In general, I find physics to be the closest science to philosophy; I think we sell ourselves short to say that the one cannot inform the other.

The discussion so far has lacked a recognition of the importance of the philosophical struggle that has taken place in science in relation to the one in the greater society. In “The Ten Assumptions of Science” and “The Scientific Worldview” I framed that struggle, not as a battle between materialism and idealism, but as the opposition between determinism and indeterminism. I did this to establish a modern determinism (univironmental determinism) as the philosophical goal for scientists as well as for those interested in the scientific worldview. We can discard indeterminism altogether, but we can never discard idealism. We just need to put it in its proper place.

One decoherence argument looks at e.g. randomly-varying, relative phase shifts between different instances of a run of shots of single photons into a Mach-Zehnder interferometer. Their case goes, the varying phases cause the output to be random from either A or B channel instead of any guaranteed output (into e.g. A channel), that is otherwise dictated by interference – in the normal case where phase is strictly controlled. They tend to argue, such behavior has become “classical.” Somehow we are thus supposedly moved away from even worrying about what happened to the original superpositions that evolution of the WE says typically come out of both channels at the same time – until they get “zapped” by interaction with a detector.

Well, that argument is fallacious for many reasons. First and foremost is the very idea of using what may or may not happen in preceding or subsequent events of an experiment, to argue the status of any given event. I mean, if the phase between the split WFs happened to be 70°, then the output amplitude in channel A = 0.819…, and the output amplitude in channel B = 0.573576… . In another case, with a different relative phase, the amplitudes would be different, umm – so what? There is still a superposition of waves, and the total WF exists in both channels until “detection” works its magic. That’s what the basic equation for evolution of the WFs say. They don’t have a post-modernist escape clause that if things change around the next time and the next time you run the experiment, then any one case gets to participate in some weird “socialized wave function” (?!)

And, what about the case where we don’t have messed up phases but a consistent e.g. 70° phase delta across instances – then what? So there really isn’t or shouldn’t be a collapse then, but waves remaining in both output channels? That isn’t what happens, you know. Chad said, the other WF doesn’t have to go away (like to “another world”), they just don’t interfere anymore. But that isn’t really the issue: the issue is that the calculation says there’s amplitude in both channels – and then how the photon ends up condensed at one spot.

The use of the density matrix doesn’t really solve or illuminate any of this either. One trouble with the DM is, it’s a sort of two-stage mechanism (in effect.) First, you start with the “classical” probabilities of various WFs being present. OK, that makes sense for actual description because we don’t always know what WFs are “really there.” But then there’s mishandling of two types. First, the actual detection probabilities are usually compiled out of the WF interactions (squared combined amplitudes.) But that takes a “collapse” mechanism for granted and can’t be used later in an argument attempting to “explain” it. If we just have Schrödinger evolution, the DM would just tabulate the likelihood of having various combinations of amplitudes, and that’s all! Without the supervention of a special collapse process, the DM has to be just a tabulation of the chances of having various amplitudes, not of the “probabilities” that only collapse can create IMHO. There wouldn’t be any “hits” to even be trying to “explain.”

Briefly, roughly: the decoherence argument is largely an attempt to force an implicit ensemble interpretation on everyone, despite the clear conflict of the EI v. any acceptance of a “real” wave function each instance, that evolves according to e.g. a Schrödinger equation. Yeah, how can they “collapse”; well who knows, and cheating isn’t the right way to deal with it.

Better an honest mystery than a dishonest “solution.”

This may be regrettable in some circumstances by usually it is not, most don’t want to spend time learning failed theories, there are too many working ones to learn.

However I believe today we have tools to get around this problem – internet.

I hope that eventually we can develop a database of free scientific knowledge maintained by scientists themselves, a tree of human knowledge. Such a database should contain not only theories considered current but also other possible alternatives and reasons why they were discarded. All theories should be ranked by their plausibility but still every one should be accessible and documented with links to publications preferably.

This would allow people to see what is wrong with each approach and save them from reinventing the square wheel. This would also allow easy resurrection of abandoned theories when new data changes the picture.

Now if such a database were also accompanied by (properly organized) forums it would make a great place for exchange and discussion of scientific ideas greatly facilitating collaboration between scientists. I think such a development is inevitable eventually but the sooner the better.

Pace Lee Smolin (#56), I suspect that the reason why the great physicists of yore, through the first third of the 20th century, were better educated in philosophy than we are today is simply because now there is so much more physics – and science in general – to learn. Given the choice of learning group theory, organic chemistry, or Kripke semantics, it’s pretty clear which is least likely to impact our research careers. Indeed, I would suggest that a physicist studying QCD phenomenology, a biologist working on bacterial evolution, or a chemist working on peptide synthesis will virtually never in their entire careers find it necessary to consult what philosophers have to say about epistemology, modal logic, or even philosophy of science. When we do import ideas from outside our chosen field, it is usually from other fields of science or mathematics. One can easily find citations to the biology, chemistry, and mathematics literature in physics articles, but very rarely to anything in philosophy journals.

