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The Search for Gravitational Waves Is On

gravitational wave

Depiction of a gravitational wave from a binary neutron star. (Courtesy: LIGO)

AU joins LIGO in the search for the unseen

No one has ever detected a gravitational wave directly. It’s a bit like looking for a black cat in the dark, when you’ve never seen a cat.

Gravitational waves, ripples in the fabric of space-time, were predicted by Einstein’s theory of relativity. Any mass could make them, but detectable gravitational waves would come from acceleration on a massive scale — the collision of black holes, a supernova explosion, or the violent, early moments of our universe’s Big Bang.

Since Einstein, scientists have theorized about and attempted to spot this mysterious phenomenon that’s much weaker than others in the physical universe. Gregg Harry, American University physics professor, has been on the front lines of the search since graduate school.

Working as a member of the National Science Foundation–funded Laser Interferometer Gravitational-Wave Observatory (LIGO), Harry is part of an international collaboration dedicated to detection of cosmic gravitational waves. This fall, American University became an official part of LIGO and AU faculty are now able to join other scientists perfecting the next generation Advanced LIGO detectors, pouring over data, and honing theories to learn more about this celestial manifestation of gravity.

What are gravitational waves, anyway?

According to general relativity, gravity is a distortion along the curvature of space-time. A colossal movement in the universe (like the collision of two black holes) would cause repeated distortion in the fabric of space-time, or waves.

Harry puts it more simply.

“If you think about space-time as some sort of sheet, that bang would ruffle the sheet, and to some degree that ruffling would sort of just keep going and presumably change over time.”

LIGO collaborators are looking beyond more common, milder gravitational waves, like those some theorize could be pulling the Earth very, very slowly toward the Sun. Their focus is a cosmic milieu that should be thick with dense gravitational waves from the big daddies of astronomical collision — the stochastic background of gravitational waves.

Like astrophysical detectives, LIGO researchers have a list of likely suspects, possible sources for easier-to-find gravitational waves.

In the early Middle Ages, around the year 1000, Chinese astronomers spotted what astrophysicists now believe was a supernova. The resulting neutron star, the Crab Pulsar, rotates extraordinarily fast — 30 times per second — and it should be giving off tremendous gravitational waves, possibly at a rate of 60 times per second. Early detectors were tuned to the expected frequency of gravitational waves from the Crab Pulsar, but nothing has yet been detected.

Another option is binary neutron stars: pairs of very dense stars, resulting from supernovas, that spiral around and in on one another. The closer they get, the faster the stars move, and the stronger the gravitational wave emission. The final minute, perhaps seconds, of that cosmic dance, moments before the stars slam into each other, is what scientists expect could give a burst of gravitational waves that could be detected here on Earth. That is, if the binary stars were close enough to our planet.

This is one of the reasons that waves from the Big Bang are significant to the search. There, it would not be a matter of timing or proximity. “Any gravity waves you have that were created at the Big Bang . . .” explains Harry, “more or less, they’re still around.”

But more to the point, if these waves could be detected, it would be the most stringent test of Einstein’s theory of general relativity. Moreover, “details of these waves would tell us about conditions moments after the Big Bang,” explains Harry, “something no other technique can even try to do.”

It would be a window into the origins of the universe.

How to detect a gravitational wave

Early efforts to detect gravitational waves used a simple and rather expensive trial-and-error method. Physicists in the 1970s and 80s hypothesized that gravitational waves would travel at particular frequencies. Much like building a tuning fork to the right pitch, they thought, if you could build an instrument to ring at the proper frequency, you’d know when a gravitational wave passed by. Since the frequency of gravitational waves is unknown, this meant building a number of expensive cylindrical bars at various frequencies. A few early claims that waves had been detected this way were later debunked as other scientists were unable to replicate experiments with the acoustical bars.

The first generation LIGO detectors used a more advanced but streamlined method. LIGO built three detectors (interferometers) in two distant geographic sites — Washington state and rural Louisiana. The distance between is to help distinguish false readings caused by mild earthquakes or rumbling trucks from cosmic gravitational waves. The detectors are sensitive to motions at the level of one thousandth the size of a proton width. (That’s about the same precision as measuring the distance to the nearest star accurate to the width of a human hair.)

Harry’s central research is a very practical matter of refining the optics on the detectors. At either end of the detectors’ four kilometer-long vacuums hang mirrors dangling from thin, half-millimeter thick fibers. Between the mirrors is a laser beam. If a gravitational wave moves through, the mirrors should wobble together and apart. The time it takes for the laser to bounce between mirrors will change, and LIGO researchers will be able to measure that time change.

As part of the search to capture the heretofore unseen, Harry is part of the astrophysics version of building a better mousetrap. Noise and distortion on the mirrors could give false readings, or worse, hide signals of real waves. For ten years at MIT and now in a physics lab at AU, Harry is upgrading optical coatings and creating epoxies in an effort to dampen unwanted frequencies. These improvements will be part of an upgrade to the project, with the next phase, Advanced LIGO, due for completion in 2015.

Harry edited the recently released book, Optical Coatings and Thermal Noise in Precision Measurements, a compilation of lessons learned by Harry and LIGO colleagues through a decade of optics research for gravitational wave detection.

“When I first started working on this, ten years ago,” explains Harry, “nobody else in the world had studied coatings at that level and even really had thought about it.” Today the optical coatings that are being perfected for LIGO have an array of applications.

At the National Institute for Standards and Technology, precision timing is dependent upon the use of lasers and even at this pinnacle of time measurement, noise and distortion have been a limiting factor. Coating expertise derived from LIGO will be applied to NIST’s next generation instruments. Micro and Nano Electrical Medical Systems rely upon small detectors (and clear optics) for making biological measurements. LIGO technologies for thermal noise reduction are even being used to test quantum theory on a microscopic level — a jump in scale that brings subatomic theory to something as relatively large as the width of a human hair.

“On the one side,” says Harry, “the quantum mechanics experiments are really studying the small scale, but with gravitational wave detection, there you’re looking on a cosmic scale.”

The past decade’s evolution in optics technology is making both possible.

You never know what you’ll find

There are plenty of theories concerning the frequency of gravitational waves, how they will behave, if we ever find any. But that doesn’t make the search for discovery futile.

“In the whole history of astronomy, every time someone has come up with a new instrument, a new telescope, and they’ve turned it on the sky, usually they’re looking for something . . . Whatever they wanted to do may work fine,” his voice grows hushed “but they’d find all this stuff there that nobody even suspected was there — and that ends up being the really interesting thing.”

It’s a matter of finding what else may be lurking in the black beyond.

“The suspicion is, once LIGO starts seeing things, sure, we’ll see some of these neutron stars, maybe we’ll see this Big Bang thing, but if history’s any guide, there may be all sorts of crazy stuff happening that we just didn’t even think of — and that will end up being the really interesting bit that gets all the headlines.”