When in 1959, Paul Dirac lectured at the Lindau Meetings for the third time, he choose a hot topic of the day which still is a very hot topic. The title of the talk is ”Gravitational waves”, but most of the lecture actually concerns the theory of gravitation in general. As Dirac points out, after the publication of Albert Einstein’s general theory of relativity in the early 20th Century, there was a lot of interest from mathematicians, physicists and cosmologists. After some time the interest waned until a revival started in the 1950’s. The reason was the development of both mathematical and observational tools, in particular radio astronomy. As a young student of theoretical physics in the early 1960’s, I also remember the impact of satellite navigation on the interest in gravitational theories. One of my friends worked in this field and used to glue large sheets of paper together to be able to write down explicitly all the mathematical expressions needed. After an interesting introduction, Dirac’s lecture mainly consists of a rather technical mathematical derivation leading to a set of approximate equations of motion for gravitational waves. He doesn’t have to glue papares together, though, since he uses a blackboard, and you can clearly hear the scratch of the crayon. Dirac introduces the so-called Hamiltonian formalism with the stated intention that one day the gravitational theory might be fused with quantum mechanics. This is, of course, together with the true nature of dark matter and dark energy, still one of the main unsolved problems of physics. He also mentions a set of international conferences on gravitaional theory and as I remember it, from the 1960’s and on there were attempts to detect gravitational waves with a kind of resonance equipment, somewhat resembling church bells tuned to the frequencies expected. Today several very large detectors use laser techniques to detect the extremely small deformations of space-time that gravitational waves are expected to give rise to. As far as I know there has been no clear observation yet, but the effect of gravitational waves have been clearly seen in the slowing down of a double pulsar, the discovery and study of which lead to the 1993 Nobel Prize in Physics to Russell Hulse and Joseph Taylor “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation”. Let me end my comment by making some advertisment for a biography of Dirac which is now available as a pocket book: “The Strangest Man. The hidden life of Paul Dirac, Quantum Genius”, by Graham Farmello. Don’t forget to look up “Lindau” in the index!

Anders Bárány

Paul Dirac (1959)

Gravitational Waves

Paul Dirac (1959)

Gravitational Waves

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