Interview with Dieter Brill

0:00 I'll try writing them, too, as we go. So, as I said, I was interested in the development of the shortwave expansion. Yeah. So, yeah, specifically on the shortwave expansion, that was... I'm trying to think. There was, of course, the Geon paper that Wheeler had written, I think just before I got to know him or be a student. and I don't remember whether he would talk about the gravitational geon. I would imagine, I mean it would have been a natural thing for him to talk about gravitational geons because at that time he wanted to consider classical field theory and see whether it can, in some sense, explain all of physics, as it used to say. You know, X without X, where X was anything in positive physics. And so, many of us always thought of the Geon as a model of some kind of a body. Now, there are all sorts of disclaimers in the Geon paper, you know, that certainly it isn't meant to be an elementary party was just meant to be an example of how you can make something yeah but anyway he needed to use electromagnetism and I suppose it would have probably been considered even nicer back then if he could have done it all just out of gravity you know this idea was always just space-time is not the arena space-time is everything and so I there was a talk about that sort of thing, saying, well, just like one can do it with electromagnetism, there ought to be gravitational geons. So I think there was, at least in the back of people's mind, that thing as a kind of a challenge. And of course, we started working on it because Jim Harlow was a senior and he wanted to do his senior thesis. I don't

2:30 know how I got into the act, but anyway I didn't suggest that. I suppose it wasn't because I thought deeply about what is really the next step that we need to take in physics, but it was something that was kind of in the air yeah so but we were very much after the geon we really didn't care so much about fundamental questions about gravitation waves or something right it was just more as catch as catch can how can we make this thing way that we can we could somehow construct something that would hold itself together yeah some gravitation configuration that would stay together. So the gravitational radiation was just so that it would be purely gravitational? Yeah, I think, I think Wheeler had always talked about, you know, how versatile the gravitational field is, that space curvature here on a large scale means attraction of some large mass, and rippled on a small scale, it corresponds to waves propagating. Words like that I'm sure were floating around. So this idea of small scale ripples was around. And then I guess somehow when I had the idea how to actually do it, you know, sort of first order ripples and then the second order background influence, but at that time we recognized that it was sort of a general scheme by which one could maybe treat these circles in general, but the main interest was in constructing this gravitational genome, and at least I didn't see it as a big industry that could discuss all sorts of vehicles, and that was Isaacson's couple of two years. So you really weren't affected in any way by concerns of gravitational radiation per

5:00 You weren't? Well for the Geon construction that was sort of incidental I would say. Well, I guess now if you'd asked us, can you make a geon without gravitational waves? I don't know. Well, we might have said, well, maybe you can somehow knot up space in some tiny wave that isn't really small ripples. I don't know whether you call that gravitational waves or not. We would have maybe, I think in the spirit of the times, we would have said, yeah, anything that has a total mass and doesn't have any other source, particularly sourceless gravitational fields that have a total mass, that is gravitational waves. That's how we define it. But if you ask us just where is this gravitational wave, we couldn't put our finger where it That was good enough. The spirit at that time was very much, you sort of think globally, you don't have to pinpoint everything to a place. And I guess there was really quite a contrast between the Willerian group, to whom it was that, you know, you could have gravitational fields that were asymptotically force-fieldered and they had a total mass, and there was no question about that. If asymptotically that's how it looks and you could put a body in orbit around that thing, then yes, of course it has a mass. It was always a little puzzling when you would get to these conferences where the other school of thought would come and talk about pseudotensors and whether you can transform them away and this and that in co-ordinate ways and so on, and to me anyway, I think that always sounded like much too local in thinking, so I think at that time, this more global view was maybe something rather new, even to Ludo's school was maybe propagating without explicitly knowing it very much. I mean, he never, I don't think we ever particularly, you know, beat the drum for this, for global differential geometry or anything like that. And that came

