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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?
(Score: 0) by Anonymous Coward on Thursday March 09 2017, @01:18PM (2 children)
supposedly the detector is anchored to earth in a complex gravitational system AND rotating.
i suppose to be conclusive the detector (mirrors?) would need to ping pong light between the lagrang points?
as long as the gravity wave detector is anchored to earth and detection can be interpreted as light NOT having a constant speed and thus not being a universal constant?
(Score: 0) by Anonymous Coward on Thursday March 09 2017, @03:15PM
Your post has interesting technical jargon that, when combined, do not make for a comprehensible statement/question. Are you saying that to be conclusive you need to be in some sort of universal rest frame? I don't know where to go with the last sentence.
(Score: 4, Interesting) by Immerman on Thursday March 09 2017, @04:26PM
I think I kind of get where you're coming from, and the answer is basically no.
All the rotational behavior of Earth happens on a nice regular cycle - if light had a non-constant speed, such as if it had a preferred direction of travel, the interferometer would show that as it rotated into and out of alignment with that direction, and you'd get a constant low-frequency signal with a 24-hour cycle. We're not detecting that.
Alternately, if the speed of light varied with time but not space, then the interferometer would detect nothing at all since the photon in each arm would remain traveling at the same speed as it's tin in the other. So long as they both make the round trip in the same amount of time, it doesn't matter how fast they're moving.
Instead what was detected was an brief, high frequency signal that rapidly increased in intensity and suddenly vanished - the "death scream" of a pair of black holes spiraling into a rapidly deepening gravitational field until they made contact and merged.
Basically, if an interferometer detects a signal then you know one of two things: either the length of one arm changed with respect to the other, or the speed of light changed between the two arms arm. It's not beyond imagining that light speed could change like that, but we have no theoretical basis to believe that it's possible. And if that somehow *were* the case, the signal detected would mean that every once in a long while something happens to cause the speed of light to oscillate rapidly, with a definite directional bias or highly localized effect(only one arm), and a rapidly increasing magnitude, and then suddenly stop.
You're offering a wildly speculative explanation without any theoretical basis as an alternative explanation for a signal that matches exactly what the theoretical predictions say that the gravitational waves from a black hole merger would look like.
It's not *impossible* that you're wrong, and looking for alternative explanations is an important part of science, but in this case you'd need to offer a pretty solid alternative theory with a strong amount of experimental evidence (because General Relativity already has a whole lot of evidence backing it) to have any chance of being taken seriously.
As for Lagrangian interferometers - I don't see that they'd really solve anything, other than determining whether the signal was confined to the planet itself. There are some decent arguments in favor of space-based interferometers, but they mostly revolve around the fact that bigger detectors detect lower-frequency waves, and that to detect the gravitational waves emitted from sources visible in other ways, such as known binary stars within our galaxy, we need an interferometer with arms ~10x longer than the distance between the Earth and the Moon.
Going all the way out the the Lagrange points would actually be an easy way to maintain at least approximate alignment over time, but would make such a stupendously large interferometer that it would be detecting too low a frequency to pick up on any of the predicted strong sources we can see.
Plans such as LISA call for more cleverness in maintaining alignment at the desired size - it calls for putting three solar-orbiting satellites in a giant equilateral triangular formation trailing behind the Earth in it's orbit, but all in different orbital planes so that they form a triangle that kind of wobbles it's way through space while maintaining constant distances. It's actually kind of wild to watch the animation near the top of the Wikipedia article:
https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna [wikipedia.org]
https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna [wikipedia.org]