This is Part 2 of a two-part feature on the search for gravitational waves. Read Part 1 here.
All of these techniques are exquisitely sensitive, seeking out minute changes. But gravitational waves might have a much stronger impact on matter than previously assumed, thanks to resonant frequencies. It’s something Alexander Graham Bell noticed as a young man: strike a chord on one piano and it will be echoed by a piano in another room. The effect is known as “sympathetic resonance.” Objects like a piano’s strings vibrate at very specific frequencies. If there is another object nearby that is sensitive to the same frequency, it will absorb the vibrations (sound waves) emanated from the other object and start to vibrate in response.
That’s the essence of a new paper in the Monthly Notices of the Royal Astronomical Society: Letters, proposing that certain stars could absorb energy from gravitational waves that ripple by. Should that happen, the stars would show a temporary marked increase in brightness from that excess energy that could be measurable.
Co-author Saavik Ford of CUNY’s Graduate Center compared stars to the bars on a xylophone, each of which has a natural resonant frequency, just like piano strings. Striking those bars in sequence, moving from lower to higher frequencies, is akin to how two merging black holes produce gravitational waves of gradually increasing frequency. “If you have two black holes merging with each other and emitting gravitational waves at a certain frequency, you’re only going to hit one of the bars on the xylophone at a time,” Ford explained. “But because the black holes decay as they come closer together, the frequency of the gravitational waves changes and you’ll hit a sequence of notes. So you’ll likely see the big stars lighting up first followed by smaller and smaller ones.”
Perhaps the Earth itself could be used as a gravitational wave detector: it, too, could vibrate like a bell in response to gravitational waves rippling through. Set up a global array of highly sensitive seismometers, and one could conceivably find evidence of such waves in the data That was the gist of a 1969 paper by physicist Freeman Dyson.
Dyson’s work was the inspiration for Harvard graduate student Michael Coughlin and a colleague, Jan Harms of the National Institute of Nuclear Physics in Florence, Italy, who were working with seismic data relating to LIGO with an eye toward reducing the noisy background so that a signal would be more easily detected. Coughlin recalled Dyson’s paper and thought such an approach could be useful for setting some vital constraints on background noise, and he and Harms did an initial analysis of terrestrial seismic data.
Then another professor recalled his earlier geophysical work with instruments placed on the moon during the Apollo missions to track so-called “moonquakes.” Those instruments collected lunar seismic data from July 1975 to March 1977. Intrigued, Coughlin and Harms analyzed that older dataset as well, correlating it with their earlier terrestrial analysis. They published their findings in Physical Review Letters earlier this year.
Coughlin and Harms didn’t find any evidence of gravitational waves in their analysis, nor did they expect to. One reason is that there is a lot of seismic noise from other sources cluttering up the data. The moon might not have Earth’s plate tectonics, atmospheric fluctuations, or volcanic activity, but asteroids routinely hit the moon, causing it to “ring” for weeks from the impact. There is also background noise generated by thermal heating from the Sun and tidal forces.
Cornish pronounced their work a good analysis but said it is unlikely to lead to direct detection of gravitational waves, even if NASA placed upgraded seismometers on the moon with far greater sensitivity to get a better dataset. He suggested that the best way to search for gravitational waves in that frequency range is the space-based LISA, now known as the Next Gravitational-wave Observatory (NGO), another very large and pricey collaboration similar to LIGO (in that it uses a similar laser interferometer array) that is still several years’ away from completion. Meanwhile, LIGO is currently undergoing upgrades, including an additional mirror to increase its sensitivity to other frequencies of gravitational waves, like those produced by binary pulsars.
Still, there remains much uncertainty in the various proposed models for gravitational waves. Coughlin’s and Harms’ null result has helped further constrain the range in which we should expect to see gravitational waves in Earth’s vicinity. “If we thought we knew what the source distribution of gravitational waves looked like in the universe, then it wouldn’t be quite such a useful exercise,” Coughlin said. “Since we don’t, and the cost is relatively low, I don’t see why we shouldn’t try it.”
Author’s picks for further reading
Galileo’s Pendulum: Will We Ever Detect Gravitational Waves Directly?
Matthew Francis explains how LIGO and similar detectors are advancing the search for gravitational waves.
Nature: Wave of the Future
Alexandra Witze previews the launch of the new advanced LIGO.
TED: The Sound the Universe Makes
In this video, astrophysicist Janna Levin explains how gravitational waves are made and LIGO’s role in searching for them.