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It’s a sunny summer day in Geneva, Switzerland. The birds are singing as lovers canoodle near the Jet d’Eau. Somewhere, someone is listening to techno music a little too loudly.

Nearby, 570 feet belowground, the Large Hadron Collider, or LHC, hums away at full power, whipping up lead atoms to near the speed of light before slamming them into protons to see how they explode. The 17-mile tunnel generates 14 trillion electron volts at a collision point six thousandths of an inch across. It is the largest and most powerful machine ever built by humans and the signature scientific experiment of the 21st century.

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Suddenly, the ground begins to rumble. It shakes and then rends apart, as if ruptured by a monster deep within. There isn’t even time to scream before the earth gives way—pulled inward at terrifying speed—carrying lovers, fountain, and techno music down into the horrible maw of blackness that was once our planet’s core. In seconds, it’s over. Earth has evaporated into an empty black nothingness.

This was the doomsday scenario put before physicists in 2008, as the LHC was revving up for its first run. Fed by news reports morbidly fascinated by apocalyptic scenarios, the public was genuinely worried that humans were about to create microscopic black holes, or otherwise bizarre matter, that could destroy the world in mere seconds.

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Almost all physicists say this scenario is beyond unlikely. As far as we know, black holes form when a massive star collapses on itself in a huge implosion once it burns up all its fuel—not when subatomic particles are slammed together at high speed. But unlikely as it was, the prospect touched all of us—the whole planet. With such high stakes, the concept of risk takes on a whole new meaning. So, physicists studied the apocalyptic hypothesis through a series of three unlikely events: that tiny black holes could form in the first place; that they could last longer than a millisecond; and that they could stick around and swallow everything nearby.

The public was genuinely worried that humans were about to create microscopic black holes, or otherwise bizarre matter, that could destroy the world in mere seconds.

The first unlikely event—creating an ultrasmall black hole—would be a revelation, says Don Lincoln, a physicist at Fermilab near Chicago. He would actually be delighted if it were a scientific possibility. “Oh, my God, I’d be wiggling like a puppy. Somebody’d be going to Stockholm over that one,” he says. “If they form, we can learn something deep and insightful and really central to our understanding of how the universe came into existence.”

That “something” would be the existence of extremely small extra dimensions in our universe beyond the usual four (three spatial, plus time). It’s a little hard to imagine what a miniature dimension would look like or how it would function, but theorists have used the concept to explain why gravity in our universe is so weak (theoretically, it should be stronger). But when it comes to black holes, gravity is everything. Theoretically, if there are five to seven dimensions hidden within the universe, one might squeeze a tiny black hole into being for a fraction of a second.

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That’s where the second unlikely event comes into play. Everything we know about black holes says that they give off a type of weak energy called Hawking radiation, named after Stephen Hawking. However, remember that Einstein showed that mass equals energy—so you can’t lose one without losing the other. Therefore, if tiny black holes could be born, they would immediately puff out of existence.

But let’s pretend for a moment that Einstein and Hawking were either wrong or just didn’t understand the physics of the infinitesimal multidimensional universes. There’s the third unlikely event that needs to happen: The minuscule black hole must consume everything around it. But the nearby atoms would be huge compared to that black hole, so it wouldn’t find “food” to consume.

It wasn’t the first time that physicists pondered particle-induced apocalypses. In 1979, a group of Berkeley scientists wondered if their newly retrofitted collider in the California hills posed some kind of threat. They decided that the very existence of Earth was proof it did not.

Their reasoning was that Earth—and everything else in the universe—is constantly pelted with cosmic rays, which are essentially charged particles not unlike those used by the LHC. There’s not much difference between a particle slamming into the Earth and two particles slamming into each other, scientists thought. After billions of years of cosmic bombardment, no black holes or other strange matter have destroyed our planet; so one or more of the three unlikely events must, in fact, be impossible. For instance, black holes may indeed form from such collisions, but even if they did, they might then blow harmlessly out into space—the same way, say, neutrinos do—without touching a single atom.

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This idea prevailed until the late 1990s, when the LHC’s predecessor, the Relativistic Heavy Ion Collider (RHIC, pronounced “Rick”) was coming online in Suffolk, N.Y. As scientists contemplated potential dangers, two small cracks formed in that safety theory. The first came from a legendary black hole physicist, William Unruh, who said that if one further twists what we know of black holes, Hawking radiation might not actually exist. Granted, he told The New York Times physics “would really, really have to be weird” for this to work. It would be like waking up tomorrow to find gravity no longer works, or that land and ocean had switched places. Nonetheless, The European Organization for Nuclear Research (CERN) formed a team to investigate the dangers and put the whole end-of-the-world business to rest.

If tiny black holes could be born, they would immediately puff out of existence.

