The relative gravitational acceleration of nearby free particles reveals the presence of gravity. In general relativity, gravity is not thought of as a force. A neutron star is the burnt out corpse of a star. A pulsar is a neutron star with an intense magnetic field. As the neutron star rotates, beams of radio waves emanating from its N and S magnetic poles sweep around the sky.
Pulsars rival the world's ensemble of atomic clocks in timekeeping stability.
Evidently they are very old, with magnetic fields and radio emissions weakened by eons of decay, yet with ultra short rotation periods, in the 1-100 ms range. Because the magnetic stresses and radiative energy losses are so low, they rotate extremely steadily. These neutron stars rotate furiously around each other in an orbit not much bigger than the Sun's diameter, with a period of eight hours. As the pulsar swings around its orbit, the pulse arrival rate increases and decreases owing to the Doppler shift, and the apparent pulsar time periodically advances and falls back. According to general relativity, Mercury's perihelion should precess at a rate of 43 seconds of arc every century.