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Interview of Carroll O. Alley by Joan Bromberg on 2006 May 19,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview discusses the "delayed choice" experiment carried out in the first part of the 1980s. Also touches on Alley's relations with John Archiball Wheeler, equipment invented for the experiement or adapted for it from previous work on lunar ranging and atomic clocks, and the experiment's role as precursor to Yanhua Shih's experiments using photons apron spontaneous parametric down convers in.
This is going to be an interview with Professor Carroll O. Alley, A-L-L-E-Y, of the Physics Department at the University of Maryland, College Park, and it is going to be recorded on Friday, May 19, 2006. The interviewer is Joan Bromberg.
I’m was going to say that the EPR experiments using spontaneous parametric down- conversion had their origin in the delayed choice experiment. I was informed by Vladimir Braginsky, of Moscow State University, on a visit here when we were starting up the delayed choice of work by David Klyshko in Moscow, on calibrating absolute quantum efficiencies, measuring absolute quantum efficiencies using the two photons that are produced simultaneously in this down-conversion process. By having two detectors you know that if a photon is incident on one, another one is incident on the other detector. And, just by counting you can get the absolute quantum efficiency.
So, that sounded to me like a very neat trick. Yanhua Shih was just beginning his research with me so I asked him to make a realization of that in the laboratory so we could calibrate our photomultiplier tubes for the delayed choice. And he did that, and I had had some discussion of this with Braginsky, that there would be some interesting quantum physics to be explored on a fundamental level using the two photons. And, when it came time for Yanhua Shih to do a thesis, I proposed that he see if we can make the Einstein-Podolsky Rosen-Bohm correlations using the spontaneous parametric down-converted photons. We worked for several years and we did a number of things that we could have done better and we finally realized it. And that had a big influence on the study of EPR and entangled states, and so on. It was picked up by the group at the University of Rochester and we told them some of the techniques to use. Oreste Piccioni, when I first announced this at this meeting in ‘86, I guess, at the Hitachi Company, he thought it was the biggest progress in fifty years in studying EPR. So, Franson picked up on it and many other people picked up on it but it had its roots in my wanting to know the absolute quantum efficiency for the delayed choice.
Okay, so I’m going to ask about delayed choice. I want to know this, your initial exposure to Wheeler and his delayed choice ideas, how far does that go back? You spoke in your 1981 paper that is in the Wheeler and Zurek volume, you spoke of having heard him speak at the Einstein Centennial Lecture, which I guess was in 1979?
Some of them were delayed a year. I think his may have been one of those, but it was certainly ‘79 or ‘80. That’s right. And, I’d heard him speak at other places as well. You know, my association with Professor Wheeler goes back to when I became a graduate student at Princeton in 1948. I think it was in the fall of ‘48 that I attended his lectures on analytical dynamics. And then I attended his initial lectures on general relativity in ‘53, I guess, after he came back from Project Matterhom. But, specifically on the delayed choice, I can’t be sure, but it was in the late ‘70s that he began to emphasize this. He thinks it’s the strangest part of quantum mechanics that, what you determine to measure now determines what shall have been. One of the few uses of that past tense. Although it all has to be pretty much qualified.
One thing that you did not bring out in your discussion Monday, Wheeler even applied the delayed choice to measuring the interference produced by gravitational lensing of distant galaxies beyond the clusters of galaxies, where you can decide to measure which path, whether it went around this way or that way, or whether you got interference — it’s never been realized in an astronomical sense. But he emphasized that the scale didn’t really matter. You could have very long, very large distances involved. And, there is this very strange connection that the delayed choice seems to bring out. There are some ideas as to how to understand this in a proper way, which are really due to Professor Hussein Yilmaz, with whom I have been privileged to collaborate closely for the last couple of decades. But, that’s another issue. I do feel very strongly that modem physics tends to lose sight of the goal of physics to understand or comprehend phenomena and not just be able to describe them. And, you know, we had a go-around Monday with Michael Fisher on some of those. But, I think that’s of great merit, Wheeler’s insistence on how we don’t understand things rather than what we do understand. Did you look at the letter?
