Interview with Thibault Damour (contd.)

0:00 ...the Doppler effect, then again you get agreement with the lowest order radiation reaction predictions to condensed subjects. So this is still, I mean, the binary pulsar could still provide surprises. I mean, if one day there is a real discrepancy. I should say that the new thing also is that the discovery of the second binary pulsar with the same quality as the binary pulsar has now allowed observations to measure orbital decay on the different systems. and this is presently in accord with theory within 20% which is the error bar. So this is very good and important because up to now we have only one system seeing one phenomenon for the radiation reaction. So it's important to have a different system somewhere else in the galaxy observed in a different way. But binary pulsar probably will not be able to probe at all higher order effects in radiation reactions at all. So from that point of view, then, would you regard the efforts to detect gravitational waves, LIGO, VIRGO, and so on, as being a possible future testing ground for such effects? It's not clear. I mean, this is put in the motivations for Virgo-Ligo, but first one should be happy if they see anything, if they detect gravitational waves. To use them as probes, really, of radiation reaction effects, it's not clear whether it would be possible to do so, in fact. but which is not that important, in fact, to do it. For instance, the Virgo-Lygo people tend to minimize the interest of the binary pulsar, say, using words like indirect, that binary pulsar is a very indirect way of seeing gravitational waves and things like that, which to me is not true because this is a very direct way of the binary pulsar to see what Krakauer was talking about,

2:30 which is that gravitational radiation propagates with a finite velocity. This is directly what creates the effect you see in the binary pulsar. You see the gravitational interaction of the companion on the pulsar having been propagated with the velocity of light. Now, when an interaction propagates with finite velocity, you call it a wave. You have not seen the wave structure, but you have seen directly the propagation with the velocity of light. This is as direct as what LIGO Virgo will see, which will be after a complicated analysis and getting rid of the noise and using a lot of the theory. Maybe you will have data which are compatible with having seen the gravitational waves, but you will not see really big waves moving your detector around. I mean, it will be such a complicated thing that you will use a lot of the theory to extract what you want to get out of it from a big background of noise. indirect in that sense. Which is not a criticism of the thing, but in physics it's not as clear-cut as one thinks between what you direct. I mean, you always need a lot of theory to extract anything from experiment. And binary process is one example and like Obirgo will be another example and really that will be important as a new, I mean, suggesting new problems in gravitational research and observing new objects in the universe. As a testing ground, I'm not sure it would be important to see directly the velocity of propagation, the electricity structure of this probably they will be able to do. But then they will enter the problems of when you have complicated systems like coalescing binary neutron stars, all the effects of the equation of state, idle dynamics and things like that will enter and you will compare computer code to what you observe but it's not very clear that you will be able to test the theory in interesting regimes because it's so complicated beyond what can be done analytically in spiraling binary system to go really to the coalescing and the fusion to neutron stars will be so complicated and numerically loaded that it's not clear you will be able to extract clean tests the binary pulsars will stay for years to come as the cleanest test of generativity,

5:00 like the hydrogen atom of generativity. Interesting. Well, speaking then of this clean aspect of the binary pulsar, I wonder, well, in some sense, I'm not sure if you agree even with the premise here, The agreement between theory and experiment in the case of the binary pulsar seems reminiscent of the earliest, most famous test of general relativity, that is the perihelion advance of Mercury, in that it's a case where the theory seemed to have no room for errors, it were no obvious fudge factor, which could be varied, and it happened to agree very precisely with the data which had been extracted by a very careful process from a given observational system. And therefore, it's not only a very impressive test for the theory, as was especially noted in the case of the per-healing shift end and the radiation reaction case, but also a very impressive testament to the experimenters in that clearly all sorts of perturbation effects in the case of the perihelion shift and obviously all such data extraction in the latter case had to be taken into account. This means in some sense that should anything else turn up later on, that would throw the agreement out. For instance, in mid-century or so, Dickey proposed that maybe there was another perturbation to do with the obesity of the sun, which might throw out the agreement in the case of the perihelion shift. So how much effort went in and how much of a problem do you think it was or is to consider other possible effects, which might be dissipative effects, say, in the case of the binary pulsar. which had to be allowed for and which could, throughout agreement, have had been found. Yeah, it's true that usually one starts being naive in the sense that when there is an agreement, then you don't want to think that it could be a coincidence.

