Hidden Space Dimensions May Permit Parallel Universes, Explain Cosmic Mysteries By Tom Siegfried / The Dallas Morning News http://www.dallasnews.com:80/science/0705sci1branes.htm   Hidden space dimensions may permit parallel universes, explain cosmic mysteries Imagine a mansion with a secret room - the perfect setting for a mystery. Now imagine that the room is vastly bigger than the mansion itself - and contains more mansions. That would make the mystery pretty bizarre. But it's very much like a story that many scientists are beginning to tell about the universe. In what amounts to a real-life episode of The Twilight Zone, physicists have realized that nature may be concealing extra dimensions - not of sight or sound, but of space itself. If so, the known universe may be just one of many "mansions" residing in the secret room - space's hidden dimensions. "It's just really frighteningly weird," says cosmologist Rocky Kolb. "It strikingly flies in the face of everything we thought was true." Apart from evoking the science-fiction fantasy of parallel universes, the new view of space offers possible solutions to several cosmic problems. In a flurry of recent research papers, physicists have explored the hidden dimensions for clues about the nature of gravity, the origin of the universe, and the identity of the elusive "dark matter" thought to lurk throughout the cosmos. Response to the hidden-dimension idea has ranged from skepticism to fascination. "When I first heard it I thought it was really crazy," said Dr. Kolb, of the Fermi National Accelerator Laboratory in Batavia, Ill. "But it's still hanging around." Joseph Lykken, another Fermilab scientist, says the hidden-dimensions idea has attracted researchers from many realms of physics. "It has really caught people's imaginations, people from quite a variety of backgrounds," he said. "It's the first thing that I've worked on that my wife is interested in." Born from studies of matter on the smallest scales, the new ideas imply that the standard picture of space on all scales is ready for a radical revision. "Clearly this is a new approach to trying to understand the universe both in the small and in the large," says physicist Nemanja Kaloper of Stanford University. The hidden-dimension mystery features a cast of characters known as branes, objects that occupy the unseen extra dimensions. The term is a play on "membrane" - a two-dimensional surface, or two-brane. Three-dimensional spaces - such as the known universe - are called three-branes. Physicists therefore refer casually to the universe as "braneworld." "All the standard particles - photons, quarks, leptons - live on a three-dimensional subspace, a three-brane, or our brane," says Savas Dimopoulos of Stanford. Branes reside in the hidden dimensions, known as "the bulk." While matter and light stick to the branes, gravity traverses both branes and bulk. The hidden dimensions cannot be seen because only gravity can go there. The current frenzy over extra dimensions began with an analysis appearing on the Internet a year ago in March. Dr. Dimopoulos, Nima Arkani-Hamed and Gia Dvali of Stanford proposed a new explanation for why the standard unit of mass in subatomic physics is surprisingly huge (by atomic standards) - about the mass of a speck of dust. That mass would be much smaller, they found, if some hidden dimensions were millimeter-sized. "The framework that we are proposing changes the way we think about some fundamental issues in particle physics and cosmology," the scientists wrote in their paper, which has since been published in the journal Physics Letters B. Since the early 1980s, many physicists had suspected that space contains more than the familiar three dimensions, corresponding to directions in which movement is possible (up-down, forward-backward, and sideways). Three directions seem enough for everyday life, but not enough to explain how gravity fits together with the other forces of nature, physicists found. Their favorite theory for including gravity describes the basic particles of matter and force as tiny vibrating loops known as superstrings. But the math of superstring theory gives nonsense answers unless space contains some extra dimensions beyond the usual three. Those extra dimensions were believed to be too tiny to worry about - smaller than a virus to the same degree that an ant is smaller than the universe. Motion in an extra dimension would return to its starting point in too short a time to be noticed. And experiments designed to detect such tiny dimensions would require energy far exceeding that available in the most powerful atom smashers. In 1996, though, Dr. Lykken suggested that superstrings might have effects detectable at much lower energies. If so, other physicists calculated last year, the hidden superstring dimensions could be much bigger than originally thought. In fact, hidden dimensions could be the size of a small ant - roughly a millimeter, or about one-25th of an inch, across. That's nowhere near big enough to hold a mansion, let alone the whole universe. But the visible universe is huge only in the familiar three dimensions of space. In additional dimensions, the universe would be extremely thin - the way a sheet of paper is big in two dimensions but thin in a third. In the hidden dimensions, the visible universe's thickness would measure on the order of a 10-millionth of a billionth of a millimeter. So countless such universes could fit in the extra dimensions, the bulk. Such parallel 3-D universes, or three-branes, might contain unusual forms of matter, possibly forming stars, planets and strange people - all less than a millimeter away from the home brane of the sun, Earth and Dennis Rodman. "The specific laws of physics would be different in each of these branes," said Dr. Lykken. "Their law of gravity would be the same as ours, but everything else would be different. . . . But maybe they could form galaxies and stars and planets." There's no danger of bumping into the alien universes, however. Nobody can reach out and touch, or see, or send laser beams carrying messages to parallel braneworlds, because matter and light are confined to each brane. "We are built of particles that cannot fall off and probe the extra dimensions," said Stanford's Dr. Kaloper. In other words, matter, light and other forces are imprisoned in one mansion and cannot travel to others through the secret room's hidden dimensions. "The key to this whole new idea is that extra dimensions can be hidden from us because we're trapped on this brane," Dr. Lykken said. Brane scans There is hope, however, of detecting the presence