ABSTRACT
This chapter is about the gravitational interaction. It begins by discussing orbits and Kepler’s laws of planetary motion, showing that orbits take the form of conic sections and are most commonly elliptical in shape, characterized by their eccentricity. Newton’s law of gravity is described and the difference between mass and weight is explained. Einstein’s theory of general relativity is shown as a reinterpretation of gravity, not as a force but as distortions in spacetime caused by massive objects. Various tests of general relativity are described, including the anomalous precession of Mercury’s orbit, bending of light observed during a total solar eclipse, and gravitational lensing in general. Microlensing as a means of detecting exoplanets is mentioned, and experiments to measure gravitational redshift are described. Gravitational radiation arises due to the acceleration of massive objects, and its indirect detection from changes in the orbits of binary radio pulsars is described. The direct detection of gravitational waves from merging black holes by the LIGO and Virgo detectors is discussed. Finally, the concept of a quantum theory of gravity is considered, mediated by hypothetical particles called gravitons.
