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Scientific Progress Goes “Chirp”!
Northern leg (x-arm) of LIGO interferometer on Hanford Reservation. By Umptanum – wikipédia, CC BY-SA 3.0.
Ladies and Gentlemen, we have detected gravitational waves! We did it!” And with that, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the discovery of a strange feature of General Relativity that’s been hiding since the 1960s.
Gravitational waves are ripples in the fabric of space, caused by the acceleration of mass. The bigger the mass and the faster it’s accelerated, the better, and there’s nothing that better fits that description than two black holes orbiting each other. These waves expand and compress space, which slightly changes distances between objects in their path. Two objects floating freely in deep space far from any forces, would find themselves carried a little towards each other and then a little away from each other and back and forth, as the wave passed over them. If you could measure the distance between them accurately, like with a laser range-finder, you’d detect that oscillation. But this is tough to do on the Earth, because you can’t have the two objects floating freely any more. The best you can do is to hang them from fine threads, like a swing. They’re not free to move in every direction, but they can swing back and forth.
So, how did we find them? LIGO shines laser beams down two 4-kilometer (2.5-mile) vacuum pipes, arranged in an “L” shape. At the end of each pipe is a mirror that reflects the beam back and measures its length, with an accuracy of 10^-18 m, a thousandth the width of a proton. This always amazes me, because the mirror itself is made out of atoms, which are bumpy and much larger than protons. And yet, it works because the bumps average themselves out. When a gravitational wave comes by, it shrinks the distance along one pipe while expanding the distance along the other (as they’re at right angles) and then it switches.
What they’ve announced today is the detection of gravitational waves coming from the merger of two black holes, spiraling into each other a billion light-years away from us. At 29 and 36 times the mass of the Sun, these aren’t very big for black holes: the ones in the centers of galaxies have millions or billions of solar masses. But the smaller ones are more plentiful, which means they’re more likely to collide. As the two in question orbited each other, the gravitational waves carried away more and more of their energy, making them spiral inward, until they finally collided and released massive amounts of energy (the equivalent of the mass of three Suns converted directly into energy).
Take a look at the wave patterns that LIGO felt:
The vibration is vertical, and time goes to the right. Early on, the waves are slow (i.e., broad) and weak (i.e., short). As the black holes spiral inward, the waves strengthen and increase in frequency until they make a loud “chirp!“ when they collide. Then, there’s a faint wiggling at the end, as the new, combined black hole rings like a bell until it settles down.
This pattern is so clear, you can see it without needing the computer to sort it through for you. What are the odds of detecting this? My wife — who works in numerical relativity (the computer simulation of the waves) — tells me that it was entirely possible they’d run this for decades before they heard anything. Instead, they found this within days of turning the machine on. That suggests that these black holes are colliding all over the place, and we’re going to find plenty more.
So what good is this? The press is mostly writing headlines shouting, “Einstein is proven right!” This is true in a simplified sense, but not too many of us were expecting otherwise and that’s not really why all of this was built. With more detections and better signals, we’ll get to the point where we can test theories that go beyond General Relativity, which is interesting. But I think the real value of this discovery will be observational.
Until now, astronomy has almost exclusively relied on what we can see with light: i.e, electromagnetic radiation, ranging from radio waves on one end to gamma rays on the other. We can see the gas falling into black holes — glowing x-ray hot before it disappears — and we can see supernovas from the light they emit. But there’s a lot going on down in the compact centers of these things that we can’t see with light. Gravitational waves will give us an entirely different way to probe them.
Already, the gravitational wave people and the visible light astronomers have teamed up to do joint searches for flashes of light that might accompany a gravitational wave burst. My wife, for example, is working on the computer simulations for these. This photo shows her working out the algorithm for her models.
The discovery of gravitational waves has been a long time coming. Searches started in the 1960s, and there’s even a detector on the Moon, placed there by Apollo 17. But the evidence up to this point had always been indirect. Now, we’ve finally got something both direct and obvious.
That’s big news.
