The most viable answer so far is proposed by string theory, a model dating to the mid-1970s. String theory explains all forces and particles as vibrating, one-dimensional loops of energy, and it proposes the existence of multiple dimensions—10 to be exact, including time—hidden within the apparent 3-D fabric of space. In this model, gravity alone of the four forces operates in all the dimensions. It’s as strong as the other three forces, but because its effects extend undetected into hyperdimensional space, it appears to be attenuated. The apple falling from the tree, the moon circling Earth are just the tip of the iceberg. “What we feel in our paltry three dimensions is a weakened version of gravity,” says Goldberg.
String theory works pretty well on paper, but physicists have struggled to identify experiments or observations to test it. The detection of microscopic black holes would shore up the theory by showing that gravity can act at the quantum level, as the model predicts. A Swiss particle accelerator called the Large Hadron Collider, due to be completed in 2007, could produce them. By hurling protons together at 14 trillion electron volts, it will create the kinds of high-energy collisions that are supposed to generate microscopic black holes. “If [black holes] are there, the LHC is going to see them,” says particle physicist Luis Anchordoqui, also of Northeastern University. Anchordoqui’s latest calculations predict that the collider could generate a black hole every few months or so. If it does, “you have to think of our entire universe floating in dimensions that only gravity can penetrate,” Anchordoqui says.
A new Argentinian observatory might get the goods even sooner. The Pierre Auger Cosmic Ray Observatory is netting signals from the most energetic particles in the universe: ultrahigh-energy cosmic rays, which slam into the atmosphere at speeds no accelerator can match, sparking air showers of subatomic particles and ultraviolet light. A fraction of those collisions could generate microscopic black holes, which Goldberg and Anchordoqui think would produce a unique brand of particle showers. “You could say not just that this is anomalous but that it has the signature of a black hole,” says Goldberg.
Feng and theoretical physicist Alfred Shapere of the University of Kentucky estimate that the Auger observatory could detect up to 10 such showers a year—“black holes appearing over your head, for free,” says Feng.
Shapere, like many string theorists, is excited by the mere prospect of actual data. “This is the first time I’ve been involved in a project that had experiments going on right now,” he says. “It could be longer than my lifetime before we have experimental proof that string theory is right or wrong. If we could observe it in action experimentally, we’d be back to the good old days, when experiments led theory.”
Yet detecting microscopic black holes can take string theory only so far. They will prove the presence of extra dimensions, but those extra dimensions may not behave according to string theory. And quantum gravity could still exist at much higher energies that cannot be produced experimentally, so the absence of microscopic black holes won’t discount the theory either. But at the very least, the upcoming experiments should establish boundaries on the size and energy level of hidden dimensions. “The scientific community will be more willing to accept us if we’re setting limits on our theories and ruling some out than if we’re jumping up and down saying we’ve proved string theory,” says Shapere.
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