You had a copy of that already?
Where he wants to “re-arouse the sense of desperate puzzlement,” of the comprehension of these quantum phenomena.
Well, when you were talking over. . . by the way, do you have any particular memories of that particular lecture that we ought to talk about, that he gave, or about what he said to you about doing the experiment or about your conversations with Wickes? Or...
Well, that was, that was when my vision was still good enough that I didn’t record lectures so I could see them better afterwards. Let me try to think. Wheeler is a, is a poet of physics. He’s a consummate wordsmith. I mean he can make brilliant analogies and it’s almost like Time Magazine. “Backward-ran sentences until reeled the mind.”
Yeah, I know. I mean, from reading.
He’s always trying to get a unique slant on a way of putting things. And, it occurred to me, you know, that we had devoted many efforts, many years and much effort, learning how to detect single photons and time them accurately. Because we had to do it for the lunar laser ranging. The idea of putting reflectors on the moon originated at Princeton in Professor Robert Henry Dicke’s group, and I was persuaded by Professor Dicke to be the principle investigator of the NASA experiment that we finally got done. And we’ve ended up with three NASA-placed reflectors on the moon, the first one by Neil Armstrong himself, and the other two on the Apollo XIV and XV missions.
And then, I told my colleagues in Russia, first Prokhorov, I guess, in a meeting in Paris that we were planning this lunar ranging and they wanted to be involved. And so, they got some reflectors made by the French, and put two of them on the moon with the Lunahod. Lunahod roving vechiclesl and 2. Lunahod 2 has worked. Lunahod 1 may have worked in the beginning but has not been used regularly. But, we had to learn, because of the very weak signals that come back from these reflectors, how to detect and time individual light quanta.
I’ve never been happy with the understanding of the light quantum. It was a brilliant introduction by Einstein in 1905. You probably know the history of that? He was led by entropy considerations and analogies with the dynamical theory of gases. Yanhua Shih tells the story that he talked to a number of physics professors at Maryland, he was interested in the light quantum also, and everybody knew what the light quantum was except me. And so, he preferred to work with me rather than some of the others. [Laugh] But, even apart from comprehending the light quantum we realized that we had the experience and the techniques to perhaps realize this delayed choice.
Now, did you, were you able to take that timing equipment right over —
to delayed choice —
or did you have to —
make it work?
We used event timers in the delayed choice.
And it’s just, you know, just as they were in the circuits?
Well, one of the things I added to the experiment was the randomness. Because of all the discussion which sort of traces back to Eugene Wigner, who was another great influence on me. He was one of my teachers at Princeton and on my thesis committee. This “Wigner’s friend” idea, you know — does the measurement reside somehow finally in the consciousness. And so I decided that to eliminate any considerations of that sort, we would have an automatic random choice that would be made by the apparatus rather than, in advance, programming in some change or something of that sort.
Did Wigner react to this whole experiment? What was his...
He was shown the experiment. We had a Wigner symposium, I think, in about ‘83 when this experiment was well underway, and I have a picture of Eugene Wigner and Arthur Wightman. Wightman was another one of my teachers at Princeton, and I actually worked with him for a couple of years on abstract quantum field theory before I decided that I could make more of a contribution to physics by doing experiments, although I never lost my taste for theoretical physics. But, no, Wigner never did — there was this conference in Bad Windsheim in 1981, organized by Marlan Scully. I take it you have talked to Marlan?
Because I wrote this article about him. So, I talked to him at some length.
Yes, I’d be happy to get a copy of your article.
If you’d make it available. But, Marlan wanted me to talk about the lunar ranging experiments and about the proper time experiments that we had done with atomic clocks. My thesis at Princeton with Professor Dicke was on the rubidium optically pumped atomic clock. And that now, fifty years later, is the best performer, that type of clock is the best performer in the global positioning system, and it’s being vigorously developed using some of the techniques that we explored at Princeton in the mid ‘50s. Now the European Galileo Global Positioning System. Let’s see....
