Sean Carroll produced a Tweet storm on free will to be found beginning here.
Sean is a wonderful science communicator and educator, turning lots of people onto the fascinating and sometimes weird world of physics. He has come around to embracing the “many worlds interpretation” in his explanations and in doing so occupies a rare position among physicists with large public profiles. For example in his recent interviews - such as with Joe Rogan - he explicitly invoking our best understanding of quantum theory in some of his explanations. See here for my discussion of that: https://www.bretthall.org/the-multiverse.html).
What he does reject in many places Popperian epistemology (he is in good company there, of course) and instead regards Bayesianism as the way in which science actually works (which is consistent with almost all other science communicators and physicists with large public profiles). It is interesting that one explicit reason he does reject Popper’s epistemology (in particular Popper’s discovery of the true line of demarcation between science and non-science as being the “experimental test” or “the falsification criterion” is because at times he has written that many-universe-theories are not testable. Sean wrote an entire academic paper titled “Beyond Falsifiability” which I responded to in Tweet form as it contained empiricism and some misreading of Popper:
Ultimately, it is no useless thing to do metaphysics. At least some metaphysics is needed: laws of physics exist and things do not “just happen”. That’s a metaphysical claim: it’s a claim about physics. Cosmological theories that invoke other universes with different physical laws are, presently, metaphysics. But so what? If someone can devise a test that in principle (not in practise this moment - just in principle one day) would allow us to make any observation whatsoever that could rule out “many universes” or “single universes” then we meet Popper’s criterion. Until then, the fact some physicists work on the border of “experimentally testable” and “purely mathematical model” is hardly an insult to their work.
But the many worlds in quantum theory - Everettian quantum theory is testable. Now. As we have been at pains to point out both in David Deutsch’s original paper on this topic: shorturl.at/izBL0 or my explanation of that paper here for non-physicists: https://www.bretthall.org/philosophy-of-science.html
Sean’s recent tweet storm on free will is interesting because once again I find myself agreeing with so many of the basic sentiments: determinism does not rule out free will. Putting aside that, I guess I would disagree with Sean on precisely what we mean by free will. And yet he seems to straight up reject the notion that quantum theory is deterministic and he even mentions the many worlds interpretation.
But he does go on to Tweet that the determinism is “hidden”. I find this a needlessly confusing move in the explanation. I think to the non-physicist (aren’t most of us?) that claim would read as, once again: “The world is not deterministic.” Which is indeed the third sentence of his first tweet there. And I think most people will read that as: The world is not, actually, deterministic.
And so we perpetuate a misconception as old as quantum theory itself. Because quantum theory is deterministic. It’s not deterministic in the classical sense but that’s because, well, it’s not physics in the classical sense. But it’s still physics. There are still laws of physics and the laws of physics determine - which is to say mandate or predict or postdict or in principle can tell you what can happen and most importantly of all tell you (which is to say determine) what cannot possibly happen in the physical universe. So - the laws of physics are deterministic - quantum theory is deterministic and what happens in the many worlds or in the Everettian quantum multiverse - under unitary quantum theory is deterministic. As David Deutsch writes on the very first page of that paper I referred to earlier “the theory (i.e: Everettian Quantum theory) is deterministic: it says that the evolution of all quantities in nature is governed by differential equations (e.g. the Schrödinger or Heisenberg equations of motion) involving only those quantities and space and time, and thus it does not permit physically random processes.”
As I like to say and have said many times - it is subjectively unpredictable - but that is a very different thing indeed. What trick the magician is going to do next in his act or what joke the comedian is going to tell in his routine is subjectively unpredictable to you. But it’s determined. Entirely. By him or her. And the laws of physics determine what they do. Now perhaps Sean wants to call this “hidden determinism”. But, again, if one is new to all this, it would seem to be having a bet each way and invites the reader to ask: what do you mean “hidden”. It would seem under that explanation there is not-determinism and then…some other kind of determinism (hidden). But there’s not. There’s just determinism. But determinism does not mean you yourself can always predict what will happen next. But that was already true under classical physics anyway. It’s even true of what is going to happen next in a book written years ago that you have never read. It’s all fully determined there in ink on the page but it is unpredictable - to you!
A Sean himself says, “3) The whole point of compatibilism is that free will is compatible with deterministic (or indeterministic) laws. You are welcome to disagree, but "the world is deterministic" certainly doesn't count as a refutation!”
Indeed. And claiming the determinism is “hidden” does not count as a refutation of the claim “The world is deterministic.”
The world is deterministic.