January 12, 2007
Gravity Detector Could Provide Clues to the Shape of the Universe
ST. LOUIS -- The laboratory is cluttered with dirty dishes, Dorito crumbs, aluminum foil and needle-nose pliers _ the detritus of graduate students whose 600-pound steel and wire Tinkertoy is rising in the middle of the room.
With the apparatus, Ramanath Cowsik, a Washington University physicist, will poke and prod at some of the most daunting problems remaining in physics:
Why is gravity so weak and so different from the other basic forces in the universe? And what is the true 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.
He says the torsion balance will cost about $100,000. When complete, it could measure gravitational forces as small as the weight of a bit of a salt grain cut into 60 billion pieces.
"Nobody has built an instrument this sensitive," Cowsik said. "It is a probe into the unknown."
Late in his career, Cowsik, 66, is joining a handful of laboratories around the country that are chasing gravity down to small spaces. In those spaces, the scientists suspect, the formula for gravity might not hold.
Measuring an anomaly in the gravitational force would be evidence for new subatomic particles, or an indication that the universe has more than four dimensions.
"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."
The first person to directly measure a gravitational force was Henry Cavendish in 1798. All objects with mass exert a gravitational force, though it can be incredibly weak. The only gravity that people feel is that of Earth.
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.
Cowsik may even freeze the balance with liquid helium and put it in a quiet place, maybe a cave, to keep it utterly still for everything but gravity.
While Cowsik makes design decisions, it's up to people such as graduate student Kasey Wagoner and machinist Tony Biondo to put it together.
On a recent day, Biondo came up from his machine shop and handed Wagoner two stainless steel screws that took two days to make. Wagoner set the screws on a back counter, where they rolled amid other tools and parts.
"Days and days go into just getting one part to work," Biondo said.
Measuring gravity is a new challenge for Cowsik, who, after a distinguished career in India, came to St. Louis in 2002. He previously had a series of sabbaticals at Washington University and had developed a relationship with the physics department.
He left a legacy behind in India, including picking the site and directing the construction of the Indian Astronomical Observatory in the Himalayan mountains, which at 14,760 feet contains the highest telescopes on Earth.
A speaker of seven languages, Cowsik projects the calm demeanor of someone who spends much of his time deep in thought, conjuring up theories.
But now the professor, with hair whitening behind his ears, is reinventing himself as experimental physicist, devoted to careful measurements.
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.
When he does, he hopes to detect a breakdown in the existing formula for gravity, which specifies that the force between two objects increases as they get closer together.
A breakdown _ such as the force being even greater than expected _ would be evidence of a new force-carrying subatomic particle. Or an indication that the universe has more than four dimensions, maybe the nine or 10 suggested by a well-known physics theory called string theory.
It would put physicists closer to a so-called theory of everything that unifies the fundamental forces in the universe. Without such a theory, gravity sticks out like a sore thumb because it is so much weaker.
"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.