7:30 into people's consciences, I think, with Hawking's work. I think it was at Hawking's theorem. But, in fact, I think that the spirit of it probably was there, more in Buda's groups and in some of these others. Yeah, that seems so. And so you were saying that at conferences you would sort of encounter this more sort local outlook but that that was more yeah i mean certainly people thought it very important to uh to have a local description of uh of gravitational energy they were always proposing yet another pseudo-tensor. They somehow wanted it to have a sufficient number of reasonable properties without having to assume too much. I guess Müller was one that was going at this for quite a number of years, going at conferences, he would always speak about it. And I guess he would introduce some additional structure, maybe some frame field, I forget what it was, but you know then you could argue whether that was reasonable or not or so on. But those questions, to me, and probably Gubart, really never seemed to sort of hit the essence of general relativity. Come to think of it, that's kind of interesting, because I guess at the same time Hitler was propagating this business of, yeah, the whole thing was embedded in a formalism where we did slice up space-time into space-like surfaces, which in a way are extraneous to just pure four-dimensional geometry that Einstein told us to think about. So, alright, I don't really understand it. This was written back how we could say, well, that's a natural thing, whereas

10:00 these other constructs that people were putting in space-time, in order to allow them to have of the local energy that those are somehow not natural. I don't know. I've never thought about that, but now thinking back of it, that seems like a bit of a puzzle. Probably the whole buzz of those views at once. I suppose, of course, that within a given group of people, everyone is forced to develop one's own constructs. Yeah, I'm, at least, I myself, I'm always astonished afterwards how much the sort of the general ideas sort of carry along particularly when we were a student you know the things that are current then you just sort of accept yes of course that's the way it is without that they were always a few guys that said oh no we have to go back to the foundations and basis for this, how do you know such and such has to be so. But those were, I don't think the ones that really paid attention to it too much. Some people that were working with Willow who did that and who never did get their degree at Princeton. And I think because they were always trying to go back to the foundations rather than actually getting on with the work and doing something new. You know, it's just building the foundations for something. Well, you know, we've heard this term daring conservatism, and there's a lot of daring in this. You know, felt by everybody, and let's just go ahead and do it. There certainly seems to have been a sort of a tension in general relativity between

12:30 the desire on the one hand to push to push the thing forward, especially into areas where they're not physically out of it and then the desire on the other hand of people to say well, let's see if the fundamentals of the the fundamentals of what we're doing or why we're doing it and so you had this sort of impression maybe, at least within Wheeler's group that there was a problem that people looking at the fundamentals were sort of not getting anywhere well some people weren't getting anywhere let me see I remember discussions where, you know, they were asking what really is a field and what distinguishes a gauge, a dependent quantity from one that isn't and so on, how odd nature would be described. been, I suppose if they'd done it, if they'd asked the right questions, they could have invented gauge theory at that point. I forget when gauge theory was, I mean it was invented around that time, but we didn't know about it. But instead, well it was also sort of mixed in with quantum mechanics. And, you know, if you're interested in the foundations that's certainly a place where you can argue about foundations a lot. So, maybe that held them up more than anything in the classical foundations. That they would say, well, maybe and try to understand classical space in a very deep point of view, because really it's all quantum, and so we've got to understand the quantum first, and then they would start with the basic, you know, the sort of Einstein-Bohr arguments about basic interference experiments

15:00 and so on. And, well, that's a long way from some calculation of something in general relativity, so, you But on the other hand, of course, there was Dickey's group who were also thinking about the foundations, but in the sense of what kind of experiments can we do that test the foundations of general relativity. And those who were all... I mean, I was very interested in that. I would very regularly go to Dickey's, he had sort of informal groups, mainly experimentalists, but Dickey was experimentalists and theorists at the same time, so it was a mixture of things, and that was almost getting us down to earth again, although you were thinking about very basic things, properties of space-time, you were doing it in terms that you could actually make experiments. So, those things I consider very good and useful. So, isn't it? You know, all the basics would just be brushed aside. Right, sure. Just as a side-by I was going to ask, how big was Wheeler's group at that time? Well, let's see, the year that I graduated there were three others that got the degree from the Wheeler, and I don't know whether it was one or two years before that Charmin Charlie Misner got his degree, and I don't know how many others may have been. There was Arthur Comer at around that time, but I can't say who else there was, but it always seemed like a big group, not so much because, you know, we'd get together and find that it's a real life, Carl can't fit in a room or something, but because it was kind of hard to to get a hold of Wheeler. He seemed to be busy with a lot of students and a lot of other projects as well. But just the fact that four of his students graduated in one year must

