Interview with Felix Pirani (contd.)

0:00 The second edition came out in 1954, I guess. But mostly one was grounded in what was in Eisenhower. so I knew a good deal of what was in Eisenhower plus what I learned from Singh neither of us knew about lead transport along a flow of curves we were discussing it one day and we both went home and came back the following morning and we both derived the equation of lead transport along a essentially along a pair of curves work for a flow. And then if you differentiated that again, you got back to the equation of geodesic deviation, along with extra terms, which you could get rid of if it was geodesic, but if it wasn't, you'd have. So that, you know, there were all sorts of stimuli pointing you towards the curvature, or pointing me towards the curvature. It was a business about which I've mentioned, was knowing that free particles under suitable assumptions moved on geodesics. There was this apparently new way of getting the geodesic deviation equation from the re-transport equations, and there was Fermi transport. If you didn't have geodesics, there was a lot of things going on. There was this business about interpreting observations. There had been a lot of rubbish published, I guess in the 40s, about how you couldn't say anything about anything in general relativity because the co-ordination system could always be completely arbitrary. and the way to get out of this was evidently to have a frame of reference you know, you went along your world line whatever it was and you carried a set of orthogonal axes with you and that was a tetrad

2:30 we called it in those days you had this tetrad that you carried with you and it was perfectly clear that if you referred everything to your local orthonormal tetrad locally to effectively be doing special relativity around your neighborhood of your origin you could do special relativity and you could work out the deviations from that because of it being a Riemannian space-time instead of being flat yeah so this was trying to push aside these guys who said you know you can't say anything because you can always choose the call in the system arbitrarily was one of the things that was in the air to straighten things out to do everything invariantly Singh was certainly the salesman for doing everything invariantly stroke covariantly that he possibly could so you could see all sorts of things that were wrong like the quadruple formula was quote wrong unquote because it wasn't an invariant varying calculations. Even if you couldn't see how to repair it, you could see that there was something wrong with it. One of the ways to find interesting problems and solve interesting problems was to look at a problem that was very coordinate dependent. Say, how can I get the coordinate dependence out of this? And it was my looking at McVitie's coordinate dependent attempt to get gravitational waves that suggested doing it with the curvature tensor because that was a coordinate three way of looking at things i mean you know nowadays what is it it's 40 years later all this seems like it's very obvious of course you want to make it coordinate dependent independent but then it was much less obvious that that was the way to go I was very interested in your account of how you came to pretend to go that way because it's clear that there had been tremendous amount of anxiety really in some quarters about issues like the energy momentum of pseudotensor and relocation waves and so on. So people had these descriptions of the ways, and yet they couldn't quite figure out how to describe them in some way that seemed suitable to me.

5:00 That's right, that's right. So I'm interested in that. That's right. in fact just as an aside I was interested to note that this that issue of the energy momentum pseudotension has been brought up recently a paper by Fred Cooperstaff I have him with him but he he has gone back to something very similar to the argument that Rosen made, or at least, well, to addressing the problem that was worrying Rosen and others. And, in fact, it's made an argument that the thought experiment put forward by Bondi and a couple of others, that this is not as fallacious. It's a conclusion because the stick would move just exactly like the other speed that and therefore there would be no function. It's interesting that some of these issues were being raised again. He was essentially saying that you had to define the energy in the field, you have to feel there in a way or whatever you like, by a tensor, and that therefore, if you did so, therefore, in a vacuum, that this tensor would be zero, and therefore there could be no way of energy. Well, you know, you could forget about the stick and have a spring which got heated up by being expanded and contracted, all sorts of ways you could get round that. There's something you said which... I love it. Come on, Mom. Okay, it's gone. Something connected with his business as a student as well. Cooperstock, what came before Cooperstock? Let me see, I was... Wow, okay.

7:30 Oh, I know what it was. It was that one of the things which followed on, what started to happen then, was that because you had the curvature tensor as a way of discriminating space-times, the interest in finding new vacuum solutions, blossomed then. I think there was a lot of work then about finding new vacuum solutions because you had a tool for discriminating one kind from another namely to look at the curvature tensor. People tried to do more sophisticated things like looking at the Bianchi tensor first covariant derivative Riemann's answer and probably there's an algebraic classification of that so you'll know. I don't remember what it would be And there were Bell tensors, have you talked to anybody that mentioned Bell tensors? The Bell tensor was a four-index tensor built out of the curvature, which had certain, it's B-E-L, it was a Spaniard who worked in Paris. Louis Bell. Louis Bell, yes. Yes, I have come across, he worked with Tagore and Darrowell, and they were doing this work which led up to Tagore's paper. He's still around? I guess he could. I think he's still there in the past, yeah. You didn't see Papa Patriot? No, no. I was quite busy in Paris, which he was pleased. And one of my graduate students worked on trying to generalize the Bell tensor. Other people call it the Bell-Robinson tensor because I think Robinson, I discovered it independently. I worked on it as well, the Bell-Robinson tensor. I'm quite interested in this question of the classification of the space is based on the Riemann tensor. So could you sketch again for me briefly the process that published that? Was that, you mentioned that you were inspired by classification schemes in the electromagnetic case to look at it?

