The continued existence of the Moon, in the form we know it, despite billions of years of cosmic-ray exposure, provides powerful empirical evidence against the possibility of dangerous strangelet production.

As the second millennium of the Christian era ended, many people feared that the world would, too.

Some anticipated Armageddon. Others were obsessed with Y2K. And a few of the more scientifically minded among the worriers dreaded the impending creation of strange quark matter.

Of course, strange quark matter had nothing to do with the end of the millennium. It was mere coincidence that a powerful new atom smasher on Long Island was scheduled to be up and running by late 1999. (And the millennium didn’t really end until the end of the year

2000, anyway.) Nevertheless, in the months before its inaugural collisions, whispers began to spread that the new machine possessed the power to destroy the Earth—if not the whole universe.

Some of the whispers made it into print in popular media, as in a Newsweek article about the atom smasher, the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory. Concerned with public reaction to such disaster rumors, Brookhaven’s director, John Marburger, appointed four premier physicists to analyze any threat that the collider, known as RHIC (pronounced “Rick”), might actually pose to the planet.

By October 1999 the experts posted their report on the Internet for anyone to see. Their analysis identified three possible catastrophes: RHIC might create a small black hole (that would suck the Earth in); RHIC could “freeze space” throughout the universe, wiping out everything that had existed up till then; or RHIC might accidentally create a “strangelet,” a small lump of matter made of an unusual mix of quarks.

By far, the strangelet scenario was the most serious to fear. The other two derived from an utter misconception about RHIC’s real power.

RHIC acquired its risky reputation because it would be smashing together atoms of gold—hundreds of times heavier than the particles commonly smashed in such machines. Therefore the energy generated in the collisions would be higher than in any previous subatomic experiments. Fear of that unknown energy realm provoked predictions of apocalypse. With more energy available than ever before, perhaps RHIC would produce a black hole capable of swallowing the Earth. Or worse, perhaps the vacuum of space is not in the most stable possible state but is on the edge of transformation, like a supercooled liquid, poised to freeze at the slightest disturbance. Maybe RHIC’s collisions would disrupt the fragile condition of space itself, sparking a phase transition that would in essence cause the whole universe to go poof!

But Brookhaven’s expert panel dismissed those concerns. What’s important is not the total energy, but the concentration of energy. Lower-energy collisions with smaller particles have already produced higher-energy concentrations than RHIC will with the relatively big gold atoms. Total energy is simply not the right thing to worry about. “If it were,” the RHIC panel wrote, “a batter striking a Major League fastball would be performing a far more dangerous experiment than any contemplated at a high-energy accelerator.”¹

Therefore the black hole concern can be readily dismissed. Generating a black hole requires an enormous concentration of energy in a very small region of space. RHIC’s energy would fall short by at least a factor of 10 billion trillion. As for triggering a sudden “freezing” of the vacuum of space, cosmic rays have already collided in deep space many times with a greater energy concentration than RHIC could muster. So the vacuum of space is surely stable enough to withstand any blips on RHIC’s energy scale.

On the other hand, the strange quark matter scenario was a little more disturbing, perhaps because RHIC’s very purpose was to create a new state of matter, best described as “quark soup.”

Unless you were around to witness the birth of the universe, you’ve never tasted quark soup. Cooking it up requires temperatures something like a billion times hotter than in the sun, higher than anything the universe has seen since a few millionths of a second after the big bang. To generate that much heat, RHIC flings gold atoms through a ring 2.4 miles around, smashing the atoms together at nearly the speed of light, cramming the matter tightly enough to reach densities 30 times greater than a gold atom’s nucleus (comparable to cramming all the matter in the moon into a ball that would fit in a backyard swimming pool). The dense heat melts the gold nuclei, squeezing

out the quarks and gluons inside to create the soup of a time long past.

Technically minded physicists call quark soup a “quark-gluon plasma”; it is, after all, more like a gas than a liquid and does contain gluons as well as quarks. In today’s cooled-down universe, quarks are the main building blocks of matter, congregating in triplets to form the protons and neutrons of an atom’s nucleus. Gluons are the nuclear equivalent of Velcro, forcing the quarks within each nuclear particle to stick together.

