Brett Altschul

Bio: Brett Altschul is an academic researcher from University of South Carolina. The author has contributed to research in topics: Lorentz transformation & Lorentz covariance. The author has an hindex of 20, co-authored 75 publications receiving 1654 citations. Previous affiliations of Brett Altschul include Indiana University & Massachusetts Institute of Technology.

Precision measurement of the hydrogen 1S-2S frequency via a 920-km fiber link.

Abstract: We have measured the frequency of the extremely narrow $1S\mathrm{\text{\ensuremath{-}}}2S$ two-photon transition in atomic hydrogen using a remote cesium fountain clock with the help of a 920 km stabilized optical fiber. With an improved detection method we obtain ${f}_{1S\mathrm{\text{\ensuremath{-}}}2S}=2466\text{ }061\text{ }413\text{ }187\text{ }018\text{ }(11)\text{ }\text{ }\mathrm{Hz}$ with a relative uncertainty of $4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$, confirming our previous measurement obtained with a local cesium clock [C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011)]. Combining these results with older measurements, we constrain the linear combinations of Lorentz boost symmetry violation parameters ${c}_{(TX)}=(3.1\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ and $0.92{c}_{(TY)}+0.40{c}_{(TZ)}=(2.6\ifmmode\pm\else\textpm\fi{}5.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ in the standard model extension framework [D. Colladay, V. A. Kosteleck\'y, Phys. Rev. D. 58, 116002 (1998)].

Vacuum cerenkov radiation in lorentz-violating theories without CPT violation

Abstract: In theories with broken Lorentz symmetry, Cerenkov radiation may be possible even in vacuum. We analyze the Cerenkov emissions that are associated with the least constrained Lorentz-violating modifications of the photon sector, calculating the threshold energy, the frequency spectrum, and the shape of the Mach cone. In order to obtain sensible results for the total power emitted, we must make use of information contained within the theory which indicates at what scale new physics must enter.

Lorentz Violation with an Antisymmetric Tensor

Abstract: Field theories with spontaneous Lorentz violation involving an antisymmetric 2-tensor are studied. A general action including nonminimal gravitational couplings is constructed, and features of the Nambu-Goldstone and massive modes are discussed. Minimal models in Minkowski spacetime exhibit dualities with Lorentz-violating vector and scalar theories. The post-Newtonian expansion for nonminimal models in Riemann spacetime involves qualitatively new features, including the absence of an isotropic limit. Certain interactions producing stable Lorentz-violating theories in Minkowski spacetime solve the renormalization-group equations in the tadpole approximation.

Gauge invariance and the Pauli-Villars regulator in Lorentz- and CPT-violating electrodynamics

Abstract: We examine the nonperturbative structure of the radiatively induced Chern-Simons term in a Lorentz- and CPT-violating modification of QED. Although the coefficient of the induced Chern-Simons term is in general undetermined, the nonperturbative theory appears to generate a definite value. However, the CPT-even radiative corrections in this same formulation of the theory generally break gauge invariance. We show that gauge invariance may yet be preserved through the use of a Pauli-Villars regulator, and, contrary to earlier expectations, this regulator does not necessarily give rise to a vanishing Chern-Simons term. Instead, two possible values of the Chern-Simons coefficient are allowed, one zero and one nonzero. This formulation of the theory therefore allows the coefficient to vanish naturally, in agreement with experimental observations.

Search for New Physics with Atoms and Molecules

Abstract: Advances in atomic physics, such as cooling and trapping of atoms and molecules and developments in frequency metrology, have added orders of magnitude to the precision of atom-based clocks and sensors. Applications extend beyond atomic physics and this article reviews using these new techniques to address important challenges in physics and to look for variations in the fundamental constants, search for interactions beyond the standard model of particle physics, and test the principles of general relativity.

Varying Constants, Gravitation and Cosmology

Abstract: Fundamental constants are a cornerstone of our physical laws. Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. Thus, it is of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy. We detail the relations between the constants, the tests of the local position invariance and of the universality of free fall. We then review the main experimental and observational constraints that have been obtained from atomic clocks, the Oklo phenomenon, solar system observations, meteorite dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic microwave background and big bang nucleosynthesis. At each step we describe the basics of each system, its dependence with respect to the constants, the known systematic effects and the most recent constraints that have been obtained. We then describe the main theoretical frameworks in which the low-energy constants may actually be varying and we focus on the unification mechanisms and the relations between the variation of different constants. To finish, we discuss the more speculative possibility of understanding their numerical values and the apparent fine-tuning that they confront us with.

Quantum-Spacetime Phenomenology

Abstract: I review the current status of phenomenological programs inspired by quantum-spacetime research. I stress in particular the significance of results establishing that certain data analyses provide sensitivity to effects introduced genuinely at the Planck scale. My main focus is on phenomenological programs that affect the directions taken by studies of quantum-spacetime theories.

Gravitation And Cosmology

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