Giant lasers, super vacuums and the search for gravity waves

By Aiona Bones on August 11, 2011

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LIGOinterior2Most ExNET exhibit sets can be found at science and discovery museums in the center of cities, but every rotation cycle we ship one set out to Louisiana, far from any large metropolis, to a very high tech research center hidden behind a tangle of dense wet forest. This is LIGO, a Laser Interferometer Gravitational-wave Observatory. They host Exploratorium exhibits at their Science Education Center and invite school groups to visit and learn about the science that happens there, but the rest of their facility is dedicated to a large scale endeavor to detect gravitational waves. Here’s how it works:

To search for these tiny signals, LIGO continuously measures the lengths of its two perpendicular arms, and looks for slight differences that might indicate that space itself is fluctuating. This very precise measurement is done with the use of an enormous interferometer. The interferometer takes a laser beam, splits it in two, shines the beam down two perpendicular, 2.5 mile long vacuum tubes with mirrors at the end, and compares the distance traveled by each beam by inspecting the interference pattern formed when the two beams meet back up again.

LIGOmachineGravity waves are created whenever a mass moves through space time, but most of them are not strong enough for LIGO to pick up. The largest waves they expect to find will be those created by massive celestial collisions, but by the time these waves reach the earth, even that signal is not expected to exceed an amplitude 1/1000th the diameter of a proton. In order to make their system sensitive enough, the LIGO scientists have had to engineer some very complex systems. The most obvious of these are the sealed metal tubes and chambers surrounding the entire apparatus. Air particles in the path of the laser beam would interact with the light and distort the signal, so the lasers must pass through a vacuum. The LIGO vacuum has an air particle density eight times lower than the emptiest void between here and the moon, and they continue to capture stray air particles with a liquid nitrogen system that removes molecules by freezing them.

LIGO also physically stabilizes their systems with a collection of sensors that detect motion in the ground and physically move the machinery in the opposite direction. Without these mechanisms, LIGO would be overwhelmed with signals from earth quakes on the other side of the planet, traffic on highways miles away, and the scientists themselves walking near the equipment.

One of the most interesting things about LIGO is that so far they have never actually seen a gravity wave. The entire facility is still in prototyping phase, although the end is in sight, and the current upgrade should bring it up to its final version. Meanwhile the LIGO scientists are checking and double checking all systems, and even the LIGO scientists themselves are being tested. A few of the senior scientists have been given the ability to inject false information into the data to simulate a detection. The rest of the scientists have to wait until the end of the year to find out if their findings were real or merely a test. This year they opened the white envelope containing the injection dates, only to find that the paper they had already written and were ready to publish, was based on false data.

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Learn more about LIGO here.

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