Pulsars are spinning, highly magnetized neutron stars that are left after stars go supernova. Some spin thousands of times per second, sending a signal to Earth on each rotation. "Millisecond pulsars have extremely predictable arrival times, and our instruments are able to measure them to within a ten-millionth of a second," says NANOGrav's Maura McLaughlin. "Because of that, we can use them to detect incredibly small shifts in Earth's position."

Right now the team is monitoring 54 pulsars, but most of them are in the northern hemisphere. As such, it is enlisting the help of teams in Europe and Australia "in order to get the all-sky coverage this search requires," says JPL's Michele Vallisneri. Once that happens, the team is confident they'll see evidence of low-frequency gravitational waves within 10 years. Meanwhile, a space mission called eLISA that can detect higher-frequency waves is scheduled to launch around 2028, so by then, they may be old hat.