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ORAL HISTORIES

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This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.

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In footnotes or endnotes please cite AIP interviews like this:

Interview of Franco Selleri by Olival Freire on 2003 June 25, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/28003-2

For multiple citations, "AIP" is the preferred abbreviation for the location.

Selleri’s background includes undergraduate and graduate studies at the University of Bologna under G. Puppi. His idea of the one-pion-exchange model and his work at the Instituto Nazionale di Fisica Nucleare and CERN. His stay in the US at Cornell University. The conversion of scientific interest to foundations of quantum mechanics [FQM]. Reading of B. d’Espagnat’s Conceptions de la physique contemporaine. Suggestion to Italian Society of Physics to organize a Varenna’s summer school dedicated to FQM. Realism, social responsibility and science. His moving to the University of Ban, and his research and career there. The role of local realism in physics. His interaction with Louis de Brogue, Jean-Pierre Vigier, and Emilio Santos. His argument on the loopholes of Bell’s inequalities experiments. Proposal of experiments to test L. de Broglie’s wave and particle duality. Conference on FQM in Bari in the early 1980s, with Karl Popper and Leonard Mandel in attendance. Experiments held by L. Mandel. The role of journals like Nuovo Cimento, Foundations of Physics and Epistemological Letters as outlets for papers on FQM. Italian attitudes towards FQM, the role of Enrico Fermi. Students and collaborators like A. Garuccio and Rapisarda. Proposal of Bell’s tests in particle physics. His proposal of a new theory of relativity.

Transcript

We are in Bari. Today is 25 June, 2003 and this is the tape number 3. So, I would like to resume a part of a former question. Could you talk about your political views, roles and activities and their intersection, if any, with your philosophical and physical views.

Surely. The time at which I started being interested in the foundations of quantum mechanics were also times of great political unrest in the universities - you know, the famous years 1968 and 1969. So in all universities, in particular in Ban, there were students occupying the institutes and organizing political debates. I was then a young teacher, and for me it was very natural to side with the students. I thought they were right and they should be helped to improve our society. From that started for me a political activity that later for some time was also as a member of a party. On one side for me it was a very important activity because I felt I made a cultural jump of quality. Before I understood nothing about man, and after I had the feeling that I started understanding how we work, how we reason, how we react — not always rationally, of course. But then it was also very frustrating because I was too naive in my approach. My method was to declare what I wanted to achieve openly, and then my behavior was as much linear as possible to achieve openly those aims. So it was very easy to shoot me down, a thing that happened several times, yes, because some people did not like what I wanted to do and could easily find all the information they needed to contrast my actions.

So from that point of view it was frustrating, but (I think, I hope) it was not really because I was too stupid. I thought that this was the right methodological approach, that you should be completely honest and transparent every time you try to do something. It does not work, I tell you. Anyway, for sure I tried to carry out this political activity. This lasted a few years, it was three years including those spent as director of the Ban INFN section, until I understood that I was going nowhere. I was losing my time and so I gave up. So I gave up politics very soon and I started doing my job, physics, full time teaching and research. I thought that there was a strong connection, because it seemed to me that the ideas of renewal of society and of physics had to go together. There was (and is) a need for a completely different approach in physics and also for a different approach in society. So my approach also in physics has been always to be open and rational and to be honest and so on. But then in physics it is not so dangerous, because few people care, few people understand anyway what you are doing, so it is much more difficult to be shot down in physics than in politics. If the scientific proposals are sound it helps too.

Okay. A related question concerning the same time, what were the reactions to your ideas from the people you were in contact with? So the reactions of the people at the end of the ‘60s.

Already in Bologna before coming to Bari I started saying I did not believe anymore that quantum mechanics was correct. And the reactions were negative. I think that it was also from that point of view very positive that I came to Bari because in Bari everybody was young at the time. Very few people were older than I was, so I had a lot of space so to say, cultural space — yes? — that in Bologna I would not have had, because there were older professors with very different ideas, ideas opposite to mine, especially in theoretical physics. And so I believe that those people would have been a terrible obstacle, very difficult to overcome. Instead nobody like that existed in Ban and here my activity was easier.

Okay.

