Episode 24: Kip Thorne on Gravitational Waves, Time Travel, and Interstellar

0:00:00 Sean Carroll: Hello, everyone, and welcome to the Mindscape Podcast. I'm your host, Sean Carroll. And I'm sure that most of the listeners remember back, just February 2016, two-and-a-half years ago, I guess, by now, when scientists announced the first direct detection of gravitational waves from elsewhere in the universe. The LIGO Observatory, the Laser Interferometric Gravitational Wave Observatory, announced that they had seen these signals of black holes 30 times the mass of the sun spiraling into each other a billion years ago, giving off gravitational waves, and we have finally detected them. They were actually detected back in September of 2015, and then announced in February 2016. And enormous excitement because of this. It's a truly groundbreaking discovery, even though we've been anticipating it for years and years. It's one of those things which will go down in the textbooks and in the history books as a real cornerstone of how we think about the universe. What is less clear is the enormous amount of, not only work, but perseverance that went into this discovery.

0:01:02 SC: It's always a lot of work to do an enormous experiment or observation in physics or astronomy. If you discovered the Higgs boson, you have to build the Large Hadron Collider, and that takes $10 billion, and many, many years, and thousands of people working. The difference being that, I don't wanna, in any way, disparage the people who found the Higgs boson. I did write a book about them and I admire them enormously, but they kind of knew it was there and how to find it, right? They were following a tradition of high energy particle physics, they had to invent new technology and so forth, but the basic path had been laid. Whereas with gravitational waves, there were a lot of people who thought that there just weren't any gravitational waves of sufficient magnitude to be found, or even more people who thought that even if there were, we wouldn't be able to build a detector with a sufficient sensitivity to actually find them.

0:01:53 SC: Today's guest is Kip Thorne, who, along with Ray Weiss and Ron Drever, was a driving force behind the LIGO Observatory collaboration all the way since the 1970s. And so much effort involved, not only intellectually, as a theorist, Thorne would have to figure out how many sources of gravitational waves there are in the universe, how strong they would be, what they would look like in the detector and so forth, but also navigating the waters of getting funding, and getting support from the community over a course of decades to build this enormous machine. And LIGO, in particular, had a history where the first version of LIGO wasn't really expected to see anything. They spent a billion dollars to build something that they didn't expect to see anything in the sky, and they were right. They had to upgrade it, to what is called advanced LIGO, before they eventually saw something. So it was really a testimony to human kind in some sense because we really stuck with it, but there's certain particular examples, exemplars of humankind who made it happen, and Kip Thorne is absolutely one of them.

0:02:58 SC: Of course, Kip is a very famous scientist for many reasons. He's written books, he's done enormous amounts of research in other areas of gravitational physics, that's his expertise. One of his famous results was introducing the idea that you can use wormholes as time machines; as long as you first find a wormhole, you might be able to travel backward in time. That was Kip and his students who originally came up with that idea. And this led to his second career as a movie writer and producer. Kip was also one of the guiding forces behind the movie Interstellar. So, over the course of the podcast, we'll talk about LIGO, of course, and gravitational waves, but also quite a bit about time travel, and movie-making, and things like that. This is a really good one, I think you're gonna enjoy it. And before we dive in, let me just remind you that we have a Patreon page, if you want to pledge a little bit of support per episode to Mindscape. And because of popular demand, I've added a PayPal page as well, so if you don't like this recurring payment kind of idea, you can just donate in one lumpsum whenever you want to.

0:04:00 SC: It's all on the podcast webpage, preposterousuniverse.com/podcast, you can find the links on the right-hand side. And I greatly appreciate it. Sometimes I say, "We greatly appreciate it," but really, the whole operation is me here in my office. So thanks for keeping the lights on here at Mindscape. It's very, very gratifying to see the support. So, let's go.

[music]

0:04:38 SC: Kip Thorne, welcome to Mindscape Podcast.

0:04:41 Kip Thorne: Wonderful being here.

0:04:42 SC: I mean, among your other distinctions I'm sure that you'll be very proud to know, you are the first Nobel Prize winner to be interviewed for the Mindscape Podcast.

0:04:50 KT: I'm not sure that that's an honor or not. Nobel Prize winners are usually "has beens."

0:04:55 SC: No. Well, that's not true, 'cause I was going to say, I came very close to getting Frances Arnold on the Podcast.

0:05:01 KT: That would be really cool.

0:05:02 SC: There you go. You see, like all the other Nobel Prize winners are good.

0:05:06 KT: There are a few, yes.

0:05:07 SC: Yeah. So you won last year, not this year, in 2018, but it was last year, right? 2017.

0:05:13 KT: Yes, 2017.

0:05:14 SC: The Nobel for gravitational waves. I'm sure people know the general story. But before we jump into the physics, was it mostly relief that you've helped when you finally discovered the gravitational waves? Or were you excited? I mean, it's a long-term project.

0:05:31 KT: Yeah. For me, it had been a half a century.

0:05:33 SC: Right. That's...

0:05:33 KT: It's kinda long.

0:05:36 SC: Very long. Yeah, you started young. It was good.

[laughter]

0:05:38 KT: It consumed, essentially, all of my career. Though, as a theorist, I was able to do a lot of other things on the side, but more than half my career was spent on it, in terms of time. For me, when I woke up in the morning and I had an email from Christian Ott, a good friend and a member of the collaboration, saying, "Hey, go look at this such and such a website. We may have a detach." And I went and looked at it. It was obvious, it was too good to be true. Something had to be wrong. It must have been what we call a blind injection to test the system.

0:06:11 SC: Right. This is you're "faking yourself", right?

0:06:13 KT: You're faking yourself.

0:06:13 SC: Intentionally putting in fakes.

0:06:15 KT: That's the best way to test the system, is an end-to-end test where a group of about something like three or four members of the collaboration are assigned the test, go in electronically wiggle in mirrors in this instrument, puts his chip precisely what the gravity wave would do. And then the wiggling in mirrors, send a signal through the whole system all the way through the electronics to the computer, and then through the computer to the human beings, and through the human beings to the point of writing a paper, and then after the paper is written, you discover, "Well, no, that was a blind injection." So we've been through that. And I figured this is a blind injection, obviously, and... Well, it wasn't, but we were absolutely sure for several months, really.

0:07:02 SC: So even you don't get told, right?

0:07:03 KT: Oh no, not even... I might be the last one to be told.

[laughter]

0:07:11 KT: So it gradually became clear that this was probably the real thing. And my reaction was simply one of profound satisfaction that I had indeed put a huge fraction of my energy, of my career, in the right direction, and direction that really paid off.

0:07:30 SC: So there's a kind of excitement you feel when you're told that your experiment has found something amazing. What about the excitement you feel when you hear you've won the Nobel Prize?

0:07:40 KT: Well, let me tell you about that. So it was October 3rd of last year, 2:15 in the morning. A telephone call came in, from... I think it was the Secretary General of the Swedish Academy of Sciences. I was asleep, sound asleep. It woke me up. Though I had been expecting it.

0:08:00 SC: Okay.

0:08:01 KT: And...

0:08:02 SC: By the way, for the audience out there, everyone in the physics world thought that LIGO would win the Nobel Prize.

0:08:08 KT: Yes. So, at the other end, he announces himself. He says, "It will not surprise you. That we are awarding the Nobel Prize to Rainer Weiss at MIT, Barry Barish in New York Caltech." And I responded, "It does not surprise me, but I'm exceedingly disappointed." And he was a bit taken aback.

0:08:30 SC: Right, that's not usually what he hears.

0:08:32 KT: This is not usually what he hears. But I said to him, "This prize should have gone to the LIGO team who pulled this off, and not to just three people. In this case, this could never have been done by the three of us. It really was a team effort. And I thought you had learned your lesson about this in the case of the Nobel Prize for the Higgs Boson several years ago, it should have gone to the team."

0:08:58 SC: Well, it went to three theorists and zero experimenters.

0:09:01 KT: That's right. It should have gone to the experimenter team, at least, maybe some theorists too, but...

0:09:06 SC: Yeah.

0:09:07 KT: And so he said, "Well, we've been discussing that, but we don't do it. We don't give it to teams." And he said, "But we can continue this discussion in Stockholm."

[laughter]

0:09:18 KT: So we went into a cycle and we did continue.

0:09:21 SC: Right.

