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Quantum Mechanics for Programmers

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Re: Quantum Mechanics for Programmers

#51

Post by Razengan »

adrianratnapala wrote: post #49
Razengan wrote: post #46

Earlier quoted context omitted.

I'm a layman and I don't really understand much of this, but I'm really intrigued: What's the general consensus in your field on the existence of more than 3 spatial dimensions? Seems to me, that there are a few phenomena that appear to be more or less random. Could they be perturbations caused by activity in dimensions we cannot perceive?

For first question: I'd say pop-science does a surprisingly good job of conveying the consensus on extra dimensions -- they are a totally reasonable possibility, but their effects are to be felt in realms far beyond the realms that experimental physicists can study well. But if they exist, they probably have important (but unknown) effects. And your proposal that they explain randomness is a good example of that. But…

I must clarify that by "activity" I don't mean directed actions (i.e. not talking about ghosts or gods) but fundamental elements — the waves and particles and fields — possessing properties and movement in more than 3 dimensions (not including time).

Like something poking through or pulling on a fabric from the other side, if such a property has, say, wavelike motion in a 4th spatial dimension, it may produce regular effects that manifest as fundamental features of our 3D space, or seem random to us if a bunch of things are bumping and deterministically interacting with each other in extra dimensions. Has this ever been considered in explaining things like [0] and [1]? Might have something to do with "dark" matter/energy too; i.e. stuff is there, just occupying other dimensions.

[0] https://en.wikipedia.org/wiki/Pair_production

[1] https://en.wikipedia.org/wiki/Quantum_foam

Re: Quantum Mechanics for Programmers

#52

Post by sideshowb »

hota_mazi wrote: post #36

It's puzzling to see the author call himself multiple times a scientist while lending so much importance to Occam's razor (which is spelled differently in the article, not sure if it's an alternative spelling in his language or a mistake). Occam's razor is not a law. It's not a fact. It's a simple suggestion if you're looking for a starting hypothesis. Not sure which way to start to investigate a phenomenon? Pick the…

Ock/cham's razor is a heuristic, not a law. It also runs into weirdness combined with MWI. Are infinite universes the simplest explanation, or have you just literally broken Occam's razor in the most egregious way possible?

Re: Quantum Mechanics for Programmers

#53

Post by sideshowb »

A slightly wacky article, but I do think there is lots more scope for explaining quantum physics via programming concepts. I got what little understanding I have of QM by creating a game that simulates it. I even made my own wacky article as well :) https://linkingideasblog.wordpress.com/2016/04/25/learning-q...

Re: Quantum Mechanics for Programmers

#54

Post by tomonl »

monktastic1 wrote: post #34
codethief wrote: post #25

Earlier quoted context omitted.

> Another professor […] once said that the single particle wave function is more fundamental than the multi-particle wave function. Nevermind […] his research […] where the molecular wave function can't be well approximated using a product of single particle wave functions Depends on what you mean by "multi-particle wave function". The way it is usually understood (I think), it includes all possible tensor products o…

Tensor products only describe the separable (i.e., unentangled) states.

As I understand it, that is only true when dealing with density matrices. For example, |0>⊗|0> + |1>⊗|1> is entangled and has tensor products. I think that Xcelerate is correct in saying that all combinations of the basis vectors form the basis of the multi-particle Hilbert space, as a single-particle wavefunction is just a vector/ket.

Re: Quantum Mechanics for Programmers

#55

Post by [deleted] »

monktastic1 wrote: post #34
codethief wrote: post #25

Earlier quoted context omitted.

> Another professor […] once said that the single particle wave function is more fundamental than the multi-particle wave function. Nevermind […] his research […] where the molecular wave function can't be well approximated using a product of single particle wave functions Depends on what you mean by "multi-particle wave function". The way it is usually understood (I think), it includes all possible tensor products o…

Tensor products only describe the separable (i.e., unentangled) states.

[deleted]

Re: Quantum Mechanics for Programmers

#56

Post by adrianratnapala »

foldr wrote: post #48
Xcelerate wrote: post #14

Earlier quoted context omitted.

> 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 t…

>So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem. At the risk of sending things off an a huge tangent, it's interesting to see physicists recognizing that an infinite regress is, at least sometimes, unsatisfactory (even though there is of course nothing incoherent per se about the concept of an infinite sequence). Physicis…

The point is that you can prove that a thing is self-consistent without proving it is true. And I think what are calling an "infinite regress" is, in this case, self-consistency.

