UH-OH, the mad scientists are at it again. In their determination to extract nature's secrets, physicists in America have built a machine so powerful it has raised fears that it might cause The End of The World As We Know It.

"Big Bang machine could destroy Earth" ran the headline over a story in The Sunday Times last month. It claimed that our planet was in peril from a vast new American particle accelerator on Long Island, the Relativistic Heavy Ion Collider (RHIC), which will collide pairs of gold nuclei at high energies. According to the article, RHIC could trigger a catastrophic event: the creation of a black hole or a ravenous "strangelet" that could swallow up our entire planet.

Within 24 hours, the laboratory issued a rebuttal: the risk of such a catastrophe was essentially zero. The Brookhaven National Laboratory that runs the collider had set up an international committee of experts to check out this terrifying possibility. But BNL director John Marburger, insisted that the risks had already been worked out. He formed the committee simply to say why they are so confident the Earth is safe, and put their arguments on the Web to be read by a relieved public.

Even so, many people will be stunned to learn that physicists felt worried enough even to mull over the possibility that a new machine might destroy us all.

In fact, they've been fretting about it for over 50 years. The first physicist to get the collywobbles was Edward Teller, the father of the hydrogen bomb. In July 1942, he was one of a small group of theorists invited to a secret meeting at the University of California, Berkeley, to sketch out the design of a practical atomic bomb. Teller, who was studying the reactions that take place in a nuclear explosion, stunned his colleagues by suggesting that the colossal temperatures generated might ignite the Earth's atmosphere.

While some of his colleagues immediately dismissed the threat as nonsense, J. Robert Oppenheimer, director of the Manhattan Project, set up to build the atom bomb, took it seriously enough to demand a study. The report, codenamed LA-602, was made public only in February 1973. It concentrated on the only plausible reaction for destroying the Earth, fusion between nuclei of nitrogen-14. The report confirmed what the sceptics had insisted all along: the nuclear fireball cools down too far quickly to trigger a self-sustaining fire in the atmosphere.

Yet in November 1975, The Bulletin of the Atomic Scientists claimed that Arthur Compton, a leading member of the Manhattan Project, had said that there really was a risk of igniting the atmosphere. It turned out to be a case of Chinese whispers: Compton had mentioned the calculation during an interview with the American writer Pearl Buck, who had got the wrong end of the stick.

Even so, the Los Alamos study is a watershed in the history of science, for it marks the first time scientists took seriously the risk that they might accidentally blow us all up. The issue keeps raising its ugly head.

In recent years the main focus of fear has been the giant machines used by particle physicists. Could the violent collisions inside such a machine create something nasty? "Every time a new machine has been built at CERN," says physicist Alvaro de Rujula, "the question has been posed and faced."

One of the most nightmarish scenarios is destruction by black hole. Black holes are bottomless pits with an insatiable appetite for anything and everything. If a tiny black hole popped into existence in RHIC, the story goes, it would burrow down from Long Island to the centre of the Earth and eat our planet—or blow it apart with all the energy released. So why are physicists convinced that there's no chance of this happening?

Well, the smallest possible black hole is around 10-35 metres across (the so-called Planck Length). Anything smaller just gets wiped out by the quantum fluctuations in space-time around it. But even such a tiny black hole would weigh around 10 micrograms—about the same as a speck of dust. To create objects with so much mass by collisions in a particle accelerator demands energies of 1019 giga-electronvolts, so the most powerful existing collider is ten million billion times too feeble to make a black hole. Scaling up today's technology, we would need an accelerator as big as the Galaxy to do it.

And even then, the resulting black hole wouldn't be big enough to swallow the Earth. Such a tiny black hole would evaporate in 10-42 seconds in a blast of Hawking radiation, a process discovered by Stephen Hawking in the 1970s. To last long enough even to begin sucking in matter rather than going off pop, a black hole would have to be many orders of magnitude bigger. According to Cliff Pickover, author of Black Holes: A Traveler's Guide, "Even a black hole with the mass of Mount Everest would have a radius of only about 10-15 metres, roughly the size of an atomic nucleus. Current thinking is that it would be hard for such a black hole to swallow anything at all—even consuming a proton or neutron would be difficult."

So we needn't lose sleep about creating an Earth-eating black hole in an accelerator. But according to John Wheeler of Princeton University, there is another way: detonating a big hydrogen bomb. He showed that the pressures generated by a suitable explosion could crush matter to the densities needed (around 1017 kilograms per cubic metre) to stand a chance of creating a black hole. However, Wheeler estimated that a "suitable" H-bomb would require all the heavy water in the oceans, and weigh many billions of tonnes. Some bomb.