By the way, I keep saying the following in my talks, although I say it only once and shyly, but I say it: “I believe quantum mechanics is not correct.” Of course I cannot know a priori how the world is made, and it could even happen that I’m wrong, that quantum mechanics is correct. That would be a terrible surprise for me. If I can make a conjecture then I would say that quantum mechanics is not correct because it is in contradiction with local realism. Local realism is so natural a philosophy that it is difficult to believe it is not true in nature. Of course everything is possible. We have not created the world, you and I for example. What I can see is that quantum mechanics was created with some strong philosophical input — and not only strong but very strange, very unusual for physics. It was created mostly by Bohr and by Heisenberg, and also by Pauli and so on. And that philosophical input is strongly tied to the mathematical apparatus to the point that the mathematical apparatus plus the empirical rules giving physical meaning to the symbols is in contradiction at the empirical level with the predictions of local realism. This is Bell’s theorem. So you have a strongly philosophically charged theory, which is philosophically charged of the philosophy of Heisenberg and Bohr, chosen by them as a philosophy — not for scientific reasons but for philosophical taste. Then this theory should be the true theory of nature. From a strictly logical point of view everything is possible, but I don’t really think so. I believe the theory to be wrong. When people will finally carry out serious experiments about Bell’s theorem they will find it is wrong. So the reactions to this were very, very negative, yes. I was looked down as a sort of crazy man who has very strange ideas. But I never changed my opinion.

So it would be interesting now to talk a little about the experiments in the history of the controversy of quantum mechanics. I have some questions but I think that we can begin asking you about your reactions to Aspect’s experiments in 1981 and how did you see the experiments, how did you react to those experiments, what is your point of view, what was your point of view at that time, and after that how did you develop your ideas concerning those experiments.

Excuse me. If I may, I would go one step backwards, I would start from 1974.

Excellent.

I would start from the Fry experiment. Because I was in Erice when Fry gave a talk in which it was clear that there was a confirmation of the 1972 results by Freedman and Clauser. So it seemed that Bell’s inequality was really violated. That was for me very difficult to take. By the way, Garuccio was with me. We went together to Erice, and after we came back we were so impressed by this experiment that we spent maybe three months working hard at a different approach to Bell’s theorem. We knew of course the proofs of Bell’s theorem and I had even published one original too, so it was not a mathematical problem. Still one had the idea that perhaps something was wrong and that perhaps after all Bell’s theorem did not exclude all possible forms of local realism. In other words perhaps Bell’s theorem was not general enough. If we had to accept those experiments; still we would try not to give up the local realism. We would rather give up Bell’s theorem. So we started building models which we never published, but we built many, many models of simple situations: tubes with balls rolling inside them two ways and half of the tube closed by an oscillating half circle, and the ball passing or not passing depending on whether it arrived in the right moment — many, many, many classical situations to see if we could invent something which was according to the philosophy of local realism but in violation of Bell’s theorem. And we could not. So that failure was for me a great strengthening in the belief in the generality of Bell’s theorem. We tried the hard way to produce the opposite and we could not succeed, so Bell’s theorem is really very general.

After that I invented many other proofs, some probabilistic. The strongest proof is probabilistic and based on Popper’s propensities. That is the most general proof I know about Bell’s theorem, because you do not even need elements of reality in a single object. Local propensities for statistical ensembles, they are enough. If you don’t have that form of reality you have no reality at all. It is ridiculous to abolish reality like that. So my conclusion was that Bell’s theorem was general, totally general, and that we should understand then better the experiments. Then started a long struggle against the naïve acceptance of the experiments, and finally I understood what was wrong with the experiments, was the fact that Bell’s theorem is something like this. It is a statement that a quantity B should be within minus 2 and 2. This B is a linear combination of correlation functions. And quantum mechanics says that B is 2.83. So we can make a linear graph and say this is minus 2, this is 2, according to Bell’s theorem the quantity B must lie in this region. Instead quantum mechanics says it should be there, outside. But then we read carefully the original 1969 Physical Review Letters paper by C.H.S.H. and we discovered the existence of additional assumptions in their proof. That Physical Review Letters formulation of Bell’s inequality is not equivalent to the original one because it has one additional assumption, one more. And in fact the quantity B is restricted to be between plus and minus 0.015, it is restricted to be in a very, very small region. And then experimentally it is found to be here outside this small region of the stronger inequality. I call the original one “weak inequality” and I call this “strong inequality”, for obvious reasons. The strong inequality is violated experimentally, the weak inequality has never been violated by any experiment. So what has been violated by experiments are probably the additional assumptions that bring you down from 2 to 0.015.

