Atoms, the Equivalence Principle, and Dueling Laureates

Good to know that our Secretary of Energy, Steve Chu, is still able to unwind from a long day of bureaucracy by thinking about atom interferometry and the Principle of Equivalence.

Equivalence Principle and Gravitational Redshift

Michael A. Hohensee, Steven Chu, Achim Peters, Holger Mueller

We investigate leading order deviations from general relativity that violate the Einstein equivalence principle (EEP) in the gravitational standard model extension (SME). We show that redshift experiments based on matter waves and clock comparisons are equivalent to one another. Consideration of torsion balance tests, along with matter wave, microwave, optical, and M\”ossbauer clock tests yields comprehensive limits on spin-independent EEP-violating SME terms at the $10^{-6}$ level.

The Principle of Equivalence says that, if you’re in free fall, there’s no way of detecting the gravitational field around you in a local region of spacetime. (You’ve seen Inception, right?) Unlike electromagnetism, with gravity there’s no local “force” that can be detected by comparing what happens to particles of different charges. In other words, all particles feel the same “charge” as far as gravity is concerned; they all fall in the same way.