'Brane'-Storming

What if our universe were just one of many floating around in a larger mega-universe?

Though it may seem out of this world, this conjecture by University physics professor Lisa Randall and Stanford University professor Raman Sundrum is now a generally accepted theory that explains forces in the universe. Randall and Sundrum's hypothesis, first introduced in a paper a year-and-a-half ago, provides an imaginative approach to the "hierarchy problem" — which attempts to describe why gravity's pull is substantially smaller than forces such as electromagnetism.

The research builds on established particle-physics theories, according to Randall, who arrived from the Massachusetts Institute of Technology in September 1998. It is based on the 25-year-old Superstring theory, which seeks to combine all forces under one theoretical umbrella. Each of the four matter and energy particles — protons and neutrons (matter), photons (light) and gravitons (gravity) — play a part in what Randall describes as a metaphorical orchestra to create one coherent "sound."

Scientists believe these four particles in string theory are objects with one spatial dimension. Each object, however, also has a time dimension, and so in theory, particles have two dimensions. These strings, except gravitons, are attached to branes, which are objects with three spatial dimensions.

"In string theory, the protons, neutrons and photons are associated with open strings and stick easily to the brane. Gravitons, however are closed strings that have no ends and thus don't stick," Randall explained in an interview last week.

Randall — who has studied particle physics at Harvard University for more than 10 years — explained that the difference between the gravitons and other particles can be observed. "You can imagine all the other charges as on something, but gravity is everywhere. It isn't stuck to one particular location," Randall said.

The branes — where most of the particles are attached — float around a hyperspace of more dimensions, according to string theory. The strings-branes-hyperspace system is the equivalent of a string in a cubic box, which in turn, is located in a multidimensional space. In terms of string theory, the one-dimensional strings are particles — protons, neutrons, photons and gravitons — the three-dimensional branes are universes and the multidimensional hyperspace holds many universes. But this hyperspace cannot be seen. Because photons are attached to different branes, they exist only on the branes and not in hyperspace.

But there are two problems with string theory that physicists have struggled to solve for the past 25 years — and which Randall and Summon may have just resolved with their research.

First, according to string theory, gravitons cannot be attached to branes. Therefore, they must be free to wander in the unseen hyperspace. But, if gravitons could exist in more than three-spatial dimensions, gravity's force would be much stronger than it actually is in our three-spatial-dimensional universe.

Second, if hyperspace does exist, string theorists generally believe that its extra dimensions would be too small to be detected. In addition, string theorists suggest that as many as eight-spatial dimensions could exist in hyperspace. The extra dimension cannot be seen in laboratory experiments with today's technology. Thus, no one has been able to verify string theory, and the theory's validity has been easily questioned.

But with mathematical equations, Randall and Sundrum have hypothesized that — with a particular curvature — hyperspace could actually consist of one infinite extra dimension, which physicists can see in laboratories. This finding has resuscitated string theory.

Get the best of ‘the Prince’ delivered straight to your inbox. Subscribe now »

"Previously, people just thought of extra dimensions as little tiny balls in the size of a Planck scale," Randall explained. A Planck length is a hundred million trillion times smaller than the width of a proton. "Now what's happened is people have recognized that by putting matter stuck on a brane, you get much weaker constraints on the size of the dimensions. You can actually have an infinite dimension. If you were able to walk it, you would be able to do it infinitely."

"That's really very remarkable because if you have this extra dimension, you have to ask why can't you see it," she continued.

In Randall and Sundrum's mega-universe, many universes float within a space-time hyperspace with an infinite fourth spatial dimension. Protons, neutrons and photons are confined to the branes, just as in the string theory — but branes interact with each other through gravitational forces. From one brane — the brane of the universe, for example — another brane cannot be seen because light cannot travel through the hyperspace. Nevertheless, the other brane's gravitational pull can be felt because gravitons can travel.

The hyperspace itself could consist of more than four spatial dimensions, and nobody would ever know it.

"Fundamentally, stars might be five-dimensional," Randall said. "It could be that true universes are five-dimensional or higher, but we see things as looking as four-dimensional because photons are stuck on branes."

Thus, according to Randall and Sundrum, only the hyperspace separates parallel universes. Because protons, neutrons and photons are all particular to a brane, each universe could be ruled by different laws of physics.

For some astronomists, this part of Randall and Sundrum's theory may explain the existence of dark matter — which is invisible and makes up 90 percent of our universe. Dark matter emits or absorbs no light and is evident only through its gravity. It could simply come from another universe from which we can sense gravitons.

In addition, Randall and Sundrum's theory can explain why dark matter is usually found in the halos around galaxies. According to the theory, large masses on different branes are attracted to each other through hyperspace with mutual gravitational pulls. Thus, a galaxy on our universe may be mirrored by a galaxy from another universe, with only the gravity from its edges apparent.

Because of its curvature, the hyperspace envisioned by Randall and Sundrum would allow some gravitons to roam, but restrict the majority from escaping the branes. Gravitons are produced on a specific brane, the so-called "mother brane" — from which some escape and subsequently migrate to other branes. Because most gravitons are kept on a three-spatial dimensional brane, their forces would remain weaker than those allowed in the four-spatial dimensions of the hyperspace.

"What we've shown is that the graviton is attracted to the brane, so you're left with four-dimensional — three spatial and one time — gravity," Randall said. "The graviton is localized on the brane. If you go a little away from a brane, you see a much weaker cupping of the graviton." She added that this particular finding persuaded them to name their hypothesis the "localized graviton theory."

The hyperspace around the "mother brane" would be curved like a funnel so that it would channel most of the gravitons back to the brane.

Theories are good on paper, but they must also be substantiated by evidence in the lab. Though string theory cannot be tested, evidence of the four-spatial-dimensional hyperspace that Randall and Sundrum envision someday may be found. According to the theory, gravitons that escape from branes and occupy hyperspace would be heavier and stronger than the ordinary variety. In testing for evidence of a hyperspace, physicists can look for these gravitons.

"By colliding together protons, anti-protons, quarks and anti-quarks, you can make a graviton," Randall explained. "If you can make the heavier graviton — a kaluza-klein — you then have evidence. If you have things in the extra dimension like gravitons, we will see it as a heavy state. So these are things we look for."

The procedure sounds simple, but it requires a large amount of energy to accelerate particles to the speed necessary for collision.

"In principle, it is possible, but we haven't seen any evidence," she noted. "The question is what energy level you need to get to these states."

An accelerator in Geneva, Switzerland, might be ideal because it operates with the highest energy level. In the United States, the Firm National Accelerator Laboratory in Batavia, Ill., contains the country's most powerful energy accelerator.

The notion that Randall's theory can be tested has revolutionized the study of string theory. The localized graviton hypothesis has sparked theories from other physicists. For example, one physicist claims the different laws of physics governing each brane may be offspring of a universal set of laws governing the whole mega-universe. Each brane would operate under a variation of this universal set, similar to local variations of a language.

Substantiated by other physicists' research, Randall's theory has gained support during the past year. Randall said this acceptance has surprised her, especially because she and Sundrum began developing their ideas by simply brainstorming about applications of string theory.

"I like solving problems and putting things together," Randall explained. "[The theory] is kind of nice because it ties into the string theory."

"There was definite suspicion in the beginning, but right now it is generally accepted. The major criticisms have been addressed at this point. Now the major problem is convincing string theorists that it really can happen," she said.

"So it is possible in principle," she added. "But is it really the world we live in?"