There’s been a bunch of talk recently about a poll on quantum interpretations that showed physicists badly divided between the various interpretations– Copenhagen, Many-Worlds, etc.– a result which isn’t actually very surprising. Sean Carroll declares that the summary plot is “The Most Embarrassing Graph in Modern Physics, which I think is a bit of an overreaction, but not too much of one. I do strongly disagree with one thing he says in explaining this, though:
Not that we should be spending as much money trying to pinpoint a correct understanding of quantum mechanics as we do looking for supersymmetry, of course. The appropriate tools are very different. We won’t know whether supersymmetry is real without performing very costly experiments. For quantum mechanics, by contrast, all we really have to do (most people believe) is think about it in the right way. No elaborate experiments necessarily required (although they could help nudge us in the right direction, no doubt about that). But if anything, that makes the embarrassment more acute. All we have to do is wrap our brains around the issue, and yet we’ve failed to do so.
My problem is with the emphasized words, which are emphasized in the original. This plays into a pet peeve of mine, which I’ve ranted about before, namely the idea that experiments are somehow an afterthought, just cleaning up the loose ends once theorists have done the hard work of thinking about things.
This is emphatically wrong. Experiment is at least an equal partner in this, and every other scientific question. If we ever do determine that there is One True and Correct Interpretation of quantum mechanics, it will be because that intepretation produces makes concrete predictions that are testable by experiment. Full stop.
Everything we know in quantum foundations– and science generally, but wer’re talking particularly about quantum foundations here– is ultimately grounded in experiment. The reality of photons? Confirmed by experiment. The wave nature of matter? Confirmed by innumerable experiments– I particularly like this one. All that Alice and Bob stuff that has theorists tying themselves in knots about “firewalls”? Confirmed by experiment.
Experiment is not now and never will be some ancillary activity that maybe provides a “nudge” or two for the deep thinkers. Without experiment, nothing is ever settled. A subject where you try to settle questions only by thinking really hard is indistinguishable from theology, and it never ends.
In fact, there are times when over-reliance on deep thinking is part of the problem. Sean argues that the lack of consensus on interpretations is embarrassing because quantum mechanics has been established for eighty years now. He dates this from John von Neumann laying out the mathematical foundations in 1932, which is a little ironic, because about thirty of those eighty years of inaction can be laid at von Neumann’s feet. When he laid out his formulation of quantum mechanics, von Neumann asserted that hidden-variable theories were ruled out mathematically, and his reputation was such that most physicists regarded this as a settled question on that basis. The problem is, he was flat wrong on this point, relying on a mathematical theorem that didn’t actually say what he claimed it did.
The question was eventually re-opened in part by people thinking about it the right way– in particular David Bohm, and then John Bell. But what blew it open into an active field of research, and prompted all the exciting research in quantum information and so on was experimental work. First John Clauser and then Alain Aspect did experiments that showed unequivocally (well, mostly) that quantum reality is not compatible with local realism. All of the dramatic progress since the early 80’s is a direct result of people realizing that these questions are experimentally accessible.
So don’t be so quick to discount the role of experiment in this question. If it’s ever going to be settled, it will ultimately be settled by experiment, just like everything else in science.
(Now, there are a couple of dodges available to argue that I’m being too harsh on this. One would turn on the “elaborate” modifying “experiments.” While I agree that quantum optics experiments aren’t as expensive as particle physics (by about three orders of magnitude), if you think they aren’t “elaborate,” I will just point you to the ThorLabs catalog and invite you to duplicate Anton Zeilinger’s experiments. And if we’re going to argue about that, I’m also going to object to that first sentence, so let’s not go there, OK?
(The other out would be to say that “thinking in the right way” includes thinking about experimentally testable consequences of interpretations. But if you think you can hold that while also relegating experiment to the subsidiary role of providing the occasional “nudge” to theory, well, that’s an awfully narrow needle to thread.)