Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session B13: Tests of General Relativity and Gravitation |
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Sponsoring Units: GGR Chair: Steven Carlip, University of California, Davis Room: Key 9 |
Saturday, April 11, 2015 10:45AM - 10:57AM |
B13.00001: Improved Measurements of Short Term Variations in the Earth's Mass Distribution Peter L. Bender The first use of high-precision laser interferometry between satellites is planned for the NASA-DLR GRACE Follow-On mission, that is expected to be launched in 2017. The GRACE mission has successfully monitored changes in the Earth's mass distribution since 2002, with very important applications in hydrology and other areas of earth science. It is based on K-band measurements of changes in the separation of two satellites about 200 km apart in the same nearly polar orbit. On GRACE Follow-On a laser interferometry system between the satellites will be added in parallel to the K-band system to improve the relative velocity precision to 50 nm/s. However, studies are still going on to understand better how much of an improvement can be expected in measuring changes in the mass distribution. One particular analysis approach called Ocean Calibration that has been suggested but not yet tested will be described briefly. The problem is that it takes 10 to 15 days for the satellite ground-tracks to cover the globe with close enough spacing to match the desired spatial resolution, but the mass distribution changes more rapidly. This can be corrected for to some extent using partial information from other sources on the mass distribution changes. However, it remains the main limitation on the accuracy of the final results. [Preview Abstract] |
Saturday, April 11, 2015 10:57AM - 11:09AM |
B13.00002: Inflight alignment and calibration of SR-POEM Robert Reasenberg, James Phillips We are developing SR-POEM, a payload for detecting a possible violation of the weak equivalence principle (WEP) while on the free-fall trajectory of a sounding-rocket payload. We estimate an uncertainty of $\sigma (\eta ) \leq 10^{-17}$ from a single flight. The experiment consists of calibration maneuvers plus eight 120 s drops of the two test masses (TMs). The instrument orientation will be reversed between successive drops, which reverses the signal but leaves most systematic errors unchanged. The TMs are unconstrained during drops. They are surrounded by capacitance plates that allow both measurement and control of TM position and orientation with respect to the physics package. The calibration maneuvers are carried out both before and after the series of drops. The pre-drop calibration maneuvers define the ``centering'' of the TMs, nominally at the payload center of mass. Each of the eight drops then starts with nonmoving TMs precisely at this location. The post-drop calibration maneuvers detect changes in the location of the payload center of mass and serve to bound the corresponding contribution to systematic error. [Preview Abstract] |
Saturday, April 11, 2015 11:09AM - 11:21AM |
B13.00003: Ultra-sensitive force measurement using optically levitated microspheres Alexander Rider, David Moore, Giorgio Gratta We have demonstrated a novel technique for measuring microscopic forces acting on optically levitated dielectric microspheres. The radiation field at the focus of a laser beam is used to levitate a microsphere in a harmonic trap where its displacement can be determined by the pattern of scattered light. Optical levitation isolates the microsphere from the surrounding environment at high vacuum, making thermal noise negligible. We have demonstrated a preliminary sensitivity of $ 5 \times 10^{-17} N Hz^{-1/2}$ for forces acting on $5\mu m$ microspheres and expect to be able to improve this by several orders of magnitude once non-fundamental sources of noise are eliminated. The electric charge of a microsphere can be determined by applying an electric field and measuring the resulting force. We have demonstrated the ability to discharge the microspheres with single electron precision, which eliminates the most significant electrostatic backgrounds from force measurements. As a demonstration of this technique we have searched for the presence of unknown charged particles with charge $>5 \times 10^{-5}e$ bound in our microspheres. Here we discuss the apparatus, the charged particle search, and outline our plans for future measurements including gravity at $\mu m$ length scales. [Preview Abstract] |
Saturday, April 11, 2015 11:21AM - 11:33AM |
B13.00004: Spontaneous Scalarization: Dead or Alive? Emanuele Berti, Luis Crispino, Davide Gerosa, Leonardo Gualtieri, Michael Horbatsch, Caio Macedo, Hector Okada da Silva, Paolo Pani, Hajime Sotani, Ulrich Sperhake In 1993, Damour and Esposito-Farese showed that a wide class of scalar-tensor theories can pass weak-field gravitational tests and exhibit nonperturbative strong-field deviations away from General Relativity in systems involving neutron stars. These deviations are possible in the presence of ``spontaneous scalarization,'' a phase transition similar in nature to spontaneous magnetization in ferromagnets. More than twenty years after the original proposal, binary pulsar experiments have severely constrained the possibility of spontaneous scalarization occurring in nature. I will show that these experimental constraints have important implications for the torsional oscillation frequencies of neutron stars and for the so-called ``I-Love-Q'' relations in scalar-tensor theories. I will also argue that there is still hope to observe strong scalarization effects, despite the strong experimental bounds on the original mechanism. In particular, I will discuss two mechanisms that could produce strong scalarization in neutron stars: anisotropy and multiscalarization. [Preview Abstract] |
