Greatest Mysteries of Gravity – Brian Greene & Kip Thorne (Transcript)

And it wasn’t the only reason, but it was part of the reason why people like Einstein resisted taking this solution as seriously as, for instance, we take it today.

KIP THORNE: Yeah. So Einstein trying to sort this out—imagine taking a star and shrinking it down to very small size. And he couldn’t shrink it down all the way to that radius. And he began to believe that maybe everything below there was in some sense, fictitious. He just couldn’t understand what was going on.

It’s not that Einstein was dumb. It was that it was a real, real hard thing that did subsequently take decades to sort out.

Einstein’s Resistance and Oppenheimer’s Breakthrough

BRIAN GREENE: And so, as I recall, I think in the late 30s, Einstein even wrote a paper where he tried to specifically model a bunch of masses that would be in some spherical configuration. They were moving, and he tried to model them collapsing inward.

KIP THORNE: He moved them in slowly. He didn’t have the wherewithal. He hadn’t even asked the question about a dynamical collapse. He said, “Let me shrink it smaller and smaller.” And once it got down to something a little bit larger than this Schwarzschild radius, he couldn’t shrink it any farther.

That was one of the key reasons that he was skeptical about what was going on below there. It then required Robert Oppenheimer and his student Hartland Snyder, to actually, for the first time, do a dynamical collapse of a ball of dust down through that radius and down to the Schwarzschild radius, and then change coordinate systems, change the actual way the mathematics was being done, and carry it all the way down inside there and discover, in Oppenheimer’s own words, “We see that the star shuts itself off from universe.”

BRIAN GREENE: Kind of cuts itself off. Yeah.

KIP THORNE: It cuts itself off from the rest of the universe, which we, in the modern time, we recognize has gone through the horizon of a black hole. It’s left behind the horizon of the black hole. But that aspect of it wasn’t understood until the 1960s.

BRIAN GREENE: Sure. Now, in the Oppenheimer and Snyder work, if I remember correctly, part of the way they made a breakthrough was much like Schwarzschild making simplifying assumptions that somehow still capture enough of the physics to give you real insight.

The Art of Simplification

KIP THORNE: This is mainly one of the—I learned over the years the greatest minds of the physicists were the greatest minds that I have met. And this included Oppenheimer, included Yakov Borisovich Zeldovich in Moscow. They could somehow understand what simplifications capture the correct physics without losing the essential physics.

Because most simplifications, you may throw away the essential physics or you may not even be capturing correct physics. But Oppenheimer and Snyder succeeded in this.

BRIAN GREENE: And I guess, in retrospect, right? And part of their assumption, I guess, was they assumed there wasn’t undue pressure within.

KIP THORNE: So for simplicity, they just assumed this was dust. So there’s no pressure at all. And of course, skeptics then immediately said, “Well, that’s not how things work in the real world. You put pressure in and it’s not going to keep shrinking.”

BRIAN GREENE: They could say that the very reason it’s shrinking is because you’ve neglected the very thing that could push.

KIP THORNE: That’s right, push outward. But in fact, it turned out that’s not the case. Once you put the pressure in, this still happens. But Oppenheimer and Snyder did not have the technical know-how to put the pressure in and keep going.

BRIAN GREENE: So what did it require for people to get to that point?

Penrose’s Revolutionary Mathematics

KIP THORNE: Well, I think what required—the key thing that happened, I think, was the insights of Roger Penrose, for which he actually got the Nobel Prize a few years ago, a half a century after he did the work, more than half a century in which he introduced—he brought into physics a branch of mathematics called differential topology, which had the capability to prove theorems that were so powerful that the theorems were correct, whether you had dust or whether you had pressure.

And he was able to prove that, indeed you really, even with pressure, that there would be circumstances in which he would still form this horizon.

BRIAN GREENE: And did that convince the community at that point, or was it still…

KIP THORNE: Well, it was really quite interesting that it ultimately convinced the community. But his techniques were sufficiently alien to physicists—they were familiar to some mathematicians and sufficiently alien to physicists—that it took a few years for the community to come around, particularly the community in Moscow.

I had very close relationships with the Russian physicists early in my career and was commuting back and forth. So I knew these people. This happened before I came on the scene, but Yevgeny Lifshitz, one of the great physicists of that era, said to me, “Kip, you cannot understand how alien to the human mind this business of what Oppenheimer Snyder saw was, nor how alien to our minds the mathematics that Roger Penrose was introducing, because it was just so different from what we were doing than it had ever done.”

And basically, he said, it took us a while to really come around and understand how—meant three, four years.

BRIAN GREENE: But it is funny how what’s alien to one generation just sort of becomes the vernacular of the next.

KIP THORNE: It’s just common sense, you know.

Two Perspectives: Inside and Outside the Black Hole

BRIAN GREENE: But part of what was alien too, I presume, was there are two different narratives, two different stories that you need to tell. If you’re watching something fall to a black hole versus the story that will be told by the individual, unfortunately, who is passing into the black hole. Can you just take us through that? Because that’s also deeply unfamiliar.

KIP THORNE: Yeah, yeah. So if I—I’ll be the one that goes through that way. If I fall into the black hole or I ride on the surface of a star that’s shrinking to form a black hole and you’re outside, time for me—you see time, my time, slowing to a halt. So you see me going to slow motion, then freeze right at the horizon because my time is halted compared to the rate of flow of your time.

On the other hand, from my point of view, time just flows willy-nilly forward and I go through the horizon and I keep on going. But once I’m inside the horizon, time flows in what you would have thought was a spatial direction toward the center. Which is why one reason I can’t go back and I can’t send signals back, because nothing can move backward against the local forward flow of time.

Now, time travel might be possible, but only by going out in space and coming back before you started. You can’t go backward against the local flow of time. Just can’t. So I go in and I like to say inside the black hole there’s a down-cascading time is flowing inward. The technical phrase is that the light cones point inward.

But this is all invisible to you because you’ve seen me go down. As far as you’re concerned, I’m plastered on the horizon and frozen there. Except that the last photon I emitted to tell you what was going on has long since reached you. And there are no more photons because photons are discrete. No more photons left to communicate to you what happened to me.

BRIAN GREENE: So there’s two things worthy of emphasis there. One, your motion as you described, once you pass the edge, the horizon toward the center of the black hole, locally is motion through time. So it is as inevitable that you’re going to go toward the center as, you know, tomorrow becomes today.

KIP THORNE: Absolutely right.

BRIAN GREENE: And the second thing, I mean, that’s hard to grasp in its own right, but the second thing is how long or how do people reconcile these two radically different pictures? I mean, we’re used to say from special relativity, you know, your clock is going slow, my clock, you know, we can get our minds around this one just seems so radically bizarre.

KIP THORNE: Well, I think we generally do it by finding a third point of view where everything fits together in a beautifully elegant way.

BRIAN GREENE: Like Finkelstein.

The Beauty of Spacetime Geometry

KIP THORNE: Yeah, so-called Eddington-Finkelstein coordinate system. And so you basically, if you adopt this idea that space and time are unified and you think in terms of four-dimensional spacetime and its properties instead of thinking in terms of space and time individually, then everything, if you think about it for a little while and go read some decent textbooks, modern textbooks, it just becomes natural.

It’s obvious, it’s beautiful, it’s elegant, and it’s a marvel to behold and fits beautifully. There’s no paradoxes, no problem at all.

BRIAN GREENE: Right, with the two stories, but there is still a problem that remains about what actually happens right at that center point, that sort of end of the time journey.

The Singularity Problem

KIP THORNE: Down in the center, there is a singularity. Again, in the Schwarzschild solution, where we say the curvature of spacetime goes to infinity, the strength of gravity truly goes to infinity in any kind of measure that you might try to introduce to measure the strength of gravity.

And this is also something that Roger Penrose basically predicted. His theorem said something goes wrong because anything that falls in there can no longer exist. Basically proves using his topological techniques. But again, it then took quite a while after that to really sort out what was going on.

It turns out that that singularity in there is unstable. And that’s not really what you wind up with. When stuff falls into it, it changes its whole character. So by contrast with the horizon of a black hole, which is highly stable, you perturb it and it vibrates a bit and then settles back down. It’s just so stable, you can’t blast it apart.

By contrast, what’s going on down beneath there is highly unstable. And when stuff falls in, the gravity of the stuff that falls in completely changes what’s going on down inside the black hole. So as best we understand it today—and this is part of the backstory of the movie Interstellar. I know you don’t want to talk about that yet, but we will get there for sure.

But there are three singularities in there, as far as we can best tell, three regions with three different characters where Einstein’s general relativity theory completely fails. And you’ve got to replace it with—and it completely fails in a way that is intimately entwined with the laws of quantum gravity. And the laws of quantum gravity take over and govern whatever happens in there.

So three regions with weird breakdown in the laws of physics as we know them. So it’s much richer interior to a black hole than we ever knew a few decades ago.

BRIAN GREENE: And so if you go back to Einstein’s hesitation, and this may not be historically accurate, but if you imagine that part of the dominant hesitation was the fact that the curvature goes haywire at the true center of the black hole, you can’t fault him for that.

Oppenheimer vs. Wheeler: A Historic Confrontation

KIP THORNE: No, not at all. And this is also a very interesting story between two of my Princeton colleagues, Robert Oppenheimer and John Wheeler. John Wheeler looks at the Oppenheimer-Snyder analysis and says, “You’ve got a singularity at the center. There’s something wrong with your analysis. And when there’s a singularity, all bets are off. I don’t believe your analysis, because that singularity is a signal that we’ve got to rethink the whole thing.”

And Oppenheimer says—and there’s a big confrontation between the two of them, what’s called a Solvay Congress in ’56 or ’57—in which Oppenheimer says, “Well, it’s very simple. The collapsing star cuts itself off from the rest of the universe. And what happens down inside there has no influence on the external universe, so why worry?”

And so they have this radically different viewpoint. And in fact, sorting out what happens with the singularity turns out to be absolutely crucial for modern physics. John Wheeler is quite correct, and Oppenheimer is also quite correct that it has no influence on what’s going on on the outside.

BRIAN GREENE: Now, John Wheeler, if I—again, please correct me—but he posited the possibility that there would be quantum processes at the center that would generate outward streaming radiation, carrying away enough mass and energy to avoid the formation of the black hole itself. Wasn’t that his vision?

KIP THORNE: That was his vision, and that was what he was pushing when I was his graduate student. That’s what he was pushing when we were writing a book together called Gravitation Theory and Gravitational Collapse. And that’s where I told him, “We can’t put this in.”

“Oh, really?”

“We can’t put this in our book. It can’t be right.”

BRIAN GREENE: And what did he say?

KIP THORNE: Well, he argued with me. And so I went and got all the big guns that I could find among his former students and postdocs and colleagues to lean on him, and finally he gave in. And it does not appear in there. It appears in his other writings, but in our little book that we did together, it’s not in there.