17:30 mean something. Let's see, I don't know who it was. It was myself, Clotter, John Fletcher, and Bobby Wima. Well, there was Richard Lindquist around that time, too. I don't know when he got his degree, but it was in the year of that. Who else? That Reviews in Modern Physics of 57, I think it was a report of the Chapel Hill Conference, which was in a way the first General Relativity Conference, the zeroth one or something like that. that had just about all of the Wheeler students in there, so you can judge pretty well how many there were. Touching on the Chapel Hill conference, at that conference, looking at the transcripts, there was some discussion about the question of whether gravitational waves count energy. Well, Felix Prang, I guess, gave a talk about his work at the time was sort of introducing the equation of g-music deviation and the description of the Rheumann tensor and the ways propagating. But there's a record of a discussion Feynman, Bombi, and Gold, I think, about the question of whether the waves carried energy, and you can see them sort of discussing versions of the thought experiment that Bombi later published, where you say one version of it is a little stick with some rings on it, and if a wave passes by, it rubs the rings up and down and so in other words

20:00 judging by the transcripts anyway to some extent this question of whether gravitational waves carry any energy or say just some sort of coordinate effect was in the air but do you remember it as being much of a general discussion or a transcript that could easily be just that this was the only time it came up or well it wasn't one of the big It's funny, I mean, in retrospect, that I, anyway, or other people would carry away from it, you know, the sort of thing that you would discuss for hours or days after the conference. Yeah. It was more like something that everybody should have always known. I guess we, I remember at some point, sort of going through the calculation, repeating maybe what Bondi said, that you write down this gravitational wave and I guess in one gauge the particles don't move in the coordinates. They sort of stay at fixed coordinate values, but then you look at the proper distance that would be like this stick, and you find that the proper distance does vary, I mean the proper distance from between two or two of these particles does vary. So for me that was an interesting thing to teach me about that coordinates really don't meanly thing, that they can totally fool you about the motion of things that you have to look at invariance and so on. But it seemed to me like something that everybody should have, that I should have known already should have paid attention. Not like some big revelation that I didn't even realize that I may have gotten it at Chapel Hill. The thing that I remember in Chapel Hill, I think, is Feynman's, is something that Feynman said, namely, I guess it was the question, maybe it was probably the same thing whether gravitational field should be quantized and he gave an argument that at least it

22:30 has to be described by probability amplitudes because you can you can amplify some you know atomic events and decay of Adam yes amplification that change the gravitational field and he was saying something like, look, this is what happens all the time. Some experimenter makes an experiment on some elementary particle and he finds something and he writes it up and he sends it to the physical review and it gets printed and it gets sent out and some guy opens the journal and he reads it and he jumps and moves the gravitational field. That's what Bachmann said. you know, that was something I remember much more right yeah, I see that in the transcripts and I think is that in the transcripts? I was always wondering whether somebody wrote that down what he said there because it was so funny the way he said that I can't remember if the precise there is this discussion of course talking about amplifying a lot of the text from the field. And in fact, it's sort of out of that, at least in the transcripts, that the discussion with himself and Blondie about the current vision of Asian films. But I can't remember if the part about the physical review. I remember him always said, you know, acting like a printing press, you know, where amplification had gotten to something really classical and then he had the sun guard jumping. So anyway, I guess that sort of comes on from that you were saying that really in the work within Wheeler's group, you weren't particularly motivated by any of these doubts about the pseudotension and that sort of thing that was concerning other people. Yeah, right, right. Mainly, I was motivated with my thesis because we just had proof to me

25:00 that these waves have positive energy. There was no question that they had energy, but the problem was to show that the energy is positive, you can't have negative energy states. Because I guess it was clear to us that if you have negative energy states then by scaling you could make them arbitrarily negative and that would be disastrous. The vacuum which presumably has zero energy could decay into something gravitational with negative energy then it would be a bad thing. So that was the big motivation for one thing the physical problem the other thing because we doesn't prove this so so you were concerned it that for your own purposes we don't was concerned with the question of defining energy in the way but not necessarily with this question of whether it's only for me right yeah I think everybody believes that one was not just a coordinate effect, but then, sort of because it's not just a coordinate effect, then this question became very important to show that it can't be negative. So, now here I'm not sure, this question of whether the wave has negative energy, is this, this is different, I guess, from the question of the direction of propagation of the energy, because when around a similar time, around the childhood conference, for instance, when people like Havas and Goldberg were starting to do a radiation reaction in fast motion consummations, they came up with these results after, well, not unexpectedly, came up with these results, where they would discover an antigone effect, in which the waves seemed to be essentially giving energy. Yeah, that's right, I remember these things. And he was saying that one of his papers, for instance, he said that, well, this is, strictly speaking, possible from a mathematical point of view, if you say that, well, the waves are probably getting energy down from infinity into the system, But this is something different from the question you were talking about, where you were saying that the actual wave itself is negative energy, meaning essentially that it's a kind of anti-energy.

