A spacecraft set to test Einstein’s General Theory of Relativity is now on the launch pad, with the world’s most accurate gyroscopes stowed away inside. NASA has a launch window of just one second early on 19 April to launch Gravity Probe B into a polar orbit precisely aligned with a reference star. The gyros, a million times more accurate than the best on Earth, will measure how the rotating Earth drags on the fabric of space-time.
General relativity is a cornerstone of modern physics and holds that gravity warps the fabric of space and time. Observations during a 1919 solar eclipse confirmed that the Sun’s gravity bent starlight by warping space. A 1976 NASA mission called Gravity Probe A – related only by name – sent an atomic clock 10,000 kilometres into space, confirming the theory’s prediction that gravity slows the flow of time.
Gravity Probe B aims to measure a weaker and even stranger effect called “frame dragging”, a warping of space-time by the gravity and angular momentum of a spinning body. In principle, it is possible to measure it by monitoring the spin axis of a gyroscope orbiting the Earth. The axis should change its orientation relative to that of a distant star.
Once we can measure the effect in the Solar System, says Kip Thorne, a gravitational physicist at the California Institute of Technology, “we can definitely understand how the same phenomena are working in the distant Universe and around black holes,” which cause much stronger warping. The probe should measure frame-dragging to within one per cent. It also will measure a much larger space-warping effect called the geodetic effect to one part in 10,000, the most precise test yet of any relativistic effect.
To meet these exacting requirements, Gravity Probe B contains four spinning balls of fused quartz, each perfectly spherical to within 40 atomic layers. Each one is coated with a 1.27-micrometre layer of niobium, and cooled to 1.8 degrees above absolute zero to keep the metal film super-conducting.
The superconducting layer creates a tiny magnetic field that pinpoints the axis of the spinning sphere. The experiment therefore requires shielding that reduces the strength of external magnetic fields by a factor of a trillion, electric fields to suspend the spinning spheres, and 2441 litres of superfluid helium coolant. The technical challenges had appeared so severe that NASA tried to cancel the project several times, only to have scientists rescue it by lobbying the US Congress. The experiment will have cost $700 million when the data is in and finally analysed. The launch had been planned for 17 April, but the latest in a long series of minor technical glitches forced a delay. However, sitting atop a Delta rocket and 46 metres off the ground, Gravity Probe B is now closer to space than it has ever been before.