And I forever regret this because John Wheeler had tremendous physical insight. He didn’t have the whole story correct, but he was basically intuiting Hawking radiation, which, as we now understand it—one of the deepest ways to understand Hawking radiation was one that Stephen Hawking and his colleague James Hartle worked out in which there actually is a connection to the singularity inside that gives rise to the Hawking radiation.

Hawking Radiation and the Information Problem

BRIAN GREENE: But that’s interesting because the Hawking radiation—and maybe you’re going to enlighten me in ways that, you know, usually we describe it as pair production near the event horizon of a black hole. One particle falls in, the other streams away, and so forth. And it’s the disconnection between that process and the center that sort of sets up the so-called information problem. You were describing more of a link.

KIP THORNE: If you want to really prove that you’re right on Hawking radiation, you’ve got a problem because the Hawking radiation is basically triggered by, in Hawking’s original calculation, vacuum fluctuations, just fluctuations of electron-positron pairs fluctuating in and out of existence.

With photons, they flow onto the star, onto the black hole when the black hole is being born. And they suck energy out of the expanding space as the black hole is born. And all that radiation is created just above the horizon as the black hole is born. And then it takes millions of years to leak out.

But there’s a real problem that the stuff that leaks out a million years later, it has humongously high energy at the beginning, far, far beyond where the laws of quantum gravity have any right to. And so it was Hartle and Hawking that went in and they did a very different calculation in which they basically turned time into imaginary time.

And so they were setting up a different way of thinking about it. But in this different way of thinking about it, a key role is played by the singularity at the center. They’re basically having virtual particles that go in and out of the singularity at the center. Virtual particles can go faster than the speed of light. No information flow.

But that analysis was the thing that finally, I think, got people—people who were highly skeptical—to throw in the towel and say, “Okay, you don’t have to posit, pretend to think that this Hawking radiation forms with humongously high energies and then waits a million years trying to climb out with its energy decaying lower and lower and lower until it finally comes out.” Feels like nonsense.

So there was this period—that was in the ’70s when this was all being sorted out—but when the tools that were used to sort it out were closely related to John Wheeler’s 1950s and ’60s viewpoint. Oppenheimer—that I did everything I could to prevent John from publicizing it.

BRIAN GREENE: And so did John ever say, “Hey, Kip, I wish we would have included that”?

KIP THORNE: No, no, no, no. But by contrast, because of this idea, he was totally skeptical about black holes. He was totally skeptical about Oppenheimer and Snyder until work by a student of Charlie Misner, by Bekenstein, who did a different way of analyzing Oppenheimer and Snyder’s work that suddenly made it obvious that this made sense.

And this is 1962. Quasars were discovered in ’63. There’s a big Texas symposium on relativistic astrophysics in ’63 where people are struggling to understand quasars. And John Wheeler gives a talk about gravitational collapse and the formation of a black hole as being what’s really behind powering quasars.

And John views this as his public apology to Oppenheimer because he is now saying, “Oppenheimer is right and you really do form a black hole.” He’s not using the word “black hole” yet. But Oppenheimer sits out in the hall at that meeting. He doesn’t deign to go into Wheeler’s—there’s been enough bad blood over the years between the two of them.

BRIAN GREENE: Oh, he skipped the talk?

KIP THORNE: Skipped the talk, really. And Wheeler comes in—I was there. I was with Wheeler. Wheeler comes out and sees that Oppenheimer’s out in the hall, and he sort of wilts. I can imagine his apology and Oppenheimer’s—and so it was a momentous moment in the history of science, of miscommunication between these two great men.

BRIAN GREENE: And was it, you know, an arrogance on Oppenheimer’s part or just feeling bad or what do you think the emotion was?

KIP THORNE: I think he was just—I don’t know. I didn’t know. I knew Wheeler better than I knew Oppenheimer. Certainly Oppenheimer is capable of arrogance, but I don’t think so. It’s more my impression that it’s just that he had sort of had it with these arguments with Wheeler, but I don’t know.

BRIAN GREENE: Right, right. Just sort of fed up a little bit. Now, you said that—excuse me—Wheeler wasn’t yet using the term “black hole.” Ultimately, he does. He’s actually credited in some stories as having named them. Is that—

The Birth of the Term “Black Hole”

KIP THORNE: Well, the phrase “black hole” was used by others earlier in this context, but Wheeler was unaware of it and it didn’t catch on. And Wheeler introduced it in 1968, as I recall, in an article that he wrote. And he doesn’t say, “Let’s call it a black hole.” He just—it’s vintage Wheeler. He just writes it as though we had always called it a black hole. And suddenly people always—everybody does.

BRIAN GREENE: Now, I heard a story that there was a conference on 112th Street and Broadway at the Goddard Institute.

KIP THORNE: You were there? I was there.

BRIAN GREENE: And the story that I heard was that somebody in the audience, as John Wheeler’s talking about these objects, says, “Oh, that’s like a black hole.”

KIP THORNE: So I don’t remember that. I was there. I’ve heard the story, and that may be, but certainly it didn’t stick at that time. And I don’t think it probably had any influence on Wheeler at that time. This was probably ’67, I think. I think it was a year before John started using the phrase.

John understood the power of words, and he spent a lot of time crafting phrases and words to describe things. And as he described it to me, he liked to lie in the bathtub, in a warm bathtub and just think about what is the right phrase to use.

Now it could be that he was triggered by that a year or so earlier and didn’t remember it at all, but the back of his subconscious remembered it.

BRIAN GREENE: So what was it like then, working with John Wheeler, who cares so much about language and words, on a book that winds up being, I don’t know, 1,200 pages dense with words and pictures?

Writing Gravitation: Physical Intuition Through Pictures

KIP THORNE: And equations. And so the second biggest book I ever wrote. Yeah, no, that was a real pleasure. That was an enormous pleasure. This book, Gravitation, was a textbook on relativity. And we wrote it—we finished it in 1973, published it in ’73. We began at about ’68, so it took about five years.

When we began it, general relativity was truly, and had been for the preceding several decades, more a province of mathematicians than of physicists. John felt, and I certainly agreed, as did Charlie Misner, that there was an issue that mathematicians—it’s great, and the mathematics is crucial, but you really need physical intuition as a tool if you’re going to make rapid progress.

Calculations can be slow and tedious and complicated. You’ve got to be able to decide what’s worth calculating. And so you develop physical intuition as a tool for making those decisions of what’s worth calculating. And then you intuit something, and then you go in and do a calculation. You see whether you’re right or not, and you get the details right.

And the foundation for physical intuition, for John and for me and for Stephen Hawking, was pictures, mental pictures or pictures, diagrams that you draw, as well as words that go along with those pictures.

And so the ratio of words and pictures to equations in that book is absolutely never been seen before in any relativity textbooks. It’s a book that is designed to try to teach physical intuition to a new generation of physicists who are just beginning to get interested in relativity.

Because quasars and pulsars have recently been discovered in the cosmic microwave background, suddenly it’s relevant to the astronomical world. And so we need to build a generation that thinks physically. So that’s what the purpose of the book was. So Bill Press, who was a student of mine at the time, he was—

BRIAN GREENE: My advisor when I was an undergraduate at Harvard.

The Joy of Working with John Wheeler

KIP THORNE: Okay, so you’re my grandson. Grandpa. So Bill comes in. We’ve just distributed to the people in my research group a draft of a chapter from this book. And the chapter has a dialogue between Salvatius and Sagritis about what is a black hole. It is very much a Wheelerian thing.

Bill slaps that down on my desk and says, “Why the hell did you let John Wheeler write something like this in your book?” I said with great pride, “I wrote that. I’m capable of writing in Wheelerian style.” Fantastic. And I agree with John that we need to be building up physical intuition and this is part of it. Absolutely. And so that was one of the joys of working with John Wheeler.

BRIAN GREENE: Now, one thing in that book which is interesting to me is that when physicists typically learn the mathematical methods of general relativity, differential geometry to be concrete, most physicists learn it in a so-called coordinate form, which is the sort of more nuts and bolts ingredients necessary to really carry out certain kinds of calculations.

You’re at great pains in that book to do both the coordinate version and the coordinate-free version, which is perhaps maybe the way more mathematicians think about things in a more sort of global perspective. It’s powerful to have both. But was this your bread and butter or did you and John—did you need to study to learn that stuff, or was it already how you were thinking about this?

KIP THORNE: Well, that was the way Charlie Misner was thinking. Charlie was very, very deep in the mathematics. And John was an enthusiast for mathematics and he was capable of doing deep mathematics. Let me just tell you a little side story, please.

So one day John had a reputation in my era and going back a decade or two for not doing much mathematics, a reputation for functioning on physical intuition. And so one day I was sitting at Caltech at a party with Richard Feynman, who had been John’s student several decades before me. And we were both a little inebriated and he was reminiscing.

He said, “Yes, I remember when Professor Wheeler”—he always called him Professor Wheeler. I called him Johnny. So Feynman says, “I remember one day Professor Wheeler and I were doing a calculation together, and Professor Wheeler went from this step to that step, and I didn’t see how he got there. And so he showed me. And as he showed me, he said, ‘Little steps for little people.'”

Now, John Wheeler was the most polite person I ever knew. And he never—I never saw him do anything impolite like that. Now, it’s clear that Feynman was full of himself as a graduate student and Wheeler felt he needed to be taken down a notch or two. And Wheeler did. But the fact that he could was an indication that he really himself was quite deep in the mathematics.

BRIAN GREENE: That is so interesting.

KIP THORNE: And he could out-think Feynman. Of course, he had a number of years behind him, and Feynman was just getting started, but still.

Feynman’s Mathematical Prowess

BRIAN GREENE: But Feynman is, you know, I mean, there’s phenomenal. If I can just give one story that you may be familiar with too. I was taking quantum field theory with Sidney Coleman, and Sidney Coleman, I think this is an act that he did every year that he taught quantum field theory. I don’t think it was spontaneous, but he put up some problem on the board and he said, “Let me first show you how Feynman would solve it.”

And he turns dramatically to the board, puts his hand on his head and then writes down the answer. He said, “You know, Feynman, he can just do all those calculations in his head. We little people have to do step by step.”

KIP THORNE: So for Wheeler to sort of play—

BRIAN GREENE: The same on Feynman is sort of astounding.

KIP THORNE: It is astounding, but it really showed. And Feynman still remembered this with chagrin. It showed what he was capable of. And so back when we were writing the book together, he was embracing the new mathematics. Well, what for physicists was new mathematics but coordinate-free notation, thinking about things without coordinate systems, which is part and parcel of the unification of space and time, that was central to really understanding what was on inside black holes.

So Wheeler was enthusiastic about it. He was embracing it. Charlie Misner was much deeper than either of the two of us in the mathematics. And I was learning it from the two of them. I brought to this an understanding of the astrophysics, but not the kind of depth in mathematics that the other two.

