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
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
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
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.