In recent years there has been (in my opinion) a positive trend whereby philosophers are first receiving high level (e.g. PhD) training in science before moving into philosophy, or independently make a serious effort to learn the relevant science. Professor Callender (#64) is an example of an academic philosopher who is admirably well-versed in physics and mathematics. (Another who comes to mind is John Earman.) Such philosophers are in a good position to seriously engage with the scientific literature, and I am hopeful that there will be increasing opportunities for exchange between our communities.

We scientists already reason “philosophically”, and, we like to think, abductively (i.e. inference to the best explanation). We even employ modal logic, reflecting on necessity and possibility, and have been doing so since before Kripke. The formalistic approach to logic employed by many modern philosophers may be stimulating but I suspect it is rather barren in terms of its potential for physics. And just look at the sort of nonsense that passes for rigorous philosophy of religion today: http://commonsenseatheism.com/?p=1992#more-1992 . (To their credit, many philosophers view philosophy of religion as an unwanted stepchild.)

Ultimately, I would endorse the view Einstein articularted shortly after he broke with the verificationists, that scientific theories are “free creations of the human spirit” (see http://plato.stanford.edu/entries/einstein-philscience/). I’m an instrumentalist at heart — I believe that the test for any theory is how well it explains and predicts observations. If a given theory is astonishingly predictive but metaphysically troubling, I would say that our metaphysics is in need of a tune-up. Our prephilosophical notions are, very plausibly, a partial product of our evolutionary path. Is there, to this very day, a philosophically tidy and uncontested interpretation of quantum mechanics?

Getting back to Einstein, I agree with him that the creative act — the “spark of genius” — defies any philosophical description or categorization. It seems quite possible that, for a variety of reasons, a crucial scientific advance might result from a blunder, or from not following the philosopher’s rules of inference. Our philosophizing about science may help us better contextualize our work, but it rarely if ever is responsible for essential *scientific* insights.

I think the problem here is that yes, learning philosophy might improve your thinking, but so can learning math, engineering, chemistry, and a lot of other stuff. Such answers do not take into account that our time for learning is limited.

The question should be posed differently, you are a physicist and you have limited time to study, would it be a good idea to skip a semester of physics and learn philosophy instead? Will it make you a better physicist? Based on the comments above the answer to this question seems to be no.

The latter-day Feynman himself keeps the sentiment up from beyond the grave here: http://xkcd.com/397/

Both science and philosophy are losing out in this!

Philosophy is still (largely) in a Newtonian world. Traditional logic may be a thing of the past. Quantum logic may be the more general paradigm.

Scientists, for their part, are too busy keeping up with developments to consider the broader aspects of their theories. Clearly there is great need for a middle ground – maybe accademic programs need altering.

First: was Feynman really anti-philosophy? There’s his famous quote about not asking “but how can it be like that?” But this quote was directed at students, for which it was actually very good advice; he later disregarded his own advice. When I was at Caltech, around 1980, he gave a lecture about how maybe negative probabilities could solve the EPR paradox and get around Bell’s inequality. It obviously didn’t work, since he didn’t publish this result, but I think the fact that he was thinking about it means he wasn’t really anti-philosophy at heart. He may not have said anything positive about the philosophers of physics who were his contemporaries, but I’m not sure I can really fault him in this.

Second: Does philosophy help? Maybe in some circumstances it hurts. Bohr was certainly well grounded in philosophy, and it (logical positivism in particular) seems to have played a role in his development of the Copenhagen Interpretation. When I think about the question of “why wasn’t quantum information theory discovered earlier,” I think some part of the answer has to do with the degradation of the Copenhagen Interpretation into what Mermin labeled the “shut and and calculate” interpretation. Of course, this may be because later generations of physicists didn’t have any background in philosophy (and later generations of philosophers didn’t have any background in physics).

For example:

- Newtonian mechanics does not always have to be deterministic – this is surprisingly little known, see e.g. Norton’s dome example . Or see Earman’s Primer on Determinism for wider discussion on the compatibility of determinism with general relativity and quantum physics.

- Interpretations of QM. Lots to choose from, but for example dealing with the problem of probabilities in the many-worlds interpretation, or the discussion of the assumptions and results of Bell’s theorem, and whether it only poses a problem for hidden variable theories. Chapter 8 of Huw Price’s book is very good at discussing these two.

OK, these may not exactly be at the practical end of physics, but they still look like proper physics to me.