Dr. Thomas White: At 5:51 on the morning of September 14th, 2015, a team of scientists witnessed something many thought was impossible: the direct detection of gravitational waves. These ripples in space-time caused by violent cosmic collisions, in this case, the collision of two black holes had been predicted by Einstein over 100 years earlier, and even he thought there would never be proof of their existence. Welcome to the Discover Science Podcast Series, where we sit down with some of the country's leading scientists and researchers for a conversation about the incredible discoveries that shape our world. I'm Thomas White, a professor of physics here at UNR.
Dr. Richard Plotkin: And I am Richard Plotkin, a professor of astronomy here at UNR.
White: Today we sit down with one of the scientists who first detected that great cosmic collision, Dr. González Gonzalez of the Laser Interferometer Gravitational-Wave Observatory or LIGO for short.
Plotkin: Gabby is a professor of physics at Louisiana State University. She was elected as the spokesperson for the LIGO collaboration from 2011 through 2017. She's the winner of numerous awards, including the 2019 SEC Professor of the Year, the Bruno Rossi Prize from the high energy astrophysics division of the American Astronomical Society, which was awarded to both her and the LIGO collaboration, and she is also a member of the US National Academy of Sciences and she's a fellow of the American Academy of Arts and Sciences. Gabby, thank you very much for joining us. We're really thrilled to have you here to kick off our podcast series.
Dr. Gabriela González: Thank you, it's an honor to be here.
Plotkin: You're, of course, a world-renowned expert on gravity and gravitational wave astronomy. For the listeners at home, can you just explain in simple terms, what's a gravitational wave?
González: Like you said, this is something that Einstein predicted right after publishing his theory of general relativity, which is a theory of gravity. It has a fancy name, but it's just a theory of gravity. It says that bodies attract each other because they deform the space-time, they curve the space where they leave weaker the space. I will say because we have mass.
Plotkin: Hopefully not too much.
González: Not too much. Not as much as the sun does and that's why the earth goes around the sun. It's not because there's a force of gravity like we learn in school from Newton's theory, it's because it follows the shortest path which is the curved one. Following that theory, that means that when the bodies move like the earth around the sun, then the space-time also changes, and if the bodies are moving in an oscillatory fashion like the earth around the sun, then the space-time has curvature like waves, and those are gravitational waves.
Plotkin: So is that like a boat disturbing water as it goes through?
González: That's right, except that it's in all three dimensions and time, so clocks are oscillating too.
White: These gravitational waves, they were predicted, like you said, over a century ago, and the LIGO project itself is now a decades-old project.
We were just wondering, what is it that kept you motivated this so long in this search because you must have had years where you were finding nothing, and not even ensure if you would find something, so what was your motivation through that period?
González: I have decades in this, not many decades. Almost three, but there are people, the pioneers in this field, they began thinking about this way of measuring gravitational waves in the '70s, so this is a question you should ask them, especially Ray Wise in MIT. He was the one who thought that if you use lasers measuring distances between mirrors, and if you put those mirrors kilometers apart, and if you build a vacuum system for the laser to travel, then you might be able to measure gravitational waves.
White: That's what you do, right? That's basically how LIGO works?
González: That's basically how LIGO works. Actually, what we do is we make the laser go through a beam splitter, so it splits in two, and then it travels four kilometers, that's two and a half miles on each side, and then bounces on mirrors all in a vacuum system, and then those lasers come back. When they come back, if the distances are different, which is what a gravitational wave would do, then the beams are not in face anymore. That's why we call this an interferometer because we measure the face or the interference of these beams.
White: When Dr. Weiss came up with this in the '70s, did he imagined that this could be done? The technology at the time surely was not advanced enough for this.
González: It was not and actually, even in those times when we read those original proposals to the National Science Foundation, they said this is probably going to take two faces. We need to be build two big facilities because if you see a signal in one, you need to confirm it with seeing the same signal in another facility. So two observatories were built, one in Hanford, Washington, and the other in Livingston, Louisiana very close to where I live. I'm from Argentina, so my accent is much more Southern than Louisiana. Yes, we have these two observatories.
