New Carroll Podcast to Defend Many-Worlds

Sean M. Carroll is frustrated by physicists preferring Copenhagen over many-worlds, so he posted a podcast with his views:

Solo: Looking Quantum Mechanics in the Eyeball | Mindscape 355

One of the major obstacles to understanding quantum mechanics is the difficulty we have in simply accepting what the theory itself is telling us. The problem is that we know what the everyday world looks like -- stuff, arranged in space, evolving through time. So we can't resist the temptation to impose that picture on the quantum description, even if it's not actually there. In this solo episode I talk about what it means to take quantum mechanics at face value, and the difficult work involved in understanding how the everyday world of our experience fits into the picture.

For a more technical description, he refers to his paper, Reality as a Vector in Hilbert Space.

This podcast is a defense of many-worlds theory. He says people believe in textbook QM because they are stupid:

And I suspect that most of the people in the physics survey who said that they're Copenhagenists don't actually have that view themselves, but mostly because they haven't thought about it very carefully.

He has learned that he loses his audience if he talks about the parallel universes too much.

[24:35] Many worlds by contrast and we're not going to get into the worlds aspect of many worlds that much because it's actually not relevant for what we're talking about today. I've often said that many worlds is not mostly about the worlds. The worlds come along. They're there, no doubt. But what many worlds is really about is saying there's no such thing as collapse.

I think the problem here is that he makes several serious conceptual errors.

First, he misunderstands probability. Probability is not a real physical thing, like energy or temperature, but it is essential to all scientific theories. Let me repeat. All scientific theories are based on probability predictions.

You might say: No, when Nasa sent the Artemis II rocket around the Moon, it was all deterministic Newtonian physics.

I say: Not correct. Nasa calculated the splashdown location as a probability distribution.

Next, a probability is a prediction that something will happen, with other things not happening. If I say a coin toss has a 50% chance of heads, I am referring to getting heads, and not getting tails. If I get heads, I can then disregard any possibility of that toss being tails. For example, if I toss a coin three times, and the first is heads, then I can immediately eliminate the possibility of three straight tails.

The core concept behind many worlds does not even have anything to do with QM. It is: Throw away probabilities. Throw away definite outcomes. Assume all possibilities happen, in parallel universes.

If you toss a coin and get heads, it just means that you are in the universe with heads, and a parallel universe has tails. You cannot even say that the universes are equally likely.

When Carroll says (above) that "there's no such thing as collapse", he means that if you toss a coin and get heads, you cannot say that tails did not happen.

Carroll has no evidence for anything he says. His big argument is that it somehow simplifies QM to never predict any probabilities, never do any experiments, never accept any outcomes, and never reject any possibilities.

By eliminating QM's relations to the real world, he says we get more a pure and abstract theory where we do not have to worry about issues like what defines a measurement.

At 1:42:20, he denies that there should be any experimental test for many-worlds theory. He says "that's just not how science works."

On YouTube, a comment points out:

A very good introduction to the subject, but you dodge all the important critiques:

- Probability (Born Rule) is a huge problem for Many Worlds, but I know you are working on that.

- The Schrodinger equation is obviously wrong because spacetime is an assumed background. You dealt with space/locality a few times. There is work on entanglement creating space, and I agree so far, but how can QM/QFT get time to be emergent when it is a parameter not observable? (momentum is just a thick slice of time for mass position? - someone needs to handle that. What is the calculus of position and momentum in background-free local (infinitesimal) proper time?

- It seems that Hilbert Space changes dimension with the number of 'entities' in the system. But we know that 'entities' are variable (n particle quantum number is not defined). So we must immediately leap to the HS of the whole universe: a total matter/field equation that must be fixed, which defines everything (Wheeler/DeWitt). Anything less becomes some strange variable dimensional complex space - I don't buy that. [My personal world is computation and variable size matrices are just ... mathematically impossible. ] What happens with particle creation/destruction and second quantization - hint total hack!

- I don't see any contact with gravity - yes, AdS/CFT and holography, maybe perhaps with actual finite dS boundary defined by SR light cones, etc. but that has to emerge from large (in)finite HS, I don't buy it (yet). My tendency is to trust/believe the criticism from GR gravitationalists (Sir Roger, et al). I don't think you have addressed any of those criticisms (yet). You have to admit that background-free GR is much much more beautiful than background-dependent QFT (yeah, okay, subjective).

- Of all the symmetries in the SM, matter/antimatter and chirality asymmetry are the frontier. Yes, RH/LH neutrinos are vague/undecided/outside SM, and (maybe heavy dark) RH neutrinos could be the answer (cue Neil Turok) but GR people also have tentative answers for the background (Twistors, PIN decompositions, etc) I don't hear solutions coming from pure QFT formalisms.

I could go on, but enough already.

Carroll's excuse for ignoring all these points is "I have to confess that the whole idea of steelmanning is not really my vibe". [10:00]