Tuesday, February 20, 2018

Interpreting QM to doubt quantum computing

A comment said:
Scott is being a good sport here and telling the truth about quantum computers. They can't work without MWI! RIGHT!
In a later posting, Aaronson says:
Which interpretation of QM you espouse (e.g., MWI, Copenhagen, or Bohm) has no effect—none, zero—on what you should predict about the scalability of quantum computation, because by explicit design, all interpretations make exactly the same predictions for any experiment you can do on any system external to yourself.
This is contrary to the opinion of others like David Deutsch, who say that the many-worlds interpretation is what justifies quantum computing.

So Aaronson would presumably deny that he jumped on the many-worlds (MWI) bandwagon in order to justify quantum computing.

This comment gives an example of a very famous and respect theoretical physicist not believing in quantum computing:
As another approach, there’s a somewhat weird book by Gerard ‘t Hooft that you probably know about (warning, 3MB download):


It explicitly (p. 87) says it’s incompatible with large-scale QC and that if such QC happens then the book’s proposed theory is falsified. So at least it says something concrete :).
'tHooft was one of the main masterminds behind the Standard Model of particle physics, but he also has some funny ideas about super-determinism. So I think he is probably right about large-scale QC being impossible, but I am not endorsing his reasoning.

It is funny to warn about a 3MB download, as that is the average size of a web page today.


  1. But the rest of his comment says basically the same thing because he said in a previous comment that he doesn't take seriously other "interpretations" that say something beyond the math:

    "There are 'interpretations' like dynamical collapse that aren’t interpretations at all, but proposals for new physical theories. By all means, let’s test QM on larger and larger systems, among other reasons because it could tell us that some such theory is true or—vastly more likely, I think—place new limits on it! (People are trying.)"

    He continues the comment you cited:

    "The one thing in foundations of QM that does matter for QC, is simply whether you believe QM is literally true or whether you think it needs to be modified. As I never tire of pointing out (because others never tire of forgetting it), if QM did need to be modified, that would be a far greater scientific breakthrough than a mere success in building scalable QCs, and we can only hope that the quest to build QCs would terminate with such an exciting outcome. In any case, though, we should be clear that this discussion is completely orthogonal to the main discussion on this thread, which was between two groups who both provisionally accept that QM is true, and one of whom cares about it being explained without the Exponential Parallelism Fallacy."

    But he is taking a position about the literal nature of the math itself:

    "This has nothing to do with the QC’s performance advantage, which is all about being able to choreograph interference patterns in a Hilbert space of exponentially large dimension."

    He just doesn't think he's making an interpretation that isn't tested. There is no empirical test to demonstrate what he is saying, except for quantum computers. However, we can't strictly prove a negative, if they can't get them to work. He went MWI because he had to. It's basically conceding the point.

    1. Scott believes in blobs: "In other words, to whatever extent a collection of universes is useful for quantum computation, to that extent it is arguable whether we ought to call them 'parallel universes' at all (as opposed to parts of one exponentially-large, self-interfering, quantum-mechanical blob). Conversely, to whatever extent the universes have unambiguously separate identities, to that extent they’re now 'decohered' and out of causal contact with each other. Thus we can explain the outputs of any future computations by invoking only one of the universes, and treating the others as unrealized hypotheticals. To clarify, I don’t regard either of the above objections to Deutsch’s argument as decisive, and am unsure what I think about the matter. My purpose, in setting out the objections, was simply to illustrate the potential of quantum computing theory to inform debates about the Many-Worlds Interpretation."

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  3. No interpretations of quantum mechanics have ever made sense to me. QM doesn't make sense, because it does not explain how the wave function collapses. What does it mean physically to "measure something"? QM doesn't explain this.

    Nevertheless, quantum mechanics works in that it makes predictions about experiments.

  4. By the way, the cellular automata hypothesis is completely idiotic. If it was such a natural explanation, then why is it so hard to emulate such systems in hardware? It's another random abstraction like strings. Having taken automata theory from Ullman, I can safely say I'm completely unimpressed with the little gimmicks they have created outside of compiler design. Conway's Game of Life is completely idiotic and comical. The primitives of cellular automata only appear to be simple but they aren't, and furthermore, they show very little expressive power in terms of life processes or physics. I have programmed evolution games with simple primitives that work much more naturally and could express WAAAYYY more advanced phenomena, such as life processes. They get something to blink and think it’s an accomplishment.

    FPGAs are one of the most complicated pieces of hardware out there. Trust me.

  5. Hype alert: https://www.sciencenews.org/article/google-moves-toward-quantum-supremacy-72-qubit-computer