But in the good old (old, old) days, when the universe was hot enough, quarks and gluons flowed freely through the plasma-soup. Only when the universal thermostat dropped below 10 trillion degrees did the quarks coagulate into protons and neutrons.

Along with the electrons (which are quark-free), protons and neutrons make up all the ordinary matter of everyday life, such as rocks and people, water and popcorn. Until mid-way through the twentieth century, protons and neutrons were regarded as the “uncuttable” components of the atomic nucleus, more similar to the ancient Greek concept of atoms than atoms themselves. But then along came a man who used math to see inside protons and neutrons—a guy who drives around today in an SUV with a license plate that reads QUARKS.

His name is Murray Gell-Mann, and he became the driving force in subatomic physics in the 1950s, a time when physicists were bewildered by all the particles they were discovering.

Leon Lederman, a Nobel winner in the particle physics game, recalls those days as a time of particle plenty. “It was almost routine: You set up your apparatus in front of a new machine and you found a particle and you passed Go and you collected two hundred dollars,” Lederman reminisced when I visited him in 1997 at the Fermi National Accelerator Laboratory in Batavia, Illinois (popularly known as Fermilab,

home of the nation’s most powerful atom smasher). In those early days, particles proliferated too rapidly even for Enrico Fermi, one of the century’s greatest physicists.

“We were standing in lunch line, and I had to make conversation with the great man,” Lederman recalled from a conference in the fifties. “I said, ‘What do you think of the evidence for the V-zero-2 particle’ that was just presented, and he looked at me and said, ‘My boy, if I could remember the names of these particles I would have been a botanist.’”²

The particle explosion dismayed many physicists who hoped that their new atom smashers would reveal nature’s underlying simplicity. “Instead we were beginning to count hundreds of particles,” Lederman said. “It was a whole attic full of discoveries that came in so fast that we didn’t know what to do with them. Except do what the botanists do, which is just classify them, organize them, and look for patterns.”

Murray Gell-Mann was the best pattern finder of them all. Born in New York City in 1929, he was, to understate it, a bright child. By age 15 he entered Yale, and then he earned his Ph.D. at the Massachusetts Institute of Technology (MIT) by age 21. After a short stay at the Institute for Advanced Study in Princeton, Gell-Mann spent a few years at the University of Chicago in the early fifties, where he took the first giant step toward clearing up the particle muddle by inventing the idea of “strangeness.”³

It was unusual in those days to give a new physics concept such a whimsical name. But Gell-Mann had precisely the right idea. After all, many of the new subatomic particles popping out of the atom smashers were strange; the V-zero-2 being an example. They weren’t like the other particles that had become familiar by then, like protons and neutrons. The new particles differed somehow, in ways then dimly understood, from other particles. It seemed perfectly natural to describe them as strange, meaning they possessed a property called strangeness.

Strangeness was more than just a clever word, of course—it was a

number. The strange new particles could be assigned a “strangeness number” that helped make sense of the situation. Typically the new particles appeared in pairs; if one of the particles had a strangeness number of 1, the other must have a strangeness value of –1, so that the total strangeness remained zero.

Strangeness was an original and fruitful idea. It eventually led Gell-Mann (who had moved on to the California Institute of Technology, known as Caltech) to the next step in understanding the basic units of matter—a pattern of particle properties that he called the Eightfold Way.⁴

In essence, Gell-Mann showed how to organize the species of the particle zoo (by now, nearly 100 particles had been discovered) into groups of 8 (or in some cases, 10). Amazingly, the properties of the particles could be described by the obscure (at the time) mathematical notion called group theory.⁵ A family of 8 related particles, in Gell-Mann’s scheme, corresponded to what a mathematician would have called a “group of eight.” That’s why Gell-Mann called his scheme the Eightfold Way.

Of course, he did not mean to imply that any eastern mysticism was involved in particle physics. (“I meant it as a joke!” he once exclaimed.)⁶ His insights were much more in the spirit of the ancient Greeks, who two and a half millennia earlier had conceived of ultimate basic particles they called atoms, their word for “uncuttable.”

In a way, the Greek idea of atoms represents one of the earliest great prediscoveries, the imagining of something to be discovered only much later. But the Greek concept was not so clear by the standards of modern physics. For a long time there was a lot of confusion about what the Greeks’ idea of an atom really meant.

