We review the early history of the general theory of relativity and its subsequent decline to the backwaters of physics and astronomy. We describe the renaissance of the theory during the 1960s and the renewed effort to subject it to experimental tests using laboratory experiments, the solar system, binary pulsars, and finally in 2015, gravitational waves. We then discuss future directions for experimental tests in the strong-field and dynamical regimes.

We describe the general characteristics of metric theories of gravity, and review the equations of non-gravitational physics in curved spacetime. We introduce the Strong Equivalence Principle, which generalizes the Einstein Equivalence Principle to situations where local gravitational interactions are important, and discuss why general relativity may be unique in conforming to this principle.

We discuss tests of the Strong Equivalence Principle. We derive the observable consequences of the Nordtvedt effect, a violation of the equality of acceleration of massive, self-gravitating bodies, that occurs in many alternative theories of gravity, although not in general relativity. We discuss the bounds obtained on this effect via lunar laser ranging, and via measurements of pulsar-white dwarf binary systems. We derive a number of observable consequences of preferred-frame effects in binary orbits and in the structure of self-gravitating bodies, and review the bounds that have been placed on the relevant PPN parameters by a wide range of observations.

We discuss a range of metric theories of gravity and their post-Newtonian limits. We begin with a general recipe for calculating the post-Newtonian limit in generic metric theories, and then turn to specific theories. Included are general relativity, scalar-tensor theories, vector-tensor theories (including Einstein-Aether and Khronometric theories), tensor-vector-scalar theories (including TeVeS), quadratic gravity theories (including Chern-Simons theory), and massive gravity theories. We also review the fate of theories of gravity that were featured in the first edition of this book, but that are no longer considered viable or interesting, including Whitehead’s theory and Rosen’s bimetric theory.

We begin with a historical overview of the problem of motion and gravitational radiation in general relativity, and then describe the current status of gravitational wave detection, based upon laser interferometry and pulsar timing. We discuss the properties of gravitational waves in alternative theories of gravity, including their speed and polarization states. We discuss the general method for analysing the generation of gravitational waves, primarily in compact binary systems, and discuss the results for the gravitational waveform, energy and angular momentum flux, and gravitational radiation reaction in general relativity and scalar-tensor theories.

We develop the parametrized post-Newtonian (PPN) formalism, which encompasses the weak-field, slow-motion regime, known as the post-Newtonian limit, of a wide range of metric theories of gravity. Ten PPN parameters are introduced, whose values depend upon the theory of gravity under study. We show that general properties of metric theories of gravity may be reflected in specific values of the PPN parameters, including the presence or absence of a preferred universal frame of reference, and the presence or absence of global conservation laws for energy, momentum and angular momentum.

We discuss the foundations of general relativity and all modern gravitational theories, based on the Einstein Equivalence Principle (EEP). We show that this principle is the basis for all metric theories of gravity including general relativity, in which gravity is a consequence of spacetime geometry. We review experimental test of the three pieces of EEP, the Weak Equivalence Principle, Local Lorentz Invariance and Local Position Invariance, and describe a number of general theoretical frameworks used to analyse EEP and its consequences.

We describe experimental tests of the effects of spinning bodies, which include precessions of spins as well as orbital perturbations. We give a technical and historical review of Gravity Probe B, a space experiment to measure the precession of orbiting gyroscopes, and the LAGEOS measurements of orbital perturbations, induced by the spinning Earth. We review experimental tests of post-Newtonian conservation laws, and the bounds on the relevant PPN parameters.

We describe tests of gravitational theory in the strong-field and dynamical regimes. Beginning with binary pulsars, we carry out an arrival-time analysis that reveals the relativistic effects on the time of arrival of radio pulses measured by an observer. We then describe the Hulse-Taylor binary pulsar, the double pulsar, a number of pulsars with white-dwarf companions, and the pulsar in a triple system, and describe the tests of gravitational theories that have been carried out using them. We describe the inspiral of compact binaries in general relativity and scalar-tensor theories, and the tests of gravitational theory that have been carried out using data from the gravitational wave detections of 2015 - 2017. We discuss future tests of general relativity in the strong-field regime, including tests using observations of stars orbiting the black hole at the center of the galaxy, tests involving accretion of matter onto black holes and neutron stars, and cosmological tests.