An important question in physics is how the macroscopic world arises from microphysics. In thermodynamics there is the question of how irreversibility arises from reversible microphysics. In quantum theory there is the measurement problem - if you try to describe a measurement in quantum terms you need to add something extra. I've written more about this at http://quantropy.org/18/, but essentially, I think it would be good to be able to model the microphysics->macrophysics transition in the case of reversible microphysics, something which I would say has never been done.

The idea would be to model a macroscopic machine such as a measuring apparatus or a heat engine. I think that cellular automata provide the best approach. Work has been done in this area, such as http://arxiv.org/abs/0802.4365, but it's not there yet - in that paper the authors talk of modelling a 'protocell' (i.e. a macroscopic machine) as the next step.

I think that the understanding that such a model would bring would be very valuable, in particular for understanding thermodynamics, but also for understanding quantum theory (even though it's not a quantum model), as it might show that some things we thing are due to quantum weirdness are more due to the microscopic->macroscopic transition.

What do people think? Is such a thing feasible? What would be the best way to implement it? Would it bring the benefits that I think it would?

## Microphysics -> Macrophysics in a cellular automaton

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### Re: Microphysics -> Macrophysics in a cellular automaton

To the best of my knowledge, "quantum weirdness" can't be explained any way other than with quantum physics (or a suitably consistent expansion of it, of course). Trying to explain it in a deterministic model, no matter how sophisticated, is intrinsically impossible, due to the Bell inequalities (unless you accept some very weird things about the universe).

So no, a CA won't help here.

So no, a CA won't help here.

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### Re: Microphysics -> Macrophysics in a cellular automaton

quantropy wrote: I think it would be good to be able to model the microphysics->macrophysics transition in the case of reversible microphysics, something which I would say has never been done.

Are you certain about this? I'm pretty sure that using statistical mechanics you can model a microscopic to macroscopic transition starting from a quantum mechanical description. I think that irreversible processes can also be modeled in a similar manner. It's basically the same, except that it works out that the probability of the process reversing is zero, or near enough so as it doesn't matter.

### Re: Microphysics -> Macrophysics in a cellular automaton

Xanthir wrote:To the best of my knowledge, "quantum weirdness" can't be explained any way other than with quantum physics (or a suitably consistent expansion of it, of course). Trying to explain it in a deterministic model, no matter how sophisticated, is intrinsically impossible, due to the Bell inequalities (unless you accept some very weird things about the universe).

So no, a CA won't help here.

Well I'm not trying to explain all of quantum weirdness, just measurement problem (collapse of the wavefunction or whatever). What I'd be trying to show that the same thing might occur is a classical(ish) model. I wouldn't be trying to explain Bell's inequality.

But since you mention it, Bell doesn't demonstrate indeterminism, and certainly not the idea that you can save locality if you're willing to do away with determinism. Quite the opposite, one of my favourite quotes is "Locality=>Determinism" (J.S Bell in "Sixty-two years of uncertainty"). The point being that you can have locality and determinism if you accept those very weird things about the universe. Otherwise you have to accept nonlocality.

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### Re: Microphysics -> Macrophysics in a cellular automaton

Well it depends what you mean by 'model'. Yes, you can show in a general way that macroscopic properties emerge from the microphysics. However, this is the sort of thing that people tend to argue about endlessly - you can never be quite sure that you're not missing something important. What I want is a model of some macroscopic machine, such as a measuring device, in terms of the microphysics.LaserGuy wrote:Are you certain about this? I'm pretty sure that using statistical mechanics you can model a microscopic to macroscopic transition starting from a quantum mechanical description. I think that irreversible processes can also be modeled in a similar manner. It's basically the same, except that it works out that the probability of the process reversing is zero, or near enough so as it doesn't matter.

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### Re: Microphysics -> Macrophysics in a cellular automaton

Are you familiar with Von Neumann measurement? That's a closely related idea you might find helpful.