Published in Science & Technology
Just curious what else would happen when the black holes merge? Is there the mother of all gamma ray bursts or some other massive release of energy?
Hi, everyone. I’m traveling this evening and won’t be able to reply right away, but I wanted to let you all know that I’ll be responding to all of the questions that you’ve asked, if they haven’t been already answered. Ricochet obviously has a good crop of scientists, engineers, and all-around educated laymen who know a lot about these things.
And then there’s anonymous, who’s in a category by himself!
Talk to you all a bit later.
MichaelC19fan—Very likely. In fact, a colleague of mine just linked to a paper by the Italian Fermi space telescope people, who saw a gamma-ray flash at the same time,in about the right direction. I’ll post a link when I get the chance. Iffy but could be very exciting.
Well, that’s a relief…especially given the Lileks Multiple Kardashian Theory of Cosmology. There many aspects of life that I’d hate to live through all over again…disco music, Gilligan’s Island, the Beanie Baby craze, puka shell necklaces…
Come on, don’t you have an appreciation for the solid feel of chalk on slate? Actually, she’s got a whiteboard in her regular office, but this was taken while we were on sabbatical at Pitt. As for myself, I prefer the look of the chalkboard, because then you get to imagine yourself in place of Einstein or Feynman in one of those famous pictures.
This does let us put some limits on the speed of gravitational waves, because we know that a flash of light would be seen no more than 10 msec apart in these two locations. (That’s the light travel time between them; Wolfram|Alpha is a great service!) So if a gravitational signal were detected more than 10 msec apart by the two LIGO facilities, we’d conclude that the “speed of gravity” is less than the speed of light.
If we work with the theory that they travel at the same speed, then we can use the time difference between them to find the direction the signal came from, although it gives us an arc across the sky.
I don’t think so, at least not in practice. They’re just too weak, far from the source. And to be strong enough to work, I’d think the compression and expansion they cause would be dangerous. But now that you mention it, I wonder if there would in principle be some way of extracting energy from them.
That’s actually an interesting idea—whether or not you could “ride” this kind of wave, I think you might be able to extract energy from it. Now I should suggest that to my wife as a paper to write up, just for the fun of it.
Inteferometry is actually pretty cool stuff. We’ve got a couple (hmm… twelve? maybe more) machines in the factory that use it to make precise measurements. I mean, not LIGO precise, or even Michelson-Morely precise, but good enough.
Dangerous how? If gravity is compressing/expanding space along with the objects therein, what sort of stresses would that produce?
Unless you’re talking about the strength of gravity varying quite a bit across the size of an object then yeah. Hey waitaminute, that was a Larry Niven story too.
I know what you mean. This kind of engineering precision is amazing. Maybe they use atomic clocks on each of these sites, but I don’t know how everything is synchronized. Actually…in practice, this experiment might not need unbelievably precise timing, down to the scale of the effects you’re talking about. The gravitational wave patterns have details around the millisecond scale, so maybe they don’t need more precision than that.
The big thing is to measure changes in distance at unbelievable precision, and that is handled by the interference pattern in the laser. I still have trouble imagining how they get a precision of 10^-18 m!
Thanks for finding those papers. I’m intrigued by the latter and its reference to “Doppler friction” in the abstract.
I would think that the mechanical strain the wave produces could also dissipate some energy, when it passes through solid matter. I’m trying to picture a thought experiment involving a spring separating two otherwise free-floating masses. In free-fall, the masses’ separations would oscillate. But I think solid matter doesn’t expand and compress as much from the wave. I’m stuck there at the moment…
To a factor of pi? Z, you’re giving us a lot of credit. We feel good when we get to within a factor of 10, some times!
But our arguments over the value of the Hubble constant, H, were a factor of only 2x for many years: 100 km/s/Mpc vs. 50 km/s/Mpc. Yeah, we had that universe expansion rate nailed! ;)
I’ve had this back-of-my-mind dread of the heat death since my freshman physics professor told us about it. Cosmologist Roger Penrose has an interesting idea on how the universe can be “reborn” in different epochs and get around the problems posed by entropy, but it’s controversial. Still, it’s hopeful.
