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February 09, 2016

Big? Rumor Says Scientists Have Finally Detected Elusive "Gravitational Waves"

Gravitational waves are important for a few reasons. One of those reasons is that they would prove the existence of black holes, which are "proven" now only by mathematics plus indirect indications.

The idea is that when two black holes begin orbiting each other, they speed up more and more and start pumping out gravitational waves.

In addition, gravitational waves are predicted by Einstein's relativity equations. They themselves are predicted, but not proven. So just detecting the waves themselves would be big deal.

The LIGO system is a neat way to detect these gravitational waves. They cut a long, long shaft beneath the earth. They hanged two weights at the end of this long shaft.* (I think this shaft is like a kilometer long. I mean, it's long.) A laser is bounced between the weights (well, mirrors on the weights). One side has a sensor of some kind on it.

The idea is that the weights are insulated enough from other forces that the only thing that can disturb them would be gravitational waves, crossing the galaxy from a swirl of black holes light-centuries away. Well, there'd be earthquakes, gravity from planets, etc., disturbing them, but those inputs are themselves measured and then subtracted out of the math. If you can measure that noise, you can subtract that noise out of your measurements.

Because each perturbance would be actually undetectable -- way too small for even the most sensitive sensors to detect -- the perturbance will be indicated by the overlay of many, many laser bounces on to each other. It is believed that if a tiny disturbance moves one weight, you would be able to see that if, say, 3000 laser lines showed a flutter, when their signals were all piled on to each other.

That's the "interferometer" part. The laser lines interfere with each other's waves, and should produce a detectable signal from what would otherwise be an undetectable nudge.

Kind of the way you can't detect Obama's lies from a single instance but when you pile up a thousand of them the pattern becomes clear.

This LIGO (which stands for Large Interferometer Gravitational-Wave Observatory) has been bouncing laser lines for a while now, and checking to see how their interference patterns might indicate a disturbance.**

Now they say they're prepared to make a big announcement.

* Actually I think they cut a long "L" shaped shaft in the earth, and have lasers bouncing via a mirror at the corner of the L from one weight to the other, but, you know. I'm not big on details.

** Laser light, having the same sinuous waveform, should just overlap upon itself perfectly, assuming you have the laser bouncing at an exact multiple of its wavelength. That is, say you have a laser with a wavelength of 1 mm. If the laser is being bounced back and forth at a distance of exactly 1 m (exactly 1000 mm), then each peak of the laser should overlay perfectly with every other peak, and every trough should overlay with every other trough.

Now, if this distance is slightly altered -- like if a gravitational wave nudges a mirror slightly -- the laser wave-forms will not overlay perfectly. You'll see them go out of sync.

They'll go out of sync so slightly you couldn't tel from a few back-and-forth runs of the laser, but when you keep piling up the runs, the out-of-sync situation should become clear.

Well, not to the eye. You have to do a bunch of math. But then you'll see it.

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posted by Ace at 05:44 PM

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