Matt Leifer, whose blog I hadn't previously encountered, has a long and fascinating post on evaluation criteria for quantum interpretations. "Interpretation" here means the stuff of countless "Isn't Quantum Mechanics weird?" books-- Copenhagen, Many-Worlds, Bohmian hidden variable theories, all that stuff. These are the "meta-theories" that are used to explain how you get from all that weird and messy wavefunction stuff up to the reality that we see and observe in our experiments.
The list is explicitly modeled after the well-known DiVincenzo Criteria for quantum computing (see also Quantum Optics lecture 19), though at the moment, Matt's list is too long and not punchy enough for people to start talking about the "Leifer Criteria" and making road maps and the like (after the cut):
- An interpretation should have a well-defined ontology.
- An interpretation should not conflict with my direct everyday experience.
- An interpretation should explain how classical mechanics emerges from quantum theory.
- An interpretation should not conflict with any empirically established facts.
- An interpretation should provide a clear explanation of how it applies to the "no-go" theorems of Bell and Kochen Specker.
- An interpretation should applicable to multiparticle systems in nonrelativistic quantum theory.
- An interpretation should provide a clear explanation of the principles it stands upon.
- No facticious sample spaces.
- An interpretation should not be ambiguous about whether it is consistent with the scientific method.
- An interpretation should take the great probability debate into account.
- An interpretation should be consistent with relativistic quantum field theory and the standard model.
- An interpretation should suggest experiments that might exhibit departures from quantum theory.
- An interpretation should address the phenomenology of quantum information theory.
The actual post contains a great deal of explanation of what each of those points means, and the interested reader is encouraged to follow the link and read, as they say, the whole thing.
This sort of research on the foundations of quantum theory is fascinating stuff, and one of those odd areas of science that gets both too much and not enough attention. These are Big Questions-- every bit as Big as the questions addressed by cosmology and string theory-- and more people ought to be working on figuring out what really goes on in the transition from quantum to classical. More sane people, anyway-- as it stands, there are probably more kooks than serious researchers in the field.
In some ways, I think this stuff is actually more important and interesting than particle physics and string theory. It's at least within ten or twelve orders of magnitude of being relevant to my daily life... Then again, string theory might have a better chance of experimental confirmation, as interpretations of quantum theory are almost by definition experimentally indistinguishable.
Comments
Thanks for the plug. I was wondering where that traffic spike was coming from.
If you're looking for a punchy list that's more like the diVincenzo criteria then I suggest just taking the first six and possibly rephrasing them to make them sound a bit more sexy.
Posted by: Matt Leifer | June 29, 2006 2:20 PM
as interpretations of quantum theory are almost by definition experimentally indistinguishable.
At the risk of sounding like a broken record, I think it's worthwhile to distinguish between interpretation which I'd interpret as an ontology for traditional quantum mechanics and the various physical additions to traditional quantum mechanics which are also sometime termed interpretations.
The former is either about philosophy or the nature of consciousness. The latter can in principle be experimentally tested.
Posted by: Aaron Bergman | June 29, 2006 3:26 PM
Such distinctions can be made, but I don't think it is as clear-cut as you suggest. Of course, there are those that view this subject as pure philosophy, but if it is to be thought of as physics then the hope has to be that any viable suggestion would eventually have empirical consequences. By this I mean that it should at least suggest novel ways in which the theory might be modified, or new ways of applying it to areas where it has not yet been successfully applied, e.g. quantum gravity. If this comes about, it will be less clear whether what we had originally was just an ontology for traditional QM or a genuine empirical alternative. For this reason, I dvocate a fairly broad interpretation of the word "interpretation". Basically, anything that adds novel concepts to the basic theory is an interpretation in some sense.
I think that for physicists like myself who are studying the subject, the potential for new physics has to be one of the primary motivations, since it is fairly clear that most philosophers are better at doing philosophy than physicists are.
Posted by: Matt Leifer | June 29, 2006 4:08 PM
Well, one purpose of experiments, and the list, could be to constrain and rule out as many interpretations as possible. A comment from Matt makes me think the current purpose of the list is to be as inclusive as possible. I would like to see an explicit naturalistic criteria to rule out interpretations that has consciousness as a basis for QM, since it seems to give a dualistic description of nature and the mind. (The reverse, consciousness from QM, is pretty much ruled out by observations.) But that's me.
Posted by: Torbjörn Larsson | June 29, 2006 7:35 PM
Rather than being used as a means to rule out interpretations, the criteria are supposed to be used as a guide to what issues need to be addressed in any proposal. Just like the diVincenzo criteria there need not be a unique proposal that has the potential to satisfy them. Different ideas will get into trouble with different subsets of the criteria and so they are mainly intended as a means of identifying the weak and strong points of the different possibilities.
A criterion ruling out dualism might be thought necessary by some, and indeed not everyone will agree that all the criteria I have included are strictly necessary. I'll admit that the whole enterprise is a good deal more subjective than the diVincenzo criteria. However, I'd still advocate the top six as a minimal set that everyone should hopefully be able to agree with.
Posted by: Matt Leifer | June 29, 2006 11:53 PM
Philosophers of science had a major discussion of this topic back in the mid-80's, trying to forumulate criteria for choice of interpretation. What makes the whole deal tricky is that every interpretation is supported by all of the same evidence since they are interpretations for exactly the same formalism. The hope was that there could be an empirical aspect to the question. The most promising route that emerged was that while the same evidence may be cited in support of all interpretations of a given theory, it is not necessarily true that the evidence provides the same degree of support. Invoking something along the lines of a theoretical version of Occam's razor, the claim was that the more metaphysical baggage an interpretation required, the more support that would be necessary to make it equally as well supported as its more ontologically thrifty competitors.
The usual example was to contrast the standard version of General Relativity with a second version that included a flat spacetime and a dynamic gravitational field ala Newton/Poisson. The idea is that for Einstein's version with everything packed into the geometry, there was one fewer component than in the flat spacetime/gravitational field interpretation, making it better supported by al the same evidence. As such, we ought to prefer interpretations that answer exactly the sort of issues you mention, but that does it with the fewest moving parts.
Posted by: SteveG | June 30, 2006 11:29 AM