Weber: You have said that we shouldn't give up so easily, we're not there yet, there may be other roads. Now some people would call that a promissory note. What are you looking to and why do you think that there are these possibilities?
Bell: There are two lines of research in quantum mechanics which I would
like to see pursued. One is the line which David Bohm was
presenting in 1952, "the hidden variable interpretation" of quantum
mechanics, as it was called. Now "hidden variables" is a very bad
name. In my opinion the picture which Bohm proposed then completely
disposes of all the arguments that you will find among the great
founding fathers of the subject, philosophers of the subject. In
some way, quantum mechanics was a new departure of human thought
which necessitated the introduction of the observer, which
necessitated speculation about the role of consciousness and so on.
All those are simply refuted by Bohm's 1952 theory. In that theory you find a scheme of equations which completely reproduces all the experimental predictions of quantum mechanics and it simply does not need an observer. There are objects in that theory which are bigger than others and if you care to call those big objects pieces of measuring equipment, then following the laws of that theory, they will reproduce the predicted quantum mechanical results, the results of measurement. So I think that it is somewhat scandalous that this theory is so largely ignored in textbooks and is simply ignored by most physicists. They don't know about it.
Weber: Why is that?
Bell: Let's not go into that. That's another question. That's the
psychology and history of physicists. I think most of the arguments
for romanticizing quantum mechanics - I use this word romance as a
shorthand for saying that somehow these physical problems are
pointing outside of physics, to the mind, to God, to whatever you
like - the arguments for romanticizing in this way are simply
refuted by this theory. Now what is wrong with this theory, with
David's theory? What is wrong with this theory is that it is not
Lorentz-invariant. That's a very technical thing and most
philosophers don't bother with Lorentz invariance and in elementary
quantum mechanics books the paradoxes that are presented have
nothing to do with Lorentz invariance.
Those paradoxes are simply disposed of by the 1952 theory of Bohm, leaving as the question, the question of Lorentz invariance. So one of my missions in life is to get people to see that if they want to talk about the problems of quantum mechanics - the real problems of quantum mechanics - they must be talking about Lorentz invariance.
Weber: Can you give a brief description of Lorentz invariance?
Bell: Lorentz invariance is a kind of mathematical heart of the theory
of relativity. It says more or less that the laws of physics are the same
regardless of the velocity of your laboratory. When that is combined with
the observed fact that light always travels with the same velocity relative
to the laboratory, that leads to consequences which are a little bit
surprising. And this idea that every laboratory in uniform motion is as
good as every other laboratory in uniform motion proves to be rather
difficult to incorporate into quantum mechanics in a deep way. And in
particular, in this 1952 theory of Bohm's, it is very hard to do that.
Now one has to distinguish between a kind of fundamental Lorentz invariance and a superficial Lorentz invariance. The 1952 theory of Bohm can be formulated in such a way that it agrees with all the experiments that are normally said to support the theory of relativity, but nevertheless deep down there is a preferred laboratory, and the observers that are moving are mistaken in their observations. Nevertheless, they are mistaken in such a way that they cannot detect that they are moving.
That's a very strange way for God to make a world. To sort of cover up his scaffolding, his fundamental preferred frame. So it's sort of incredible. But what I want to insist on is that this difficulty is not the one you find in most accounts which say: "Look how crazy quantum mechanics is" and all the craziness which you do meet in those accounts is disposed of by Bohm's 1952 theory. So I think that much of the argument is simply missing the point.
Weber: That still raises the question why physicists - who surely read the literature and know of Bohm's book - didn't pick up on this?