The more discerning mad scientist might instead opt to pick a black hole "off the shelf". One left over from the Big Bang or an exploding star, for example. The temptation is certainly there, for as the Oxford mathematician Roger Penrose showed 30 years ago, black holes make wonderfully clean sources of energy. Just throw a skipful of junk at a black hole in the right way, Penrose discovered, and it will eat up all the junk and then hurl the empty skip back out again with more energy than it had before.

Fortunately, there's not much chance of bringing a black hole to Earth any time soon. After all, they would be rather unwieldy and the nearest one is likely to be many light years away.

It was while dismissing the black-hole threat in last month's Scientific American that theorist Frank Wilczek of the Institute for Advanced Study in Princeton mentioned an altogether more exotic form of killer blob: "strangelets".

Strangelets are chunks of matter made from "strange" quarks as well as the usual "up" and "down" types of ordinary matter. It might be possible to make them in particle accelerators like RHIC. The risk is that a strangelet might consume nuclei of ordinary matter and convert them into more strange matter, transmuting the entire Earth into a strange-matter planet. But having raised this appalling prospect, Wilczek quickly dismissed it.

And quite rightly, says the world's leading expert on strangelets, Robert Jaffe of the Massachusetts Institute of Technology. "Strangelets are almost certainly not stable, and if they are, they almost certainly cannot be produced at RHIC," he says. "And even if they were produced at RHIC, they almost certainly have positive charge and would be screened from further interactions by a surrounding cloud of electrons." Every one of these steps in the argument would have to be flawed for strangelets to be a risk.

Blown to smithereens

But don't heave a sigh of relief just yet. The Brookhaven scientists have also considered an even more alarming possibility than the destruction of the Earth. Could their mighty machine trigger the collapse of the quantum vacuum?

Quantum theory predicts that the Universe is filled with a seething melee of so-called vacuum energy. That might seem an unlikely threat to civilisation. After all, it's simply the average energy of the mess of particles that flit in and out of existence all around us. As the Universe expanded and cooled, that vacuum energy dropped down to the lowest possible level.

Or did it? What if the Universe is still "hung up" in an unstable state? Then a jolt of the right amount of energy in a small space might trigger the collapse of the quantum vacuum state. A wave of destruction would travel outwards at the speed of light, altering the Universe in bizarre ways. It would be rather bad news for us, at least: ordinary matter would cease to exist.

In 1995, Paul Dixon, a psychologist at the University of Hawaii, picketed Fermilab in Illinois because he feared that its Tevatron collider might trigger a quantum vacuum collapse. Then again in 1998, on a late night talk radio show, he warned that the collider could "blow the Universe to smithereens".

But particle physicists have this covered. In 1983, Martin Rees of Cambridge University and Piet Hut of the Institute of Advanced Study, Princeton, pointed out that cosmic rays (high-energy charged particles such as protons) have been smashing into things in our cosmos for aeons. Many of these collisions release energies hundreds of millions of times higher than anything RHIC can muster—and yet no disastrous vacuum collapse has occurred. The Universe is still here.

This argument also squashes any fears about black holes or strange matter. If it were possible for an accelerator to create such a doomsday object, a cosmic ray would have done so long ago. "We are very grateful for cosmic rays," says Jaffe.

But RIHC is special, goes the counter-argument, because it collides gold nuclei together. What if some subtle unforeseen physical effect makes collisions between heavy nuclei particularly dangerous? Fortunately, there are some heavy nuclei among the multitude of cosmic rays that fly through the Solar System. "We believe there are relevant cosmic ray "experiments" for every known threat," says Jaffe. "Even if one insists on gold-gold collisions, there have been enough such collisions on the surface of the Moon since its formation 5 billion years ago to assure us that RHIC experiments are safe."

So until we can build atom smashers so powerful that they can exceed the energy of the punchiest cosmic rays, we needn't lose any sleep over them. Paranoiacs should look elsewhere, and a good place to start would be in the pages of journals like Physical Review Letters, which have carried schemes for extracting energy from the quantum vacuum. The worry here is that no-one knows how much energy might be unleashed: calculations give answers anywhere between zero and infinity. Arthur C. Clarke once raised the possibility that some of those vast explosions we see in the cosmos may be smart-alec alien scientists getting their comeuppance for tinkering with the quantum vacuum: "they might be industrial accidents" he said.

Those of a nervous disposition should stop reading now. For some top physicists are toying with the idea of recreating the birth of the Universe right here on Earth (see "Cosmos-making for amateurs"). One of the big names backing this idea is cosmologist Andrei Linde of Stanford University. He admits that he has no idea how to trigger a little big bang, yet insists that the experiment would not be catastrophic.

But then, as the Russian theorist Lev Landau once said: "Cosmologists are often wrong, but never in doubt." Perhaps Linde's reassurance will turn out to be the very last Famous Last Words.

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