7:30 And myself, I guess, I didn't think hard about that. I was assuming this was a clean system. The observers spent a lot of effort trying to at the dispersion data that the system was clean in the sense there was not too much material in the system that would disperse electrons. So there was already from the beginning, from the observational point of view, an effort to prove internally in the system that the system was clean. But it is true that after the beautiful agreement and all the work of the theory the agreement was so nice that one did not think back what's wrong could happen in the system. And that focusing back on this, when the disagreement started showing up, which was this two or three sigma, and this is in a sense, I saw a little bit of a shame, but people are human beings, and they kind of always think that everything could be wrong, and it reaches by chance, but when this happened, that there was a disagreement, then, with, and this was a joke with Joe Taylor, we did not try only to find one way of explaining it, but really to have as exhaustive, as complete survey of all possible things, which have been discussed in the literature already, or which have not been yet discussed, but look at them very deeply, knowing all the information that one, and in the meantime, the modernization of the system, the evolution of the system, for each possible source of error, reasonable possible source of error, I mean, nearby stars accelerating the system, many other things, to put limits on them, and we try to do that. So most of the paper is to do that and to conclude that for all these effects, they are much smaller than the error bar, except for one that you have to take into account, which just re-establishes the perfect agreement between this and this. And this shows two things, that there will be limits to the binary pulsar test from this point of view of the radiation reaction in the sense that there is a noise, incompressible noise, due to all these things, which means that when the hour bar will be too small below the noise, then in fact you will not test the theory anymore, because you know that many dirty astrophysical facts could enter, so it will be the end of the binary pulsar as a test of radiation reaction. It will not be the end

10:00 as a test of other things, because the strong field tests are independent of many of these things, so it can still do all for other things, but for, and also what was useful in this thing is that there is the binary pulsar, the original one, PSR 1913 plus 16, but new binary pulsars have been discovered, and this study can be applied to the other things, and can localize what other binary pulsars, other binary pulsars could be better, and be able to be better tested in the long term of radiation reaction, cleaner in several different ways and this is what one can conclude in fact that in principle 1534 plus 12 could be much better for aspects like that. I mean, if we take time to get the data. So this, at least in this case, the analysis has been done up to a certain level of confidence. Just to have brought something quite different I mentioned that I'm also interested in looking at an earlier period, for instance in the 50s when Infeld and some others expressed doubts about whether the radiation reaction existed at all in, say, binary systems were any of these doubts ever an issue that you recall during the period when you were interested in this No. To me, no, and I guess to nobody else, too. I mean, it was known that Imfeld had defended this very unorthodox view. It was also known that, in the end, he turned his mind around, in the sense that there had been this paper by Imfeld, and this is Michalska Trautmann who is the wife of Trautmann Trautmann was involved with aspects of radiation reaction at infinity and this work with Michalska Trautmann both on the radiation reaction in the system so where Infeld himself in a sense changes mind under the arguments of many of his friends for me I was aware of that I mean I read the book of Infeld and Klevonsky on the problem of motion which is somewhere here, because this was the continuation of Einstein for Lofman,

