Up until a year ago, gravitational waves were a theoretical construct, a consequence of the theory of relativity. We had indirect evidence that they were real, as energy was lost from binary star systems just as Einstein predicted. But directly observing them took the upgraded Laser Interferometer Gravitational-Wave Observatory (LIGO). As soon as we detected them, however, astrophysicists were quick to point out that they gave us a completely new window through which we could view the most energetic events in the Universe.
As a sign of just how seriously that claim is being taken, European physicists just opened up a new detector. Called VIRGO, it will combine with LIGO to give us a better picture of where events are taking place. And Virgo isn't the end. Researchers used the meeting of the American Association for the Advancement of Science to discuss the next generations of gravitational wave detectors—as well as the continuation of what's now a decades-long experiment that has come up empty so far.
VIRGO and LIGO work based on the same physical principles. Laser light is sent back and forth between mirrors at the ends of two perpendicular 4km long arms. After a sufficient number of trips, they're recombined in a way that lets small changes in the distance between the mirrors to be detected. Passing gravitational waves ripple the fabric of space, changing that distance infinitesimally.
How small is the movement? According to Nobel Prize winner Takaaki Kajita, a strong gravitational wave will change the Earth-Sun distance by 10-8 centimeters. So, the laser light has to bounce between the mirrors many, many times (about 280, to be exact) to increase the effective distance traveled. And, even then, random noise dominates the data obtained.
Having won the Nobel for his role in creating an underground neutrino observatory, Kajita was on hand to talk about Japan's work on moving gravitational wave detectors underground. The KAGRA gravitational wave observatory is being built in the same mine (Kamioka) as the neutrino detector (called Super-Kamiokande). While the KAGRA is going to have shorter arms than LIGO—3km as opposed to 4km—Kajita said that moving it underground cuts the seismic noise by a factor of 100.
But KAGRA is also doing something about additional noise of its key parts: the mirrors that repeatedly send the light back and forth down each arm. In KAGRA, these will be sapphire mirrors that are cooled to 20 Kelvin, which should get rid of lots of thermal and vibrational sources of noise. Expectations are that KAGRA will begin operations in a couple of years. That will make for four active observatories, which will provide detailed positioning information on the source of any gravitational waves. That, Kajita said, will allow for follow-up observations using light, which should provide a more complete picture of events like the merger of neutron stars.
LIGO in space
Of course, making longer arms is still possible, and plans are in the works to make them much, much larger: about a million kilometers apart. Obviously, that can't be done on Earth, which eliminates the issue of seismic noise. Stefano Vitale of the LISA project said that arms of this size would also allow the detection of gravitational waves at much longer frequencies than LIGO does.
This has some significant advantages. For example, the two merging black holes detected by LIGO started producing gravitational waves that gradually increased in frequency as they got closer to each other. LIGO only picked up a few seconds' worth starting right before the black holes merged. LISA, Vitale said, would have spotted the system over a year before LIGO saw anything, meaning we could have followed the black holes' death spiral.
Larger black holes, like the supermassive ones at the center of galaxies, also produce low-frequency gravitational waves as they spiral in toward merger. Since this should only happen through galaxy mergers, Vitale said LISA would allow us to build "a natural history of the Universe," watching how the dynamics of mergers changed over time. We'd also be able to follow a small black hole orbiting a supermassive one 10,000 times as it spiraled in, allowing us to map the event horizon.
All of which means that LISA will be able to detect plenty of sources, but it will be seeing many of them simultaneously. I asked Vitale how you would separate out these overlapping signals. His response? "Very carefully!" He went on to say that simulations indicated we would be able to tease apart any signal that stood out at least 20-fold more than background noise.
So, what would it take to get LISA to work? The plan for LISA is to set up three spacecraft in a triangular configuration, each sending lasers to the other two. At these distances, even lasers spread out considerably, so the spacecraft would have to have two telescopes to focus the light onto the hardware. The big challenge, however, comes from the fact that we can't stabilize spacecraft sufficiently for our needs—they're constantly bumping into photons, particles, and dust that jostle them slightly. Instead, the spacecraft will act as a shell and shield for hardware floating freely inside. The craft will maneuver enough to keep from jostling the experimental hardware while absorbing all the bumps from the environment.
Obviously, this requires some exacting hardware requirements. So the ESA has already started the preliminary testing through the LISA pathfinder mission. Even with the test hardware, the stability was close to what would be needed for the full LISA mission. Two rounds of tweaking in response to the data they've collected mean it's now operating better than we'd need to get LISA to work. Sadly, however, the earliest launch date isn't until nearly 2030.
Who needs a laser?
(Everybody but the sharks, obviously.)
Of course, for some people, a million kilometers just won't do. Going bigger gets even longer frequencies, some of which enter into the territory where you can test some versions of string theory. At those distances, however, we can't make a laser that's sufficiently bright, and we can't get a mirror out to the far end anyway. So we rely on a light source that nature has created: pulsars.
Pulsars are rotating neutron stars that sweep their magnetic poles through the location of the Earth, bathing it in a burst of radio waves in the process. The timing of their rotations is ridiculously precise. Sarah Spolaor of the National Radio Astronomy Observatory said, "It's an incredible measurement error—I get really excited about that," while showing a slide cluttered with zeros. While I didn't get the precise number down, the variations of other pulsars have been measured down to 10-13Hz.
As with LIGO, a gravitational wave passing between the pulsar and Earth would expand and contract the space in between, shifting the timing slightly. By observing two pulsars in different directions from the Earth, the effects of gravitational waves could be detected. The idea is so simple that astronomers have developed an entire array of appropriately located pulsars, and they have been making regular observations for decades. Doing so, however, has turned up nothing.
But that's a good sort of nothing. The pulsar timing arrays should be able to pick up merging supermassive black holes well before LIGO or LISA, when they're still about a dozen light days apart. The number we'd detect is going to be proportional to how long two supermassive black holes spend within this distance before merging. Non-detection, Spolaor said, means that they're not spending that much time, which means they have to be losing energy and spiraling inward rapidly. Which in turn means that they're losing energy by one or more mechanisms in addition to radiating away gravitational waves. Which tells us something interesting must be going on in galactic cores following mergers.
Either that, or some alternative theory of gravity is in operation.
While the session was intended to build interest in going beyond LIGO (and, from my perspective, worked brilliantly at that), LIGO isn't done yet. Its spokesman, LSU's Gabriela González, was in the audience. She let slip that in its second operational run, LIGO had already triggered two alerts for astronomers, telling them there was something they might want to point their telescopes at. More details will have to wait for the analysis of the full dataset obtained from run two.