Bell: It can't relate. Similar ideas were put forward by de Broglie in
1925, but de Broglie was not a very articulate person, not very
good at repartee, and he was more or less trampled on by much
stronger personalities. So he more or less abandoned his ideas
until Bohm arrived. And the people who did establish the orthodox
way of thinking about the subject were already polarized by the
time de Broglie advanced his ideas. They were already convinced
that the atomic world was essentially unintelligible and that we
are obliged simply to talk about how our apparatus behaves; that we
cannot form a picture of what is happening on the atomic level, but
we can say how it affects our equipment and how what we do to one
piece of equipment emerges as a result of another piece of
equipment. They felt that this kind of superficial description of
physics was forced upon us by the phenomena. They were already
convinced of that before de Broglie started to speak, so they
didn't listen to him.
Also, this way of doing physics was enormously successful, there is good pragmatic justification for ignoring Bell's Theorem. It may have implications but it is about things that we don't need to know of, about some picture of what is real at the smallest level, and we can get along without such a picture.
Weber: Get along in the pragmatic sense, yes. But suppose we want to understand nature as much as is humanly possible. How does Bell's Theorem change our present interpretation of quantum mechanics?
Bell: Well, if you are interested in such things, if you feel that you need a deeper understanding than is given by the rules for applying the theory, then I think the so-called Bell Theorem is important because it tells you that the picture that you will make is very different from the traditional picture, the one which Einstein espoused, for example, in which chains of cause and effect do go from place to place less rapidly than light. Somehow you will need a picture in which either you admit that things can go more quickly than light, or that in some other way the distant regions of space are coupled to one another.
Bell: The 1952 theory of Bohm does that. It's not something which, when you see it, you say: "That's the explanation." You only say: "So that's the mathematics," and what it does is give you a picture of particles interacting with waves, and these waves do not propagate in ordinary space but in so called configuration space. And this introduces the necessary element of non-locality.
Weber: In this configuration space, which is not three-dimensional space - it's a multidimensional space, right? - are you postulating that what we call "particles" in this space are continuous or in touch with one another literally?
Bell: No, they are not in touch with one another literally. But they are each in touch with something, namely, the wave function, which is just not local, something which does not exist at a point but fills the whole world.
Weber: Is it spread out infinitely?
Bell: Right. It is spread out infinitely.
Weber: And they are the outcome of that and yet they are rooted in that?
Bell: The particles interact with the wave, the wave occupies the configuration space, and when you come down to [physical] space it means that the different positions in [physical] space are in some way in touch with one another.
Weber: Because they get this information in configuration space?
Weber: So in a sense they are tuned in on each other . . .
Weber: . . . because they emerge from that source.
Weber: . . . where there is no separation.
Bell: Right. I think that's a good way to put it. Now whether you say you have achieved a deeper understanding or not - well the word "understanding" can be discussed at great length - as a professional theoretical physicist I like the Bohm theory because it is sharp mathematics. I have there a model of the world in sharp mathematical terms that has this non-local feature. So when I first realized that, I asked: "Is that inevitable or could somebody smarter than Bohm have done it differently and avoided this non-locality?" That is the problem that the theorem is addressed to. The theorem says: "No! Even if you are smarter than Bohm, you will not get rid of non-locality," that any sharp mathematical formulation of what is going on will have that non-locality, the theory of quantum mechanics.
Weber: Are you saying that there are no separate particles in configuration space, or that there are potentials for separate particles in the wave and they are in touch with one another?
Bell: Let me first of all insist that what I am describing is a
picture. And the merit of the picture is that it is coherent, it's a sharp
picture, there is no creaking in the machinery, . . .
. . .
Weber: . . . you take the option that there is no need to "transmit" the information because these things are linked, and so they "know." The information is shared in this inward dimension, in configuration space.
Bell: Yes, I think that's what I think. That's what I have in mind, yes. But I want to insist again that this 1952 model of Bohm - which for me is a tremendously important thing and which I want to become better known and more discussed - is not Lorentz-invariant. Somehow, it is in conflict with something else which we have come to believe in very deeply, namely, that it should not be possible to say that a laboratory is absolutely at rest. It should not be possible to say that of all the moving objects, somehow that one is really moving and that another is at rest. According to Relativity Theory there should not be a preferred frame of reference. The Bohm theory is set up with a preferred frame of reference.
