EXPERIMENTAL TESTS OF GENERAL RELATIVITY

 

Selection of Viewgraphs

Testing Scalar-Tensor Gravity

We studied the motion and gravitational-wave generation from compact binary systems in a general class of massless scalar-tensor theories, using the formalism of the Direct Integration of the Relaxed Einstein Equations (DIRE) adapted to scalar-tensor theory, coupled with an approach pioneered by Eardley for incorporating the internal gravity of compact, self-gravitating bodies. We calculated the explicit equations of motion for non-spinning compact objects through 2.5 post-Newtonian (PN) order, including the 1.5 PN and 2.5 PN contributions to gravitational radiation reaction, the former corresponding to the effects of dipole gravitational radiation (Mirshekari and Will, 2013). For binary black holes we showed that the motion through 2.5 PN order is observationally identical to that predicted by general relativity. For mixed black-hole neutron-star binary systems, the motion deviates from that of general relativity by terms that depend on a single parameter involving the scalar-tensor coupling constant and the structure of the neutron star. We then obtained the tensor contribution to the gravitational waveform through 2 PN order, and the scalar contribution to 1.5 PN order, as well as the energy flux to 1 PN order beyond the GR quadrupole approximation (Lang 2014, 2015). For binary black holes, the gravitational radiation is also observationally identical to that predicted by general relativity. With Anna Heffernan and Ryan Lang we are working to complete the energy flux through 2 PN order.

Testing the "No-hair" Theorems with the Galactic Center Black Hole SgrA*

We showed that future high-precision observations of the orbits of stars very close to the galactic center black hole could test whether the hole's quadrupole moment matches the no-hair requirement that Q=-J^2/M (Will 2008). Whether such a test is feasible depends in part on whether perturbations of the orbit of a target star due to other stars in the central cluster will mask the relativistic effects. We studied this using both numerical simulations (Merritt, Alexander, Mikkola and Will 2009) and semi-analytic orbit perturbation theory (Sadegian and Will, 2011), and showed that, given reasonable assumptions about the distribution and number of stars in the central cluster, and for a target star within a few tenths of a milliparsec of the black hole, the relativistic precessions would be larger than those induced by stars.


Tests of the Weak Equivalence Principle
Tests of Local Lorentz Invariance
Tests of Local Position Invariance
Bounds on the PPN parameter gamma
Summary of Bounds on the PPN parameters
Bounds on variations of fundamental constants

Bounding the Mass of the Graviton

We studied the possibility of placing a bound on the mass of the graviton using gravitational-wave data from inspiralling compact binaries (Will 1998). The potential bounds were refined by incorporating better LISA noise curves, the effects of spins and the contributions of higher harmonics in the waveforms [see Will and Yunes (2004), Berti, Buonanno and Will (2005a,2005b), Arun and Will (2009), Stavridis and Will (2009)]. This idea came to fruition in 2015 with the first detection of gravitational waves by LIGO, where they placed a lower bound of 10^13 km on the graviton Compton wavelength!

The Multiple Deaths of Whitehead's Theory of Gravity

In a jab at some philosophers of science, who continue to love Whitehead's theory, despite evidence presented in 1971 that it violates experiment, Gary Gibbons and I (2006) became serial killers, and showed that the theory violates 5 different kinds of experiments.

 
Review Articles

The Confrontation between General Relativity and Experiment. A Living Review (here)

Was Einstein right? A centenary assessment (here)

The 1919 measurement of the deflection of light (here)

Resource Letter on Precision Tests of Gravity (here)

 

Other Review articles:

  • Ingrid Stairs on Binary Pulsars (here)
  • David Mattingly on tests of Lorentz Invariance(here)
  • Jean-Phillippe Uzan on Varying Constants (here)
  • Dimitrios Psaltis on Strong-Field tests of GR (here)
  • Neil Ashby on Relativity in GPS (here)
  • Benoît Famaey and Stacy McGaugh on MOND (here)
  • Claudia de Rham on Massive Gravity (here)
  • Nicolás Yunes and Xavier Siemens on Gravitational-Wave tests of GR (ground based and pulsar timing) (here)
  • Jonathan Gair et al on Gravitational-Wave tests of GR (space based) (here)
  • Stephen Merkowitz on Lunar laser ranging tests of GR (here)
  • Slava Turyshev and Viktor Toth on the Pioneer Anomaly (here)
  • Antonio De Felice and Shinji Tsujikawa on f(R) theories (here)

 


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