27:30 Yeah, so, let's see, so what it would mean would be that you could have, well we were very much thinking always in terms of just pure gravity without sources, but I mean if If you had sources, you would say, well, you could have sources that would emit gravitational, positive gravitational energy and themselves pick up negative gravitational energy, and this could go on forever if they were no bound. Of course, in a way it's true, right? I mean, you can have a source, some sort of a star orbiting, a pair of stars orbiting, and then it emits gravitational waves, and itself gets more, and then the gravitational energy contribution to that system becomes, the change becomes negative, because from an atoning point of view, the attractive force means the negative energy. But, you know, for finite-sized bodies, that can't go on forever. Sure. And now if you do it with, in the spirit of times, you said, well, these bodies maybe are really geons. Well, you don't know what happens when geons merge or something like that. At that time, of course, we didn't know about black holes and horizons. I mean, nowadays we know that what happens is when these things get too close together then a horizon forms around them and then that's, you know, just becomes a black hole. But at that time, you know, it wasn't clear that if you somehow get these clumps of waves very close to each other, that then these attractive forces that must exist between in them is because gravity is attractive. It wouldn't mean that they pick up a lot of negative energy as they get closer and closer and more compact, and that it would overwhelm

30:00 the sort of positive energy that each clump has individually. And if that were possible then you could do this thing. I mean, the gravitational waves classically at least don't have any limit on how much you can compact them. And so then you could get more and more energy out of them and make the thing more and more compact. So the question that you saw... So we want to show that in fact that it's impossible and in fact that this negative interaction energy overwhelm the sort of positive intrinsic energy of each clump. Except we realized of course we can't talk that way that's only some kind of a picture because we can't separate the clumps from the interaction. Okay so it was sort of looking at the question of the interaction of the geoms. Yeah in a sense that the interaction energy Yeah, if you look at some of the early writings of Dr. Misner, you'll find some of that discussed, I think. More or less in those terms, what happens to the very concentrated matter. Somehow we're using this principle that the gravitational energy is the source for more gravity. it makes it reasonable that it shouldn't ever become negative but as you were saying so at what point it's always good for me to be sure at what point then you were saying And people didn't understand that the question of the era of, say, have a kind of horizon about if the gender shouldn't...

32:30 Right. Well, that also came up around... Well, let's see. Horizons were a little later, I guess, but even the Kruskal solution, the Kruskal diagram extension of the metric. Let's see, when was that? It must have been around 1958 or 59. I mean, just around the day when I was getting my PhD. And that was just a Schwarzschild solution. I don't think people have the idea that something like this horizon that Kruskal had shown exists and choice for the space-time, that that was a general thing that you could expect to develop. That was happening about the same time. Yeah. Well, I just wanted to get back for a minute, you were saying that about the, the, the two wave of the two-land scale expansion that essentially it more or less developed out of this sort of feeling about the two-land scales involved at the small and the large-scale background effect. And certainly this kind of expansion parameter went out to play a big role in radiation theory later on, but you were saying that at the time you were pretty much just concerned with the problem of the gravitational genome. Right. Of course, the electromagnetic gene sort of taught us that it's appropriate to think of of two wavelengths scale right I mean you you have the electromagnetic waves and of course there you also average them yeah and then the gravitational field just sort of response to that average stress energy I mean that that's a good good enough approximation because it varies so rapidly you know the details of where the energy is

35:00 geom sphere but the large-scale field just just notices the average of that so that was already contained in the electromagnetic geom in the way we just had to imitate that Yeah, it was a very natural thing and it wasn't somehow that we had to think very hard that there ought to be two scales involved in the steering, but I wasn't the phrase in that case. and so really in the case of Yeah, the general theory of the waves. Right, I really didn't follow that too much more. But if you look at the detailed assumptions about the scale on which various things happen, it isn't quite the same for the Gion problem as it is in Isaacson's treatment. I guess Isaacson was more interested in some sort of a gravitational wave group that was however still propagating, not really being held together, but it potentially is so strong that it bends them all the way around to come back, and, you know, in the Geon the balance is really rather delicate between, it isn't just a question of the wavelength of the waves