Just one question. Could you talk a little about those additional assumptions? More a little.

Yes. All what I am telling you I have written in books and in papers and it depends on what you want from me. If you want details I can give you.

No, not details. Just an idea to record.

There are four additional assumption proposed by different authors for deducing the strong inequality. Additional assumption 1, additional assumption 2, additional assumption 3, additional assumption 4, were proposed by different authors. They are equivalent; conceptually they have been shown to be all the same. Their implication is always the same. There is no difference. But let’s say if we take one of them, for example the second one, aa2, it is a statement comparing different setups that could be used to study the problem. Suppose you have a region in which there is a polarizer and there is a detector. The polarizer has a certain orientation, and suppose that a photon just passed through the polarizer, so the photon is now in this region between the polarizer and the detector, the photomultiplier. Well, aa2 states that the probability for the photon to be detected is independent on the orientation and even on the presence of the polarizer. That is, the probability to detect this photon should not depend on having crossed the polarizer. This is a very strong limitation of the hidden variables because if you have a hidden variable you expect it to be modified by the polarizer. This is very important in particular if it is the same variables that makes the photon cross or not cross the polarizer and makes the photon to be detected or not detected in the photomultiplier. So if it crosses it will probably be modified. So the situation generated by the additional assumptions is like saying: now we have to fight, but you bind my hands behind my back, and after we fight. Obviously you win.

Okay. So retaking the time line, you were talking that a little after 1974 you and Garuccio have arrived to understand the existence of additional assumptions in Clauser, Home, Shimony and Holt demonstration.

Yes.

So after that.

After that?

So after that, related with the Aspect’s experiments that came later.

When Aspects experiment came there was already this distinction between weak and strong inequalities available. I was not surprised anymore that Aspect found a violation of the inequality because it had already been found by Fry and by Freedman and Clauser. The answer was exactly the same. And of course I told Aspect about this, but he did not seem to be very interested and he continued to go around in Europe and to give seminars in which the point of the limited meaning of his experiment was not made. They have been carefully silent about this difficulty. And you can see what people believe by whether they talk or not talk about additional assumptions. Everybody knows. I mean if you take an expert, he really knows about this. But he does not like to talk about it because he hopes that quantum mechanics is correct and that hidden variables aren’t there.

That is the amazing reaction. So with Aspect it was exactly the same. We published more careful papers, this time with Lepore, another one of my pupils who is now a high school teacher in Ban and Lepore helped me a lot in publishing a paper in which we had the analysis of the additional assumptions one by one showing the consequences one by one why the same criticism applied to all four of them. So people know that I do not believe that experiments of Bells theorem are final and that newer experiments are needed — for example the experiments about which I gave you a couple of papers yesterday, with K zero, K zero pairs which allow one to avoid the additional assumption. So for those particles the strong inequality and the weak inequality — perhaps they are not identical but they are very near for kaon pairs, so it is easy to discriminate quantum mechanics really. My feeling is that (talking on the average of course every man lives the situation in a different way) on the average people do not want to discover the truth about Bells theorem. I think that is the average case. Although maybe this average does not correspond to any single human being, as it is very difficult to imagine a physicist who thinks, “Probably Bell’s theorem shows that quantum mechanics is wrong, but this I will cover up.” This is clearly not the right psychological way. But finally it comes out to the same.

Okay. As you spoke about your proposal of experiments in particle physics, I would like to know if there was any reaction among physicists in the sense that somebody could say it’s possible to do these experiments. Have you had any reactions of this type?

Well, I was invited to a conference in Rome. In the year 2000 there was an international conference at the University of Rome devoted to such matters. More recently I was invited in Pisa in the year 2001 at the conference Kaon 2001. That is the title of the conference, Kaon 2001, and I presented this idea to the particle physicists who were there and so on. I presented my ideas to the particle physicists and they were interested and there was positive reaction at that moment, but I do not know if any real plan for experiments is going on to test this. I still hope that eventually it will be done. It so happens that the best place in the world to do such an experiment would be Italy, Frascati, because we have a phi factory accelerator called Daphne. You know the phi particle?

