Experimental Tests of the Equivalence Principle and Newton's Law in Space

Tests of the Weak Equivalence Principle (WEP) by means of torsion balances have confirmed the WEP nearly to the 10-13 level. At this level, the coupling to changing gravitational environment on earth as well as seismic and thermal noise seem to establish a severe threshold for the improvement of accuracy. In order to overcome these difficulties, experiments in space offer new ranges of accuracy: Space platforms and satellites in a nearly free fall along geodesics offer unique possibilities to improve the accuracy of WEP-tests by several orders of magnitude which is important as some quantum gravity inspired theories predict a WEP-violation. At present, two satellite projects are planned and scheduled to test the WEP for neutral bulk matter: the French mission MICROSCOPE approved to be launched early 2008 is designed to measure on the 10-16 level and the ESA/NASA mission STEP which is still under study, will be precise on the 10-18 level. Modern space technology is not only interesting for WEP tests of neutral bulk matter, but can be used to carry out comprehensive EP-tests (e.g. Lunar Laser Ranging). Beside tests with bulk matter, complementary tests with comparable results for other kind of matter or couplings (charged particles, atoms, neutrons, anti-matter, or spin) are required. New ideas for space experiments to test the WEP for anti-matter or charged matter have been discussed recently. Space also enables to test Newton's Law on very small and huge distances. In the range below 0.1 mm, one can make use of the advantage of the weightlessness environment. For long range measurements above 1 astronomical unit, deep space satellites are and will be used to probe the gravitational field with improving accuracy.