When Apollo astronauts landed on the moon, they left flags and footprints, yes, but also dozens of scientific instruments. Among them was a network of seismometers originally meant to study moonquakes. Forty years later, data from these seismometers are still helping physicists understand how to detect elusive gravitational waves — a challenge even with our fancy modern technology.
What are gravitational waves and why do we care about finding them in the first place? Well, it goes back to a guy you may have heard of called Albert Einstein. Einstein's theory of general relativity says that gravity is caused by warps in the space-time continuum, and the warping also creates vibrations we call gravitational waves. These gravitational waves are tiny amounts of energy rippling through the universe.
Primordial gravitational waves that originated from the Big Bang may or may not have been detected earlier this year, but gravitational waves can also come from things like black holes merging or two stars orbiting around each other. There's evidence for these waves, but we've never directly detected gravitational waves of any sort.
But there are indirect ways, and that's where the moon comes in. As gravitational waves ripple through a celestial object, its energy causes the object to vibrate. The Earth is rife with seismometers that could theoretically detect this vibration, but the Earth's crust is constantly moving, drowning out the gravitational wave signal. The moon is seismically quieter. And conveniently, between 1969 and 1972, four Apollo missions left a network of seismometers that operated until 1977.
A couple of physicists had the bright idea to sift through this decades-old data. (Their paper was uploaded to the preprint repository ArXiv, and the Physics ArXiv Blog has a wonderful write-up about it.) The seismometers couldn't actually detect any gravitational waves, but this lack of data was scientifically illuminating. We know the sensitivity of the moon seismometers; that they couldn't pick up gravitational waves means the activity of the waves must be below a certain threshold — a threshold that turns out to be 1000 times lower than previously limits for waves of a certain frequency.
To detect gravitational waves directly, we still need to build modern detectors — on Earth or in space. But it's a pleasant surprise that data from Apollo missions long ago can still tell us something about a 21st century cosmological mystery. [Physics ArXiv Blog, ArXiv]
Picture: Alan Bean of Apollo 12 deploying instruments on the moon. NASA