Dr. Avshalom Cyrus Elitzur (Hebrew: אבשלום כורש אליצור; born 30 May 1957) is an Israeli physicist, philosopher and professor at Chapman University. He is the founder of the Israeli Institute for Advanced Physics and obtained his PhD working directly under Yakir Aharanov. Elitzur became a household name among physicists for his collaboration with Lev Vaidman in formulating the “bomb-testing problem” in quantum mechanics, which has been validaded by two Nobel-prize-winning physicists. Elitzur’s work has sparked extensive discussions about the foundations of quantum mechanics and its interpretations, including the Copenhagen interpretation, many-worlds interpretation, and objective collapse models. His contributions have had a profound impact on both physics and philosophy, influencing debates about measurement, the role of observers, and the ontology of quantum states. Elitzur has also engaged in discussions about consciousness, the arrow of time, and other foundational topics, including a recent breakthrough in bio-thermodynamics and the “ski-lift” pathway.
* The following text has been mildly edited from the full presentation and interview, which may be viewed here and below.
1. Welcome, Professor Elitzur. Can you tell us about your presentation today?
1. Thank you very much my dear friends, José Luis, distinguished guests, ladies and gentlemen. What I’m going to do in this talk, I shall not assume that you are well informed about quantum mechanics or even physics uh and I’ll take it as an advantage because I think ─ and I hope that I’m not being too arrogant ─ I think that you can use the interaction free measurements (popularly referred to as the Elitzur-Vaidman bomb testing experiment), the subject of this talk, as an introduction to Quantum Mechanics. So, if you don’t know almost anything about auto mechanics what I will do, I’ll begin just from the very beginning and then I’ll take you, for example, there is the double-slit experiment and other experiments which demonstrate how unique quantum mechanics is. I’ll take my own discovery, together with Vaidman, as an introduction to Quantum Mechanics. I do believe with all modesty that it’s a no less good introduction than the others. Let me say that this is part of a course that Yakir Aharanov and I are going to give which will be both an academic course and a popular course. That is you will be able to register and then get credit but you will also be able to just skip all the mathematics and enjoy the philosophy and the conceptual layer. Together with producer Tzachi Schiff from Israel we are preparing a very interesting course and this is the chapter actually discussing IFM (Interaction-free measurement).
Was there a breakthrough moment behind your discovery?
The key was the moment I asked, “What happens… if you give up one detector?” Nothing genius, nothing strange, just make the experiment slightly more simple. That’s it. I remember it was Thursday, and I was at the beginning of my journey. I said, “Come on, no way that nobody can think about it now after so many years.” You know all these people, Bohr, Einstein, Feyman, Wheeler, all of them would play with this. I can’t believe that none of them thought about the possibility of just omitting one detector. I’m saying this to the young students among you. If sometimes you encounter the thought, “Oh, I’m sure that somebody thought about it once.” Well, don’t be so sure.
So here’s what I was asking myself: I have only one detector. What is going to happen in half of the cases? This single detector is going to click, which means that I know where the photon was. It took the right path; it did not take the left path, and I have to pay for it. I’m going to get a click in the upper two detectors because interference was destroyed. That’s fine. But in the other half, fifty percent of the cases, no click is going to appear. What do you know now? The detector is silent. What do you know? You know that it has taken the left path. Remember, as crazy as quantum mechanics is, it obeys conservation laws, so it must be somewhere. If it’s not on the right, it must be on the left. So you know that it’s on the left. Now here is the question: Would you pay the price for this knowledge by destroying interference? Would interference be destroyed too? Let me give you a clue: If it had not been destroyed, I would get an invitation to Stockholm. When you find the Nobel Prize for discovering the uncertainty principles, if you find a way to disprove themselves in the principle, it’s another Nobel Prize. Was I going to get it? As you probably know, I never received a Nobel Prize, and I was not expecting to get it. But I have thought about the other possibility, which is almost as nice.
What is the other possibility you saw?
