You've heard the controversy. Particle physicists predict the world's new highest-energy atom smasher, the Large Hadron Collider (LHC) near Geneva, Switzerland, might create tiny black holes, which they say would be a fantastic discovery. Some doomsayers fear those black holes might gobble up Earth--physicist say that's impossible--and have petitioned the United Nations to stop the $5.5 billion LHC. Curiously, though, nobody had ever shown that the prevailing theory of gravity, Einstein's theory of general relativity, actually predicts that a black hole can be made this way. Now a computer model shows conclusively for the first time that a particle collision really can make a black hole.
"I would have been surprised if it had come out the other way," says Joseph Lykken, a physicist at the Fermi National Accelerator Laboratory in Batavia, Illinois. "But it is important to have the people who know how black holes form look at this in detail."
The key to forming a black hole is cramming enough mass or energy into a small enough volume, as happens when a massive star collapses. According to Einstein's theory of general relativity, mass and energy warp space and time, or spacetime, to create the effect we perceive as gravity. If a large enough mass or energy is crammed into a small enough space, that warping becomes so severe that nothing, not even light, can escape. The object thus becomes a black hole. And two particles can make a miniscule black hole in just this way if they collide with an energy above a fundamental limit called the Planck energy.
Or so physicists have assumed. Researchers have based that prediction on the so-called hoop conjecture, a rule of thumb that indicates how much an object of a given mass has to be compressed to make a black hole, says Matthew Choptuik of the University of British Columbia in Vancouver, Canada. A calculation from the 1970s also suggested a particle collision could make a black hole, Choptuik notes, but it modeled the particles themselves as black holes and thus may have been skewed to produce the desired result.
Now Choptuik and Frans Pretorius of Princeton University have simulated such collisions, including all the extremely complex mathematical details from general relativity. For simplicity and to make the simulations generic, they modeled the two particles as hypothetical objects known as boson stars, which are similar to models that describe stars as spheres of fluid. Using hundreds of computers, Choptuik and Pretorius calculated the gravitational interactions between the colliding particles and found that a black hole does form if the two particles collide with a total energy of about one-third of the Planck energy, slightly lower than the energy predicted by hoop conjecture, as they report in a paper in press at Physical Review Letters.
Does that mean the LHC will make black holes? Not necessarily, Choptuik says. The Planck energy is a quintillion times higher than the LHC's maximum. So the only way the LHC might make black holes is if, instead of being three dimensional, space actually has more dimensions that are curled into little loops too small to be detected except in a high-energy particle collision. Predicted by certain theories, those extra dimensions might effectively lower the Planck energy by a huge factor. "I would be extremely surprised if there were a positive detection of black-hole formation at the accelerator," Choptuik says. Physicists say that such black hole would harmlessly decay into ordinary particles.
"It's a real tribute to their skill that they were able to do this through a computer simulation," says Steve Giddings, a gravitational theorist at the University of California, Santa Barbara. Such simulations could be important to study particle collisions and black hole formation in greater detail, he says. Indeed, they may be the only way to study the phenomenon if space does not have extra dimensions and the Planck energy remains hopelessly out of reach.