Bohr told Wheeler that it was a pile of crap because it was a pile of crap. In particular, the "splitting of the worlds" made no sense. Even today, in 2018, it makes absolutely no sense and no fan of these Everett ideas can tell you anything whatsoever about the question whether the worlds split at all, when they split, why they split, how many branches there are. You may suggest several answers to each questions, none of them can be completed to a convincing let alone quantitative theory, and in fact, none of them has a significantly greater support among the Everett fans than others. They don't seem to care. ...Motl is correct. I think a lot of people have the misconception that MWI has some way of calculating which worlds or outcomes are more probable, but it has nothing of the kind. MWI does not make any testable predictions.
On top of that, even if you solved these problems in some way, the many worlds theory will have nothing to do with science – with predictions. All predictions of quantum mechanics have the form of probabilities, continuous numbers assigned to possible results of experiments, or their functions or functionals. And no Everett's fan has an idea how these probabilities could be written into the many worlds, or extracted from the many worlds. It's just not possible. If this many world theory predicts something, it's the wrong prediction that all probabilities should be rational – the number of worlds would be the denominator because if several worlds obviously exist, they should be "equally likely". Well, the actual outcomes in quantum mechanics are not. It just doesn't make the slightest sense. And all predictions in quantum mechanics are functions of these continuous probabilities. Because the many worlds philosophy can't be reconciled with the continuous probabilities at all (or it seems to predict wrong probabilities), it can't be reconciled with the predictions as such – it cannot possibly have anything to do with science within the quantum mechanical framework.
Peter Woit writes:
The calculation [of the spectrum of the hydrogen atom] in Many Worlds is exactly the same textbook calculation as in Copenhagen. It’s the same Schrodinger equation and you solve for its energy eigenvalues the same way. That is the problem: there’s no difference from the standard QM textbook.This is just wrong. There is no known way to do a MWI calculation that matches some real world object like a hydrogen atom.
Jim Baggott says:
All this really shouldn’t detract from the main point. The formalism is the formalism and we know it works (and we know furthermore that it doesn’t accommodate local or crypto non-local hidden variables). The formalism is, for now, empirically unassailable. All *interpretations* of the formalism are then exercises in metaphysics, based on different preconceptions of how we think reality could or should be, such as deterministic (‘God does not play dice’). Of course, the aim of such speculations is to open up the possibility that we might learn something new, and I believe extensions which seek to make the ‘collapse’ physical, through spacetime curvature and/or decoherence, are well motivated.In a sense, this is correct. If an interpretation reproduces all the calculations used to test the theory, then whether to accept it is a philosophical issue, not an empirical one.
But until such time as one interpretation or extension can be demonstrated to be better than the other through empirical evidence, the debate (in my opinion) is a philosophical one. I’m just disappointed (and rather frustrated) by the apparent rise of a new breed of Many Worlds Taliban who claim – quite without any scientific justification – that the MWI is the only way and the one true faith.
... this endless debate over interpretation is really a philosophical debate, driven by everybody’s very different views on what ‘reality’ ought to be like. And, as such, we’re unlikely to see a resolution anytime soon…
For example, the solar system has geocentric and heliocentric interpretation, and preference for heliocentric is philosophical, not scientific.
The trouble with this is that most of these QM interpretations are not really interpretations. MWI does not reproduce any calculations of QM, and does not have any empirical support.
MWI is just like QM except for (1) MWI has no way of making quantitative predictions (like the Born rule), and (2) MWI postulates parallel worlds where all possibilities exist and no world has an effect on any other world.
These two properties make MWI completely disconnected from any scientific analysis. With no predictions, it cannot be tested. And the parallel worlds are just subjective fantasies, with no relation to our world.
More and more, I see physicists argue that the MWI is the only scientific interpretation of QM, because the Copenhagen interpretation somehow fails to solve the "measurement problem" or to define what is "real". Whatever you might think to be shortcomings of the CI, the MWI does not solve any of them, and does not even qualify as a scientific theory. It is a mystery how otherwise-smart physicists could fall for something so ridiculous.
Tim Maudlin attacks Woit:
It is a bit hard to know how to comment on a discussion of a book called “What is Real?” when it has been asserted thatNo, when quantum philosophers ask "what is real?", they are not asking about existence or physical consequences. They are usually searching for a nonlocal hidden variable theory that is supposed to match their nonlocal intuition. They subscribe to a belief that QM is defective, and a hidden variable theory would be better.
“I’d rather do almost anything with my time than try and moderate a discussion of what is “real” and what isn’t.
Any further discussion of ontology will be ruthlessly suppressed.”
The question “What is real?” just is the question “What exists?” which is in turn just the question “What is the true physical ontology?” which is identical to the question “Which physical theory is true?”. Peter Woit begins by writing “Ever since my high school days, the topic of quantum mechanics and what it really means has been a source of deep fascination to me…”. But that just is the question: What might the empirical success of the quantum formalism imply about what is real? or What exists? or What is the ontology of the world? To say you are interested in understanding the implications of quantum mechanics for physical reality but then ruthlessly suppress discussions of ontology is either to be flatly self-contradictory or to misunderstand the meaning of “ontology” or of “real”. That is also reflected in the quite explicit rejection of any discussion of two of the three possible solutions to the Measurement Problem: pilot wave theories and objective collapse theories.
At least Maudlin is not defending MWI. But pilot wave theories are nonlocal, and objective collapse theories are hard to reconcile with experiment.