12:30 but this book I always found very unclear and not very good. So for me it was never this line of work, dance about the reality of gravitational waves was not a real concern at all. So that touches on something that we mentioned earlier, but you mentioned perhaps that it wasn't so much an issue for you but concerning the general reception by the community of the binary pulsar data do you have any thoughts that you mentioned for instance that you felt that in America people weren't as interested in many of the details of it but how do you regard that the general from the point of view of radiation reaction to the general reception of the boundary of what they're doing, or its effect on the communities and that. Okay, one should say that probably it has always been anywhere, a small group, which was dealing really with a quadruple controversy, and this group dissipated away fast when methods converge. As I mentioned, there are still people belonging to this, although which have not been convinced, which is not so surprising in fact. So I guess the community at large has never been... I mean, when you go to a conference and you just want to hear the final word, you are not so interested in fine prints and things like that. So when you feel people agree, then you forget about the controversy has never been a big thing for physics at large. I mean there are controversies, there are big problems of physics which are totally unsolved because they are too difficult and I guess there are from time to time more fields conjectures which prove to be wrong. We are talking anyway about rather small community when you compare to physics at large. So this is normal in the sense that I mean I mean it's normal that this is yeah don't need too many people to work on this so the question of the reception is maybe to

15:00 look alike what is good is that many physicists at large understood that what can be extracted, that the bionicle star is a fantastic object, and that this is really a threshold in experimental relativity. So this has been certainly recognized by the moment that tries to help to discover it like on Centeno, and so it's not good. Yeah, certainly it obviously had sufficient significance that I guess it was the first tries to be given. Yeah, it's something connected to generativity. Which means that all this work had really convinced physicists at large that there was no doubt and that the final observation of Baner-Prosse established the reality of gravitational radiation also. Yes. because this was one of the I mean the novel prize that is given to a discovery but a discovery which has an impact on physics at large and this is the case well it's something that well regarding the for instance the number of people I guess I can't remember immediately references I guess a number of people have used the phrase quadruple formula controversy or something like that you mentioned that first of all of course this was within quite a small community and so on it had a certain course which it certainly replicated in other similar cases of other branches of physics that is there was certain disagreements between those people as to the course these disagreements either dissipated or went away eventually do you perceive it this stage of the controversy leaving aside earlier doubts

17:30 of the reality of radiation reaction do you perceive it as being anything that was out of the ordinary in the physics sense or that it was just a normal course of the way this work was done, this is that? Ah, so, um... Well, yes, that's true. It has been a very useful thing, I mean, to have these people contesting the thing, insisting on the fire. do work on the controversy direct work like Jürgen Ellos that suggested a lot to work on it and insisted on the problems and this has always been the very useful work. I should say that also even the work which my opinion was simply plainly wronged, technically wronged by Arnold Rosenberg has played a very useful role and I guess this is why for instance consciously, I mean conscious of that because it was published in FISRAV Letters, accepted to publish the work of Rosenblum, although I guess there are doubts about the correctness of it, but because they thought it was very useful for pushing people to solve this problem, to publish this as a provocation, and this has been very useful. So it is a very healthy way in which science progresses. so from that point of view controversies, even if the people that participate in the controversies are not necessarily the best and have clear ideas the fact that people fight on it and put some heat in it is a very useful thing and leads to clarification of problems and progress in science regarding then some of the within the history of the subject some people have that I've seen have alluded to a difference of approach between certain workers in relativity for instance you alluded yourself to the failure of some earlier people working in this particular problem to deal with observables to remain attuned to strictly physical phenomena and quantities.

