The quest for the sexiest signal in physics—ripples in space and time called gravitational waves emanating from stellar sources—is once again underway. This past weekend, physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) began their first observing run since they rebuilt their instrument in Livingston, Louisiana (shown in this video), and its twin in Hanford, Washington. Advanced LIGO is already three times as sensitive as the original LIGO, researchers say. Even so, there's only a small chance that it will spot anything in this first 3-month data-collection run. The real news may be that, in spite of all its complexity, Advanced LIGO has kept up with an observing schedule researchers laid out for it in 2013.
"Progress is coming faster than we expected," says Gabriela González, a physicist at Louisiana State University, Baton Rouge, and spokeswoman for the more than 900 scientists in the LIGO Scientific Collaboration. "We have a sensitivity that's at the higher end of what we expected at this point."
Each LIGO instrument, or interferometer, looks for stretching of space and time, or spacetime, by using laser light to compare the lengths of an interferometer’s two 4-kilometer-long arms, which are set at a right angle. Researchers aim to compare the lengths of the arms to a precision of 10-19 meters—a billionth the width of an atom. To do that, they must damp out myriad vibrations caused by seismic waves, human activity, and other sources. LIGO physicists' benchmark source would be a pair of neutron stars spiraling into each other. The original incarnation of LIGO, which cost $360 million and ran from 2002 to 2010, reached a sensitivity high enough to detect such a source if it was up to about 65 million light-years away. Advanced LIGO should eventually be able to detect such sources out to 650 million light-years, increasing the volume of space probed by a factor of 1000.
Right now, however, the LIGO interferometers are sensitive enough to see such binary neutron stars out to just between 200 million and 260 million light-years, says Frederick Raab, a physicist at the California Institute of Technology in Pasadena, who leads the Hanford observatory. That factor of three improvement over the original LIGO gives physicists "an outside chance" to see something, Raab says. The big question is how many sources there might be. In their 2013 plan, LIGO researchers estimated that, optimistically, with the current sensitivity, they might spot three binary neutron stars in 3 months. Pessimistically, they might have to run LIGO for 600 years to see one.
Advanced LIGO's chances should be much better in subsequent runs, physicists say. "As we become more familiar with the machine it will get tuned to higher sensitivity," Raab says. If things go as planned, LIGO should run next year for 6 months, and be able to detect binary neutron stars about two times farther away than now; the year after, the plan is to run for 9 months looking out another 50% farther. Starting next year, LIGO will also be joined by Europe's similarly revamped VIRGO detector near Pisa, Italy.
But even coming up empty in this run would be revealing, González says. That's because some very optimistic models predict that Advanced LIGO will soon see pairs of more massive black holes spiraling together, and "testing those would be the science goal of the first run," she says.
If Advanced LIGO does make a discovery, it will take time for researchers to validate the signal. "We've set ourselves a goal being able to make an announcement within 3 months of seeing something in the detector," Raab says. But if there is a discovery, will LIGO leaders be able to keep it from leaking out? "We worry about that a lot," González says. "We have all signed confidentiality agreements."