The multiverse worries me more.
So, to make a black hole you need a certain amount of energy density.
Let’s talk Event Horizons. That’s the limit, right? Anything going the speed of light or less can’t get out of the black hole past that horizon. To form an event horizon you need a stupendous amount of gravity to stop light in it’s tracks. A bigger black hole has a bigger event horizon, and a smaller one has a, well, smaller one.
If you start with an event horizon in mind (what’s your price range?) you can figure out how much space there is in that sphere, and how much energy you’d have to cram in there to get it to, ah, stick. For large masses (star sized and up) you can get that with a supernova. For things smaller than that we just don’t know of anything remotely energetic enough to get it to work. Atom bombs don’t even come close.
The idea of quantum black holes is that, for very very tiny black holes you could potentially find something energetic enough to do the trick. If you do the math (which no one does because it’s difficult and disappointing) the Large Hadron Collider is orders of magnitude away.
I attended a colloquium once at UW Milwaukee. My mechanics professor was giving a talk about his research. He was arguing that, if string theory is correct, a quantum black hole would have it’s event horizon extended into the extra dimensions (which you’ll recall must be rolled up and tucked away somewhere down there), which would reduce the energy density necessary to something with might possibly see from the LHC.
And then he gave a slide on why it wouldn’t actually destroy the world. Given the energy density of gluon space (inside a proton. We’re talking 2-3 orders of magnitude smaller than that.) these black holes would evaporate due to Hawking radiation well before they’d get big enough to consume the planet.
And then I go back to his mechanics class where I’m having a difficult time wrapping my head around where billiards balls go when you hit them. In my experience, anywhere but the pocket.
I remember one lecture in college where they describe a calculation of what you’d expect in (a first guess at) theory, to observation, and getting a 120 order of magnitude error. “Which is too much for even an astronomer to forgive.”
Don’t remember a thing about the problem, but I remember the joke. I was always great at the nerd jokes.
“Heisenberg gets stopped by a police man. Do you know how fast you were going? No, but I know where I am!”
I think (think!) that the way it works is that if a wave expands (or, alternately, compresses) space itself by, say, a factor of 2x, a solid object is not simply 2 times larger. Freely-floating objects away from any forces would be separated by 2 times their original distances, but the atoms in solid matter are bound together with forces—the molecular bonds—and these make the object retain pretty close to its normal size. But the wave would be felt as an expansion or compression force (a mechanical stress) applied to the object. If the wave were large enough, then the applied stress could be dangerous.
The key is that while space expands and contracts, physical objects don’t expand and contract by the same amount. (And here I have to check with the Mrs. to be sure I said that right. Others here might know.)
Ahh 120 orders of magnitude—that’s the cosmological constant problem. The standard prediction of what the constant should be is 120 orders of magnitude off from what it actually is. Which indicates there’s probably a quantum-mechanical screening effect that prevents us from feeling the full force of the cosmological constant. Which in turn leads to one of the great “fine tuning” problems—why is this effect exactly strong enough to allow our universe to produce life?
True story: eons ago when, as a young pup, I started working at an analytic shop doing math I was given a new office and asked what kind board I wanted for it. I replied “blackboard (please)”. “And four (4) please, one for each wall. Actually, the wall I look at from my desk has room for two boards, so five (5) in all. Thanks.”
They did it for me. I know they thought me crazy, but, yes, they did it for me. And my colleagues (different company) today never let any of our new hires go a week without trotting out that old chestnut to confirm how weird they explain I am.
Guess how much I actually used those boards? Uh, huh.
Darn, I probably just revealed my true identity to a number of former colleagues who might be fellow Ricos now. Well there goes my CIA cover.
You don’t know how much of a relief this will be to many Ricos here.
Questions like this always intrigue me. If the effect wasn’t exactly strong enough to allow our universe to produce life, we wouldn’t be discussing it.
