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13.4Waves extension
Gravitational waves
Einstein presented his general theory of relativity in 1915. This was a revolutionary theory that replaced Newtonâs theory of gravitation and predicts and explains new phenomena that Newtonâs theory cannot. The basic ingredient of the theory is that the presence of matter curves the space in which the matter finds itself. One prediction of the theory is that a new kind of wave should exist: gravitational or gravity waves. (The history around gravity waves, including Einsteinâs own involvement in it, is absolutely fascinating, and you may want to find out more about it.) These waves would be generated whenever very large masses such as neutron stars and black holes collide with each other. Like electromagnetic waves, gravity waves are transverse and travel at the speed of light. The passage of a gravity wave through space distorts space, stretching and compressing it. So if a gravity wave were to travel into the plane of this paper where a ring is placed, the ring would oscillate as shown in Figure 13.28.

Figure 13.28: A gravity wave moving into the page would make a ring oscillate as shown.

The scale in the diagram is greatly exaggerated. The displacement in either direction for a 1 m diameter ring is typically of the order of 10â21 m! This explains why it has been so difficult to detect these waves. Success came in 2015 by LIGO (the Laser Interferometry Gravitational wave Observatory) in the United States. The group detected gravity waves that originated in the collision of two black holes of mass 29 and 36 solar masses some 1.3 billion light years away. Indirect evidence for gravity waves was observed earlier, in 1974, by Hulse and Taylor in the study of a binary pulsar star system: the two stars orbiting each other were losing energy by radiating gravity waves and so their period of revolution around each other was changing; this was measured. The change in period was consistent with gravity waves being emitted.
Matter waves
Quantum theory demands that all particles have wave properties. The waves corresponding to particles are called matter waves. These are waves of probability: the displacement of the wave is related to the probability of finding a particle near a particular position. Details about these waves can be found in ChaptersÂ 21â25.