I was asking you what Wigner’s...
Yes, what Wigner...
.. . thought about this experiment.
Yes. Yes. And at that conference that Marlan organized at Bad Windsheim, Wheeler urged me to also give a talk about the delayed choice. We had not finished our work, I mean, but we had it, you know, well underway. And, it was there that Herb Walther heard about it, and unbeknownst to me decided to do something on his own. But, Wigner was in the audience, and he heard about the delayed choice plans, but I don’t remember any explicit discussion he had with me. I do remember that when I talked about the atomic clock, you know, again these were the, the first careful real measurements with actual clocks of the relativistic effects on time.
When a clock moves, it runs slow. When it’s raised up in the gravitational field it runs fast. And we were able to do this with the help of the Navy, with the Navy P-3C. In fact, these experiments were supported by the Defense Department because of their impact on the global positioning system. You have to do relativity right in the GPS for it to work. And when I mentioned that fact in one of my lectures at Bad Windsheim, Eugene Wigner beat on the table with his hands. You know, in Europe applause is sometimes, enthusiastic applause is beating on the table. And also, at that meeting Professor Wigner came up to me and said, “Please call me Eugene.” I was very flattered. You know, as a graduate student, Wigner was the great man in theoretical physics at Princeton.
So, you had arrived?
I was very much pleased by his asking me to use the familiar form of address. But, on the delayed choice, I don’t remember any reaction from Eugene.
Now, as far as Schleich’s experiment goes, now it’s in the 1986 Hitachi Conference proceeding?
I think so. Yes.
And, the first part of it is just like yours except they put in an optical fiber to extend the interferometer.
And the second part is a beats experiment, they’re doing atomic beats. How did that, do you have any idea how that related to what you were doing?
Well, it was inspired by Herb Walter was impressed by what we were doing as reported at the Bad Windsheim conference in ‘81. And, without telling me they went ahead and did some things. The student who was working with me was a little slow to complete the experiment. Well, all graduate students have to learn and Oleg Jakubowicz was more interested in computers than he was in physics and so he had elaborate computer programs to take the data from the event timers and assemble them. And, at the first Hitachi Conference on the quantum, was it the Foundations of Quantum Mechanics in the Light of New Technology? So the title bears on your theme, we didn’t have data. We had the experiment in great detail, which I described, again at the insistence of John Wheeler. He thought it should be described. It was Wheeler’s idea to use cold neutrons, which he passed on to Warren Miller.
Miller had been a graduate student at Texas. But that never got done.
I was wondering about, that.
Well, it was the Rauch work which is very good. Let’s see you were asking me about the work of...
Well, I was asking about you the Wolfgang Schleich work. At one point in his autobiography there’s a picture of Wheeler surrounded by some of his Texas group and Wolfgang Schleich is in back.
Yes. He came to work with Wheeler after that work, I think. I’m not sure of the date. I think of Schleich more as a theorist, and I had forgotten that he may have been involved in that experiment. Marlan Scully, of course, was affiliated with Herb Walther in Garching. Marlan has always worn several hats and I don’t remember the time frame.
But, getting back to remembering Wheeler, this talk at Maryland, Bill Wickes was an astronomer, was a physicist but he was working on astronomy. He’d made observations of binary systems, again at Princeton with Professor Dicke, and had come to Maryland to work primarily with Douglas Currie on interferometers to study binary systems. But he was a friend of mine. And after the talk he and I discussed whether we might be able to realize Wheeler’s delayed choice, and Oleg Jakubowicz had also heard the talk. I’m not sure just when he came to me to ask if he could join in the efforts.
So, it wasn’t that Wheeler came and said, “Look guys,”
“I know you’ve been doing this laser ranging”?
No. No. No. No. It wasn’t that. And I think I discussed with John whether this would really be a satisfactory example of what he had in mind. And he really thought that it would, and encouraged me to do it. And so we wrote this up and the first write-up we couldn’t do exactly what we said. That was what was put in the book by Wheeler and Zurek. But we tried another approach, which we gave up, and we finally ended up with the one that is published in that Hitachi Conference.
