Session R16: Tests of General Relativity and Gravitation

Sensitivity Considerations for a Short-range Test of the Gravitational Inverse-square Law

The gravitational Inverse-Square Law (ISL) has been verified from infinity down to the 0.1 mm regime. Several theoretical scenarios predict possible violations of the ISL at short distances. At HSU we are developing an experiment that will test gravitational interactions below 50 microns. The experiment will be approximately null by using a stepped torsion pendulum and a large attractor plate. Hence, in the approximation that the attractor mass is an infinite sheet of matter, the Newtonian gravitational force is independent of separation distance between the pendulum and attractor. The experiment will measure the torque applied to the pendulum as the attractor mass is oscillated nearby. The size and distance dependence of the torque variation will provide a means to determine any deviations from the ISL at untested scales. The mass distribution of the pendulum and attractor determine the sensitivity of the experiment. This talk will focus on the investigation of the ISL and the experimental sensitivity. Gauss' Law of Gravitation, the infinite plane approximation, Yukawa potential, and Newtonian vs. Yukawa torque will be discussed.   

Tests of the Weak Equivalence Principal Below Fifty Microns

Due to the incompatibility of the Standard Model and General Relativity, tests of gravity remain at the forefront of experimental physics research. The Weak Equivalence Principle (WEP), which states that in a uniform gravitational field all objects fall with the same acceleration regardless of composition, total mass, or structure, is fundamentally the result of the equality of inertial mass and gravitational mass. The WEP has been effectively studied since the time of Galileo, and is a central feature of General Relativity; its violation at any length scale would bring into question fundamental aspects of the current model of gravitational physics. A variety of scenarios predict possible mechanisms that could result in a violation of the WEP. The Humboldt State University Gravitational Physics Laboratory is using a torsion pendulum with equal masses of different materials (a ``composition dipole'' configuration) to determine whether the WEP holds below the 50-micron distance scale. The experiment will measure the twist of a torsion pendulum as an attractor mass is oscillated nearby in a parallel-plate configuration, providing a time varying torque on the pendulum. The size and distance dependence of the torque variation will provide means to determine deviations from accepted models of gravity on untested distance scales.   

A Sounding Rocket Payload to Test the Weak Equivalence Principle

We are developing SR-POEM, a payload for detecting a possible violation of the weak equivalence principle (WEP) while on a sounding rocket's free-fall trajectory. 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. Each TM comprises three bars and a Y-shaped connector. The six bars are in a hexagonal housing and stand in a plane perpendicular to the symmetry axis (Z axis) of the payload and close to its CM. At a distance of 0.3 m along the Z axis, there is a highly stable plate that holds six of our tracking frequency laser gauges (TFGs), which measure the distances to the bars. The TMs are surrounded by capacitance plates, which allow both measurement and control of TM position and orientation. A central theme of the design is the prevention and correction of systematic error. Temperature stability of the instrument is essential and, during the brief night-time flight, it is achieved passively.   

Error Budget for SR-POEM, a Test of the Weak Equivalence Principle

SR-POEM is a test of the weak equivalence principle (WEP) using free fall provided by a sounding rocket. The differential motion of two test masses (TMs) will be measured during eight drops of 120 s each to reach the planned accuracy, $\sigma (\eta ) \leq 10^{-17}$ . During each drop, the payload is inertially oriented. Payload inversions between each pair of drops are a central tool in the control of systematic error. Another key tool is the rapid measurement enabled by our Tracking Frequency laser Gauge (TFG). This is a unique advantage of SR-POEM over other planned missions. The TFG will measure the length of an SR-POEM resonant cavity to 0.1 pm in 1 s. The rapid measurement allows superior thermal control by inexpensive, passive means. It also allows the TMs to be unconstrained, eliminating both systematic error and noise due to constraints or springs. The sounding rocket reduces mission cost and has a near-vertical trajectory, which reduces Coriolis error. We discuss the errors due to distance measurement, Coriolis and related pseudo-accelerations, gravity, electric fields, magnetic fields, gas, and radiation pressure.   

Astrophysical Limits on Superluminal Electron and Neutrino Velocities

The observation of two PeV-scale neutrino events reported by Ice Cube allows one to place constraints on Lorentz invariance violation (LIV) in the,neutrino sector. First, I derive an upper limit for \textit{the difference} between putative superluminal neutrino and electron velocities of \textless\ $\sim$5.6 x 10$^{-19}$ in units where c $=$ 1. I then derive a new, strong constraint on superluminal electron velocitiy $\delta_{\mathrm{e}}$ \textless\ 5 x 10$^{-21}$. One then obtains an upper limit on the superluminal neutrino velocity \textit{alone} of $\delta \nu $ \textless\ 5.6 x 10$^{-19}$, many orders of magnitude better than the time-of-flight constraint from the SN1987A neutrino burst. However, if the electrons are \textit{subluminal} the constraint on \textbar $\delta_{\mathrm{e}}$\textbar \textless\ 8 x 10$^{-17}$, obtained from the Crab Nebula $\gamma $-ray spectrum, places a weaker constraint on superluminal neutrino velocity of \textless\ 8 $\times$ 10$^{-17}$.   

Theoretical Suggestion of Realistic Experiment on the Earth's Orbit to Test Quantum Effects in General Relativity

We show theoretically that quantum fluctuations result in the existence of seldom events, where the equivalence between energy and passive gravitational mass is broken for the simplest composite quantum body -- a hydrogen atom [1]. We suggest to conduct experiment on the Earth's orbit, where such seldom events can be observed by measuring electromagnetic radiation, emitted from a tank of pressurized hydrogen molecules or helium atoms placed in a small spacecraft or satellite. It could be the first experiment where quantum effects would be directly observed in general relativity. \\[4pt] [1] A.G. Lebed, Cent. Eur. J. Phys., v. 11, p. 969 (2013).   

Conformal Gravity Rotation Curve of the Milky Way Galaxy

Galactic rotation curves have proven to be the testing ground for dark matter bounds in galaxies, and our own milky way is one of many large spiral galaxies that must follow the same models. Here, we present synthesis mass models of milky way rotation data and apply the fitting procedure of Conformal Gravity. We find that like the other already published 200 plus galactic rotation curves, the Milky Way galaxy follows the same trend observed that a fantastic fit can be achieved without the need for involving dark matter. Specifics of the fitting procedure will be discussed as well as comparisons to other alternative gravitational theories such as MOND.