“They had two different concepts contained in the notion of an

atom,” Gell-Mann remarked over lunch when I interviewed him in Santa Fe in 1997. “One is the smallest unit of some substance, and the other was that it was—as the name indicates—uncuttable. But those turned out not to be the same thing.”

Today’s atoms are, in fact, the smallest units of chemical elements. But these atoms are clearly cuttable, or at least smashable and splittable, as they are made of still smaller pieces. In the sixties, Gell-Mann was searching for the true uncuttables, the most basic of matter’s building blocks.

His Eightfold Way revealed the mathematical formulas corresponding to groups of subatomic particles. Understanding that math allowed him to arrange the known particles in charts similar to Dmitri Mendeleyev’s periodic table of the chemical elements. The idea of positioning particles in a table like Mendeleyev’s had been expressed years earlier by the physicist Abraham Pais. “The search for ordering principles at this moment may indeed ultimately have to be likened to a chemist’s attempt to build up the periodic system if he were given only a dozen odd elements,” Pais wrote.⁷ Gell-Mann, on the other hand, told me he wasn’t explicitly setting out in Mendeleyev’s footsteps.

“I was playing around with the particles,” Gell-Mann said. “He was playing around with the elements. It was natural to make a comparison between them, although I think Mendeleyev’s work was much more important.”⁸

Anyway, just as Mendeleyev had used gaps in his table to predict the existence of undiscovered elements, Gell-Mann predicted that new particles should be found to fill in some of the empty slots in his Eightfold Way charts. The new particles were found with just the properties he anticipated. In fact, when experimenters at a 1962 conference reported the discovery of new particles fitting into his scheme, Gell-Mann realized immediately that yet another new particle must exist, which he called the omega-minus. The mass of the

new particle, Gell-Mann asserted, should be 1,685 million electron volts (MeV), and its strangeness number should be –3.⁹

Nicholas Samios, of Brookhaven, returned from the meeting determined to find Gell-Mann’s new particle. It was not an easy experiment to do. But by December 1963, his team had succeeded in recording the particle collisions that Gell-Mann suggested might produce his omega-minus. In February 1964, the Brookhaven team published its report, announcing the discovery of a new particle with a mass of 1,686 MeV—give or take 12. The strangeness number: –3. Gell-Mann had been right on the money. The Brookhaven experimenters concluded that they were “justified in identifying” the new particle “with the sought-for omega-minus.”¹⁰

It was one thing, though, to predict more particles that fit into established groups. It was something else again to imagine an entirely new sort of particle never previously encountered. But that’s what Gell-Mann did next.

Doing so proved difficult, though. The math told him that the heavy particles in nature (known as the hadrons) could be built from a set of three basic particles. But this picture was blurred by a problem with electric charge. The math required the building-block particles to have electrical charges only one-third or two-thirds of the smallest unit of charge believed possible (the charge of an electron or proton). No experiment had ever produced a particle with fractional charge.

“I ignored the fractional charge possibility—it seemed so crazy,” Gell-Mann told me. But at one point—in 1963—another physicist, Robert Serber, asked Gell-Mann why he hadn’t used the triple-particle approach. Gell-Mann said that he had tried.

“I drew on a napkin a picture showing him the equation, showing him the charges would be fractional,” Gell-Mann recalled. That seemed to satisfy Serber, but Gell-Mann pondered the three-particle approach some more. “During that day and the next day, I thought

about it and I decided, well, maybe they don’t come out, maybe they’re trapped inside” the protons and neutrons and other hadrons.

So he pursued the idea of trapped, fractionally charged particles, to which he gave the name he now uses on his license plate: quarks. He referred to the quarks as “mathematical” or “fictitious,” meaning, he said, that they would always be trapped inside a bigger particle and would therefore never be seen alone.

