Free-Fall Satellite Test of the Equivalence Principle (Free-Fall STEP)

A Proposed Mission to Measure the Universality of Free Fall with Completely Free Test Masses to Accuracies between 10^-19 and 10^-21 g

It is proposed to fly a satellite mission to test the Universality of Free Fall (the Weak Equivalence Principle) in a room temperature satellite using free-falling test masses consisting of a heavy-metal sphere inside of a less-dense spherical shell.

Because of the technique of DC or AC Cancellation, a previously impossible experiment consisting of a sphere inside of a spherical shell is now possible. Either DC or AC Cancellation solves the two problems which have previously prevented a truly free-fall space experiment:

the fact that an EP violation cannot be distinguished from a semi-major axis error and

the fact that a response to an EP violation only grows as t not as t².

Using completely free-falling test masses, accuracies between 10^-19 and 10^-21 g may be obtained.

For more information read the papers: DC Cancellation or the same paper in the physics arXiv gr-qc/005073.

Also see the University of Washington Department of Physics Eöt-Wash Group's Definitions of the Equivalence Principle or their page on Laboratory Tests of Gravitational Physics.

For a good discussion of the importance of testing the Equivalence Principle, see Testing the Equivalence Principle: Why and How? by Thibault Damour.

As pointed out in the above reference, an important part of any test of the EP requires that different materials be compared. Recent experience, especially with GP-B and Globalstar, shows that it is cheaper and easier to do this with multiple satellites rather than to attempt to design a single experiment with a comparison of many materials in a one satellite.

Furthermore, it makes good sense to first use a simple design to discover the size of any EP violation and then to do the comparison with several different materials.