I remember when the Wheeler-Zurek book came out. It was about the time Wheeler had persuaded me to organize a birthday celebration for John Toll. You know, John Toll was Wheeler’s student. He was his right-hand man in the Matterhom Project, and when Wheeler was asked to become chairman at Maryland he declined but recommended John Toll. So, he became chairman at the age of twenty-eight.
In 1953 or so. But, how did I get off on that?
I don’t know. We were talking about Hitachi. Was that an interesting conference?
The ‘83 one.
Oh, the ‘83. That’s right. Wheeler called me up one day and said “We should celebrate the sixtieth birthday of John Toll.”
He thinks it’s a good thing to do these celebrations, and he persuaded me to do it. And, it was at that celebration, I think, that Wheeler gave me a copy of the book that he and Zurek had put together, and he inscribed it to me with the phrase, “Who makes things happen.” [Laugh]
I have the book down in my library, downstairs. So, Wheeler has always viewed this “great smoky dragon” experiment as sort of the main delayed choice. I mean the interpretation of Alain Aspect’s experiment and others, they weren’t consciously involved with the time. Whereas we were, and I think perhaps the biggest influence of the delayed choice was in the later EPR, spontaneous parametric down conversion work. Yanhua Shih realized Marlan Scully’s quantum eraser using the down-converted, you know —Yanhua Shih has done marvelous things with this EPR, with this entangled state production.
That - well, we were...
But, we were indebted to David Klyshko for first using that technique for the calibration. And then Klyshko became very interested in the foundations of quantum mechanics, inspired by the experiment that Yanhua Shih and I had done. He has a great book on photons — I can’t remember the name, but it’s a very fine book on quantum optics. Klyshko died of a brain tumor about a decade or more ago.
Was it Wheeler who invited you to speak at Hitachi, at the Hitachi Conference? How did you come to go there?
He urged me. He urged me to speak there. That’s right. He wasn’t organizing it, but he informed me of the existence of it. I didn’t know about the conference. And so, he informed me about it and urged me to present there. Yes, he pushed the delayed choice. [Laugh]
He wanted you...
He wanted to have it exposed at a conference. [Laugh] Yes. So, even though we didn’t have data I decided I would describe what we were actually doing.
I see. One of the things that I’ve always been wondering about and somebody has to find out, is why these guys at Hitachi suddenly decided to get involved? Was it just that they, sort of a nationalist thing to run this conference?
Well. They were interested in electron microscopy and there was a good experimenter there, whose name escapes me now.
Tanemura? Isn’t that it?
I think so, yes. Who saw that he could measure the Bohm-Aharonov effect using some of the techniques that he had. And so, I think Hitachi was proud of that, and they were aware of the interplay between foundational questions and newer techniques. And so, they decided to have this conference. There have been a number of them since then. There was one every three years. There was one in ‘83, one in ‘86. I think...
You went to, in ‘86 too, didn’t you?
Yes. Yes. Yes. And, I think we sent Yanhua Shih in maybe ‘89 or some of the subsequent ones. I haven’t been back to any of the subsequent ones, but Yanhua Shih has gone to some of them. Well, I think, yes, there’s probably some nationalism in Japan, and the pride of Hitachi in its technological prowess, and the connection to fundamental physics is very real.
It is my fervent hope, and I think John Wheeler’s also, that these developing techniques with single atoms and single-light quanta and all of the marvelous fractional Hall effect studies in solids, might finally reveal the real nature of what’s really going on in quantum mechanics. Yes, I think I mentioned at your talk that the creators of quantum physics, Einstein, de Broglie, Schrödinger, all resisted this imposition of the Copenhagen formalism as being premature. But, I was told by Vigier that de Brogue had told him that those three were literally shouted down at some of the Solvay Conferences and other conferences. That’s not made much of in the literature. [Laugh] The students don’t know about that. There’s this sociological aspect to the history of physics.