Saturday, April 11, 2015 11:33AM - 11:45AM |
B13.00005: Projected Constraints on Lorentz-Violating Gravity with Gravitational Waves Devin Hansen, Nicolas Yunes, Kent Yagi Gravitational waves are excellent tools to probe the foundations of General Relativity in the strongly dynamical and non-linear regime. In this talk I will consider one such foundation, Lorentz symmetry, which can be broken in the gravitational sector by the existence of a preferred time direction, and thus, a preferred frame at each spacetime point. This leads to a modification in the orbital decay rate of binary systems, and also in the generation and chirping of their associated gravitational waves. I will examine whether waves emitted in the late, quasi-circular inspiral of non-spinning, neutron star binaries can place competitive constraints a proxy of gravitational Lorentz-violation: Einstein-\AE{}ther theory. I will show that a gravitational wave detection consistent with General Relativity can only place competitive constraints on gravitational Lorentz violation when using future, third-generation or space-based instruments. I will also show that a single electromagnetic counterpart to a gravitational wave detection is enough to place constraints that are 10 orders of magnitude more stringent than current binary pulsar bounds, even when using second-generation detectors. [Preview Abstract] |
Saturday, April 11, 2015 11:45AM - 11:57AM |
B13.00006: Short-Range Test for Lorentz Violation in Gravity Rui Xu, Quentin G. Bailey, V. Alan Kosteleck\'y Lorentz symmetry is an essential property of modern physics. However, some candidate theories of quantum gravity have solutions that break Lorentz symmetry, and signals may be detectable in current experiments. By investigating in the general effective field theory for gravity, called the gravitational Standard-Model Extension, we find that short-range experiments can detect Lorentz violation. This talk discusses corrections from Lorentz violation to the Newtonian gravitational force and the corresponding effects in short-range experiments, including recent measurements. [Preview Abstract] |
Saturday, April 11, 2015 11:57AM - 12:09PM |
B13.00007: Projected Constraints on Scalarization with Gravitational Waves from Neutron Star Binaries Laura Sampson, Nicolas Yunes, Neil Cornish, Marcelo Ponce, Enrico Barausse, Antoine Klein, Carlos Palenzuela, Luis Lehner Certain scalar-tensor theories endow stars with scalar hair, sourced either by the star's own compactness, or by the companion's scalar charge, or by the orbital binding energy. Scalarized stars in binaries have different conservative dynamics than in General Relativity, and can excite a scalar mode in the metric perturbation that carries away dipolar radiation. As a result, the binary orbit shrinks faster than predicted in General Relativity, modifying the rate of decay of the orbital period. Scalar-tensor theories can pass existing binary pulsar tests, because observed pulsars may not be compact enough or sufficiently orbitally bound to activate scalarization. Gravitational waves emitted during the last stages of compact binary inspirals are thus ideal probes of scalarization effects. In the work presented here, we analyze the types of constraints the gravitational wave measurements from the advanced LIGO detectors will be able to place on these types of scalar-tensor theories. [Preview Abstract] |
Saturday, April 11, 2015 12:09PM - 12:21PM |
B13.00008: Testing Einstein's theory of gravity in a millisecond pulsar triple system Anne Archibald Einstein's theory of gravity depends on a key postulate, the strong equivalence principle. This principle says, among other things, that all objects fall the same way, even objects with strong self-gravity. Almost every metric theory of gravity other than Einstein's general relativity violates the strong equivalence principle at some level. While the weak equivalence principle --- for objects with negligible self-gravity --- has been tested in the laboratory, the strong equivalence principle requires astrophysical tests. Lunar laser ranging provides the best current tests by measuring whether the Earth and the Moon fall the same way in the gravitational field of the Sun. These tests are limited by the weak self-gravity of the Earth: the gravitational binding energy (over $c^2$) over the mass is only $4.6\times 10^{-10}$. By contrast, for neutron stars this same ratio is expected to be roughly $0.1$. Thus the recently-discovered system PSR J0337+17, a hierarchical triple consisting of a millisecond pulsar and two white dwarfs, offers the possibility of a test of the strong equivalence principle that is more sensitive by a factor of 20 to 100 than the best existing test. I will describe our observations of this system and our progress towards such a test. [Preview Abstract] |
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