I think he imagined that this could be big, he knew it was going to take time, but the quest for doing these and building such a sensitive instrument was so exciting. It wasn't just discovering gravitational waves. This started when you asked what kept me going. I think what kept me, and he, and all the people going on these, they are very proud, they are very excited in contributing to the sensitivity of these instruments. You said that we discovered them, but I haven't told you yet how small these gravitational waves are.
Plotkin: Please tell us.
González: The first one that we discovered is still the largest one, it's still has a record of largest amplitude. What we measure was these two and a half miles getting longer and shorter, longer and shorter a few times by a distance that was smaller than an atom, smaller than a proton, it was four parts in a thousand of the proton.
White: Oh my, that is incredibly small.
Plotkin: I can't even imagine that, it's insane.
González: It's like comparing an atom to the distance between the earth and the sun.
Plotkin: I remember many years ago, I got to tour the Livingston LIGO Detector in Louisiana. Things I remember was they're telling me they could essentially hear trees falling down from miles and miles away because this thing was so sensitive to logging in the area.
González: That's right. Of course, sometimes those trees fall closer than that because the detector is in the middle of a logging forest, so if they log very close, then sometimes we have to stop, but that doesn't happen very often.
White: In retrospect, would you have positioned the detector somewhere else?
González: We know now a lot better about what to avoid, and we probably would have looked for some more solid ground, rocky ground, not so close to the coast. One of the noises that we hear apart from trees falling, that is actually once in a while, so it's not so bad, but the noise that bothers us the most is produced by the surf waves on the coast and they are resonant waves.
White: Wow, how far are you from the coast?
González: We are quite a few miles from the coast. We are like 60 miles depending on how you measured of course. Louisiana is a swamp, so it doesn't really have a coast, a well-defined coast, but this is something that you can measure in a seismometer anywhere in the world, but if you're closer to the coast, then that kind of noise is larger.
White: I think the next detector should be built in Nevada then. Large, flat, nowhere near the ocean.
Plotkin: That's quite a problem.
González: We are thinking about what we call third-generation detectors, which are longer, better, not cheaper. They are actually more expensive, but we would like to make facilities that are 10 times longer, 40 kilometers instead of four kilometers long. The longer you make the detector, the more you have to compensate for the curvature of the earth.
White: That's amazing.
González: Because the laser travels in a straight line, but the earth is not flat, we know it's not flat.
Plotkin: So even over only 40 kilometers, you have to worry about that.
González: Then you have to worry about that, you have to build some ground so that the laser is going to be straight.
Plotkin: Let's talk about the exciting moment, the first discovery of when you saw the first retrocession gravitational wave. This is in September of 2015 at 5:51 AM, very early Eastern Daylight Time. A signal is detected in both twin detectors. Often we have this image like we'll see in movies of a scientific discovery of this eureka moment.
The one I always think of like in the movie Contact when Jodie Foster is in the New Mexico desert listening to radio signals from outer space, and she finds aliens and she jumps into her car, back to control center screaming out coordinates, "Point it here, point it here."
I imagined those dramatic things are amazing if that's how it works, but it was mentioned it's mixed with something mundane at the time. What were you doing?
González: I was sleeping. Actually, it is an interesting story because we were preparing to take data, we hadn't begun taking data regularly like we do now in what we call observing runs, where we try to take data 24 hours a day, 7 days a week. We were preparing for doing that and we were doing tests in interferometer pushing the meteors, simulating gravitational waves, calibrating the output of the detector. People were introducing noise in the detector to see how much noise cars introduce when they break, for example, and they had been working until very late in the night and they had stopped working.
When they stop working, then we begin testing the analysis algorithms. We take data and we do the analysis, computers do the analysis automatically, and then computers put the results on web pages that when people wake up at that time, now we have alerts and they receive phone messages, but at the time since we were testing, then these web pages appeared then people were looking at that. At 5:51 in the morning Eastern Time and Louisiana Central Time, so it was 4:51 in the morning, but people in Florida who wake up early and people in Germany where it was close to noon already, they saw these web pages saying there is something here.
Plotkin: They receive this in real-time or they process it afterwards?
González: No, it was hours later. They all thought that this was another test, that this was like the tests we had been doing in the previous days. They called up the operators on side and said, "Are you still doing tests?" They said, "No, everything is fine."
White: They thought it was a fake signal.
González: They thought it was a fake signal and it took us a day to convince ourselves that this was not a test.
