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
Laser beam quality as shown in the figure. However, when a multimode beam is focused, Equation (3) becomes M cannot be determined from a single beam profile measurement. The ISO/DIS 11146 define that M should be calculated from a series of measurements as shown in the figure below. M is measured on real beams by focusing the beam with a fixed position lens of known focal length, and then measuring the characteristics of the beam waist and divergence. These measurements can be taken with a laser beam profiler. The multiple measurements ensure that the minimum beam diameter is found and enable a "curve