The status of experimental tests of general relativity and of theoretical frameworks for analyzing them is reviewed and updated. Einstein’s equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

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The Confrontation Between General Relativity and Experiment

Quantum mechanics

We summarize our current knowledge of neutron-star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the past few years, leading to a large number of mass measurements. These discoveries show that the neutron-star mass distribution is much wider than previously thought, with three known pulsars now firmly in the 1.9–2.0-M⊙ mass range. For radii, large, high-quality data sets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 10–11.5-km range and shrinking their uncertainties, owing to a better understanding of the sources of systematic errors. The combination of the massive-neutron-star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk p...

Masses, Radii, and the Equation of State of Neutron Stars

One century after its formulation, Einstein's general relativity (GR) has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that GR should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of GR. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einstein's theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

/pdf/testing-general-relativity-with-present-and-future-3ik25z2kf9.pdf

Testing general relativity with present and future astrophysical observations

We summarize our current knowledge of neutron star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the last few years, leading to a large number of mass measurements. These discoveries show that the neutron star mass distribution is much wider than previously thought, with 3 known pulsars now firmly in the 1.9-2.0 Msun mass range. For radii, large, high quality datasets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 9.9-11.2 km range and shrinking their uncertainties due to a better understanding of the sources of systematic errors. The combination of the massive neutron star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk properties of cold nuclear matter at densities higher than that of the atomic nucleus, a major unsolved problem in modern physics.

Masses, Radii, and Equation of State of Neutron Stars

Precision timing and multiwavelength observations of a millisecond pulsar in a triple system show that the gravitational interactions between the bodies are strong; this allows the mass of each body to be determined accurately and means that the triple system will provide precise tests of the strong equivalence principle of general relativity. Millisecond pulsars act as high-precision celestial clocks, and astronomers can use them to test aspects of basic physics and astrophysics. A triple system containing a radio pulsar could provide measurements of the interior structures of the bodies and a test of theories of gravity, but the only previously known system with a millisecond pulsar shows only weak interactions. Scott Ransom et al. report precision timing and multiwavelength observations of a unique object, the millisecond pulsar PSR J0337+1715, in orbit with two white dwarf companions. Strong gravitational interactions are apparent in this triple system, making it possible to estimate the masses of the pulsar and the two white dwarf companions, as well as the inclinations of the orbits. The surprisingly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from known stellar systems. Gravitationally bound three-body systems have been studied for hundreds of years1,2 and are common in our Galaxy3,4. They show complex orbital interactions, which can constrain the compositions, masses and interior structures of the bodies5 and test theories of gravity6, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs 7, 8; with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13)
 , where is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15)
 and 0.4101(3)
 ), as well as the inclinations of the orbits (both about 39.2°). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity.

A millisecond pulsar in a stellar triple system

(with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of sev-eral decades), shows only weak interactions. Here we report precision timing and multi-wavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triplesystem with two other stars. Strong gravitational interactions are apparent and providethe masses of the pulsar (1.4378(13)M , where M is the solar mass and the parenthesescontain the uncertainty in the ﬁnal decimal places) and the two white dwarf companions(0.19751(15)M and 0.4101(3)M ), as well as the inclinations of the orbits (both ˘39.2

A. Schechtman-Rook

Papers

A millisecond pulsar in a stellar triple system

A Millisecond Pulsar in a Stellar Triple System