Einstein put to the ultimate test

A satellite launched this week may finally prove the scientist's general theory of relativity. Dick Ahlstrom reports on the ambitious Gravity Probe B mission.

A giant vacuum flask and four crystal balls were lifted into orbit this week, part of an elaborate attempt to test two fundamental predictions made about the universe by Albert Einstein.

Gravity Probe B was launched from Vandenberg air-force base, in southern California, at about 6 p.m. Irish time on Tuesday. Now some 660 kilometres above Earth, it will use four extremely sensitive gyroscopes to measure how gravity warps and twists the space-time in which our planet sits.

Developed by Stanford University, NASA and Lockheed Martin, the mission was a long time coming. Originally proposed in 1959 as a test of general relativity, the idea had to await the arrival of a level of technological competence that did not exist at the beginning of the space age. Lack of funding also held up the project, which is now well past an earlier launch target of 2002.

All that is so much history now, provided, of course, that the probe can deliver on its promise and bring definitive proof of general relativity.

Einstein devised two key relativistic theories that have shaped physics over the past 80 years, special relativity and general relativity. The former blends space and time into a single related entity and describes how travel at near light speeds can affect space-time. The latter discusses space-time as a field and how gravity can make space-time apparently curve and twist.

In particular the probe should prove two main relativistic assumptions, the geodetic effect (how much Earth's gravity warps space-time) and the frame-dragging effect (whether Earth's rotation is able to drag space-time along with it as it turns).

Being able to achieve these goals depends entirely on the four astoundingly accurate gyroscopes on board the probe. Each is built around a 3.8 centimetre niobium-coated quartz crystal ball, polished to within 40 atoms of a perfect sphere.

These will spin about 9,000 times a minute while sitting in a three-metre-long flask filled with liquid helium, designed to keep the gyros at just a few degrees kelvin above absolute zero.

The elaborate construction should reduce all unwanted influences on the gyros, bringing extraneous disturbances to below the levels likely to be caused by the gravitational tug on space-time. This will help ensure a sensitivity that almost defies comprehension.

The four gyros are so precise that non-relativity effects will cause them to drift by no more than a third of a milli-arc-second during a full year. To put this tiny arc into perspective, the angle formed by sighting along the left and right edges of a human hair placed 30 kilometres away would measure half a milli-arc-second.

The satellite will orientate itself by sighting an onboard telescope on HR8703, a binary star in the constellation Pegasus. Over a period of months the probe will measure the tiny disturbances to its gyros caused by geodetic and frame-dragging effects, with luck providing the proof of Einstein's general relativity.

There is a strong Irish connection to Gravity Probe B. Prof Susan McKenna-Lawlor's Space Technology Ireland, based in Maynooth, fabricated and developed the Energetic Proton Monitor hardware also carried by the probe. The device measures the proton bombardment that the gyros will experience, allowing this to be factored into the results.

You can get more details about the experiment by visiting http://einstein.stanford.edu