ts 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 scientists call this gravitational wave signal as well.
The signal itself may not look like much, but it contains multitudes. The shape and strength of the signal depends on the individual masses of the black holes, how far away they are from Earth, and also how quickly they merge. From all this the LIGO scientists (along with their VIRGO collaborators) found that they were 31 and 19 times the mass of the Sun each.
What gets the hair on the back of my neck standing on end is what happens at the moment of coalescence. The equations governing relativity predict that a large fraction of the masses of the black holes is converted into gravitational wave energy. The thing is, a tiny bit of mass converts into a lot of energy, thanks to that whole E being equal to mc2 thing. “c2” means the speed of light squared, and that’s an unholy huge number. So when you take a lot of mass and convert it to energy, the numbers get, well, terrifying.
Two times the mass of the Sun was converted to energy in this event. The mass of the Sun is 2x1030 kg, or, if you prefer, two octillion tons. Converted to energy, that’s 2x1047 Joules. How much energy is that?
It’s two thousands times the amount of energy the Sun emits over its entire lifetime. And these merging black holes blasted that much energy out in less than a second. In fact, for that brief fraction of a second, just this single event radiated away roughly as much energy as all the stars in the Universe combined. All of them. Everywhere.
I’m glad they were three billion light-years away. By the time the signal got here, it had weakened to the barest whisper of the roar it once was. But near the event, right there next to those black holes, this was the single most powerful event in the entire Universe. At that moment.
I have no words for this, and in fact I cannot comprehend it. Oh, I can do the math, put the numbers together, compare it to various other things. But to truly grasp this, to viscerally internalize it, is far, far beyond what our puny ape minds can handle.
The Univer