Spacequake

Spacequake

The discovery of gravitational waves is a seismic event for physics and astronomy, and not only in the metaphorical sense. Just as earthquakes are the release of energy stored in subterranean faults, these gravitational waves were set in motion when two black holes – faults in space itself – became a single, stable object. Call it a spacequake.

We measure earthquakes on the Richter scale; at magnitude 3 an earthquake is big enough to feel while a 9 is a major disaster. The magnitude reflects the energy released by the quake; with each step up the scale the quake's energy grows by a factor of 32. But even the biggest faults can only store so much energy, and the best guess is that earthquakes can't get past a 10. That may be comforting news, but other, bigger catastrophes can be included: the asteroid impact that wiped out the dinosaurs was around magnitude 13, while a 16 on the Richter scale would be a planet-shattering cataclysm.

So how big is a spacequake? The merger LIGO saw produced a black hole 60 times more massive than our sun, but three solar masses smaller than the pair of black holes that preceded it. The missing mass is the energy released in the spacequake (via E=mc2). Plugging that energy into a handy formula tells us that this spacequake came in at Magnitude 33 on the Richter scale. And that's a boatload of energy.

If the black hole merger produced visible light rather than gravitational waves, the flash in the sky would have been briefly been brighter than the full moon. So why were the gravitational waves so hard to detect? The answer is that space is remarkably rigid. Gravitational waves stretch space but even a huge amount of energy still produces a tiny amount of stretching. Yes, space bends and stretches, but it is more like a steel plate than a trampoline: a steel plate will bend if you stand on it, but not by very much. Consequently, physicists' favorite metaphor for the stretchiness of space, the ball on a rubber sheet, is a little bit misleading...

Today the entire history of gravitational wave astronomy consists of a single flash in the sky. But that flash tells us the story of an epic spacequake that really did happen a long time ago, in a galaxy far, far away. Astronomers are eagerly awaiting the sequels.


CODA: A quick google reveals that the term "spacequake" has also been used to refer to a disturbance in the Earth's magnetic field but it doesn't seem to have caught on.