Thread by @jimalkhalili

Musings on quantum mechanics (a thread with a few technical terms):

It's interesting how the big questions in science are always there, but they jostle for position as to which is fashionable at any given time and for any generation of scientists and philosophers. 1/17

If you ask philosophers of physics or physicists working on what are called 'foundational problems' what is the big issue they are grappling with today many will likely tell you it's either the origin of the arrow of time or the nature of quantum reality. 2/17

The latter of these (what's actually going on in the quantum world) has been something I have been thinking about for forty years. And, spoiler alert, I have not reached a conclusion to be revealed at the end of this thread. 3/17

As a PhD student in the 80s, I would complain that I hated the Copenhagen view because it explained nothing - it was no more than a recipe, a set of rules to make sense of the results of experiments and said nothing about what an electron is doing when I’m not looking. 4/17

My supervisors would tell me to, well, 'shut up and calculate' [a famous phrase used to describe the Copenhagen 'positivist' view]. An electron isn't a particle or a wave, they would say - an electron is an electron is an electron. Now go and calculate that S-matrix. 5/17

I remember in the late 80s attending a lecture by John Bell at Queen Mary College London and feeling relief that here was a world leading physicist who had the same concerns as me. I wasn’t aware of the, at the time largely underground, industry in foundations of QM. 6/17

Fast forward to today and I would argue that the majority of those people I mentioned earlier (philosophers of physics or physicists working on foundational problems) have (the way I like to think) come round to my way of thinking… 7/17

…that Bohr, great though he was, has a lot to answer for in persuading generations of physicists to accept that there is nothing more we squeeze out of quantum mechanics in the way of an explanation of reality beyond what we can actually measure. 8/17

So, what is the growing consensus today? I would say that people are coming round to siding with Einstein in the famous Einstein and Bohr debate - not that Einstein's hope that quantum mechanics was incomplete and that there had to be LOCAL hidden variables… 9/17

… that has now been quashed by both theory and experiment, but that the spirit of his position is the right one, that physics should be about how the world actually is (ontology) rather than only what we can say about the world (epistemology). 10/17

So, this brings me to the issue of what is exactly IS going on. If we acknowledge that, as the philosopher Tim Maudlin says, the Copenhagen view is just a recipe and what we want is a physical theory of reality, then most would agree that there are three frontrunners. 11/17

All of them work, in the sense of agreeing with the results of every experiment so far devised, and all explain in very different ways what is going on in the quantum realm, but we have yet to find a way of choosing between them that we can all agree on. 12/17

They are: (1) Everettian many worlds (2) Bohmian hidden variables and (3) spontaneous collapse models. All give descriptions of a physical reality underpinning the mathematical formalism of quantum mechanics. 13/17

All have their advocates, and all have their problems, which are of course highlighted by their opponents. But none of them have problems that are complete deal breakers. And people are working hard on all fronts to solve these issues to see which comes out on top. 14/17

We don't know which of these (or even a completely different interpretation) will eventually emerge as the correct one, but that's for physicists to figure out. Nature behaves in a certain way: either the world branches into different realities or it doesn't; … 15/17

…either the quantum state collapses randomly and spontaneously, or it doesn't; either there are real particles with definite positions and momenta at all times that are guided by a quantum wave or there aren't. 16/17

Now that I teach a masters course on decoherence theory and head a research group on quantum foundations I can legitimately start thinking about and working on this stuff. And no one is going to tell me to shut up and calculate. 17/17

18/17 (!) Oh I forgot to add: I am what’s called a realist, but agnostic on the different interpretations. As people who know me know, I have a soft spot for de Broglie-Bohm pilot wave theory, for reasons that I might explain in another Twitter thread one day.