20:00 Feynman, in about the period when Infeld and others were casting doubt in the existence of gravitational radiation, had some trenchant things to say about some of the people that he felt had a wrong-handed opinion. Some people have put this as sort of a difference between those who are more inclined to deal with problems from a mathematical point of view and those who are more physical in their thinking. Is that a kind of view of it that you share, or do you think there is a dichotomy between two different methods of working in relativity or physics in general? No, I would share this opinion, but it's worse than that in the sense that it's not only that they look at it in a mathematical way, We are not very good mathematicians either, because you can do good mathematics and generativity, but now this is very hard. For instance, one person who, I mean, if one goes back to the history of gravitational waves, important work was done to clarify things in France by Lichnowitz, and André Lichnowitz, and Madame Choquet-Borat, and people from their school, like Louise Bell, who guided some sense of Lichnerovitz. And so Lichnerovitz and Madame Schochet are mathematicians, but they had a clear idea from the mathematical point of view of what structures they could speak about and not. So if they were discussing shockwaves, they had precise definitions, and either they were finding the word shockwaves or they were not shockwaves. And then they knew that that existed in a geometrical sense, which means they existed in a physical sense, because they existed absolutely. So, it's not mathematics versus physics, in fact. This is often that, I mean, it is true that people like King Feld, although he worked with Einstein, had not the physical intuition of Einstein. And when Einstein asked questions with Rosen on the reality of gravitational waves, this was in a more difficult problem where they were looking at the fully non-linear waves, Einstein-Rosen, and there they had some doubts and they asked questions, but Einstein was not stubborn in these things.

22:30 I mean, if somebody had clarified the problem at the time, he would have agreed on that. And Einstein had clear ideas, rather clear, in spite of some mistakes, in the problem of motion, where I think Infeld was simply not as good a physicist as Einstein. He did not, certainly. And this is because of that, that back in his country, which played maybe an important role, I mean, he was back in Poland, which means he was a big chief there. I think he arranged to be the big chief there by arranging that other people would not get, maybe better people would not get positions, that's why Klevonsky had to go away and end up in Mexico and things like that. So this was maybe part of the problem in communist countries, that you have these big chiefs and they have their groups, and then they do what they want and there is no contestation of this. So from that point of view, it is true that controversy is very good, Because it allows any people to contest orthodox ideas, and they should be answered, except if this is really too long a question, why maybe Infel had too much power and not enough contestation in his own group. And other works also which were not good, they were works, for instance, by Ku, who was Chinese, who tried technically to do something very difficult, and that just stayed in the literature, and then he, I guess, he quitted more or less the field and things like that, because nobody else was interested in that. So it was not put in the focus of lively discussions, as the problem should be. So the controversy lasted for a long time because of the structure of these groups and lack of interest within the system. So on the one hand, you feel that the problem of the lack of interest was to a great extent overcome by the advent of the binary pulsar, had the motivational force of a certain amount of disagreement and people like Ehlers pushing people to. I should say as a bonus to Infeld and Plevonsky together, I don't know who is responsible for that, one of the places where I learned the fact that you

25:00 have really to think always in terms of observable, which is not always clear in the early literature, in Einstein's own words and things like that, is in fact in the book of Infeld and Plevonsky where they discuss elementary facts like period precession and things like that and they try to say that the way to discuss it correctly to be independent of the problem also of coordinate systems because if you have a peri-alien or peri-astron in general which precesses the problem is precesses in a certain coordinate system and are you sure that this is really what you observe and things like that and they propose explicit ways of saying you have to project that by light trace to infinity see at infinity. So in this book there were very unclear statements about veritation of radiation and reality, but there were also clear statements that you should work with observables for some aspects of it. So maybe Spedonsky was more accessible for that than Infel, or maybe Infel was schizophrenic in the sense that he's not applying his own thing. And also it was a difficult problem. I mean, one really tackles nominalities it's in the field so you feel that there that there are people who are inclined to work from a more mathematical point of view so there are people who approach it more physically but that perhaps a more crucial distinction is between those who do good mathematics or bad mathematics or good physics and bad physics I don't know about statistics whether generativity has attracted more, relatively speaking, bad physicists as other fields or not. I mean, there are fields, probably not, because the other fields, there are many more people working on them, and so the statistics must contain also, quote, bad people in that. And I can see from other things I know in particle physics that even bad people can ask very good controversies. I mean, they can raise points which are finally subtle, and they can say very wrong things, but still, this is useful. so maybe one needs anyway a question that might be to pinpoint