“That was the defining moment . . . after Bob Serber asked that question,” Gell-Mann said. “I thought that maybe these things can’t come out and therefore there’s no problem with experiment.”¹¹

True enough, there was no problem with experiment then, and there hasn’t been since. No compelling evidence has ever materialized for the existence of a free fractionally charged particle. Quarks are indeed trapped. Experimental evidence did come soon, though, for the reality of quarks trapped within other particles. Using the Stanford atom smasher, physicists in the late 1960s fired electrons into protons and found curious deflections of the electrons’ paths— just the kind of deflections, in fact, to be expected if a proton was made up of smaller particles. It took a while for the physics community to reach a consensus that the proton’s parts were the same as Gell-Mann’s quarks. But by the 1980s, nobody seriously doubted it, and in 1990, Stanford physicist Richard Taylor, with MIT colleagues Henry Kendall and Jerome Friedman, won the Nobel Prize for their discovery. Gell-Mann’s prediscovery of quarks had been verified.

In his original math, all the hadrons could be made from only three kinds of quarks—the up quark, designated by u, the down quark, symbolized by d, and a third quark abbreviated with the letter s—not for sideways, but for strange—the quark conferring the property that Gell-Mann had years earlier called strangeness. Protons and neutrons, being “ordinary particles,” contained no strange quarks, just ups and downs. A proton contains two ups and a down; a neutron possesses two downs and an up. Thus a proton could be abbreviated as uud, a

neutron as udd. Particles exhibiting strangeness contain combinations that include an s, or strange, quark.

Nowadays physicists recognize six quarks—the up, down, and strange quarks identified by Gell-Mann, and the charm, bottom, and top quarks discovered years later.

For many years after Gell-Mann invented them, nobody worried very much about strange quarks. After all, the oddball particles containing strange quarks didn’t have a very prominent role in real life. The “strange” particles were created only under unusual conditions, and they didn’t live very long. They were kind of like termites, unseen deep within the walls of nuclear science. But they were about to chew their way out.

In the early 1980s, a young physicist injected a strange new plot twist into the quark story. Edward Witten, at the time at Princeton University, was about to become the Murray Gell-Mann of his generation—a creative and critical thinker and the intellectual leader of an entire community of physicists.

It is fascinating to hear other physicists marvel at Witten’s brilliance. I’ve attended numerous lectures where a speaker expressed awe at some insight Witten had provided to illuminate an important issue. One such comment came from Willy Fischler, a physicist at the University of Texas, when discussing a peculiar point that Witten had clarified involving string theory. Fischler admitted that he had no clue to how Witten had arrived at his conclusion. “I was not in his brain,” Fischler said, “so I don’t know.”¹²

In his best-selling book The Elegant Universe, the theoretical physicist Brian Greene implies that Witten might be the greatest physicist who ever lived. I asked Greene if he really believed it. “I didn’t say that,” he said. “I said ‘some would say’ that he’s the greatest physicist of all time.”

“So do you agree?” I cross-examined.

“I think he’s phenomenal,” was Greene’s evasive reply.¹³

In any event, when Edward Witten talks, other physicists listen. As they did in 1984, when he wrote a paper examining the idea that quarks need not always be confined in protons and neutrons. Under extreme conditions—say, within the superdense matter of a neutron star—perhaps quarks could arrange themselves differently, forming “quark matter.”

At first glance, it seems like a far-fetched idea. We (and most of the matter we know about) are made of protons and neutrons (in this context, it is OK to ignore electrons). That must be because quarks like to stick together. Free-swimming quarks have energy to spare, a situation that physicists refer to as “unstable.” Everything in nature seeks a condition where the energy required to maintain it is minimal. Sooner or later, energetic objects relax to the lowest-energy, stable state. Rocks roll down to the valleys below, living things die, and quarks coagulate to make protons and neutrons.

But suppose there is some more stable, lower-energy condition that quarks can find. In that case they might not need protons and neutrons. As early as 1971, Arnold Bodmer, of the University of Illinois, pointed out that up and down quarks might be stable if enough strange quarks joined them. With roughly equal numbers of strange, up, and down quarks, dense matter (inside stars, say) might actually prefer to stay in the form of a cluster of quarks, without congealing into protons and neutrons. Thus was born the novel idea of “strange quark matter”—either “strange matter” or “quark matter” for short.

Nobody paid much attention to Bodmer, although a few other physicists played around with the idea. But it became a hot topic in 1984, when Witten analyzed the strange situation.

Witten suspected that quark matter might solve a major astronomical mystery—the identity of the invisible “dark matter” that lurked throughout the cosmos, betraying its presence only by exerting gravitational effects on visible matter.