Plotkin: Did you know from the outset what you had, that would be a Nobel Prize-winning result?
González: Actually, when we saw it, it was so large in amplitude, I mentioned, this is still the record largest amplitude we've had. We could tell from the frequency of the signals that if they were produced, if it was a gravitational wave, it had been produced by two big black holes of a size that nobody had measured before. That was another reason to think this cannot be real. This is too good to be true.
Plotkin: There was no known physical explanation for how you could, that blackhole thing.
González: That's right, there were no black holes known of that size. Before our discovery was 20 solar masses, 20 times the mass of the sun, and this signal, it was produced by two black holes, each one having 30 times the mass of the sun, creating in the end, a bigger black hole.
White: What is the frequency or infrequency of the collision of two black holes of such magnitude? How often do we expect to see such a collision?
González: Of course, these things are happening all the time, but they're too far away, and then the signals are very small. The red gravitational wave is going through the earth all the time. The problem is that they were too small for our detector to measure. Now, this first discovery was in 2015. We have improved the sensitivity of the detectors quite a bit in the last observing run that we started on April 1st of this year. We have discovered 33 candidates in the first six months, so about one week.
White: Wow, pretty good.
Plotkin: That's amazing.
White: If I remember correctly, the press release was around five months after the discovery, so I wondered if you could comment on your feelings in between those two points in time. How did you keep it under wraps? Did you want to talk to people about it? What were your instruction?
González: As I told you in the beginning, we could not believe this was real yet, so we had to be really, really sure before we claimed we had discovered something. We had not started taking data with a detector. We didn't know where the data was like and we know the detectors are very noisy, so it was unlikely that it could be that the new detector decided to have noises that looked like gravitational waves. We couldn't know that until we took some data. That first day, we decided, well, now we have to begin taking data, so we stopped everything else. We began taking data.
We were going to take data for seven months, but we said we're not going to
wait seven months, weren't going to take data for one month, analyze that, and then see whether the signal is still significant with respect to the noise. That took a month to take the data, another month or two to analyze it, to deduce what were the masses of the black holes, to do all the analysis. We were going to write a scientific article and send it for peer review and wait for the response before we had a press conference because we wanted a thousand people looking this over, but we all wanted to see a gravitation wave there, so we were very afraid that we had forgotten something.
We waited for the peer review, it was positive, and then it was on February 11th, 2016 when we went on stage saying we did it.
Plotkin: You were famously a part of that press conference. What's that kind of experience like? I guess what I wonder is is that a fun experience or is it nerve-wracking?
González: It was nerve-wracking. In fact, we had been rehearsing this almost every day and it had been getting worse, and worse, and worse, and worse, so we were so nervous that we were sure this was going to be a disaster, but we have to do it. Then, in the end, I think it went okay.
Plotkin: I can tell you from an outsider's view that it went amazingly well. It was one of the best presentations of research that I've ever seen. I remember the next day, I was working in Australia at the time, so the press conference came very early in our mornings. I didn't get to see it live, but our entire work shut down. It's all everyone was talking about. We were watching replays all of it. It was a big deal.
González: It was a very big deal. There are so many coincidences in this history. We didn't plan it for this, but that day was the first day that was celebrated as the international day of women and girls in science, and there we were two women scientists among five people. That's not a fraction of women scientists, it was nice to represent that.
Plotkin: Yes, it was fantastic.
White: Outreach and science communication is important to us at UNR and I do lots of outreach events here. Happy Hour With a Scientist was one that we did recently. I wondered if you could give your opinion on why you think science education to the general public is important, especially given your position as the spokesperson for LIGO for six years.
González: I think it's an obligation that we have as scientists. We do our science, which is really expensive. People need to know what we do with money and people need to know that we do cool things, and they need to know also, and I think appreciate that, that we do this kind of science, not because it is applied right away for creating useful technology, which eventually happens, but we do it to understand the universe and that's in the sense of being human. Humanity is almost defined by being curious.
Plotkin: The example that I always remember was when they were going to service the HoloSpace telescope and it was the public, I feel, that just had the general outcry in saying, "We love these images. We love seeing what's out there in outer space."
González: That's right and they weren't asking what are going to get out of that, we were going to get images off the sky
White: Now we have some questions that was submitted from some students in the physics department. The first one is from Emily Chao, a final year physics student and she asks, what is it like working a LIGO? What do you do day-to-day?
