New type of gravitational wave detector proposed

The existence of gravitational waves is one of the central predictions of Einstein’s theory of general relativity. Gravitational waves have never been directly observed, though the indirect evidence is so good that Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland, recently said

“We are so confident that gravitational waves exist that we don’t actually need to see one.”

But most scientists for good reasons prefer evidence over confidence, and so the hunt for a direct detection of gravitational waves has been going on for decades. Existing gravitational wave detectors search for the periodic stretching in distances caused by a the gravitational wave’s distortion of space and time itself. For this one has to very precisely measure and compare distances in different directions. Such tiny relative distortion can be detected very precisely by an interferometer.

In the interferometer, a laser beam is sent into each of the directions that are being compared. The signal is reflected and then brought to interfere with itself. This interference pattern is very sensitive to the tiniest changes in distance; the longer the arms of the interferometer, the better. One can increase the sensitivity by reflecting the laser light back and forth several times.

The most ambitious gravitational wave interferometer in planning is the eLISA space observatory, which would ping back and forth laser signals between one mother space-station and two “daughter” stations. These stations would have distances of about 1 mio kilometer. The interferometer would be sensitive to gravitational waves in the mHz to Hz range, a range in which one expects signals from binary systems, probably one of the most reliable sources of gravitational waves. eLisa might or might not be launched in 2028.

In a recent paper now two physicists from Tel Aviv University, Israel, have proposed a new method to measure gravitational waves. They propose not to look for periodic distortions of space, but periodic distortions of time instead. If Einstein taught us one thing, it’s that space and time belong together, and so all gravitational waves distort both together. The idea then is to measure the local passing of time in different locations with atomic clocks to very high precision, and then compare it.

If you recall, time passes differently depending on the position in a gravitational field. Close by a massive body, time goes by slower than farther away from it. And so, when a gravitational wave passes by, the tick rate of clocks, or of atoms respectively, depends on the location in the field of the wave.

The fineprint is that to reach an interesting regime in which gravitational waves are likely to be found, similar to that of eLISA, the atomic clocks have to be brought into distances far exceeding the diameter of the Earth and more like the distance of the Earth to the Sun. So the researchers propose that we could leave behind a trail of atomic clocks on our path around the Sun. The clocks then would form a network of local time-keepers from which the presence of gravitational waves could be read off; the more clocks, the better the precision of the measurement.

Figure from arXiv:1501.00996

It is a very ambitious proposal of course, but I love the vision!