Interview with Herman Bondi (contd.)

0:00 From the beginning thought it was alright. I found it less persuasive than when a doubter comes round, right? Like Alas. Right. It was interesting, of course, as you say, when the death becomes satisfying. So, I guess a certain amount of the discussion that took place around that subject in the 70s concerned that the fact that some people thought that the problem was solved or others didn't. And so, um, for instance, I guess that Ehlers was concerned to point out that there were issues that were outstanding. So that, um, so that you also felt concerned that some people were, uh, unaccountably for your going to be inclined to. Now, of course, about my involvement, I don't know how well you know my career, but from the early 60s, of course, I would cease to be a proper academic. I mean, I became full-time something else only in 67, but my time was increasingly taken up by other work. Well, so that, in a sense, took you away from the direct participant. Correct. This is something that you may not have any particular memory of, but I'm interested in some of the people at that time who were, speaking of the 50s, the early 60s, the people to express doubts about Christian gravitation and you also mention in the letter to nature there work by McVitie and you also, but specifically you also mention Scheidegger who I think was a student. I can't remember him, but McVitie I've known of course for many years.

2:30 He had produced a paper concerning the existence of gravitational planes or in the existence of gravitational planes. So were there other people that you remember from that time who were inclined to be skeptics on the question of the existence of? I think Infeld was the chief skeptic. There was, of course, that other school of Fock, which looked at the relativity only through the looking glass of the Donder condition. And, now, they felt that the wave problem was well defined already, but they didn't actually have a solution for it at the time, I can't remember. Do you remember that Fock gave three co-made lectures in Fisrael, there in Fisrael in the late 50s, I think it was. The first half of the first, I think, is absolutely brilliant. and then he thinks what to me is the morass of the Deuteronomy condition I mean my outlook on general relativity I don't know whether you know my sort of popular writings on this I mean I always derive it directly from Newtonian theory shall I talk about that for a moment? is suppose Newton lived in an age as skeptical as ours of anything that is not directly measurable. What would he regard as the observable of the gravitational field? Only something that is observable in any and all circumstances, which is the relative acceleration of neighboring particles, the tide-raising force. And so the simple Newtonian equation would have been the tensor equation.

5:00 Delta Fi is Aij delta Xj, the relative acceleration vector delta Fi being linearly dependent on the relative position vector delta Xj through a tensor, three-dimensional tensor, of course. You then, by the usual trick of looking at rotations, show that this tensor must be symmetrical, because otherwise you spin up things in the computation field. And you then say, that's all very good for Newton's day, but if I assume, as Newton would have done, that Aij is velocity-independent. It would be incompatible with special relativity because it could accelerate particles through the speed of certain light. So Aij must be velocity-dependent. One then plays about with a little, and lo and behold, you get delta Fis, Rij,kl, delta Xj, Vk, Vl, which is the equation of Q-DC graded deviation and that is the simplest generalization of Newtonian theory compatible with special relativity and you come to directly to the measurable curvature tensor through the it's measurable through the equation of Q-DC deviation it's an observer and then And then you have your theory. I mean, you don't have any, make any new assumptions. Newtonian theory plus special relativity gives you a curved metric. I'm interested in your emphasis on the GDC deviation in that context, because I'm wondering also if it influenced your early work on gravitational waves, where in both your work and Pirani's... Well, it was Pirani who taught me GDC variation. So he had been an influence in that? Oh, very much so. But the importance of Judaism, I know from you.

7:30 And so I did that, so to return then, for instance, to the, well, to things like the idea of the particles on the stick. That, I think, was very much my way of thinking. When people doubted, is there any energy in there? That if you have relative acceleration, there is energy. That's very much my engineering sense. Sure. Oh, yes, I think so. But does that derive from thinking about the equation of GD6 deviation? Yes, but the GD6 deviation is a real thing. It doesn't really give you use the relative acceleration. That's quite right. And so if, for instance, whether we're ignoring the question And as it was a subject of debate at the time, the question of the energy, pseudo-energy momentum, pseudo-tensor, the energy momentum, pseudo-tensor. Yes, pseudo-tensors have never had anything to talk with. So ignoring that, but dealing with the fact that the rheumatic, the rheumatic tensor is non-zero in the way of the machine. You have an effect due to the equation using them within our system. The pseudo-tensors are a lot of balls, to express myself clearly, and I've written this in some places. I mean, why is the energy momentum tensor only co-verically conserved? Why isn't it conserved? with it via green steel. And the answer, of course, is that in physics it is in itself. The energy momentum tensor does not contain gravitation. In the hydroelectric power station, you convert gravitational energy into energy of flowing water into electricity. So the energy of the non-gravitational energy by itself is not conserved. In Newtonian theory, you get over that by introducing the fiction of potential energy.

