In the Newtonian view of the world, binary star systems should remain in a stable orbit in perpetuity, no matter how massive the objects or how close the orbit. But with general relativity, that changes; energy gets carried away from the system in the form of gravity waves, which gradually causes the orbit to decay, ultimately leading to a merger.

By observing binary systems of massive objects, we've determined that general relativity gets it right. These systems behave just as general relativity predicts, giving us confidence that the theory is correct. What's missing is the other half of the confirmation: gravity waves. We haven't detected any originating from these systems. In fact, we haven't detected any, period.

It's not for lack of trying. For nearly a decade, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, searched for gravity waves from astronomical events, like the merger of two black holes (a number of other detectors have also engaged in the search, but all have come up empty). Now, scientists are readying a worldwide network of LIGO-like detectors that should start coming online in 2017. A short perspective in Science outlines the project's plans.

LIGO's design is very simple. Laser beams are sent down long, perpendicular tunnels where they bounce off mirrors before heading back to the source. When they return, the light from the two beams is allowed to interfere, which creates a distinctive pattern. If a gravity wave passes through the observatory, it will distort the space occupied by this setup ever so slightly and, since the two beams are perpendicular, the effect will be different on each. As a result, the beams will interact differently, and the interference pattern will change.

To increase sensitivity, the LIGO project also operated two detectors, one in Louisiana, the other in Washington. Due to the large separation, the waves should arrive at each at a slightly different time, allowing some rough estimate of the direction to the event that produced them.

If LIGO didn't detect anything, there were two potential explanations. Even the most energetic events, like the merger of two black holes, produce very weak gravity waves. As a result, the LIGO detectors' four kilometer tubes would only shift by about four billionths of a nanometer when one passed. The other problem is that these events are extremely rare, with estimates that only one occurs in the average galaxy over about 10,000 years.

Both of those issues are now being addressed. Using the lessons learned from the first detector, work is underway on an Advanced LIGO that may be operational later this year. But increased sensitivity will only get us so far if the events are rare. The solution there is to look deeper into the Universe, and to do that requires more detectors. Europe already has one in VIRGO, located outside of Pisa. These detectors will eventually be joined by LIGO-India and the KAGRA detector in Japan. With three detectors in operation, their combined sensitivity will let us sense events out to about a billion light years; with all five, our reach will extend out to nearly 2.5 billion. (For context, Andromeda, the nearest galaxy, is 2.5 million light years away.)

By 2017, we should be able to detect about 10 events a year. Perhaps just as importantly, we'll have a better sense of where the events we see are taking place. With only three detectors, the errors on locating events would be about 50 degrees; with all five, that figure drops to six degrees. This result might give us a chance to point a telescope at it to see what it looks like.

Of course, to some extent, we have to know what we're looking for, and in the case of a merger of black holes, that's not necessarily obvious. The author of the perspective, Mansi Kasliwal, indicates that theoretical physicists have been busy with trying to understand the sort of energy that might be released. Initial models put the optical output of neutron star mergers somewhere in between a nova and a supernova and suggest that they will emit reddish light for hours or days. That may be enough to allow an infrared survey telescope to spot one.

For now, gravity waves remain rooted firmly in theory, with only indirect evidence for their existence. But if all goes according to plan, that status will have changed by the end of the decade.

Science, 2013. DOI: 10.1126/science.1235956 (About DOIs).