Well, that famous anecdote of Schrodinger’s about how he was in bed with Bohr sort of standing over him. [Laugh]
That’s right. And he says, “If he had known he would get into that he would never have studied the subject.” [Laugh] Or “worked on the subject.”
But, I want to go back one moment to the Hitachi Conference. When you read the proceedings it sounds as if you have a bunch of foreigners you...
Anyway, I was saying that I get this sense that you, and Wheeler, and Shimony, and Greenberger, and you know, all these people, I guess, — I guess Aharonov was there too?
I’m not sure. Aharonov? Yakir Aharonov?
Oh, Aharonov. Yes. Yes. He may have been. Yes.
That you were...
Frank Yang was there. C.N. Yang.
Right. That you were all talking to each other and the Japanese were just listening because in the discussion part it sounded — and Rauch, Helmut Rauch was there too.
I wondered if that impression really...
I didn’t quite have that impression. I was there hoping to learn, get some insights, [Laugh] and as well as present things. Wheeler was sort of the star of the affair. He was paid much attention to by the Japanese. And, yes, I didn’t have that sense.
Okay. Then I’m just wrong.
But, it was all in English, but it’s hard to understand Japanese speaking English sometimes. [Laugh] And, you know, I don’t think you’re wrong. It was a forum.
But in all of these discussions the simpler the experiments you can do that bring out some of the strange things, I think, the better it is for ultimately grasping the nature. That’s one of the reasons I wanted to realize the simple form of the delayed choice. I mean, it does have a Rube Goldberg aspect to it. [Laugh] But, it’s graspable, I think, whereas, one of the experiments done in Munich, at Garching, seemed to me to be a little — you need to know a lot of physics to grasp the essence of it, whereas the Mach-Zehnder interferometer was a realizable thing and it’s something like the two-slit in that you have a beam splitter and two routes for things to go. The “welcher-weg” experiment, I guess I should call it.
What happed to Jakubowicz? Did he go into computers?
He went into computers. That’s right. He was at the State University of New York at Buffalo, and he did come to the conference that Dan Greenberger held in New York City, at the World Trade Center, I think. Was that in.
Eight-six, that one?
Eight-five or ‘86, ‘86 is when it was.
That the New York Academy of Science...
New York Academy of Sciences, that’s right.
Then there was a second conference that Dan Greenberger organized and it was held at the University of Maryland, Baltimore County, because of the presence there of Yanhua Shih, carrying on these two-photon experiments. Yes, the New York meeting was in the World Trade Center.
I didn’t realize that.
That was certainly a very interesting meeting, that ‘86 one.
It was. Was that one dedicated to Wigner or to Wheeler? I’m not sure.
I don’t remember that either.
I think that was dedicated to Eugene Wigner.
But, I know that Vigier spoke. Wheeler spoke, and. . . this whole thing about the world being created in order that there be a community of observers. [Laughj And Vigier spoke, you know, on the entirely opposite end of things. I think that’s where he said that “This idea of a phenomenon is only a phenomenon if it’s an observed phenomenon is a complete tautology if you realize the word ‘phenomenon’ means appearance.” So, it must have been quite an interesting meeting between the two of them. I don’t know, but one wonders.
I don’t know how much personal interchange there was. I may have tapes of that meeting. I started to record things even before my vision got worse, just to have a record. You know, I thought the video camera was a marvelous asset and I’ve got lots and lots of these recordings. Jeff Bub organized a conference over on the eastern shore many years ago, Foundations of Quantum Mechanics, and I have the tapes on that. They’re not well organized, but they could be resurrected. I’ve been wondering whether the Institute of, whether the Institute of Physics might like these at some point.
Well, I’m going to make a little plug that you ought to talk to them at some point about your papers in general. They are very interested to make sure that parts of physicists’ papers are preserved. And you should perhaps just talk to the archivist and say you’ve got tapes of this and that, and papers, probably, on the ranging experiment.
Yes. There’s a lot of material on that lunar ranging. I have not published as much as I should have. It’s not easy for me to write things down. That’s a good suggestion. Thank you.
