[Physics] Wouldn’t angular momentum of a binary star system decrease

angular momentumbinary-starsgeneral-relativitygravitational-waves

Consider a binary star system, as these stars go around one another they would emit gravitational waves. Since, the graviton is a spin 2 particle. Wouldn't the angular momentum of the stars decrease? And would this decrease be observable?


although the angular momentum change was predicted by GR, could there also potentially be an extra angular momentum effect caused by the quantum mechanical spin 2 effect of the gravitons (which could be looked for for a proof of quantum gravity

Best Answer

Yes indeed to all your questions: mutually orbitting binaries do spin down, the system's orbital angular momentum thus decreases with time and the loss of energy and angular momentum is almost certainly owing to the emission of gravitational waves.

Look up the Hulse-Taylor binary system: its spin-down has been carefully observed and measured since its discovery in 1974 and the observed spin down carefully compared with the spindown foretold by General Relativity (one calculates, by GTR, the gravitational wave power emitted). So far, as you will quickly learn, the agreement between GTR gravitational wave power loss model and the observations has been astounding. This is considered by mainstream astronomy to be very strong evidence for the existence of gravitational waves and was the first experimental evidence for them. Early this year, direct observation of gravitational waves in the early cosmos is thought to have been made by the BICEP2 experiment when frozen ripples in the CBR.

Edit: as Warrick pointed out (thanks Warrick), a worthwhile piece of physics history to add is that Russell Hulse and Joseph Taylor were awarded the 1993 Nobel prize for physics for their analysis of the system, the wording of the prize was

"[for the discovery of] ... new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."

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