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. |