Scott Aaronson on QM

“Look, if you teach an introductory course on quantum mechanics, and the students don’t have nightmares for weeks, tear their hair out, wander around with bloodshot eyes, etc., then you probably didn’t get the point across.”

—Scott Aaronson

Scott Aaronson cracks another wrong intuition of mine open yet again. (Ah, the thrill!) Here’s Scott on why “many great thinkers have found QM so hard to swallow”:

Why have so many great thinkers found quantum mechanics so hard to swallow? To hear some people tell it, the whole source of the trouble is that “God plays dice with the universe” — that whereas classical mechanics could in principle predict the fall of every sparrow, quantum mechanics gives you only statistical predictions.

Well, you know what? Whup-de-f@#%ing-doo! If indeterminism were the only mystery about quantum mechanics, quantum mechanics wouldn’t be mysterious at all. We could imagine, if we liked, that the universe did have a definite state at any time, but that some fundamental principle (besides the obvious practical difficulties) kept us from knowing the whole state. This wouldn’t require any serious revision of our worldview. Sure, “God would be throwing dice,” but in such a benign way that not even Einstein could have any real beef with it.

The real trouble in quantum mechanics is not that the future trajectory of a particle is indeterministic — it’s that the past trajectory is also indeterministic! Or more accurately, the very notion of a “trajectory” is undefined, since until you measure, there’s just an evolving wavefunction. And crucially, because of the defining feature of quantum mechanics — interference between positive and negative amplitudes — this wavefunction can’t be seen as merely a product of our ignorance, in the same way that a probability distribution can.

Today I want to tell you about decoherence and hidden-variable theories, which are two kinds of stories that people tell themselves to feel better about these difficulties.

And that’s basically how he starts off Lecture 11 of his now-famous course PHYS 771 (Quantum Computing Since Democritus), which is now available as a book. Go there for more!

Note: The quote above sounds nice as a matter of course, but it conflicts with my personal stance regarding the attitudes I want to have when I learn e.g. QM, expressed by Eliezer here:

I amnot going to tell you that quantum mechanics is supposed to be confusing.

I am not going to tell you that it’s okay for you to not understand quantum mechanics, because no one understands quantum mechanics, as Richard Feynman once claimed.  There was a historical time when this was true, but we no longer live in that era.

I am not going to tell you:  “You don’t understand quantum mechanics, you just get used to it.”  (As von Neumann is reputed to have said; back in the dark decades when, in fact, no one did understand quantum mechanics.)

Explanations are supposed to make you less confused.  If you feel like you don’t understand something, this indicates a problem—either with you, or your teacher—but at any rate a problem; and you should move to resolve the problem. 

I am not going to tell you that quantum mechanics is weird, bizarre, confusing, or alien.  QM is counterintuitive, but that is a problem with your intuitions, not a problem with quantum mechanics.  Quantum mechanics has been around for billions of years before the Sun coalesced from interstellar hydrogen.  Quantum mechanics was here before you were, and if you have a problem with that, you are the one who needs to change.  QM sure won’t.  There are no surprising facts, only models that are surprised by facts; and if a model is surprised by the facts, it is no credit to that model.

It is always best to think of reality as perfectly normal.  Since the beginning, not one unusual thing has ever happened.


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