As I say, the archivist — I mean, you’re right here, and the archivist there who is overseeing collections of physicists all over the world. His name is Joe Anderson and he could really sit down with you or talk to you on the telephone, say.
No, I don’t think I’ve ever met him. Who’s the librarian there?
You know, I think that he is officially the head of the library.
There are a number — the only librarian I know there is Sandy Johnson, but there are a number of people there who are in the library who are very helpful. But, usually people leave their papers, you know, deposit their papers at their own institution.
Wheeler has his papers at the American Philosophical Society in Philadelphia.
But, usually people arrange to, you know, put them in the university archives.
Yes. Did Ken Ford help him organize his papers?
I have no idea.
He helped him organize that autobiography.
Well, anyway, that’s just a little plug I’m putting in for the sake of historians. Well, did you have — now, in that experiment you’ve got the timing equipment, you’ve got the laser. You’ve got the Pockel’ s cell? Was that all pretty straightforward? You’ve got computer readout? Was there anything that needed a lot of development? You said the experiment took — What had to be done to make that experiment work? That is really what I’m asking.
Well, I had to...
I mean, I assume Oleg J. was doing it and you were overseeing it?
Well, it wasn’t Oleg just by himself, It was my whole group. That’s another thing that ought to be emphasized. The lunar ranging development, where we had to develop the ground station and the timing techniques as well as the corner reflector on the moon required a staff of people. And so, happily, we had those people around and I had an electronics engineer full time and several technicians. We had also done the proper time experiments with atomic clocks in aircraft in the period ‘74 to ‘77. And, some of the timing equipment had been developed for that, as well as for the lunar ranging. And, I guess the main new thing, the laser that I used to realize Einstein’s comparison of distant clocks — you know, we had a laser on the ground at the Patuxent Naval Air Test Center, and we flew our clocks in a Navy P3-C which was an anti-submarine patrol which could stay up for long periods. We’d fly fifteen hours, round and round the Chesapeake Bay. And we would do the light pulse time comparison of the airborne clocks with the ground clocks, and I had gotten that laser from the, I guess, a program that the Navy had had in technology development. So, that was on hand. The main new thing, I wanted to have a random choice so we had to develop a random choice.
I see. So that was portable?
That was the new thing. But, most of the other equipment we had on hand. But, we had to learn how to use the Pockel cells. Pockel cells were a commercial product because of their value in Q-switching lasers but we still had to learn how to really use them and how to make it all fit and work together. But, the new thing was the random choice developed by my electronics engineer Charlie Steggerda, who had also developed the event timer that was useful in the lunar ranging and also our proper time experiments, and then used in several relativity experiments that we’ve done. One problem was the interferometer itself. I had wanted to have it stable so that we could deliberately introduce a phase difference between the two arms, but we could never thermally stabilize it sufficiently. So we finally had to resort to accepting a drift so that there would be a continual change of phase between the two arms. And I would alternate data taking with weak light, one photon every twenty laser firings, with a few minute periods of sampling with strong light. So we just resorted to measuring the phase and it changed in a fairly regular way. Any real experiment gets you into very practical considerations. And so, it’s the more mundane techniques of temperature control and things like that that you have to devote so much attention to rather than the more exotic new techniques.
One thing occurred to me I should tell you about. You probably know that the development of quantum theory by Planck was really a result of the desire of the German technologists to make better light bulbs.
You know, that’s why there were the measurements of the blackbody spectrum made very carefully in the infrared regime. And, the data didn’t fit, you know, the existing theories, and Planck had to do this interpolation, which again he did with entropy. But, the technological drive that prompted — I mean, there were the interests of the theorists in understanding blackbody radiation, but the detailed measurements, as I understand it mainly from the writings of Paul Forman — no. No. Not Forman.
Kuhn. Kuhn. That there was this practical side to wanting to know exactly how the energy was distributed from the radiating filaments of light bulbs. I try to tell the students that, you know, there is this interplay.
Well, this interplay shows so strongly in the work, your work that we’re talking about now between practical and extremely philosophical — I mean it seems to me.