Perhaps even in this case when there was no click, interference is destroyed just because you know there was no interaction, single detector, there was no interaction. But you know that it’s taken on the left. But this is really strange. Think about it. Suppose just to show you how strange it is because actually something happened. The left-side detector up there clicked just because there was no click in the lower right-side detector. Suppose that the single detector’s battery is off. Would interference vanish? You understand that there will be no click here. Okay, the battery is off. No, it will not. There will be no click, and interference will not vanish. You will have zero percent on the right and zero percent on the left. Now suppose that you’ve returned the battery, and you get no click this time. There will be a difference. So do you understand the problem? You have two silences. In both cases, you had silence. There was a case in which you had silence because there was no… I know that you think that this guy is nuts. What is he talking about? But this is science. I showed you two cases: one in which there is no battery in the detector, so it can’t click, and it doesn’t click. Interference remains intact on the right; zero percent on the left when the photon emerges from this interferometer.
Now, I’ll show you another case in which a detector does not click, but it could have clicked because there is a battery. That’s enough to destroy the interference when it goes out. So we have to distinguish between two kinds of nothing. Right? There was no click, so the one kind of no click we should call nothing is the classical one, but we have to invent another word for the other kind of no click, which is quantum mechanical. I called it in Yiddish “gunisht.” I think that in Luis’s native tongue, it would be “NADA.” So can we say or can we now use “nothing” rather than “nada”? This is interaction-free measurements. You have nothing, but you have another, in whatever language, another word for nothing, NADA . But that nothing is something very classical, very physical, and that brings us to the question of the bombs.
You believe we have entered a new phase in the history of physics. Please explain.
In the history of physics, we’ve witnessed a fascinating evolution. Classical physics, exemplified by the cannonball, was all about determinism. You could precisely calculate the cannonball’s trajectory by knowing its initial conditions – angle and velocity. Newton applied this to planets’ orbits. Then came quantum mechanics, shaking things up. It discarded determinism for particles; when you emitted one, it turned into a wave spreading everywhere. Yet, intriguingly, even a single particle created interference patterns, depending on which slits it went through. This apparent unpredictability led to uncertainty but opened doors to technological advancements.
The two-state vector formalism added a new layer of complexity. Unlike the straightforward predictability of a cannonball, particles lacked such certainty in their final positions. However, once detected, you could backtrack to compute their past positions. This redundancy didn’t merely complement; it unveiled a unique feature. Quantum physics illuminated that, unlike classical physics, which knew everything from start to finish, here, you could only piece together the complete story in retrospect.
Further mathematical exploration revealed exotic physical variables, from negative mass to strange numbers, hinting at uncharted territories within quantum physics. This newfound complexity, albeit puzzling, offered fresh insights and potential breakthroughs, especially when considering brief moments of exotic physics occurring between measurements. While some may dismiss these ideas, taking them seriously might help complete the puzzle of quantum mechanics. In this evolving landscape, students, and young minds should be encouraged to question and explore, acknowledging that quantum mechanics remains incomplete, and its mysteries are yet to be fully unraveled.
What is the relevance of the two-state vector approach?
“Very briefly, in the quantum world, time runs backwards. Let me give you one example. Think about the EPR experiments. What do you have in the EPR? You have two particles coming from the same source, going very far away, and then Alice and Bob are measuring them. With Bell’s theorem, you prove that whatever Alice did to her particle affected Bob’s distant particle. We know that, but it’s strange. They are very far away. It is not so strange if you believe that the act of measurement goes backward because they have a common origin in the past. The two particles have been emitted by the same atom. So Alice and Bob may be measuring them on Monday, but on Sunday, they came from the same atom. How about some zigzag in four dimensions in Einstein’s four-dimensional world, space and time? The other things are going backward to the past, just like in science fiction movies. How come there’s something here and it’s here if you go out of space and time?
And probably this is a story. We have more than three spatial dimensions; you should also take time, and perhaps there is even more. So it looks like questions of non-locality are better explained by this time-symmetric formalism. And as I said, counterfactual events, an event that could happen but did not happen, but just because it could happen, it leaves a trace, as in the bomb non-explosion case. This, I think, can be best explained by this new approach. So this is why I think there is new physics here. I believe that there will be many future experiments in which people will show that very strange forms of matter and energy exist very briefly within the wave function, and it is this physical phenomena that give rise to the odd quantum mechanical phenomena that we see. Basic principle, perhaps basic laws of nature, who knows what is waiting for us there?