Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
Chapter 11. Microscopic Black Holes
A Boon to Science or a Boom
for the World?
Human minds weren’t meant to picture something that was smaller than an atom, and yet weighed megatons. . . . Something ineffably but insatiably hungry, and which grew ever hungrier the more it ate.
—DAVID BRIN, EARTH (1990)
Now I am become Death, the destroyer of worlds.
—ROBERT OPPENHEIMER, QUOTING THE BHAGAVAD GITA FOLLOWING THE TRINITY NUCLEAR TEST
Not all scientists are lunatics. On the contrary, despite cinematic depictions ranging from Dr. Caligari to Dr. Evil, and aside from a smattering of harmless eccentrics, genuinely mad scientists are few and far between. Yet the cultural stereotypes persist, and drive particularly nervous members of the public to assume that the average laboratory researcher would think nothing of taking chances with the fate of the world.
We refer here to civilian scientific research—designed to press forward human knowledge. In an age of consent forms and the omnipresent potential for lawsuits if events go awry, experimental scientists today are generally extremely careful not to expose the public to hazards. People do sometimes make mistakes, but, if anything, scientists are particularly thorough. “Nutty Professor” stereotypes aside, if you read in the paper about a chemical explosion—which would be the more likely cause, an industrial accident or a botched scientific experiment? I would venture to say that it would be the former in almost every case.
It’s true that in times of war, scientists have been recruited to conduct far, far riskier experiments. The expectation that war incurs horrific dangers makes that a completely different story. Those involved in the Manhattan Project, for example, knew that they were constructing and testing weapons of unmatched destructive power. In the Trinity test, in which the plutonium bomb was detonated for the first time in the aptly named region of New Mexico called Jornada del Muerto (Journey of the Dead Man), nobody knew for sure what would be the effect of a nuclear explosion. Would the blast be confined to that desert basin or would it race out of control and cause untold damage—perhaps even destroy the world?
In a macabre kind of gambling, right before detonation Fermi offered to take bets on whether a chain reaction would occur that would vaporize the atmosphere. Wagerers could decide if just New Mexico would be wiped out or if it would be the whole Earth. In retrospect, the idea that an experiment could be conducted with an unknown impact on the fate of the entire planet is shocking—and physicists joking about apocalyptic outcomes is highly disconcerting.
As we know now, although the bomb test lit up the sky like a “thousand suns”—as J. Robert Oppenheimer described, borrowing language from the Bhagavad Gita, the Hindu holy book—it did not annihilate the world, of course. The blast produced a crater something like 10 feet deep and 2,400 feet in diameter. Its energy was estimated to be approximately 20 kilotons, equivalent to 20,000 tons of TNT.
Explosions are virtually never measured in terms of TeV, because that unit represents a far smaller quantity of energy. Nothing stops us from making the conversion, however, and we determine that the nuclear explosion at Trinity was equivalent to 5.0 × 1020 TeV. That’s 5 followed by 20 zeroes—an enormous figure—something like the average number of stars in a billion galaxies. Thus, even the most rudimentary atomic bomb releases astronomically more energy than any of the particle collisions we’ve discussed.
Only a few weeks after the Trinity test, bombs were dropped over Hiroshima and Nagasaki, leading to the devastation of those Japanese cities and the end of World War II. The dawn of the atomic age brought increased apprehension about the possibility that scientific miscalculation, combined perhaps with political blunders, would lead to Earth’s apocalyptic demise. It didn’t help matters that brilliant scientists such as Edward Teller and Herman Kahn would dispassionately discuss the effectiveness of new types of nuclear weaponry in scenarios involving mass casualties.
In an era of trepidation, horror films provided a welcome release valve for anxiety. Fictional threats from aliens that could be warded off through concerted effort were easier to handle emotionally than factual dangers from ourselves that seemed to defy ready solution. The 1958 movie The Blob, about a rapidly growing creature from outer space, offers a case in point. Its plot is simple: an alien meteorite delivers a gelatinous cargo that turns out to be a ravenous eater. Each time the blob ingests something, it becomes larger. Soon it is humongous and pining for human “snacks” found in a movie theater and a diner. People flee in terror from the gluttonous menace until the film’s hero, played by Steve McQueen, manages to freeze it using fire extinguishers.
