Michael Hohensee

Equivalence principle and gravitational redshift.

We investigate leading order deviations from general relativity that violate the Einstein equivalence principle in the gravitational standard model extension. 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 Mössbauer clock tests, yields comprehensive limits on spin-independent Einstein equivalence principle-violating standard model extension terms at the 10(-6) level.

Limits on Isotropic Lorentz Violation in QED from Collider Physics

We consider the possibility that Lorentz violation can generate differences between the limiting velocities of light and charged matter. Such effects would lead to efficient vacuum Cherenkov radiation or rapid photon decay. The absence of such effects for 104.5 GeV electrons at the Large Electron Positron collider and for 300 GeV photons at the Tevatron therefore constrains this type of Lorentz breakdown. Within the context of the standard-model extension, these ideas imply an experimental bound at the level of -5.8x10{sup -12}{<=}{kappa}-tilde{sub tr}-(4/3)c{sub e}{sup 00}{<=}1.2x10{sup -11} tightening existing laboratory measurements by 3-4 orders of magnitude. Prospects for further improvements with terrestrial and astrophysical methods are discussed.

A Clock Directly Linking Time to a Particle's Mass

Linking Mass and Time The precision of atomic clocks is based on the transitions between two well-defined energy levels—the frequency of oscillation. We know from relativity that mass and energy are equivalent and from quantum mechanics that energy relates to frequency. Therefore, the ticking of a clock can be related, in principle, to the mass of a particle. The oscillation frequency of a particle is known as its Compton frequency and, because of the high frequency involved and stability of atoms, it has been argued that a clock linking mass and time would offer very high precision. Ordinarily, the Compton frequency is extremely high and not accessible to direct excitation. Lan et al. (p. 554, published online 10 January; see the Perspective by Debs et al.) demonstrate the operation of a Compton clock exploiting a related parameter, the phase accumulation rate of cold cesium atoms. Using an atom interferometer and an optical frequency comb to bring the Compton frequency into an experimentally accessible regime, mass and time could be directly linked. A clock is demonstrated wherein the ticks are related to the mass of a cesium atom. [Also see Perspective by Debs et al.] Historically, time measurements have been based on oscillation frequencies in systems of particles, from the motion of celestial bodies to atomic transitions. Relativity and quantum mechanics show that even a single particle of mass m determines a Compton frequency ω0 = mc2/ℏ, where c is the speed of light and ℏ is Plancks constant h divided by 2π. A clock referenced to ω0 would enable high-precision mass measurements and a fundamental definition of the second. We demonstrate such a clock using an optical frequency comb to self-reference a Ramsey-Bordé atom interferometer and synchronize an oscillator at a subharmonic of ω0. This directly demonstrates the connection between time and mass. It allows measurement of microscopic masses with 4 × 10−9 accuracy in the proposed revision to SI units. Together with the Avogadro project, it yields calibrated kilograms.

Limits on violations of Lorentz symmetry and the Einstein equivalence principle using radio-frequency spectroscopy of atomic dysprosium.

We report a joint test of local Lorentz invariance and the Einstein equivalence principle for electrons, using long-term measurements of the transition frequency between two nearly degenerate states of atomic dysprosium. We present many-body calculations which demonstrate that the energy splitting of these states is particularly sensitive to violations of both special and general relativity. We limit Lorentz violation for electrons at the level of 10(-17), matching or improving the best laboratory and astrophysical limits by up to a factor of 10, and improve bounds on gravitational redshift anomalies for electrons by 2 orders of magnitude, to 10(-8). With some enhancements, our experiment may be sensitive to Lorentz violation at the level of 9 × 10(-20).

Particle-accelerator constraints on isotropic modifications of the speed of light.

The absence of vacuum Cherenkov radiation for 104.5 GeV electrons and positrons at the LEP collider at CERN combined with the observed stability of 300 GeV photons at the Tevatron constrains deviations of the speed of light relative to the maximal attainable speed of electrons. Within the standard-model extension, the limit -5.8x10(-12)<or=kappatr-4/3ce00<or=1.2x10(-11) is extracted, which sharpens previous bounds by more than 3 orders of magnitude. The potential for further refinements of this limit with terrestrial experiments and astrophysical observations is discussed.

Force-Free Gravitational Redshift: Proposed Gravitational Aharonov-Bohm experiment

We propose a feasible laboratory interferometry experiment with matter waves in a gravitational potential caused by a pair of artificial field-generating masses. It will demonstrate that the presence of these masses (and, for moving atoms, time dilation) induces a phase shift, even if it does not cause any classical force. The phase shift is identical to that produced by the gravitational redshift (or time dilation) of clocks ticking at the atoms Compton frequency. In analogy to the Aharonov-Bohm effect in electromagnetism, the quantum mechanical phase is a function of the gravitational potential and not the classical forces.

Equivalence principle and bound kinetic energy.

We consider the role of the internal kinetic energy of bound systems of matter in tests of the Einstein equivalence principle. Using the gravitational sector of the standard model extension, we show that stringent limits on equivalence principle violations in antimatter can be indirectly obtained from tests using bound systems of normal matter. We estimate the bound kinetic energy of nucleons in a range of light atomic species using Greens function Monte Carlo calculations, and for heavier species using a Woods-Saxon model. We survey the sensitivities of existing and planned experimental tests of the equivalence principle, and report new constraints at the level of between a few parts in 10(6) and parts in 10(8) on violations of the equivalence principle for matter and antimatter.

Improved Constraints on Isotropic Shift and Anisotropies of the Speed of Light Using Rotating Cryogenic Sapphire Oscillators

We demonstrate that Michelson-Morley tests, which detect direction-dependent anisotropies in the speed of light, can also be used to place limits upon isotropic deviations of the vacuum speed of light from c, as described by the photon-sector standard model extension parameter {kappa}-tilde{sub tr}. A shift in the speed of light that is isotropic in one inertial frame implies anisotropic shifts in others. Using observer Lorentz covariance, we derive the time-dependent variations in the relative resonance frequencies of a pair of electromagnetic resonators that would be generated by such a shift in the rest frame of the Sun. A new analysis of a recent experimental test of relativity using this result constrains {kappa}-tilde{sub tr} with a precision of 7.4x10{sup -9}. This represents the first constraint on {kappa}-tilde{sub tr} by a Michelson-Morley experiment and the first analysis of a single experiment to simultaneously set limits on all nine nonbirefringent terms in the photon sector of the minimal standard model extension.

Erratum: New methods of testing Lorentz violation in electrodynamics [Phys. Rev. D 71 , 025004 (2005)]

In an earlier paper [1] (hep-ph/0408006), the bound on the Standard Model Extension photon-sector parameter

Lineshape asymmetry for joint coherent population trapping and three-photon N resonances

\tilde{\kappa}_{tr}