As noted in the previous post, quantum mechanics is weird. If we try to have a realist understanding of what’s happening, it forces bizarre choices about which aspects of common sense reality we throw under the buss.
The central mystery is the wave function collapse. Quantum particles move like waves, mathematically described by the wave function, but when the particles hit and leave a mark, that is when it’s measured, it’s always as a localized point-like ball, a classical particle. People have been trying to find an explanation for this for almost a century now, which is why we have so many interpretations of what’s happening.
One approach is to regard the wave function as not real, as simply a mathematical contrivance, one that aids in predictions, but doesn’t reflect actual reality. The problem I’ve long had with this move is the very reason we have a wave function in the first place, the interference effects between the portions of the wave. Something is causing those effects.
But once we accept any degree of wave function realism, we hit a dilemma. Where does the rest of the wave, all the other possible outcomes, go on measurement? One answer is that they are simply annihilated from reality. In other words, the wave function collapse isn’t just an epistemic event, but an objective physical one.
I’ve never been particularly enthusiastic about this option. It’s long seemed like too much of an added assumption about how physics works, one not sufficiently motivated by the data. But one strength of the view is that it’s ultimately a testable proposition.
However as Philip Ball discusses in a Quanta article this week, it’s a test that most objective collapse models appear to be failing. Ball notes in his piece that these experimental failures don’t completely close the door on objective collapse models. It’s always possible to tweak the models. But as the tweaks add up, they start to look increasingly less well motivated and more convoluted. That appears to be where we’re getting to with objective collapse theories.
Where does that leave us? Well, pilot-wave theories, where there’s both a particle and a wave the entire time, remain, although that option is arguably already problematic due to the inability to reconcile it with quantum field theory. (And any modifications to quantum field theory would have to be compatible with all the data accumulated from the LHC and earlier experiments, no easy feat.)
There are also superdeterministic theories like the ones Sabine Hossenfelder is trying to find. And there are outright retrocausal theories like the Transactional interpretation. Although both of these seem to need additional assumptions, hidden variables, to work. Adding to the mathematical structure of the theory without disturbing its generalization into quantum field theory seems like a tough challenge.
And of course, there’s always the mad uncle in the basement, simply accepting the mathematical structure as is without any form of collapse. As experiments continue to rule out the other options, this one is becoming increasingly harder to ignore. But, due to its many-worlds implications, it’s probably the option people find the most disturbing, so it’ll likely be resisted for a long time.
Or we can just throw up our hands, give up on realist options, and simply fall back to observable and mathematical predictions. Although I suspect making progress on quantum gravity will require taking into account how the theory can be reconciled with other aspects of physics, and that will likely require a realist approach, at least to some degree.
Unless of course I’m missing something?