What I hope to do in these sessions is offer an invitation to the foundations of quantum mechanics, an account of the sense in which I think there are big, interesting questions at stake here. Let me talk in a little more detail about what I want to do.
What is primarily interesting about the story of quantum mechanics, about the foundations of quantum mechanics, is that it represents a unique episode in the history of modern science, by which I mean science since the Scientific Revolution. It was widely thought, and widely announced, that the discoveries about the bizarre behaviors of subatomic particles that led to the development of quantum theory represented a truly decisive collapse of the traditional aspirations of the scientific project.
Over and above being a set of discoveries about the behaviors of subatomic particles and the kinds of mathematical apparatus required to describe those behaviors, what interests me primarily and in the first instance about this story is how it was received by the people who discovered it. Those discoverers understood it as a powerful demonstration that the traditional aspirations of the modern scientific project had collapsed and were now exposed as naïve, impossible, and presumptuous. Those aspirations, stated crudely, were to give us a straightforward, literal, realistic, and complete account of the physical world.
What I would like to convey in these sessions is, first, what the phenomena were that seemed to justify such a conclusion, and second, how that conclusion has since been pushed back against. That is, in what sense one can say that, notwithstanding these developments, the original aspirations of the scientific project are still very much alive and represent a going concern.
This is quite an astonishing story, astonishing in the following sense. It goes without saying that there is a long history of what you might call philosophical critiques of, and concerns about, the completeness of the scientific project. There are worries to the effect that science cannot refute skepticism. There are worries about the image of ourselves that fundamental physics presents to us, worries about where our conception of ourselves as rational agents, or as free agents, would fit into such a picture. There are worries about what sense one can make of reference and intentionality in the context of the picture of the world that physics seems to be presenting us with.
What happened in the case of quantum mechanics is of an utterly different kind. What happened is that something amounting to a radical critique of the scientific project emerged within and in the course of the scientific project itself. Milo once said to me, quite nicely I thought, "This is the occasion when the call is coming from inside the house." The idea here was not that there were external critiques that may or may not be devastating to various aspirations of the scientific project, but that the scientific project did something more along the lines of committing suicide, in the course of quantum mechanics.
Astoundingly, not because of any kind of philosophical reflection, but because of where pointers on measuring devices ended up pointing at the conclusions of various straightforward physical experiments, it could be argued, on the basis of how these experiments came out, that any attempt to tell an objective, literal, third-personal, realistic story about what was going on behind the curtain, about what was going on under the table, that would produce an intelligible account of why these experiments came out the way they did, was doomed to fail. This was argued by people like Bohr and his circle in Copenhagen: that any such attempt would inevitably collapse into paradox or self-contradiction.
It is quite astounding to think that anyone could even have imagined that the bare results of certain experiments, the mere positions of pointers on measuring devices, somehow demonstrated that no realistic story about what was going on behind those experiments could possibly succeed.
The first thing I hope to do here is to tell a story that makes it understandable, even if I don't think it turns out to be right, that people could have drawn such shocking and radical conclusions from the way some experiments on subatomic particles came out. What is striking about this episode in the history of science is that this is not a critique arising from general philosophical reflection. You have these working physicists minding their own business, minding what everybody agrees is their own business.
They are not trying to figure out how agents work, how rationality works, or how freedom works. They are not trying to understand being qua being or anything like that. They are simply trying to tell an intelligible story about what happens to rocks when you pass them through various kinds of measuring devices. And all of a sudden, things get weird, and you are confronted with claims from smart people to the effect that this particular attempt to understand these particular kinds of very small rocks shows that the entire project was misguided.
The very aspiration to tell this traditional kind of scientific story about what these rocks are doing in between the time they go into a machine and the time they come out is not something that can be done without collapsing into paradox, contradiction, or some other kind of madness. The first thing I want to do is make it plausible that smart, reflective people could have arrived at such conclusions as a result of how these experiments came out. The opening part of these sessions will be devoted to telling that story.
I want to move on to the very early attempts by figures like von Neumann, Schrödinger, and Wigner to push back against this position, to explore what it would look like to construct the kind of realistic account that Bohr and his circle had declared to be impossible.
I also want to show how those initial attempts at realism run into a formidable obstacle, which is known in discussions of the foundations of quantum mechanics as the measurement problem. Setting that up is the second main thing I want to accomplish here.
The third thing I would like to do is report something we have learned quite definitively about what any attempt to tell this story realistically is going to look like. I am referring here to the famous discoveries of John Bell, which grew out of arguments by Einstein, Podolsky, and Rosen regarding the question of locality.
We have always approached the world with a particular conviction, and by "always" I mean prior to modern science, prior to science altogether. This is a conviction with which dogs approach the world, and mice approach the world. It is the conviction that conditions at some particular point in space and time can only be directly and physically affected by conditions that are immediately adjacent to them in space and time. In situations where something that happens here affects something that happens over there, that influence is always accompanied by a process of propagation through adjacent spatiotemporal points, stretching continuously from the initial cause to the final effect.
What we now know about quantum mechanics is that any attempt to push back against the radical instrumentalism I described earlier, any attempt to push back against that radical anti-realism, is going to result in a story that violates this deeply held intuition in a genuinely shocking way. I want to tell that story as well. And all of the stories I have referred to so far, I hope to tell in a way that is both accessible and logically and conceptually rigorous. I believe it is possible to tell these stories clearly without sacrificing logical or scientific rigor.
I want to go on and talk in a little more detail about the attempts to come to grips with the measurement problem, the attempts at being realistic, the attempts at pushing back against the claims of Bohr and his circle. There are three interesting kinds of attempts to solve the measurement problem that I want to discuss.
A fully rigorous treatment of those attempts would require a good deal more mathematical apparatus than I want to drag into these proceedings, so the discussion is going to be considerably sketchier than the presentation of the topics I have covered so far. But this material is, in the end, very accessible. There are a number of good books one can read that will complement the latter parts of these lectures with far more detail about how those proposals work than you are going to hear here.
A few books are worth mentioning here. First is a book I wrote some years ago called Quantum Mechanics and Experience, published by Harvard. There is also a wonderful book by Tim Maudlin called Philosophy of Physics: Quantum Mechanics, published by Princeton, and a fine book by Jeff Barrett called The Conceptual Foundations of Quantum Mechanics, published by Oxford. Travis Norsen has written a very good book as well, Foundations of Quantum Mechanics: An Exploration of the Physical Meaning of Quantum Theory, published by Springer.
All of these are accessible treatments of the subject. They will require you to absorb some mathematical material, but each of them introduces all the mathematics you will need in a fully self-contained way.
That, in any case, is a crude outline of what I hope to accomplish in these sessions.