Volume 3, Issue 24 December 2006/January 2007
Catching Gravitational Waves
Don Backer and graduate student Paul Demorest work on optimizing detection methods for gravitational waves. (Photo by Andrew West)
Don Backer goes about his days attuned to some of the faintest rhythms in the cosmos. As a UC Berkeley professor of astronomy, he's not interested in the proverbial music of the spheres. Rather, he aims to detect surges rippling the very fabric of spacetime: gravitational waves.
Gravitational waves were first predicted by Einstein in 1916. According to his General Theory of Relativity, extremely massive objects deform three-dimensional coordinates of space as well as time. Imagine dropping a bowling ball onto a trampoline; the resulting dimple is how a hefty object such as our Sun warps spacetime. Two stars rotating around one another, also known as a binary system, will not only distort the surrounding space, but also send out waves of distortions through this fabric. The effect is much like a rotating lawn sprinkler spewing out spiraling streams of water. Those space ripples are known as gravitational waves.
Princeton University astronomers Joe Taylor and Russell Hulse discovered a stellar binary made up of two hyperdense "neutron stars" in 1973. This binary led to the proof of the existence of gravitational waves. Measurements by Taylor and colleagues over the following decade showed that the orbit of the two stars about each other was slowly shrinking. The rate at which their orbit decays exactly matches the predictions of Einstein's theory. The discovery won Hulse and Taylor the 1993 Nobel Prize in Physics.
The Arecibo telescope in Puerto Rico is used for the pulsar timing array experiment. The average power from a pulsar collected by this telescope is a billion-billion times fainter than a household lightbulb. (Courtesy NAIC - Arecibo Observatory, a facility of the NSF)
Yet to this day, no one has managed to detect gravitational waves directly affecting detectors on Earth. Don Backer seeks to change this.
Backer is working to detect the relatively strong gravitational waves generated by massive black hole binaries. These objects, which are found at the center of virtually all galaxies, tip the scales at billions of times the mass of the Sun.
Invisible, endless, and trillions of times fainter than the radiation emitted by the average toaster oven, gravitational radiation can't be "seen" directly. However, its effects can. So Backer tracks how gravitational waves alter the precise, metronomic signals received from objects known as millisecond pulsars. Backer and colleagues discovered the first of these highly magnetized neutron stars in 1982. Spinning like tops at nearly 1,000 times per second, millisecond pulsars act like cosmic lighthouses, sending out beacons of electromagnetic radiation from each pole. The signals are so regular that millisecond pulsars are considered among the universe's most precise clocks.
"When a pulsar signal is sent through space distorted by a gravitational wave, there is an effect," Backer says the blips will arrive sooner than expected, then later, in a repeating pattern. For that reason, he says, "we have to be exquisitely precise about any perturbations of that clock." With observations from the world's most powerful radio telescopes, he can time the arrival of each few-minute batch of pulses with microsecond precision .
Black holes orbiting one another produce ripples in space-time called gravitational waves. These ripples cause the signals traveling from two pulsars (P1, P2) to the telescope (T) to arrive faster or slower than they expected if they were traveling through flat space. (Courtesy JPL/NASA)
Black hole coalescence events emit broad, low-frequency gravitational waves that require up to ten years to cycle from beginning to end. Backer has observed one pulsar in particular for more than 17 years, with detector technology he has made increasingly sensitive over the past two decades. At the same time, he and colleague Andrew Jaffe have been fine-tuning theoretical models of what gravitational waves from coalescing black holes across the Universe will look like, enabling him to further perfect his detectors.
Idiosyncrasies in any one pulsar's timing are rare, but possible. To rule out such anomalies, Backer and colleagues are watching more than a dozen millisecond pulsars scattered across the sky as part of his Pulsar Timing Array experiment. "We're looking for a predictable pattern of perturbations amongst the different objects, which are spinning totally independent of each other, that we can't explain otherwise."
The signal itself, says Backer, "will be a cacophony of radiation coming in from all directions." Chaotic though it sounds, the data will open a window into a mysterious property of the unseen universe. "It will inform us about the overall rate of black hole coalescence events. We can't see particular sources. And it doesn't tell us whether there are more little ones than big ones. But it does reveal how much spacetime is being perturbed by the aggregate effects of waves from these distant events flowing through the solar system."
The Allen Telescope Array under construction at Hat Creek Radio Observatory near Mount Lassen in Northern California. These antennas are among the first 42 currently being outfitted and commissioned. The project goal is an array of 350. (Photo by J. R. Forster)
His discovery would also further the next generation of gravitational wave analysis. NASA and the European Space Agency are developing the Laser Interferometer Space Antenna (LISA), which should begin operating in space by 2015. The instrument is designed to detect the brilliant bursts of gravitational waves released by individual black hole mergers. Backer's gravitational wave data would tell astronomers how often these merging events are likely to occur.
Meanwhile, Backer continues to hone his pulsar detecting equipment and techniques. "We just have to make our measurements more precise with new equipment and new telescopes such as the Allen Telescope Array being built jointly by Berkeley's Radio Astronomy Laboratory and the SETI Institute at the Hat Creek Radio Observatory; find new pulsars to track; and dig deeper in the sensitivity."