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
External beam radiotherapy treatment. This means that rather than radiation entering the patient through only a small number of fixed angles, it can enter through many angles. This can be beneficial for some treatment sites where the target volume is surrounded by a number of organs which must be spared radiation dose. The intensity of the X-rays produced in an megavoltage linac is much higher in the centre of the beam compared to the edge. To counteract this a flattening filter is used. A flattening filter is a cone of metal (typically tungsten); after the X-ray beam has passed through the flattening filter