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Gravity on the Brane
March 6, 2010  |  by Michael Anft

The Universe works. It has to. And yet humanity remains humbled: Mysteries endure about how, exactly, the wheels on The Big Machine turn. Our knowledge is sketchy, a dark picture punctuated only by bits of theoretical light. So, physicists ask Big Questions: What makes up the force that pushes the Machine outward faster and faster? Why are the tiniest subatomic particles so light, when calculations say they should be much heavier? And one that particularly frustrates physicists and sets them in motion: Why does it take whole fields of gravity to hold us to Earth and keep the planets apart, when the other three forces of nature invariably announce themselves with much more bravado?

Scientists, like nature, abhor a vacuum. They gleefully jump into the breach, furrowing their brows, cursing the darkness, and creating their own mathematical explanations of how the whole shebang—the size of particles, how they interact, how they formed the universe—fits together. They’ve extrapolated, postulated, and triangulated—a handful of brainy verbs that have yet to yield some solid nouns with clear definitions.

A querulous bunch, they are so enamored of their own intellects—despite their lack of answers—that they make it a point to challenge Albert Einstein, the god of modern physics. They’d like nothing better than to prove the old genius wrong. Einstein, of course, got the ball rolling in the modern era of highbrow positing when, in 1915, his general theory of relativity explained how gravity worked to curve space, a finding that changed the rules about gravity and raised exciting new questions about the masses of particles and the nature of space. But even Einstein knew that his thinking fell short. He would work the last 35 years of his life on a Theory of Everything, one that accurately delivered a formula explaining how the four forces work together with particles, and fail.

In the decades since Einstein’s death in 1955, particle physicists described how the three natural forces other than gravity—electromagnetism, a “strong force” that holds nuclei together inside atoms, and the “weak force”—interact with each other and with elementary particles that make up matter. Those interactions are together known as the “the standard model” of particle physics, the best attempt to date to Explain How It All Works.

But as standards go, the model sets the bar pretty low. Physical experimentation has exposed its flaws. The model breaks down when exposed to high energies, meaning it doesn’t meet a major criterion for a viable physical law: Will it hold up under all conditions? And most vitally, it can’t get at the crux of what ails it: Why doesn’t gravity behave like the other forces? Could there be something unique about the interactions between gravity and the elementary particles that accounts for why it is the 98-pound weakling of the cosmos? Is there a previously unseen detail that could complete the whole picture? Or a new angle or wrinkle—a new way of looking at the problem—that could lead us to an answer?

Particle physicists continue to take shots in the cosmic dark. Scattershots. String theorists propose that the universe is not made up of particles but of infinitesimal strings that may be billions of light-years long, and are part of an endless string of universes, collectively called “the multiverse.” Quantum loop theorists favor an idea that rhymes a bit with string theory but envisions endless loops instead of strings. Proponents of “supersymmetry” see the universe filled with tiny particles, each coupled to an as-yet-undiscovered companion particle, and with collectively enough mass to square the calculations of physicists.

Even among those working hypotheses, one that envisions an extra, infinite dimension in nature stands out for its sheer audacity. Concocted by Raman Sundrum, a professor of physics at Johns Hopkins, and Lisa Randall, who holds the same title at Harvard, this version of the universe could go a long way toward explaining why particles appear to be so lightweight, and why gravity is so puny. If proven right, the Randall-Sundrum models (or RS models, as they’re known in the trade) could go a long way toward augmenting the working standard model.

Photo Courtesy CERN

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