Can you tell me a little about your background, I know you were born in Russia, how is it that you came to Israel?
Prof. Vaidman: Ever since I was a child, I knew that Israel was my real home. In 1973, at the first opportunity, my family and I emigrated. At the time, I was studying physics at Leningrad University (now called Saint Petersburg State University). After arriving, I continued my studies for a year in the south at Ben Gurion University. I received my undergraduate degree in Physics and Mathematics after I transferred to Hebrew University in Jerusalem.
After that, I did my requisite three-and-a-half years of military service and then enrolled at the Weizmann Institute in Rehovot, where I received my Masters degree studying the calculation of electromagnetic corrections for sum rules in quantum chromodynamics. When I was deciding what to do for my Ph.D., I wanted something less specialized than the work I had done at the Weizmann Institute. I ended up studying quantum mechanics under Professor Yakir Aharonov
at Tel Aviv University.
What drew you to Physics?
Prof. Vaidman: I wasn't a very good student in high school. Fortunately, I had an excellent teacher named Dimitry Vinnizky. He made me fall in love with physics. I ended up transferring to a specialized school affiliated with Leningrad University and in my senior year, I took first place among Soviet high school students in the 1972 Physics Olympiad. Had I not won, it would have been almost impossible for me to enroll at Leningrad University, because I was a Jew.
One of the areas you're known for is interaction-free measurement. Was there an "aha!" moment when you realized that you could use the properties of quanta to do it? Where did that idea come from?
In fact, the idea came from Avshalom Eliztur
, the original idea was a joint effort between he and I. In fact, he came to me with the question and together we kind of realized how to do this.
But the original question came from Eliztur. He's a very interesting guy. At that time he didn't have a high school degree but had already been published. Now he has a Ph.D. in philosophy of science and he continues to work in the field of foundations of quantum mechanics, psychology and others.
I'm not sure this is the most important work of mine. I think most citations come from the work on teleportation
. I wasn't the who discovered it, but I was the first to introduce teleportation of continuous variables.
Can you tell me how the two ideas of teleportation and interaction-free measurements are related?
Somewhat indirectly. I have one paper that relates both of them to my belief in the Many-Worlds Interpretation. I think that both lead me strongly towards the Many-Worlds Interpretation
(MWI) of Quantum Physics.
Prof. Vaidman: Yes, I think that both of the ideas lead me to the MWI, because otherwise quantum mechanics is very paradoxical. When doing interaction-free measurements, you can get information from a place in which you have never been and nothing from there has ever come to you. So you've never visited the place, nothing has come from there, not even a photo or anything but still you're getting information that there is something there. This seems extremely paradoxical.
Now if I accept the Many-Worlds Interpretation, I say that all our interaction is about physics which encompasses all the worlds, and (referring to the bomb in the Elitzur-Vaidmann Bomb Tester) in one world, I find that the bomb is present without having any contact with it. But in another world, the photon encountered the bomb and it exploded. If my intuition is for physics which encompasses all the worlds, then the result is not so strange. In one of the worlds I know that I paid the price in another world where there was an explosion.
It would be a lie if I said that I don't sometimes struggle with your branch of Physics, but let me ask you something about your original thought experiment and how the MWI interacts with that: I think I basically understand the nutshell idea, which is that when the particle hits the beam-splitter, according to Everett, there are two worlds that arise from that moment--one where it is deflected, and one where it passes through. But the bomb is either a dud, or a live bomb in both.
Prof. Vaidman: There will be three worlds, but in the certain world--our world, I know there is a bomb, I know it is good, although I didn't touch it and I didn't blow it up.
But if we back up, the bomb itself is the same in each world, right? According to the MWI, the moment the bomb is built, there is one world where the bomb is built and it's a dud, and one where it's not a dud, but we're past that moment. Therefore the different worlds that come about in the experiment, with the beam splitters and detectors the bomb is a dud in all of them, or it's not a dud in each of them. Is that fair?
First, you are talking about interaction-free measurement presented in Roger Penrose's
book. In the book "Shadow's of the Mind", he uses this language to describe Interaction-free measurement. He talks about duds and good bombs and whatever. It's problematic. It's works up to a point. In the original experiment, either there is a bomb inside or there is no bomb inside. In that experiment, there is only one world where there is a bomb inside. In the this one, there only one in which the bomb is inside. Now we conduct the experiment. When we do so, we will be split into three worlds. In one there is an explosion. In two others, there will be no explosion and the photon was detected by the other detectors. Because, in fact, the bomb is just a detector, like the other photon detectors in the experiment. The single photon will be detected either by a bomb, or by one of the other two detectors of the interferometers. Now, if there is no bomb inside, there will be only one world. So, if we start in the world where there is no bomb, then the world will not be split. only in the world with a bomb inside, will the world be split into three.
