Back in the day

How it all began,

Heisenberg: It starts with Einstein.

Bohr: It starts with Einstein. He shows that measurement—measurement, on which the whole possibility of science depends—measurement is not an impersonal event that occurs with impartial universality. It’s a human act, carried out from a specific point of view in time and space, from the one particular point of a possible observer. Then, here in Copenhagen in those three years in the mid-twenties we discover that there is no precisely determinable objective universe. That the universe exists only as a series of approximations. Only within the limits determined by our relationship with it. Only through the understanding lodged in the human head.

That’s from Michael Frayn’s Copenhagen. Much to disagree with in the passage above. Steven Weinberg nails it with his trademark succinctness:

All this familiar story is true, but it leaves out an irony. Bohr’s version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wave function (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from?

Considerable progress has been made in recent years toward the resolution of the problem, which I cannot go into here. It is enough to say that neither Bohr nor Einstein had focused on the real problem with quantum mechanics. The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wave function, the Schrödinger equation, to observers and their apparatus.

Einstein started it. Einstein didn’t like it. Now we all know better. Physics progresses. We move on.


2 thoughts on “Back in the day

  1. Joshua Job says:

    While I agree that the Copenhagen interpretation is flawed, it’s not flawed for the reasons Weinberg states. The Copenhagen interpretation is fundamentally an epistemology — it states that entities are wholly what we see them as, that there is quite literally nothing more to an object than our sense-experience of it. As a consequence of this, asking questions such as “what is a measurement” are literally meaningless, as measurement is identical, to them, with “experienced” by some being like ourselves. It rejects the notion of an objective, independent reality and adopts completely an instrumental epistemology.

    The debate between Copenhagenists and people like Many Worlders and Bohmians is that Copenhagenists say physics is about describing and explaining our experience of the world, and that anything not knowable through experience is by definition meaningless. This leads naturally to views like the relational interpretation and Copenhagen, which fundamentally reject the idea that there is a universal wavefunction (for different reasons — in particular the relational interpretation points out that a universal wavefunction must specify more information than can be known by any entity in the universe), and that in some sense the “state” of an object is wholly epistemological. The QBists, Copenhagenists, and relational QM people all agree on the point that physics is basically about describing appearences and that states are not objective properties of any object.

    On the other hand, Bohmians and Many Worlders (and the like) demand that no, physics isn’t about just describing appearences. It’s about describing what is honest-to-God really happening *out there*. There has to be an objective observer independent reality about which we gain information, they say, and our laws of physics should be about that thing. Just because we have a theory which predicts measurements or our sense experiences in a hodge-podge fashion, we as physicists haven’t done our jobs. We need to describe what is really going on, and then relate that to our sense experience. Of course they have very different descriptions.

    And on this point Bohr and Heisenberg, Einstein and Schrodginer stood on very different sides. Bohr and Heisenberg embraced instrumentalism and variations on Kantianism, Einstein and Schrodinger rejected instrumentalism and demanded that we can know the real world.

    Physicists are too quick to dismiss philosophy, and that makes them have all sorts of muddled arguments. Until you decide what you demand — that the world is objective and observer independent, or if physical theory is only about sense-experience and all else is to be relegated to the noumenal world — you can’t make any progress. And once you do,a lot of the interpretational jumble falls away.

    • Good point. I was oversimplifying.

      I guess you’re referring to the ontic vs. epistemic debate. If I understand you correctly, you’re saying that if the quantum state can be interpreted epistemically (i.e. as a state of knowledge), then there will no measurement problem: the “collapse” is akin to a Bayesian update. This approach is championed by Fuchs; and Spekkens provides a very convincing toy model which reproduces many, but not all, counter-intuitive features of quantum theory.

      The epistemic vs. ontic is a fairly deep and technical debate which I’m not conversant with. There were a few recent results providing evidence against epistemic interpretations: the PBR result, the ABCL result, and this Leifer result. So, epistemic models seem to be taking a beating recently. Though also see this ESSV result that is a criticism of PBR.

      Needless to say, I’m completely out of my depth here. I feel though, that the ontic interpretation is definitely the more intuitively natural: always in physics, if we claim that we’ve said all that we can possibly say about a system, then we equate that to reality. Further, I’m not entirely sure that if the epistemic picture is right, then the measurement problem disappears. Of course, in the ontic picture, the measurement problem is much more urgent.

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