In a poll about which astronomical objects the public deemed most bloblike, black holes, the compact relics of massive stars, would surely come in first place. The image of such an astronomical object stealthily entering a theater, absorbing all of the patrons, enlarging itself, and moving on to another venue might not be all that far from the popular stereotype. Several black hole properties bring blobs to mind. If a black hole is near an active star, for instance as a binary system, it can gradually acquire matter from the star, due to its mutual gravitational attraction, and become more massive over time. Nothing is mysterious or unusual about this process except that black holes form particularly steep gravitational wells. Astronomers observe this accumulation of material through images of the radiation emitted as it falls inward toward the black hole.
The physics of black holes derives from Einstein’s general theory of relativity. In 1915, with the ink on Einstein’s gravitational theory barely dry, German physicist Karl Schwarzschild, while serving on the Russian front in the First World War, discovered an exact solution. He solved Einstein’s equations for a static, uniform, nonrotating ball of matter, and mapped out the geometry of the space surrounding it. The Schwarzschild solution, as it is called, represents the gravitational influence of simple, spherical astronomical bodies. It describes precisely how a sphere of matter, such as a star or a planet, dents the geometry of space-time and forces nearby objects to move along curved paths. Whether these objects flee, orbit, or crash depends on whether their speeds exceed the escape velocity required to take off. Those nearby objects without enough escape speed, such as an insufficiently fueled rocket, are doomed for impact.
One curious aspect of the Schwarzschild solution that at first seemed simply a mathematical anomaly, but later became the basis of serious astronomical consideration, is that for dense enough objects there exists a spherical shell, called the event horizon, within which nothing that enters can escape, not even light. That’s because the escape velocity within the event horizon is faster than the speed of light. Therefore no object can reach such a speed and flee. In the 1960s, John Wheeler coined the term black hole to describe such a perpetually dark, ultracompact object. Black holes represent the ultimate dents in the fabric of space-time—the “bottomless pits” of astronomy.
The idea of inaccessible regions of space raises profound questions about the laws of physics in such enclaves. Are physical principles the same inside and outside a black hole’s event horizon? How would we know, if no one could venture inside and come back to tell the tale? Wheeler was puzzled in particular by the question of what would happen to disordered matter entering the point of no return. According to the long-established law of entropy, for any closed system, any natural process must either preserve or increase the total amount of entropy. Entropy is a measure of the amount of disordered energy, or waste, in a physical system. Thus, although natural processes can convert ordered energy into waste (such as a forest fire turning a stately grove into ashes), they can never do the trick of transforming waste energy completely into fuel. Although it is an open question whether the law of entropy applies to the cosmos as a whole, Wheeler was troubled by the idea that we could fling our waste into black holes, it would vanish without a trace, and the total fraction of orderly energy in the universe would increase. Could black holes serve as the cosmetic of cosmology, gobble up signs of aging, and make the universe seem more youthful?
In 1972, Jacob Bekenstein, a student of Wheeler’s, proposed a remarkable solution to the question of black hole entropy. According to Bekenstein’s notion—which was further developed by Stephen Hawking—any entropy introduced by absorbed matter falling into a black hole would lead to an increase in the area of its event horizon. Therefore, with a modest increase in entropy, the event horizon of a black hole would become slightly bigger. The signs of aging in the universe would thereby manifest themselves through the bloating of black holes.
