The concept of extra dimensions, dismissed as nonsense even by one of its earliest proponents nearly nine decades ago, may soon help solve seemingly unrelated problems in particle physics, cosmology and gravitational physics, according to a panel of experts who spoke Feb. 15 at the American Association for the Advancement of Science annual meeting in Denver.
"It doesn't happen often that you get a confluence of ideas and experiments that come together and it's something that obviously would change your whole way of looking at the universe," said one of the panelists, Joseph Lykken, Professor in Physics at the University of Chicago and a scientist at Fermi National Accelerator Laboratory.
Even though scientists lack direct evidence of extra dimensions, "we have a number of hints from experiments and theoretical ideas that make us think they're probably out there. That's why we're so excited about looking for them," Lykken said.
On the theoretical side, string theory, developed over the past two decades, requires that space-time has extra dimensions if it is to include gravity. "It's just built into the way that string theory works," Lykken said.
Experiments, meanwhile, have produced the standard model of physics to describe the most elementary particles and the forces that hold them together. Physicists have come to suspect that something is missing from the standard model.
"There seems to be more particles and forces than we really need, and they operate in more complicated ways than they need to," Lykken said. But extra dimensions may ultimately help explain these data complications.
"That standard model itself may be our biggest hint that there's this world of extra dimensions," he said.
New experiments at Fermi National Accelerator Laboratory are producing data that just don't fit the standard model, said Maria Spiropulu an Enrico Fermi Fellow at the University of Chicago. "We have things in the data that leave our mouths hanging," she said.
Whether extra dimensions or some other phenomenon emerges to clarify these murky data, scientists seem certain that they stand only a few years away from a scientific revolution.
"What's going on right now in particle physics, gravitational physics and cosmology is like when quantum mechanics started coming together," Spiropulu said.
Quantum mechanics, developed in the 1920s, describes the physics of objects at the atomic level and dominates the concepts of modern physics.
Spiropulu, who organized the AAAS session on the physics of extra dimensions, spoke at the session along with scientists from Fermi National Accelerator Laboratory, Harvard University and the universities of Chicago and Washington.
Another panelist, Sean Carroll, Assistant Professor in Physics at the University of Chicago, said that extra dimensions could help solve two mysteries in cosmology: what were the initial conditions of the universe and what is the mysterious dark energy that is accelerating the expansion of the universe.
The idea of an inflationary universe, one that expanded rapidly just moments after the big bang, has gained wide acceptance among cosmologists to explain how conditions in the early universe could be unexpectedly different from what they later came to be. But inflationary cosmology tells scientists nothing about the initial conditions of the universe. This is where extra dimensions come in, even though they might be microscopically small.
"If you had extra dimensions, then when the universe is very small at early times, the extra dimensions weren't small compared to the rest of the universe," Carroll said. "They must have played a big role. What was that role? Could the role have something to do how we perceive the initial conditions?"
Extra dimensions may also explain dark energy. Physicists conjecture that dark energy is governed partly by occurrences in the familiar four dimensions and partly by occurrences in the extra dimensions, Carroll explained.
"There is the tantalizing possibility that a complete change of perspective makes all of the problems collapse at once," he said.
University of Chicago
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