Quentin's Research Interests

Attention Space Physics students with an interest in theory! I have several research projects related to my research area which I have designed with the Space Physics curriculum in mind. Anyone interested is encouraged to come and talk to me (email:baileyq@erau.edu).

I am interested in foundational tests of our best current fundamental theories of physics, the theory of gravity, General Relativity, and the Standard Model of particle physics. So far, I have been studying the theoretical and experimental aspects of testing the Einstein Equivalence Principle, a foundation of General Relativity. In particular I have focused on tests of Lorentz symmetry, the spacetime symmetry of Special Relativity, and also the discrete spacetime symmetry called CPT. The motivation for this work is twofold. First, spacetime symmetries are cornerstone of modern physics. As such, it should be an experimental precedent to test these principle in as many ways as possible. Second, recent work on fundamental theories of physics, that attempt to unify the Standard Model of particle physics and General Relativity, has pointed to the possibility of deviations from perfect spacetime symmetry. In the ongoing search for new physics in turns out that high-precision, typically low-energy tests of Lorentz and CPT symmetry offer a promising alternative to conventional high-energy accelerator experiments.

This area of research has grown considerably in the past decades and there is now a dedicated research center at Indiana University called the Indiana University Center for Spacetime Symmetries (IUCSS).

My own work, in the publications below, has been on the electromagnetic, nuclear, and gravitational sectors of a general effective field theory framework for testing Lorentz and CPT symmetry called the Standard-Model Extension (SME). Information about the SME theoretical and experimental program, including links to refereed journal articles, books, and popular magazine articles, can be found at this link: Information about CPT and Lorentz Violation.


In the SME formalism, Lorentz violation for a given particle type (species) is described by its coefficients for Lorentz violation. In certain special cases, we can visualize these coefficients as a background field of arrows, pointing in some direction, that affects our measuring apparatus (rods and clocks) as they move or rotate through the background. This is illustrated in the animation above for blue and green rods and clocks. As the two sets of rods and clocks rotate their relative lengths and ticking rates will change if Lorentz symmetry is violated. If deviations from perfect Lorentz symmetry occur in nature, they must be miniscule. This implies that the best method for finding Lorentz violation is to use the most sensitive "rods" and "clocks" available with today's technology.

In practice, a variety of real physical systems can be used as effective rods and clocks. For example, some of the systems that have been used to test Lorentz symmetry include hydrogen atoms, cesium atoms, torsion pendula, superconducting gravimeters, electromagnetic resonant cavities, the Earth-Moon system, and even distant light propagating from the early universe.

For weak gravitational fields, there are nine independent coefficients for Lorentz violation in the pure-gravity sector of the (minimal) SME. These coefficients would vanish in the limit that (local) Lorentz symmetry holds for gravity. Searches for nonzero gravity coefficients include a variety of laboratory experiments, solar-system observations, and beyond. For example, analysis of lunar laser ranging data can place stringent constraints on these coefficients. In the figure below, Lorentz violation (arrows) could cause the Moon to deviate from its usual elliptical path.

Kostelecky and Tasson have analyzed matter-gravity couplings in the SME framework. Their work reveals new types of unexplored signals for Lorentz violation in gravitational tests:

  • Matter-gravity couplings and Lorentz violation, Alan Kostelecky and Jay Tasson link
  • Prospects for Large Relativity Violations in Matter-Gravity Couplings, Alan Kostelecky and Jay Tasson link
  • More recently, Bailey, Kostelecky, Mewes, Tasson and Xu have studied the gravity sector nonminimal SME in a series of publications. This introduces a large number of coefficients classified by the mass dimension of the operators appearing in the lagrangian. These coefficients can be measured in precision short-range gravity tests as well as gravitational wave measurements. The publications on the nonminimal gravitational SME include:

  • Short-range gravity and Lorentz violation, (with V.A. Kostelecky and Rui Xu), Phys. Rev. D 91 , 022006 (2015) link
  • Constraints on Lorentz Violation from Gravitational Cherenkov Radiation, Phys. Lett. B 749, 551 (2015) link
  • Testing local Lorentz invariance with gravitational waves, Phys. Lett. B 757, 510 (2016) link
  • There are nineteen independent coefficients for Lorentz violation in the "minimal" version of the photon sector of the SME. Astrophysical observations and laboratory resonant-cavity tests have probed for these photon coefficients. Lorentz violation could also affect other known particles, such as neutrinos, and many experiments have already been performed.

    So far, no statistically convincing evidence exists that any coefficients for Lorentz violation are nonzero. However, future experiments may dramatically improve existing sensitivities, and may yet discover miniscule Lorentz violation.

    Gravity sector experimental/observational analyses using the SME framework

    Other SME experiments

    A summary of the current experimental constraints on the many coefficients for Lorentz violation in the SME can be found here.

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