It seemed then (and still does) that the amount of dark matter is oddly similar to the amount of ordinary matter in the universe. Not exactly the same amount, but maybe seven times as much—to astrophysicists, not much of a difference. Maybe, Witten realized, dark matter was similar in quantity to ordinary matter because it was made of the same stuff (quarks), just arranged in a different way—in the form of quark matter, rather than as protons and neutrons. In the moments following the explosive big bang that gave birth to the universe, he suspected, the most stable arrangement of up, down, and strange quarks would have been within lumps of quark matter.

“I got the idea that if quark matter was stable at zero pressure, . . . perhaps the big bang was a good place to make it,” he told me many years later.¹⁴ If so, it raised the possibility that today, chunks of strange quark matter might be hanging out in the universe, accounting for the unseen dark matter that astronomers can’t identify.

Witten realized that entertaining the idea of stable strange quark matter required a heavy dose of speculation. “The odds are against it,” he acknowledged. Strange matter might be stable under astronomically high pressures, but in the zero-pressure environment of empty space it’s not a good bet. But then again, long shots some-times win.

“It’s just barely possible that strange matter is stable even at zero pressure,” Witten said. “Very hard to make, but stable once you make it.” So Witten explored the notion of making strange quark matter in the big bang. Alas, he ran into a severe problem.

“There’s really a very good objection . . . which comes up right away, which is that, unfortunately, the big bang was hot,” he said. “If you accept at face value that the big bang was hot, it’s really almost impossible to make quark matter.”¹⁵

At high temperatures, you don’t get lumps of strange quark matter, you get quark soup (or quark-gluon plasma). When it cools it wants to congeal into protons and neutrons, the process known to

physicists as the quark-hadron phase transition (since protons and neutrons are hadrons).

In any event, the prospect that the dark matter is strange matter seems dim. Yet the idea of strange matter remains alive. It may not like the heat, but it loves pressure. Perhaps there are pressure cookers somewhere in the universe in which strange quark matter could thrive. One obvious possibility would be in the middle of a neutron star, where pressures dwarf anything ever encountered on Earth.

“We do have good reasons to believe that under sufficiently high pressure, quark matter would be stable,” Witten told me. “But it’s very difficult to estimate the pressure. . . . We don’t know whether at the center of a neutron star the pressure is big enough. It’s possible that it is, and that neutron stars are mostly strange matter. But it’s also possible that even at the center of a neutron star you don’t have quark matter.”¹⁶

But if you did, then the possibility does exist that some strange matter might escape its neutron star prison—and if so, chunks of strange matter might indeed be found flying through space.

“If it’s stable at zero pressure you could make it in neutron stars and it’s conceivable that some catastrophes involving neutron stars would eject some into the universe,” says Witten. “That’s the best chance I can see.”

So there is some small chance that some lumps of strange quark matter might be zipping through the universe, perhaps headed this way. Perhaps it would be worth looking for. “But only,’’ says Vic Teplitz, “if you already have tenure.”

Teplitz is a physicist who spends most of his time in an airplane, shuttling back and forth between Maryland, where he lives, and Dallas, where he works.¹⁷ He came to Texas in 1990, eager to participate

in the century’s grandest physics experiment, the Superconducting Super Collider (SSC). In 1988 the government selected a site south of Dallas, surrounding the quaint town of Waxahachie, as the future center of high-energy physics. There physicists planned to oversee the construction of a 54-mile-around racetrack where protons would smash into each other with more energy than any atom smasher had ever before generated.

Five years later, not yet half-finished, the SSC was dead, killed by political bickering in Congress. In the meantime, though, Teplitz had revitalized the physics department at Southern Methodist University (SMU), a school more famous for football than science, tucked into the community of Highland Park, an “endoburb” entirely surrounded by the rest of Dallas.

It’s not that SMU was without significant science, though. One of its most prominent programs involved seismology, where a nationally recognized expert named Eugene Herrin worked on problems like figuring out how to tell earthquakes from underground nuclear explosions.

