Surf’s Up

gravitational waves.jpg

If you ponder the mysteries of the universe, check out morning’s LA Times story about LIGO — the Laser Interferometer Gravitational-Wave Observatory Caltech operates in Hanford, Washington and an identical twin managed by MIT in Livingston, Louisiana — which scientists hope will allow them to demonstrate the truth of Einstein’s theory that “large bodies moving through space would give off waves of gravity, traveling at light speed, that would shrink and expand space-time itself.”

After Einstein, our conception of the universe changed. It is not empty space, it is a fabric. Space, and everything occupied by space, can be bent and stretched by waves of gravity, which Times’ author John Johnson Jr. likens to the ripples from a spoon stirring milk, or the indentation a bowling ball would make on a trampoline.

Today, such waves exist only in theory, the product of cosmic events like supernovas or pairs of neutron stars whipping through each others’ orbit and then smashing into each other.

According to theory, if our planet came close to the source of a gravitational wave, Earth would stretch to twice its normal size, then shrink in half before returning to its original shape — a scenario worthy of a Road Runner cartoon. Have no fear, however. The waves that could reach Earth are very weak, too weak to be measured — until last November when LIGO “reached a level of sensitivity at which (Caltech physicist Kip S. Thorne) and other experts believe they might detect waves.”

Here’s how Johnson describes what’s involved for LIGO to measure gravitational waves:

Down a twisting side road, LIGO appears out of the Russian cheatgrass and mustard plants, a bulky apparition with two tubes extending at right angles into the desert.

LIGO sites.jpgThe 2.4-mile-long tentacles are the heart of LIGO. They are at right angles so that incoming gravity waves will shrink one arm while lengthening the other. An identical facility sits in a forest in southern Louisiana, so that the readings made at one observatory can be cross-checked almost 2,000 miles away.

(snip)

Inside the arms is a laser interferometer, which works by splitting a laser beam and sending one of the two resulting beams down each arm. The beams then bounce around 100 times on a set of mirrors before being sent back to a photodetector.

The two beams should recombine at exactly the same time since they travel an identical distance.

But if a gravity wave passes by, the beams will be thrown off as the arms are alternately stretched and squeezed.

Detecting such a minute signal has required extraordinary steps.
Because the site had to be as flat as possible, satellites were used to survey the land, which was eventually graded to within three-eighths of an inch over five miles.

To get around the problem of air molecules shaking the mirrors, workers sucked the air out of the tubes down to a billionth of an atmosphere. But that still wasn’t good enough to make sure the speed of light would be constant throughout the tubes. So the team had to get the tubes down to a trillionth of an atmosphere.

The surface of the four 10-inch mirrors in the arms is so smooth it doesn’t vary by more than 30-billionths of an inch. Thirty control systems keep the lasers and mirrors in alignment. The vibration isolation system is so sophisticated, the only thing approaching it is the mechanics used by semiconductor chip makers to etch circuits on the chips.

Read the whole thing.

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