37:30 versus the scale of the background but there's also the angular momentum parameter involved now in a sense that measures the there's the frequency of the waves and then there's the you know, the L in the spherical harmonic, and both of those have somehow to be large, so we take a limit of large values for those things, but if you... but I think they don't just bury together, there's some funny powers. I remember there were lots of funny powers that ended into there. You know, that maybe the frequency went up like the two-thirds power of the L in the spherical harmonic, or some crazy thing like that. And then there was the scale in the background. Well, there are several scales there. There's a large scale, just a Schwarzschilding behavior, and then there's this thing that they call the active region, where the metric coefficients change rather rapidly. and again the size of the active region and things like that scale with some other funny power of the spherical harmonic order say so it was a it was kind of a doubt but it's the same thing as for the other community you know it's just a kind of a specialized thing because you want to construct this thing to host together uh and in general you would you wouldn't do it that way and I think the way Isaacson described his circles it was it was different in that respect that you just forget exactly how his parameters scale but I remember that it isn't the same. Both of them are valid in the sort of situations where they are valid, but the june is very special because you have to construct this trapping region so that the waves are trapped in kind of a very narrow potential well. And I think Isaacson's description is

40:00 more appropriate to a situation where you don't have this very narrow well it's more spread out and distribution of gravitational wave energy So, to go on with the, or to touch briefly on sort of the layers yet, although I've seen papers by people, for instance, still worrying about the, um, the student, Einstein's student tensor and energy in the way, I think for the most part, following works such as what we're discussing, most, certainly I think it's fair to say most people were happy about at least the existence of gravitational radiation. But then there was a lot of attention, focus on the Quadruple Forum and questions of the radiation, whether the radiation reaction problem specifically was under control, especially during the 70s and into the 80s, I guess. Do you remember much about the sort of the debate surrounding that, or was it anything that you were involved in? No, I didn't really contribute much to any of that. I mean, the stuff that's going on nowadays that is sort of a logical continuation of what I was doing then are these attempts to define quasi-local energy, which you've probably heard about. But that's really, again, rather different from the question of prodigal formula. Yeah, that's good. And... No, I remember there was somebody who came and told us about postnatal... Oh, it was, who's the guy from Poland, who was working on such things.

42:30 I think he's in Mexico now. Yeah, right. I think he came and gave us a talk about it. about the Einstein and have been from half enough it and so on it none of us had ever looked into that and it was well it was sort of good to understand better how it's done but it also I don't I don't think it led to any work at Princeton, using that. What was that? Was that when he was still in Poland? Oh, because he came here for a year or so. Yeah, I think he came from Pound to Princeton. And it was around 1960. I might be able to look it up. I have notebooks from way back where I wrote down most of the talks that we went to. But they're not very well indexed. I suppose actually those notebooks are of some historical interest. I mean, occasionally I look up things for people, like, you know, they will wonder how many Stevens meetings have there, you've heard about the Stevens meetings, right, how many have there been, or how many were there in a certain year, or did so-and-so talk, or the Stevens meeting, well, I can often look through my notebooks and find out those things. Yeah. So, you know, if you had that specific question where important I could probably find out. Yeah. No, I, but I hope to talk to the basket sometime, maybe if I get a chance, and I'm sure I could look up certain details of the time, so... Marcel Ries was another one that visited. I don't think he was doing any work in that sort of thing. But for some reason he was visiting me through this group. Several times I think.

45:00 And he was doing some very fancy mathematics that I don't think I understood very well. The other crazy thing is that, of course, at that Chapel Hill conference, I talked about this paper with Willa on neutrinos and gravitation fields, so we had to think about how to treat spinning particles in the interaction of gravity. And then the very next thing that I was supposed to do for my thesis was this business of the positive mass, gravitational radiation. Well, if I'd only had the insight of Witten, I could have put the things together because, of course, The spinners are the ideal way to get a handle on the positive mass question, as Witten showed. But, you know, I had all the necessary tools, but I just didn't put them together. That's one of those humbling friends to the backhand.