Quantum Mechanics for Programmers

I'm a quantum physicist (well, we don't say that, I'm an atomic physicist, but for anyone not aware it is 99% quantum mechanics we do all day), and the MWI isn't universally accepted, but it's not universally rejected either. Plenty of important physicists interpret quantum mechanics that way, and I do too (I am not important though). That's not to say I'm confident it's correct, just that it's the most sensible way to understand the theory as we know it so far.

It might not completely solve the measurement problem, in that the purported derivations of the Born rule are hotly contested, but neither does any other interpretation - it gets further than most. I don't see anyone else except pilot wave theory (which violates locality) deriving the Born rule either.

"measurement is not special, and observers are just quantum systems like any other" seems like it ought to be the default assumption to make, in the absence of any evidence to the contrary, and honestly I think the reputation of MWI as a bit crazy comes about for purely historical reasons. Namely, Bohr didn't like it and he held a lot of sway.

Physicists are human, and parallel universes is pretty strange, so most people reject the idea without thinking about it in much detail.

Most physicists' opinions shouldn't count for much anyway, because the measurement problem simply doesn't come up in our day to day work, so most haven't thought about it much.

Furthermore, plenty of physicists don't actually grasp the fundamentals of their own field - they specialise a lot and can use specialised theories to get what they need done without understanding in detail where it came from.

So I really think the decoherence people, as cranky as their website looks and as oddly as they write, ought to be the ones whose opinions count on the matter.

Edit: seriously, check out their website, straight outta the nineties:

http://decoherence.de

> that decoherence does not completely solve the measurement problem

It's kind of funny how the problem keeps getting pushed to higher levels of "meta":

If you consider the experimenter and his system, measurements of (non-eigenstate) quantum systems appear indeterministic to him. However, the state of [experimenter + system] is governed by an entirely deterministic equation that follows a reversible, unitary path through time. Great! But the problem is that you then have another experimenter who measures that composite system, and the outcomes he sees likewise appear indeterministic. So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem.

> Since you are a programmer, you do not know what science is, even though you may believe you do. A hint: Computer science does not contain science, just mathematics.

He probably wrote this half in jest, but it is actually a serious issue in fields like Natural Language Processing and Computer Vision that are have their intellectual roots in Computer Science.

Papers in CV and NLP are about data structures, algorithms, software engineering methods, neural networks, statistical models, and so on. They do not discuss anything in particular about the structure of language or the properties of images. This is because they are descended from CS. In CS, exemplar results are ideas like QuickSort and Dijkstra's algorithm. These algorithms work on any list or graph; you do not have to know anything special about the particular properties of the list or graph you are operating on.

As an illustrative anecdote, I went to an NLP talk given at MIT by a well-known Google/Berkeley parsing researcher. He gave a talk about a system that used neural networks to do sentence parsing. In the QA session, I mentioned the idea of verb argument structure, and asked how the system would learn, for example, that a verb like "persuade" or "argue" can take a that-complement, while other verbs cannot. He didn't really have an answer, because it wasn't the kind of thing that he worried or thought about. My guess is that he did not consider such a question to be relevant to his field.

This book looks very promising. I just bought the kindle version on amazon. Thank you.

Hey, something I can comment on properly for once ;). Handed in my final Quantum-field-theory homework yesterday (or blood-sweat-and-theory as I called it, great fun) and finishing a MSc. in Computational science 'soon'.

From what I can follow, most of it is, unfortunately either varying degrees of wrong or just confusing as all hell. :\

Some assorted quotes:

>The electron is not in a single place, but instead spread out over all the positions, more or less. This is called "superposition".

No, the superposition principal states that individual states (contributions) can be summed in a linear fashion. I know this is not a very clear way of wording it, but it is one of those things I think is quite hard to word but very easy to understand once you see it.

(Although one could say this is a form of superposition, namely a sum of infinite delta functions in position-space. But this would be the most confusing example to use)

>The smart programmer would guess at a model containing more complex math that will get the array to model several particles, but no such thing exists.

Quantum field theory.

> These examples have used cubes with a width of 1000 voxels, and of 1000 time instants. The Universe use a width of something like 10^70 voxels. The same goes for time.

I assume this is (roughly matches) size-of-observable-universe/plank-length. But this is misinterpretation of the planck length. As far as we (I) know, spacetime is continouis. (string theorist migth disagree, I am not familiar).

Furthermore, the many particle approach is not at all reasonable. One would (usually) use lattice-QFT. where one simulates a field for each type of particle (the field can have several components) and particles are identified as excitation of this field. The most well known is lattice-QCD.

>Quantum Electro Dynamics

Quantum Electrodynamics ;)

>Richard Feynman got the nobel prize for figuring out a way of doing this. His Quantum Electro Dynamics is a sort of dynamic programming method

This part is right (I have no idea what the next few sentences are trying to say). Feyman diagrams (those fun squilly drawings) represent the results of some awful, awful integrals. The real analytical answer is integrated over two infinite spaces. But one can do a taylor expansion to get a answer which can be computed. Feynman noted that you can read off a few rules from this approximation and assign drawings to them. The answer can then be computed by summing all (topologically distsinct graphs) instead of the terms in the Taylor sum. It's much easier than it sounds (and sure as hell more fun than doing integrals), and you can copy-paste entire sections of your diagrams as long as the in and out-puts match up. perfctly suitable for dynamic programming.