One day Teplitz called me about a column I’d written for the Dallas Morning News’ Monday science section. He was ecstatic that I’d written about a paper in Physical Review Letters describing strange quark matter. Some physicists in England and France had speculated that nuggets of strange matter might have survived the big bang and could be floating through space today. Apparently Herrin read the column, and it inspired him to call Teplitz to discuss a project Teplitz had been pushing Herrin to pursue. Teplitz wanted to build a seismic telescope—to search for those strange quark nuggets.

Since he had tenure, Teplitz figured he was free to pursue strange matter, but he needed the help of Herrin’s seismic expertise to “build” a seismic telescope that might reveal strange quark visitors from space impinging on our planet. Basically, the seismic telescope he envisioned consisted primarily of the Earth itself.

As the sun and solar system sweep through the galaxy, Teplitz explained to me, the Earth might encounter quark nuggets in its path. A typical nugget might weigh 4 tons yet be smaller than a millimeter across. Traveling at 150 miles a second, it would shoot through Earth like a bullet through butter.¹⁸

The Earth would shudder at such an insult, with ripples from the passing nugget shaking the Earth’s interior much like an earthquake or nuclear explosion. But an earthquake or explosion sends its waves outward from a single spot. The quark nugget’s vibrations would emanate from all along its path through the planet. It should therefore be feasible to analyze readings by seismic recording stations around the globe to find strange patterns of signals from strange quark nuggets.

Fortunately, the U.S. Geological Survey collects seismic signal reports from stations around the world. For the years 1981 through 1993, about 9 million such signals are on record. And at least 2 million of those signals cannot be connected to known earthquakes. So with a little luck, and some superior computer programming skills, the SMU scientists might be able to analyze the mystery signals and find a pattern in which seven different stations recorded waves at the right time intervals to match the pattern expected of a quark nugget.

It turns out that programming a computer to analyze all that data and spot just the right combination of seismic tremors is a far from trivial task. And seeking strange quark matter is hard to justify as a full-time job. So merely preparing the computer has taken years, and analyzing all the data could take years longer.

“Nobody’s been looking for strange quark nuggets passing through the earth, so things haven’t been set up in an optimum way,” Teplitz told me. And even in a best-case scenario, out of the 2 million or so events to sift through, there are probably no more than a handful of nuggets.

Of course, there might also be no nuggets at all. But Teplitz and Herrin are not alone in searching for them. Many experimenters have

expended considerable effort seeking signs of strange matter in meteorites, moon rocks, and the debris of atom-smasher collisions. So far, no luck.

There are other attempts, though, to detect strange quark nuggets striking the Earth. If the nuggets are a lot smaller than the millimeter-sized lumps considered by Teplitz, they might not shoot through the Earth but could be stopped by the atmosphere, behaving a lot like ordinary cosmic rays.

Shibaji Banerjee and colleagues at the Bose Institute, in Calcutta, India, even suggest that smaller nuggets, or strangelets, may already have been detected. A giveaway feature of a moderate-sized strangelet is an unusually small electric charge compared to the mass of the particle. An ordinary heavy atomic nucleus carries an electric charge not too much less than half its mass. A typical uranium nucleus, for example, has a mass of 238 (the sum of its protons and neutrons) and a charge of 92 (the number of protons). But cosmic ray detectors have on occasion reported signs of unusual nucleus-like fragments, larger than uranium, but with a much smaller electrical charge—as low as 14. Banerjee and friends have calculated that such strange charges could in fact be explained by strange nuggets just massive enough to slice through the Earth’s atmosphere down to the altitude of mountaintops.¹⁹

So there’s hope that further analysis of cosmic rays may someday offer evidence of strange matter. Still, it may be that strange nuggets will remain forever strangers to the Earth. In that case, the best remaining bet for finding strange matter will be in space, based on the hope that strange matter lurks inside neutron stars—or more precisely, what appear to be neutron stars. For it may be that neutron stars are really made almost entirely of strange quark matter and would therefore better be known as “strange” stars.

The ultradense balls of nuclear matter known as neutron stars were discovered in 1967 by the British astronomer Jocelyn Bell, fulfilling a prediscovery from decades earlier. In 1932, the Russian physicist Lev Landau, inspired by the recent discovery of the neutron, suggested that stars made of neutrons might lurk in space. Two years later, Fritz Zwicky and Walter Baade proposed that the transformation of an ordinary star into a neutron star might underlie the phenomenon of exploding stars known as supernovas.

