The step payload and experiment Original Research Article

The foundation of modern gravitational theory is the Equivalence Principle. General Relativity is incompatible with theories of other fundamental forces such as QED, suggesting that it is incomplete. For example, there may be additional forces coupled to baryon number or spin. In this case the Equivalence Principle may be violated below the experimentally verified level of one part in 1012. A violation could provide crucial information for new theories. A team of US and European scientists has assembled to do the Satellite Test of the Equivalence Principle (STEP) with the goal of improving this measurement to 1 part in 1018. In STEP two or more test masses “fall” around the earth in a drag free satellite. A difference in the rate of fall appears as a periodic difference in their acceleration. The test masses are cooled to less than 2K and are supported by frictionless superconducting bearings. Ultra-sensitive SQUID position sensors measure their relative motion and their common motion is removed by adjustments during acceleration maneuvers. Any Equivalence Principle signal is separated from major disturbances by rotation of the spacecraft. STEP is planned to be launched by 2004, with nominal mission lifetime of 6 months.