrom introduced the notion of a fifth dimension as a way to better understand the then-new concept of four-dimensional space-time. A decade later, Swedish physicist Oskar Klein--who drew on ideas from mathematician Theodor Kaluza of Germany--used the concept to unite the forces of electromagnetism and gravity within a single theory.

The quest to unify the two forces became more complicated later in the century when scientists discovered two additional ones--the weak force, which governs the radioactive decay of atoms, and the strong force, which welds protons and neutrons into atomic nuclei.

Since the 1970s, scientific interest in extra dimensions has surged as physicists developed what's now known as string theory. That theory, also called M theory, postulates that all matter and energy is composed of excruciatingly minute filaments called strings and membranous entities called branes. If such objects exist, then every point in our apparently four-dimensional universe is a tiny volume with six or seven extra dimensions. Those volumes are so small, the theory holds, that 10 trillion trillion of them could fit into the space occupied by a single atom. Unfortunately, that tininess would make these dimensions undetectable with current methods.

About 4 years ago, three theorists came up with a bold proposal. Perhaps some of those extra dimensions weren't so tightly confined, suggested Savas Dimopoulos of Stanford University, Nima Arkani-Hamed, now at the University of California, Berkeley, and Georgi Dvali, now at New York University. Given that no experimental evidence precluded the possibility, an extra dimension might be even as relatively huge as a millimeter in radius, or roughly the size of a poppy seed, they argued.

In this new hypothesis of so-called large extra dimensions resides a possible solution to a long-standing puzzle: Why is gravity so much weaker than the other forces? In attempting to resolve the question, the hypothesis raises the remarkable possibility of wee black holes right in our own neighborhood. It's all a matter of how minutely scientists examine gravity.

Although electromagnetism and the weak and strong forces are comparable in strength to each other, they are as much more powerful than gravity as a mountain is larger than one of those fantastically teeny extra dimensions of string theory.

To bridge that vast gap, Dimopoulos and his colleagues hypothesized that not only are there large extra dimensions but that gravity is the only force that permeates all the dimensions. Consequently, "gravity is not really so weak," explains Greg Landsberg of Brown University in Providence, R.I. Rather, "we feel it so weakly because gravity actually lives in many dimensions.... Gravity is diluted by this enormous extra space that we don't feel."

Conversely, at length scales not much smaller than the poppy-seed-span of the proposed, relatively large extra dimensions, gravity would operate at a strength comparable to those of the other forces, says Feng, who is also of MIT. In other words, gravity would become a real brute within those very confined boundaries of the extra dimensions. That enormously amplified strength--normally hidden to our four-dimensional view--could scrunch matter and energy into minuscule black holes.

Wilczek notes that by extrapolating from what is already known in physics, theorists have shown that such a boost in the force of gravity may happen. However, it would only take place at truly minuscule scales, not the macroscopic ones that Dimopoulos and his coworkers have proposed.

Also troubling, says Wilczek, is that the existence of large extra dimensions doesn't jibe with certain findings in experimental physics and cosmology. "There are various tricks and dodges you can try" to explain away those discrepancies, he says, "but it's really difficult." For instance, experimenters have found no evidence that protons can spontaneously decay. Yet the theory of large extra dimensions implies that such disintegrations would have been detected already, he asserts.

Dimopoulos disagrees. Because extra dimensions allow particles to interact with each other in novel ways, proton decay "is definitely not an issue," he claims. Although the large-extra-dimensions hypothesis may not provide an exact fit to everything known in physics, it does about as well as other theories on the cutting edge of particle physics, he argues.

TALL ORDER As a rule of thumb in particle physics, energies correspond to

sizes--the smaller the dimension to be probed by smashing particles together the higher are the accelerator energies required. But the mathematics of the large-extra-dimensions hypothesis doesn't dictate exactly what size the surplus dimensions must be. So, Dimopoulos and his colleagues chose a size that loosely corresponds to a collision energy--roughly, a trillion electron volts--that particle colliders are just beginning to reach.