But in 1967, nobody realized at first that Bell’s discovery was a neutron star, because it showed itself in an odd way—in the form of a pulsing radio signal, much like the beacon of a lighthouse. Soon the astronomer Thomas Gold realized that the pulses could be explained by the rotation of a neutron star surrounded by a magnetic field. Neutron stars emitting such signals became known as pulsars.

As their name implies, neutron stars, presumably, are made of neutrons. “Neutron matter” would be extremely dense, of course— something like a trillion times as dense as water, denser the deeper you go. In its earlier life as a shining star, a neutron star would have been more massive but less dense. Death comes to such a massive shining star when it has burned up all its nuclear fuel. It stops shining. And that means no pressure emanates from within to keep the star’s mass from collapsing inward. The star’s solution to this problem is to explode. As the star collapses upon itself, its inner core (made of iron) shrinks from a few thousand miles wide to a mere 20 miles across in about a second. Much like a compressed tennis ball, the iron core then rebounds, blasting the star’s outer layers away (that’s the supernova) the way a depressed trampoline would boost a gymnast into the air. All that’s left is a dense core; that’s the neutron star.

Basic physics suggests that this core must be even denser than an ordinary atomic nucleus (about 300 trillion times water’s density). A chunk of such matter the size of a Fig Newton would weigh as much

as 10 billion Nate Newtons (a very weighty football player). At such densities, a neutron star weighs a little more than the sun but would fit inside the interstate highway loop surrounding a big city.

Densities so high make the pressure in the middle of a neutron star enormous. And as you may remember from Chapter 1 (after all, you’re still in Chapter 1), higher pressure raises the possibility that strange quark matter is stable. So suppose that somehow or other, conditions within a neutron star generated a bit of strange matter. The question is, could strange matter coexist inside the star with ordinary neutrons?

Maybe so, some analyses indicate. But the first appearance of strange matter might instead initiate a chain reaction. In minutes, the ordinary matter could all turn into strange matter, turning the neutron star into a “strange” star, releasing copious amounts of energy in the process—so much, in fact, that mysterious flashes of gamma rays from deep space might be produced in this way, some scientists suspect. If so, these gamma-ray bursts may actually be signals of strange matter in space.

Of course, there are other explanations for gamma-ray bursts. And in truth, nobody knows for sure whether strange matter is the most stable form of nuclear matter within neutron stars. There are even some indications that it isn’t. In some pulsars, for example, the pulsing beams exhibit temporary hiccups, or glitches, indicating that the star’s spin rate changed momentarily. Strange matter shouldn’t be able to do that, conventional wisdom holds. But Norman Glendenning, of the Lawrence Berkeley Laboratory in California, and colleagues from Germany say dismissing strange matter for that reason is premature. It is possible, their calculations show, that strange stars could have glitches.

Still, the best evidence that strange stars exist would be their direct detection, and there are possible signatures of strange stars that could be observed from Earth. For one thing, extremely rapid

pulses—on the order of 2,000 times a second—would be evidence of strangeness. A star made of neutron matter could not spin that fast without breaking apart. And Teplitz and Herrin (with additional collaborators from Virginia and California) have suggested yet another way to detect strange stars—namely, by tuning in to the right radio channel.²⁰

If they exist, strange-matter stars just might be sending a radio signal advertising their presence. Teplitz, Herrin, and colleagues calculate that strange stars could emit radio signals with a wavelength of about a millimeter or two, right in a popular range of frequencies studied by radio telescopes.

Emission of such radio signals would be expected because strange-matter stars should acquire a thin crust of ordinary matter, cushioned from the strange interior by a layer of electrons. It is possible that this crust would vibrate back and forth with respect to the center of the core. Since the crust would be an electrical conductor, its motion would cause the cushioning electrons to slosh back and forth, generating a radio wave.

“A detectable signal could be achieved for pulsars as far away as 15 kiloparsecs (about 300 million billion miles),” the scientists wrote in their 1997 paper. More than 100 candidate pulsars are known within that distance.

