Catching A Space Wave

Is our universe rippling with gravitational waves? Scientists studying the nature of space, time, and gravity believe that it is, and they are on the hunt to detect one of these waves directly. So what exactly is a gravitational wave? Imagine sliding yourself slowly into a still pond. As you glide down and immerse yourself in water, the glassy surface remains largely undisturbed. Now, picture flinging yourself from a rope swing and cannonballing down, crashing through the water's surface at full force. Large waves slosh up around you, getting smaller as they make their way onto the banks of the pond and outward across the water.

That's the idea behind gravitational waves. Einstein predicted that they happen all the time as bodies of mass splash through the fabric of space and time. And when big events happen--when two super dense, massive stars collide, for instance--cannonball waves course through spacetime. Now, extremely sensitive detectors around the world, and maybe even out of this world, are waiting for those gravitational waves to wash over them. And when that happens, scientists will not only have more evidence for Einstein's already hugely successful theory of general relativity; they will also have a new tool for mapping our cosmos.

General relativity tells us that space is like a vast pond. Space distorts around mass just the way water warps around a swimmer. Celestial bodies slowly doggy-paddling through the spacetime pond don't make too many ripples. Butterfly-stroking black holes, on the other hand, cause quite the disturbance. But by the time their ripples reach the Earth, they're so small they are practically impossible to observe.

So how do you detect the near-undetectable? Gravitational waves distort the space they push through; as space lengthens in one direction, it contracts in another. So in the 1970s, scientists at Cal Tech and MIT used their combined brainpower to secure funding for what they called LIGO, the Laser Interferometry Gravitational-Wave Observatory. (They hyphenated "Gravitational-Wave" to make sure they wound up with a cool acronym.)

Laser interferometry is a technology used in many branches of science, but the idea is always the same. Split one beam of light into two. Shoot those two beams off in two different directions. Each beam travels the exact same distance before it bounces off of a mirror and makes it way back to the detector. Because the speed of light in a vacuum is always 186,000 miles per second, the beams should return to the detector at exactly the same time. But if something disturbs the paths of those light beams, the beams interfere with each other. That disturbance will be reflected in a distinct pattern in the return-trip data--thus the name "interferometer."

LIGO uses this principle on a huge scale. Both of LIGO's locations, in Hanford, Washington and Livingston, Louisiana, are home to detectors with two 2.5 mile-long arms stretching out in different directions. (The two distant locations help the researchers confirm that any motion they detect is in fact due to gravitational waves, and not local geologic movement.) The mirrors and the detector are surrounded by stabilizing devices designed to isolate them from typical Earth-bound jostling. If a wave comes sailing through, one arm will contract while the other stretches. The light bouncing off the mirror at the end of the shortened arm will return to the detector sooner than the beam shooting down the lengthened arm. When the LIGO scientists see this time discrepancy, they will have seen a gravitational wave.

So far, they haven't seen anything at all. LIGO has been up and running since 2001 and there has been nary a trace of a gravitational wave. But scientists aren't taking this to mean that these waves don't exist; their instrument just isn't quite sensitive enough to catch them. LIGO is now in line for upgrades that will make it ten times more sensitive. When the modifications are complete (tentatively expected in 2014), LIGO will become the so-called Advanced LIGO, and scientists predict they will be swimming in positive results.

But an interferometer need not patiently wait for waves here on terra firma. Space agencies around the world are looking into devices designed to detect gravitational waves from space. An orbiting interferometer would consist of a triangle-shaped system that would circle the earth, scanning the skies for the tidal-wave signals of huge gravitational events. Thanks to tight budgets, though, these projects are stuck on the drawing board.

Why dedicate all this time and technology to gravitational waves? Einstein's theory of general relativity has been thoroughly tested since it was proposed in 1915. Why do we need another test of its validity? "Seeing" gravitational waves will give us more than yet another verification of relativity. The information from the signals coming in to the detectors can paint a picture of what is happening in distant reaches of the universe. Scientists could piece together the collision of massive black holes. They could see the explosive birth of an incredibly dense neutron star. They could even detect the remnants of the gravitational wave that shot off at the Big Bang, giving them a clearer picture of the events that gave rise to our universe. Gravitational wave detectors could clarify the skies by illuminating the many massive wonders wading through space.

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Stephanie McPherson

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