Track One and Track Two: A Pedagogical Innovation

BRIAN GREENE: Yeah, what a wonderful experience, though, to be learning and contributing and creating something timeless as a book. Now, one of the other things that I deeply appreciated about that book is you had track one and track two. Track one was for maybe the beginner, track two, the person wants to go more deeply, which is a great structure.

From that point on, when I read your book for years, when I would give a technical lecture, I would do, here are the track one slides, and here are the track two slides, and try to appeal to a broader group of individuals who could follow the ideas, but perhaps maybe not the mathematics. I think it’s a powerful way of going about it.

But back to black holes, then. So by what would you say the community had pretty much been convinced through all of the things that we’ve discussed historically, that these things should be real?

KIP THORNE: I think it depends on which community. There were people like Phil Morrison at MIT, very highly respected, a superb astrophysicist, who never, ever embraced black holes. And so after they died, the community was convinced.

BRIAN GREENE: Is that famous adage that physics progresses one funeral at a time kind of thing?

KIP THORNE: I hadn’t heard that one, but yes. But certainly the younger generation, people who were learning stuff and thinking stuff through, my generation, had all embraced this and had, I think, a very clear understanding by the early to mid-1970s. And it took some of the older generation another few years beyond that, but certainly by the 1980s, it was pretty universal, except for a very small number of people who hung on until death.

Observational Evidence for Black Holes

BRIAN GREENE: And psychologically, for those who were convinced, were they waiting for observational confirmation or it was just a—

KIP THORNE: Well, the observations were coming in on things related to black holes. There were a number of black hole candidates in the sky, and the gravitational waves didn’t come along until decades later. But electromagnetic observations, we were learning about the cosmic microwave background, and we were learning about black holes through quasars and pulsars and jets sticking out of galactic nuclei and so forth.

The theories that were being developed and the models that were being developed were, by the 1980s, pretty much right on.

BRIAN GREENE: And so, for instance, additional data that ultimately was awarded the Nobel Prize, observations of stellar trajectories in the center of our galaxy. Was that viewed as just adding to the mountain of evidence?

KIP THORNE: Well, I think there was always a worry—I would say a worry, a hope—that there was something wrong. And I saw over my career some huge surprises where we were wrong. For example, the acceleration of the universe, which I didn’t believe until there were several different, very different pieces of data that insisted the universe was accelerating its expansion.

And so I saw things like that that were sacred to me, that it was just obvious that there was no acceleration of the universe. We really did understand the cosmology of the early universe, and my nose was rubbed in it. And that has associated with it huge potential surprises that we still haven’t sorted out.

And so I’m always hoping that something will similarly be wrong with black holes. There’s something going on at the horizon and we’ve got it wrong. And so going all the way up to the present time, I’m certainly a strong advocate of going in and really pushing the observation really hard to be sure that we’re right and hope that we’re wrong because it’ll lead to a revolution.

The Event Horizon Telescope

BRIAN GREENE: And the Event Horizon Telescope, the images, when you first saw those, what was that?

KIP THORNE: Well, they were fabulous. Just to see it. To see it. But for me the key thing there is to see movies. There has been various skepticism and complicated plasma physics arguments over how the jets are launched from the vicinity of a black hole. As we see jets sticking out of black holes, how are they launched?

And there are very interesting and probably correct theories related to it and models related to it. Begin with the so-called Blandford-Znajek effect. But there are skeptics in the plasma physics community and skeptics. I can’t be absolutely sure the skeptics are wrong. And so I want to see a movie of the launching of these jets.

And so that’s what I think we will ultimately get from the Event Horizon Telescope. And so I’m just waiting for them to come out with movies now. Not of the black hole in M87 because that’s such a humongously big black hole. The movie is just too long a time scale. But the black hole in the center of our galaxy. But that is technically very, very difficult. But that’s where the really exciting payoff from the Event Horizon Telescope is going to come. And it could come most any time. I don’t know precisely where they stand.

BRIAN GREENE: Yeah, I’ve had conversations on occasion with Shep Doeleman and movies has certainly been the top next step. But timescale, I don’t know. But it’s interesting that you’re still open to the possibility that our understanding may need to be fine-tuned or changed.

KIP THORNE: In some way may be much more than fine-tuned. As I say, I’ve seen several huge surprises. And so I have come to believe that there are situations where we’re not as smart as we think we are and where we’re quite sure of things that will turn out to be wrong.

You take any given thing and I look at it and say the odds that we’re wrong there are pretty damn small. But we’re going to be wrong somewhere among things that we hold near and dear to ourselves. And we should work very hard to search for a failure.

String Theory and the Fuzzball Proposal

BRIAN GREENE: And by any chance there is a proposal in string theory—I don’t know if you’re familiar with the fuzzball proposal. Samir Mathur, Ohio State University, this idea that maybe black holes are not the thing that we’ve long thought they are, maybe they’re closer to an entity. It could be an agglomeration of strings and other membrane-like objects within string theory.

And it can mimic, he finds it can mimic the features of a black hole. But if you really got in and looked at it with adequate resolution, it wouldn’t be what Schwarzschild wrote down per se. Is that a thing that you can imagine?

KIP THORNE: Well, now, I don’t know much about that. I do know other speculations about what goes on in the horizon. Let me just make a remark about string theory. I think that string theory and the laws of quantum gravity are the most important and the most interesting area of physics. They have been for several decades and they will be for a few more decades.

I haven’t touched them with a ten-foot pole because I need elbow room. And I’m not happy if I’m working in a community where there are huge numbers of people working. They’re all really smart. In my mind, works slower than any of theirs do. So I need elbow room. And so I’ve stayed clear of that. But I admire you and your colleagues who do this. And I follow what goes on at sort of a semi-popular level and keep hoping for big surprises.

BRIAN GREENE: Sure, yeah, we are hoping too. And yeah, you know, it goes both ways. When you do have a lot of people working so closely, we find that things do go in fads, right? An idea catches on, it spreads through the community through no other reason than excitement and wanting to make progress, which weirdly, online you do have these people having conspiracy theories about how we’re trying to shut out—you know, it’s not. It’s excitement and it is a group of individuals that do work closely.

The elbows are touching, you know, as you’re saying. And that’s sort of a good thing and a bad thing about it all. But with that, maybe we could turn to the next arena in which you’ve had a tremendous impact, which is gravitational wave physics. We can’t go as far back, I don’t think, as John Michell or Laplace or anybody here, but we can go back to Einstein in 1916, 1918.

He writes a couple of papers that suggest this possibility, but he himself, I don’t know, is a little confused on—I mean, it goes back to coordinates and how to really determine what’s real in your mathematics. Maybe you can just give us a sense of what that story is.

Einstein and Gravitational Waves

KIP THORNE: Yeah. So Einstein, 1916, he formulates his general relativity theory in 1915. He writes his first paper on gravitational waves in 1916. He’s pretty confused in that paper. And he recognizes it rather quickly and basically in some sense, retracts it and does it right, aside from maybe a factor of two, in 1918.

But at that point, I think it’s my impression that he really believes in these as physical gravitational waves. But there is a lot of confusion over that. And he has moments later on when he has misgivings, but I think they’re only moments.

BRIAN GREENE: Oh, really?

KIP THORNE: That’s certainly my impression.

BRIAN GREENE: Yeah, I got the impression that he was a little more skeptical, but I haven’t really delved into it in detail.

KIP THORNE: Well, it’s certainly—that’s my impression that there are moments, but moments a month or two when he’s having some cold feet. The central thing was, how do you describe the gravitational waves in a way that is absolutely clear physically, that it has a physical reality?

And this ultimately comes from something called the equation of geodesic deviation that is formulated around 1956, ’57, ’58 by Felix Pirani in London. He identifies something called the Riemann curvature tensor as the thing that stretches and squeezes. And he says if you begin with two particles and they’re running along side by side with a fixed distance between them, then this Riemann curvature tensor, which has nothing to do with coordinates, it is a true geometric object. And everybody agrees on that, that it actually squeezes and stretches the space between the particles. They move back and forth relative to each other.

But it’s only then, I think he publishes it in ’58. He has it in ’57 or ’56. He talks about it at a famous conference on general relativity in Chapel Hill, North Carolina, in ’57, and then publishes it the year later. But it then becomes the foundation for Feynman and for Hermann Bondi and others to start thinking about this in a totally physical sort of a way.

Joseph Weber and Early Detection Attempts

BRIAN GREENE: And then people—well, at least one person starts to think seriously in those older times about detecting these way before LIGO: Joseph Weber.

KIP THORNE: And Weber is—he has already just before this—well, he in some ways is a disciple of John Wheeler. He is a professor of electrical engineering at the University of Maryland. But he’s interested in gravity and gravitational waves. He goes with Wheeler to Leiden when Wheeler has a sabbatical in Leiden, Netherlands. And they’re there, and so is Charlie Misner and a few other people around Wheeler, and they’re thinking about gravitational waves together.

We have no evidence that he’s thinking about any detection methods in Leiden yet, though he might have been, but there’s no evidence of it. And right a few months after Leiden is this conference in Chapel Hill where Wheeler and Weber together present analysis of gravitational waves, but not of an experiment, but a mathematical analysis that emphasizes the physical reality of the waves.

And then it’s immediately after that that he is thinking about gravity wave detection and that we have evidence he’s thinking about it. So ’57, ’58, he’s beginning. He even writes a little textbook of his own on general relativity. Really, that is a nice little textbook, unconventional approach, but one in which it’s clear that he’s thinking about it from a point of view of experiment.

And the only thing is, it will measure the stretch and squeeze of space. He then actually starts to put together an instrument, gravitational waves, and works on this until ’69. In spring of 1969, there’s a conference in Cincinnati, Ohio, at which he announces evidence for having seen gravitational waves with his instrument, which is a large cylinder in which he has mounted around the equator of the cylinder crystals called piezoelectric transducers, which, when they’re squeezed ever so slightly, they develop a voltage across them. And then he’s instrumented to see this voltage, this twist, squeezed back and forth by gravitational waves.

BRIAN GREENE: It’s a clever idea.

KIP THORNE: It’s a very clever—it’s clever to the point that other people keep pushing on that particular technology until we and LIGO have better sensitivity than they do. But that’s not until about 2006, 2007. So this technology that he invents, that he conceives, keeps getting pushed by some very good experimental physicists from then in the ’60s all the way up to 2006—40 years, basically.

BRIAN GREENE: And just to give people a sense of the challenge, what sort of squeezing and stretchings do they need to be able to detect?

KIP THORNE: So they were hoping that if they stretched and squeezed a bar that’s a meter long by an amount that is, say, a sizable fraction of the nucleus of an atom, that would be enough. We know today it was not enough, that you need something that is quite a bit smaller than that. But that was sort of where he was headed. And it was astounding that he could do as well as he actually did. But it wasn’t nearly enough. In retrospect, we know it wasn’t nearly enough.