10:00 But potential energy is unacceptable in a relativistic theory, because potential energy has no location. And since all energy has mass, you cannot, in a relativistic theory, have an energy that is not localized. Right. And so the non-conservation of the energy-momentum tensor may be an inconvenient fact, but it's a fact, without which I was saying we couldn't have a hydroelectric power station. and then to try to get this right by the energy moment by the pseudo tensor is totally pseudo precisely because it suggests that there's energy which in some sense is not localized which is an absurdity and I never And it is only the people who hanker after the analogies with electromagnetic theory who are constantly invoking the pseudotensor. And frankly, it's almost died out now. It's never been any used to man or beast. Yes, it doesn't. It hasn't paid any dividends at all. Yes, it's not much involved. Um, the, uh, someone I haven't spoken to yet, but, uh, what I may do is, uh, So, you mentioned Pirani's contribution, his work on the Judaistic Deviation, and Ben

12:30 Robinson had done some work, I guess. Oh, yes. In one sense, he had anticipated what we jointly did. But since nobody, least of our feelings, or I could understand a word of what we're saying. But in one sense, he was the first. but communicating in such a form that it didn't ring a bell in either of our minds I mean you could say that our joint paper was an effort to bring Robinson's discovery down to earth but he's very brilliant but at least in those days he wasn't very good at making himself clear I see. But he had, so as you say, in a sense you were trying to make more concrete what you'd be working on. Well, make it clear ourselves. And I think our paper is really quite a clear paper. where with any train wave you have to ask what happens a long way off I'm not sure that he published any work on it early, he gave some talks and as far as I understand, nobody but nobody could understand Although certainly he's someone that I also want to get in touch with, I was interested because you mentioned Havash as being someone at that time who was involved, and he certainly played a prominent role with Ehlers later on in the radiation reaction controversy pointing out the problems that existed. What was his role at that time, say, in the 50s or in the early 60s? If I may express it very rudely, not a very constructive role. I mean, he's always been valuable in criticizing, but not in a way that was very helpful to get things right. I think it was... Ehlers turned his criticisms into solid stuff.

15:00 Right. So there was sort of a progression there by which... ...Havash would prompt the Ehlers maybe to then... Would then actually do something about it. Yeah. That's my impression. Sure, sure. I'm trying to remember something something then that that you and I guess a number of the people that you were working with at that time did and obviously it's central to what we've been talking about was the expanding the field, the gravitational field of some isolated source out in powers of 1 over R. Yes, well, of course, some of that is basically in my radiation paper of 62. And then, Irindland, I have a tiny paper of about three years ago, not very distinguished, but it's there. And you also said that Newman and Tanya did... Well, they did the beginnings of the Spinner analysis. Yeah. Now in I was just rereading this this is your letter to nature in 1960 What did I say in 1960? I must have been very vulnerable Well in this you dealt with that work I guess specifically Yes that was the trailer for the 1962 paper You mentioned the fact that you hadn't worked much of it out in 1960 already. And so, you... Yes, this was the trailer for the paper that appeared in 1962.

17:30 And here, in fact, again... It is the correct metric already. Right. And here again you mention the question of our dependence. And again you mention that the existence of such a class of emotions is related to influence in this that was a specific case where the the curvature tensor falls off like 1 over r cubed yeah so that's what I was interested in so what you did or introduced in this and I guess development 62 the following results have been found one, if a system is static before summits and then undergoes motion and eventually again returns to a static state then its mass in the final state is less than its mass in the initial state I was very pleased when they came out to do work it showed that something really had happened in that case it seems quite concrete that as you say something had happened that some radiation must have come away So, uh, and so obviously that work at that time, uh, I think it must have had obviously a great deal, but clearly in some sense the issue was settled as much as I said, insofar as the existence of gravitation. Let me see, you've really eliminated a lot of my ideas about what I read from your papers. So I hope you'll be. And I'm just trying to think of other questions that I should ask. In case I move back, I don't have particular importance, but you've already, as I said, answered my questions as you went along.

20:00 One thing that I'd return to, I don't know if it's of any importance, but you mentioned the connections that you established within those groups in Warsaw, and I guess you mentioned Pirani went there, and to Chiav and Plebensky came first. Yes, I spoke to Plebensky and Stanford during the summer, and he was telling me in fact that in his book with Infeld, Motion and Relativity, that Infeld had inserted the chapter discussing the non-existence of gravitation and radiation without telling you, who didn't really share his view, exactly. Interesting. Did you have much personal contact with Inflat himself? Yes, yes, perfectly. Because I'm very well. And what were your impressions of his view in the subject of his general objections to the... You must remember, these were the days of the communist empire. Where the succession of things had been in the thirties, relativity was considered ideologically unsound because it was too idealistic, they dealt too little with real things and ascribed to an unobservable space, various properties, there was this sort of Eddingtonian formulation. This was then put right by Fock, who showed, with his first Copenhagen lecture, that relativity in fact is a very, very physical theory. He then fell into the trap of this de Donde condition, and Infeld saw it as his task to show that relativity was really general. You could use any coordinates you liked, as long as you knew what you were doing. And so Infert upheld the

