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posted by janrinok on Thursday March 09 2017, @01:17AM   Printer-friendly
from the hot-rod dept.

The last major prediction of Einstein's theory of General Relativity, gravitational waves, was the most controversial and difficult to verify of them all. It wasn't until 1993 that gravitational waves were indirectly observed in the behaviour of neutron star binaries, and not until 2015 that they were finally directly detected. Even Einstein himself for a time had doubts that they were real, and he even attempted to publish a paper that tried to argue that gravitational waves were a mere artefact of the mathematics, which turned out to be flawed. Oddly enough, it was Richard Feynman, who is much better known for his work on quantum electrodynamics, who came up with an argument that convinced many of the doubters. Rather than arguing the mathematical subtleties of relativity, he came up with a physical explanation that not only demonstrated that gravitational waves must carry energy, but later inspired the design of LIGO, the first apparatus that detected gravitational waves directly. Paul Halpern has an article where he tells the whole story. From the article:

Enter Richard Feynman, who had distaste for unnecessary abstraction. If gravitational radiation is real, it must convey energy. Rather than debating the technical question of whether or not the pseudotensor definition of gravitational energy was correct, he turned instead to a far more intuitive line of reasoning, what has come to be known as the "sticky bead argument."

In his thought experiment, Feynman imagined a thin stick on which one mass is fixed and a second mass, slightly separated from the first, is free to slide back and forth, like a curtain on a rod. These two masses would be analogous to a pair of charges embedded in a vertical receiving antenna used to pick up radio signals. Just as a pulse of electromagnetic radiation would cause such charges to oscillate, the same would happen in the "gravitational antenna" if a gravitational wave passed through—with the maximum effect occurring if the wave were transverse: at right angles to the stick. Upon the impact of a gravitational wave, one of the masses would accelerate relative to the other, sliding back and forth along the stick. The rubbing movement would generate friction between the free mass and the stick, releasing heat in the process. Therefore the gravitational radiation must convey energy. Otherwise, how else did the energy arise?


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  • (Score: 2, Informative) by Anonymous Coward on Thursday March 09 2017, @03:06AM

    by Anonymous Coward on Thursday March 09 2017, @03:06AM (#476845)

    1. If the science is correct (there's always a chance it's not ...) then these were stellar black holes, i.e. remnants of an earlier binary star system. Except for a few leftover planets and asteroids (whose total mass is typically very insignificant when compared to their stars), there's literally nothing else around that could produce electromagnetic radiation. The black holes sure won't, by definition. And even if there's a lot of planets and stuff: they'd have been circling the binary for a long time now, why should they stop doing so and turn into radiation? They're only concerned with the mass at the center of their orbits, and that mass just became *smaller* by ~3 solar masses.

    Your demand could only, possibly, be fulfilled if one/both black holes had accretion discs (and possibly not even then). But accretion discs are *very* unusual for stellar black holes, to say the least.

    BTW: For a few weeks there was an uncertain candidate for a very weak GRB, but further analysis ruled that out quite conclusively (we're very uncertain about where in the sky GW happened, so there's a lot of probabilities involved ...)

    2. I also do not see how there is circular reasoning: relativity gave a prediction of and details for BH-BH mergers even though *all* of its more egregious predictions (black holes per se, gravitational waves, time dilation, contraction of lengths ...) were unknown at the time and had never been observed. Now, decades later, an observation has been made that pretty exactly matches the prediction.

    I do not see the circularity here, how did you reach that conclusion?

    3. Not getting a Nobel Prize directly after your discovery is the norm.
    It typically takes between 10 and 20 years after a discovery to receive a Nobel Prize in physics. 2013's Nobel for the Higgs Boson was a fluke in that regard (discovery in 2012, initial theory in 1964, Nobel was for both). Einstein, on the other hand, got his Nobel in 1921, 16 years after publishing the original article (for explaining the photoeletric effect, not for relativity)

    I'm mostly certain that there's a Nobel in the making for the detector teams, give it a few more years, let them find a few more GW events first, let a few more detectors come online giving us actual astronomical capabilities. Einstein would perhaps get his second Nobel for relativity too, but he's dead already and thus not eligible anymore.

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