27:30 but you mentioned of course that in any case previous work even where it was quite wide of the mark was obviously useful in focusing people's opinion on questions how useful were some of the previous derivations by land on the Schitts and so on of the Schittler-Quadrifold formula in a less rigorous way. How useful were these, and what do you feel was the earliest reasonable and useful derivation of the clinical formula or treatment of radiation reaction? But here, again, For the work on radiation reaction in the binary pulsar, nearly nothing of that has been useful at all, except what has been useful there was really the approach of Einstein and Karl Hoffmann and his continuations, which means many works, like by Kerr and others, with important technical things that were done there. because, for instance, was technically useful the work of a Japanese group which, independently of what was going on in the rest of the world, continued working on the problem of motion and going to high orders of the post-internet interaction. And although finally there was a technical mistake there and some conceptual lack of clarity in what they were doing because they were discussing a lot of gauge dependence and it was not very clear what they were doing. This work has been useful, certainly very useful to me. In fact, I met one member of this group in the same Varena school where I met Joe early on, so I was aware of this work and it was the fact that they could push the calculations very far and we needed later to push the calculations very far. survival of, let's say, technical things, that technical things, even in a wrong conceptual context or even containing mistakes, is as a permanency which is extremely useful for the progress of things, and that's why I always found useful myself to know the literature and to look at it fast, because there are things which are definitely not useful, but

30:00 other things, even other words, wrong things, which are very useful, but not going back. But now, if your question is, what is the first reasonable derivation of the particle formula from the Farfield point of view, which is the emission of gravitational waves, then the original derivation of Landau-Lifschitz is fine as not very high level of rigor, but they know what they are doing and they deal with intervals which are converging in fact, they know very well what they are doing and of all what they do if you cut them off enough and you just stop at what they say and you got some heuristic is correct so this is for me and I should say that and I insisted in some of my favorites on that in fact, that there are two early derivations which are good and, in a sense, a better one by Fock, which is already trying to match what happens at infinity and using the problem of the logs, the logarithms that appears in the harmonic gauge and renormalizing in a lot of ways. And this part of the work of Fock I found now technically more useful for what we have been trying to do, which is to go beyond the chronopold formula, now for emission, not radiation reaction, to needed to integrate all the previous work and find, in fact, better ways of doing it. And, in my opinion, the Fock method has been very useful at this point, together with the later work by Bonner and Kipthorn, and this is, by generalizing this, that we have been able now to compute first the post-Newtonian thing which is in my opinion in fact the first rigorous one if you want a good proof of the quadruple formula it has to be 1989 one proof the post-Newtonian one which as a subpart contains a very clear way of having the quadruple formula plus an error term because this is the first time also one should say one computes an error term the next term as a measurement of the as an estimate of the error so this gives a clear limit of the validity of the formula which was which is never clear in derivations where you stop at the term that you compute and you never know what is the error and what you are doing

32:30 but these are let's say technical improvements and now one can push this to the 2pm second post-euternal level processes. Conceptually, it uses the same concept as before. So, in developing your work in dealing with the binary pulsar and developing the radiation reaction problem, you found, first of all, that there was a history of valuable technical work on the problem of motion, which influenced and the work, and that there was a certain, also on the radiation reaction side then, what came useful from there was the various efforts and improvements that had gone in into dealing with the problem of matching between the organisms. Yeah, the matching thing, it's true that the work of but that was later, in fact, in the work of Burke, which was done at Caltech a long time ago, I always judged critically, because not much, in my opinion, is achieved. He's playing a useful role of transferring things from hydrodynamics where people are using matching things to generativity, so he played a very useful role there. I don't think that in his work he's really proving anything, but knowing that was useful, but not so much for the radiation reaction where the matching was very different. wave zone to the near zone, but the matching was more directly linked to reflections after Einstein and Ferloffman, which is matching to what is the structure of a body and the near field, not near field in the sense of wave zone or anything like that, but field just near a body, near a neutron star, independently of the problem of whether there is a wave zone or not, which is a problem that we try to evacuate attacking the problem in a different way. But still, it's true that and I've tried in my Les Houches lecture in fact to put together all the different threads which I had read and which I had found useful conceptually to have a, I don't know, in my opinion a better way of attacking the problem. And this means many papers in fact. A quick question again then.