BRIAN GREENE: Was he ever convinced that he had not detected gravitational waves?

KIP THORNE: No, I think he wasn’t. In the end, up until his death, he believed—up until his death, nobody has precisely replicated my experiment. I’m seeing something. I don’t know for sure it’s gravitational waves, but I’m seeing something. And nobody has precisely replicated what I’ve done. And we really need to sort out what it is I’m seeing.

BRIAN GREENE: And you’re convinced though that he was not?

KIP THORNE: In no way, yeah.

BRIAN GREENE: And so when was it—it took a few years?

KIP THORNE: It took a few years to get to the point where I was convinced that he wasn’t seeing them, and negative experiments by a number of different groups. I was hopeful that—it was fairly quick that I began to think he probably wasn’t.

BRIAN GREENE: Were you rooting for him or did you want it not to be so the field would still be open?

KIP THORNE: I was absolutely rooting for him.

The Birth of Laser Interferometry

BRIAN GREENE: And then where did the idea of a new technology that ultimately succeeded come from?

KIP THORNE: So the new technology was that you have mirrors that hang from overhead supports and you have laser beam bouncing back and forth between the mirrors, and you use a technique called laser interferometry to monitor the motion of the mirrors.

That idea was first conceived by Mikhail Gerzenstein and Vladislav Pustovoit in Moscow just a few years before I started developing my close collaboration with the physicists in Moscow. So I knew them well. They were theorists. They didn’t really understand the noise sources that this instrument would face. They just had the idea, and they had the idea clearly before anyone else did. So the idea was fairly obvious once you began to think about it. And it was sort of in the air.

But it was Rainer Weiss at MIT who had the idea completely independently in the mid to late ’60s, who then carried out, after doing some experimental work to see various things like whether he could get a laser down to its so-called shot noise level and some things that would be required—he wrote a technical paper in 1972 in which he described this kind of gravitational wave detector.

And he analyzed—he identified all the major sources of noise that it would face. For each major source of noise, he conceived a way to deal with it. He then analyzed how far down he could push that noise source by the method he conceived of doing it. He concluded then that if he were to make this instrument a few kilometers long, he might have a shot at seeing gravitational waves.

This paper, interestingly, it’s an absolutely classic paper. It’s the most prescient paper in this field that was ever written, as far as I can see. The most important paper, really. He didn’t publish it in a journal. He published it in an internal progress report series at MIT because he thought you shouldn’t publish until you’ve actually built the detector, maybe seen gravitational waves. This is Rai, who became one of my dearest friends.

But he sent this out to colleagues. So I got a copy, sent it to me. It was precisely at the time that I had been thinking about gravitational waves and their detection and what you could do with gravitational waves for astronomy for a few years. Bill Press and I had just written or were just finishing our first paper on a vision for gravitational wave astronomy.

And so this thing comes in from Rai Weiss. And I look at it cursorily, quickly. And I am at the same time—we are about to send to the publisher the manuscript of our book “Gravitation” with Charlie Misner and John Wheeler. I look at it and it just seems to me crazy to think that it could succeed.

And so in this manuscript that’s about to go to press, I write a brief description of this technique. And then it’s an exercise for the reader to explain why it’s not very promising. Our book goes off to press and then on and off over a period of about three or four years. I keep thinking about this. I have long conversations with Rai Weiss. I have long conversations with Vladimir Braginsky in Moscow. And I gradually become convinced that it has a shot at succeeding.

BRIAN GREENE: But a shot at succeeding—it’s still audacious.

KIP THORNE: Well, it’s going to be very difficult. But by 1978—’76, I guess it is. By autumn of ’76, I think, November of ’76, I’m convinced it has a good enough possibility of success. That because the payoff would be so enormous if it does succeed, that I decide Caltech ought to get into this business.

The Vision: A New Window on the Universe

BRIAN GREENE: Now, when you say the payoff, I guess there’s two ways to look at it. One is you’re confirming a prediction of general relativity, or you’re opening a new—that’s what I’m wondering. Was that the driver or was it opening a new window of astronomy?

KIP THORNE: Totally opening a new window in astronomy. Just confirming—this is gravitational waves. What was going to be several decades of work and enormous effort. I was going to spend most of my career in that of my students.

BRIAN GREENE: But what about Rai Weiss? Was it similar vision?

KIP THORNE: Similar vision. Similar vision. The driving issue is to create gravitational wave astronomy. The issue was that Galileo 400 years earlier had built a little optical telescope, pointed at Jupiter, seeing Jupiter’s four largest moons and initiated instrument-based electromagnetic astronomy which blossomed and revolutionized our understanding of the universe.

There’s only one other kind of wave that we knew about at the time and that we know about today that could be created in the distant universe and bring us information about what’s far away, and that’s gravitational waves. So the vision was to do for gravitational waves like Galileo did and over periods of decades to centuries that that will revolutionize our understanding of the universe.

BRIAN GREENE: That certainly is motivation, framing it that way.

KIP THORNE: So that was truly the motivation. But it was going to be very difficult. When I could see that we had a reasonable shot at success, better than a 50-50 chance, I then wrote a document proposing that Caltech get into this game. Gave it to the chair of physics, math and astronomy at Caltech, who at the time was Maarten Schmidt, who was the discoverer of quasars.

And Maarten and I had discussed it before. He said, “You write it up.” I wrote it up. And he appointed a committee of Caltech physicists and one radio astronomer who were deep in the relevant physics of these instruments to look at this proposal in depth and make a recommendation.

BRIAN GREENE: And there were some skeptics.

KIP THORNE: Oh yeah, there were skeptics on the committee.

BRIAN GREENE: Was Dick Garwin part of that?

KIP THORNE: This was a later thing. This is ’68 at NSF. This is—I mean that was ’76 at NSF. This is ’60—just a minute, I confused my decades. This is ’76 and that is ’86, Garwin.

And so this is at Caltech. And this is at a time when NSF has invested in Rai Weiss, I think, $56,000. That’s the total amount of money he’s gotten from NSF and total amount of money he will get in the 1970s and when MIT won’t give him the time of day. And so there’s just everywhere there’s skepticism.

And I couldn’t fault the skeptics because I had been skeptical for four years, but I had come around. And so this committee is appointed. The committee—we as a committee, it includes Barry Barish who becomes the key director of LIGO. But the committee then talks to everybody who’s working in this field and a lot of other people who have knowledge about the technology.

And after a year decides we should go into the field. Spends a year trying to identify who we might bring to Caltech to help start. It identifies Ronald Drever in Glasgow, Scotland. And our recommendation gets enthusiastically endorsed by the physics, math, and astronomy faculty, and by the Caltech administration. I even do presentations to the board of trustees so they don’t have to embrace it. They don’t make a decision. And so it’s embraced at Caltech.

And Murph Goldberger, who is the new president of Caltech, he phones up Bob Dicke back at Princeton—and that’s where Murph comes from. He says, “What do you think of this?” Well, Rai Weiss had been Dicke’s postdoc, and I had been a graduate student in Dicke’s group, although I did my thesis with John Wheeler. Dicke is the great experimental physicist in relativity of that era, and he gives it his enthusiastic blessing. He knows Rai, he knows me, and he knows the field.

As a result, Goldberger says, “Okay, we will invest what we need to get this started.” And that turns out to be that he forks over $2 million of Caltech money, which inflates to $14 million today.

BRIAN GREENE: $14 million in today’s dollars. Wow.

KIP THORNE: To get it started at a time when NSF has invested $56,000 and MIT has invested zero.

The Bold Move Forward

BRIAN GREENE: So that’s a bold.

KIP THORNE: Bold. It’s a very bold move. But Caltech’s a unique institution. It’s small, it’s intimate, and it’s a place where you can make things happen. The relationship between the trustees and the faculty and the administration is very strong. And the trustees are the sources of startup money like this.

BRIAN GREENE: Was there any outcry? How can you put that much money into this?

KIP THORNE: No, no. This had been embraced by the PMA faculty. And Murph Goldberger is a physicist. He got the blessing from Bob Dickey.

BRIAN GREENE: And so off you go.

KIP THORNE: NSF sees this and says, well, we’ll do our own study. And they do their own study quickly over a period of about four months, and come up with the same conclusion. They start investing money both in Caltech and in MIT, and we’re off and running.

BRIAN GREENE: And so what’s the first thing you do when you—

KIP THORNE: Well, so Caltech starts building with Ron Drever, whom we have hired. Bring Stan Whitcomb from Chicago. Ron is commuting back and forth between Caltech and MIT. Stan is a superb experimental physicist who becomes ultimately effectively chief scientist, although he didn’t carry that title. He was at various times deputy director, acting director of LIGO. But he’s just absolutely superb, and he really makes things happen in the laboratory.

BRIAN GREENE: And is MIT and Caltech sort of in partnership now?

Building the Prototypes

KIP THORNE: Well, not yet, but R and D is now underway vigorously at MIT with a 1.5 meter prototype. At Caltech with a 40 meter prototype in Glasgow, Scotland, a 10 meter prototype. That is Ron Drever’s group. He’s commuting back and forth and in the Max Planck Institutes in originally in Munich and then in Garching. They moved to Garching. They built a 3 meter prototype, then a 30 meter prototype.

These four groups interacting with each other are pushing hard to—and what we know is we have to—I know approximately how strong the waves are. And it’s clear to me already by 1978 and I assuming the source at—

BRIAN GREENE: That it didn’t matter.

KIP THORNE: You’re just saying by ’78 it was black holes and neutron stars.

BRIAN GREENE: You’ve already concluded that that’s the—

KIP THORNE: Yeah, that’s right. And I had zeroed in. I knew my best guess was right on 10 to the minus 21 strain. And with an uncertainty that was pretty clear by 1992, about a factor of 100 between 10 to the minus 20, 10 to the minus 22, which becomes important later.

But so we know where we’re going and we know where we’re beginning. And we have to improve our displacement noise on the mirrors by a factor of a million. And the team, by 1986, after about 10 years of effort, has gotten a factor of 1,000. So they’re in logarithmic scale. They’re about halfway there. So the prototype work is going pretty well, our colleague said.

BRIAN GREENE: Is logarithmic the way to think about it psychologically?

KIP THORNE: You think so? Yeah, it was in that era. In that era, because obviously you hear—

BRIAN GREENE: A thousand to a million to us, you’re like, oh, you got a ways to go.

KIP THORNE: Well, and no wonder I was skeptical. I mean, that’s why I was skeptical. Factor of a million, you’re going to do that. Yeah. But as I say, I came around.