Can you talk about some of the real world experiments that have come about from your idea?
Prof. Vaidman: If you have a hope that it can be used in an x-ray without radiation for people, I'm not very optimistic that it can happen.
You're talking about being able to obtain internal images of people without x-rays? I have read about that, but I'm talking more about actual experiments that have been done.
Prof. Vaidman: The actual experiments which have been done describes seeing in the dark without physically seeing. we can really find this object. It's kind of an impossible task. It uses a detector and a single photon. Finding this detector is like seeing in the dark. There have been good experiments, but not the experiments from which you can get practical advantage.
But they supported your underlying theory, right?
Prof. Vaidman: There are relatively many papers which criticize and discuss the name of "interaction-free". You have to ask the right question. After you ask the right question, it took us some time to do this, but it's not so difficult. I think that the greatest achievement of Avshalom Elittzur was the biggest one. He asked the right question. Can you find an object without touching it or disturbing its internal state in any way whatsoever? Quantum mechanics tells us that you can--in our particular world. My explanation is that in another world, it has been touched. But for us, it has not been touched. There was an experiment that actually did this.
There was another experiment which made interaction-free measurement's with "gray" objects
. The original idea was talking about objects that were completely opaque. The bomb was such an object in that the photon cannot pass through it. If the photon encounters the bomb, the photon is completely absorbed and it doesn't continue. The question is can you find objects that are gray? That some light comes through them. What was found is that you can find this object with less radiation than otherwise. So, it's also has a kind of chance for practical application for gray objects. But it's not so efficient. This is why x-rays for humans is not a very plausible idea, even though the body is a kind of gray object. It's not black and white. When light encounters a human body, some of it is completely absorbed and some of it passes through it.
Relating directly to MWI, are these experiments getting information from a parallel universe?
Prof. Vaidman: Well, in a sense, yes. Because in a parallel universe there was an explosion. Or in a parallel world--there is only one universe. In the MWI there is one physical universe and many worlds which look like ours. There are many different stories. In one story, there is an explosion. In another, there is no explosion. So, in a sense, with interaction-free measurement, in one world we get information about the bomb which exploded in another one. So, in some sense, it's close to what you are saying.
I know that physicists are very conservative...
In fact, it may sound strange, but I might consider myself very conservative. But being conservative, the MWI is the best option. Because If I don't believe in the MWI, I have to believe in the collapse of the wave function and there is no plausible explanation for the collapse of the wave function
. So, being a conservative physicist I have to believe in the MWI.
Has there been some changes in your own position regarding MWI since your original ideas? I seem to detect that in some of the papers I've read.
Prof. Vaidman: It's grown stronger and stronger. I am convinced that this is the strongest interpretation. This resolves all the paradoxes has grown stronger and stronger. There are many paradoxes in quantum mechanics, and I find it important to investigate them and try to understand them, and it is only in the framework of MWI that I resolve all of them. Everything is there.
Do you think this work is very important?
Prof. Vaidman: Which one IMF? or which one?
Specifically, your work in Interaction-free measurement and as it relates to the MWI of Hugh Everett and others like Tegmark?
For me, it's important for the MWI. Interaction-free measurement is interesting for some of the ideas, I do hope for some kind of application in the future. I do think it's a good argument for MWI, but I don't think it's the but the crucial one. I think that the fact that MWI resolves of the the Einstein-Podlosky-Rosen Paradox
is more important.
What does it tell us about "time's arrow", about the asymmetry of time? I know you wrote a paper about "symmetrical time" and the results of interaction-free measurment?
We've already talked about interaction-free measurement and teleportation. Time symmetry in quantum mechanics is another thing I've worked on with Yakir Aharanov. Aharanov is the creator of the Time Symmetric Two-State Vector Formalism
. The work we did together was in the area of "weak measurement"
. It has lead to some important applications in recent years. Aharanov's time-symmetrical formalism is just that: a formalism. It doesn't tell us if there really is symmetry in nature. In fact, Professor Aharanov takes it quite seriously. He really tries to say that there is real symmetry in nature and that in addition to the quantum state that is going forward in time there is one going backward in time.
