This is Part 1 of a two-part feature on the search for gravitational waves. Read Part 2 here.
Physicists are poised to make the first-ever direct detection of gravitational waves. Will the detection come from a big-budget experiment already a decade deep into the search? Or will one of a handful of dark-horse experiments win an upset?
When Albert Einstein published his general theory of relativity in 1916, he revolutionized physics and reenvisioned the nature of spacetime and gravity: he showed that spacetime was dynamic, not static, and reimagined gravity as the bending and warping of spacetime by massive objects. He also made the startling prediction that gravity travels in waves. Just as objects moving through water cause waves to ripple outward, objects moving through space should produce ripples in spacetime. The more massive the object, the more it will churn the surrounding spacetime, and the stronger the gravitational waves it should produce.
Most of these ripples would be small, and would dissipate too quickly to be detected, Einstein predicted. But certain bodies, such as merging black holes, supernovae, or orbiting pairs of neutron stars, are massive enough that they might produce detectable gravitational waves. Physicists have devised a number of ingenious methods to detect gravitational waves directly, from extremely precise laser interferometry to clever schemes for using stars, pulsars, even the Earth and moon as gravitational wave detectors.
The big-ticket project is the half-a-billion-dollar Laser Interferometer Gravitational Wave Observatory (LIGO), which boasts two highly sensitive laser interferometers in Louisiana and Washington state. Here’s how it works: Split a laser beam in two and send each beam down one of two long, perpendicular tunnels, each with a mirror at the end. When the laser beams strike the mirrors they will be reflected back to the same spot, where they will recombine and cancel each other out. But if a gravitational wave happens to be passing through, it will warp the space between those mirrors ever so slightly. One beam will travel a longer path than the other, and when they meet up again, they won’t cancel each other out, producing light that will be picked up by a detector.
LIGO ran from 2002 to 2010—almost a decade—yet failed to detect any gravitational waves. Furthermore, LIGO is sensitive only to a small fraction of the gravitational wave spectrum. Just as optical, radio, x-ray, infrared, and gamma-ray telescopes each reveal different, and complementary, electromagnetic views of the cosmos, says Montana State University physicist Neil Cornish, it will take more than one kind of gravitational wave telescope to “see” the full gravitational wave spectrum. “You can only see [the waves] in their particular [frequency] bands because the frequency they emit is set by the mass of the system,” Cornish explained in an interview. “We need to open up the entire gravitational wave spectrum just like we’ve opened up the entire electromagnetic spectrum [in astronomy].”
LIGO’s range centers on stellar remnant black holes and other celestial objects of similar mass. Tackling the lowest end of the gravitational wave frequency spectrum are the headline-grabbing BICEP2 and Planck experiments, which are looking for imprints left by gravitational waves from the earliest moments of our universe in the polarization of the cosmic microwave background radiation.
To detect higher-frequency gravitational waves, like those produced by supermassive black holes, astronomers are using pulsars—rapidly rotating neutron stars that beam out regular radio pulses—like beacons on the sloshing sea of spacetime. One such effort is the North American Nanohertz Observatory for Gravitational Waves (NanoGRAV), part of an international consortium that also includes the European Pulsar Timing Array, and the Parkes Pulsar Timing Array in Australia.
The first pulsar was discovered in 1967, when Jocelyn Bell Burnell and Antony Hewish noticed strange, highly regular radio pulses coming from a fixed point in the night sky. They cheekily dubbed the mysterious object LGM-1 (for “little green men”). The signals weren’t coming from alien transmissions, however, but from a rapidly rotating neutron star. Pulsars form when stars more massive than our Sun explode and collapse into neutron stars. As they shrink, they spin faster and faster, because angular momentum is conserved. (Think of what happens when you swing an object around your head on a string: the more you shorten the tether, the faster it goes.) Pulsars also blast out radiation that can be picked up on Earth whenever that beam sweeps into our direction, like the rotating beam of a lighthouse.
The fastest pulsars, spinning hundreds of times per second, make excellent clocks—on par with the best atomic clocks. “That regular rotation of a pulsar is like the swing of a pendulum,” said Cornish, and it enables astrophysicists to precisely time all kinds of astronomical systems. Pulsars have helped astronomers identify distant exoplanets, and provided the first indirect evidence for gravitational waves back in 1982, when astronomers observed energy leaking out of a binary pulsar system—probably in the form of gravitational radiation.
The NanoGRAV network uses data from telescopes at the Arecibo Osbervatory in Puerto Rico and the Green Bank Telescope in West Virgina to monitor 19 pulsars in the Milky Way that serve as a galactic-scale gravitational wave detector. The method is described on NanoGRAV’s Website as a “cosmic Global Positioning System… looking for tiny changes in the position of the Earth that are due to the shrinking and stretching effect of passing gravitational waves,” although Cornish said the analogy is imperfect. The GPS employs multiple satellites to triangulate the three dimensions of space, thereby pinpointing the location of the source of a signal. NanoGRAV is looking for a common effect in the form of a telltale signature: a “shimmering” effect produced because pulses affected by gravitational waves should arrive slightly earlier or later in response to those ripples in spacetime. While no detection has yet been made, the collaborators are currently combining data from all three arrays to further improve accuracy and precision, according to Cornish. Those results should be released in the next several months.
“Compared to the cost of LIGO, this is the bargain basement way of detecting gravitational waves,” says Cornish. “The NSF has made this major investment using laser interferometers. For a tiny fraction of that, they have a chance to enable detection using pulsar timing. As far as bang for buck, it’s the cheapest way to go about it.”
Author’s picks for further reading
Galileo’s Pendulum: Will We Ever Detect Gravitational Waves Directly?
Matthew Francis explains how LIGO and similar detectors are advancing the search for gravitational waves.
Nature: Wave of the Future
Alexandra Witze previews the launch of the new advanced LIGO.
TED: The Sound the Universe Makes
In this video, astrophysicist Janna Levin explains how gravitational waves are made and LIGO’s role in searching for them.