The Push for Collaboration

And so the folks at NSF, the key people, Marcel Bardon, who’s head of physics, and Richard Isaacson, who’s head of Gravitational Physics, reports to Marcel. They understand two things. One, that there’s no way that Caltech and MIT separately can build up enough manpower, enough technical strength to pull this off. They’ve got to collaborate. And two, there’s no way Congress is going to give money to a field of multiple groups competing where there are two different groups competing. And so they tell us, you’ve got to build a collaboration.

And Ed Stone, who’s now the head of Physical Math and Astronomy, says, absolutely, yes, we got to collaborate. John Deutsch who’s the dean of science and becoming the provost at MIT, says, no way. We have no interest in this. This project is going to fail. We have no interest in supporting this. So MIT administration has no interest. Caltech is enthusiastic.

There’s a troika of leadership: Ray Weiss, Ron Drever and me. Ray and I say, of course we collaborate. Ron Drever says, no, I can’t collaborate. Ron is a very interesting guy. He’s highly creative. He invents set of things that are in the final interferometers that are really crucial for ultimate success, that are improvements on what Ray began with. So he’s made some substantial improvements, but he cannot psychologically somehow function efficiently and enthusiastically unless he’s in complete control over everything that he’s involved in.

But he’s also the most disorganized physicist I ever met. There’s no way he can be the head of this project. Just no way. So this is a whole recipe for dysfunction. Somehow, however, we do build a collaboration. We define it. I had a steering committee of the three of us. Troika, we were called. And we get a lot done over that period.

Then from ’84 to ’86 in 1986, we’ve got this factor, a thousand improvement in sensitivity. Not just us, but the four research groups that are doing this. And it’s 1986 that we’re beginning to talk with NSF about going big, going up to kilometer scale.

Richard Garwin’s Intervention

Richard Garwin, the most influential, politically influential physicist of that era, who was at IBM—

BRIAN GREENE: I used to be a summer student, so I knew him from my days as a summer kid at IBM.

KIP THORNE: Yeah, no, he played a huge role in the hydrogen bomb development process. But he had also built one of the detectors to check Weber, a bar detector in that era. So he had credibility from that point of view and had very quickly built a very good but very rapidly built experiment and not seeing gravity waves.

He writes a letter to NSF, to Marcel Bardon, and he says basically that if you’re thinking, really thinking about spending 40 or $100 million on this, I advise against it. And if you’re going to do that, you should appoint a summer study committee to look at this in depth of people who are from outside the field and give advice.

And so Bardon does the best he can. He can’t get it together for a summer study. He does it in the autumn and appoints a committee that Garwin is on. He puts Garwin on because Garwin goes in as a total skeptic and the other committee members are highly respected experts in the technology, plus one person from the gravitational community, which is Saul Teukolsky, who at Cornell is at Cornell at the time. A theorist, yes, a superb physicist, just a theorist, but who could represent the committee and provide understanding for the committee about the thinking of the gravitational wave community in the committee’s deliberations.

But the real heart of the committee is this whole set of something like eight other experimenters. And so the committee does a one week long study. It brings in all the people from around the world who are working on this, grills them, looks in depth at the technology and comes out with a report that says this is likely to succeed. It is likely to revolutionize our understanding of gravity and of astrophysics.

You should move forward now. You should build two instruments separated by at least a thousand kilometers. Even though the Europeans are thinking of building several instruments because a network of the European instruments and the US instruments is going to be crucial for getting all the science out that can be gotten out. And as I said, you move forward now.

BRIAN GREENE: Were you surprised by their report, relieved by it?

KIP THORNE: I was certainly relieved. I was surprised that it was so strong and so specific and particularly with regard to build two, even though the Europeans are aiming to build. But it was in fact a blueprint for what actually happened over the subsequent decades. It just foresaw this.

BRIAN GREENE: So sometimes committees work.

Moving Forward Despite Opposition

KIP THORNE: Well, these committees worked and this was the secret to our success with funding in subsequent years was these kinds of committees that were experts in the technology. And so I’m told that Richard Garwin, when LIGO actually discovered gravitational waves, well, I know he was very enthusiastic. Barry Barish saw him shortly thereafter and he was euphoric, Garwin. But I’m told he also took great pride in having triggered this 1986 study that enabled it to go forward.

And so over the subsequent few years we submitted a proposal and proposal got funded. It was crucial time and again that we have this kind of review committee because we had eminent astronomers who were fighting us tooth and nail in Congress, totally skeptical. And I couldn’t fault them because I was so skeptical initially, but totally skeptical about this and thinking that this money should go for conventional astronomical telescopes, not for LIGO.

BRIAN GREENE: And so you get the go ahead there and you settle on two locations in the United States, Washington and Louisiana. You go forward. But there’s still enormous challenges to overcome. So you’re building the plane kind of while you’re flying it to some extent.

Leadership Transitions

KIP THORNE: Yeah, that’s right. And so we also have a sequence of several directors. This committee had also said as part of their report, get rid of this troika, this semi-dysfunctional troika of leadership, and get a single director. So they bring in Robbie Vogt, who had been the Caltech provost. He’d been the first Chief Scientist at Jet Propulsion Laboratory and defined the job of chief scientist. He was tough, tough, charismatic.

And he forced the Caltech and MIT to start collaborating. And he was central to getting Congress to provide the funds, central to winning the battle against the astronomers who were trying to prevent us from getting the funding. But due to some misunderstanding between him and NSF, he wound up with a budget that was not adequate to have a robust management. And so he was running a lean management because there had been some misunderstanding and he was stuck with—I think lean management was also probably his preference. But he recognized that it would be safer to have a robust management. The difference between lean management and robust management in this case was several tens of millions of dollars in cost just to have a robust management.

BRIAN GREENE: And robust, just more people, more people, more layers and so on.

KIP THORNE: More layers, more documentation, more continuing response to queries that come from NSF and elsewhere. So NSF gets cold feet once we’re ready to start construction. And when they get cold feet, then Congress gets cold feet and they come back and they say, you’ve got to have a robust management. Robbie says, I don’t have the money for robust management. They won’t give him more money because they don’t really trust him to do a robust management, although he’s done a superb job up to this point.

And so the president of Caltech, who has the authority to change the director, he fires Robbie, basically, and brings in Barry Barish to take over the leadership. And Barry asks for an additional something like $30 million for the robust management, which NSF will give to him.

A Strategic Partnership

But then the really interesting thing, Robbie Vogt, who has never functioned very effectively under somebody else, he and Barry making an agreement that he will take over at Barry’s request, to be in charge of the design of the first gravity wave detectors leading into their construction, working under Barry Barish’s director. And there’s a huge respect between these two people. And Robbie does that for two years of transition. You have this team that is very loyal to Robbie, and he’s just been fired, but he stays on in this capacity. And then with Barry, he smooths everything out.

BRIAN GREENE: So strategically, this is a great move.

KIP THORNE: It’s a great move. And it’s something that I would not have believed that Robbie was capable of, but he did for two years. And then when things were really solidly in place, Robbie left the project and moved off and started doing other things. And Barry was solidly in place.

And Barry carried it forward through the absolutely crucial period of—we knew that—so Barry identifies—Robbie had not thought much about advanced detectors. We knew that the initial detectors, as we were planning to design them, would probably not see gravitational waves and we would have to go beyond. Yeah. And we knew that with Robbie. And that was part of our—when we submitted a construction proposal, we said that.

BRIAN GREENE: And people bought into this idea of building a machine that you think is likely not to succeed as a stepping stone.

KIP THORNE: That’s right.

BRIAN GREENE: Okay.

Planning for Advanced Detectors

KIP THORNE: But this was also part of the problem with NSF and the robust management business was that they didn’t see adequate planning for the advanced detectors while the initial detectors were still under design, much less under construction. But Robbie didn’t have sufficient budget to be doing much on the advanced detectors.

So when Barry comes on board, he analyzes the whole thing and he says, look, there’s no way that if the advanced detectors see nothing and the first detectors have seen nothing, we’re dead in the water. And so the advanced detectors have to have a very high probability of success.

BRIAN GREENE: Because you can’t fail twice, I guess.

The Two-Interferometer Strategy

KIP THORNE: And so the initial interferometers become a precursor, a test bed, someplace where you can find out what the problems are going to be in order to design the advanced interferometers to deal with that. And so you don’t expect the first interferometer to see anything but the advanced interferometers. They’ve got to see something.

BRIAN GREENE: And did you clue Congress in enough? I mean, to say, we’re going to come back to you and say this is going to fail first?

KIP THORNE: Yes, yes, absolutely. And so we had—when Barry then formally came on and took over the project, and he had gone through and redone the budget and seen how much more he needed, he needed to get buy-in from NSF and then from Congress to restart the project. And he did so in the context of this two-interferometer strategy.

We met—so Barry and I met with the National Science Board that oversees NSF, and then they would make a report that Congress would see. And when we met the National Science Board, folks at NSF were really quite nervous. I was supposed to be sort of the guru on where the gravity waves were, how strong they were going to be.

And I was very blunt and said the odds are the first interferometers will not see anything. But the advanced interferometers will have a high probability of success and we have to be prepared for that. And we had a detailed discussion with the National Science Board over this and the National Science Board bought into it and then Congress bought into it.

And part of that then was that Barry went in and started asking for funds for the R&D and the extraction for components of the advanced interferometers several years before the initial interferometers reached their design sensitivity. And so you’re not even doing—

BRIAN GREENE: You’re not doing stage one, stage two, you’re doing stage two while stage one—

KIP THORNE: That’s right. So it was a very bold, bold move, but it was essential because it would have driven the cost way up because of the stretch-out time if you were not ready with the advanced interferometers just as soon as you had done a search with the initial interferometers.

Barry Barish’s Management Genius

BRIAN GREENE: And how did Barry—and it was Barry, of course, particle physics. He had already managed big—was this part of his own experience?

KIP THORNE: This is quite different in some respects, but Barry was a superb strategist and the designer and manager of big projects. Look, I’m not an expert in this, but it’s certainly my impression that he’s the best we’ve ever seen.

And he was dealing with a situation where you were doing technology development hand in hand with then building, designing and building instruments in two generations. You don’t do that in particle physics. It’s just not done. So it was a unique situation.

Similarly, it was very interesting with the first interferometers. He was building the facilities to house the interferometers, big vacuum systems and so forth. And then he was going to start installing the interferometers in them as soon as they were ready. And the preparation for the installation of the first interferometers was all going on.

So he ran two organizational structures in parallel. And if you were working on facilities, you were in one organizational structure which was very pyramidal. There was a boss and like you do on a big construction project. But if you were working on the interferometers in preparation for beginning installation of components, you’re in a very flat organizational structure.

And he successfully ran the two simultaneously for a couple of years. It’s amazing. It was quite amazing. That’s the first thing I saw that was in the 1990s and on into the early 2000s. And then this business of initial interferometers and advanced interferometers running along with only a time lag between them, but the advanced was very far developed before the initial interferometers reached design sensitivity.

