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Slingshot test for general relativity

04 Apr 2001

Physicists are hoping to make a new test of Einstein's General Theory of Relativity by measuring the effect of the Sun's gravity on a passing spacecraft. James Longuski of Purdue University and colleagues have devised a new formula to calculate the distortion of space from the deflection of a spacecraft's path as it skims the Sun (J M Longuski et al 2001 Phys. Rev. Lett. 86 2942). The researchers are optimistic that imminent refinements in measurement technology will allow them to carry out the most sensitive test of the theory yet.

In 1915, Einstein proposed that gravity actually distorts the fabric of space. His calculations perfectly explained certain astronomical observations that Newtonian physics could not account for – such as the apparently exaggerated precession of Mercury’s orbit. “General relativity is at the heart of everything in cosmology”, says team member Ephraim Fischbach, “so it’s very important we continue to test it to make sure its predictions are correct”.

The Small Interstellar Probe mission currently under consideration by NASA needs to perform a close fly-by of the Sun to gather enough momentum to propel itself outside the solar system. This ‘slingshot’ manoeuvre will take the craft to within 4 solar radii of the Sun. The probe’s chief goal is to establish the composition of interstellar space, but Longuski and colleagues hope it will be able to include their experiment. “We can’t get as precise a measurement by merely observing the planets in their orbits”, Longuski told PhysicsWeb, “but we can control this experiment by selecting a trajectory for the spacecraft”.

Other tests of general relativity have involved measurements of redshifts, the deflection of light by stars and galaxies, and tests of the inverse square law of gravity. Technological advances – such as improvements in the accuracy of atomic clocks – have improved our understanding of general relativity in recent decades.

To accurately determine the effects of relativity, Longuski and colleagues must eliminate other influences on the spacecraft’s motion, including radiation pressure, the solar wind, interplanetary dust and magnetic fields. The team believes that with existing technology the experiment will estimate key mathematical parameters in the theory to within an accuracy of 16% – but this will plummet to 0.2% with expected improvements in the measurement of the spacecraft’s position. As Longuski says: “If we find that Einstein was wrong, it would be big news”.

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