As Hawking demonstrated, Bekenstein’s theory offers startling implications about the ultimate fate of black holes. Although nothing can escape a black hole’s event horizon intact, Hawking astonished the astrophysics community by theorizing that black holes gradually radiate away their mass. Hawking radiation, as it is called, is a natural consequence of another conjecture by Bekenstein. In addition to defining the entropy of black holes, Bekenstein showed that they also have temperature. Because anything in nature with finite temperature, from lava to stars, tends to glow (either visibly or invisibly), Hawking speculated that black holes radiate, too. To evade the event horizon’s barrier, this would be a quantum tunneling process, akin to how alpha particles escape the strong attraction of nuclei. A painstakingly slow trickle of particles, much too protracted to observe, would emerge over the eons. The more massive the black hole, the lower its temperature, and the longer it would take for it to completely evaporate. For a black hole produced by a collapsed star ten times the mass of the Sun, it would take an estimated 1070 (1 followed by 70zeroes) years for the whole thing to radiate away—far, far longer than the age of the universe. Because of such a prolonged time span, Hawking radiation has yet to be observed.
Less massive black holes would evaporate more quickly. These would need to be produced, however, through a completely different mechanism than stellar collapse. Stars of solar mass, for example, end up as the faint objects called white dwarfs, not black holes. Rather than collapsing further, their internal pressure supports their weight and they simply fade away over time.
Nevertheless, the Schwarzschild solution makes no mention of a minimal black hole mass. Rather, it defines a Schwarzschild radius—the distance from the center to the event horizon—for any given mass, no matter how small. The lighter an object, the tinier its Schwarzschild radius.
A black hole ten times solar mass, for instance, would have a Schwarzschild radius of almost nineteen miles, allowing it to fit comfortably within the state of Rhode Island. If an incredibly powerful force could somehow squeeze Earth so that it was smaller than its Schwarzschild radius, it would be only the size of a marble. A human being shrunk down to less than his or her Schwarzschild radius would be billions of times smaller than an atomic nucleus—clearly below the threshold of direct measurement.
In 2001, Savas Dimopoulos, along with Brown University physicist Greg Landsberg, published an influential paper speculating that microscopic black holes could be found at the LHC. These would have Schwarzschild radii comparable to the Planck length, less than 10-33 inches—or one quadrillionth of the size of a nucleus. Basing their work on theories of large extra dimensions, the researchers estimated that the LHC would churn out ten million microscopic black holes each year, similar to the annual rate of Z particles that were produced at the LEP.
Dimopoulos and Landsberg pointed out that any microscopic black holes produced at the LHC could be used as delicate tests of the number of extra dimensions in the universe—potentially verifying the braneworld hypothesis that gravitons leak into a parallel realm. That’s because the mass of these minute compact objects depends on how many dimensions space contains. Because Hawking radiation vanquishes lighter bodies at a faster rate, they would evaporate almost instantly, decaying into potentially detectable particle by-products. Their discovery would thereby present an ideal way to study the process of Hawking radiation, as well as examining dimensionality.
A simulation of the production and decay of a microscopic black hole in the ATLAS detector.
The existence of microscopic black holes is at this point purely hypothetical. Stellar-size black holes are still not fully understood, let alone theorized miniature variations. Dimopoulos and Landsberg emphasized that their calculations involved “semiclassical arguments” set in the nebulous zone between general relativity and certain theories of quantum gravity—particularly string theory and M-theory. “Because of the unknown stringy corrections,” they wrote, “our results are approximate estimates.”1
When a subject is as little known as the application of quantum theory to gravity at the smallest scales in nature, it is hard to say for sure which theoretical predictions will yield tangible results. The brilliance of detectors such as ATLAS and CMS is that they are general purpose. Data they collect will be analyzed by various groups all over the world and matched up against all different kinds of hypotheses. Until then, microscopic black holes remain fascinating to consider but highly speculative.
If microscopic black holes do pop up, they would have virtually no time to interact with their environment, which would just be the evacuated, low-temperature, hermetically sealed collision site. Produced by interacting quarks from two colliding protons, they would immediately decay into other elementary particles. During their brief existence, they would weigh little more than heavy atomic nuclei and would be far enough away from everything else that their gravitational interactions would be negligible. No fireworks, or even a blip on a screen, would announce their appearance. The only way anyone at the LHC would recognize that they came and went would be through meticulous data analysis that could well take many months.
