How to catch a ripple in spacetime

How do you build a real-world machine to test the most abstract of theories? Janna Levin talks with Rai Weiss, one of the original designers of LIGO, the four-kilometer-long instrument that has now twice detected the distant reverberations of two black holes crashing into one another.

Janna Levin is a theoretical physicist — she works with pen and paper to turn the elegant rules of the universe into theory (and also uses that pen and paper to write books about science; her latest, Black Hole Blues, was recently published by Alfred A. Knopf). Rainer Weiss, or Rai, as he’s known, is an experimental physicist — he thinks about how to find and measure something that may or may not exist outside of theory. Weiss was part of the group that designed and built LIGO, the detector that, in September 2015, for the first time “heard” the sound of two black holes crashing into one another, producing a gravitational wave that stretched spacetime and proving something that Einstein first floated as a theory almost 100 years ago. Earlier this week, the LIGO team announced that on December 26, 2015, they had detected another rippling, spacetime-stretching gravitational wave that erupted 1.4 billion years ago when two other black holes collided at half the speed of light.

In a sparkling conversation held at a Brooklyn art space, Pioneer Works, Levin and Weiss (with help from journalist John Hockenberry) talked about how they work, how LIGO works — and the moment when they realized Einstein had just been proven right. An edited version of the discussion follows.

The 50-year, billion-dollar bet

Janna Levin: The first detection made with LIGO was of a pair of black holes that collided 1.3 billion years ago. The success of the experiment was not only the detection of gravitational waves. That was the first time we detected a pair of black holes, because they’re dark and we can’t see them with telescopes. So it was really an astronomical discovery as well.

The detection happened in September 2015. In August 2015, Rai and I were talking and he said — this after the experiment cost a billion dollars, people had spent 50 years building it — he said, “If we don’t detect black holes, the thing is a failure.” I thought that was a really bold thing to say on the eve of starting the detection, because most people in the scientific community did not think we would detect black holes, at least not for a long time.

Rai Weiss: What was behind that statement — it is slightly facetious, of course — but you have to understand what black holes ultimately mean to people, and what they mean to the theory of gravity that Einstein put together. It’s the one thing that is so Einsteinian, we couldn’t have gotten it any other way. The whole idea of gravity curling up space, that is the epitome of what is going on in a black hole. I would’ve loved to have seen Einstein’s face if he were presented with the data that we actually discovered such a thing, because he himself probably didn’t believe in much of it.

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time—and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time. Image courtesy T. Pyle/Caltech/MIT/LIGO Lab

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time—and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time. Image courtesy T. Pyle/Caltech/MIT/LIGO Lab.

When black holes collide

JL: I’m a theoretical physicist. I only work pen-on-paper. I think that’s why I became so enamored of the physicality of the LIGO experiment. It’s one thing to say, “Oh, two black holes are going to collide,” and work it out on paper. It’s quite another thing to build giant L-shaped instruments that span four kilometers on two coasts.

RW: The equations don’t tell you much until you put actual numbers into them and you find out how much energy is involved in making a distortion of space. A gravitational wave is a very slight stretching in one dimension. If there’s a gravitational wave traveling towards you, you get a stretch in the dimension that’s perpendicular to the direction it’s moving. And then perpendicular to that first stretch, you have a compression along the other dimension. It compresses that way, and it stretches that way.

The problem is the energy that’s needed to make that very tiny little distortion in the fabric of spacetime. Space is much stiffer than you imagine; it’s stiffer than a gigantic piece of iron. That’s why it’s taken so damned long to detect gravitational waves: to deform space takes an enormous amount of energy, and there are only so many things that have enough.

JL: Those two black holes were orbiting each other probably for a few billion years, and they were creating gravitational waves the whole time, but the waves were weak because they were still far apart. It wasn’t until the black holes got within a couple of hundred kilometers of each other, in the final 200 milliseconds of their life together, when each one’s going at nearly the speed of light, and they collide to make a new black hole. It’s only that last fraction of this very long process that they’re able to detect at LIGO.

How to measure spacetime, stretching

RW: Here’s the fundamental problem when it comes to measuring the stretching of space: We’re looking for a motion that is 1,000th the size of a proton. It’s 10^-18, which is a decimal point, 17 zeros all in a line, and then a one. You can make the measurement by using a lot of light and an interferometer to look at things that are about 10^-18 meters of motion. You can do that. That’s the easy part. The hard part is how not to detect everything else around you — the people next to you, the leaves blowing on the trees in Louisiana, the winds blowing and howling up in Hanford, Washington. If you’re lucky, if the building is a good building, the amount of motion of the ground you’re dealing with is about a micron, it’s about 10^-6 meters. But you have to somehow get rid of that motion.

John Hockenberry: What were you doing when you thought of the first prototype of LIGO?

RW: I was teaching a course. I was trying to explain what Joseph Weber, the man who first did these gravity wave experiments, was doing, how he had devised a technique I couldn’t understand, and I couldn’t explain it to the class. So I sat and sat and sat, and thought: How can I make the most pristine, simple-minded explanation of how you could detect a gravitational wave? It was very straightforward — at least maybe to me it was: Let me launch a big mass off in space, and put a good clock on it, and over here, I’ll launch another one with another good clock. Then we’ll measure how far they are apart — we’ll send a light beam from one to the other, figure out when it hits, measure it and see what the clock says. Then let the gravitational wave come down, which stretches and compresses space, and see how that changes that time. That was the basis of the idea. I gave that as a problem to the students in the class, and they all were able to do it, and then I put it aside. But in 1971 or so, that’s when I took this idea seriously — I said, maybe this idea is okay. So then it became, how would you do it in a practical way?

JL: When I first went to see the LIGO facility, I was blown away. There’s a sort of corner lab, and it’s a cold room, a clean room, so you go in in bunny suits and laser goggles. And that holds the apex of the machine where a laser comes out and it gets split and sent down two arms. The arms punch right outside the lab in an L-shape, so they cut right out into the — in the case of Hanford, the desert, and in the case of Louisiana, the swamp. Those arms are four kilometers long in each direction, and they just go. There’s not much between the apex and the end stations. So you take a truck, you drive four kilometers down to the end station, and there’s another much, much smaller lab which houses a chamber for the mirrors.

As in Rai’s original thought experiment, one beam of laser light is split into two, and the two beams head off in different directions toward mirrors at the end of each arm. The two beams should take the same amount of time to go out and come back to the apex. But a ripple in spacetime actually changes the distance each laser beam has to travel — so they don’t perfectly meet up back at base. By monitoring the interference pattern where they meet up, we can record the stretching and compression of spacetime.