0:09:21 KT: And I told him in no uncertain terms, the Nobel committee has an obligation to educate the public about the importance of collaborations. There are some kinds of major scientific breakthroughs that can only be done by a big collaboration, and that the process of collaboration is absolutely crucial for success. And you're not doing a good job of educating the public about that. And he said, "Well, yes," he said, "I've been sensitive to this. There are a number of members of the committee that don't agree that our principal goal with the Nobel prize is to educate the public about the importance of science, the value of science, what has been done, and three individuals are better icons for science than a big team." And so that's how the conversation went. And they're still struggling with this question of...

0:10:11 SC: I'll be honest. I get that argument. I don't have strong feelings one way or the other. I think that if your goal of the prize was to do the best possible job honoring good science, then it's clear that the larger team should win. But I mean, obviously winning it does change the lives of the individual winners, but maybe the best public thing that it does is bring the excitement of science to a wider audience and identifying some real human faces with that is the right way to go. I really don't know.

0:10:44 KT: Yeah. Well, that may be the case, but the breakthrough prize, which is only in physics, but it is a huge prize...

0:10:54 SC: I think there's some biology version too.

0:10:56 KT: Yeah. That's right. I guess you're right. You're right. Anyway, the breakthrough prize does it in a manner that they, and with LIGO, they gave two thirds of the prize to the collaboration, and one third to three individuals. And I thought that worked well.

0:11:13 SC: Yeah. They give the Peace Prize to...

0:11:14 KT: Organizations.

0:11:15 SC: Groups. Peace prizes, and best for other reasons, but... And I know other prizes, my friend Brian Schmidt who helped discover the accelerating universe sort of insisted for some prizes that, rather than it be himself, it should be the team, but the Nobel said, "No, we don't work that way." But it might change. I don't know. Do you think that they're open to changing down the road?

0:11:35 KT: I think some of them are open to change. And I was told they do have the power to change. This is not set in stone legally by Alfred Nobel's will.

0:11:47 SC: Yeah. Apparently, Alfred Nobel's will is very different than what they actually do anyway, right? So.

0:11:52 KT: Yeah. Yeah. Yeah.

[laughter]

0:11:53 SC: Do you think, overall... I know that Richard Feynman had a story or an article called Alfred Nobel's Other Mistake. Do you think that overall the Nobel's good for science?

0:12:01 KT: I think it is. It is more effective at reaching broadly throughout the world to non-scientists and giving them some sense of the importance, the power, and the beauty of science. More effective than anything else that we have.

0:12:22 SC: Yeah. And so 50 years ago, what were you thinking? What do, what was it... Well, I guess I wanna ask you what was the opinion about gravitational waves and so forth, but for those out there in podcast land, what is a gravitational wave?

0:12:35 KT: A gravitational wave can be described heuristically as a ripple in the fabric or the shape of space or of space and time, that is produced, in our case, by two colliding black holes, travels across the universe bringing information about its source. The word ripple is meant to evoke the idea of a ripple on the surface of a pond. If you throw a pebble into the pond, and it's a very quiet pond, you see the waves propagating out. In fact, these waves are quite different, where as the surface of the pond is disturbed, it goes up and down and up and down in a wave. Here, what happens is space is stretched and then scrolls in one direction, perpendicular to the direction the wave is propagating. And then in the other perpendicular direction, it scrolls and it stretches, that it's a stretch on one direction, and a squeeze on the other. Now, what does it mean for space to be stretched, and squeezed?

0:13:39 SC: Yeah.

0:13:40 KT: We could just think of particles or little tiny asteroids out there floating in space as the wave goes by, and they're at rest with respect to each other initially, and they each ride on the stretching and squeezing space, and so they get pushed a part, and then together, apart, and together, and so the distance between them changes.

0:14:02 SC: And this is something where people had talked about it for a long time. Einstein had... It sort of went back and forth, as I recall. His opinion about whether or not these were even a real thing changed over time.

0:14:13 KT: It did. There were times when he lost faith in gravitational waves. And very quickly, however, he recovered, and realized well, yeah, they ought to be real. But it was controversial among theorists who worked in relativity theory, at least among some fraction of the community, all the way up into the 1980s. And so, really, quite surprising it took so long for the community to totally sort this out.

0:14:41 SC: And what was... What's so difficult about figuring it out?

0:14:44 KT: The difficulty is you have a theory that's a mathematical theory, and it's a question of truly understanding the physical consequences of the mathematics. And to me, it was obvious, but I was of the younger generation.

0:15:02 SC: Yeah.

0:15:02 KT: And so I was taught in, what I would call, the right way by my mentor, John Wheeler, to think about it. But to people of an earlier generation who struggled to understand the mathematics, it was not totally clear until after we started planning these gravitational wave detectors.

0:15:23 SC: Wasn't there a famous thought experiment by Feynman that was trying to show that gravitational waves are physically real?

0:15:28 KT: Yes, that's right. And so that's one of the compelling things in which he said, let's take a stick and we'll put some beads on the stick, and there's a little bit of friction between the beads and the stick. And when the gravitational waves go by, they move the beads back and forth because they can slide. The stick is stiff and it resists being stretched and squeezed, so it doesn't move hardly at all because of its resistance. The beads don't resist. So the beads go back and forth, and they rub on the stick, they heat the stick up, and if you have strong enough waves, they might even start a fire.

0:16:03 SC: I just loved that example for so many reasons. Everyone else is sitting there with equations, trying to figure out what the symmetries are, what's going on, and he has this stick with beads.

0:16:12 KT: But he is relying on a particular equation called the "equation of geodesic deviation". It was another physicist, mathematical physicist, Felix Pirani, who first said, "Hey, this is an equation we should be using to discuss gravitational waves." So Feynman's remark about this was at a conference in Chapel Hill, North Carolina in 1956 or '57.

0:16:40 SC: I think it's '57.

0:16:41 KT: '57, okay.

0:16:43 SC: It might've been '56.

0:16:44 KT: Anyway, Felix Pirani was almost that same time, just within a year or so, that same time. So the people hadn't identified, really, the right way, mathematically, to discuss waves until then, and it was Feynman, as far as I'm aware, who first gave this beautiful description, but it did come from the mathematics, and that's a remark about Feynman, who, of course, was a close friend of mine. Feynman would, at conferences like this, he would come up with some really remarkable statement that would startle people, and this was not the only one. And if you would then afterwards ask Feynman, "Where did this come from? Where did you get this sudden insight?" He would say, "Well, I did some long calculations back several years ago just trying to understand how things work. And once I understood, I was satisfied. I didn't need to write a paper describing this, I just understood and I was satisfied."

0:17:42 SC: Just wait till the right moment of the conference to pull it out as a sudden insight, right?

0:17:46 KT: But he was driven by his own personal curiosity. And he had enormous storehouse of information that came from the curiosity-driven questions he had asked to himself over the decades.

0:18:00 SC: I know about this, among other things, that this famous conference in Chapel Hill did, is it helped launch the many worlds interpretation of quantum mechanics which I'm thinking about that.

0:18:08 KT: Indeed, it did.

0:18:09 SC: 'Cause John Wheeler was Hugh Everett's advisor and Richard Feynman's advisor, and your advisor, so...

0:18:14 KT: Yeah, and John Wheeler, who promoted this, didn't believe it.

0:18:17 SC: He didn't believe it. No, that's right. But it...

0:18:19 KT: Anyway, that's a separate story.

0:18:20 SC: Separate story, that's another podcast. So there was skepticism about gravitational waves, there was arguably even more skepticism about black holes back in the day.

0:18:29 KT: Indeed. Indeed and for the same reason, the issue of really understanding what the mathematics were saying was even harder in the case of black holes than gravitational waves. So that skepticism also, well, that lasted I think up until the early 1970s when it was finally laid to rest.

0:18:47 SC: And that's about the time you started thinking about LIGO or gravitational waves more generally?

