A Beginner's Guide to Antimatter

According to Einstein, Earth makes a dimple in the spacetime around it--something like a bowling ball sitting on a sheet of Spandex. Because Earth spins, this "dimple" is twisted into a shallow vortex. Gravity Probe B is orbiting Earth, right now, in search of these distortions.

GP-B senses the distortion of spacetime around our planet using gyroscopes. (There are four of them onboard the spacecraft.) Francis Everitt, principal investigator for GP-B and a professor at Stanford University, explains:

More Gravity Probe B orbiting Earth: an artist's impression

"Gyroscopes moving through curved spacetime will gradually change their direction of spin (i.e. tilt) with respect to the stars. GP-B will measure that tilting motion with extraordinary precision and from that measurement we can calculate the structure of space near the Earth."

Everitt will give a presentation about Gravity Probe B in April at the "Physics for the Third Millennium: II" conference hosted by NASA's Marshall Space Flight Center in Huntsville, Alabama. The conference is part of the World Year of Physics 2005, a United Nations-endorsed series of events to recognize the 100th anniversary of Einstein's seminal work and to raise public awareness of big issues in modern physics.

In addition to giving a status update on GP-B (in short: so far, so good), Everitt plans to explain how GP-B will measure gamma, an important physics variable used by scientists in their quest to go beyond Einstein's relativity. Roughly speaking, gamma corresponds to the curvature of three-dimensional space.

If Einstein's theory matched reality perfectly, gamma ought to be exactly equal to one. Measuring a value for gamma that's even slightly different from one would be the "first shot" that physicists have been waiting for.

"Gamma is the most sensitive way of measuring any possible deviation from Einstein, because it is sensitive to [any kind of unknown field]," says Thibault Damour, a professor at the Institut des Hautes Etudes Scientifiques, France, and an expert in theories that could replace relativity.