In his lecture, Weinberg expressed a newfound sympathy for the critics of quantum mechanics.
“I’m not as happy about quantum mechanics as I used to be, and not as dismissive of the critics. And it’s a bad sign in particular that those physicists who are happy about quantum mechanics, who see nothing wrong with it, don’t agree with each other about what it means.”You can watch the full lecture here. (The above quote is at 17:40.)
It’s become a cliché that physicists in their late years develop an obsession with quantum mechanics. On this account, you can file Weinberg together with Mermin and Penrose and Smolin. I’m not sure why that is. Maybe it’s something which has bothered them all along, they just never saw it as important enough. Maybe it’s because they start paying more attention to their intuition, and quantum mechanics – widely regarded as non-intuitive – begins itching. Or maybe it’s because they conclude it’s the likely reason we haven’t seen any progress in the foundations of physics for 30 years.
Whatever Weinberg’s motivation, he doesn’t like neither Copenhagen, nor Many Worlds, nor decoherent or consistent histories, and he seems to be allergic to pilot waves (1:02:15). As for qbism, which Mermin finds so convincing, that doesn’t even seem noteworthy to Weinberg.
I learned quantum mechanics in the mid-1990s from Walter Greiner, the one with the textbook series. (He passed away a few weeks ago at age 80.) Walter taught the Copenhagen Interpretation. The attitude he conveyed in his lectures was what Mermin dubbed “shut up and calculate.”
Of course I as most other students spent some time looking into the different interpretations of quantum mechanics – nothing’s more interesting than the topics your prof refuses to talk about. But I’m an instrumentalist by heart and also I quite like the mathematics of quantum mechanics, so I never had a problem with the Copenhagen Interpretation. I’m also, however, a phenomenologist. And so I’ve always thought of quantum mechanics as an incomplete, not fundamental, theory which needs to be superseded by a better, underlying explanation.
My misgivings of quantum mechanics are pretty much identical to the ones which Weinberg expresses in his lecture. The axioms of quantum mechanics, whatever interpretation you chose, are unsatisfactory for a reductionist. They should not mention the process of measurement, because the fundamental theory should tell you what a measurement is.
If you believe the wave-function is a real thing (psi-ontic), decoherence doesn’t solve the issue because you’re left with a probabilistic state that needs to be suddenly updated. If you believe the wave-function only encodes information (psi-epistemic) and the update merely means we’ve learned something new, then you have to explain who learns and how they learn. None of the currently existing interpretations address these issues satisfactorily.
It isn’t so surprising I’m with Weinberg on this because despite attending Greiner’s lectures, I never liked Greiner’s textbooks. That we students were more or less forced to buy them didn’t make them any more likable. So I scraped together my Deutsche Marks and bought Weinberg’s textbooks, which I loved for the concise mathematical approach.
I learned both general relativity and quantum field theory from Weinberg’s textbooks. I also later bought Weinberg’s lectures on Quantum Mechanics which appeared in 2013, but haven’t actually read them, except for section 3.7, where he concludes that:
“[T]oday there is no interpretation of quantum mechanics that does not have serious flaws, and [we] ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is merely a good approximation.”It’s not much of a secret that I’m a fan of non-local hidden variables (aka superdeterminism), which I believe to be experimentally testable. To my huge frustration, however, I haven’t been able to find an experimental group willing and able to do that. I am therefore happy that Weinberg emphasizes the need to find a better theory, and to also look for experimental evidence. I don’t know what he thinks of superdeterminism. But superdeterminism or something else, I think probing quantum mechanics in new regimes is best shot we presently have at making progress on the foundations of physics.
I therefore don’t understand the ridicule aimed at those who think that quantum mechanics needs an overhaul. Being unintuitive and feeling weird doesn’t make a theory wrong – we can all agree on this. We don’t even have to agree it’s unintuitive – I actually don’t think so. Intuition comes with use. Even if you can’t stomach the math, you can build your quantum intuition for example by playing “Quantum Moves,” a video game that crowd-sources players’ solutions for quantum mechanical optimization problems. Interestingly, humans do better than algorithms (at least for now).
|[Weinberg (left), getting some|
kind of prize or title. Don't
know for what. Image: CASW]
I don’t think for example that numerological coincidences are problems worth thinking about – they’re questions of aesthetic appeal. The mass of the Higgs is much smaller than the Planck mass. So what? The spatial curvature of the universe is almost zero, the cosmological constant tiny, and the electric dipole moment of the neutron is for all we know absent. Why should that bother me? If you think that’s a mathematical inconsistency, think again – it’s not. There’s no logical reason for why that shouldn’t be so. It’s just that to our human sense it doesn’t quite feel right.
A huge amount of work has gone into curing these “problems” because finetuned constants aren’t thought of as beautiful. But in my eyes the cures are all worse than the disease: Solutions usually require the introduction of additional fields and potentials for these fields and personally I think it’s much preferable to just have a constant – is there any axiom simpler than that?
The difference between the two research areas is that there are tens of thousands of theorists trying to make the fundamental laws of nature less ugly, but only a few hundred working on making them less weird. That in and by itself is reason to shift focus to quantum foundations, just because it’s the path less trodden and more left to explore.
But maybe I’m just old beyond my years. So I’ll shut up now and go back to my calculations.