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Register a new userThe Equivalence Principle at Work in Radiation from Unaccelerated Atoms and Mirrors
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The equivalence principle is a perennial subject of controversy, especially in connection with radiation by a uniformly accelerated classical charge, or a freely falling charge observed by a supported detector. Recently, related issues have been raised in connection with the Unruh radiation associated with accelerated detectors (including two-level atoms and resonant cavities). A third type of system, very easy to analyze because of conformal invariance, is a two-dimensional scalar field interacting with perfectly reflecting boundaries (mirrors). After reviewing the issues for atoms and cavities, we investigate a stationary mirror from the point of view of an accelerated detector in Rindler space. In keeping with the conclusions of earlier authors about the electromagnetic problem, we find that a radiative effect is indeed observed; from an inertial point of view, the process arises from a collision of the negative vacuum energy of Rindler space with the mirror. There is a qualitative symmetry under interchange of accelerated and inertial subsystems (a vindication of the equivalence principle), but it hinges on the accelerated detectors being initially in its own Rindler vacuum. This observation is consistent with the recent work on the Unruh problem.
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Atom interferometry tests of universality of free fall based on the differential measurement of two different atomic species provide a useful complement to those based on macroscopic masses. However, when striving for the highest possible sensitivities, gravity gradients pose a serious challenge. Indeed, the relative initial position and velocity for the two species need to be controlled with extremely high accuracy, which can be rather demanding in practice and whose verification may require rather long integration times. Furthermore, in highly sensitive configurations gravity gradients lead to a drastic loss of contrast. These difficulties can be mitigated by employing wave packets with narrower position and momentum widths, but this is ultimately limited by Heisenbergs uncertainty principle. We present a novel scheme that simultaneously overcomes the loss of contrast and the initial co-location problem. In doing so, it circumvents the fundamental limitations due to Heisenbergs uncertainty principle and eases the experimental realization by relaxing the requirements on initial co-location by several orders of magnitude.
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Light is known to exert a pushing force through the radiation pressure on any surface it is incident upon, via the transfer of momentum from the light to the surface. For an atom, the interaction with light can lead to both absorption as well as emission of photons, leading to repulsive and attractive forces, respectively. For classical light, these two processes occur at the same rates. Therefore, a thermal ensemble of atoms at a finite temperature always experiences a net pushing force. In this paper, we show that when treated quantum mechanically the pulsed electromagnetic field interacting with the thermal ensemble of atoms leads to unequal transition rates, again resulting in a non-zero net force. However, the signature and the magnitude of the force depends upon the intensity of the light, the number of atoms, and the initial temperature of the ensemble. Thus, even at finite temperature, controlling the parameters of the electromagnetic pulse and the number of particles in the ensemble, the net force can be changed from repulsive to attractive, generating negative radiation pressure in the process. Quite counterintuitively, this negative radiation pressure arising out of pure quantum character of light gets stronger for higher temperatures.
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