Physicists probe the fifth dimension

Alan Boyle Science editor • Profile • E-mail

SEATTLE - The cosmos would make perfect sense … if it turns out we're living in a 10- or 11-dimensional realm where gravity is bubbling off a different plane entirely. At least that's what's emerging as the hottest concept on the frontier of physics.

Though these sound like virtually unverifiable claims, physicists are trying to come up with ways to gather evidence to back up or disprove the extradimensional theories currently in vogue. But it’ll take several years to get that evidence, if it can be gotten at all.

The claim that the cosmos has more than the four dimensions we can perceive — that is, three spatial dimensions plus time — is exotic enough. But the quest to prove that claim brings in a virtual menagerie of mysteries: mini-black holes and dark matter, gravitational waves and cosmic inflation, super-high-energy particle collisions and ultra-powerful gamma-ray bursts.

Even the physicists behind today's most-talked-about extradimensional theory, Harvard University's Lisa Randall and Johns Hopkins University's Raman Sundrum, aren't yet exactly sure whether the approaches will pay off.

"Nothing comes with a money-back guarantee," Sundrum told MSNBC.com.

So why bother? Physicists aren't just spinning out these tales of 11 dimensions for the amusement of science-fiction writers. Rather, unseen dimensions seem to offer the best hope for solving the kinds of problems that have frustrated theorists since Albert Einstein's day.

The incredible lightness of gravity
For decades, physicists have puzzled over the weakness of gravity in comparison with the other fundamental forces of nature.

"A tiny magnet can lift a paper clip, even though all the mass of the earth is pulling it in the opposite direction," Randall noted in her book on the search for extra dimensions, titled "Warped Passages."

Einstein tried to come up with an overarching theory that could apply equally well to gravity and the other forces, but just couldn't do it. In fact, the theories that govern gravity and quantum mechanics are totally separate, and totally incompatible in the four-dimensional world we know.

Over the past couple of decades, Einstein's successors have focused their quest for a "theory of everything" on string theory — the idea that the fundamental constituents of matter are tiny stringlike objects vibrating at different frequencies. String theorists could come up with equations to cover gravity as well as quantum effects, as long as they were given 10 or 11 dimensions to work with.

Straining to explain branes
The theories work even better if you can think of our four-dimensional space-time continuum as a type of membrane, or "brane," embedded in a "bulk" that takes in even more dimensions. Randall and Sundrum found that gravity's comparative weakness was perfectly understandable if particles called gravitons could leak off a brane into a five-dimensional bulk. In fact, they said, it could well be that gravitons are leaking across the bulk into our own brane (the "Weakbrane") from an extradimensional brane nearby (the "Gravitybrane").

Admittedly, this sounds like a made-up world straight from "Alice in Wonderland" — and indeed, Alice has been invoked more than once by theorists themselves. The only thing that could save extradimensional physics from the fiction shelf is the prospect of finding real-world evidence to support the braneworld concept.

Although there are no guarantees, Randall and Sundrum are holding out hope that ambitious experiments will soon produce precisely that kind of evidence. "Within the next five years, we might actually encounter these extra dimensions," Randall said during a talk last week in Seattle.

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