For me, it is a more pragmatic type of thing. When I consider a quantum system, between two measurements, I say they are described by two state vectors, one from pre-selection--the first measurement--and one from post-selection--the second measurement. It's just useful. It fits very well with my view of the MWI. Because in a particular world, I get the particular result of my first measurement and in a particular world I get the result of my second measurement. So, in this world, I can discuss things in the middle. This is one of the world in which I get this "forbidden click" when conducting interaction-free measurements, and I know that the bomb is present. I can say that in this world, my photon never went near the bomb, But nevertheless I was able to find it. For me, this kind of formalism helpful in the analysis of information-free measurement, but this is not the only way to think about it.
Let me ask you about information as a physical "thing". How it is important to your work, and how critical is it to physics and how physics actually unfolds in the universe?
Prof. Vaidman: This is a very popular idea. I've been to several conferences which devoted sessions to physics and quantum physics from information and other things. It doesn't tell me much. Information of whom? For me, I don't like this approach at all. From my point of view, everything that exists is just a big universal wave function. There's no humans or objects--well there are humans inside, but this is not a description of nature or reality. Information is our concept that helps us discuss all sorts of things. Like, our concept of everything else. I don't think it has anything to do with basic laws of nature or physics. It's a popular idea, but I don't subscribe to it at all.
Are you familiar with Craig Hogan, and his experiment at the Fermilab in Chicago? He's trying to determine whether or not the information that describes the universe is infinitely dense or not. Whether the information in the universe is ultimately reducible to a finite amount of information to describe things.
Prof. Vaidman: I'm not a specialist in cosmology and the early universe but there are literally thousands of papers on the many paradoxes in quantum mechanics. There are even more on the topics of the interpretations and foundations etc... From my point of view, I try to explain what's happening in a small box with a small number of particles. From that, maybe I can imagine what would happen in a bigger box, and maybe in the room I'm in. When that happens, I like to say "Okay, this explanation is good for the whole universe."
I cannot say much about the whole universe, because I hope that nothing particularly new will come up just because your considering a bigger system. Small systems I think I understand completely using quantum mechanics and the theory without the collapse but with many parallel stories. I hope that our explanation stays the same, no matter how much larger the system is.
You sound like someone who truly believes that the best supported position is MWI.
Prof. Vaidman: Yes.
Based on your own knowledge and your own experience.
Prof. Vaidman: Correct.
Do you feel that that changes your philosophy in your own life? Do you look at the world around you differently than others, from those who have never heard of quantum physics or Hugh Everett?
There are some questions that are different. On my homepage, you can find a link to a "World Splitter"
.(note: this page is not always available outside the TAU community
). A quantum device which sits in the basement of Tel Aviv University. And it's not completely clear to me if it is or isn't a good idea to use it. For example, if I go to a restaurant and there are two cakes that look good to me. I cannot eat both of them. Now, I can decide by tossing a "classical" coin which one to take. Or, I can go to my home page and split the world in two. In one world, I will eat the first cake and in the other, I eat the second. It's not clear to me that this is different or better. Except, if you believe in the many worlds interpretation, I can take seriously the idea of performing such an experiment. If I don't take it seriously, then there's no difference if I take a classical coin or the "quantum coin", which is in my laboratory in Tel Aviv.
There might be a difference in my behavior. If something bad happened to me, I can think "oh, there is another world where something good happened to me."
Why don't you get the same world splitting effect from a "classical" coin?
Prof. Vaidman: When you toss a classical coin you give to it an initial velocity such that, in principle, you could calculate what the outcome will be. While doing the actual calculations is beyond one's abilities, there is only one outcome and it's fixed.
Many years ago I had an idea for a portable device with quantum measurement which can be sold as a world splitter. I thought about getting a patent, but I couldn't get financial backing. Nicolas Gisin
from GAP Optique in Geneva manufactures a quantum random generator (these generators don't just split the world in two, they split the world many times and very quickly).. I understand that he's also involved in the $1.99 I-phone application which allows users to split the world by connecting to his laboratory.
What about parallel worlds where events in history are different? As a Jew, I can't help but ask, whether you think about those ideas sometimes.
Prof. Vaidman: It's not clear how many different worlds there are. If I make a quantum experiment, I know that both outcomes are realized. If there is a world where the Soviet Union is still intact? I don't know the answer to that.
For Israel, there is no guarantee that it will survive. This is one of those questions for which the answer is not so clear. Maybe it's a good idea for the Israeli Prime Minister, before he decides on the next withdrawal or the next war, to use the World Splitter, so that in at least one world, Israel will survive. I'm unaware of any politicians actually using the World Splitter, so I don't know if this kind of splitting actually occurs. Again, this the kind of question that arises and I think it's relevant. Even if the answer is unclear.