0:18:52 KT: That's about the time I started thinking about the experiments. So I was thinking about theory of gravitational waves beginning in the mid 60s, when I was John Wheeler's student and just finishing working with him. It was after Joseph Weber, in '69, had announced tentative evidence for gravitational waves that I really began thinking deeply about it, fairly deep, as deeply as I could, about experiments. Although I had been a champion enthusiast of Weber earlier, it was really only triggered by that. And it was 1972 that I wrote with Bill Press, a student of mine at the time, my first paper about a vision for what you can do with gravitational waves if you could detect them. And it was that same year that Rayner Weiss, or Ray, as I call him, at MIT, he wrote a marvelous technical paper where he described his invention of the kind of gravitational wave detector we would ultimately build, identified all of the major things that could go wrong, all the major noise sources, as we say, and described ways of dealing with each one and estimated what kind of accuracy you can get after you've dealt with them all, toward a force. And that was really the beginning, I think, '72, for him and for me, of the research that led directly to LIGO.

0:20:25 SC: And s, a quick footnote. LIGO, for those who don't know it all, if you've not been paying attention, just probably a tiny fraction of the audience, the Laser Interferometric Gravitational Observatory, that found gravitational waves, but also, for people who are not physics experts, let's talk a little bit about the difference between being a theorist, such as yourself, and being an experimenter or an instrument builder like Ray Weiss.

0:20:48 KT: Yeah. Well, there's not a very clean distinction between the two, but most physicists work essentially entirely on theory, developing models for the universe or for things that happen in the universe, mathematical models, trying to understand the consequences of experiments that have been done in the past and the implication for things you might do in the future, but working with mathematics and with physical intuition to try to deduce what's going on in the universe, from observation and mathematical laws of nature. An experimenter designs experiments, builds apparatus to perform them in order to investigate the laws of nature and their predictions. So, Ray Weiss, a consummate experimenter, who also understands theory quite deeply, but focused his career on experiment, primarily, was the primary designer and inventor of these gravitational wave detectors, though the basic idea underlying them had been also found independently by other people, even earlier. But he was the person who really built these and made them happen.

0:22:06 KT: I was a theorist who set some vision for where we might be going, but I also spent, along with my students, a lot of time and energy helping identify things that could go wrong in these gravitational wave detectors, and through theoretical calculations with the mathematics and physical understanding, identifying ways to deal with them, but I didn't go in and actually build the apparatus.

0:22:30 SC: Right, that's right. Now, that was a long sentence that I think you might...

0:22:32 KT: Yeah, sorry.

0:22:33 SC: No, no, it was good, except I think that you might've said "most physicists are theorists". And I think that you wanted to say most physicists are theorists or experimenters.

0:22:39 KT: Or experimenters, yeah.

0:22:40 SC: Well, there's probably a lot more...

0:22:40 KT: Physicists are theorists or experimenters, but in my case, and I think this was essential, when I was a PhD student at Princeton, I had made a decision I was going to work in relativity. Relativity was a wonderful field. I thought it was going to take off. It had been more abound for several decades. And I knew it could really only take off if it had some experimental underpinnings. And so I actually spent much of a year working on experiment, just to prepare myself so I could interact with experimenters, didn't matter what kind of experiment. I happened to work in nuclear physics using a machine called a cyclotron. And then, in terms of research groups at Princeton, I had worked within a theory research group led by John Wheeler, but I also participated in the weekly group meetings and experimental research group that Ray Weiss was a postdoc in.

0:23:37 SC: Oh, okay.

0:23:38 KT: Who was led by Bob Dickie, a different professor. And so I prepared myself for the possibility, the likelihood that I would be able to work at the interface between theory and experiment.

0:23:50 SC: Dickie's famous gravity group was partly responsible for major research in the microwave background technically.

0:23:54 KT: Precisely.

0:23:55 SC: Yeah.

0:23:56 KT: Yes.

0:23:56 SC: Yeah. So let's skip ahead, and then we can come back, but what was the final product of all this thinking? Explain to us the LIGO, what it is, how it looks.

0:24:06 KT: So LIGO is a set of instruments called "gravitational wave detectors" or "gravitational wave interferometers" that are designed to detect these gravitational waves coming from the distant universe and extract the information they carry. So we could use that information to learn about the universe. Each of these instruments is something that measures the stretching and squeezing of space, monitors the stretching and squeezing of space, monitors the pattern of stretch and squeeze. What we have is, we have four mirrors. Each of them weighs about 40 kilograms, 100 pounds.

0:24:51 SC: That doesn't sound so bad. I could do this in my backyard, right? And I wouldn't spend so much money.

0:24:56 KT: That's right, that's right. Yeah and they need to be... They're perfect mirrors and they hang from overhead supports by Quartz fibers.

0:25:09 SC: I don't have any of those, but Amazon will probably sell me something.

0:25:12 KT: That's right, that's right. And you put two mirrors along one arm of an L, and the other two along the other arm, so you have two arms that are perpendicular with mirrors at each end of an arm. And when the gravitational wave comes along, it pushes the mirrors on one arm together, while it's pushing the ones on the other arm apart. The amount of the push and squeeze is the relevant thing. And for what we finally did detect, it was 1/100th the diameter of a proton or the nucleus of an hydrogen atom.

0:25:53 SC: That seems very small.

0:25:55 KT: That is about roughly a trillion times smaller than the wavelength of the light that we use to make the measurement.

0:26:01 SC: And these mirrors are?

0:26:03 KT: These mirrors?

0:26:04 SC: How far apart?

0:26:05 KT: They are 4 kilometers apart.

0:26:06 SC: Is it underground or just in tubes?

0:26:08 KT: They're in tubes.

0:26:09 SC: That you shoot the laser down.

0:26:10 KT: And shoot the laser down the tube. And so you use laser beams to monitor the stretching and squeezing using this technique, interferometry. But the key thing is, how the hell do you measure motions in mirrors that are four kilometers apart when the motions are about a trillion times smaller than the wavelength of the light that you're using? How do you do it when that size is a hundred times smaller than a proton and a proton is a hundred thousand times smaller than an atom? And there, you got lots of individual atoms in the face of this mirror. So we're talking about 10 million times smaller than the individual atoms that make up the face of the mirror that are jiggling around in the mirror all the time 'cause they're warm by the distances or aptitudes of motion that are also huge.

0:27:08 SC: So if someone said, "Can you find the location of this proton to within this accuracy?" You would say, "No, that's impossible." But you're measuring the changes in location due to the gravitational wave.

0:27:19 KT: But you're measuring the changes in location of huge numbers of them. You're measuring actually the average position of all the atoms in the mirror. And that's a big part of why it's hard. You have to design your experiment, as Ray showed us how, so that you're using light that bounces off the face of these often jiggling faces, thermally jiggling faces of these mirrors. But you want the signal that's put under the light to only be influenced by the motion of the center of the mirror, and not by the jiggles of the faces.

0:27:55 SC: One of the things you had to worry about is just that any graduate student nearby who sneezes, is gonna make the mirror jiggle by much more than the gravitational wave would jiggle it by, right?

0:28:03 KT: Yeah. So to that graduate student better not be too close.

0:28:07 SC: But there must be like baffles and noise-cancelling.

0:28:10 KT: Yeah. There's huge numbers of things to deal with. And so that's why this thing, this whole experiment, it cost over a billion US taxpayer dollars.

0:28:21 SC: Right, that's right.

0:28:22 KT: A huge amount, because there are so many things that can go wrong. One way to describe how many things can go wrong is the number of data channels carrying information out of the instrument and out of the environment, that each of which could tell you something was going wrong, is not a hundred, not a thousand, not 10,000 but 100,000. 100,000 data channels telling you things, monitoring things that might go wrong.

0:28:51 SC: That might go wrong.

0:28:52 KT: And one data channel carrying gravitational wave information about the pattern of stretch and squeeze.

0:28:58 SC: Yeah, that's about right. That's about the challenge you face. And how much of this was clear in 1972, or how quickly you did come to it.

0:29:05 KT: No, it wasn't clear it was going to be that complex. But one of the best experimenters I ever worked with, aside from Ray, and Ron Drever in Caltech, was Vladimir Braginsky in Moscow. He was absolutely superb, became a close personal friend. And he was, aside from Joseph Weber, the other person, the second person to really jump into this field and make a mark. He was enthusiastic about this, but said it wasn't for him because there were so many things that can go wrong. That it was very dangerous to pursue this approach, so he pursued a simpler approach. Through the 1970s and 80s, he came to visit Caltech and MIT where we were working on LIGO in the late 1980s, looked at the progress, looked at the plans, went home and shut down his whole operation, and joined LIGO. He joined our effort. But he was superb and he was just so very skeptical for about 15 years because of the complexity that these instruments would have to have.