Closing the Sensitivity Gap

BRIAN GREENE: Now, when we left the numerical sensitivity a little while ago in our conversation, we were at a thousand versus a million. A factor of a thousand to go. How is that gap closing through all this?

KIP THORNE: That gap is closing through continuing R&D in the four-kilometer-long arms. The initial interferometers are installed and they are being driven down over a period of about four or five years, being driven down to their design sensitivity. And that’s picking up this next factor of 1,000.

BRIAN GREENE: So you see the gap closing?

KIP THORNE: You see the gap closing. And that was the plan. That’s basically what we said in 1986 is how you would do it.

BRIAN GREENE: So when does the first LIGO actually turn on?

KIP THORNE: So the first interferometers, they begin installation around 2001. They get close enough to design sensitivity to start doing very serious gravitational wave searches around 2005, 2006.

We have a delay because of a no-new-start policy in the George W. Bush administration. So we have a several-year delay. By 2006, 2007, we’re at design sensitivity and the advanced interferometers can’t be installed with a new start until 2010. So what do you do?

Well, there’s this graduate student of Ray Weiss named Rana Adhikari, who proposes a detailed plan to get another factor-two sensitivity beyond the original design sensitivity, and then do another run. And that plan is accepted.

And they do get this other factor two, at frequencies above about 150 Hz, something like that, or 200 Hz, a little less improvement at the lower frequencies. But I mean, it’s quite remarkable. That means you’re seeing twice as far into the universe. Eight times bigger volume, eight times bigger event rates for things in the higher frequency band than the original interferometers were even designed to have. And we picked that up during this period.

The Weight of Responsibility

BRIAN GREENE: But you mentioned graduate students and I’m just wondering—so in this whole period there are graduate students involved. Their careers in some sense are hanging on this idea. What sort of pressure did you or does Ray feel that you brought these young researchers into a field and obviously it’s their choice to take this chance. But it’s a big chance, right?

KIP THORNE: Well, so when gravitational waves were finally discovered in 2015, this shows up in our reactions, our emotional reactions. Ray and I have long since become very close buddies. I called him my transcontinental soul mate.

And when it’s finally clear we’ve seen gravitational waves, my reaction is profound satisfaction that we had made wise choices in a number of places along the way. Ray’s reaction is profound relief because of just this—that he had convinced hundreds of students and postdocs, not just at Caltech and MIT, but around the world.

Because Barish had expanded LIGO in order to have this succeed to what in the end became 15 countries, 80 institutions, 1,000 people in order to succeed, by the way. And Barry had actually left the leadership of LIGO about 2006 when the first detectors were reaching design sensitivity because the high-energy physicists needed him back to lead the biggest international collaboration that they had ever conceived.

He was told this won’t go forward unless you come lead it—the so-called design of the International Linear Collider. And so he leaves LIGO, but he leaves it in the superb hands of Jay Marks, who we bring down from Berkeley to do that. And then Dave Reitze who comes in from Florida to take over after that.

So we have a series of superb subsequent directors with Barry down to 20% time on the project while he does high-energy physics. But anyway, so there’s all this goes on, but by the time we finally see the gravitational waves, as I say, Ray’s reaction is profound relief and the younger generation is just euphoria.

September 14, 2015: The First Detection

BRIAN GREENE: And so take us through the period September 2015 to February 2016. So you get a signal, I forget the exact date, September 14th or something of 2015 at like 5:21 in the morning. You know the details better than I do.

KIP THORNE: Yeah.

BRIAN GREENE: You’re not convinced immediately that this is a real signal.

KIP THORNE: I have an email from Christian Ott, a young colleague of mine, who says go to such and such an internal website. “We may have a detection.” And the signal has come in. It’s been entirely processed by an automated system untouched by human hands.

And this automated system has laid out what the data are and is basically announced to everybody that this is a likely detection. It seemed too good to be true. And this has come in three or four days before we’re going to do our first serious gravity wave search with the advanced interferometers.

The advanced interferometers have been installed. They have been brought down to a good sensitivity, good enough to do a search. They have a sensitivity that by then is—I’ve forgotten—something like five times better than the initial interferometers ever were. So a substantial improvement beyond the initial interferometers. And they’re just being tuned and the signal comes in.

So Dave Reitze announces our first search has just started. “Freeze the interferometers.” And the signal—but for the initial interferometers, the team had done blind injections in which they went in and they applied electrostatic or magnetic forces to wiggle the mirrors back and forth in both sites in Louisiana and in Washington State in a pattern that corresponds to a particular source, say a binary black hole merger, particular location on the sky. Just mocking it up.

And then you see whether or not the team can find the signal and analyze the data.

BRIAN GREENE: And then you keep everybody on their toes.

KIP THORNE: That’s right. So I said—I responded to Christian, I sent him an email back and I said, “This is obviously a blind injection.” And Christian responds, “No, I’m on the team that does blind injections and we didn’t do it.”

And so then I began to wonder if this is really true. But there is then a big issue of being absolutely sure that this is the real thing.

BRIAN GREENE: Were you afraid of hackers?

Ruling Out Every Possibility

KIP THORNE: Yes. And so that was the big worry. Hackers. When it was pretty clear that this appeared to be quite a clean signal—but the instrument had been designed so that every component in the instrument, you could probe that component, ask what was it doing at that component.

So there are 100,000 data channels you could get out if you wanted because—or 100,000 critical components inside, but also in the environment, much fewer than that, say 1,000 or so that are really interesting. But I mean, when I was told there are 100,000 data channels, I said, “How is that possible?” The answer is every component was built with a data channel.

But that was crucial because you could then go in and query everything. And the very best experts, hands-on experts of this younger generation among the experimenters—I’m not involved in this at this point—but they cannot conceive how to hack the system without leaving fingerprints in a number of different data channels. They just don’t see any way you could do that.

And there are no fingerprints anywhere. And if they can’t do it, then if there’s a hacker that did it, this hacker is super smarter than everybody, right?

BRIAN GREENE: Right, right.

KIP THORNE: And so that’s the point at which we’re really convinced. And the committee that investigates this spends about six weeks doing all this probing, pushing and probing. But then a bit after that, another signal comes in the day after Christmas.

It’s not a big strong signal, but it’s another signal that for anybody who had any questions, that was the clincher. But we don’t announce—the team does not announce that signal until sometime later that year. It’s just an internal thing that gives confidence.

So then we go into mode of preparing for a press conference, an announcement, which ultimately then happens in February.

BRIAN GREENE: And the world takes notice.

KIP THORNE: And the world takes notice.

The Computational Challenge

BRIAN GREENE: Now, one final thing that I want to talk about in this story is there’s a whole computational side to this story where you’ve got to reverse-engineer signals to figure out what source created this particular wiggle in Washington State, Louisiana. My understanding of that is that was also a bit of flying the plane while building it, too.

KIP THORNE: Yes. So let me tell you again from a personal perspective.

BRIAN GREENE: Yeah, please.

The Challenge of Quantum Noise and Computer Simulations

KIP THORNE: So about 1992, I am really thinking very hard. I’ve been thinking through this whole thing. I sit on the sidelines as a theorist who, however, is very close to the experiment. I’ve done some design and some pieces of the apparatus, for example, baffles to control scattered light and beam tubes together with an applicant. So I’m pretty knowledgeable about the experiment, but not a real experimenter.

But I have enough understanding of the astrophysics, of the theory of gravitational waves, and of the experiment that I can think about the whole picture. And I become concerned about two things.

The first thing I’m concerned about is that I am already, by then, thinking it’s likely the first thing we’ll see is two black holes merge. This is not a popular viewpoint because we didn’t have nearly as good a handle on the event rate for binary black hole mergers as we did for neutron star mergers, because neutron stars show up, many of them, as pulsars, and the black holes don’t show up as anything you can observe. So you have to get a handle on this through more indirect means.

But I have this view that the black hole binaries are going to be roughly 10 times heavier than the neutron star binaries, which means you will see them ten times farther away, which means you’ll see them over a volume of the universe that is 10 cubed. So a million times bigger volume of the universe. And it seemed to me just very likely that that factor of a million would outweigh the greater rarity of black hole mergers and neutron star mergers.

And so it seemed to me very likely that the first thing we would see would be binary black holes. It also seemed pretty clear to me that if we were going to really understand the waves when we see waves from a binary black hole, we would have to have computer simulations of the black hole mergers, because the details of the waves from the merger phase could only be understood with numerical relativity simulations. Solve Einstein’s equations on a computer.

So I start pushing very, very hard in 1992, as does Richard Isaacson, who has a similar view at NSF on the computer simulations.

Then there’s a second thing that at about the same time concerns me. Based on all we know about the sources, I have this range from 10 to the minus 20 to 10 to the minus 22 sensitivity for the first detections, 10 to the minus 21, which is where it actually was in the end as best guess. But in the vicinity of 10 to the minus 22. If it’s 2, 10 to the minus 22, maybe even 3, 10 to the minus 22, quantum noise in these gravity wave detectors is going to kill us.

Each mirror, these detectors are designed to monitor the center of mass motion of mirrors. And we spent $100 million or more to guarantee that you see only the motion of the center of mass. So each mirror is like a 40 kilogram particle.

BRIAN GREENE: You’ve got the uncertainty principle.

KIP THORNE: You’ve got the uncertainty principle, not the uncertainty principle for an atom or electron, but for a 40 kilogram particle. And that uncertainty principle says that there will be fluctuations at the level of 10 to the minus 18 meters, a few 10 to the minus 19 meters that will hide the gravity waves. To get down to this level of 10 to the minus 22 sensitivity, you can’t—it’s going to hide the signal.

And so we have to invent and build and make robust technology to circumvent Heisenberg’s uncertainty principle. Quantum technology. Quantum measurement technology. It’s now called quantum precision measurement technology.

It was Vladimir Braginsky in Moscow whom I mentioned before, who first pointed out this was going to be a problem. He points it out in 1968, before Weber sees his first signal. He says, however you build whatever kind of instrument you build, you’re going to have to face this.

And by this time, fortunately, the fundamental idea for how to do this technology that you need has been conceived in 1981, about 11 years earlier by a student of mine.

BRIAN GREENE: This is the quantum non-demolition.

KIP THORNE: This is the quantum non-demolition technique. It’s a technique called squeezing or frequency dependent squeezing. Carlton Caves has conceived this and what’s required. And Jeff Kimble, a superb experimenter down the hall from Carl, has invented the necessary technology by something called degenerate parametric down conversion in a nonlinear crystal. If you want to be fancy, he’s invented it.

By 1992, the ideas are basically there and the issue is to bring it into the LIGO frequency band and make it robust and implement it and you’ve got to do that before it’s needed. And so that’s when I started pushing very hard in LIGO. We need a parallel effort in this quantum non-demolition to get there. Because if nature is unkind, we will have to have this to see the first waves.

