The discovery of gravitational waves

TITLE: First predicted by Einstein in his General Theory of Relativity, gravitational waves are ripples in the fabric of spacetime. Caused by cataclysmic events in the universe, such as black holes merging, the waves carry information about their origins and the very nature of gravity itself. These waves had never been detected arriving at Earth until LIGO (Laser Interferometer Gravitational-wave Observatory).

KIP THORNE, Richard P. Feynman Professor of Theoretical Physics, Emeritus, Caltech: I came to Caltech in 1966 on the faculty working on relativity. I began thinking in great detail about the future of gravitational wave physics and astronomy. Ray Weiss had been the primary inventor of the laser interferometer gravity wave detector. In his classic paper, written in the early 1970s, just lays it out and says here's what you have to deal with, here's how you deal with it.

RAINER WEISS, Professor of Physics, Emeritus, MIT: The important thing to do in this field was to do experiments. To test the theory. And those are hard experiments, because turns out gravity, relative to all the other forces we know about, is a pipsqueak. It's nothing.

THORNE: Between 1980 and 1983, NSF funded Caltech to build a 40-metre prototype. And NSF funded MIT to do a feasibility study. So each of these efforts then running in parallel became crucial inputs.

WEISS: In 1989 we made the proposal to built LIGO. It was a Caltech/MIT joint proposal. 

DAVID REITZE, Executive Director and Research Professor of LIGO, Caltech; Professor of Physics, University of Florida: And in 1992, NSF, after very careful review, said okay, we're going to go forward with this. We're going to fund the project. And this was a bold thing. I mean, NSF took a bold step.

THORNE: NSF bought in because they had faith in Barry Barish, the most brilliant director of large projects that physics has ever seen.

REITZE: He immediately saw that some of the things that needed to happen should be happening quickly, like you need to build buildings, you need to build observatories, and it took years. Hanford got started first, then Livingstone. And then of course once the facilities were constructed, you have to start putting in the detectors. In 1999, there was a dedication ceremony.

Interferometers were working, they weren't working anywhere near their design sensitivity, there was a lot of work that needed to be done before we were ready to go into the first observing run. Nobody had ever made something like this before, so there was a lot of technological challenges that needed to be overcome.

Barry Barish realised that if you're building these big interferometers, you'd better have a community.

LAURA CADONATI, Associate Professor of Physics, Georgia Tech: The LIGO scientific collaboration is about 1,000 people. We all want to detect gravitational waves, we all want to start doing gravitational wave astronomy.

GABRIELA GONZÁLEZ, Professor of Physics and Astronomy, Louisiana State University; Spokesperson, LIGO Scientific Collaboration: We didn't have two collaborations saying, who detects it first, who detects it better. We work together.

REITZE: We ran our intial detectors from 2002 to 2010.

WEISS: We saw nothing. Now, you'll say, that's a terrible defeat. That's not true. The LIGO scientific collaboration was in existence and they were deeply involved with the data analysis, and tracked down everything that we didn't understand about that detector, and found out what it was. We had already proposed that, in the 1989 proposal, that we were going to do a two-stage thing. We would build the initial detector, and then we would build a follow-on detector with the experience we'd gotten from the initial detector, called advanced LIGO.

REITZE: We rebuilt our detectors. We'd been designing them for about twelve years or so.

ALBERT LAZZARINI, Deputy Director of LIGO, Caltech: We had more light, better suspensions, and better isolation from the ground, and it's that combination that allowed LIGO to become ten times more sensitive.

REITZE: It was Monday morning, September 14th. 

LAZZARINI: I knew that there was something going on because I subscribe to the logs.

REITZE: This particular log pointed to something that looked like it might actually be a gravitational wave. 

THORNE: What I saw is what's called a time frequency plot called a chirp. And it was strong. It was unbelievably stronger than anything I expected to be a first detection. It was so strong you could see it by eye, and here is the chirp at Hanford Washington, and there was the chirp at Livingston Louisiana, and I thought, my God. This looks like it's it.

REITZE: It was just perfect. In fact, it was almost too good to be true. When I looked at it, I said, somebody must have done something wrong and injected a signal.

WEISS: Nobody believed it. Everybody thought it was a fluke, it was too good. And it took us a while to get to the point where all of us believed it.

GONZÁLEZ: It's monumental. It's like Galileo using the telescope for the first time.

THORNE: When our descendants look back and they ask what is the legacy of that era for humanity, I think it will be rather similar to us looking back on the era of the Renaissance when we say that the legacy was great art, great music. So I think LIGO and gravitational waves, along with the electromagnetic study of the universe, will be a huge part of our legacy for future generations.