0:30:08 SC: And one of the worries was certainly, like what if the instrument work perfectly well, but there's nothing out there making large enough gravitational waves to see, right?

0:30:17 KT: No, I wasn't worried about that?

[laughter]

0:30:20 SC: You're not hooked up to a polygraph right now.

0:30:23 KT: It was worry in the community, but it seemed clear to me, not with 100% confidence. But up in the 95% confidence level at least, that we would have black holes that orbit around each other and merge, the neutron stars that orbited around each other and merged. And those already became our primary sources that we were planning to go after that dictated things about the design of these instruments already by about 1980.

0:30:58 SC: And were you surprised at the exact thing that you saw. Was it what? 2016, when they saw the first event?

0:31:04 KT: Yeah, 2015.

0:31:05 SC: 2015. Was announced in 2016.

0:31:07 KT: That's right. It was precisely what I had expected, except the black holes were a little heavier than I expected.

0:31:14 SC: 30 times the mass of the sun, each.

0:31:16 KT: Yeah. And I figured maybe 15 times the mass of the sun each. But again, this is what I had been expecting since about 1980. We knew enough about the universe that that seemed like a pretty good bet, about the universe and about these instruments. A key part of that knowledge was that heavier objects aim at stronger waves. And so black holes, if they were 15 times heavier than the sun, that makes them 10 times heavier than a neutron star. And our primary sources, we thought, were in fact the things we have seen, neutron stars orbiting around each other colliding, black holes doing the same thing. With the black holes, 10 times heavier, you can see them 10 times farther. That means the volume of the universe you can see them through is 10 cubed, the cube for the three dimensions of space. A thousand times greater volume. And it just seemed very likely to me that the number of black holes, it would be less than neutron stars, but it would be less by a factor, maybe 100 not 1000 and so that would be the first thing we saw that is what we saw, and that's what I was aiming for as the most likely thing to happen.

0:32:34 SC: And these events are quite rare on a galaxy by galaxy basis, right? I mean, in our galaxy, how often do two 30 solar-mass black hole coalesce? Do you have a guess?

[laughter]

0:32:46 KT: One in a millions years.

0:32:47 SC: Yeah. Okay. Right. But there's a lot of galaxies out there. That's how we learn, right?

0:32:50 KT: That's right. Precisely.

0:32:51 SC: What have we learned about the universe so far by these experimental results?

0:32:56 KT: Well, we've learned that black holes do collide and merge, that they do form binaries as we expected, similar in neutron stars.

0:33:04 SC: But let's not forget, we truly didn't know that for sure until then.

0:33:06 KT: No, we didn't know that for sure. To me, one of the most interesting things we have learned is the fact that when two black holes collide, you're colliding two objects that aren't made from matter. They have no solid surface. They have nothing solid in them at all. They're made only from warped space and warped time. And so when they collide and merge, they create a veritable storm in the shape of space in the rate of flow of time while they're oscillating rate of flow of time while they're oscillating shape of space like the surface of the ocean in a huge storm out at sea. And we didn't know anything about storms and the fabric of space and time, even theoretically, until maybe three or four years ago. And just a little bit before we discovered gravitational waves from these things, the supercomputer simulations solving Einstein's equations on supercomputers, began to tell us about these storms. We have now seen the waves from these storms. The agreement between the predictions of the simulations and the observations is absolutely remarkable. And so for the first time now, we have both a theoretical understanding and an observational verification of storms in the fabric of space and time.

0:34:31 SC: And I think that there's an aspect of the story that maybe isn't as popularly appreciated as it could be. We have general relativity, Einstein's theory of space and time. He gives us what we call an equation, Einstein's equation. But in some sense, it's really multiple equations talking to each other and it's very complicated to solve, and you can't get complicated, interesting solutions on pencil and paper, you need a computer. And just a number of years ago, the idea of getting accurate real-world useful numerical calculations from Einstein's equation seemed very, very difficult. Maybe not feasible, right?

0:35:06 KT: Precisely. And the action and the effort to create the technology and the techniques for these simulations for solving Einstein's equations on computers lasted just as long as our experimental effort, lasted half a century. And in the early 2000s, well in the 1990s, I was following that effort with great interest because I was expecting that the first things we would see would be colliding black holes and we wouldn't be able to say for sure what we were seeing, unless we had predictions, and predictions required these simulations. We would need the predictions to compare with the observations in order to deduce the details of what the black holes were doing. The details of these storms and the progress was painfully slow. I was chairing an Advisory Committee to the collaboration of all the leading groups in the world and we're trying to do these simulations. And as the chair of their advisory committee, I was a aghast at the slowness of that progress.

0:36:13 KT: I mean it was very hard, it wasn't they were dumb they are the best people in the world, but it was very, very hard. And so, in fact, I personally left the LIGO project in terms of day-to-day involvement in the early 2000s in order to start a effort at Caltech and doing these simulations, in collaboration with what I viewed as the strongest group in the world. A group of Saul Teukolsky at Cornell in the early 2000s. It's not that I wrote a code, computer code anymore than I built the apparatus for LIGO, but at least I had some sense of where we needed to be going, and what questions needed to be asked, and what accuracy is needed to be achieved and so forth, in these simulations. And so that's where I was putting my effort when the gravitational waves were discovered. I was not a major participant in the end game experimentally.

0:37:03 SC: But we did make amazing, we, the royal we, made quite impressive progress over the last 10 years.

0:37:07 KT: That's right. And huge progress on the simulations and huge progress experimentally and the two came together on just the time scale that was required, and it was the marriage of the simulations with the observations that really led to our being able to understand in-depth what was being seen.

0:37:26 SC: So where does a 30 solar mass black hole come from, much less a pair of two 30 solar mass black holes right next to each other ready to coalesce?

0:37:35 KT: No, you tell me.

[laughter]

0:37:36 KT: These were not expected. They were not expected. Of course, in many, maybe in most cases, when something unexpected is seen, theorists have... Can come up with explanations.

0:37:50 SC: Right. We have options, we just don't know which one's right.

0:37:52 KT: Yeah.

0:37:53 KT: That's right. Coming up with options. And so my personal... The option that I think is most likely to turn out to be true is that you begin with smaller black holes that form inside, what we call globular clusters, or big clusters of stars. And they sink to the bottom of the cluster through gravitational interactions with the smaller stars, with less massive stars. They sink to the bottom, they find each other, they collide and merge, and then that merged hole, merges with another merged hole, and you build up fairly quickly to two 30 solar mass black holes.

0:38:38 SC: And these are very different than the supermassive black holes we have at the centers of galaxies.

0:38:42 KT: They are very different. The super... So we get... We're dealing with 30 solar mass black holes. The ones at the center of our own galaxy is about four million times the mass of the sun. The one at the center of the Andromeda galaxy, the nearest big galaxy to our own, is more like 100 million times the mass of the sun. So they are completely different kinds of beasts.

0:39:03 SC: Is there any hope for some day seeing those in gravitational waves?

0:39:06 KT: Yes. I'm sure we will see them.

0:39:09 SC: Will LIGO see them?

0:39:10 KT: No.

0:39:11 SC: LIGO has and does have a prayer. LIGO, through... So the bigger the black holes when they merge, the slower the oscillations of the waves as they pass, which means the longer the wavelength of the waves. And on the Earth, when you get down to those oscillations that are this slow, and now we're talking about 10 minutes, an hour, 10 hours. Noise on the Earth is horrendous.

0:39:40 SC: Yeah, there's a lot of things happening with our time scales.

0:39:41 KT: A lot of things happen in our time scales... This is hopeless on Earth. You have to get away from the Earth, and far away from the Earth and noise on the Earth that comes from weather, humans, and so forth. And so that has to be done out in interplanetary space.

0:40:00 SC: And also, you would like to have your mirrors, or the equivalent of your mirrors, roughly 10 light minutes apart, right?

0:40:06 KT: That right. That's right.

0:40:08 SC: And how big is the Earth in light-second?

0:40:10 KT: Well, it's about a 10th of a light-second.

0:40:15 SC: Okay. There you go.

0:40:18 KT: A few tenths. About a tenth or a few hundredths of a light second.

