The Satellite Test of the Equivalence Principle, commonly known as STEP, is a project
sponsored jointly by NASA and the European Space Agency to greatly improve the test
that Galileo conducted at the leaning tower of Pisa: Do all objects fall with the
same acceleration due to gravity? The answer to this question relates to whether an
object's gravitational mass is the same as, or equivalent to, its inertial mass.
The experiment plans to fly several pairs of masses on a drag-free satellite in low
Earth orbit, with a launch planned for after 2006.
It may surprise you to learn that so great a thinker as Aristotle believed that heavier
objects would fall faster, the rate of acceleration being proportional to the weight.
These thoughts from the Greek Golden Age prevailed for about 2000 years until courageous
scientists like Galileo began to challenge the orthodoxy of physics beliefs. Galileo's
purported test in 1620 at the Tower of Pisa to drop a musket ball and a much heavier
cannon ball, with the result that the two landed "within a hand's-breadth" of each other,
showed two results: If wind resistance is small, then lighter and heavier objects of the
same material will fall at the same rate; and, this result is independent of the
compositions of the two objects.
This test is of such significance to our understanding of gravity's force that it has
been repeated over the centuries by such eminent scientists as Isaac Newton (~1680),
Baron Roland von Eotvos (~1900-1922), and Robert Dicke (1962). Measurement accuracies
have improved from parts per 100 by Galileo, parts per 1000 by Newton, parts in a billion
by Eotvos, and parts per trillion by Dicke and coworkers. The most recent report by a
group headed by Eric Adelberger at the University of Washington achieves an accuracy at
the parts in 1013 level; a similar uncertainty is reported by the
Lunar Laser Ranging experiment. The importance of the question
to the science community is apparent in the many efforts to improve the measurement result.
Now STEP will push the precision to the level near a part in 1018, a gain of more than
100,000 in resolution over the ground-based research. At these improved levels of
measurement STEP can better test theories that propose new composition-dependent
forces acting between two bodies.