22:30 purity of the system. But of course if you insist, rightly in my view, that you can use any coordinate system. Then the difficulty of extracting real effects from effects due to your choice of coordinates are very substantial. And I think he went a little over the top in that direction and felt that the waves were due. And you see the other was of course this glibness of Landau Lipschitz who look too much at the linearity. What Inferno, of course, Follern-Einstein and Feldhoffman and the like had always stressed and were so much agreed with him that the non-linearity of general relativity is absolutely basic, just that its non-Euclidean characters, absolutely basic. I mean, either of these are optional add-ons. Right. so it's interesting that Troughton, for instance, when I spoke, you mentioned In the sense, Infeld and Falk were arguing on two fronts, one over the question of the boundary condition, and then on the other hand, Falk was, I can't believe your point is the word, in the existence of gravitation and radiation. In fact, he had done work on radiation. Right, right. Intel doubted it. Because Foxworth was dependent on the debate, Infeld wasn't at all convinced that it could be translated into the general situation. So the two issues were really quite intimate. Correct. And I don't know, you probably wouldn't have a feel for this, but I thought I would mention that the other thing that Troutman said was that he felt that info-skepticism perhaps derived from

25:00 some doubts entertained by Einstein. Yeah, I think Einstein had doubts, too. I know you've got to be influenced by that. Correct. So, it occurred to me that I wanted to return to something that you said earlier, which interested me because I've heard it I've heard people concerning the Hollywoodians principle in gravitational wave theory, and And I've heard at least one person raise the question of, because Hagen's principle doesn't exist in the gravitational wave theory, that, I think this is slightly different from the point that you had been making there, that gravitational wave detectors might have difficulties because they... because it might not be possible to... because of the absence of coordinates principle, I guess, because of tail effects and so on, it might not be possible to reconstruct the motion of the source and the wave actually received, because... But I don't think you can do it with cylindrical sound waves. I mean, the moment... I don't know how it spins and it doesn't apply, this becomes an infinitely more difficult problem. It's reversing a convolution, which is not always easy. Right. So, as you say, the problem exists in the gates of sound, really? Yes. Syrritical sound. So is this something that you would regard as a problem for, for instance, gravitational wave detectors, for example, such as LIGO or even existing fire detectors and so on?

27:30 I mean, of course, the wave fronts are very clear. how much you can deduce from the way France with clarity is then a question. I mean, it's not something I thought about. I was interested because of the comparison, but to return to your own point concerning Hogan's principle and the fact that assistant can't return to a static stage in the gravitational case. What is your am I correct in thinking that you regard this as being of a A problem from the physical point of view, I mean, that it raises. I mean, so you have a system that is static, and then at some point there's a disturbance and so on, it starts emitting gravitation. Radiation. And how does it become a purely passive system again? How do I know that it has become passive? Because, so for instance in the case of a binary system... Well you see there the difference between active and passive is more difficult. you see if we take a binary system and neglect the radiation then it goes on forever then certainly nothing new happens if it spirals in changes its orbits under the effect of gravitation that would certainly affect the equation of state the bodies themselves they were going to previously irrelevant regions and does something unexpected happen at some stage or not does of them one of them become a supernova or not and how you describe it when there's nothing much happening at least i'm i think there's

30:00 But I certainly don't know what the solution is. So, supposing the two bodies merge... Certainly something big happens there. Obviously a lot happens. So here you have a lot of news and so on. And physically it seems as if the two bodies having merged will tend to become axisymmetric. Yes, through dissipation. Through dissipation in a number of different ways. And it seems plausible, of course, that they might axisymmetrize, and therefore there's no further emission of gravitational radiation. But it's the problem that, um... I haven't been into these questions. It's really for my own interest, I'm trying to... It would be nice if you worked on it. Yeah. There's a lot to be fetched, basically. Yeah. The, um... So, but in any case, your feeling is that it's not clear how, or it doesn't seem possible for the system to ever become actually static again. As a spherical symmetric again. I mean, just ask, can the spherical symmetry ever return? Well, I don't see how, but maybe, I mean, again, if one could show that at least asymptotically it becomes spherical, it would be nice. Yeah. But I haven't been able to show that. That's right. I think it's a surprise, it's a moment I haven't really considered before, so I was interested to make you raise it on, so to make sure that I understand the differences there. It's certainly true, of course, but it's not entirely clear that it would ever damp down completely, but in my opinion, it shows you say that it would have less asymptomatic. In any case, no, it's interesting to get it clear in my head. Well, see, I think that you've answered. Very good. Well, shall I rapidly run you back to the station? Thank you very much. Thank you.

32:30 Thank you. Thank you. Thank you. Thank you.

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