Thursday, February 11, 2016

LIGO Detects Gravitational Waves from Merging Black Holes!

If you've been on social media at all today and you're either friends with or following a physicist, you've probably seen them mention something—or, perhaps, a lot of things—about gravitational waves and wondered "what the heck are they so excited about?" Rest assured, this is the most scientifically exciting discovery since the Higgs Boson (at least within the realm of the physical sciences). Today, the folks at LIGO (Laser Interferometer Gravity-wave Observatory) have announced the first ever direct detection of gravitational waves. Better still, those gravitational waves were produced by the merger of two black holes each roughly 30 times the mass of the Sun into a single black hole. This is also the first time we have detected such an event.

Gravitational waves are a prediction of general relativity, one of the few remaining ones that had eluded us (until last September). Gravitational waves are created as objects accelerate (their velocity changes either in direction or magnitude) in space. This is pretty much the same as the idea from Maxwell's Laws governing electromagnetism, in which accelerating charges create electromagnetic waves. The only difference is that gravitational waves specifically move through spacetime, distorting it slightly as they go.
LIGO is actually a pair of facilities—one in Livingston, Louisiana, one in Hanford, Washington—specifically designed to detect gravitational waves. They do this by splitting a laser beam from a single source and projecting it along two perpendicular arms (depicted below), each precisely 4000 meters long. The lasers are reflected back toward their origin, where they are brought back together. If these two beams travel exactly the same distance, then they will cancel one another out, and no signal will be detected at the end. However, if a gravitational wave passes through the detector and slightly distorts the length of one of these arms, the two beams will be less out of phase, so you'll briefly get a signal.
Aerial image of the LIGO site in Livingston, Louisiana. Image courtesy of ligo.org
Turns out, this is precisely what happened on September 14, 2015. Just as the new and recently improved Advanced LIGO came online (in engineering mode, actual observations weren't scheduled to happen for another 4 days!), they saw a blip in the data at the Livingston site. This, of course, would have been no big deal had another signal not been detected at the Hanford site 7 milliseconds later! The comparison between the two is shown below.
Figure 1 from Abbott et al. 2016
The top plots show the distortion of spacetime caused by the gravitational waves as detected by LIGO at each of the LIGO sites. The panels beneath that show models of what we expect the distortions from binary black hole mergers to look like, and the panels below those show the difference between the top two plots. Specifically, the "strain" is the fractional (relative) change in the length of those two 4 kilometer laser beams that make up LIGO's detectors. The bottom panels show the change in the signal in frequency space over time, while the color shows the intensity of the signal.

LIGO also has a system for getting follow-up observations on suspected astrophysical gravitational wave sources. In short, they can figure out roughly (to about within 100 square degrees on the sky), and ask a bunch of other telescopes to look for anything interesting going on in that region. As it just so happens, my friend and officemate Lea Hagen is a science planner for NASA's Swift Gamma Ray Burst Mission, so I was able to ask her how this whole procedure works. Swift, which I have talked about before, is an awesome little space telescope that conducts rapid follow-up observations of things that explode in space using on-board x-ray and optical/ultraviolet telescopes.

At 9 PM on September 16, the Swift team got an alert from LIGO requesting follow-up observations on a region of sky around the Large Magellanic Cloud. Swift's procedure when given a large target area like this is to focus observations on some of the more likely targets in the given field, those being the nearby galaxies within that field of view. For more details on this exact procedure, you can check out this paper from 2015 (thanks Lea!) Swift, along with the other follow-up observations, detected no visible counterparts in the region, as one would expect for two objects that we can't see merging with one another.

Based on models of merging black holes (to reproduce the signal), the pair of black holes in question were likely around 29 and 36 times the mass of the Sun apiece, and formed a black hole of about 62 times the mass of the Sun. This missing 3 solar masses were converted into pure gravitational energy. That's roughly 1047, or one hundred billion billion billion billion billion, Joules of energy. Based on these models, astronomers were also able to determine that this "new" black hole is 1.3 billion light years away from Earth.

This is the beginning of an incredibly exciting new phase in the history of astronomy, and frankly, I'd be surprised if there aren't already a few more detections the LIGO team will be coming out with in the next year or so. This marks the first glimpse of a field we've hoped to be able to observe basically since the beginnings of general relativity. I, for one, look forward to gravitational wave posters and talks becoming commonplace at future American Astronomical Society meetings, to the point where it's just another way we can do astronomy. Until then, however, this detection is something special, so it's absolutely worth the hype.

For an even better write-up, check out Phil Plait's post over at Bad Astronomy.


Yo mama's so fat, we detected her gravitational waves before upgrading LIGO.

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