May/June 2008

Catching Einstein's Waves

Continued from page 1

By Katherine Bourzac, SM '04

"As gravitational waves travel, they stretch and squeeze space-time," says Mavalvala. If one passed through you right now, you might get a little taller, then a little shorter; a little wider, then a little thinner. Similarly, if a gravitational wave passes through two objects, the distance between them varies. LIGO is designed to measure this effect.

It so happens that the things LIGO will be good at detecting, such as black holes colliding with neutron stars, can't be seen well with telescopes. But while such phenomena are interesting, the real value of studying them, says Hughes, is that they will let us test the laws of physics in areas of the universe very unlike our own.

Newton's laws work well in our solar system, where gravity is weak. But near black holes, says Rainer Weiss, "space is so strongly gravitating that it's not flat anymore; it's curled up on itself in horrible ways." With LIGO, he says, "we're going to be seeing things from regions in the universe where Einstein is the whole story. Newton you can forget about." LIGO, physicists hope, will open up what Hughes calls an extreme laboratory. "Measuring gravitational waves will give us insight into the deepest nature of space and time," says Edward Bertschinger, head of MIT's physics department. "Until we thoroughly study them, we haven't understood gravity."

It took a long time, however, for the physics establishment to believe that technologies designed to measure gravitational waves were worth investing in. Weiss, who has spent his entire career at MIT, played an important role in turning the tide.

The origins of LIGO
Even Einstein recognized that gravitational waves would be difficult, if not impossible, to measure. Although he believed they really existed, in the 1930s physicists began to think of gravitational waves as mathematical curiosities. And with no way to test his ideas about them, Einstein himself backed down from his earlier claims.

But in 1960, a man whom Weiss calls "courageous" and "an imagi­native nut" decided to try to measure gravitational waves. Joseph Weber, a professor at the University of Maryland, built a detector that worked something like a metal xylophone bar; but instead of vibrating when struck by a mallet, it would vibrate when struck by gravitational waves. Weiss says that Weber "saw all sorts of wonderful things" and claimed he'd detected what Einstein predicted.

The problem was, no one could duplicate his results, though Weber, who died in 2000, stuck by them. Weiss says that a more careful physicist would have been more skeptical of his own conclusions; he speculates that Weber's machines may have detected such things as lightning strikes or problematic phone lines, but Weber didn't investigate other possible explanations for his data. Without independent confirmation of Weber's waves, "the field went into a terrible state," Weiss says.

Weiss, no less imaginative than Weber, was also interested in gravi­tation from an experimental perspective. He had flunked out of MIT in the 1950s but was given another chance by the legendary physics professor Jerrold Zacharias, who hired him to work in his lab. After earning his PhD at MIT, Weiss spent time at Princeton University in the lab of Robert Dicke, a leading expert in gravitation.

Soon after he came back to MIT as a professor, Weiss was assigned to teach a graduate-level class on relativity. It was the spring of 1966, and Weber's detector was up and running. "I couldn't understand what Weber was doing," Weiss recalls, so he decided to explain gravitational waves to his students by "devising the most simple-minded explanation of how you could detect one that I could imagine." His idea was to use an interferometer, an L-shaped configuration of equally spaced mirrors that uses laser light to precisely measure distance. As gravitational waves pass objects, they stretch and compress space-time in such a way that the distances between those objects change. The greater the initial distance between two objects, the greater the change. The greater the change, the easier it is to measure.