The other participants are David Albert, Sheldon Goldstein, and Rüdiger Schack, with the conversation moderated by Brian Greene. The group is not merely a randomly-selected collection of people who know and love quantum mechanics; each participant was carefully chosen to defend a certain favorite version of this most mysterious of physical theories.QBism is essentially the same as the Copenhagen interpretation, as promoted by Bohr and Heisenberg around 1930.
David Albert will propound the idea of dynamical collapse theories, such as the Ghirardi-Rimini-Weber (GRW) model. They posit that QM is truly stochastic, with wave functions really “collapsing” at unpredictable times, with a tiny rate that is negligible for individual particles but becomes rapid for macroscopic objects.
Shelly Goldstein will support some version of hidden-variable theories such as Bohmian mechanics. It’s sometimes thought that hidden variables have been ruled out by experimental tests of Bell’s inequalities, but that’s not right; only local hidden variables have been excluded. Non-local hidden variables are still very viable!
Rüdiger Schack will be telling us about a relatively new approach called Quantum Bayesianism, or QBism for short. (Don’t love the approach, but the nickname is awesome.) The idea here is that QM is really a theory about our ignorance of the world, similar to what Tom Banks defended here way back when.
My job, of course, will be to defend the honor of the Everett (many-worlds) formulation. I’ve done a lot less serious research on this issue than the other folks, but I will make up for that disadvantage by supporting the theory that is actually true. And coincidentally, by the time we’ve started debating I should have my first official paper on the foundations of QM appear on the arxiv: new work on deriving the Born Rule in Everett with Chip Sebens.
GRW has the virtue of being potentially testable, but there is no evidence for it yet. Albert likes it mainly out of a philosophical rejection of positivism, and a belief that GRW somehow better satisfies his beliefs in scientific realism.
The argument for Bohmian mechanics is even more philosophically misguided. It is based on a belief that a theory of nonlocal hidden variables with action-at-a-distance is somehow more scientific that a theory based on observables and experiment. The truth is nearly the opposite.
The many-worlds interpretation adds mysterious unobservable parallel universes, without any scientific payoff.
Quantum mechanics has certain paradoxes, but so do other theories. Relativity has the twin paradox, the reality of the length contraction, and the fact that two events can appear simultaneous to one observer, and not to another.
We do not have reputable physicists running around complaining that relativity does not match their intuitions. Relativity explains the observations, and if it upsets your intuitions, then there is something wrong with your intuitions.
Likewise quantum mechanics matches the observations. For Albert, Carroll, Greene, and Goldstein to complain that it is not scientifically realistic or has some other philosophical defect, they are just showing their stubbornness to accept what was established in 1930.
The many worlds "interpretation" doesn't allow us to calculate – or imprint – the generally different probabilities of different outcomes into the "strands" of the film. Even the fans of this religion admit it's the case but some of them say that they are "working on a fix" which is supposed to be enough. (A similarly "modest" fix makes Genesis compatible with all the detailed data about the cosmic microwave background, the DNA, and genetics.)You might think that that an event would be consider more probable if it occurs in most of the parallel universes. However the MWI advocates deny any scientific meaning to counting the universes. Mitchell Porter explains:
What they don't realize is that the inability of this theory or "interpretation" to calculate any probabilities isn't just a moderate vice or disadvantage. It is a complete, rigorous proof that this philosophy has nothing to do with the empirical data or science whatever and people who are defending it don't have the slightest clue what they are talking about. Everything that modern science predicts are probabilities or their functions. If your ideas don't predict any probabilities, they don't predict anything at all. They have nothing to do with science.
Moreover, there can't be any fix. There can't even exist a candidate theory that would "extract" the probabilities from something else.
If you are a many-worlds theorist, and you want to explain e.g. why QM says event A is twice as probable as event B, the logical explanation is that event A is twice as common as event B, when all the parallel worlds are considered. But Deutsch, Wallace, and now Carroll and Sebens, all reject this approach.They simply don't believe in probabilities of outcomes because they believe all outcomes happen. They are denying modern science as we know it, and so are Albert, Greene, and Goldstein.
Carroll and Sebens explicitly recommend against trying to count parallel worlds / branches of the wavefunction. For example, on page 15 they speak of “the unrealistic assumption that the number of branches in which a certain outcome occurs is well-defined”.
Lumo writes again:
The people who are trying to revive the objective reality – whether they are assuming Bohmian "real particles plus guiding waves" or various forms of "objective GRW-like collapses" or "many worlds" or any other classical visualization what's going on – are not closer than proper quantum mechanical physicists to genuine science. Instead, they are stubbornly defending the indefensible, a notion (of the objective reality) similar to a tooth fairy that cannot be extracted from our real perception of observations and that may actually be shown to be wrong by a careful enough (but not excessively complicated) analysis of several representative processes in the microscopic world.I would not phrase it that was, but I do think that belief in those interpretation is rooted in some faulty idea about what science is all about. Instead of accepting that science is about observations, they have a belief that science is about hidden variables instead.