The Eöt-Wash Group:
Laboratory Tests of Gravitational and sub-Gravitational Physics

What measurements have we made?

We have tested the Equivalence Principle (more precisely the Universality of Free Fall or UFF) for the following cases:

  1. Be, Al, Cu and Si test bodies using the Earth as attractor. Our null results consitute the most precise laboratory tests of the UFF. Our results showed that the proposed "fifth force" and its natural generalizations did not exist.
  2. Be, Al, Cu and Si test bodies attracted toward the Sun. The goal here was to complement the lunar laser-ranging data that, in effect, tests the UFF for the Earth and Moon falling toward the Sun. The laser-ranging data are used to test whether gravitational self-energy obeys the Equivalence Principle, because the Earth's mass has a 4 × 10-10 contribution from gravitational binding energy but the contribution to the Moon's mass is only 2 × 10-11. However, the laser-ranging test probes a combination of two effects, the differing gravitational self-energies and also the differing compositions (the Earth has an Fe-Ni core while the moon does not) of the Earth and Moon. We are doing an experiment where we compare the accelerations toward the sun of special test bodies that have compositions very close to that of the Earth's core and to that of the Moon's surface. We expect to do this well enough to take full advantage of the precision of the laser-ranging data.
  3. Be, Al, Cu and Si test bodies attracted toward the center of our Galaxy. The idea here was to test { in the laboratory} whether the only significant force between dark matter and ordinary matter is gravitational (i.e. obeys the UFF). Our work showed that for Galactic dark matter this was indeed the case. We are developing the Eöt-Wash III instrument to extend our test to cosmological dark matter; because the relevant accelerations are smaller, we need better experimental sensitivity.
  4. Cu and Pb attracted toward a compact 3-ton 238U attractor. The goal here was to test the UFF at short ranges, to use an attractor with a very different N/Z ratio than the earth, and to close the "gap" at ranges between 10 km and 1000 km where Eötvös-type experiments using the earth attractor have little sensitivity. Uranium was used because of its high density; this allowed us to place a substantial mass close to the torsion balance. This instrument provided useful constraints for ranges down to 1 cm, i.e. exchange-boson masses up to 2 × 10-5, covering the "Turner window" where astrophysical constraints are very weak.

© 1987-2008 Eöt-Wash Group. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation, DOE or NASA. Trouble? Comments? Contact cah49#at#u.washington.edu