OK, it sounds like another long shot. But even if all the searches for strange matter fail, there might still be some payoffs. All the unaccounted-for signals from inside the Earth studied by the “seismic telescope” may not be caused by strange nuggets but by some other interesting and previously unknown geological phenomenon. Furthermore, failing to find strange matter in cosmic rays or neutron stars would in itself tell a lot about its properties. It may sound strange, but understanding more about strange matter could put scientists on a path to better appreciating the universe’s simplicity. As John Wheeler, the physicist who named black holes, has often re-

marked, “We will first understand how simple the universe is when we recognize how strange it is.”

Meanwhile, fear of universal destruction from RHIC has faded. But the prospect of deadly danger from strange quark nuggets was worth considering. Strange quark nuggets are not the sort of thing you would want for a pet rock.

If somebody did create strange matter, it might pose the same danger to Earth as it would to a neutron star. A chunk of such strange matter forming within a neutron star could begin “eating” the neutrons surrounding it, converting them into additional strange matter—ultimately digesting the entire neutron star. So a strange quark nugget, or strangelet, does pose a certain danger to anything around it. You see why some people got worried about RHIC. As the team of experts studying RHIC’s dangers acknowledged, a strange quark nugget is not something you’d want to meet in a dark alley, especially if it carried a negative electrical charge.

“If such an object did exist and could be produced at RHIC,” wrote the analysis team, “it would indeed be extremely dangerous.’’

A whole chain of unlikely events would have to happen to wipe out the Earth, though. To begin with, RHIC would somehow have to produce a negatively charged strangelet that stays around at least a hundred-millionth of a second. If that happened, the strangelet would quickly be captured by an atomic nucleus in the vicinity. Once inside the nucleus, the strangelet would swallow other nuclear particles, beginning to grow into an even larger strangelet. For a moment, the strangelet’s negative charge would switch to positive. But then it would begin gulping down negatively charged electrons in the vicinity of the nucleus, reversing charge again, back to negative, growing bigger all the while. Then it would capture more

nuclear particles, and the process would repeat itself. Soon the strangelet would be 100 times the size of an ordinary atomic nucleus.

That would be the time to watch out. The electric charge of the strange quarks would initiate the creation of electrons and their antimatter counterparts, positively charged positrons. The positrons would stick to the strangelet, surrounding it with a cloud of positive charge. Any atom passing by would find its electrons annihilated by the positrons, and the naked nucleus remaining would be gobbled up by the strangelet.

“This process would continue,” the RHIC experts wrote, “until all available material had been converted to strange matter. We know of no absolute barrier to the rapid growth of a dangerous strangelet.”²¹ In other words, once a strangelet starts growing, it doesn’t stop. Pretty soon the entire Earth would be just one big fat strange nugget.

It would be a very strange way to destroy the planet. But while this strangelet scenario is scarier than The Blair Witch Project, it’s even more implausible. The RHIC experts conclude that there’s really nothing to worry about.

First, there is no real evidence that strange matter can exist long enough for this to happen, they pointed out. Second, other particle smashers had tried to make strange matter and failed. And if RHIC’s collisions could make strange matter, so would have cosmic rays colliding with the moon. And the moon is still up there.

Besides, if RHIC did accidentally make a strangelet, it would almost certainly have a positive electrical charge, and only negatively charged strangelets are dangerous. In other words, worrying about strangelets is like fearing your dog’ll bite you to death, when your dog has no teeth, and you don’t even have a dog.

As it turned out, RHIC didn’t get going on schedule, anyway; it was mid-2000 before it began smashing gold atoms together in earnest. By then Y2K fears had fizzled, along with any worries that

RHIC would destroy the universe. But the search for strange quark matter continued.

In fact, in April 2002 two teams of astronomers reported new evidence for strange quark stars, announcing at a NASA news conference that data on two supposed neutron stars pointed to the conclusion that they were made of strange quark matter. “Stars suggest a quark twist and a new kind of matter,” proclaimed the headline in the New York Times.

And a day before the NASA news conference, Teplitz and Herrin submitted a paper for publication, reporting the possible “sighting” of two strange quark nuggets in their analysis of seismic data. So by the time you’re reading this book, my promise in the preface that at least one of the strange matters will someday be discovered may already have come true.

In the meantime, Teplitz has been spending some time looking for other potential prediscoveries in a mirror. Or more precisely, in a mirror world.