Session T11: Big G and Tests of Gravity

Live

Newtonian gravitational constant \textit{G} is poorly determined in both precision and accuracy (significant discrepancy between existing results) when compared to other universal constants. To improve the situation, a new torsion pendulum device is currently under construction in our lab with two major modifications. One is enlarging the scale of the system as the metrological uncertainty, the major contribution of the systematic error, decreases linearly as the scale increases. The other one is allowing the apparatus to measure \textit{G} with three different modes ({\it i}) the angular-acceleration feedback method, ({\it ii}) the time-of-swing method with large amplitudes on the pendulum for two positions of the attractor masses, and ({\it iii}) extracting the resonance frequency of the pendulum through noise measurements for two positions of the attractor masses. The idea is to address the potential Kuroda effect by comparing \textit{G} determined by different methods in the same apparatus. The progress of the apparatus construction with preliminary test results and other modifications, such as using better attractor masses, will be shown in the presentation.   

Live

The universal law of gravitation has undergone stringent tests for a long time over a significant range of length scale, from an atomic scale to a planetary scale. Of particular interest is the short distance regime, where modifications to Newtonian gravity may arise from axion-like particles and extra dimensions. We have constructed an ultra-sensitive force sensor based on optically-levitated microspheres with a force sensitivity of $10^{-17}$ N/$\sqrt{\rm Hz}$ for the purpose of investigating non-Newtonian forces in the 1-100 $\mu$m range. Microspheres interact with a variable-density attractor mass made by alternating silicon and gold segments with periodicity of 50 $\mu$m. The attractor can be located as close as 10 $\mu$m to a microsphere. I describe the characterization of this system, its sensitivity, and some preliminary results. Further technological developments to reduce background are expected to provide orders of magnitude improvement in the sensitivity, going beyond current constraints on non-Newtonian interactions.   

Live

Light-pulse atom interferometry is a versatile and powerful tool for conducting precise measurements of fundamental constants, testing general relativity, searching for signatures of new physics, and investigating quantum mechanics on a macroscopic scale. For atom interferometry, pulses of light are used to create the atom optics equivalents of beam-splitters and mirrors. Recent advances in atomic clocks have illustrated the advantages of using strontium, an alkali-earth atom over the typically used alkali atoms. We present progress toward the realization of a two-meter atomic fountain at Northwestern University and discuss the advanced atom optics techniques and noise mitigation strategies that enable large spatial separations of atomic wave packets and sensitive phase detection. Prospects for future gravitational measurements using this apparatus will also be discussed.   

Live

We explore the interplay of matter with quantum gravity with a preferred frame to highlight that the matter sector cannot be protected from the symmetry-breaking effects in the gravitational sector. Focusing on Abelian gauge fields, we show that quantum gravitational radiative corrections induce Lorentz-invariance-violating couplings for the Abelian gauge field. In particular, we discuss how such a mechanism could result in the possibility to translate observational constraints on Lorentz violation in the matter sector into strong constraints on the Lorentz-violating gravitational couplings.   

Live

There are serious inconsistencies in the current understanding of the universal gravitational constant, $G$ despite the long history of measurements. The scatter in experimental results could be an indication of the incompleteness of general relativity, the current accepted description of gravity, or due to underestimated biases in the metrology of small forces. The metrology of test and source masses, typically made of high density metals, is of prime importance. There are however, some inherent limitations in the previous evaluations of systematic uncertainties associated with them. We propose to address these by developing high density transparent materials such as $PbWO_{4}$, for use in the next generation of experiments. This is motivated by the fact that density variation in glass and single crystals are significantly smaller than in metals and can be measured nondestructively. We have developed a laser interferometer for the measurement of the internal density gradients of these masses. An independent measurement was also completed using the cold neutron beam and the grating phase neutron interferometer at NIST. The preliminary analysis and results for two 2x2x12cm$^3$ $PbWO_{4}$ samples will be shown.   

Multi-Mode Apparatus to Determine Newton's Constant G

The Newtonian gravitational constant, G, is a fundamental constant in nature not linked by any complete theories to other forces of nature. Compared to all other fundamental constants, G is known with the least precision. Over the last 200 years, its value has been repeatedly measured, and even the world's leading experiments have produced values which are incompatible with one another. Recently, two experiments have measured consistent results at the 12 ppm level. After examination of the methodology used in previous measurements, the research group at IUPUI, in collaboration with Humboldt State University and Syracuse University, will use multiple approaches to determine G within a same torsion pendulum apparatus. We expect to obtain a measurement at the 2 ppm level using these new methods. By continuing the use of a torsion pendulum apparatus, we also hope to better understand the current discrepancies among previous experimental results. This talk will explore the experimental configurations and give a current update on the optical system of the experiment.   

Optically Cooled Quantum Dots for Extreme Weak Force Sensing

Optically trapped nanoparticles placed in the quantum mechanical ground state would be a very valuable tool for ultrasensitive force detection, such as probing weak gravitational interaction. Increasing the power of the optical force trap presents a trade off between cooling the center of motion of the targeted dielectric nanoparticle and increasing its internal temperature. Therefore to attempt to address this limitation, the current project looks to trap a quantum dot while optically cooling it. The theoretical prediction is that with an additional combination of an ultrahigh vacuum environment and parametric feedback cooling of the trapping laser should allow the quantum dot to be cooled to its ground state mode of the harmonic potential optical gradient trap. Ultimately if it proves successful, the proposed scheme provides a way to further the search for non-Newtonian forces and explore other short-range interaction physics.