January 12, 2007

Gravity Detector Could Provide Clues to the Shape of the Universe





In an era of big science _ billion-dollar space telescopes and atom smashers _ Cowsik's approach is refreshingly small. The apparatus, called a torsion balance, is cheap and based on a centuries-old idea.


"Nobody has built an instrument this sensitive," Cowsik said. "It is a probe into the unknown."



"There is the potential for something huge," said Sylvia Smullin, a postdoctoral student at Princeton University who previously measured gravity for a leading Stanford University team. "It would change our understanding of how everything in the universe works."


To detect something so weak, Cavendish had to use the world's first torsion balance: a wooden rod, hanging from a wire, with metal masses on each end. When he brought heavy lead spheres near the hanging masses, the small gravitational force between them was just big enough to twist the wire a tiny bit.

Knowing that gravitational force allowed Cavendish to eventually calculate Earth's mass for the first time, and, in turn, the mass of the sun, moon and other planets.

Fast forward two centuries, and things aren't all that different in Cowsik's lab. His torsion balance is just much more sensitive. The stainless steel wire is as thin as a human hair. The balance will be kept in a vacuum chamber to keep stray gas particles from twisting it. The inside of the chamber was polished with acid to remove dirty molecules.


While Cowsik makes design decisions, it's up to people such as graduate student Kasey Wagoner and machinist Tony Biondo to put it together.


"Days and days go into just getting one part to work," Biondo said.





Usually, experimentalists collect data that lead to new theories. Albert Einstein and his theory of relativity happened the other way around. Einstein's theory sprang fully formed and led to experiments that confirmed it later. Cowsik says Einstein glamorized theory for many students.

"Everybody wants to be a theorist. To be a great theorist requires the same attention to detail," he said.

With the luxury of tenure, Cowsik is taking his time. He has four experiments planned with two versions of the torsion balance. In several months, he will begin gathering data from a first experiment _ measuring the way Earth rings like a bell after an earthquake. But he says he probably won't get to the main experiment, measuring gravity, until next year.




"Gravity is the big mystery," said Eric Adelberger, a University of Washington physicist in Seattle.

String theory suggests that the universe's extra dimensions are curled up in a way that we don't notice. Adelberger uses the analogy of an ant walking on a wire. We see a one-dimensional wire. But the ant notices length and width. Measuring gravity at such tiny distances is a way of gaining the ant's perspective.

Adelberger's group, having precisely measured gravity to within a hair's width, has come the closest. But so far, they've found no breakdown in gravity.

Cowsik says his torsion balance should be even more sensitive than Adelberger's. Of course, that doesn't guarantee Cowsik will find anything. And that's fine for an experimentalist.

"We have to pan _ maybe some gold is there," he said.