In our universe, gravitational waves are produced in many different ways. Almost every occasion in which masses are accelerated leads to the generation of travelling space distortions, be it two heavenly bodies orbiting one another or stellar matter jettisoned into space in a gigantic explosion.
However, all of the gravitational waves that reach us from the depths of space are very weak, since as such a wave propagates away from its source, it spreads out farther and farther, and the distortions get weaker and weaker. In order for our detectors to measure a gravitational wave, on the other hand, it must be comparatively strong (and that will be the case only for waves generated in some of the most violent situations our universe has to offer).
Promising situations include two orbiting neutron stars, or a neutron star orbiting a black hole, or even two black holes orbiting one another. Such objects (which will be described further in the following chapter, Black holes & Co.) are extremely compact (i.e. for objects of their mass, they are of extremely small size). It is this compactness that makes these binaries excellent sources for strong gravitational waves.
While gravitational waves have not been directly detected so far, there is strong indirect evidence. The smoking gun is a system of orbiting neutron stars with the catchy name PSR1913+16. Einstein's theory predicts that gravitational waves carry away energy. For a system of orbiting stars, such a decrease in total energy leads to an ever faster and closer orbit. Over decades, radio astronomers have monitored the time that it takes the stars of PSR1913+16 to complete each successive orbit, and lo and behold: this orbital period decreases over time exactly as predicted by general relativity. This is strong evidence that the speed-up is indeed due to the radiation of gravitational waves, and the reason Russell Hulse and Joseph Taylor were awarded the Nobel prize for physics for the year 1993.
With orbiting objects drawing nearer and nearer (as in the case of PSR1913+16), the end is inevitable: There will be a collision, and if neutron stars or black hole collide, a huge amount of energy is radiated away in the form of gravitational waves. The following simulation by scientists of the Max-Planck-Institute for Gravitational Physics shows the spatial distortions effected by these waves as expanding, coloured regions. In this example, the collision and merger involves two black holes.
[Image: W. Benger AEI/ZIB. Animation size 93 kB; please
allow time for loading.]
Supernovae (violent explosions of dying stars, in which huge amounts of energy are freed and huge amounts of matter ejected into space) are also promising gravitational wave sources.