0:40:24 KT: Okay. And so we had this plan, when I was a kid, there was this plan called LISA to put satellites in space that would bounce lasers off of each other and people then decided it was too expensive. But then you and your friends discover gravitational waves and there's an effort to bring it back. Do you have any idea how that's going?

0:40:40 KT: Well I think the history is a little different from that.

0:40:43 SC: Oh okay.

0:40:43 KT: I think the problem is that the NASA had a cost, a big cost overrun on James Webb Space Telescope, and NASA pulled out of a signed agreement with the European Space Agency to do this mission.

0:40:55 SC: Yeah, okay.

0:40:55 KT: It was not the first time that NASA pulled out of agreements with the... So the Europeans were getting accustomed to this.

0:41:01 SC: The US is not a great partner for some of these big long term missions.

0:41:04 KT: It's a result of the nature of our political system. It's not something that NASA can control. It's a result of the congressional election system and the way things swing with new administrations, unfortunately for science. But anyway, so NASA pulled out, and then it was too expensive for the Europeans alone to do right, so they scaled the mission down. They made it less robust in a way that was really quite dangerous, but in order to be able to do it. Now, as you were saying, now the gravitational waves have been seen by LIGO but fast gravitational waves, rapidly oscillating, not the slow ones that LISA would see. Now that that's been seen and the Europeans have flown test apparatus that verifies that the most serious sources of noise that had been identified are under control. We expect LISA to be on a fast track, very likely with NASA rejoining as a junior partner, not an equal partner.

0:42:13 SC: Well, that's what they get.

0:42:14 KT: Yeah, that's what happens. And in parallel, the Chinese are pushing very hard to do a simpler analogue to LISA faster.

0:42:23 SC: Oh okay.

0:42:24 KT: So they are the first ones to see these gravitational waves in space.

0:42:28 SC: But that can often be very, very useful right?

0:42:30 KT: Yeah, that's right.

0:42:30 SC: Just a quick dirty thing just to see the scope of the land and then go back and do it right.

0:42:34 KT: Yeah.

0:42:35 SC: Yeah. And this is one of the things that, again, that we take for granted, but is worth emphasizing is here's Albert Einstein, 100 years ago, almost a little bit over 100 years ago, from very thought experimenty kind of inputs, right? Like there was no data that said, "Oh, you have to throw out Newtonian gravity," roughly speaking. But he had principles, he knew about special relativity, he knew about other things that he wanted to be true, the principal equivalence, then he came up with general relativity, which both you and I have written textbooks about, by the way. We're competitors in that way.

0:43:09 KT: Well, hardly competitors. My textbook was written decades before your textbook.

0:43:13 SC: But you sold a lot more copies than I have.

0:43:16 KT: Yeah. We don't sell today nearly what you sell, I think.

0:43:20 SC: Though, they're making younger ones I'm sure.

0:43:24 SC: And he was right. And in some sense, not only was he right about the predictions for the experiments, which it took us, like you say, years to figure out, but the feature, the fact that space-time is curved and that curvature is gravity, we're still learning about what that really means at a deep level, is that fair to say?

0:43:41 KT: Yes, I think that's very fair to say.

0:43:43 SC: What do you think is the future of... What do we get need to learn about space-time and general relativity even though we have the equation written down?

0:43:50 KT: Well, thus far, the key thing, the recent thing and the thing that excites me for the next five years, maybe a little longer, is understanding these storms in the shape of space and time. So for example, we've come to understand, first through computer simulations and then understanding it in the equations directly, that sticking out of each spinning black hole is a twisting vortex of space.

0:44:18 SC: Okay.

0:44:20 KT: It's very much like the twist of a tornado. So two tornados sticking out of a black hole. One of them, at the North Pole of the black hole, it has a counterclockwise twist of space; the South Pole, a clockwise twist of space. And these vortices, when you have two black holes collide and merge, you wind up then with four vortices sticking out of a merged black hole plus two more vortices that were created by the orbital angular momentum. So you can have as many as six vortices sticking out. Black holes don't like to have any more than two vortices...

0:44:55 SC: Right.

0:44:56 KT: And so these vortices have to fight with each other in some manner and do a shakedown to two vortices. So it's very interesting behavior of empty space.

0:45:06 SC: Yeah, yeah. I mean...

0:45:07 KT: We had no idea about this until we saw some of this in simulations and then started thinking about it theoretically.

0:45:14 SC: So these black holes are spinning very rapidly. We say those words, but really just like you said it's empty space.

0:45:20 KT: Yeah.

0:45:20 SC: It's as if they were spinning very rapidly, right? So if I fall into one of these black holes, can I travel across the universe?

0:45:27 KT: Probably not.

0:45:29 KT: Almost certainly not.

0:45:31 SC: Well, how would I then travel across the universe if I wanted to do that? I think this is a leading question, 'cause I know a famous colleague of yours back in the day asked a similar question of you.

0:45:40 KT: Yeah. Well, he's a guy, a close friend, Carl Sagan.

0:45:45 SC: Yeah.

0:45:45 KT: Had written a novel called Contact. Well, hes originally written the screenplay, and then he turned it into a novel...

0:45:51 SC: Oh, I didn't know that.

0:45:51 KT: When the screenplay was getting made, into a movie very fast.

0:45:54 SC: Why would anyone make the movie like that.

0:45:57 KT: Anyway, so he wanted his heroine, who became Jodie Foster, to travel through a black hole to the vicinity of the star Vega. And so he sent me the page proofs of his book. He was already, he...

0:46:17 SC: He waited a long time. Yeah, I know.

0:46:20 KT: And he said, "You know, I realize I might be in trouble. Can you help me?" And I read them driving up to see... I think it was driving up to see my daughter graduate from UC Santa Cruz.

0:46:33 SC: Okay.

0:46:33 KT: I read them in the car and fired off a response, and said, "Well, you can't do it with black holes. She's gonna die inside."

0:46:43 SC: That's a short movie.

0:46:44 SC: Yes.

0:46:45 KT: And so you want to use a wormhole. And a hypothetical object that is somewhat similar to a black hole in the sense that it has a spherical mouth, but it's sort of like the horizon of a black hole, except you can travel two ways through this mouth. You can travel in and back out, and you go through it, and it leads to another place in the universe.

0:47:08 SC: And just so we're not in getting in trouble with the physics police here, black holes really exist. Wormholes, not only are hypothetical, but is it... I mean, what are the chances that wormholes exist in the real world?

0:47:19 KT: I think the chances that they exist naturally are exceedingly small. The chances that they can be made by a very advanced civilization, are bigger, but still small. And if they get made by an advanced civilization, they probably implode before you can travel through them. The chances that the civilization can stabilize them so you can travel through, again, are small, but not zero.

0:47:49 SC: But not zero.

0:47:49 KT: And so the issue is that motivated by Carl Sagan's question and by my suggesting that he use wormholes in what became the movie Contact. I and other of my physicist colleagues started working hard to try to understand, do the laws of physics allow them? And it was obvious already from the beginning that it would, unless you did something very strange with them, they would self-destruct. They would implode. And trying to sort that out. And to my surprise, we couldn't get it sorted out fully, but the work that has been done points rather strongly to a conclusion that, probably, they can't exist. And if they can exist they very probably can't exist naturally.

0:48:46 SC: Yeah. So this was circa late 80s, early 90s you were thinking about this?

0:48:48 KT: So the work on this continues to today, but a small level. The bulk of the research on this was done late 80s, early to mid 90s, but it still continues because the answer isn't in.

0:49:01 SC: Right. We still don't know. So Carl Sagan wants to let Ellie Arroway travel across the galaxy. You say it should be a wormhole, not a black hole. And then you realize, wormholes were, again, coined by John Wheeler. Is that true? The phrase?

0:49:14 KT: So the phrase was coined by John Wheeler. The concept actually goes back to Herman Vile, about 1922.

0:49:25 SC: I did not know that. I would have given Einstein credit but...

0:49:27 KT: Yeah, so it was conceived perhaps independently by Einstein and his colleague, Rosen, Einstein and Rosen, in the '36, but you go back and back and you find it in Herman Vile in about 1922. But it was John Wheeler who really pushed hard to understand these initially, because he had intuition, which may have been right, that on very small scales, what we call the "plank length", the scale where space and time as we know them must become probabilistic. They fluctuate, like anything in the universe, fluctuates due to quantum physics. On those very small scales, John Wheeler argued that you would likely find a froth of fluctuating wormholes.

