Lecture 29 - The Gravitational Radiation

On 25 November 1915, Einstein published the gravitational field equations of the General Theory of Relativity (GTR). Almost exactly a century later, on 14 September 2015 came the first direct detection of gravitational waves, made by an extremely sensitive instrument called the LIGO (Laser Interferometer Gravitational-Wave Observatory), providing the most direct proof of GTR. The observed gravitational waves were generated by the merger of two massive Black Holes some 1.3 billion years ago!

This lecture is about the recent detections of gravitational waves. Just as Maxwell's Theory of Electromagnetism predicted the existence of transverse electromagnetic waves, Einstein's General Theory of Relativity predicted the existence of ``Gravitational Radiation". Although this should have been regarded as a `straightforward conclusion' of any `relativistic theory of gravitation', serious doubts about their existence persisted for more than forty years, with Einstein himself being one of the `doubters'. These doubts were finally laid to rest by a series of remarkable papers published in the 1950s and early 1960s. The first observational evidence for gravitational radiation came in the late 1970s. The shrinking of the orbit of the Hulse-Taylor binary pulsar agreed precisely with the prediction from General Relativity. Meanwhile, heroic efforts were underway to build a Laser Interferometer that would actually detect gravitational waves. This was an incredible symbiosis between physics, engineering and technology. On 14 September 2015, almost exactly a hundred years after Einstein's paper, the laser interferometer detected a burst of gravitational waves from two coalescing black holes.