Physicists testing Albert Einstein's relativity theory need abundant patience and exquisite measurement skills. The theory-testing Gravity Probe-B satellite, carrying the most nearly perfect rotating spheres ever made, is rewarding both of those virtues. It took 40 years of work to finally get the spheres into orbit three years ago. This month's update of the analysis of the spherical gyroscope data promises significant scientific results.
Meanwhile, a plan to seek Einstein's predicted gravitational waves, which are ripples in space itself, passed a major milestone June 18. In a ceremony at the Paris Air Show, the heads of National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) signed a formal agreement to proceed with the first stage of this joint project. For the project to succeed, physicists must be able to guarantee that a mass can float freely in space completely undisturbed by anything but gravitational waves. Engineers must also be able to control the spacecraft position to within a few millionths of a millimeter. The first step is to launch a satellite to test those skills in 2010.
Scientists keep poking at Einstein's theory because it is central to 21st-century physics. They want to confirm it down to the last detail or find a flaw that opens up new revelations.
Two questions stand out. In the theory, space and time meld into a single seamless fabric called space-time. Is Einstein right in predicting that a mass, such as Earth, distorts space-time? Second, is Einstein right when he predicts that powerful events, such as a stellar explosion, send ripples through space-time – ripples called gravitational waves?
Gravity Probe B tackled the first question. Does Earth distort local space like someone sleeping weighs down a mattress? That's called geodetic distortion. Also, does the rotating planet drag the space-time fabric around like a restless sleeper drags his or her bedclothes? That's called frame dragging.
Both effects should show up in the way the probe's rotating spheres tilted. Analysts have found the geodetic distortion to within an accuracy of 1 percent. That's about as good as previous measurements made by other means. But the team expects to improve their precision 10-fold by December. The restless-sleeper frame dragging is harder to pick out. Again, the team expects – but can't guarantee – it will find the effect in the data by year's end.
The NASA/ESA program is also a multidecade affair. The final experiment will consist of two masses falling freely through space while scientists measure their relative motions. With no other influence to worry about, any effect from gravitational waves would show up in that motion. But first, project scientists have to show they have the skills to pull off that extremely precise measurement.
The overall program is called LISA. That stands for Laser Interferometer Space Antenna. The final step is to orbit a satellite called LISA Pathfinder to test the technology and spaceflight skills that will be involved.
Even though it can take much – or even all – of their working lifetime to complete, scientists involved with these projects say the eventual payoff makes the effort worthwhile. LISA Pathfinder scientist Tim Summer at Imperial College in London says that his group "has been working hard on this mission for the last 15 years" because it provides "an enormously rich new discovery opportunity."