At the same time I push very hard that we need the computer simulations, as does Richard Isaacson.

BRIAN GREENE: See, these are two ways that you could fail.

KIP THORNE: Two ways that you could fail. Well, we would see the signal, the gravitational—

BRIAN GREENE: We wouldn’t know what to do with them.

KIP THORNE: Yeah, you wouldn’t know what to do.

BRIAN GREENE: In fact, I’m wondering about that. If you didn’t have the numerical simulations, let’s say everything else were quantum and non-demolition and you see the signal, would you be convinced that it was a gravitational wave signal?

KIP THORNE: I think we would have been convinced and I think we would have been convinced it was black holes merging. We would not have had a good handle at all on the masses of the black holes or the spins of the black holes. But we knew enough about signals that I think we would have had confidence of what it was as a type of source, as binary black holes.

The September 2015 Detection

BRIAN GREENE: And so where are we today then? So this very clean signal that comes in in September 2015.

KIP THORNE: So the community that’s working on computer simulations, those simulations, the foundations for them start being laid in John Wheeler’s group in the 1950s before I come on the scene and they’ve been carried through and they’re not going well in the 1990s.

And so I leave the day to day involvement with LIGO in the early 2000s. I’m no longer needed. I’ve trained several generations of young theorists who can work close to experiment and play the roles I was playing. And so I turned my attention to simulations. I’m not an expert on simulations, but I know what’s needed and I know what science we need to get out.

And so we build a collaboration between Caltech and Cornell, Saul Teukolsky’s group at Cornell. I mentioned Teukolsky before.

BRIAN GREENE: Yes, my first job was at Cornell, so I knew Saul.

KIP THORNE: So we build what’s called the SXS collaboration, Simulating Extreme Spacetimes, with Lee Lindblom at Caltech, who’s on the research faculty playing a key role in getting it started at Caltech. And so we build this collaboration and Saul and I together and also in consultation with others, it becomes clear to us that in order to be confident that we’re going to have the simulations in hand before the gravitational waves come in, we need an intermediate scale effort, an effort of roughly 15 computational physicists at Cornell and roughly 15 at Caltech.

There was only one group of that kind of a scale that had been built, a group of roughly 15 at one institution, that’s in Ed Seidel’s group at the Albert Einstein Institute in Germany. That’s the only one. There’s no way NSF politically can provide the funds for this.

And so again I go to Caltech. Caltech provides the seed funds to get started and then helps me find permanent funding of about a million dollars a year from the Sherman Fairchild Foundation, which then we can combine with funding we have from NSF and NASA and other sources to pull this off. And that’s still what we have. The Fairchild Foundation has been crucial for us to pull it off, the private funds.

And so the SXS team is put together and the goal is to have the first successful simulations. The first successful simulations are by a postdoc in my group at Caltech who however is really working on his own. And he’s a prize postdoc. He has the freedom to do whatever he wants with prize money. And he conceives of a combination of techniques where everybody else had failed and wasn’t able to get mergers to go. He makes the first merger breakthrough, Frans Pretorius.

It was a big enough deal that he wound up as a professor at Princeton very quickly. And Frans was just wonderful. But to get something that was robust, where you could do hundreds of simulations over a period of a few months with different parameters for the masses and the spins of the black holes, this really required this team of 30 people under Saul Teukolsky’s leadership. And that team had that in hand, had what we needed by about 2013. The signal came in in 2015.

BRIAN GREENE: Just under the wire, so to speak.

KIP THORNE: And then the quantum precision measurement, the quantum non-demolition. It turned out it wasn’t needed in the initial detection. It’s needed today. It was needed as of about four years ago or so. The first piece of it was needed quite early in Advanced LIGO, squeezing, but not so-called frequency dependent squeezing.

But the LIGO team had worked hard to bring the technology to the point where it could do the job and had it in place when needed. It just worked beautifully thanks largely to the group of Nergis Mavalvala at MIT and Roman Schnabel in Germany and David McClelland in Australia. So three group collaboration.

The Future of Gravitational Wave Astronomy

BRIAN GREENE: And so where do you think we stand now on the original vision that pushed you forward of really having gravitational wave astronomy?

KIP THORNE: Well, we’re there. Since the beginning, I believed from the beginning that the real payoffs, the huge payoffs will come over the following decades and centuries. And I still believe that.

Now we have this quantum precision measurement in place. It’s responsible for the fact that LIGO is able to see several black hole collisions every week, whereas when we first turned on, we were seeing about one every six weeks. With this, that means we have much lower noise. That means we’re seeing details of the waves that we couldn’t see before.

And that has led us to the domain where the team is really testing the laws of black hole mechanics that were devised by, discovered by Stephen Hawking and colleagues in the 1970s. Detailed tests, for example, at the five sigma level, which is the standard of physicists to say it’s absolutely true, for proof that the surface area of the final black hole is bigger than the sum of the surface areas of the initial black holes. Those tests are being done, have been published within the last few weeks. So it’s really an exciting period, but it’s just the beginning.

BRIAN GREENE: Do you see—one of the big prizes, presumably to finally hear gravitational waves from the beginning, from the Big Bang, is that—

KIP THORNE: That’s, to me, that’s the biggest prize, the biggest prize of all that I have. Because the primordial gravitational waves—well, then, I’m going to ask you a question.

I have this view, the view that there was a Planck era in which space and time came into existence. And the conventional wisdom is that when they came into existence, that the only thing that was present in terms of fluctuations were vacuum fluctuations, the smallest, weakest fluctuation that could possibly exist of everything. Electrons, protons, photons, gravitational waves.

And that those fluctuations sucked energy out of the early inflationary phase of expansion of the universe and created all the matter and radiation we see in the universe today and created them with just the fluctuations of density and temperature that were required to make galaxies. So it’s a beautiful, beautiful picture. Do you believe that picture?

BRIAN GREENE: Well, I believe it as an effective picture in the language that we commonly use. I think it’s a good model for what happened after some kind of primordial stage. But I wouldn’t take it as the guide to that primordial stage. I think it is based upon conventional ideas of space and time, for instance.

And I think, like many of us, have become fairly convinced that space and time are emergent quantities and that there is some more fundamental description. String theory is hinting at it from various directions today, but I don’t think we have it yet.

KIP THORNE: So from a certain point on, I think that is a really good model. But I don’t take it as the truth of what’s really going on. I mean, how about you?

BRIAN GREENE: All I have is hopes. Yes, as I think I mentioned to you, I think this area of understanding quantum gravity, understanding the details of the birth of the universe, this is the most interesting and important area of all of physics and has been for several decades and will be for a few more decades.

And I’ve avoided it like the plague because I want elbow room. And so I then ask you and your colleagues for what really goes on, what you think goes on. My hope is that something along these lines is correct, but that what came off of that early era, because we don’t understand it properly yet, is somewhat different from just vacuum fluctuations.

KIP THORNE: Well, that would be astounding.

KIP THORNE: And that if that turns out to be the case, we will have by roughly the middle of this century, gravitational wave data about this in two widely separated frequency bands from the cosmic polarization of the cosmic microwave background. A polarization, the component polarization produced by the primordial gravitational waves.

Then LISA, we will be seeing the indirect gravitational waves with periods of hundreds of millions of years. And then a follow-on mission to LISA. LISA’s not going to do it itself, but a follow-on mission to LISA. There’s a study for one by Sterile Finney and the team a few years ago called the Big Bang Observer, which is a plausible follow-on direct observation of gravitational waves with periods of seconds to minutes.

You’ve got these hundreds of millions of years, seconds to minutes. You’ve got data in those two frequency bands. If there’s some huge incompatibility between those data or between those data and people’s best theories, I just had this dream that it all blows up in our faces, that there’s a huge mystery to be solved and that somehow these gravity observations, together with the struggles of brilliant theorists like you, that this will lead to a true deep understanding of quantum gravity and the birth of the universe.

BRIAN GREENE: Yeah, no, I share that dream. You know, it’s a tall order, but, you know, we actually had some real data. And if that data pointed toward things that were not quite the vanilla model that we sort of all focus, that would be astounding.

I want to turn it just in the final few minutes to a related area. You described the importance of simulations to being able to interpret data from LIGO, for instance. You have used simulations in another domain, a sort of new chapter of your career where you’ve collaborated with people like Christopher Nolan and made Interstellar. And just tell us a bit about that, because my understanding there is you actually did real simulations that pushed the boundaries of visualization there, right?

Visualizing Black Holes for Interstellar

KIP THORNE: Well, sort of. So my collaboration with Christopher Nolan is just a wonderful collaboration, wonderful experience. He is highly creative man, very deep, very different background from me, has enormous intuition about physics and the universe built up almost entirely by browsing the web. Really?

Yeah, it can be good or bad. Well, in his case it’s good because he understands limitations. But his creativity, you combine him with me, with my knowledge and coming in from almost orthogonal direction in terms of knowledge, it just turned into a fabulous collaboration.

But you asked about one aspect of it, and that is that we agreed that we would base everything involving the visualization of black holes and wormholes on computer simulations based on Einstein general relativity theory. In a universe where wormholes can exist. So there’s aspects of the laws of physics that we don’t understand very well. And so we pick a particular choice for those aspects of the laws of physics in which a very advanced civilization can build a wormhole and provide it to humans. And similarly, well, then you have the physics of black holes.

So the question then is to do computer simulations to visualize what goes on around a black hole. And so he puts me in touch with the team at what was at the time called the Double Negative visual effects company based in London, where Paul Franklin, who is the co-founder of that company, is the head of visual effects for the movie Interstellar.

His young collaborator, Oliver James, is the chief scientist and he has a master’s degree in optics and physics. He’s very deep and he’s a superb physicist in all aspects of optics, ever so much more than I know. And so I provide the equation that we need to do propagation of light, say from the accretion disk, hot gas going around a black hole. Propagation of light around a black hole and down to an IMAX camera.

And it doesn’t work because he analyzes and he figures out the problem is that if you have two adjacent pixels on the camera, these light rays go out from those adjacent pixels almost precisely parallel. They go over the black hole and down to the disk. The tidal forces of gravity, the difference in the strength of gravity along this ray and along that ray is so great because you’re near the horizon of a black hole that these light rays get pried apart. So you land very far apart.

BRIAN GREENE: So one misses the camera.

KIP THORNE: Well, you begin on the camera. There’s a lot of—

BRIAN GREENE: Oh, he’s going backwards.

KIP THORNE: Yeah, yeah, sure.

BRIAN GREENE: Yeah, yeah, sure.

KIP THORNE: Says, we’ve got to have another method. We’ve got to invent a new method. He says, “I have an idea. We want to propagate not light rays, but light beams.” And they’re going to begin as circles. Have some size to them, they have size to them. They begin overlapping. Got it. And then they go over the camera and they get spread, but they still overlap. And they collect their data from the other side, overlapping.