Proving self-consistency is a decent achievement, and is certainly error. But it is only weak evidence in favour of a position.

Re: Quantum Mechanics for Programmers

#57

Post by foldr »

adrianratnapala wrote: post #56
foldr wrote: post #48

Earlier quoted context omitted.

>So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem. At the risk of sending things off an a huge tangent, it's interesting to see physicists recognizing that an infinite regress is, at least sometimes, unsatisfactory (even though there is of course nothing incoherent per se about the concept of an infinite sequence). Physicis…

The point is that you can prove that a thing is self-consistent without proving it is true. And I think what are calling an "infinite regress" is, in this case, self-consistency. Proving self-consistency is a decent achievement, and is certainly error. But it is only weak evidence in favour of a position.

> And I think what are calling an "infinite regress" is, in this case, self-consistency.

I don't understand what you mean by this.

Re: Quantum Mechanics for Programmers

#58

Post by adrianratnapala »

platz wrote: post #9
monktastic1 wrote: post #7

Earlier quoted context omitted.

It is generally agreed (except, perhaps, by the strongest champions of the decoherence program) that decoherence does not completely solve the measurement problem. Some good references here: http://physics.stackexchange.com/questions/295527/decoherenc... It helps explain the loss of interference, but it does not resolve the question of why and how we see one particular outcome.

> it does not resolve the question of why and how we see one particular outcome. agreed. dechoherence doesn't explain particular outcomes . It explains the scale & magnitude of mixing quantum states from different systems. I thought "what kinds of physical interactions qualify as measurements" was referring to a different part of understanding QM. Decoherence doesn't explain which outcome , it does explains "part" of…

The problem here is that very much of what we mean by "scale and mangitude" boils down to the frequency of particular outcomes when situations are repeated.

So Decoherence only explains those things after you already have the Born rule (P ∝ |Ψ|^2). But that's the very thing we are trying to explain!

Re: Quantum Mechanics for Programmers

#59

Post by n4r9 »

AgentME wrote: post #37
B1tchard0 wrote: post #35

This article could have been written by an algorithm. Everyone knows the meme about Quantum Mechanics being incomprehensible, like, ~"if you understand quantum mechanics, you don't understand quantum mechanics". Quantum Mechanics requires randomness, because determinism is scary. Both probability and fate are functions of time, and time is the most interesting thing to look at. Generally, Time is ignored, or at best…

>Quantum Mechanics requires randomness, because determinism is scary. Isn't the many-worlds interpretation generally regarded as deterministic?

Yes, as is de-Broglie-Bohm interpretation. Some others make no assertions about determinism.

I interpreted "because determinism is scary" as a tongue-in-cheek aside. Either way, time is not ignored: there is serious research effort into the concept of time in quantum mechanics.

Re: Quantum Mechanics for Programmers

#60

Post by Xcelerate »

adrianratnapala wrote: post #50
Xcelerate wrote: post #14

Earlier quoted context omitted.

> 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 t…

> quantum systems appear indeterministic to him. > However, the state of [experimenter + system] > is governed by an entirely deterministic equation ... This sort of thing only makes sense in the context of many-worlds QM, and it is amusing how many professed non-many-worlders say such things. We often describe quantum systems using an entirely deterministic (Schroedinger) equation. But we don't know in what sense th…

> and it is amusing how many professed non-many-worlders say such things.

I'm not saying I do or don't believe any of this (if anything, I'm interpretation-agnostic at the moment). I'm just pointing out that there is a contradiction in having one postulate demand unitary state evolution (the Schrödinger equation) for some ill-defined "system" while another postulate says that unitarity is broken at the system/environment boundary. While there's been plenty of attempts to work around this (e.g. https://arxiv.org/abs/quant-ph/0101012), I wouldn't say that anyone has formulated a consistent set of axioms that definitively resolves the issue.

> We often describe quantum systems using an entirely deterministic (Schroedinger) equation. But we don't know in what sense that equation describes the physical state of the experimenter + system, or in what sense it is "just" a probability model.

Agreed. It's certainly a useful model but it leaves out all kinds of interesting phenomena that we observe in practice (namely, relativistic and radiative effects). Curiously though, if you turn to QFT for a better probabilistic model, Haag's theorem (https://en.wikipedia.org/wiki/Haag%27s_theorem) implies that a universal Hilbert space representation cannot describe both free and interacting fields (a problem that the non-relativistic Schrödinger equation doesn't have!)

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