In most cases, detecting the effects of gravity isn't that hard. Skydivers rush toward the ground the moment they step out of a plane, and thanks to space telescopes, you can see light being warped into stunning rings by massive groupings of galaxies. But it has proven especially hard to detect gravitational waves, ripples in space-time triggered by a powerful cosmic event.
Most attempts so far have looked for the way space-time ripples are expected to influence light and matter. Now scientists in the U.S. and Israel think we could find the waves quicker and cheaper if we look for their effects on time instead of space.
The hunt for gravitational waves has been on since 1916, when Albert Einstein predicted they should exist as part of his general theory of relativity. He made the case that space-time is like a fabric, and what we sense as gravity is a curvature in that fabric caused by massive objects. Like a bowling ball suspended in a blanket, for example, our massive planet Earth curves space-time around it.
The theory also suggests that when very massive objects like black holes merge, the gravitational blast will send ripples propagating outward through space-time. Detecting them would not only continue to validate Einstein's theory, it would open a new window on the universe, because scientists could use gravitational waves to probe otherwise invisible events across the cosmos. But proof of gravitational waves has been elusive, in large part because the waves grow weaker the farther they travel, and many gravitational wave sources are found at the edge of the universe, billions of light-years away.
Last year an experiment called BICEP2 claimed to have detected the faint signals associated with a type of primordial gravitational wave, produced by a sudden growth spurt in the early universe. The claim was premature, though, as later analyses reduced the confidence that the BICEP2 team saw anything more than swirling dust in the Milky Way.
The European Space Agency’s planned eLISA observatory, due to launch in 2034, is designed to detect a different kind of wave: millihertz-range, or low-frequency, gravitational waves generated by the merger of supermassive black hole pairs. Scientists have discovered supermassive black holes at the centers of many large galaxies, including our own. The coalescence of two such galaxies is predicted to emit gravitational waves that can propagate across the universe. To find them, eLISA will use lasers to measure tiny changes in the spacing of a spacecraft fleet that should happen when a gravitational wave passes by.
In a new paper, Avi Loeb at the Harvard-Smithsonian Center for Astrophysics and Dani Maoz at Tel Aviv University point out that recent advances in timekeeping could allow atomic clocks to detect gravitational waves faster and cheaper than eLISA. They outline a proposal for an array of atomic clocks stationed at different points around the sun that could detect a phenomenon called time dilation, when gravitational effects can cause time to slow down.
Like eLISA, their plan also requires spacecraft flying in formation and communicating using lasers. But instead of relaying information about changes in distance, the lasers will keep track of tiny discrepancies in timekeeping between synchronized atomic clocks installed aboard the spacecraft.
The predicted temporal changes are tiny: "We're talking about one part in a million trillion in timing precision," says Loeb. "To detect that kind of change, you need a clock that will neither gain nor lose only one tenth of a second even if it were to operate for 4.5 billion years, or the entire age of the Earth."
Until recently, this kind of accuracy was beyond the ability of atomic clocks that use the element cesium, which are the basis for the current international standard of timekeeping. But in early 2014, physicists at the National Institute of Standards and Technology (NIST) unveiled an experimental “optical lattice” atomic clock that set new world records for both precision and stability. These clocks operate at optical frequencies and so provide greater accuracy than cesium atomic clocks, which rely on microwaves to keep time.
In theory, optical atomic clocks can provide the precision necessary to detect the tiny time shifts predicted from gravitational waves. Loeb and Maoz argue that their design would be simpler and could be achieved for less cost, because it would require less powerful lasers than eLISA. Atomic clocks of lower precision are already being used on GPS satellites, so Loeb thinks it should be possible to send the new generation of atomic clocks to space too.
The best setup would be a pair of atomic clocks installed on twin spacecraft that share Earth's orbit around the sun. A main spacecraft would also be in orbit to coordinate the signals coming from the clocks. The clock-bearing craft should be separated by about 93 million miles—roughly the distance between Earth and the sun, or one astronomical unit (AU).
“That’s a nice coincidence, because one AU happens to be roughly equal to half a wavelength for a [low-frequency] gravitational wave, like the kind scientists think merging supermassive black holes emit,” says Loeb. In other words, that would be precisely the right distance to sense both the peak and the trough of a gravitational wave passing through the solar system, so atomic clocks positioned at these two points would experience the greatest time dilation effects.
For now such a mission isn't on any space agency workbench or budget proposal. But Loeb hopes the idea will trigger a more careful study of eLISA alternatives. The eLISA project "benefited from decades of discussion, so we should allow this alternative design to be studied at least for a few months before dismissing it.”
Loeb adds that there are numerous practical applications from having more precise atomic clocks in space, such as better GPS accuracy and improved communications. He thinks the first optical lattice clocks could be launched by businesses for commercial purposes, rather than by government agencies. “If that happens, any science we get out of it would be a byproduct,” he says.
Jun Ye, a physicist at the University of Colorado and a NIST fellow, says Loeb and Maoz’s proposal “opens a new intellectual front” on the use of optical atomic clocks to test fundamental physics, including the search for gravitational waves. “I am optimistic about further improvement of optical clocks and their eventual use in such applications,” says Ye.