Michelangelo Mangano, one of the CERN physicists charged with the examination, soon realized that there was a second flaw in the model—namely that LHC collider beams were actually different from cosmic rays. The cosmic rays that hit Earth behave similarly to a speeding train that hits a parked car—both go careening off roughly in the direction the train was heading. But LHC would essentially slam two trains into each other in a head-on collision, which would cause them to stop in place after a phenomenally powerful blow. Converting that metaphor to colliding beams, if a black hole were formed it wouldn’t careen harmlessly out to space, but might instead sit around and interact with nearby atoms—and ultimately swallow everything around it.

To investigate what happens when high-velocity particles collide head-on like trains, Mangano needed a testing ground—someplace in the galaxy where such particles actually ram into each other and come to a screeching halt. Neutron stars fit the specs, he decided. A neutron star is a type of collapsed massive star whose gravity is so strong that any cosmic ray that hit it would be stopped in its tracks like a train that hit a mountain. If black holes could form, and if they didn’t puff out immediately, then they would sit in place and gobble up the neutron star. Thus, Mangano reasoned, the existence of neutron stars was proof that such micro black holes don’t exist.

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Mangano called string theorist Steven Giddings, who had written one of the first papers suggesting such black holes could not be created, and sought his opinion. He was shocked to learn that Giddings’s thinking reflected his own, and he was looking at neutron stars as a testing ground, too. They paired up and began collaborating.

They quickly realized how hard the work would be. They had to understand not only the physics of neutron stars (super-dense objects the size of London but twice the sun’s mass), but also the behavior of different types of cosmic rays and tiny theoretical black holes. “It was extremely exciting,” Mangano says. “It was like being back as a graduate student, where every day you open a book and there is something new. But doing it with the wisdom of a seasoned researcher.”

Many of their colleagues thought they were wasting their time chasing phantoms, but the pair persisted for months until they suddenly hit a brick wall. They realized that the gravity of a neutron star was too strong for a cosmic particle to even penetrate with any speed. Essentially, as soon as a particle got close to the star, it would slow down too much to hit any other particles with LHC-like energy. All their work was for naught.

“It was more than disappointing, it was super-frustrating. It was just a terrible moment. We went through the calculations several times just to be sure that was the case,” Mangano says. “And it was a huge heartbreak because we thought we were done.”

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The nearby atoms would be huge compared to that black hole, so it wouldn’t find “food” to consume.

So, Giddings and Mangano gathered themselves up and started again. This time, the scientists chose white dwarfs—a type of collapsed star that’s the size of Earth with the mass of the sun—as their models. White dwarfs are sufficiently dense to stop a careening tiny black hole, but not strong enough to prevent the collision from happening. The duo dove in and found eight white dwarfs that were of the right mass and had been around long enough to ascertain that tiny black holes formed by particle collisions didn’t consume them. The existence of white dwarfs—particularly those eight—despite 100 million years of cosmic ray pounding was proof that the LHC was safe.

After a year of work, in 2008, Giddings and Mangano published “Astrophysical Implications of Hypothetical Stable TeV-scale Black Holes” in Physical Review D. CERN also released two reports summarizing their results, each more confident than the last. The papers were met with a combination of relief and elation—less that the world was safe than that the LHC’s funding was, and that the whole mess was over. One astrophysicist, Rainer Plaga, did raise a couple of questions, but beyond that, the paper put the scientific community at ease.

It didn’t necessarily convince the rest of the world however. Several vocal opponents without strong science credentials have tried to sue to halt the experiment, but all the cases have been thrown out either on technicalities or lack of jurisdiction. That brought to light a more mundane, though practical, question: What would such a trial look like? It’s unclear at the moment since, as a transnational partnership similar to the United Nations, CERN has immunity from most European laws. Also, it would be tricky to find independent voices for such a trial, says Eric Johnson, a lawyer who wrote a detailed review of this legal situation.

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“The scientific inquiry was driven by the project schedule of the experiment. I think you can say that it was not independent and was done to varying degrees by people with a stake in the matter,” Johnson says, adding that he is not worried about cataclysms, but rather is interested in the legal puzzle. “That’s what makes it so interesting for me from a legal standpoint: How courts and lawyers are supposed to deal with this.”

The philosophical issue also remains, for it’s essentially impossible to prove a negative. How can one prove that a natural cosmic ray hitting Earth every day would not obliterate the planet tomorrow? The same applies to the LHC’s particles, but at a certain point we have to abandon the “what if everything we know about physics is wrong?” argument and look forward to the astounding discoveries that the LHC might soon deliver. The collider will run at full power in early 2015.

“Even though it sounds like it should be a scary thing to say,” says Lincoln of Fermilab, “yes, I would like to see black holes form.” He looks forward to the groundbreaking discoveries of the 21st century. “Whatever we find—and I really hope we find five outlandish things, whatever they are—it’s not dangerous.”

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Science writer Erik Vance is based in Mexico City and has written for DiscoverHarper’s Magazine, and The New York Times.  He’s afraid of spiders, lightning, and really big fish. But not black holes.

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