Two black holes circle each other in a gravitational dance. Spiraling closer over thousands of years, they eventually get so close that they can no longer keep dancing. In a fraction of a second these two black holes merged into a single, larger black hole. It’s an event that happens fairly regularly throughout the universe. But this time, a group of humans 1.3 billion light years away measured the ripples in space and time produced during the merger.
It’s hard to overstate the significance of our first direct detection of gravitational waves. On the one hand the discovery announced in Physical Review Letters confirmed what we’ve suspected for decades: gravitational waves exist. By itself that’s not a big deal, since they are a natural result of general relativity, and we’ve had indirect evidence of gravitational waves since the 1970s. The direct detection of gravitational waves is yet another confirmation of what we’ve already known. On the other hand, this opens up an entirely new window to the universe.
The event as seen in the two LIGO detectors (above) compared to the numerical model fits (below). Credit: B. P. Abbott et al.
The paper released today has been peer reviewed, which is comforting given the BICEP2 incident. It’s also a remarkably strong result given the extreme sensitivity necessary to detect gravitational waves. The advanced LIGO experiment consists of two detectors located in Louisiana and Washington. To qualify as a real detection, there must be a nearly simultaneous event in both detectors with the same basic form. In the above image, the event in question matches up quite well. It also matches the expected signal as calculated from numerical simulations of merging black holes. This is a strong, clear signal confirming gravitational waves.
The data is good enough that we actually know quite a bit about the merging black holes. The larger black hole had a mass of about 36 Suns, while the smaller one had a mass of about 29 Suns. When the two black holes merged they formed a single black hole of about 62 Suns. You might notice those numbers don’t add up. That’s because in the process of merging, about 3 solar masses worth of energy was radiated away as gravitational waves. That’s a huge amount of energy to release in a fraction of a second, which is why we can detect it so clearly from more than a billion light years away. We also know some broader characteristics, such as how fast the final black hole rotates, roughly where in the sky the merger occurred and the cosmological redshift of the event (which is how we know its distance).
While the detection of gravitational waves is the biggest news, this is also further confirmation that black holes are real. If they weren’t black holes their merger would create a burst of light or neutrinos like a stupendous supernova, which wasn’t seen. This is also the first clear observation of a black hole merger.
In the history of human civilization humans looked up at the sky and saw light. When Galileo first raised his telescope to the sky he saw light. Over the centuries we’ve widened the range of what light we can observe. We’ve launched telescopes into space to see types of light not visible from the ground. With the exception of some neutrinos and cosmic particles, the field of astronomy is rooted in our ability to observe and analyze light.
But now we can listen to the very fabric of space and time.
Paper: B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration).Observation of Gravitational Waves from a Binary Black Hole Merger. Phys. Rev. Lett. 116, 061102 (2016)