Psychological perceptions of risk don’t always match up to actuality. Exotic threats, when matched against familiar hazards, often seem much scarier. People don’t spend their time worrying about the national injury rate due to slipping on bathroom floors or falling down basement stairs unless it unfortunately happens to their loved ones or to them. Yet there’s something about the bloblike image of black holes that stirs apprehension, even if the chances that such objects, particularly on a microscopic scale, will affect people’s lives are about as close to zero as you can imagine.
In 2008, Walter L. Wagner and Luis Sancho filed a lawsuit in Hawaii’s U.S. District Court seeking a restraining order that would halt operations of the LHC until safety issues involving potential threats to Earth were fully investigated. The named defendants included the U.S. Department of Energy, Fermilab, CERN, and the National Science Foundation. In a twenty-six-page decision, the judge hearing the case dismissed the suit, stating that the court did not have jurisdiction over the matter.
Trained in nuclear physics, Wagner heads a group called Citizens Against the Large Hadron Collider that he has established to warn against potential doomsday scenarios. One such scenario is the production of microscopic black holes that somehow manage to persist. This could happen, he conjectures, if Hawking radiation proves ineffective or nonexistent. After all, he points out, it has never actually been observed. The enduring mini-black hole would either pass right through Earth, like a neutrino, or be captured by Earth’s gravity. Suppose the latter is true. Once embedded in the core of our planet, he speculates, it could engorge itself with more and more material, grow bigger and bigger, and threaten our very existence. As Sancho and Wagner describe in their complaint:
Eventually, all of earth would fall into such growing micro-black-hole, converting earth into a medium-sized black hole, around which would continue to orbit the moon, satellites, the ISS [International Space Station], etc.2
This doomsday scenario is reminiscent of the catastrophe described in David Brin’s 1990 novel, Earth. In that science fiction epic set in the year 2038, scientists create a miniature black hole that accidentally escapes from its magnetic cage. After plunging into Earth’s interior, it is primed to gobble up the whole planet. A chase ensues to find the voracious beast before it is too late.
The anti-LHC group urges us not to wait that long. If there is even the slightest chance of a black hole destroying the world, the group argues, why take the risk? Why not rule out all conceivable hazards before the particle roulette begins? It would be a compelling case only if mini-black holes could really grow like blobs from microscopic to Earth-threatening sizes—but no credible scientific theory indicates that they could.
Another concern of Wagner’s group is the possibility of the LHC engendering “strangelets” or particle clusters with equal numbers of up, down, and strange quarks. According to the strange matter hypothesis, such combinations would be more stable under certain circumstances than ordinary nuclear matter. Like heat changing a runny egg into a solid glob, the energy of the LHC could catalyze such an amalgamation. Then, according to Sancho and Wagner’s complaint:
Its enhanced stability compared to normal matter would allow it to fuse with normal matter, converting the normal matter into an even larger strangelet. Repeated fusions would result in a runaway fusion reaction, eventually converting all of Earth into a single large “strangelet” of huge size.3
Yet another purported global threat is magnetic monopoles. These would be magnets with only north or south poles, not both. Chop a bar magnet in half and you get two smaller magnets, each with north and south poles. No matter what, there would always be two poles per magnet. Monopoles, in contrast, would have just one. Dirac predicted their existence in the 1930s, and they are an important component of certain Grand Unified Theories (GUTs).
Although monopoles have never been seen in nature, some theorists anticipate that they would be extremely massive and possibly turn up in LHC debris.
Sancho and Wagner ponder a scenario in which two massive monopoles, one north and the other south, would be produced at the LHC. Interacting with ordinary matter, theoretically they might hasten certain GUT processes and induce protons to decay. Suppose this caused a chain reaction, causing proton after proton—and atom after atom—to disintegrate into other particles. Eventually, the whole world would be a lifeless orb of inhospitable decay products.