Well, that’s true, I mean, the laser has all it’s practical advantages
But, on the one hand you’ve got the global positioning system and how that should function, all the way up to general relativity [Laugh] and to the nature of the quantum. I mean, these things are so connected in your work and in this whole episode.
Well, I guess that’s true.
That’s what the Hitachi Company is saying when they’re linking foundational things with technology.
That’s right. That’s right. I don’t know whether you, how much you’ve looked at the talk I gave at the Wheeler Fest. You said you looked at the first part of it.
I looked at your whole talk.
Oh, you did?
Yes. The next day I was able to.
The part about gravitation theory?
Yeah. And people arguing with you.
General relativity, contrary to the popular opinion, is not the greatest thing that Einstein did. It’s incomplete, and the equations are demonstrably incomplete. There’s no interaction between massive bodies in the Newtonian sense, in Einstein’s theory. I’ve come to that conclusion through my interaction with ProfessorYilmaz, who has found a completion to the Einstein work, Energy is not treated properly in general relativity. The stress energy tensor of the gravitational field turned out to be a non-tensor in general relativity, just a coordinate artifact. I don’t know how much you interest yourself in history of general relativity?
Well, I know very little about the physics. But, is this very much on the margins right now or is it pretty much accepted?
The new theory?
The theory of Yilmaz?
No, it’s viewed with hostility by the general relativity community, by and large. That was Bill Unruh who was arguing with me at the end. He’s a Wheeler student. He teaches at the University of British Columbia. He’s also done some work in quantum physics. But, I’m invited to speak at the next Marcel Grossman Conference, as is professor Yilmaz. We’re giving papers back-to-back. The — it’s ironic. I mean, the popular opinion is that Einstein has this great creation of general relativity, but he was wrong about quantum physics. I really think it’s just the opposite. I think his reservations about quantum physics were real and that all the work of this curved space time, it’s partially correct. I mean, the limited experiments I’ve done with atomic clocks and with the moon ranging, but I now know that the moon ranging cannot be adequately described by Einstein’s theory because it doesn’t have N-body interactive solutions. It doesn’t even have a two-body interactive solution. It’s a central body-test body theory. And in the moon ranging we have to take account of the earth, moon, and sun. This is done to study whether the Brans-Dicke theory of gravity had any support, which it does not seem to have. And, it was taken for granted that it would be general relativity. But, in the last few years I have realized that that’s not so. It has to be the Yilmaz modification, of general relativity.
I’ll ask you something which a little bit changes the subject. The only other person I’ve been reading in sufficiently to compare this, your attitude, was Edwin Jaynes, was he…
I knew Ed Jaynes fairly well. We were graduate students together at Princeton. He wrote his thesis with — well I guess he wrote a thesis on ferro-electricity for Eugene Wigner, and Eugene then wanted him to make it into the first book in the Princeton series in Physics. By the way, Professor Wigner wanted me to develop my thesis into a book, and to my regret I never did it. He used to call me up. He was editor of that series, he and Bob Hofstadter at Stanford. Hofstadter had been an instructor at Princeton. But yes, Ed Jaynes was a very original thinker. And, he wanted to see some alternative to standard quantum mechanics. We never interacted very much in later years. I would see him time to time at conferences. He died at too early an age. But, yes, I had great respect for Ed Jaynes. He was older than the other graduate students. He had been in the service. He was a few years older than the rest of us. Yes, he would sort of scoff at some of the belief in quantum mechanics and all of these strange things.
I want to ask you one very specific question. In my talk Monday I said that as far as I could tell by reading your papers you were very noncommittal about Wheeler’s idea about the present as influencing the past. Is that a correct characterization of what you were feeling, that one should do real experiments, but you were, it seemed to me, rather careful not to say that this proves that, you know, that there’s a great smoky dragon and so on.