Without that choice comes the tantalizing possibility that only a small increase in sensitivity of certain experiments, or a modest boost in the energy level of others, might unveil a startlingly different realm of physics from the one we now know. If so, "it's going to be magnificent," says Maria Spiropulu of the University of Chicago. "We're going to be seeing all sorts of stuff," including small black holes.

In the Oct. 15, 2001 Physical Review Letters, Landsberg and Dimopoulos predicted that the Large Hadron Collider may crank out a black hole every second. Around the same time last fall, another pair of researchers independently came to similar conclusions.

There's no danger in manufacturing such black holes, claims Andreas Ringwald of the Deutsches Electronen-Synchrotron laboratory in Hamburg, Germany. If black holes are in fact produced in future colliders, that would mean they also have been relentlessly zipping in and out of existence in the atmosphere for billions of years. People "should not be afraid," Ringwald urges. Feng also notes that the little black holes would be too fleeting to gobble anything up

Wowed by the possibility of creating black holes in a collider that will soon be operating--and tempted by the possibility that cosmic-ray detectors might beat the Large Hadron Collider to the punch--Feng and Alfred D. Shapere of the University of Kentucky in Lexington calculated rates for atmospheric black hole production from cosmic rays. Cosmic rays would produce a few atmospheric black holes somewhere in Earth's atmosphere every minute, Feng and Shapere report in the Jan. 14 Physical Review Letters.

That's enough, they say, for a vast, new cosmic-ray detector called the Pierre Auger Observatory, which is now under construction in Argentina, to detect tens of black holes each year. Other analyses by scientists including Feng and Ringwald indicate that some existing neutrino observatories also could serve as atmospheric black hole detectors.

ON THE LOOKOUT Directly detecting any kind of black hole would be a major milestone. Although most astro-physicists are convinced that cosmic black holes exist, they can only infer the objects' existence from such evidence as motions of nearby stars and gas and the presence of jets of matter or radiation. There looms the possibility, however, that gravitational ripples in the fabric of space-time itself caused by enormous astronomical black holes may be recorded in the next few years by gravity-wave detectors (SN: 1/8/00, p. 26).

Microscopic black holes would betray their presence in a different way. Despite the reputations of black holes for not letting even light escape (hence their blackness), a quirk of quantum mechanics causes them to emit so-called Hawking radiation, which makes them evaporate, scientists say. What's more, this radiation intensifies as an evaporating black hole shrinks.

While a typical astronomical black hole would give off little illumination and only slowly evaporate, a microscopic black hole about 1,000 times the mass of a proton would appear and then blast apart in just [10.sup.-27] seconds--that's one-billionth of one-billionth of one nanosecond.

At the Large Hadron Collider, each explosion of a new, human-made black hole should "light up the detector like a Christmas tree," Landsberg says. The burst would stand out partly because micro black holes would be among the highest-energy objects ever to be observed in the collider. Their demise would also be remarkable because of the unusual constellation of particles that would fly out, he adds. Some other researchers, however, suspect that black hole decays might prove tougher to pick out from the debris of less-exotic collisions, which would occur a million times more often.

Feng expects that micro black holes would be easily spotted in the atmosphere. The signature to look for would be particle showers that spread out less than those from ordinary cosmic rays do. That's because the impacts most likely to yield black holes would be caused by ultrahigh-energy neutrinos--a predicted but as yet undetected type of cosmic ray--that would interact very little with other matter and therefore would penetrate the atmosphere deeper than other cosmic rays do. Black holes would form as low as the paths of commercial jets, Feng says. The resulting particle showers would therefore fan out less extensively than typical showers do.

In the next few years, scientists will tune in to these and other possible signals of both local and distant black holes. If they're really lucky, they'll detect subtle ripples from far-flung black holes embedded in space-time as we know it, maybe even telltale particle bursts from micro black holes ensconced in hidden dimensions right over their heads.