Thursday, February 11, 2016

Scientists Finally Detect Gravitational Waves

A hundred years ago Einstein predicted the existence of gravity waves rippling through space-time. They are the last prediction of relativity to go undetected and now have been observed.

The Verge has a nice overview article which includes a short video, Scientists have finally proven Einstein’s century-old theory about gravitational waves

The New Yorker has a more detailed article Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them. It includes some pictures of the instruments and prettier visualizations.

Weiss’s detection method was altogether different from Weber’s. His first insight was to make the observatory “L”-shaped. Picture two people lying on the floor, their heads touching, their bodies forming a right angle. When a gravitational wave passes through them, one person will grow taller while the other shrinks; a moment later, the opposite will happen. As the wave expands space-time in one direction, it necessarily compresses it in the other. Weiss’s instrument would gauge the difference between these two fluctuating lengths, and it would do so on a gigantic scale, using miles of steel tubing. “I wasn’t going to be detecting anything on my tabletop,” he said.

To achieve the necessary precision of measurement, Weiss suggested using light as a ruler. He imagined putting a laser in the crook of the “L.” It would send a beam down the length of each tube, which a mirror at the other end would reflect back. The speed of light in a vacuum is constant, so as long as the tubes were cleared of air and other particles the beams would recombine at the crook in synchrony—unless a gravitational wave happened to pass through. In that case, the distance between the mirrors and the laser would change slightly. Since one beam would now be covering a shorter distance than its twin, they would no longer be in lockstep by the time they got back. The greater the mismatch, the stronger the wave. Such an instrument would need to be thousands of times more sensitive than any previous device, and it would require delicate tuning in order to extract a signal of vanishing weakness from the planet’s omnipresent din.

“It never should have been built,” Isaacson told me. “It was a couple of maniacs running around, with no signal ever having been discovered, talking about pushing vacuum technology and laser technology and materials technology and seismic isolation and feedback systems orders of magnitude beyond the current state of the art, using materials that hadn’t been invented yet.”

It took years to make the most sensitive instrument in history insensitive to everything that is not a gravitational wave. Emptying the tubes of air demanded forty days of pumping. The result was one of the purest vacuums ever created on Earth, a trillionth as dense as the atmosphere at sea level. Still, the sources of interference were almost beyond reckoning—the motion of the wind in Hanford, or of the ocean in Livingston; imperfections in the laser light as a result of fluctuations in the power grid; the jittering of individual atoms within the mirrors; distant lightning storms. All can obscure or be mistaken for a gravitational wave, and each source had to be eliminated or controlled for. One of LIGO’s systems responds to minuscule seismic tremors by activating a damping system that pushes on the mirrors with exactly the right counterforce to keep them steady; another monitors for disruptive sounds from passing cars, airplanes, or wolves.

Some of the most painstaking work took place on the mirrors, which, Reitze said, are the best in the world “by far.” Each is a little more than a foot wide, weighs nearly ninety pounds, and is polished to within a hundred-millionth of an inch of a perfect sphere. (They cost almost half a million dollars apiece.) At first, the mirrors were suspended from loops of steel wire. For the upgrade, they were attached instead to a system of pendulums, which insulated them even further from seismic tremors. They dangle from fibres of fused silica—glass, basically—which, although strong enough to bear the weight of the mirrors, shatter at the slightest provocation. “We did have one incident where a screw fell and pinged one, and it just went poof,” Anamaria Effler, a former operations specialist at the Hanford site, told me. The advantage of the fibres is their purity, according to Jim Hough, of the University of Glasgow. “You know how, when you flick a whiskey glass, it will ring beautifully?” he asked. “Fused silica is even better than a whiskey glass—it is like plucking a string on a violin.” The note is so thin that it is possible for LIGO’s signal-processing software to screen it out—another source of interference eliminated.

So here's my favorite thing from the article. "Ultimately, he said, 'We accepted that the most economical explanation was that it really is a black-hole pair.'" That means that the most likely explanation of what their instruments were telling them is that a billion years ago two black holes merged and we're seeing the result now by measuring a laser bouncing off a mirror and seeing that it's a thousandth of a diameter of a proton off. Science is fucking amazing.

The importance is not just observing the theory. It's a new way for us to observe the universe. Until now, virtually everything has been seen via the electromagnetic force. Whether we use a visible light telescope or a radio telescope or spectrophotometer, those all basically detect photons moving at different frequencies. They are all part of one of the four physical forces. This is a direct measurement of gravity, a different force. Usually when we get a new instrument (the telescope, microscope, radio telescope, electron microscope, etc.) we get interesting new observations and unexpected science.

The LIGO scientists have extracted an astonishing amount from the signal, including the masses of the black holes that produced it, their orbital speed, and the precise moment at which their surfaces touched. They are substantially heavier than expected, a surprise that, if confirmed by future observations, may help to explain how the mysterious supermassive black holes at the heart of many galaxies are formed. The team has also been able to quantify what is known as the ringdown—the three bursts of energy that the new, larger black hole gave off as it became spherical. “Seeing the ringdown is spectacular,” Levin said. It offers confirmation of one of relativity theory’s most important predictions about black holes—namely, that they radiate away imperfections in the form of gravitational waves after they coalesce.

The detection also proves that Einstein was right about yet another aspect of the physical universe. Although his theory deals with gravity, it has primarily been tested in our solar system, a place with a notably weak gravitational regime. “You think Earth’s gravity is really something when you’re climbing the stairs,” Weiss said. “But, as far as physics goes, it is a pipsqueak, infinitesimal, tiny little effect.” Near a black hole, however, gravity becomes the strongest force in the universe, capable of tearing atoms apart. Einstein predicted as much in 1916, and the LIGO results suggest that his equations align almost perfectly with real-world observation. “How could he have ever known this?” Weiss asked. “I would love to present him with the data that I saw that morning, to see his face.”

Update: Here's a nice comic explanation of Gravitational Waves

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