10:00 Singh had this classification of the Maxwell tensor into null and non-null. And the null, he showed, or he gave examples to show that you got the null case when you had a radiation field, I guess when you had plane waves. And I thought that this would be a good way to handle it, knowing that McVitie had not succeeded in picking out radiation, this would be a way to get into the radiation problems. I went looking in the literature and I found Petroff's papers, at least I found mathematical reviews which told me what Petrov had done because my Russian was negligible. And I certainly got into correspondence with Petrov and I got some of the papers and I got somebody to translate them. And he gave examples. He gave an example of a Type 3 metric for instance. But then, now where did the plane wave metrics come from originally? They had a lot of things, different people discovered them. Maybe Bondi and Robinson discovered them. Yeah, because Bondi and, that's right. I'm not sure I ought to beg you to turn the machine off. But I won't. So, Bondi and Robinson independently discovered plane waves, and Robinson, I guess, was still here at the time, before he went to Texas, and each of them felt a little bit that the other was not giving him enough credit. And in the end I made peace between them by writing the paper under our joint names. So it was a paper by Monte Pirani and Robinson in which the only thing I did was to write the paper, as far as I can remember. They didn't work. And I did the writing up because I wanted to cool the atmosphere in there.

12:30 So that, yeah, and that, whether they've seen it some, whether someone else had done it before, I don't remember now, you'd have to hunt the literature, probably the same way of metrics were known before, but anyhow, here was a nice example of a Riemann tensor which had all its invariant zero without being, all its scalar invariants were zero without being zero. And then the business bloomed, you know, there was a lot of good things about it. I think that was probably a lot of good things. And Schwarzschild was type D. Yeah. But if you rush through a Schwarzschild space-time, then it looks like type M. I wrote a paper about that in one of the ones in the King's College series. If you zip your velocity four-vector over until you're going there and there to the null count, then the curvature tensor of a Schwarzschild spacetime gets to look more and more like that. It's always naturally time D, but it can look more and more like time M. And then Wheeler told me that's just the Weitzschecker-Williams method for electromagnets and you want to do it plainly if you have a charged particle rushing by you. Which I don't have anything about that. And so as you say, then that led to a sort of, in fact, I suppose that was part of a sort of a renaissance of the work that was going on in that time, interested in these various types of literacies. Yeah, I get mildly irritated by one or two things I read last year, which said that I I think it's Clifford Will who says that the renaissance of relativity was in the 80s. There was quite a lot going on in the 50s and 60s, too, which may not have survived so well, or may not seem as important now as it did to the people that were doing it, which is understandable.

15:00 But there was certainly something going on there, too. There seems to be a clear case for making a renaissance back then, considering that there were very few people working in the field, as you said there were. That's true. Yeah, this is not relevant to what you're doing, but I would be interested to know what the feeling is in California. One of the things which Will and other people seem to do is to put in a whole cluster of parameters. If you tweak these parameters to one set of values, you get general relativity. If you tweak them to another set of values, you get brand sticky. Right. That seems to me a funny way around to do gravitation theory, and I wonder if it's popular. I think it was very popular at one time, as it might be before my time, because there was so much about this talk about whether Brian Stickey or the three were real alternatives to relativity and people were interested in experimental tests to distinguish between them as a sort of classic crucial tests between various theories so in large part I guess this parameterization was designed to facilitate this testing process. And I said, well, now we've got this number and it's zero for this and one for this. And, of course, Will has written a couple of books on the whole subject of the testing of the theory. But, at least partly because General Relativity has, you know, allegedly passed all the tests and defeated the other theories to a certain extent. Of course, since each of the theories themselves may have adjustable parameters of some kind or another, it's always available how crucial the test can be. But nevertheless, the general feeling out there seems to be that people are inclined to just do the general activity. And while there's still a certain amount of tests going on, there's, I think, a much

17:30 less level, a much smaller level of interest. My impression is that, for instance, in Kip's group in the 70s, or maybe 80s, they had some joke that, well, on Tuesdays, Thursdays, and Saturdays, we believe in the brown sticky period, on Mondays, Wednesdays, and Fridays, we believe in the period relative, and on Sunday, we go to the beach. So, but that's not really so much true. Now, for instance, with LIGO, which is, of course, an experimental issue at this certain stage. It's really, everybody just assumes relativity, and while there's some thought, that as a kind of a separate issue, one might address the question of looking at the parameter, looking at the parameters from the parameter as opposed to the formalism with LIGO, it's very much a separate thing, so that for all of your applications, astrophysical You just assume relativity. And then there's a separate issue off in the corner, it seems to me. Well, maybe we'll use it for some more or a little test. But I would say that, in my experience anyway, is that the question of parameterizing the theories is not so relevant. Simply because people are so much, I guess, are so much more confident that relativity is just a theory that they're going to stick with. Yes, I went in science a while ago that LIGO had some trouble at the top, of course. Yes, they've had... That guy from Glasgow, wherever he was, went away, didn't he? Yes, he's still in... It's a very unusual situation. He's still in Caltech, but he's not part of the LIGO team anymore. However, he's still doing work related to this topic, but the basic situation was that there was a very deal of personal friction within this LIGO team, and here I'm really speaking very much at second hand because our group has very little personal interaction with these experimental people. But they're still getting the money? They're still going ahead? Oh, as far as the money goes, they appear to be doing fine.

20:00 I gathered their problem last year, apparently, was that they were getting more money than they really wanted. So, yeah. So, I think their main fear was that all the other scientists would get angry with them if they took all the money that they were doing. But they also had these personal difficulties, which they appear to resolve by various shufflings of personnel. So for the moment I think the feeling is that the future is wild. And they feel it unlikely that they'll be cut the way the NSSC was, or some period to it. Well, that's good. Well, thank you very much. You're very welcome. I'm so patient and helpful. It was extremely interesting. Well, it was interesting to hear what's going on now. Thank you.

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