And he says, “But I need the equations for this.” And so I work out the equations in general relativity. It’s much more complicated than just propagating light rays. And I test the equations out on Mathematica. That’s my level of computing. But Mathematica is capable of doing that to get stills, not movies. And I get it. Debug, give it to Amy. He does this in C. And, you know, and it works. It works.

And so he makes the visuals for Interstellar this way. But then we say to each other, “Well, let’s just see what we can learn about gravitational lensing.” And so we take our black hole, now, our mathematical model for the black hole, Gargantua, and we surround it by a star field, and the light comes in from the stars and goes in and is lensed by the black hole.

We see all these wonderful things that I’ve been told about caustics in the past. Light cone, structure of a pixel on the camera, how the caustics lead to multiple images. We watch multiple images be created in pairs out of nothing on the screen. We watch these pair go out and this one annihilates against some other image, and that one goes out and annihilates against some other image. We see these rather remarkable things that I’ve heard about, but for the first time, we have the resolution to really be able to observe some really remarkable things about optics that we’ve not had before.

BRIAN GREENE: And beautiful images.

KIP THORNE: Beautiful images. And so we published a paper together, also with other colleagues from the team at Double Negative, Paul Franklin and von Tunzelman. We publish a paper in Classical and Quantum Gravity describing the details of our method and then applications both to the movie and to just Starfield. That paper, by a huge margin, is the most downloaded paper in the history of this journal.

The Reality of Wormholes

BRIAN GREENE: So, Kip, in Interstellar and also in various other science fiction stories, wormholes play a key role. Where do you stand? I mean, do you think they’re real? Are they a bonafide solution of Einstein’s equations. Where do you think about them?

KIP THORNE: I think the first thing that I have to say is if they’re going to be real, then they have to be compatible both with Einstein’s classical general relativity equations and the laws of quantum physics. And wormholes certainly are compatible with Einstein’s equations. They arise naturally from Einstein’s equations.

But there are reasons to suspect that quantum physics then, combined with general relativity, prevent them from existing, or at least existing naturally in our universe. So my best guess is that there are wormholes in a quantum foam at the Planck scale at very small scales. There are fluctuations in the topology of space at very small scales. John Wheeler postulated this as quantum foam. But that you cannot have macroscopic wormholes that live long enough for people to—

BRIAN GREENE: Travel through, even with some kind of exotic matter.

KIP THORNE: Well, that’s my best guess. However, I can’t prove it. And we know a lot more about this question than we did when I first started thinking about it, triggered by Carl Sagan back in the mid-1980s.

We do know that in order to hold a wormhole open, you have to have something that repels gravitationally, basically push the walls of the wormhole apart. I call it that exotic matter. We know that with pretty high confidence that you can make exotic matter through by rearranging vacuum fluctuations.

And we have a very simple example that I think is very compelling. If you take two electrical plates, I’m sorry, two conducting plates, let’s make them superconductors for simplicity. And I put handles on them, and so I’m holding them apart, that as they get closer and closer together, the electric field parallel to the plates basically gets annihilated because current flows in the plates to annihilate it.

And so you have reduced fluctuations. The component of the electric field is parallel to the plates. And the closer the plates get together, the more reduction of fluctuations there is. And there’s a resulting attractive force between the plates. And this attractive force has been measured. It’s called the Casimir force. This is the Casimir vacuum between the plates. And you could feel this with your hands. And the plates are being pulled together, and they’re doing work on your hands. Your hands are extracting energy from the vacuum between the plates.

BRIAN GREENE: It’s a purely quantum effect.

KIP THORNE: A purely quantum effect. But you’re actually getting work out, and you’re getting it out of the vacuum. And so if the vacuum began with zero energy, energy in the sense of what produces gravity, then it has negative energy, and that you have exotic matter in there that repels gravitationally.

I think it’s pretty convincing. I don’t know that it’s 100% sure, but pretty convincing that thereby you get exotic matter between the plates. The question is, can you make enough exotic matter and put it inside a wormhole to hold the wormhole open? Because without it, the wormhole will implode so fast that nothing can travel through.

And that’s where people have done a number of calculations trying to answer that question without a definitive answer. My impression is that the odds are against holding a wormhole open. But that’s just the odds. And as I like to say, I’ve been proved wrong often enough in areas where I think I know the answer that you shouldn’t take my pronouncement seriously.

Time Travel and Information Loss

BRIAN GREENE: Well, one final gut check though. Imagine that you were able to keep the walls of a wormhole open using Casimir energy or some kind of like-minded exotic matter. Do you think we’d ever be able to use these for time travel by towing the opening to a black hole and having a time warp set in between it and the other opening?

KIP THORNE: I don’t know. But this intimately ties into a related issue, that is whether or not time travel is possible at all on either macroscopic or microscopic scales. And so there is very interesting calculations done by two different groups.

One is Jim Hartle and Murray Gell-Mann have developed a variant of Feynman so-called path integral approach to quantum mechanics that is capable of dealing with regimes that normal quantum mechanics cannot deal with. And in normal regimes this path integral approach is absolutely equivalent to the standard approach of quantum mechanics. But if you have time travel then ordinary quantum mechanics can’t deal with it. Feynman approach, Gell-Mann Hartle approach can.

And so Jim Hartle did an analysis, similar analysis with a different but still Feynman-based approach done by John Friedman and colleagues at the University of Wisconsin at Milwaukee. In both these calculations they imagine that you have some region. If I run time up in space horizontally, you have some region where there’s backward time travel. There’s no backward time travel allowed down here, there’s none up there. But there’s a compact region in space-time where it’s allowed.

Using the Feynman approach, they could evolve quantum fields from initial state down here to a final state up there. They compute probabilities, probabilities are preserved. There’s no problem whatsoever. But information is lost. Or more precisely, the evolution from here to there is non-unitary, non-unitary. So you can’t go back, so you can’t go back.

And this is relevant because there is the mystery, the paradox, the question of whether information is lost. If a black hole is created through implosion of some matter and then the black hole evaporates, is information lost? And Stephen Hawking insisted at one time that information gets lost. And we even had a bet where he and I bet that can and does get lost. And John Preskill, our colleague at Caltech, bet it doesn’t get lost.

And Stephen threw in the towel. I have not thrown in the towel. I’ve not thrown in the towel. I still think it’s an open question because I think there’s a real possibility that backward time travel is allowed, is required on microscopic scales. At least that when you do a path integral approach to quantum gravity, a sum of our histories, that you have to have finite probability amplitudes for backward time travel on microscopic scales.

And that’s what eats the information, that’s what prevents the unitary evolution. And this is just saying that the true approach in the real universe to quantum gravity is a path integral approach and that the standard approach is just not broad-minded enough to deal with these bizarre kinds of situations. And I think that’s a very, very important open question.

BRIAN GREENE: So you think Hawking bought Preskill the encyclopedia of baseball too soon?

Time Travel and Self-Consistency

KIP THORNE: I think I do think that. Interesting, but on the macroscopic scale I think it is also an interesting question. Can the laws of physics accommodate themselves to backward time travel? That is the kind of question that I spent some fair amount of effort trying to understand.

And it appears to me the answer is probably yes. They can accommodate some sort of self-consistent. You can build self-consistent solutions. Always though there will be some circumstances where these self-consistent solutions cannot have a quasi-classical approximation. Some situations where you give initial data and the probability gets all spread out all over the place because you can’t have classical self-consistency.

BRIAN GREENE: So it gets sort of hidden in the quantum diffusion. But you can’t go back and kill your grandparents.

KIP THORNE: Yes, right, excellent. So anyway, that’s my view. But what happened was I was really digging deep, as deep as I could on these kinds of issues in the 1980s and the early 1990s. And then LIGO got funded off and running. I shut down all that research in my own research group. We had some non-trivial amount of research in that to focus on LIGO and I’ve never turned back. Right.

BRIAN GREENE: Well, maybe that’s the fourth act.

KIP THORNE: Maybe.

Collaborating with Artists and Inspiring the Public

BRIAN GREENE: Unfortunately we’re running a bit out of time. I could keep on going for another few hours, but I just want to close with one question. So you are now focused more on trying to create experiences for people. You’ve been involved in live events on Zimmer and visuals and stars and music and so forth, Interstellar and other projects of that sort. What do you hope to bring? What do you hope the experience of the public would be? And why is it important to you that they had that experience?

KIP THORNE: Well, let me say my motivations are several fold. One is I was a conventional professor for almost 50 years. Been there, done that. I want to do other things that are really interesting and are fun. So one of my motivations is have fun. That was also my motivation as a conventional professor too. And I had fun.

But I also see this as a means to communicate and inspire people about science through these collaborations between scientists and artists. And I think I was particularly inspired by my collaboration with Christopher Nolan. We talked. So we had in-depth discussion of his movie Tenet before he ever started any filming, when he was just beginning to conceive it.

What happens if you have a person whose direction of flow of time is opposite the rest of the universe? Entropy inside me increases in the wrong direction.

BRIAN GREENE: From my perspective.

KIP THORNE: What happens at the interface between those. If you use as a physicist, if I try to do statistical physics in that kind of universe, that’s just an intellectually fascinating question. But it also led me to give him some advice for that movie. But I would never have asked that question. But he did and it was just so stimulating.

So I get enormous amount of joy out of collaborating with people like that. With Leah Halloran, the painter that I have done this book, that book, “The Warp Side of Our Universe.” Her painting is my verse. And conceiving this new genre of tightly integrated paintings and verse as a mechanism to try to convey the essence of some pieces of science, in this case the warp side of our universe, to a different kind of an audience than I’ve ever tried to reach before. Really enjoyable. And I didn’t know that we have some success in inspiring people about science at the same time.

So I just at a phase in life where I’m getting a lot of joy out of that kind of collaboration.

Bringing Science into the Cultural Center

BRIAN GREENE: Well, look, it is a very rich realization when people can see science not just as something that scientists in laboratories or universities do, but it’s something that seeks truth in a pathway that’s true, resonate with a pathway that artists and poets and writers can follow. So it’s a wonderful way of really bringing science into the cultural center, which is vital.

So, Kip Thorne, thank you so much for this conversation.

KIP THORNE: Wonderful, Brian. Thank you.

Related Posts

• Neil deGrasse Tyson on UFO Files, Trump & Alien Existence (Transcript)
• Professor John Lennox: AI Is Humanity’s Attempt to Make God (Transcript)
• What’s Changing, What Kids Must Learn w/ Sinead Bovell @ SXSW (Transcript)
• How Emotions Are Made: The Secret Life of the Brain – Dr Lisa Feldman Barrett (Transcript)
• The AI-Generated Intimacy Crisis – Bryony Cole (Transcript)