A pair of merging black holes 3 billion light years away sent ripples of spacetime through the Earth
Presented by: Phil Plait
[Artwork depicting two black holes orbiting each other. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)]
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 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 Universe dwarfs us.
But let me add something here, because it's beautiful.
Black holes spin. They have to; they form from stars that explode (usually), and these stars are spinning. When the stellar core collapses the spin is magnified for the same reason that an ice skater's spin increases rapidly when they draw their arms in. This is called "conservation of angular momentum," and basically says that when something is spinning, that spin rate will go up if the size of the object shrinks.
Both of the black holes that merged were spinning. But they were also in orbit around one another. Now, here's an amazing thing: If the two black holes were spinning in the same direction as they orbited each other (say, they both spun clockwise looking down from above, and their orbit around each other was also clockwise), then their merging will be a bit slower. It's tougher to slough off the angular momentum they have, so the merger is sluggish.
However, if one or both of their spins are not aligned with their orbital spin, the merger happens more quickly. This spin-orbit alignment (or lack thereof) is reflected in the signal we detect here on Earth; the speed of the merger tells us this. And in this case, although it's not rock-solid, it appears as though the spins were not aligned.
That's interesting. If these two stars formed together as a pair of normal but ridiculously massive stars orbiting each other, then their mutual gravity would align their spins over their brief lifetimes. The signal we saw seems to weigh against that.
Instead, it may be they formed separately in a dense cluster of stars. They exploded as supernovae separately (though, again, see my previous post about that), and over time their gravitational interaction with other stars in the cluster dropped them to the cluster center. It was there that they matched up, eventually becoming bound to each other, circling in a dance that would, after several billion years, result in the catastrophic merging and explosion of energy we detected billions of years after that.
All this inferred from the timing of a signal so weak that it moved a set of mirrors less than the size of a subatomic particle.
This is science. This is what we humans do. We can plumb the depths of the Universe without ever leaving home, but instead explore by patiently waiting and cleverly listening in ways our ancestors couldn’t even have imagined.
And yet, despite all this, we're just getting started. In the case of gravitational waves this is literally true; we’ve been listening for decades but detected the first signal less than two years ago. Even today, a whole new field of observational physics is still possible to be opened.
What awaits us ahead? What new and amazing things have we yet to learn, yet to imagine?
It's a big Universe, and it whispers its existence to us constantly. With science, we can hear what it has to say.