Unseen Dimensions Hold Theory Aloft

String theory--the notion that everything in the universe is woven in a tapestry of 11 dimensional vibrating strings--has seduced an increasing number of physicists over the past 20 years with its sheer mathematical beauty and power to solve difficult problems.

At the same time, skeptics have found it easy to dismiss these successes as so much theoretical smoke. After all, critics argue, the vibrating strings and unseen dimensions that hold them are too tiny ever to be seen in experiments. And a theory that can’t be tested is about as relevant to a physicist as a bicycle is to a fish.

But what if the unseen dimensions were much larger than previously thought, big enough to see in relatively simple experiments? “You cannot rule out that possibility,� says Harvard physicist Andrew Strominger. “That’s astonishing.�

In fact, the idea that the strings might be big enough to perceive is rapidly gaining attention, if not outright respect, among many scientists.

If true, it would mean that “string theory is just out of reach of experiment,� according to physicist Joseph Lykken of the Fermi National Accelerator Laboratory in Batavia, Ill. Even more important, it would mean a whole new way of solving a host of thus-far elusive mysteries--ranging from the unexplainable weakness of gravity to the unaccountable existence of matter in the universe at all.

According to this new scenario, the everyday, three-dimensional universe we live in is trapped on a thin membrane--something like the world inhabited by characters playing out their lives within the confines of a movie screen. Unknown to these shallow, two-dimensional players, a larger universe spreads into numerous extra dimensions, like theaters in a multiplex.

Making Better Sense of Gravity’s Disparity

And while we are stuck as firmly in our membrane as Rhett and Scarlett are stuck on the screen, certain aspects of our universe can ooze off--leaving behind experimentally detectable tracks.

In fact, Stanford physicist Savas Dimopoulos speculates (not entirely tongue in cheek) that Bill Gates might figure out how to make a profit in the universe beyond our membrane. “There is extra space out there,� he said recently during a workshop at the Aspen Institute for Physics. “Maybe you can store things. This is a possibility that hasn’t been investigated.�

Of course, these ideas are wildly speculative, to put it mildly. But the general idea that our universe is but a thin sliver of a larger reality offers multiple advantages to theorists.

For example, one of the thorniest problems in physics is the vast disparity between the relative weakness of gravity and the strength of all the other forces, such as electricity, magnetism and nuclear forces. A tiny magnet is powerful enough to lift a paper clip off a table in defiance of the gravitational pull of every atom in the entire Earth.

Such a huge difference just doesn’t make sense.

However, it would make perfect sense, according to Dimopoulos and his colleagues, if gravity were weak only because it alone could leak off our membrane into the larger universe.

Imagine our three-dimensional universe as the skin of a soap bubble floating in a larger world. Electricity, magnetism and nuclear forces are stuck inside the skin.

In contrast, the gravitational attraction of the paper clip to the Earth gets diluted as most of the gravity oozes out of our membrane into other dimensions.

“The reason why gravity is weak is that [most of it] lives far away from us,� says Dimopoulos. “In a way, it’s a very simple idea.�

Taking another tack, MIT’s Lisa Randall and her colleagues are exploring the possibility that gravity changes strength dramatically in various parts of this higher-dimensional world; we just happen to live on a slice of it where gravity is weak.

And gravity is only the tip of the iceberg. After you introduce the idea that our three-dimensional universe is simply a slice of life in a larger world, it’s only natural to assume that other membranes lurk out there as well. Signals from these other membranes could affect our universe just as gusts of wind can deform the skin of a bubble.

In our universe, the energy infiltrating our area of the cosmos from other membranes might show up as puzzling new particles--or perhaps some unexplained property of matter. Because these other forces are extremely diluted, however--living as they do mostly in that larger, extra-dimensional universe--they would have very weak effects.

As such, they would enable scientists to explain many quantities in physics that snuggle up puzzlingly close to zero, but don’t quite amount to exactly nothing. Among them: the mass of a barely there particle called the neutrino, the exceedingly slight excess of matter over antimatter that allows us to exist, and the “weight� of empty space.

The extra dimensions also provide a logical hiding place for the long-sought “dark matter� that gravitationally pulls on clusters of galaxies but has remained otherwise frustratingly invisible.

“It’s just mind-boggling,� said Randall. “There are some hard problems out there that we haven’t been able to get at. Maybe there’s something lurking here which will help us solve some of these problems.�

Answers Could Come Within a Few Years

Physicists won’t have to wait forever to find out if these ideas have any basis in fact. Dimopoulos’ latest work predicts that previously unknown forces reaching us from membranes far beyond could be a million times stronger than gravity, and therefore even easier to detect. Energy oozing out of our membrane might show up as missing energy in particle experiments in a new accelerator now under construction in Europe, the Large Hadron Collider.

Or, new families of particles created from extra-dimensional vibrations might pop out of these experiments. If the physicists get very, very lucky, the first signs of higher dimensions could materialize at Fermilab within the next few years.

Even more imminent, if more speculative, are pending results from several tabletop experiments at Stanford and the University of Colorado to sense “large� extra dimensions. Because measuring gravity is the only way to perceive these dimensions--and gravity is uncannily weak--finding evidence will be difficult.

Moreover, it’s known that at most, these extra dimensions could be the width of a grain of rice. (Gravity has been well tested down to scales as small as a millimeter, and no evidence of extra dimensions has shown up yet.)

In Dimopoulos’ scenario, the two extra dimensions are curled up into tiny tubes, like cocktail straws, about a millimeter in diameter. An experiment sensitive enough to probe on that tiny scale could witness a dramatic change in Newton’s familiar laws of gravity.

Of course, physicists will have to explain the geometry of these extra dimensional landscapes, as well as the way they evolved: Why should three dimensions spread out while two roll up? Why a millimeter and not a yard?

The range of possibilities is almost endless. But so are the opportunities.

Indeed, the very fact that these scenarios are not impossible has stoked much excitement among string theorists. It is as if, said Strominger, humans are like water bugs skipping over the surface of a deep ocean. Everything we know is so much foam and flotsam stuck to the surface. But there may be a whole undiscovered world waiting underneath.

“This kind of structure never occurred to anybody before, but it turns out it’s very natural,� he said. “It tells us that our imagination has been very limited. It shows how little we know about the universe beyond that which we’ve actually measured.�

In upcoming Science Files: Physics’ Biggest Mysteries; and Experimenting on the Universe.

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A Universe on a Membrane

According to some radical new theories, our universe is trapped entirely on a very thin membrane that is part of a much larger universe of extra dimensions. Electric, magnetic and nuclear forces are stuck inside the membrane, while gravity leaks out. If true, this theory would explain why gravity is so weak compared with the other forces.

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Pool Table Analogy

In this analogy, pool balls struck by a cue remain on the two-dimensional surface of the table, while sound waves carry energy into a third dimension. Experiments to detect unseen extra dimensions would look for the telltale missing energy carried off by gravity, rather than sound, in particle collisions.

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Source: Savas Dimopoulos, Stanford University.