Some picture gravity as a rubber sheet–stretched taut like a trampoline. If the Sun is a bowling ball, its heft will form a bowl-shaped valley on that sheet. In its stable orbit, the Earth rolls along the edges of the Sun’s valley. But if gravity is like a rubber sheet with weights on top, what happens when those weights misbehave? What if they collide or explode, sending ripples along the rubber surface?
In 1916, Einstein predicted the existence of these gravity waves: ripples not in rubber, but in space-time, the surface of our universe. Today, almost 100 years later, gravity waves remain the last piece of his theory of general relativity that no scientist has observed directly. But a series of detectors, including two in the United States, are looking for these waves.
Rainer “Rai” Weiss is the father of LIGO, the Laser Interferometer Gravitational Wave Observatory. He first devised the instrument as a homework assignment for some of his MIT students, and it started operating in 2001. Weiss spoke last Friday night as part of a World Science Festival event in New York.
Light waves, like any waves, can interfere with one another. Two peaks can build to make even brighter light and a wave and a trough can cancel one another out to leave only darkness. Weiss and colleagues have designed and built a large interferometer, an L-shaped device with a series of lenses and mirrors. Laser light is split at the L’s joint and travels along each of the L’s two legs. At the end of each, the light reflects off a mirror, travels back along the leg, and recombines at the joint before going to a detector.
By adjusting the length of the two 2.5 mile-long arms, scientists can change the interference pattern formed by the two beams’ recombination so that the light beams just cancel one another out. If a gravity wave comes along, for example from two pulsars colliding or even two black holes, scientists can measure gravity waves as these space-time ripples change the arms’ lengths and thus the pattern formed by the light’s combination.
Their current system can detect changes in the arms as small as 10-16 centimeters, or one-hundred-millionth the diameter of a hydrogen atom. With two such interferometers, as are currently operating in Livingston, Louisiana, and Hanford, Washington, the scientists can rule out other changes in the interference pattern such as “micro-earthquakes.” Other detectors are operating across the globe, so that if scientists find a wave, they can map out where in the universe it came from.
As you might guess, the changes in the pattern are so small that seeing them will be difficult, requiring a computer to sort out the change from other “noise.” The metaphor with sound is intentional since scientists map the signal onto audio frequencies we can hear and will listen for a what they predict will be a “whooop” as two massive objects collide.
“That’s the whole challenge, to dig that sound out,” Weiss said, after asking his audience to listen for two massive objects, like black holes, spiraling around one another and then colliding. “Your ear is a very good detector. It takes a lot of computing power to get that–what you heard–out of that noise.”
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Image: Interferometer lens on display as part of the World Science Festival