General Information

The Satellite Test of the Equivalence Principle (STEP) is a joint European-U.S. space program to investigate one of the most fundamental principles in physics, the Equivalence of inertia and passive gravitational mass. Isaac Newton first recognized the identity between these two distinct properties, which represent the quantity of matter in an object, and its weight. A direct consequence of this Equivalence Principle is the 'universality of free fall' such that all objects fall with exactly the same acceleration in the same gravity field. The Equivalence Principle was reinterpreted by Albert Einstein as a consequence of an even broader equivalence between the laws of physics in different accelerated reference frames, a principle which Einstein made the basis for his general theory of relativity. STEP will advance the sensitivity of Equivalence Principle tests by five or six orders of magnitude, into regions where the principle may break down. A violation of Equivalence at any level would have significant consequences for modern gravitational theory. The STEP experiment is conceptually a modern version of Galileo's Free-Fall Experiment, in which Galileo is said to have dropped two weights from the Leaning Tower of Pisa to demonstrate that they fall at the same rate. Any difference in the ratio of inertial to passive gravitational mass of the weights results in a difference in the rate of fall. In STEP, the masses are in free fall in an orbit around the Earth, and if there is a violation of the Equivalence Principle they tend to follow slightly different orbits. The orbiting masses fall all the way around the Earth and never strike the ground, so that any small difference in the rate of fall can build a large displacement. SQUID magnetometer circuits are used to measure displacements as small as 10-13 centimeter.