## Saturday, February 13, 2016

### Gravity Waves!

Two days ago, on February 11th, the LIGO and Virgo interferometer collaborations announced the first direct observations of gravity waves (actually detected on September 14th, 2015). Why is this a big deal? Gravity waves are one of the predictions of Einstein's general theory of relativity, predicted a hundred years ago in 1916. They were also the last major prediction of general relativity not to have been experimentally verified, and have been the object of search for over fifty years, making this an especially momentous detection.

Gravity waves are caused by the finite speed of propagation of gravity, and are created when any mass accelerates (they're analogous to the electromagnetic radiation emitted by an accelerating charged particle). They're essentially ripples in spacetime, like the wake of a boat on water. However, due to the weakness of gravity, they're very, very weak unless you're talking about extreme concentrations of mass in extremely tiny regions. Thus, the best place to look for them is in binary systems of neutron stars or black holes, and indeed the detected waves most likely came from one of the latter.

You see, all orbiting masses radiate energy away in the form of gravity waves. For most systems, the amount of energy is negligible—for the Earth-Sun system, the radiated energy is about 200 watts; probably less energy than the computer I'm typing this on is using at the moment. The energy lost causes orbiting bodies to move closer together over time, though for the Earth-Sun system the distance works out to about the width of a proton per day (i.e., nothing to worry about). However, for dense masses in close proximity to each other like two orbiting black holes, the amount can be significant enough that it causes the orbits to decay much more rapidly.

Eventually the orbits shrink enough that the two objects (black holes, in this case) merge into a single object. As this happens the frequency of the radiated waves will rapidly rise, then level off in a way that gives a lot of information about the objects involved. In this case, we can say that the two objects were black holes with roughly 36 and 29 times the mass of the Sun, respectively. Unfortunately, due to the fact that only two interferometers actually detected the signal, we can't tell exactly where it came from, merely constrain it to an arc mostly within the South Celestial Hemisphere. It was likely pretty far away though, approximately $$1.3\pm0.6$$ billion light-years away.

There's an interesting feedback effect that happens with this whole process: as the orbits radiate energy away as gravity waves, they shrink, but as they shrink, they radiate more energy away, which causes them to shrink faster, which makes them emit more energy, etc. From the detected signal, it turns out that in the last 20 thousandths of a second before the merger the two black holes were radiating away $$3.6\times10^{49}$$ watts of power as gravity waves; to put that in perspective, that's more energy than is emitted as electromagnetic radiation by all the stars in the observable universe combined. (It really says something about the weakness of gravity that it took this absolutely mind-boggling amount of energy to actually be observable.)

All in all, this was a major step forward in verifying the last major prediction of general relativity, as well as opening up a whole new avenue of investigation for astrophysicists. Now that we know it's possible, hopefully we'll see more work put into additional, more sensitive interferometers in the future that'll allow us to detect weaker gravitational waves from more sources. A hui hou!