No, no.

It is a vector meson with mass thousand and twenty MeV, I believe, that can decay into two kaons.

Okay.

It can decay in K plus K minus, or in K zero K zero bar. So this is an interesting channel. Other places where my experiments could be carried out are Stanford and an accelerator in Japan, but they are not kaon producers, but they rather produce B mesons. B mesons are more difficult to analyze because they have a higher mass, many decays and it is more complicated. The best would be to do it really at this lower energy accelerator. As you see thousand and twenty MeV is just above the threshold for a two kaon production. But I am not willing to disturb people too much. I publish, I send papers, I go to conferences to present my ideas, I talk to single people when they come, but after that I am not the type of person who runs after people pushing them with too much pressure. It is up to them to understand that there is a problem and that they could do the experiment.

Okay. I would like to put two minor questions still concerning experiments. As you spoke about the 1974 Erice Conference, I would like to know if you remember the presence for instance of Anton Zeilinger at that conference and the discussions about the use of neutron interferometry in the foundations of quantum mechanics.

I don’t remember Zeilinger. I met him later and perhaps he was also there. I don’t remember his participation at that particular conference. If he was there he probably was not very relevant to me at that moment. I have done a lot to promote neutron interferometry as a tool to study fundamental questions. I was particularly connected with Helmut Rauch at Vienna and Rauch is a practical man, less philosophically inclined than Zeilinger but a very concrete physicist. And I believe that neutron interferometry shows very clearly that you have wave particle duality again, because the experiments are done with very low intensity beams and so there is no problem for having neutrons one at a time. It is true. And also you do not have neutron bunching. For photons you have a small fraction of cases with photon bunching. In my opinion it is not very important, but anyway some people believe it is. But neutrons have spin one half and you do not expect any correlation, so you really have neutrons one at a time. And still you see that the neutron interferes with itself. It is fantastic. So at every conference I organized or I was connected with I tried to do my best to have neutron interferometry represented. And that was important to me to the point that when I teach, for example this year I teach Quantum Mechanics, I always include a section on neutron interferometry to show how it is important for understanding correctly quantum mechanics. Although it is difficult there to imagine an experiment that would reveal the reality of quantum waves, in the case of neutron interferometry you have many, many experiments that can be interpreted in the causal way. Unfortunately, also in the Copenhagen way, so it is more weak. With photons you can do better. With photons you can solve the ambiguity as I have shown in several published papers.

Okay. Letts, to return to a question we talked yesterday, you came to Bari, you told us yesterday, you came to Bari at end of 160 to be the first theoretical physicist of the group. There was only experimental physics in Bari at that time. So could you talk a little about the relationship between your theoretical work and the experimental work done in Bari at that time and after that during the last years? So what connections there were between your theoretical work and the experimental work done in Bari.

No, really very little or nothing, because my ideas were on the foundations of quantum mechanics and they were about Bell’s theorem. You see we had a strong connection with Catania. In Catania there was a physicist, his name was Vittorio Rapisarda, who was willing to do experiments in the foundations of quantum mechanics. And unfortunately he died in a car accident. It was terrible because he was traveling from Catania to Ban to come to visit us. Periodically we went to Catania or he came here, and that day was a Sunday. I was waiting for him and he was much later than we had thought he would come, so I was giving up. I thought I would go out. And the telephone rang and it was the police saying, “Are you Franco Selleri?” yes, yes, “Your friend has died in an accident.” He was with a lady. The lady was badly injured but survived eventually. So I went to the place where the accident took place. And that was also a scientific tragedy, because Rapisarda was really available to implement all the variations that we thought important to the standard Bell’s theorem, and other people like Aspect were not available to do anything different or to reconsider what had been done. But Rapisarda would probably have done the experiments we suggested. So that of course in practice was the end of the Catania experimental activity on the foundations of Quantum Mechanics. Other people here, as I told you yesterday, were particle physicists. At the time it was not possible to find a connection with the foundations of Quantum Mechanics. Applied laser physics is not good for doing this type of experiments, so we had little interaction. We did not find a joint way to do research, experimental research in Ban. I was not capable of influencing them enough. But it was also very difficult because they had different lines of research.