35:00 certain types of work that were done in the 60s which more or less explicitly ignored many of the doubts that people had at the time about how to deal with with the problem of strong fields and so on and looked at the problem of the radiation reaction in its most external things, like for instance a paper by Peters and Matthews and Peters in these papers which looked at the evolution of the orbits simply saying, well, there's this energy of infinity and this must come from the system and so on. How useful, how important in general were this kind of work in the 60s and was it in any way useful or influential It was not influential at all. What is technically clear is that they had obtained, essentially, the good way of writing, although they never write the change of orbital period. No, they do, probably, because they look at the evolution. I don't know. But anyway, they have the full formula depending on eccentricity. Why, for instance, so it's an absolutely essential technical thing that it was correctly done at that point, because in the early versions of the Landau-Leafsheet, they have the general quadruple formula for the far field, but then when they apply it to the variation, to the reaction in the binary system, they do only the circular case. So the non-circular case has been worked out in detail by Peter and Matthews, And definitely, this is something that I wanted to reproduce, and I was very happy that they were not trying in any way to do the same calculations that they had done, which was at infinity, but starting from the term of order of the opposite fit in the motion, and having proven that the other terms were conservative and did not create a change of the orbital period than the last term, which looked very different, which was not connected to the quadrupole, because in what we have done, the quadrupole never entered. It's never a credible formula in any of the things we have done on the binary pulsar. But to get, in the final calculation, exactly the same result as Peter and Matthews, as dependence on the eccentricity, was certainly a very important thing.

37:30 And I was fully aware of the result of Peter and Matthews on that. Sure. It was more a case of, as you were doing it in a much more direct, explicit way, that you were pleased in the end to find out that you got the same thing. expected from the U.S.C., which I didn't, the part which I did not doubt. Once that the strong field effects within our methods had been under control, then I had no doubts that for this one should reproduce the pitot in my field. I had no doubts about that, so it was a very good technical check on what we had been doing, and a necessary check. But again, there was some question, a priori, whether the strong field effects would prove to be conservative. So I knew that one had to wait for this five million years. Well, maybe this will be my last question unless I can make it something else. Does, again, turning back to something that you emphasized earlier on about the way that there was a lack in the field in an earlier period, a lack in the whole field of general activity of experimental work, provide some sort of impetus for people to come into the field or be interested in a particular problem is that a problem that you feel has been is not as severe as it was now as it was is there enough experimental data or is there still generally a lack of vision? No, in fact, maybe the golden years of the confrontation and close interaction between the theory experiment in generativity are over now in the sense that I mean from the solar system experiments the golden years have been the 60s and 70s before the missions where activity tests were performed the binary pulsar especially for me but at

40:00 large has played a very important role for 20 years and now although this continuous this most of the important things probably are done now and gravitational waves is something very important but it's and is now driving a lot of activity in coalescing binaries and will drive this for a while but at some stage I mean the number of things you can do analytically I'm not So let's hope cavitational waves will discover entirely new objects that will suggest new problems for relativistic things. Otherwise, but then independently of that, this is also something which is slow to come. I mean, I remember myself when I first met and heard Cape Town in, I guess, December 1974, maybe not, maybe it was in 75, maybe it was November 75, in Erice, no? in 1975 in Air H.A. it was announcing it was a school on gravitational waves so it was announcing things as if gravitational waves were going to be detected in the next 5 years and we had announced that for quite a long while there are things like GPB, Gravity Probe B which started 20 years ago which has not flown yet I myself very much interested because I think this is a very important avenue for generativity in a space experiment step, satellite test of the equivalence principle, because this opens something which for me is very important, which is not problems just within gravity, but at the frontier between gravity, particle physics, and cosmology. And this is a very important frontier for the field. But these also are slow to come. I mean, one is never sure about these space experiments. They will not give much data in our many years. So maybe we are entering a period of death of data, and also a gravitational wave will be detected hopefully in, I mean, reasonably more than five years and maybe only ten years from now.