of parallel braneworlds. In dozens of papers and various conferences, including two recent meetings at Fermilab, physicists have explored possible methods of extra-dimensional perception. The best-bet strategies involve gravity. Gravity-carrying particles, known as gravitons, have the run of the cosmos. They can fly freely through the bulk. So a nearby parallel brane might be detected by gravitational effects. Astronomers should notice objects in the visible cosmos behaving weirdly, as though under the influence of gravity from an unseen source. In fact, that's exactly what astronomers have seen for decades. Galaxies spin as though they contain matter far beyond their visible edges; occasionally distant stars brighten as though an intervening massive object has "lensed" their light by gravitational bending. Astronomers have argued that such matter cannot be seen merely because it is not bright like stars; dozens of suspects have been proposed to account for this "dark" matter. But it's possible, says Dr. Lykken, that dark matter is actually "transparent" matter, residing in nearby braneworlds and therefore invisible. "This would be a new kind of dark matter," he said. "We could never see it or ever feel it by anything other than its gravitational pull." Other observations could also betray the presence of the hidden dimensions. If they exist, the strength of gravity should differ, at small distances, from the ordinary inverse-square law established three centuries ago by Isaac Newton. (The strength of gravity diminishes as the square of the distance between two massive objects - at twice the distance, the gravitational pull is one-fourth as great.) At distances of less than a millimeter, gravity should begin to grow stronger than the Newtonian law predicts. Previous proofs of Newton's law have all involved gravity measurements at distances greater than a millimeter. Deviations from Newton's law at shorter distances are now being sought. "There are table-top experiments looking for new, submillimeter forces," Dr. Dimopoulos said at one of the recent Fermilab meetings. One such experiment, now under way at the University of Colorado at Boulder, is a modern variant on the experiment performed by 18th-century British physicist Henry Cavendish, who measured the strength of gravity between two small spheres. Instead of balls, the Boulder experiment will measure the attraction between two tiny wafers suspended within a millimeter of each other. A result is possible as early as sometime this year, Dr. Lykken said. Brane smashing More evidence for extra dimensions could come early in the next century from a new European atom smasher. Although matter particles are ordinarily confined to the three-brane, the Large Hadron Collider, under construction at the CERN laboratory near Geneva, could create particles with enough energy to escape from the brane and enter the bulk. "You could actually deform your brane and produce particles that move off into the extra dimensions," Dr. Lykken said. Such escaped particles would reveal their departure through "missing energy" after all the other fragments in a particle collision had been accounted for. In fact, it's possible that physicists could find signs of such missing energy in collision data already recorded at the Fermilab atom smasher. "It's not obvious that we don't already have something in the existing data," Dr. Lykken said. Hidden dimensions also imply the exotic possibility that the CERN atom smasher could create tiny black holes. "You might produce nothing but black holes," Dr. Lykken said. "So physics could look very surprising in this scheme." Such mini-black holes would probably go poof in a instant, producing a burst of radiation that scientists could immediately recognize as a black hole's signature. "You'd say, 'Aha! I've made a black hole,' " Dr. Lykken commented. The Brane Bang Besides exploring ways to detect the extra dimensions, researchers are investigating the implications for the history of the universe. At one of the Fermilab meetings, Antonio Riotto of CERN described the possible role of branes and extra dimensions around the time of the big bang, which started the universe expanding about 15 billion years ago. In particular, the new dimensions may help explain a sudden burst of expansion, called inflation, that many experts believe was necessary to give the visible universe its current structure. Other experts suspect, however, that hidden millimeter-sized dimensions might conflict with observational data about the universe and its past. In a paper posted on the Internet last week, Katherine Freese and Daniel Chung of the University of Michigan argue that many versions of the hidden-dimension idea fail when compared with known features of the early universe. Many brane scenarios, the Michigan physicists found, are inconsistent with current estimates of the age of the universe and measurements of the amounts of various chemical elements created in the big bang. Furthermore, it's hard to reconcile certain brane approaches with a constant strength of gravity in the universe today, Dr. Freese said in an e-mail interview. "We suspect that this difficulty may prove to be quite general" with other brane theories, she said. Such objections may not apply to all versions of the brane approach, though. In fact, at this point the braneworld-view is rather fuzzy. Nobody can say for sure how many extra dimensions may be important, for example, although current theory suggests that no more than seven extra dimensions are possible. The precise size of the hidden dimensions also depends on how many there are. Dr. Kolb remains skeptical, yet interested, in the hidden dimensional paradigm. "We ride whatever ship is taking us someplace," he said. "It's a long shot, but people do buy lottery tickets." He is not ready, though, to invest in the hidden-dimension view too heavily. "Land speculation in the extra dimensions is not warranted at this time," he said. Still, the significance of the new ideas extends beyond their ultimate validity, Dr. Lykken argued. "Even if this new idea is wrong, the fact that we didn't think of it until now shows that the real new physics out there may be very different from what we're planning for," he said. And since particle physics experiments require multibillion-dollar atom smashers, it makes sense to consider all the possibilities when designing them. "If you're going to spend billions of dollars over 20 years, you better be not too smug about what you're looking for," said Dr. Lykken. "We better be a little more broad-minded in our planning for how to look for new phenomena."