Superconducting-Gravimeter Tests of Local Lorentz Invariance

Abstract

Superconducting-gravimeter measurements are used to test the local Lorentz invariance of the gravitational interaction and of matter-gravity couplings. The best laboratory sensitivities to date are achieved via a maximum-reach analysis for 13 Lorentz-violating operators, with some improvements exceeding an order of magnitude.Local Lorentz invariance is among the foundational building blocks of General Relativity (GR). Though GR provides an impressive description of the wide variety of gravitational phenomena, standard lore holds that GR may be the low-energy limit of an underlying theory that merges gravitation and quantum physics, such as string theory. Local Lorentz violation may arise in such an underlying framework [1]. Hence tests of local Lorentz invariance probe the core construction of GR and may provide clues about the structure of new physics at the quantum-gravity scale. These ideas triggered the development of a comprehensive effective field theory based framework [2,3] for testing Lorentz symmetry used in many modern searches for violations [4].Superconducting gravimeters [5] have generated a vast amount of information about the gravitational field of the Earth. Devices functioning at over 2 dozen locations around the globe generate data at minute intervals for the Global Geodynamics Project (GGP) [6]. In some cases measurements span more than a decade, and sensitivities to local variations in the gravitational field approaching parts in 10 12 can be extracted for variations with periods on the order of a day. Stability at the level of parts in 10 9 per year [5] has also been achieved. Though the primary use of the data is in geophysical applications, the nature of the data clearly also depends on the foundational theories of physics. Hence these data sets provide opportunities to test fundamental physics [7,8]. The search for preferred frame effects in gravitational physics, a particular Lorentz-symmetry violating scenario, was perhaps the first application of superconducting gravimeters to tests of foundational theory [9].In the 4 decades since those early tests, interest in Lorentz violation has surged [10], as have theoretical and experimental developments [11][12][13]. In addition to the search for preferred frame effects as a signal of alternatives to GR [14], more general types of Lorentz violation are now actively sought as a possible signal of new physics at the Planck scale [4]. Though performing Planck-scale experiments directly will likely remain infeasible for the foreseeable future, experimental information about the nature of the underlying theory can be attained by searching for tiny Planck-suppressed effects in experiments at presently accessible energies. Lorentz violation provides a useful candidate Planck-suppressed effect [1], and the gravitational Standard-Model Extension (SME) provides a field-theory based framework for organizing a systematic search [2,3,15]. While sensitivities to SME coefficients for Lorentz violation have been achieved in a variety of gravitational systems...

Limits on Lorentz violation in gravity from worldwide superconducting gravimeters

“…III. As we see, our limits for the space-space components of Lorentz violation are up to the level of 10 −10 , one order of magnitude smaller than that of atom interferometry [11], and similar to the single superconducting gravimeter result listed in [13]. And more than precision, we have also resolved the accuracy issue caused by inaccurate tidal model used at the precision of superconducting gravimeters.…”

“…The underlying idea is simple: if the force of gravity is anisotropic, then the local acceleration of free fall on the rotating earth should exhibit a modulation correlated with the earth's rotation. In analyzing such tests, the influence of the sun, the moon and the planets have to be taken out, which is done by subtracting a "tidal model" describing of these influences.However, a persisting problem has been pointed out in previous works [11][12][13] whether a simple first-principles solid-earth tidal model or a more sophisticated empirical model should be used. The simple first-principles solidearth tidal model does not include any Lorentz violation signal.…”

“…But it's not accurate enough beyond 10 −10 g with- * E-mail:zkhu@mail.hust.edu.cn † E-mail:hm@berkeley.edu out including local tidal effects like ocean tides. At the precision of superconducting gravimeters, it may produce fake Lorentz violating signals [13]. Sophisticated empirical models are a lot more accurate, but it's based on fitting of gravity measurement which itself may contain Lorentz-violating signals.…”

“…Competitive bounds in this sector have been established by various experiments and observations [24,25], such as gravimetry [11][12][13], lunar laser ranging [14][15][16] and astrophysics observations [17,18]. Among these, local gravimetry is the one of the easy-to-access and very precise ground-based method.…”

“…However, a persisting problem has been pointed out in previous works [11][12][13] whether a simple first-principles solid-earth tidal model or a more sophisticated empirical model should be used. The simple first-principles solidearth tidal model does not include any Lorentz violation signal.…”