Plato considered it first.
What if everything we hold dear is but a thin slice of some larger, unreachable reality, like a flickering shadow cast on the craggy wall of a cave? What if the moon and stars, your home, your thoughts, your cat, are but projections on this wall -- mere suggestions of unfathomable realms beyond?
In the last few years, a mathematically rigorous version of Plato's 2,000-year-old thought experiment has been refashioning the way physicists think about everything from subatomic particles to the Big Bang. The universe we see, according to this scenario, is stuck on a thin membrane of space-time embedded in a much larger cosmos. And our membrane may be only one of many, all of which may warp, wiggle, connect and collide with one another in as many as 10 dimensions. Physicists call this new frontier the "brane world."
The idea could help solve a long list of outstanding mysteries. Among them: What is the "dark matter" that seems to make up 90% of the universe? And why is gravity trillions of times weaker than electromagnetism?
The revolution was set off in the mid-1990s when UC Santa Barbara physicist Joe Polchinski determined through mathematics that branes were a surface to which things attach, like hair to skin -- except the "things" in this case were the minuscule "strings" that may well be the fundamental ingredients of the universe.
"I was just fiddling around with mathematics.... Within a week or two [other physicists] had done things with it I hadn't envisioned. It was like taking the stopper out of the dam. Things poured through."
Alan Guth of the Massachusetts Institute of Technology, creator of the currently accepted version of the Big Bang, said recently he felt a little like Rip Van Winkle -- picking up his head from a long sleep only to notice that the landscape of physics he thought he knew had suddenly, drastically, changed.
Stephen Hawking of the University of Cambridge, among others, envisions brane worlds bubbling up out of the void, giving rise to whole new universes. He ends his latest book, "The Universe in a Nutshell," with a call to explore this "brane new world."
One might well wonder why such a seemingly bizarre concept has attracted so many well-established physicists. The short answer is: desperation.
The laws of nature that describe the large-scale universe to an astonishing degree of precision (Einstein's general relativity) are incompatible with the laws that describe the small-scale universe with the same astonishing exactness (quantum theory). This means either that one of these well-tested theories is wrong (all but inconceivable) or that there is some larger, more encompassing theory that somehow accommodates both.
To date, the only theory that comes close to marrying the two is "string theory" -- a mathematically elegant set of ideas that has swept the world of physics over the last few decades. According to string theory, the basic ingredients of the universe are not point-like particles, but tiny strings vibrating in 10-dimensional space. Although still untested, string theory has scored a spectacular series of theoretical successes, earning it an ever-widening circle of admirers.
And yet string theory remains a realm apart from day-to-day physics -- lovely to behold but innately aloof.
For one thing, the strings are so small that it would take a particle accelerator larger than the solar system to create the energies needed to "see" them. This means, in effect, that strings can never be detected.
For another, the complex mathematics required to deal with the tortured 10-dimensional landscape is beyond the reach of most physicists.
Brane models change all that. Unlike in string theory, the extra dimensions in brane worlds can be big, infinitely big. "It led to a whole new bunch of possibilities that could be experimentally tested," said physicist Jim Cline of McGill University in Montreal.
What's more, branes don't require the full range of mathematical tools required for string theory, opening the door to new groups of scientists. "You can use methods that are part and parcel of more traditional physics," said Columbia University physicist Brian Greene. "So a person who's not a string theorist can jump into the field and make contributions."
This sense of promise was palpable last summer at the Aspen Center for Physics, where string theorists and cosmologists -- the scientists who study the origin and structure of the universe -- gathered for a workshop to explore links between the smallest scales in the universe and the largest. Brane scenarios popped up everywhere, enveloped in the thick fog of uncertainty that clouds the birth of new worlds.
The setting was strangely church-like. The faithful sat in rows under spires of white-barked aspens, their round leaves fluttering in the wind.