0:50:20 SC: Right.

0:50:21 KT: And so that was his central focus.

0:50:23 SC: There's this rough problem, maybe this is getting a little bit too technical, but I can't resist. So Einstein gives us this equation for general relativity, Einstein's equation, and we might wanna say, well, why can't you just solve the equation? And part of the problem is, there's a left-hand side which says spacetime is curved, and there's a right-hand side which says there's stuff in the universe, matter and energy and so forth, causing space time to curve. And the left-hand side with the curvature is very pretty and understandable, and the right-hand side with stuff is kind of a mess. Is that about why we don't understand wormholes very well?

0:50:52 KT: Well, that's maybe a piece of it. That's a piece of it because although wormholes are things that are made from warp space time, without matter, if you have them made from warp space time without matter, then they self-destruct.

0:51:08 SC: Right.

0:51:08 KT: So you have to put some kind of matter in them to hold them open, and that's where it becomes tough.

0:51:14 SC: That's were the rest of physics becomes... And that's all a messy terrible thing.

0:51:18 KT: The other issue, and this was the issue for Wheeler, is that when you get down to these very tiny length scales associated with quantum effects, that there, you don't even know the correct laws of physics at all well. And so then you have to start speculating. And that's where he gave, I think, some pretty plausible arguments that you would have this quantum foam of fluctuating wormholes. But there, the problem is you don't understand the laws of physics for a big wormhole. You don't understand the matter well enough to be sure that whether you can hold the wormhole open with it.

0:51:54 SC: But if you have a big wormhole and you can use it to travel across the galaxy very quickly, don't you worry because special relativity says, I can't go faster than the speed of light. It's like going backward in time.

0:52:04 KT: Oh, come on.

0:52:07 SC: I'm leading you down somewhere.

0:52:07 KT: Sure you can go faster than the speed of light, we all know, but not locally. So let's be more precise in things. So... The universe is expanding, as we know. You're a cosmologist, you can understand this far better than I do, Sean.

[laughter]

0:52:24 SC: And the most distant part of the Universe is moving away from us faster than the speed of light, so we can't see it 'cause light can't get to us from it, but it's doing it. And so what the speed limit really says is that if you have two objects that are close enough to each other that there is no significant warping of space and time between them, then they can't move faster than the speed of light with respect to each other. But when you've got a wormhole, you've got lots of warping with space and time, and all bets are off.

0:52:54 KT: Right, so you can just... And ever since, I don't know if Contact was the first movie to use wormholes to get people across the...

0:53:01 SC: I think so.

0:53:03 KT: And now every movie does it, so yeah. Star Trek and everybody else.

0:53:07 KT: But then, I don't know whether it was you or your collaborators, but you began to realize that if you can travel across great distances of space, maybe you can also go backward in time.

0:53:18 SC: Yeah, yeah. So that was me in this case. I mean, other people had, working in relativity had seen other ways that you might be able to go backward in time. This was just one more way, but one that became particularly popular, maybe because of Contact.

[overlapping conversation]

0:53:38 SC: Although to be more fair, I would say that it was... You could imagine building it. A lot of the other ways to build time machines started by saying, "Imagine you have an infinitely long cylinder," right?

0:53:52 SC: And at least the wormhole is contained in some region.

0:53:54 SC: Yeah, once you have a wormhole.

0:53:56 KT: Once you have wormhole.

0:53:58 SC: So, with a couple of my students, Mike Morris and Ulvi Yurtsever, I worked out how you would... If you had a wormhole, how you would use it to make a time machine. That's fairly simple. You just take one mouth and I put my wife in one mouth. She carries that mouth out through the Universe in her spaceship and comes back at high speed and time for her slows down, as seen in the external universe. And I sit on Earth, it doesn't slow down. But as seen through the wormhole, our clocks run at the same rate. And so you begin. The clocks always running at the same rate, hers and mine as seen through the wormhole, when she comes back, she's very young, and I'm very old, as seen through the exterior, as seen through the interior of the wormhole we're the same age. And so there's something crazy going on. The craziness is that you have created a time machine.

0:54:58 KT: Right.

0:55:00 SC: So that was fairly simple and fairly obvious, but I think, yes, you're right. This was something that if you had a wormhole you could actually make it. And so then the question became, what did the laws of physics say about this? Could you really do it? And there comes in the key thing that things... If you're an engineer in a very advanced civilization and you can do anything that is allowed by the laws of physics, you got to look at more than the laws of relativity. The laws of relativity say, "Yeah sure, you can make a time machine."

0:55:31 SC: Yeah.

0:55:31 KT: But you also got to look at the laws of quantum physics and the behavior of matter. There's always going to be some matter present because with quantum physics, there's always at least a little bit of fluctuating matter present. And so you... Then that's okay, now you've got a wormhole you've turned into a time machine, relativity allowed it, what did quantum physics do? What did it say? And the answer is that quantum physics says, with high probability, that the wormhole is going to self-destruct the moment you turn it into this time machine.

0:56:07 SC: It seems like from various different perspectives, the universe is kind of reluctant to let you build a time machine.

0:56:13 KT: It does seem that way, essentially.

[laughter]

0:56:14 SC: Do you... Would it bother you if we could build the time machine. I mean there are logical paradoxes involved, right? Can we rig the...

[overlapping conversation]

0:56:22 KT: I think it would be wonderful.

[laughter]

0:56:25 KT: Anything that says that the laws of nature are different than you expect is wonderful. You can learn wonderful things.

0:56:31 SC: Is that how you knew in the early 70s to think about gravitational ways, 'cause you visited back to yourself in the past gave you a hint?

[laughter]

0:56:39 SC: Well, but as...

0:56:41 KT: Yeah, no, yeah. So, yes. So then you immediately do worry about paradoxes. And so that's what I did, again, with students and colleagues. And so we asked ourselves, "Okay, suppose you do have a wormhole, you did turn it into a time machine successfully, it didn't self-destruct in the process, then how do you deal with these paradoxes? How does nature deal with these paradoxes?" And so what we did was we... Well, a colleague of mine, a dear colleague of mine and yours, Joe Polchinski, who just passed away, sadly, fairly recently.

0:57:20 SC: Right.

0:57:21 KT: A really great theoretical physicist. He had been a student of mine years ago, decades ago. And Joe sent me an email, I think it was an email. Maybe it was a letter in those days. This is 1990... The dawn...

0:57:35 SC: It's about the dawn of the early email days. Yeah.

0:57:38 KT: And he said, "Well, here's a little thought experiment. You send a billiard ball into a one wormhole mouth and it comes back out of the other mouth before it went down and it hits itself and prevents itself from going in. How do you solve that paradox?" So we called this "Polchinski's Paradox." So together with some students, I worked out what does nature say about Polchinski's Paradox? We found that there are multiple ways that nature can get out of it.

0:58:12 SC: Okay.

0:58:12 KT: The billiard ball goes in, it gets hit a very gentle blow by itself on its way in. So it goes in, it comes out on a slightly different trajectory than it was supposed to, and hits itself at a very general blow.

[laughter]

0:58:29 KT: So everything is all right. But there are a number of general ways that can do these gentle blows, and so you wind up with more than one solution, the laws of physics. Instead of just one solution, where you thought there were none and you thought there's no way, no answer to what happens to the billiard ball when it goes in 'cause there you've got a paradox. First, you say the billiard ball goes in, it comes back out, it hits itself and prevents it from itself from going in, that's the trajectory. And then you find a trajectory where it's modified itself and then you find another one and then another one. And in the laws of classical physics, there had not ought to be multiple solutions. When you post initial conditions, there should not be multiple answers. And so then you go to quantum physics and you ask what does quantum physics say. So it becomes a very interesting game, eventually. Trying to figure out how does nature get out of this. And this is much easier to deal with this with billiard balls than with human beings who go back in time and try to kill their grandparents.

0:59:32 SC: That's the more common thing. But Joe is a good physicist honed in on a simple physics problem. So it's interesting because, on basic features of sort of logical consistency, we would not want to have a true paradox or two incompatible things happen, and you are saying that it seems that, at least at the classical level, what nature gives up on is not logical consistency but predictability. There's more than one possible way out.

0:59:52 KT: Yes. But then we pay attention to the fact that the world is really fundamentally quantum at the bottom. And so where you have these multiple solutions, classically, you go in and analyze them quantum mechanically, and what is... You and I would call it a WKB approximate.