Well, you’ve got to do better than just the “great smoky dragon.” I mean, I’m loath to commit myself on beliefs. I believe in causality. I don’t think what you determine now really determines what went on in the past. I think that the past determines the future. But how do you comprehend, how do you grasp experiments like the delayed choice? And, as I alluded to recently, the greatest physicist I know, who is unsung, is Professor Yilmaz. I have seen them all from vantage points at Princeton, and I taught at Rochester for a while before coming to Maryland. And, I think something goes both ways, in all these light quantum experiments, that is the source and the detector are really related to each other, and particularly in the spontaneous parametric down-conversion. This is a two-photon situation but it really involves influence from each detector on the other. This can be put in a formal mathematical way. It was done by David Klyshko, in terms of advanced waves, which he thought just a figment, just a way of— I think there aren’t advanced waves. There are waves that go in both directions. Dr. Yilmaz has a way of using two lines in some of these optical sketches, and the photon, the energy is exchanged in quanta — well, energy and momentum are exchanged in the quantitized form, but Einstein’s light quantum of just a little entity, you know, going out in some way, you have to have something on the other end. And, these are nebulous ideas. I probably shouldn’t even be saying them, because they haven’t been developed adequately. There is a transactional quantum mechanics which has been developed by John Kramer of the University of Washington, which I think has a lot of elements of truth in it. I mean, physics as an experimental science, we have to become familiar with the way things actually work. And, the conceptualization, ideas that we can use to grasp these things, are our creation. And, Professor Yilmaz has written on the theory of perception. He spent much effort and his view towards physics has been much influenced by his thinking about how we actually perceive and interpret. Yes, I can give you some of his writings. I don’t know whether I have — he has an article in the Boston Philosophical Society proceedings. I think in the mid ‘60s or so. But, we have to infer from what we perceive and we have to get the right ideas. And, that’s one of the reasons I think it’s so important to have as simple experiments as we can, so that we don’t get confused by complexity. That’s why I think the delayed choice was worth doing in the way that we did it.
I’m really actually very confused about delayed choice in general, because on the one hand nobody thinks that it won’t work. Nobody thinks that the delayed choice experiments will be any different from the normal experiments and yet everybody seems to be suddenly talking about it or doing it. I was just wondering whether that was just a fad that went through the physics community, or Wheeler sort of overpowered everyone? Or, I mean I can see why you did it and why even Schleich and Walther do it, because that’s early, but everybody is suddenly talking about it.
Well, it’s the power of Wheeler’s imagination, and imagination has to be disciplined by the real world. And, that’s the way a lot of modem physics, modem theoretical physicists, they don’t have anything to compare their ideas with. I mean, all the string theory and superstrings, and — well. It is peculiar that in a certain sense, you can change your mind at the last nanosecond as to what you want to measure and that, in some sense, determines what was. And that turns causality on its head. But, when you look at things, you know, more carefully, you can’t really influence the past by what you decide now. It’s your interpretation of what the past was. But what all this means to me is that there’s something deep still lacking that’s eluding us in the quantum world. And, as Wheeler points out in his letter to me of 1998, which was sort of the theme of that Wheeler Fest meeting, he thinks that once we uncover this it’ll be as powerful of Darwin’s theory of evolution. I think the interaction is the key. Physics, I think, in a sense, is non-local in that not only are particles related to their source, but also to the detection of them. There’s sort of an interplay that we have to grasp. That, of course, was Planck’ s insight explaining the blackbody radiation. It was interaction between bodies, not the radiations themselves. It was Einstein that brought in the quantization of the radiation field in his light quantum hypothesis, which certainly has a lot of truth to it, it is manifested in the photoelectric effect.
Humonst Wolcott Gibbs, on Time Magazine syntax.
John Archibald Wheeler and Wojciech Hubert Zurek, eds., Quantum Theory and Measurement (Princeton University Press, 1983).
Wheeler's imaginative name for the Maryland delayed choice experiment.
International Symposium on foundations of Quantum Mechanics in the light of New Technology, Tokyo 1983 (Physical Society of Japan, Tokyo, 1984).
Daniel M. Greenberger, ed., New Techniques and Ideas in Quantum Measurement Theory: Annals of the New York Academy of Sciences, Vol. 480 (1986).