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### Re: Microphysics -> Macrophysics in a cellular automaton

I've recently found out about Gaussian Quantum Mechanics. (http://arxiv.org/abs/1111.5057) This is a restricted form of quantum mechanics, which exhibits some of the 'weirdness' of quantum mechanics (Uncertainty principle, measurement problem) but not all (Bell's theorem). The thing is that Gaussian Quantum Mechanics has a classical model - in fact it corresponds to a classical world where we are limited as to how precise our measurements can be.

My claim is that this is the situation you find yourself in if you try to fully construct a classical model - where measurements aren't just assumed to be possible to infinite precision, but need to be performed with measuring apparatus within the model.

How would the world appear to us if its ontology was that of classical mechanics but every agent faced a restriction on how much they could come to know about the classical state? We show that in most respects, it would appear to us as quantum.

My claim is that this is the situation you find yourself in if you try to fully construct a classical model - where measurements aren't just assumed to be possible to infinite precision, but need to be performed with measuring apparatus within the model.

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### Re: Microphysics -> Macrophysics in a cellular automaton

YO! There is no such thing as quantum weirdness.

All myths about QM being strange arose back before people even had a clue to what was going on.

Meditation: What is a measurement?

...

...

...

An interaction. Yes, that is right. It is an interaction between quantum thingies.

When you measure weirdly behaving quantum state X, some electron in the setup exchange a photon with some electron in your measurement apparatus, your apparatus then undergoes a radical state change, moving millions of elementary particles around, eventually resolving in a beeb of your computer, which changes an even larger amoung of particle states in your brain!

You are particles. Yes. You interact with the experiment, you are part of the system. Quantum systems are very sensitive, compared to say a pendulum, just reflecting light can change them.

QM is: Deterministc, Differentiable, CPT-reversible, Mathematically well-described, Not mysterious. It was in the 1950's, it isn't now. The measurment behaviour predicted by the Born Rule is still an open question, but we know it has something to do with the way we as humans decohere along with the system, causing the famously non-existant "collapse".

It needs nothing "extra." Feynmann's work is the rock solid foundation: the rest is about not falling into the trap of useless philosophy.

All myths about QM being strange arose back before people even had a clue to what was going on.

Meditation: What is a measurement?

...

...

...

An interaction. Yes, that is right. It is an interaction between quantum thingies.

When you measure weirdly behaving quantum state X, some electron in the setup exchange a photon with some electron in your measurement apparatus, your apparatus then undergoes a radical state change, moving millions of elementary particles around, eventually resolving in a beeb of your computer, which changes an even larger amoung of particle states in your brain!

You are particles. Yes. You interact with the experiment, you are part of the system. Quantum systems are very sensitive, compared to say a pendulum, just reflecting light can change them.

QM is: Deterministc, Differentiable, CPT-reversible, Mathematically well-described, Not mysterious. It was in the 1950's, it isn't now. The measurment behaviour predicted by the Born Rule is still an open question, but we know it has something to do with the way we as humans decohere along with the system, causing the famously non-existant "collapse".

It needs nothing "extra." Feynmann's work is the rock solid foundation: the rest is about not falling into the trap of useless philosophy.

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### Re: Microphysics -> Macrophysics in a cellular automaton

QM is: Deterministc, Differentiable, CPT-reversible

In order- usually no. often yes. usually no (the Schroedinger equation doesn't even have Galilean invariance, let alone full Lorentz invariance), what does C even mean if you have no anti-particles in the theory?

The measurment behaviour predicted by the Born Rule is still an open question

This is like saying "how to make predictions with the theory is still an open question." Hopefully you see why thats an issue.

Anyway, you should probably learn about quantum physics from a physics book and not 'less wrong'. But there are other threads where quantum interpretations are being discussed, to avoid dragging this off topic, maybe respond here viewtopic.php?f=18&t=100848 if you want to.

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