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Quantum Mechanics for Programmers

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Re: Quantum Mechanics for Programmers

#61

Post by andrepd »

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…

It's not only a heuristic, it's also a principle. It states that "when choosing between two theories which make the exact same predictions, choose the one with the fewer assumptions". Which is to say, correctness comes first, of course, but when deciding between two equally correct theories, choose the one with the fewer assumptions.

Re: Quantum Mechanics for Programmers

#63

Post by naasking »

monktastic1 wrote: post #8
platz wrote: post #4

Earlier quoted context omitted.

I thought "coherence" was the model by which quantum systems spread entanglement to other systems; the larger the system the 1st system into contact with, the bigger the effect of coherence loss in the 1st system and the larger the "measurement"

Also see this piece by Steven Weinberg: http://www.nybooks.com/articles/2017/01/19/trouble-with-quan... "One common answer is that, in a measurement, the spin (or whatever else is measured) is put in an interaction with a macroscopic environment that jitters in an unpredictable way. For example, the environment might be the shower of photons in a beam of light that is used to observe the system, as unpredictable in p…

Doesn't the probability basically follow from the uncertainty of your own eigenstate, ie. the system performing the measurement? This contextuality is why deterministic interpretations of QM also entail probabilistic measurements.

Re: Quantum Mechanics for Programmers

#64

Post by naasking »

monktastic1 wrote: post #3

Hmm he seems to imply that MWI is the "right" interpretation, and that the measurement problem is solved. Most physicists would not agree. If you follow the link to his MWI description, there's this gem: > But fortunately, I knew computer science, which most physicists do not know, with the Church-Turing thesis, which roughly states that anything physical can be simulated by a computer. But that is not what the Churc…

> But that is not what the Church-Turing thesis says. A Turing machine cannot mimic a truly random physical process, pretty much by definition.

It's not clear that a truly random process exists. There exist deterministic interpretations of QM, for instance. Certainly unpredictable processes exist, but that's an entirely different classification (whether a Turing machine halts is also unpredictable, but still deterministic).

Further, Turing machines can generate pseudo-random outputs that pass all known randomness tests.

Re: Quantum Mechanics for Programmers

#65

Post by naasking »

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…

Ockham's razor is not a heuristic, it's the only principled universal prior for Bayesian reasoning. This was formalized in Solomonoff Induction.

Re: Quantum Mechanics for Programmers

#66

Post by naasking »

andrepd wrote: post #61
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…

It's not only a heuristic, it's also a principle. It states that "when choosing between two theories which make the exact same predictions, choose the one with the fewer assumptions". Which is to say, correctness comes first, of course, but when deciding between two equally correct theories, choose the one with the fewer assumptions.

Not quite accurate. "Fewest assumptions" assumes that axioms are equally comparable, but this isn't necessarily true. Obviously one should eliminate redundant assumptions, ie. ones that have no effect on observable predictions between two theories, but this provides little guidance for selecting between two theories with drastically different axioms that differ in only a small set of predictions for which we have no data.

Ockham's razor, when formalized as in Solomonoff Induction, suggests preferring theories with the lowest Kolmogorov complexity.

Re: Quantum Mechanics for Programmers

#67

Post by naasking »

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?

IIRC, it's deterministic in a sense that isn't equivalent to the way de Broglie-Bohm is deterministic. The latter is generally what people mean by deterministic, ie. it's a classical theory with an extra term to account for quantum influences.

Re: Quantum Mechanics for Programmers

#68

Post by platz »

adrianratnapala wrote: post #58
platz wrote: post #9

Earlier quoted context omitted.

> 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!

Maybe you are trying to explain the Born rule, but I was not claiming to explain that; I've already stated decoherence doesn't explain why we get certain outcomes instead of others.

Re: Quantum Mechanics for Programmers

#69

Post by platz »

Xcelerate wrote: post #60
adrianratnapala wrote: post #50

Earlier quoted context omitted.

> 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/environ…

I feel like if I claimed the schrodinger equation was just a probability model, I'll be immediately lambasted because "the wave equation is reality" or then I'm immediately a "local hidden variables proponent"

Re: Quantum Mechanics for Programmers

#70

Post by chriswarbo »

Xcelerate wrote: post #14
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.

> 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…

Let's say I toss a fair coin; before I look at it, the outcome is indeterministic to me: there's a 50/50 chance of heads or tails. Once I look at it, I gain 1 bit of information. To another experimenter, the 'me + coin' system is indeterministic: it's either me seeing heads or me seeing tails. Once I tell that experimenter the outcome, they gain 1 bit of information. To a different experimenter, the two of us with the coin is indeterministic... and so on.

Is this scenario fundamentally different than the quantum system? Is this scenarion also a "problem"?

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