This article originally published in
Air and Space Magazine

The instrument the GP-B team has devised to detect these effects is basically a large thermos bottle, with 400 gallons of supercooled helium surrounding a nine-foot-long cylindrical "probe." The probe consists of a telescope, which keeps the instrument precisely pointed at a guide star, and four gyroscopes, each a fused quartz sphere about the size of a Ping-Pong ball and ground to specifications better than one one-millionth of an inch. Each gyro will be positioned in a quartz housing containing a superconducting loop. Any change in the direction of a gyro’s spin will produce a change in the direction of the sphere’s magnetic field that the loop will detect. The entire payload provides a high-vaccuum, lead-shielded environment with a temperature near absolute zero. The idea is that if the gyros are sufficiently isolated from such factors as heat, electrical charges, and magnetic fields, any change that occurs in their spin direction can be attributed to the curvature of Einstein’s postulated space-time fabric.

The Case for Einstein

While a number of competing theories of gravity have been knocked out of the ring, the theory of general relativity has yet to be irrefutably verified. Still, several observations and experiments over the years have given weight to what physicists consider the most beautiful and profound scientific theory of this century:

Each time the planet Mercury orbits the sun, its perihelion--the point at which the planet comes closest to the sun--advances a tiny bit. Such orbital advances could be attributed to the gravitational effects of the other planets in the solar system; however, Mercury’s advance is too great for this explanation to suffice. Instead, it appears that the massive gravity of the sun is warping the space around itself, producing a depression that the planets have to follow. Because Mercury is the planet closest to the sun, its orbit show the effects of this depression most dramatically.

Einstein believed that a massive body can also bend starlight passing near it. To test this, astronomers twice measured the locations of a group of stars: once during a 1919 solar eclipse, in which the sun was positioned between Earth and the stars, and again under normal conditions, when the sun was not in this intermediate position.

A comparison of the readings showed that the eclipse seemed to have caused the stars to shift. The explanation? During the eclipse, the stars’ light had had to pass close to the massive sun-and thus through the warped space surrounding it-before reaching Earth.

In a related series of tests conducted in the 1960s and 70s, scientists targeted radar signals at various objects in space: planets, spacecraft, and finally the Viking lander that had been left on Mars. When the researchers measured the amount of time it took for the signals to reach the targets and then bounce back, they discovered that the transit time was a bit longer than would have been the case had the signal merely traveled a straight line. The delay was attributed to a curvature in space.