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The folks at LIGO have done it again: For the third time they have detected the whisper-faint roar as a pair of black holes ended their long courtship by merging into a single, bigger black hole. By doing so, they have shown that gravitational wave astronomy is more than a novelty; it’s a fundamentally new field of astrophysics.
In this new case, two black holes three billion light-years away (!!) coalesced, their individual masses of 31.2 and 19.4 times the Sun’s mass combining to form a single black hole 48.7 times the mass of the Sun.
If you do the math, you see that two solar masses seems to be missing. Where did that go?
It was converted into the energy of shaking the very fabric of time and space itself.
Oh yeah. This is truly incredible, mind-crushing stuff. I promise you’ll be amazed by the time you finish reading about this. The energies and forces involved are brain-bending.
For background, I strongly urge you to read what I wrote the first time gravitational waves were directly detected in 2015, and then again when they were detected a second time in 2016. There’s lots of good stuff there. Also, there's the official journal paper with the new results if you want the technical details.
But, as a recap, here’s the deal.
Einstein’s Theory of General Relativity was a lot more than just E=mc2 and time dilation making twins age at different rates. It showed us that space and time were two aspects of the same thing —spacetime— and that this wasn’t just some background in which events happen; instead, spacetime was a thing unto itself, and this changes the way we interpret the Universe.
One of the most important aspects of relativity is that spacetime can be warped by matter. We perceive this warping as gravity, which (as the name implies) affects both space and time, and of course the matter within it. One of my favorite phrases in physics is "Matter tells space how to bend, and space tells matter how to move."
But it gets weirder. Objects that are accelerated can create ripples in the fabric of spacetime, much as grabbing the end of a sheet and moving it up and down rapidly creates ripples that propagate across it. These ripples in spacetime are called gravitational waves. In general they’re weak and mushy, and nearly impossible to detect … but, if you have very strong gravitational sources accelerating very rapidly, the waves are sharper and stronger, making them easier to find.
And hey, black holes are extremely strong gravitational sources. If two are orbiting each other closely, they accelerate each other fiercely. They emit gravitational waves as they do so, which robs them of orbital energy. The outcome is that they spiral together, slowly at first, then ever faster, until one day, BLOOP! They merge, forming a single bigger black hole and emitting a huge burst of gravitational energy in the form of spacetime ripples.
The Laser Interferometer Gravitational-Wave Observatory was designed to detect these waves. It consists of two separate facilities, one in Washington state and the other in Louisiana. It uses lasers to detect extremely small shifts in the positions of mirrors set several kilometers apart; as a gravitational wave passes through the Earth, it compresses and expands the space between the mirrors by a tiny bit. How tiny? Less than the width of a proton. Yeah. How about that?
And on January 4, 2017, at 10:11:58 UTC, for at least the third time, just such a ripple washed over the LIGO detectors (the signal arrived at the Louisiana site 3 milliseconds after the Washington site, a product of the time it took light to move that far plus the position of the source on the sky). The signal was characteristic of in-spiraling massive objects; as they get closer they circle each other faster, so the frequency of the gravitational waves increases rapidly, as does the signal strength (called the wave amplitude). When you make a sound that gets louder and increases in frequency rapidly, it sounds like a chirp. So that’s what s