Do Quantum Measurements Change If the Detector Moves?

For example, could a count of the number of photons in a burst of light depend on the location of the detector in an extreme gravitational field? These ideas, long pondered by physicists, might be verifiable in the lab, according to a new theory in which a Bose Einstein condensate (BEC) of cold atoms acts as a stand-in for the universal vacuum.The related notion that potential energy residing in the vacuum can influence the geometry of spacetime and thus the expansion of the cosmos could also be testable in a tabletop experiment here in Earth.

The pertinent phenomenon that would facilitate this line of research is called the Unruh-Davies effect, which suggests that a detector accelerating (not just moving at a constant speed but actually moving ever faster) through a vacuum will effectively encounter photons coming out of the vacuum. (A related phenomenon is the Gibbons-Hawking effect, in which photons, "Hawking radiation," can be detected in the gravitationally intense region of a black hole.)

In the Unruh effect the energy needed to turn virtual photons into real photons would be supplied by the accelerating detector itself. The detector would see the vacuum not as an empty space but as a thermal bath of photons. The same effect can disrupt quantum teleportation (see Update 660). The "temperature" of this bath would be proportional to the detector's acceleration.

Actually observing such a thermal bath (equivalent to an effective temperature of something like 10^-15 K for a detector acceleration one hundred thousand times more than that felt by us on the surface of the Earth) with any foreseeable manmade detector is close to impossible, but two physicists at the Leopold-Franzens-Universitaet in Innsbruck, Petr Fedichev (peter.fedichev@uibk.ac.at) and Uwe Fischer (uwe.fischer@uni-tuebingen.de), believe the effect could be probed by studying how sound waves ripple through BECs in the lab. The superfluid condensate of atoms would correspond to the vacuum and phonons would be analogous to photons moving through a curved space-time. Before the experiment can be performed, larger BECs than used so far will be needed, as well as sharper optical manipulation of atoms in the BEC. (Fedichev and Fischer, Physical Review Letters, 12 December 2003)