The Sciences

Atoms, the Equivalence Principle, and Dueling Laureates

Cosmic VarianceBy Sean CarrollFeb 23, 2011 1:17 PM


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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. So to look for violations of the EP (which are certainly conceivable, even if it sometimes just sounds like technobabble), you do experiments that look for particles doing different things in different kinds of gravitational fields. For example, you can use the EP to predict the gravitational redshift, which can be thought of as "time running more slowly when you are deep in a gravitational potential." (Not the most precise formulation, but it will do.) And therefore you can test the EP by measuring the different amount of time elapsed by sending clocks on different trajectories. A relatively new technique for performing such tests is atom interferometry. Rather than literally sending clocks along two different trajectories -- which has also been done, of course -- you take advantage of the quantum-mechanical wave nature of matter, and simply send two "wave packets of atoms" along different trajectories. Waves oscillate, and the number of oscillations they experience serves as a clock. The benefit is that the waves interfere when they recombine, and we're very good at measuring tiny deviations in the predicted amount of interference. But apparently there's been some controversy over whether the phase of the atom wave really counts as a "clock," for purposes of this kind of experiment. This new paper by Hohensee and collaborators is saying "yes, it does." A previous paper by Wolf et al. said "no, it doesn't." At a glance, I tend to agree with Hohensee et al., but I haven't gone through the arguments very carefully. I'll just note that one of the authors of the Wolf et al. paper is Claude Cohen-Tannoudji, who shared the Nobel Prize in 1997 with Bill Phillips and ... Steve Chu! Always good to see a Nobel-level tussle. I think I'll let them figure it out.

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