Given the story of the Trinity test, such dire scenarios might lead us to believe that CERN researchers are now taking bets on the fate of Earth. Could LHC workers be wagering each lunchtime on whether black holes, strangelets, monopoles, or another bizarre creation will gobble up the French soil as if it were toast, bore holes through the Swiss mountains as if they were cheese, make haste toward Bologna, recklessly slice it up, and still not be satisfied? Could there be a hidden plot to conceal the true dangers of the world’s largest collider?
On the contrary, CERN prides itself on its openness. Secrecy is anathema to its mission. Isidor Rabi, who participated in the Manhattan Project and witnessed the Trinity test, founded CERN as a way for Europeans after the war to rebuild peaceful, civilian science on a cooperative basis. He emphasized that CERN would not have nuclear reactors and that none of its findings would be classified—to preclude the possibility that its research could be used for destructive purposes.
Scientists at CERN sometimes joke about the hullabaloo over mini-black holes. With gallows humor, some jest with a wink and a smile about being at the epicenter of imminent catastrophe—then go right back to coding their software. “[Friends] know that I’m not an evil scientist trying to kill the world,” said graduate student researcher Julia Gray.4
Theorists realize that quantum uncertainty offers a minuscule chance for an astonishingly diverse range of eventualities. Why spend time worrying about these? Through an extraordinarily unlikely roll of quantum dice, Nima Arkani-Hamed remarked that “the Large Hadron Collider might make dragons that might eat us up.”5
Despite the lighthearted attitude of many of its researchers, though, the CERN organization itself, for the sake of maintaining a candid and amicable relationship with the international community, takes any public fears very seriously. It doesn’t want people to suspect that something sinister is going on in its tunnels and caverns.
Although CERN conducted a comprehensive safety study in 2003 that found no danger from mini-black holes, strangelets, and magnetic monopoles, it agreed to complete a follow-up report that was released in June 2008. The new report concurred with the original findings, offering a number of powerful arguments why microscopic black holes, strangelets, and monopoles, if they truly exist, would pose no threat to Earth.
In the case of miniature black holes, the report showed how conservation principles would preclude them from being stable. Following the maxim that anything not forbidden is allowed, if they are produced by elementary particles, they could also decompose into elementary particles. Thus, aside from the question of whether Hawking’s description of black hole radiation is correct, mini-black holes must decay.
Moreover, produced in proton-proton collisions, chances are that microscopic black holes would carry positive charge and thereby be repelled by other positive charges on Earth. Hence, they’d have a hard time approaching atomic nuclei. Even if they could somehow survive and overcome the forces of electrical repulsion, their rate of accreting matter through gravitational attraction would be inordinately slow. In short, any Lilliputian blobs produced at the LHC would not have long for this world. They’d be the goners, not us.
Strangelets, the report’s authors point out, would be most likely to be produced during heavy ion collisions rather than proton collisions. In fact, some theorists anticipated their production at the Relativistic Heavy Ion Collider (RHIC), a facility at Brookhaven that opened in 2000. Interestingly, Wagner filed lawsuits against that collider too, unsuccessfully trying to prevent it from going on line. Yet during the RHIC’s run, absolutely no strangelets have turned up. The nearby Hamptons beach resort continues to attract the rich and famous to its glistening sand and surf, untainted by strange matter. If strangelets never appeared where they were most expected, why worry about them showing up at a less likely place?
There’s good reason to expect that any strangelets produced in collisions would be extremely unstable. They’d break up at temperatures much lower than those generated in ion crashes. The report compares the chances of stable strangelet production under such searing conditions to the “likelihood of producing an icecube in a furnace.”
Monopoles were explored at length in the 2003 study, as the authors of the 2008 report point out. If they managed to disintegrate protons—an extremely hypothetical scenario supported only in certain GUTs—they could gobble up barely a fraction of a cubic inch of material before being blasted harmlessly into space by the energy produced in the decays. A tiny hole in an LHC detector would be, at most, the only souvenir of their fleeting existence.
