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Physicists Gear Up To Catch a Gravitational Wave
sciencehabit writes: A patch of woodland just north of Livingston, Louisiana, population 1893, isn't the first place you'd go looking for a breakthrough in physics. Yet it is here that physicists may fulfill perhaps the most spectacular prediction of Albert Einstein's theory of gravity, or general relativity. Structures here house the Laser Interferometer Gravitational-Wave Observatory (LIGO), an ultrasensitive instrument that may soon detect ripples in space and time set off when neutron stars or black holes merge. Einstein himself predicted the existence of such gravitational waves nearly a century ago. But only now is the quest to detect them coming to a culmination. Physicists are finishing a $205 million rebuild of the detectors, known as Advanced LIGO, which should make them 10 times more sensitive and, they say, virtually ensure a detection.
There *is* a second detector (a third even!) The second main detector is in eastern Washington state. Both will have to go ping before you accept any result.
I toured the LIGO in Eastern WA (which is a twin if this, but the article apparently doesn't know it exists). Basically they have two long tubes (2 miles each I think) at right angles. A laser is shined down them and bounced back and forth as much as they can to increase the effective length. Lasers are combined again and they can see by an interference pattern if one arm of the experiment gets shorter or longer than the other, which is what is theorized to happen when a gravity wave passes through.
A couple of years ago I think they were down to 10^-25m sensitivity, which is pretty amazing to me. They could tell when a truck rolled down the highway 10 miles away, when the hydroelectric dams in the state open their spillways, and when there are big waves from a storm on the coast (200 miles away).
There are two more detectors at the Hanford Washington site. A primary one like at Livingston, and a secondary one that's half the length.
Also, there is an European experiment in Italy, called Virgo. It's currently being upgraded to similar sensitivity to the other 3.
When they are all working, it will allow the detection to not only be verified, but the time of the events at each detector will let them triangulate the location the wave originated from.
We're pretty darn sure of gravitational waves, as a Nobel prize was awarded in 1993 for showing that the slowing of a binary pulsar was just the right amount to account for the gravitational waves it would generate.
These detectors will let us do gravitational wave astronomy much like we do with light and radio waves now.
The huge news would be if they get all of them working with their maximum sensitivity and didn't detect anything. That would mean something was very wrong with their assumptions.
LIGO works by measuring the distance between two tracks set at right angles. A passing gravitational wave would momentarily change the length of one leg or the other, or both, in characteristic ways.
It measures the distance with a laser beam. It splits the beam, and sends them down the two tracks. They bounce off mirrors, and when they return, they interfere. Changes in the length will change the interference. That means that they can detect changes at distances on the order of a single wavelength of light.
That's an interferometer, the I in LIGO. At its core, it's the same thing that Michaelson and Morley used to look for aether, and failed to find it. The trick is that this has to be even more sensitive, because the expected changes are even smaller and the contraption itself is much bigger (4 km, versus a few meters). They have to exclude all kinds of potential interference, from passing trucks to earthquakes.
I suppose it may well go "ping" when it spots a gravitational wave, and they'll end up comparing it to other experiments. But they'll get more than a ping; they'll get a signal of the changing lengths that they can use to map the size of the wave, and even a hint of its direction.
There are two more detectors at the Hanford Washington site.
Actually, there is now only one detector at Hanford, the full length H1. The half length H2 interferometer was discontinued as part of the upgrade.
"Education is not the filling of a pail, but the lighting of a fire." -- William Butler Yeats
since the deterctor only returns one bit of information
Except it doesn't return one bit of information. It returns a spectrum and relative phase of oscillation measurements, for each of two directions, and again for each of two directions at a different site. Not to mention all of the information associated with calibrating and configuring the device. GR makes predictions about how gravity waves would interact with the two arms and two sites. Calibration and analysis of the construction and background determine a noise floor.