González: Of course, now I'm traveling a lot and giving talks like the one today and tomorrow at UNR, but when I am at LIGO with my graduate students, I have a group with graduate students and postdocs and undergraduate students, we work in a very big team. Anything that we do is about 10 people at least doing it. In general, in the collaboration that I led the time of the discovery, we had more than a thousand people in 20 different countries.
Plotkin: Wow, I can't imagine those telecoms.
González: We are talking on the phone all the time in different time zones or sometimes we have to wake up at midnight, sometimes people wake up at 2:00 AM to attend telecoms, but we do it all on teams. We actually have four experiments. We have a very careful schedule of what can be done when.
Plotkin: Our next question comes from Donna Depollo who is a junior physics student, and she asks, who are the physicists or people who you looked up to throughout your schooling?
González: My two biggest mentors were my PhD advisor, Peter Saulson at Syracuse University, and actually later this week, I'm going to his 65th birthday party here. We are celebrating his retirement and his birthday. He's been a great mentor and a leader in the fields in the early times, designing the detectors, helping with the design of the initial LIGO detectors, and then the advanced ones.
Plotkin: So you still work with him closely?
González: I still, yes.
Plotkin: That's amazing.
González: I consider him and his wife my family in the US. Still, my parents, my brother, all my family is still in Argentina except for my husband, but Peter and Sara are my family in the US. Then Ray Wise. Ray Wise, after getting my PhD, I went to work at MIT with Ray Wise. His tenacity and his curiosity, I don't know how he keeps all these things in his head, but he knows about everything, and if he doesn't know, he finds it out and he writes the programs, and he is still active. I think he's long '80s and he still comes to Louisiana works with people, with electronics, with the vacuum systems, fixing things. That's who I want to be.
White: I think a physicist never retires. They just keep going and going.
Plotkin: Yes, sounds about right.
White: Trying to understand everything.
González: That context, yes.
Plotkin: Curiosity never goes away.
González: That's right.
White: Our next question is from Jacob Molina, a sophomore physics student, and he asks, of the courses you took during your undergraduate and graduate career, which ones did you enjoy the most and which ones do you think were the most useful?
González: The courses that I liked the best, well, it was probably the first product costing mechanics where one really learns how to apply the math to things that look simple but they're not so simply when you really want to describe them in detail.
White: I'm currently teaching graduate-level mechanics, so I'm really glad you said that if any of my students are listening.
González: When I taught, that's the one I like the best, and you use that all the time.
Plotkin: Andrew Sunquest, whose another physics student here, he asks, if you weren't a physicist, what do you think you would do?
González: I would be a teacher. I love teaching. That's what I thought I was going to be. My mom was a high school math teacher. I really wanted to be like her, so I wanted to be a teacher. I really love teaching, so that's the beauty of being a professor that I teach, I have students, and then I also do research, so I have the best of both worlds.
White: We have one more final question. This is not from a student. What does the future hold for gravitational wave astronomy? New detectors, new science, what's the next holy grail?
González: More than 400 years ago, Galileo used the telescope for the first time, and 400 years later, we have a number of kinds of telescopes and we are still building bigger and better telescopes, so that's going to be the field of gravitational wave. We will be building new detectors and measuring new gravitational waves from new sources, from centuries to come.
White: I can't wait.
White: Also, LIGO in space sounds.
Plotkin: It sounds amazing.
Plotkin: Let me get this straight, it's spaceships shooting lasers at each other in space.
González: It's three satellites, five million kilometers away.
Plotkin: Five million kilometers.
González: Actually, I think the latest is two and a half millions, but still.
Plotkin: Wow, give or take a few million.
González: They would be orbiting around the sun, not around the earth but around the sun in an orbit behind the earth. They would be about 20 degrees behind the earth, and there would be these three satellites with lasers shooting at each other, getting a few photos at the end of those two and a half million kilometers.
Plotkin: Amazing. Thank you very much for doing this, this was a lot of fun for us.
White: Hopefully it was a lot of fun you.
González: It was a lot of fun for me, thank you. Those were very good questions.
White: Thanks for listening to this episode of the Discover Science Podcast Series and we'll see you guys next time.
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