Test of the Weak Equivalence Principle
Was Einstein right? We have an opportunity to test an underlying assumption he made in creating his theory of gravity, General Relativity.
Our goal is to repeat the famous experiment of Galileo to drop two different masses simultaneously from the leaning tower of Pisa -- but, in our case, with picometer precision! By using two test masses made of different materials, we will carry out a unique test of the Einstein Principle of Equivalence, which underlies the generally accepted theory of gravity, General Relativity. The Principle of Equivalence is exactly obeyed in General Relativity, but any potential quantum theory of gravity seems likely to exhibit deviations, for which we hope to search. Violations of the Einstein Principle of Equivalence also tend to be predicted by quantum theories of gravity. Once the effect of gravity on matter has been fully established, research on the effect on antimatter can be started.
Version 1.0 of the apparatus, named G-POEM, was designed and built by a team at the Harvard-Smithsonian Center for Astrophysics (CfA) moved to Illinois Tech, who will update and take G-Poem's first set of scientific data. Our challenges are to get all of its systems running, as well as making improvements to it. One such issue is reducing the vibration in its motion, which was started at the CfA with the air bearing system and improvements to the linear motor. Once these systems are fully commisional and tested, then the test of the WEP can be preformed.
In the experimental setup, a vaccum chamber on a cart rides up and down in a polished granite pillar, contrained by air bearings. A feedback loop controls a linear-drive electric motor to overcome friction and keep the test masses in free fall. At the end of a 1 second period of free fall, the cart contacts a bouncer which gently but firmly reverses its downward motion in 0.3 seconds, beginning a new free fall, while a laser beam bounces between the test masses to measure the distance between them.
If the cart rides smoothly with no pertubations, the test masses launch with minimal sideway velocity or rotation. This is important in controlling systematic error so that test masses do not strike their stops before the 1 second free fall is over. If sucessful, this can produce 1 pm (picometer) precision within this one second. This will give a precision of 5 x 10^-11 m s^-2 in one toss for the differential acceleration of a pair of test masses. After one day of 50,000 tosses, the uncertainty for test substances ( constituting about 25% of the test mass) would be
9 x 10^-13 g.
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