Einstein’s brilliant work on the Theory of Relativity is renowned worldwide but have you ever wondered where and how its concepts are actually applied? One of the answers is probably in your hands right now. From a simple GPS functionality in mobile phones to the very working principle of nuclear plants, relativistic principles are used in all sorts of tech. However, before going ahead for real-world applications, one must test and prove that the theory holds true under drastic situations. Scientists have worked conscientiously to find conditions where the theory might fail but their work has only given it more credibility. One such test was the “Gravity Probe B” experiment.
Gravity Probe B (GP-B) is a NASA physics mission to experimentally investigate the General Theory of Relativity. The goal of the GP-B experiment is to measure the geodetic effect to an accuracy of 0.01%, and to measure the frame-dragging effect, to an accuracy of 1%. What are these exactly you ask?
The geodetic effect is the amount by which the Earth warps the local space-time in which it resides. Think of it as the clichéd example of a ball warping a taut fabric’s surface. The frame-dragging effect, on the other hand, is the amount by which the rotating Earth drags its local space-time around with it. It’s akin to how if you press down on the aforementioned ball and rotate it, the fabric gets pulled too and develops folds around the ball.
The project was proposed by Leonard Schiff, a popular physicist best known for his work on Quantum Mechanics and was initialized in 1963 by NASA. The project was cancelled many times by NASA due to numerous delays and expenditure. Finally, it was launched in 2004 and the results were reported in 2011, proving it to be a yet another successful test of the General Theory of Relativity. In the meantime, scientists had difficulty in measuring the data from the GP-B. The probe’s data was unexpectedly noisy due to solar flares in March 2005 that interrupted the satellite’s observations, as well as unexpected changes in gyroscope’s orientation because of which the accuracy was down to 0.5%.
The GP-B consisted of 4 gyroscopes and a telescope and was a polar-orbiting satellite placed at 642 km above the Earth. At the start of the experiment, both the telescope and the spin axis of each gyroscope are aligned with a distant reference point — a guide star. Keeping the telescope aligned with the guide star for a year, as the spacecraft made over 5,000 orbits around the Earth. The scientists then measured the change in the spin-axis alignment of each gyro over this period in both, the plane of the orbit (the geodetic precession) and orthogonally, in the plane of the Earth’s rotation (frame-dragging precession). The changes in the angles of the axes of rotation thus found, matched up with the predictions of the theory in question accurately.
The GP-B gyro rotors are now listed in the Guinness Database of World Records as being the roundest objects ever manufactured and it is one of the longest projects that has ever been done by NASA and the most stringent test of Einstein’s predictions to date.