[laughter]

1:00:09 KT: This getting jargon.

1:00:12 SC: We will footnote that, okay. It's a good approximation scheme we can use.

1:00:15 KT: And so you see that the quantum physics probably has a solution to get around this, and there's probably one unique solution. So my bottom line with this and my problem is that I was just deep into this and felt like I was making some progress in understanding how you would deal with paradoxes when LIGO got funded.

[laughter]

1:00:36 KT: And I said, "Do I spend my rest of my career now, working hard on making gravitational waves a success, or do I think about time travel with paradoxes?"

1:00:46 SC: I love time travel, but you made the right choice. [laughter] But the way you mention quantum mechanics there, just because I know probably some people know a little bit about the idea of branching the universe off because you have a time machine and creating a new timeline, and that's not what you're talking about, is that right?

1:01:03 KT: No. It's not at all what you're talking about.

1:01:05 SC: You're just using quantum mechanics to find the one consistent, most probable trajectory that would actually happen.

1:01:11 KT: And so what you really want to do is ask how do you formulate quantum mechanics in the presence of a time machine. And in fact, there is a way to do it and it's using a formulation of quantum mechanics due to our dear friend Richard Feynman. But a generalization of Feynman's ideas is due to his colleague, Murray Gell-Mann and Jim Hartland. So the tools are there...

1:01:44 SC: Very Caltechy fell about it.

1:01:45 KT: Very Caltechy feel, that's right. Well, with Hartland at Caltech and Santa Barbara. But the tools are there to do it.

1:01:51 SC: Wasn't Jim a grad student at Caltech, I mean back in the day?

1:01:54 KT: He was Murray Gell-Mann's grad student.

1:01:56 SC: Yeah, alright.

1:01:57 KT: And John was an undergraduate and post doc. But that anyway so... You can formulate quantum mechanics in a way that handles all this in the presence of what we call "closed time", in presence with time machine. But for me, the beautiful thing about this is that once you've done that, you discover that information gets lost.

1:02:23 SC: Right. Physicists don't like this.

1:02:25 KT: And physicists don't like that.

1:02:26 SC: Some physicists don't like this.

1:02:27 KT: Some physicists don't like this, but I think it's quite wonderful. But, anyway. And so then you get caught up in the so-called information loss paradox. And a view of this that I have that's iconoclastic, that differs from that of the majority of physicists.

1:02:43 SC: But it's a good little lesson about how you were fooling around in some sense, right? You weren't trying to build a time machine. You were inspired by a question from Carl Sagan, but it leads you to some maybe interesting insights into questions people really care about. Yeah.

1:02:57 KT: Precisely. Since I came to appreciate that in areas where you're dealing with physics beyond where we can actually do experiments today, someday we will, but not today, thought experiments like these, simple thought experiments like what happens to billiard balls if they collide through a worm hole that has a time machine built into it. Simple thought experiments can sometimes dig pretty deeply into the laws of nature.

1:03:22 SC: And nevertheless, I remember a footnote or the acknowledgements in one of your papers saying that the National Science Foundation wouldn't pay you to work on this anymore. [laughter]

1:03:31 KT: That was a half tongue in cheek remark, only half. So the story is that I was working on this in parallel with the gearing up to build LIGO. And we had our major funding for LIGO, in the end, $1.1 billion for LIGO coming from the National Science Foundation. And I had a conversation about this research with Richard Isaacson, the superb program director for our field at NSF. A person who I regard, Ray Liston, I regard as, really, our collaborator in Washington who really made this happen. Anyway, so Isaacson said to me, "You don't want to screw things up for LIGO if some congressman comes in and starts hacking at you for working on time machines. And maybe you can find funding elsewhere for that work, just to keep yourself safe from being attacked by some... "

1:04:38 SC: It sounds like a good advice.

1:04:39 KT: It was a very good advice. But I also tweeted back at Richard Isaacson and put in this acknowledgements this work was not supported by the National Science Foundation... It was...

1:04:50 SC: I did not know that.

1:04:50 KT: Instead it was supported by the Richard Chace Feynman Research Fund at Caltech.

1:04:55 SC: Tolman, Tolman. Richard Chace Tolman.

1:04:56 KT: Tolman. It was the Tolman Funds.

1:04:58 SC: That's right. And the...

1:05:00 KT: Wait a minute. Sequential point it was... This was in the 90s, so I became the Feynman professor, '91?

1:05:13 SC: Yeah, I don't know.

1:05:14 KT: I think it was Feynman funds, actually.

1:05:15 SC: Okay, that could be. Alright.

1:05:16 KT: I think it was Feynman funds.

1:05:18 SC: But there's another spin-off of this fortuitous conversation you have with Carl Sagan, was that you got interested in the phenomenon of Hollywood making science fiction movies, right?

1:05:29 KT: Yeah.

1:05:31 SC: And is it exactly right or maybe you can correct me, like you were not perfectly happy with how the movie version of Contact turned out.

1:05:37 KT: Yeah, that's right. So George Miller was the original director of Contact, and he was working very hard to perfect the tail-end of the movie involving wormholes and the ultimate denouement at the end of the movie, and he got canned by the studio for taking too long.

1:06:06 SC: Hollywood, yep.

1:06:07 KT: Yep... And so, they brought in Zemeckis with an order "You finish things off. You don't diddle any longer with the screenplay, you finish things off. We wanna get into production." And so he did. And basically, all the efforts that were being put into really perfecting the last part of the movie went down the tubes, so I was disappointed in that.

1:06:31 SC: So did that inspire you to say, "Someday, I'll do it right"?

1:06:33 KT: No. [laughter] No, no. I never had any intention to do that, but yeah, it was always in the back of my mind, but I got into this simply because Lynda Obst, who had been Carl's partner in Hollywood on Contact, a movie producer, a great movie producer. She called me up one day in 2004 and said, "Would you like to brainstorm with me for a science fiction movie?".

1:07:03 SC: Okay. And that's what led to Interstellar?

1:07:06 KT: That's what led to Interstellar, but I had no plan to do that.

1:07:08 SC: So we just... We have Hollywood people on the podcast, so we don't need to rehearse the entire process. But how do you go from brainstorming to seeing your name up there as an executive producer?

1:07:20 KT: Oh, that happens by having a very good attorney. [laughter]

1:07:25 SC: An important lesson for the young Hollywood strivers out there. Yes, absolutely.

1:07:29 KT: So I saw... Lynda said to me... Once we had brainstormed and she had brought on a studio, which bought an option to make the movie from the treatment that she and I had created through brainstorming. The treatment is just a description of a story, and in our case, with science embedded in the story. So she brought in a studio and she said, "You have to negotiate with the studio. You can't do it yourself, you have to have an attorney."

1:07:58 SC: Okay.

1:07:58 KT: So with the help of a Caltech president, who went to a member the board of trustees, who was well connected in Hollywood, I got a great attorney Ken Safran.

1:08:08 SC: Good to know people in high places.

1:08:10 KT: Yeah, and Ken negotiated and got me an executive producer credit off the bat, based on the brainstorming that we had done initially, and on the expectation that I would stick with the project all the way through and would be the lead science advisor and realize the vision that Lynda and I had formulated of a film with science embedded deeply into it, from the outset.

1:08:39 SC: And what happened... What was the first step, was it getting the director or the screenwriter or...

1:08:43 KT: Well, so, Lynda brought on... Well, the story is that Lynda brought on Steven Spielberg to direct it, and the two of them were both with Paramount at the time, and so then they brought on Paramount. Steven then was the director through the early creative phase when making the movie. The early creative phase lasted for a few years. They brought on Jonathan Nolan or Jonah, as he's known to his friends, to write the screenplay. Jonah had written a couple of screenplays previously with his brother, Christopher Nolan, but that was the extent of his experience in Hollywood. Since then, he has created Person of Interest and...

1:09:28 SC: Westworld.

1:09:28 KT: And Westworld.

1:09:29 SC: Yeah, he's doing okay. [laughter]

1:09:31 KT: He's doing okay. Yeah. And Westworld, with his wife, Lisa Joy. And so... But he was really pretty green and young at the time, but superb and wonderful to work with. So he went through three drafts of the screenplay, and then Steven always carries more movies in the creative phase than he can possibly make. And he came... Crunch time came, and he was going to make Lincoln.