Hi, Gaby,
Yes, but just what it is, is a little complicated and context-dependent. Her full name is Maria-Cristina Babiuc Hamilton. She’s Romanian, and since every third girl is named “Maria” there, she went by “Cristina.” But here that’s not the case, so most people know her as Maria Hamilton. Family and her closest friends call her Cristina Hamilton. And since she was publishing before we got married, she continues to publish under Maria Babiuc.
So it’s almost like Cristina Hamilton writes under the pen name of Maria Babiuc!
Here is a link to several of her papers.
Interesting! I hadn’t heard of this one before.
The simplicity with which many of these thought experiments clarify complex problems impresses me. On the opposite site, there’s one simple question that’s been asked which has not yet been really resolved. John Wheeler (I think), at a conference in the 1950s asked, if you took an electric charge and dropped it in a gravitational field, would it radiate? The room was split between the two possibilities.
One side says yes. An accelerating electric charge radiates light. If the charge falls in a gravitational field, it accelerates, so it must radiate light.
The other side says no. Free-fall in a gravitational field is, through the equivalence principle, the same as floating freely at rest, with no forces. So it can’t radiate.
Someone asked the same question fifty years later at a Perimeter Institute meeting my wife attended, and the audience was still pretty evenly split between the answers!
I kind of lean towards it not radiating, because the equivalence principle seems more fundamental to me. But the well-observed fact that charges accelerated (by other forces) radiate makes me wonder if whether you observe it to radiate or not radiate would depend on whether you are in the same reference frame of the charge.
I love this line.
Another member already answered, I see, but I’ll add that we think there may be a few different ways to do it. Mini-black holes might come from particle reactions. Stellar-mass black holes from big supernovae. Perhaps the seeds of supermassive black holes came from fluctuations after the Big Bang.
I agree that it’s embarrassing that we’ve never been able to detect a single particle of dark matter. Particle physics predictions keep getting falsified. That’s what drives the guys trying to work on alternative theories of gravity, which would eliminate the need for the unidentified “missing mass.” But over the last ~15 years, we’ve been able to map out dark matter (assuming it exists) and found structure in it. Clumps, rings, displacements, and so on. That is tough to match by changing the theory of gravity, and it gives me some confidence.
Awesome, thanks for the follow up.
Tell her feel free to cite me in her work. I am sure the physics world will be most impressed.
Yes, I think so. The first thing you can do is take the delay between the two detections. Assuming it travels at the speed of light, then from geometry, that tells you that the signal came from a certain angle, relative to a line connecting the two LIGO facilities. That means it was somewhere on a cone centered on that line.
Now, if you look at the sky map showing where they think it was, you see not a full circle tracing that cone on the sky, but only an arc. So they’re able to narrow down the location further. The LIGO detectors are not sensitive to a wave coming in perpendicular to their L-shape. Since Louisiana and Washington are at two different points on the globe, it might be that the difference in angle of the surface of the Earth is enough to localize it further. At this point, I’m not sure what clues they use to narrow it down more than that.
Oh, they don’t actually orbit at or beyond the speed of light, just very, very close to it.
Yes, they are more intense close to the source and weaken with distance. I think they weaken pretty much as 1/distance-squared (an inverse-square law). If it’s not exactly this, due to some geometric distortion from gravity, then I’m sure it’s pretty close to it. Waves, even gravitational waves, can overlap one another without causing problems, just like water waves can overlap one another. You would get an interference pattern, but that just means that two waves in synch with each other add constructively, making a bigger wave there, while two waves exactly out of synch would add destructively, canceling out the wave in that spot.
So, if they couldn’t tell which black holes — where in the sky — caused the waves, how did they know they were “a billion light-years away from us. At 29 and 36 times the mass of the Sun”? I infer from those quantities a knowledge of two specific black holes. In fact, how did they know there were only two?
So, that answers most of my questions back to Mark. And, they could not tell the direction? And, trinary mergers are theoretically impossible, or very unlikely?