Keay Davidson, Chronicle Science Writer Friday, June 9, 2006
Researchers have proposed a bizarre new theory that our solar system might be the home of thousands of very small black holes -- that is, pint-size versions of the weirdest objects in the universe.
If verified by a NASA satellite scheduled for launch next year, their claim might overturn orthodox ideas about nature and the universe. In particular, it could dramatically reinforce physicists' growing suspicion that the cosmos is pervaded by invisible alternate dimensions, never-never lands that cannot be detected with existing instruments.
Black holes are collapsed stars whose gravity is so intense that nothing can escape them, not even light -- hence the adjective "black." So far, there is strong evidence for only one type of black hole: the huge kind that inhabits deep space, millions of light-years from Earth. One example is a titanic black hole that is sucking up nearby stars at the core of the Milky Way. Scientists know the black hole is there because the galactic core is emitting intense radiation -- the death cry of devoured stars.
But black holes, like shoes, might come in different sizes. Physicists have speculated that very small black holes might have formed after the Big Bang that spawned our cosmos more than 13 billion years ago, then vaporized like dew drops after sunrise.
Now, researchers Charles Keeton at Rutgers University and Arlie Petters at Duke University have proposed that throughout the cosmos, very small black holes might have survived to the present day. Thousands of them could be drifting around our solar system like dust motes in an old house.
They base their claim on their interpretation of one of the hottest ideas in theoretical physics: "braneworld" theory, a distant cousin of string theory.
"Brane" is short for membrane. To greatly oversimplify: According to string theory, matter ultimately consists of infinitesimally tiny, vibrating strings of energy. A harp or guitar produces different sounds when its wires vibrate at different frequencies; likewise, a string of energy turns into different subatomic particles according to its vibrational rate. In brane theory, string theory is broadened to include multidimensional, vibrating membranes that are believed to pervade the cosmos.
High school physics students learn there are four dimensions in the universe: length, width, depth and time. But braneworld theorists say there are additional, unseen dimensions that can be described only with mathematical equations. Physicists hope that by computer-modeling hypothetical interactions of known and unknown dimensions, they can explain phenomena ranging from the force of gravity to the birth of the cosmos.
At the moment, one of the most publicized versions of braneworld theory is proposed by physicists Lisa Randall of Harvard and Raman Sundrum of Johns Hopkins University.
In an article in the May 24 issue of the journal Physical Review D, Keeton and Petters explain how, inspired by the Randall-Sundrum braneworld hypothesis, they concluded that the Big Bang generated zillions of small black holes -- many of which may have survived to the present day. The black holes might be very small -- perhaps as small as atoms -- yet hyperdense, hypermassive objects that weigh from a few pounds to the equivalent of the celestial flying mountains known as asteroids.
If Keeton and Petters are right, then our solar system should have its own share of little black holes, perhaps as few as 3,000 and as many as 300,000 at any given time.
What would happen if one of these black holes came to Earth and passed through a room? Could they suck a person up like the black holes depicted in science fiction?
Keeton, a member of Rutgers' department of physics and astronomy, said it couldn't happen. He told The Chronicle that a tiny black hole probably could suck up only a few atoms at a time.
Yet it might not go unnoticed. If a black hole as massive as an asteroid -- thousands of tons -- passes through a room, there would be a subtle gravitational tug.
Keeton and Petters propose a way to test their hypothesis: by using NASA's GLAST satellite (Gamma Ray Large Area Space Telescope), which is scheduled for launch next year. In theory, the black holes should distort gamma radiation from extremely distant, explosive objects called gamma-ray bursters, which are scattered all over the universe. If so, then GLAST might detect the distortions and thus, indirectly, the black holes.
Still, the search might take a lot of patience. Space is big; even if our solar system contains thousands of black holes, finding just one could be like looking for a raft in the Pacific Ocean. Keeton calculated that if the high-end estimate of 300,000 is correct, then about 15 black holes might exist between the sun and Mars -- the fourth planet from the sun -- at any given time.
Neither Randall nor Sundrum was involved with the writing of Keeton and Petters' paper. Sundrum told The Chronicle he was intrigued by their proposal.
"If their proposed test yields a positive result (finding small black holes), that would be fantastic," he said. "If it finds something, a new world opens up."
One of the nation's top string theorists, physicist Ed Witten of the Institute for Advanced Study in Princeton, N.J., called Keeton and Petters' proposal "extremely clever. ... I hope it pans out."