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Cosmology in the Lab
Martin Plenio
One of the most important effects in fundamental physics that has not yet been full understood or observed are Hawking radiation and related effects due to mirrors accelerated in vacuum and the closely related Unruh effect. While it is known that all inertial observers perceive the vacuum as is, non inertial observers will see the same ‘vacuum‘ as populated by particles. Unruh has shown that these are distributed thermally and pointed out the connection to Hawking radiation.
Fig 1: Any two observers that are moving with constant velocity with respect to each other agree on their respective definitions of vacuum. A uniformly accelerated observer however sees the vacuum of an inertial observer as filled with thermal radiation. This Unruh effect demonstrates that the concept of vacuum depends on the relative state of motion of observers.
The Hawking and Unruh effect combine three fields of physics: thermodynamics, gravity and quantum mechanics in one simple formula. The experimental verification of effects of this type are important precisely due to their dependence on three areas of physics that still suffer tensions in that there is no theory unifying them. Despite this importance none of these effects was ever measured in experiment. The challenge results from the fact that the acceleration needed to measure this effect, i.e. a sufficiently high temperature, appeared to be beyond experimental capabilities. It is proportional to acceleration and inversely proportional to the speed of light and in free space accelerations of 10^22 m.s^2 are needed to create temperatures of mK. While indeed a direct measurement and the verification of the effects in the vacuum of a quantum field in free space is extremely challenging, the measurement of the analogue of this effect in cold gases seems to be feasible. The difference results from the fact that the speed of sound in BEC (Bose Einstein condensate) is of the order of mm/sec which is much smaller than the speed of light, that a BEC is naturally very cold, around nK, and due to the fact that the interaction between the observer and the field is fully controllable. Hence much more moderate accelerations are sufficient for the observation of the effect.
In a recent paper published in Physical Review Letters [1] we propose a method to measure the Unruh effect in a Bose-Einstein condensate. In this analogy the vacuum is modelled by the BEC ground state and the observer is replaced with a moving atom dot, that is an optical potential that is very tightly confined in space to appear almost point like when compared to the BEC. The observed quantity is the number of atoms (in a different internal state to those in the BEC) in this detector. We show in [1] that current experimental methods are sufficient to measure the Unruh effect with sufficiently high precision to probe effects that are beyond the tractability of a classical computer or analytical calculations. By this we argue that Bose Einstein condensates have the potential to simulate the interplay between quantum fields and various observers. This suggests the field of cold gases as a tool to explore a new domain in physics.
[1] A. Retzker, J.I. Cirac, M.B. Plenio and B. Reznik.
“Probing the Unruh effect in Bose-Einstein Condensates”
Phys. Rev. Lett. 101, 110402 (2008) and E-print arXiv:0709.2425 [quant-ph]
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