Introduction to general relativity in a setting with comparatively strong gravitational fields. This has become possible thanks to precision observations of binary pulsars. In such a star system, two highly compact neutron stars orbit each other. At least one of them is a pulsar – an astronomical object that emits a tight beam of radiowaves. These beams strike the Earth at very regular intervals, similarly to the way that the rotating beam of a lighthouse means that an observer sees the lighthouse blink, and can be observed as a highly regular series of pulses. General relativity predicts specific deviations from the regularity of these
Neutron star explanation does not fully explain magnetic field strengths of neutron stars. The gravitational field at a neutron star's surface is about times stronger than on Earth, at around . Such a strong gravitational field acts as a gravitational lens and bends the radiation emitted by the neutron star such that parts of the normally invisible rear surface become visible. If the radius of the neutron star is 3"GM"/"c" or less, then the photons may be trapped in an orbit, thus making the whole surface of that neutron star visible "from a single vantage point", along with destabilizing photon orbits at