42:30 The amount of data will be maybe not very strong at the beginning. So general relativity, maybe as known, is golden age from the 60s to the 90s, which has been a very intense activity with the concept of black holes, discovery of new objects in the universe, relativistic cosmology. But now it will survive only by merging with physics at large, for instance. one of the most important problems where generativity has not a word to say per se, but is the central things and their context is the quantization of gravity. Unification is a different problem, it's not necessary for me to unify all interactions, but quantized gravity is an absolute necessity and disposes immediately the problem of the cosmological constant, This poses the problem of where does the universe at large come from, why is it so big, compared to particle scales, compared to the Planck scale, why is the universe so big. These are problems where generativity is an ingredient, but maybe where the essential methods of generativity will be lost in a higher context. If something like string theory is the answer to some of these problems, what is many of the things that are purely within generativity are not so important for solving these problems, and problems will be solved outside. So it's not clear this is a field which will thrive by itself. I mean, part of it will continue to be attached to astrophysics. I mean, one will discover more and more complicated systems in quasars, and one has to understand better the physics of quasars or binary systems. But maybe the community of astrophysicists will use tools from general relativity to solve their problems, and then the people of practical physics will use some of the ideas of general relativity, what they want to solve. but as a community by itself I do not think that anyway I do not think it has to survive by itself as a separate field in infinite time to start from now and maybe the golden age is going to an end but certainly some of the space experiments like step for me are very important and gravitational waves detection of gravitational waves are very important

45:00 but beyond that now one asks let's assume we have already discovered gravitational waves to observe them what will be the next step is not clear when we want to think further about that. As a little addendum to some of the issues you raised answering that question do you see we touched earlier on the possible fruitfulness of a controversy within some topic in theoretical general relativity do you see that as being also true in the experimental case? For instance it might be said that the controversy over the alleged detection of gravitational waves experimentally in the late 60s, early 70s propelled along some of the work in that field, and there was also a controversy within experimental gravity over the fifth force and so on. Do you think that these controversies were helpful in perhaps encouraging people to... This is quite true. I mean, the importance of the original work of Joe Weber had not been appreciated by anybody, I guess, until he announced something, and then people really took seriously what he was saying, and then this really propelled the pill, and the fact that this was not confirmed was sad for Joe Weber himself and for physics, because it had not been discovered, but this is really what pushed many people to continue the effort. and this is also true for the fifth force so which and then when you look at that you see that some people drop out of the field they say no it's too long too complicated I don't want to enter and some continue say no this is a very good thing some like one river where from the first thing continued and same thing for the fifth force where many people jumped on it after the announcement, and I always considered although this was even within the original thing of Fishback, it was quickly realized there were many bad things and wrong things, it has been a very useful thing, and FISRAG letters again play the useful role in publishing it and attracting a lot of interest on it. Many unclear experiments have been done, and after the qualification settled, it's true that nothing has been detected in the positive sense, but we have

47:30 And what is very important, too, the theoretical issues at stake have been re-discussed in depth, and I think this provides a good case for performing much improved experiments of this type, like, in fact, steps after I tested the equivalence principle. and this space experiment has profited very much of the existence of the controversy of the pitfalls in re-examining really that this is a burning issue that at large there could be things beyond what we know in the spin-to field of generativity and the other interactions that exist and this framework is not closed by itself Well, I think that you've very helpfully answered all the questions that I thought of when I come in. I may well, in the days to come, think of lots of things that I should have asked you. Was there anything that came into your own head as we were talking that you wanted to bring up? Yeah, yeah, it's a very interesting thing to say. Well, I've taken up a lot of your morning already, so maybe we'll stop the tape at that point.