Weber: And you feel that's a handicap?
Bell: It's incredible. Because you see, Einstein's principle of relativity has been enormously fruitful in physics. Unless you want to dismiss it as a happy accident - and there are happy accidents in physics where people have come to good things with wrong arguments - the principle of relativity has been so repeatedly fruitful that it would be hard not to believe that it is a very deep principle of nature, that there is no such thing as absolute motion. In Bohm's 1952 theory there is such a thing as absolute motion.
Weber: Have you ever discussed this with David?
Bell: David knows this very well. He thinks about it. I last saw David
two weeks ago in Paris at a fiesta for one of his collaborators, Jean
Vigier. David gave a talk and afterwards we discussed this point. He feels
that I am giving too much importance to the principle of relativity and he
may well be right. Maybe you can somehow show that the principle of
relativity emerges naturally for coarse-grained things, for physical
things, and yet is not present at the fundamental level. . . .
. . .
Weber: You feel that it has not happened and that no direction for it is presently on the horizon?
Bell: I cannot say it so quickly as that. I can see the logic of people
who say that the so-called reduction of the wave packet in quantum
mechanics must happen inside the human mind, there is a logic in that,
whereas there is really much less logic in the position of Bohr, who says
that this wave packet reduction happens in large objects, but is [not] very
specific about what the large object is. To find something distinctively
different from the material of physics, it is a very reasonable thing to
say, perhaps mind is sufficiently different to provide this interface in
which the linear evolution of the wave function is replaced by the
non-linear one. However, I think that one has not tried hard enough to
avoid this step and to see whether this step is dictated by the facts or by
our wanting a central place for ourselves. One must really try very hard to
avoid taking that step. It may be that after trying for some time and for
some generations to avoid that step, that the outcome of it is inevitable.
For example, from my own experience there is this question of non-locality. The non-locality which I first met in Bohm's 1952 theory is very odd and very hard to accept, so I resisted it. As a result of resisting it, I came to the [theorem] that it is inevitable, I cannot avoid it, so I now have to live with that. Maybe I would have the same experience in connection with this business of the role of the conscious observer in the physical world. Maybe after trying hard for some time, I would come on something like the so called Bell Theorem, which says that you cannot avoid putting the mind in physics. But I haven't got there and when I look at the people who think they have got there, I think they are wrong.
Weber: You are not objecting either to the aim or to the principle. As I understand it, you are saying: "It is too early, this should be a last resort. I am not entitled to that move until I have exhausted all the less extravagant moves."
Weber: The more sober ones?
Bell: The more unromantic possibilities.
Weber: If, having attempted to do that, it still would not work, then you will have greater confidence that that move is warranted because you will have resisted it with all your might.
Bell: Right. But let me be clear that it will not be possible to come to
a stage where you say: "Aha! The question is now answered." Each person
must judge for her or himself . . .
. . .
Bell: . . . For me it is absolutely unquestioned in my analysis of things that the real world is out there, and that I am an incident in it.
Weber: It's really the one closest to Bohm, namely realism?
Bell: It is early Bohm. Bohm of 1952. And I have the greatest respect for that theory. The trouble with it is that it is not Lorentz-invariant. I always come back to this point that if I didn't know about Lorentz invariance, I wouldn't see a big, deep mystery in quantum mechanics. I would say Bohm has shown us how to think about the subject without dividing the world into apparatus and systems, without insisting that there is a conscious observer, somehow in there reducing the wave packet. He has shown a formalism which agrees with the experimental facts and with this realist conception of the world, and it's just a technical fact of physics that Lorentz invariance has played a vital role in our discovery of the different laws of nature, and it would be very hard to regard it as a superficial affair. So I am looking for a theory in which Lorentz invariance is there at the heart of things and which still agrees with all the experimental evidence.
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