1:10:00 SC: Okay.

1:10:01 KT: Or he was going to make...

1:10:02 SC: Interstellar.

1:10:02 KT: Interstellar. And I don't know, he may have had other choices as well, but he did make the choice to make Lincoln.

1:10:09 SC: He certainly has over a dozen movies in development at one time, right?

1:10:12 KT: So that was his choice. And Christopher Nolan, being Jonathan's brother, had said to Lynda, before Steven dropped out, he had said, "If Steven drops out, I would be interested in considering making this movie."

1:10:32 SC: Pretty good backup possibility, just to have in your knowledge.

1:10:35 KT: And, so, when Steven dropped out, then Lynda and Paramount started to try to negotiate with Chris. Chris said, "No, I won't negotiate until after my next movie comes out, and has been out for maybe six months, so then I'll negotiate."

1:10:51 SC: 'Cause he is the opposite, right? He does one thing at a time.

1:10:54 KT: He is the opposite. He does one thing at a time. He doesn't even make a decision about the next thing he's gonna do until the previous movie has been out for a few months. He sees how it's been done, he's gone through all of the publicity on it, and so forth. And then he starts thinking seriously about what he'll like doing next. And so we waited for two and a half years while he did Batman: The Dark Knight Rises. And then, a few months after that came out, he negotiated seriously with Paramount. By then, Lynda had had a divorce with Paramount, as had Steven.

[laughter]

1:11:35 KT: And Chris was not about ready to work with Paramount. So Chris said, "I'll only work with Warner Brothers." And so...

1:11:45 SC: And none of your physics education had prepared you for any of this?

1:11:48 KT: Well, but fortunately, I had a great attorney.

1:11:50 SC: You had a great attorney, yeah.

1:11:51 KT: And a great partner in Lynda.

1:11:52 SC: Right. Somebody knew the ropes, yeah.

1:11:54 KT: Lynda is superb. She knows the ropes and Ken knows the ropes, so I just sat back and waited and got briefed by phone and email from time to time. And so anyway, it was obvious that Chris was the person who should make this movie, obvious to everybody. And so Paramount negotiated with Warner Brothers that Warner Brothers would have the rights, all the foreign rights, for this movie. Paramount would have the domestic US rights, and they would do it jointly, but the production would be in the hands of Warner Brothers, because that's where Chris works.

1:12:29 SC: Close to his home. Okay, and then...

[laughter]

1:12:31 KT: And then, as you say, this is not part of my training. So I watched this all.

1:12:37 SC: A wonderful education, right?

1:12:39 KT: Yeah, real education, yeah.

1:12:40 SC: And speaking of education, you figured out you had some good science projects coming out of the attempts to get the science right in the movie.

1:12:47 KT: Yeah, yeah. And so, yeah. So I very much enjoyed working. Well, I enjoyed brainstorming with the Nolan brothers. I enjoyed my interaction with the actors and particularly with the computer graphics team at Double Negative Limited London, led by Paul Franklin, who was one of the founders and leaders of Double Negative. And together then, at Christopher Nolan's request, enthusiastic request, which I was expecting, he wanted everything that involve visualizations of black holes and wormholes and astrophysical objects to be done as accurately as possible through computer simulations and solving the equational propagation of light from the source of the light through the environment to an IMAX camera through the optics of the IMAX camera, with all of the vagaries of the optics of the IMAX camera to the film at the end. And so that was how it was done, and I provided the equations for the propagation of the light to Oliver James, who was the Chief Scientist at Double Negative, and he programmed the equations. And the artist there created the images of the sources of the light, and it all came together quite beautifully.

1:14:18 SC: And I remember, if I remember correctly, you saying that not all science in the movie might be plausible, but it's absolutely compatible with what we currently know about the laws of physics.

1:14:26 KT: Not everything, almost everything.

1:14:28 SC: Almost everything.

1:14:31 KT: So, the agreement between Christopher Nolan and me, from the outset, was I told him I wanted everything to be compatible with the laws of nature and he said, "I'm enthusiastic to do that. As long as it doesn't get in the way of making a great movie."

1:14:44 SC: Which is fair.

1:14:44 KT: I think it's fair.

1:14:46 SC: Yeah, absolutely.

1:14:47 KT: And he was afraid how I would be playing the role of science police.

1:14:51 SC: Right.

1:14:53 KT: And I was afraid he wouldn't respect the science. And within a few hours of brainstorming together, it became obvious that we were on the same wavelength, that we could work together as beautifully as I had worked with his brother on the screenplay, and it became a marvelous collaboration. There did come one point when we had these wonderful images of the wormhole and of the spacecraft traveling around the wormhole and then they had to make the commuter graphics for the trip through the wormhole, and I got a telephone call from Christopher Nolan and he said, "Kip would you come over to my house? We've got a problem."

1:15:36 SC: Okay.

1:15:37 KT: I went over and he said, "We've tried various sizes and shapes of wormholes. This is what the trip to the wormhole looks like, with this shape and size and that's what it looks like with another one. None of them are very exciting. They're all pretty dull and...

1:15:54 SC: Because they would be actually, right?

1:15:56 KT: They would be. So he said, "What do I do?" I said, "You use creative license." And that's the one place where there was substantial departure for what the things would really be like. Everywhere else, there were a number of other places with little departures. If you wanna know all of the places that I was aware of at the time the movie came out, there were a few I missed where there were little glitches with regard to the laws of physics. You go to my book, "The Science of Interstellar." First, you buy the book The Science of Interstellar.

1:16:27 SC: First buy the book, and for Christmas present too, yeah.

[laughter]

1:16:31 KT: Anyway, you go to the subject index, but you look up a person in the subject index, you look up Christopher Nolan, and then under Christopher Nolan in the subject index, you'll see all of the compromises that he made with the science in order to make a great movie.

1:16:49 SC: Your conscience is clean though. You listed...

1:16:52 KT: With most books that I've written over my career, there are there little gems like this hidden in them.

1:16:56 SC: Yeah, that's a good thing to do.

1:16:57 KT: And you may find them, if you're clever, in the index, or you may be told.

1:17:07 SC: The authors have to keep themselves entertained as well as everybody else. So do you have another movie in the plan?

1:17:13 KT: I do. So Stephen Hawking and Lynda Obst and I wrote the treatment for a follow-on movie several years ago before Stephen passed away. And we're very enthusiastic about the movie, again, a science fiction movie, again, with science, real science, built into it. We... Whereas... Well, so, with this movie then, we went through the same process. Lynda identified the studio, we sold an option. Well, she and my...

1:17:47 SC: And your lawyer?

1:17:47 KT: My lawyer, Ken Safran and his team, sold an option to make the movie from our treatment to a studio. The studio hired the screenwriter that we wanted. They haven't had an offer out to the director we want. And it may go forward but I'm not allowed to say anything on that.

1:18:07 SC: No.

1:18:07 KT: And that's probably more than I'm allowed to say.

1:18:08 SC: Not even asking but... Well, probably that much is on IMDB already, right? So much goes out now. You... So you've enjoyed the process?

1:18:15 KT: I've enjoyed the process very much, very much. And I should say with Interstellar, the Nolan brothers changed the story so much. It wasn't Lynda's and my story at all. We didn't even... In the way things work in Hollywood, if you've written the story for a movie then you go and you ask the screen writers guild that you want a credit for "story by". We didn't even ask for a credit for a "story by" because they had changed the story in very positive ways. We're enthusiastic about the changes they had made. So this wound up as the Nolan's movie, but with the science that came from Lynda and me, and then additional science from the brain storming with the Nolan brothers. In this second movie, so far, it's followed pretty close to the story that we began with in the treatment.

1:19:03 SC: Very exciting.

1:19:04 KT: So maybe Stephen and I and Lynda will get the credit for "story by", we'll see.

1:19:08 SC: Well, Kip Thorne, I think that one thing everyone who has listened to the podcast will be able to say is, "You've been very successful." [laughter] "You've had a great life."

1:19:18 KT: I've had great fun, I've had great fun.

1:19:20 SC: Yeah, and I love the variety of it. We didn't even get into the various books that you've written but great science, a little bit of fun living here in LA, it's been a great conversation. Thanks so much being on podcast.

1:19:33 KT: Thank you.

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