Finally, in arguing against the dangers of black holes, strangelets, monopoles, and other hypothetical high-energy hazards, perhaps the CERN safety team’s most compelling argument is that if any apocalyptic scenarios could occur, they would have happened already in cosmic ray events. Cosmic rays are enormously more energetic than what the LHC or any other collider provides. As the report points out:
Over 3 × 1022 cosmic rays with energies of 1017 eV or more, equal to or greater than the LHC energy, have struck the Earth’s surface since its formation. This means that Nature has already conducted the equivalent of about a hundred thousand LHC experimental programmes on Earth already—and the planet still exists.6
If CERN’s reassurances aren’t enough, perhaps we can be comforted by the lack of warning signals from the future. According to Russian mathematicians Irina Aref ’eva and Igor Volovich, the LHC might have the energy to create traversable wormholes in space-time linking the present with the future. If the LHC represented a danger, perhaps, as in the case of Gregory Benford’s novel Timescape (1980), scientists from the future would relay messages back in time to warn us. Or maybe, as in John Cramer’s novel Einstein’s Bridge (1997), they would try to change history and prevent the LHC from being completed.
A traversable wormhole is a solution of Einstein’s equations of general relativity that connects two different parts of space-time. Like black holes, wormholes are formed when matter distorts the fabric of the universe enough to create a deep gravitational well. However, because of a hypothetical extra ingredient called phantom matter (or exotic matter) with negative mass and negative energy, wormholes respond differently to intruders. While matter dropping into a black hole would be crushed, the phantom matter in a traversable wormhole would prop it open and allow passage through a kind of a space-time “throat” to another cosmic region. The difference would be a bit like attempting passage through a garbage disposal versus through an open pipe.
Researchers have speculated since the late 1980s that certain kinds of traversable wormhole configurations could offer the closed timelike curves (CTCs) that permit backward time travel. CTCs are hypothetical loops in space-time in which the forward direction in time of a certain event connects with its own past, like a dog chasing its own tail. For large enough wormholes, by following such a loop completely around, an intrepid voyager (in a spaceship, for example) could theoretically travel back to any time after the CTC’s creation. Smaller wormholes would just allow the passage of particles and information. Still, they might allow people to contact younger versions of themselves.
Aref ’eva and Volovich conjecture that the LHC’s energetic cauldron could brew up wormholes that allow backward communication. LHC researchers might learn of this first if they receive bizarre messages on their computer screens dated years ahead. Perhaps their e-mail in-boxes would suddenly become clogged with spam from companies yet to exist.
As numerous science fiction tales relate, backward time travel could create paradoxes involving the violation of causality: the law that a cause must precede its own effect. For example, suppose an LHC technician discovers a message from future researchers who have discovered how to send interpretable signals backward in time through wormholes created in the machine. The message warns of the creation of a new type of particle that will start to decimate Earth. After receiving the warning, CERN administrators decide to shut down the LHC. In that eventuality, the original future wouldn’t exist. How then could the future researchers have sent the signals? It would be an effect (turning off the machine) with either a cause from the future or no cause at all.
In such paradoxical situations, that’s where parallel universes would come in handy. If, in a manner similar to the Many Worlds scenario, each time information, things, or people travel backward in time, the universe bifurcates into several versions, the cause in one strand could precipitate an effect in another without contradiction.
Currently, most high-energy physicists have more pressing concerns about the future than hypothetical global disasters or whether backward-traveling signals are possible. When you are running a machine as complex as the LHC, and planning future upgrades and projects, pragmatic considerations generally overshadow abstract speculations. The LHC detectors have so many delicate components, subject to extreme conditions such as temperatures near absolute zero, it takes an incredible amount of effort to make sure they are working properly.
In between tinkering with current technologies, if a high-energy physicist has time to contemplate the future, he or she might well be thinking about the future of the field itself. How will the LHC results—however they turn out—affect the direction of particle physics? At what level of funding and commitment will the public continue to support one of the most expensive scientific disciplines? Is it reasonable to encourage young students to pursue high-energy careers given such uncertainty? What will particle research look like decades from now?