The Gravity Probe B space mission has confirmed two predictions derived from Albert Einstein’s theory of general relativity.
Today we are accustomed to hearing about the theory of relativity as a cornerstone of science and we take for granted its validity. However its verification is often difficult because some of its effects are so complex to measure that an experimental verification requires considerable technological developments.
The first experiment to verify general relativity already came in 1919, when British astronomer Arthur Eddington measured the gravitational light deflection by photographing the stars around the Sun. Those pictures were taken during a solar eclipse because it was the only moment when such an effect could be registered with the instruments of that period.
At that time the theory of relativity was considered strange by many scientists. As Einstein developed it while working at the University of Berlin it wasn’t well seen in Great Britain, where scientists generally defended in a patriotic way Newton’s gravitational theory, also because during World War I Germany had fought against Great Britain.
Eddington and Einstein were pacifists and the war years were tough for them. The British astronomer had discovered the theory of relativity thanks to his colleague Willem de Sitter and got convinced that it could be correct so with great difficulty he started a correspondence with Einstein. The two scientists could meet in person only after the end of the war.
In 1959 MIT professor George Pugh proposed a verification of general relativity experiment much more sophisticated than the one performed by Arthur Eddington many years earlier. For its implementation however it was necessary to develop a very advanced system of gyroscopes and that took a long time.
Over the years various changes in the space missions schedules caused the Gravity Probe B space probe to be launched only on April 20, 2004.
Over time the probe had some technical problems but it succeded in carrying out its work. It was therefore possible to test the geodetic and frame-dragging effects predicted by general relativity.
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The geodetic effect is due to the space-time curvature caused by a gravitational body. With its sophisticated gyroscopes Gravity Probe B could measure their small change in direction caused by the curvature of the Earth’s gravity during the experiment.
Frame-dragging is the effect caused by a rotating body which basically drags space-time around itself. Again the gyroscopes of Gravity Probe B could measure tiny changes in the direction of their rotation.
Gravity Probe B measurements were completed at the end of 2010. Initial evaluations of the data began in 2007, less than three years after the mission start, but the complete data analysis will still go on.
The technological impact of the Gravity Probe B space mission has already started bringing improvements in technologies such as GPS management. Other applications were found for other NASA missions. It’s only the beginning because, as always, these missions not only increase our scientific knowledge but they also end up creating and enhancing technologies that can be used even dayly.