So I would like to put a last question. Your work, your theoretical work in the last years, I think that it was mainly a work in the sense of criticizing the foundations of theoretical quantum mechanics. So I think that you were not so interested in creating immediately a new quantum mechanics, or at least an alternative approach to quantum mechanics, but in a way as there were, and there are several alternative approaches, the question I would like to put is, what approach to create a modified quantum mechanics have you found particularly promising?

If we are to build a theory which satisfies the requirements of local realism it would be a theory very different from quantum mechanics. In quantum mechanics the trouble in my opinion is with the Hilbert space, the adoption of the Hilbert space. Because you see, if you have two systems and you have two wave functions, psi one and psi two, you multiply them. You make the tensor product, and this is the state vector for the system. But then of course you can also make things like this, psi one times psi two plus psi one prime times psi two prime, because if the first one is a possible wave function and the second one is a possible wave function, then the superposition principle, the Hilbert space being linear, makes their sum also a possible wave function. And this is against local realism. Such a wave function always contradicts local realism as it leads to the violations of Bell’s inequality. So we need a completely different approach. I find particularly promising the approach based on Popper’s propensities. That is to say, elements of reality for statistical ensembles. That is a most general local realistic formulation of physics.

But of course the theory needs to be done. We have worked on this and published some papers and shown that Bell’s theorem is satisfied and so on, but we did not even try to build a general theory of the type of quantum mechanics, as general as quantum mechanics. You see, perhaps I was wrong, but for many years I thought that the right approach to the problem is to convince experimentalists to do a new experiment and show eventually that quantum mechanics is wrong and that after obviously everybody will look for a new theory. I had undervalued the strength of the Copenhagen philosophy even in the head of experimentalists. So that is very difficult to achieve anyway. And then we are still waiting for an experimental clarification here. But I can tell you that the next one will not be a theory similar to quantum mechanics. It will be very different, with a completely different structure — without a Hilbert space, without operators and with a completely new approach. Of course no uncertainty relations, even though Heisenberg’s inequalities could survive with a different interpretation.

Okay. I think that we have finished with the interview, but I would like to know if there are some questions you found interesting to let your record before I finish our interview.

Thank you. I would like to make two points. One is the following. Concerning the foundations of quantum mechanics, everybody knows Bell’s theorem and Bell’s inequality. In Ban Garuccio, Lepore and I could show that actually any linear combination of any number of correlation functions satisfies an inequality that is a consequence of local realism. Bell’s inequality is the simplest one of such an infinite set of inequalities. The more complicated inequalities are again consequences of local realism as I say, but in some cases they are consequences of Bell’s inequality itself and so they give nothing new. There are also many cases in which you can easily imagine a correlation function that is compatible with Bell’s inequality, but not with the more general inequalities we have found, so that you see local realism is a very simple philosophical idea, but has a very complicated mathematical expression because it is expressed by an infinite set of inequalities. Only if all of them are satisfied then you can say you have local realism. For example if you find that Bell’s inequality is valid, then you can say quantum mechanics is wrong. But you cannot say local realism is certainly correct, because maybe the other inequalities are not satisfied. How do you know? You should check over all of them.

Okay.

So that is our first result, that it is very rarely mentioned but that we consider very important, because we have really worked out the consequences of local realism generally.

Interesting.

And then a second point I would like to make is to stress again the importance of what I have done in the foundations of special relativity. Because now we have a completely new theory which is different from relativity. You see, you know of course — you know the Lorentz transformations which are the following: X’ = (X-VT)/R. You remember these transformations.

Yes, yes.

Where Y’= Y, Z’= Z and T’= (T-XV/c2)/R. These are the Lorentz transformations. Now, I believe that they are not correct. The good transformation of the physical reality are the following ones — X’, Y’, Z’ transform in the same way as in the Lorentz transformations, but T’= TR. Much simpler. This is a theory in which you have absolute simultaneity. You have a space that is not mixed with time as in relativity, but still moving clocks slow down. And it is a theory having a privileged system so having the Lorentz ether and explaining all the experimental data. I published a book. That is my newest book, “Lezioni di Relativita” in which there are five chapters. In the first three chapters there is a review of standard relativity and in the final two chapters there is a completely new approach and there is a full development of this type of ideas. So I was unable to build a new quantum theory, but at least I built a new relativity and I strongly believe it is correct. Thank you.

Thank you.