Help with correct description of Everettian QM

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Re: Help with correct description of Everettian QM

#41  Postby twistor59 » May 04, 2016 8:17 am

DavidMcC wrote:
crank wrote:
aban57 wrote:back on topic, the first thing I thought when I read your post was this :

DavidMcC wrote:[
The reality is that QM is inappropriate to large, massive systems. It is only appropriate for very small, very low mass systems, such as atoms and molecules, unless the temperature is below a critical value. Once the energy difference between states of the whole system becomes less than thermal fluctuations, the model breaks down.


I didn't go deep into the QM field, but I knew that. I'm still trying to figure out how scientists of that level can overlook that. Or am I missing something ? :scratch:

DavidMcC is way overstating this. QM applies everywhere, it's just not practical at the macroscopic levels.

Re the bolded bit: wrong. If the coherence length is shorter than the distance between particles, then no wavefunction applies, and it is a classical system. OK?
EDIT: Also, the coherence length goes down as the mass goes up.


No, that doesn't make sense. To even say "the coherence length is shorter than the distance between the particles", there has to be a wavefunction to take the coherence length of!
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Re: Help with correct description of Everettian QM

#42  Postby Cito di Pense » May 04, 2016 8:58 am

twistor59 wrote:No, that doesn't make sense. To even say "the coherence length is shorter than the distance between the particles", there has to be a wavefunction to take the coherence length of!


So maybe the universe runs by quantum mechanics on Tuesdays, and by classical mechanics on Fridays. This is a paraphrase of somebody's famous quote, but I can't be bothered to look it up.

TBH, I doubt it's a powerful argument to say that a rendering of quantum weirdness in natural language 'makes no sense'. In fact, it does make no sense, but then, it never does. "Making sense" is a catchphrase of internet debate, and as such, it is highly overrated, due to lack of spoon-bending, unless a mathematical argument is present to make sense of. What's silly is what you point out, that talking in terms of coherence length while insisting it doesn't apply is a recipe for wibble.

This probably isn't being said as well as it can be said, but David has a point: Proposing to do quantum calculations on a system best described in classical terms is only done to prove a point, and at that, it isn't practical. Since you're an engineer, this makes sense to you, whether you'll admit it or not.

Saying that "QM applies everywhere", while it currently can be assumed, is a throwaway line nonetheless. As we review it, there's no point to saying something applies everywhere if we can't apply it everywhere, except in our overactive imaginations. That's filosofeezing, for ya.
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Re: Help with correct description of Everettian QM

#43  Postby crank » May 04, 2016 9:07 am

DavidMcC wrote:
crank wrote:
aban57 wrote:back on topic, the first thing I thought when I read your post was this :

DavidMcC wrote:[
The reality is that QM is inappropriate to large, massive systems. It is only appropriate for very small, very low mass systems, such as atoms and molecules, unless the temperature is below a critical value. Once the energy difference between states of the whole system becomes less than thermal fluctuations, the model breaks down.


I didn't go deep into the QM field, but I knew that. I'm still trying to figure out how scientists of that level can overlook that. Or am I missing something ? :scratch:

DavidMcC is way overstating this. QM applies everywhere, it's just not practical at the macroscopic levels. Really more like impossible, there are on the order of 1027 atoms in everyday objects, the equations/solutions needed for fairly simple interactions of a few electrons, photons, protons, etc, can get too intractable very quickly, 1027 particles interacting and forget about it. Newton does quite nicely. But, the macro systems are still obeying QM, it is still the theory underlying pretty much everything, it just won't make nice with relativity. Your smart phone is one of the very few things normal human beings interact with that actually require both theories to work, QM for all the electronics, and GR for the GPS.

Re the bolded bit: wrong. If the coherence length is shorter than the distance between particles, then no wavefunction applies, and it is a classical system. OK?
EDIT: Also, the coherence length goes down as the mass goes up.


So you're saying that Newton trumps QM now? Can you tell me what physicists would say this? The wave function collapsing doesn't mean QM collapses, this is a fignewtonment of your imagination.

[Edited to add quotes, this was reply to DMcC, sorry for any confusion]
Last edited by crank on May 04, 2016 12:10 pm, edited 1 time in total.
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Re: Help with correct description of Everettian QM

#44  Postby Cito di Pense » May 04, 2016 9:24 am

crank wrote:So you're saying that Newton trumps QM now? Can you tell me what physicists would say this? The wave function collapsing doesn't mean QM collapses, this is a fignewtonment of your imagination.


Do you want something that you can say 'works everywhere', crank? If that's the case, go with God, top dog in ideological warfare. If you want something that works everywhere, don't use too precise a definition of works.
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Re: Help with correct description of Everettian QM

#45  Postby crank » May 04, 2016 10:12 am

Cito di Pense wrote:
crank wrote:So you're saying that Newton trumps QM now? Can you tell me what physicists would say this? The wave function collapsing doesn't mean QM collapses, this is a fignewtonment of your imagination.


Do you want something that you can say 'works everywhere', crank? If that's the case, go with God, top dog in ideological warfare. If you want something that works everywhere, don't use too precise a definition of works.

I made it fairly clear earlier my understanding of this in post #4, where I said
Plus, it's my understanding that QM, meaning the Standard Model IS the theory for everything but the regimes requiring GR. It's not that QM doesn't apply to macro systems, it's just not feasible to do so, and Newtonian mechanics, classical mechanics, is more than adequate. At least is what I've understood for a long time. I'm obviously no physicist.


If this is not right, I'm of course open to correction, as I hope is clear in that quote. As for god, strangely, he's got his TOE shoved all the way up his ass, and can't get it out.
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Re: Help with correct description of Everettian QM

#46  Postby twistor59 » May 04, 2016 11:43 am

Cito di Pense wrote:

This probably isn't being said as well as it can be said, but David has a point: Proposing to do quantum calculations on a system best described in classical terms is only done to prove a point, and at that, it isn't practical. Since you're an engineer, this makes sense to you, whether you'll admit it or not.

Saying that "QM applies everywhere", while it currently can be assumed, is a throwaway line nonetheless. As we review it, there's no point to saying something applies everywhere if we can't apply it everywhere, except in our overactive imaginations. That's filosofeezing, for ya.


Well David's sentence was manifestly wrong, and if you want to investigate the places "where quantum mechanics applies", I don't think it'll be the case that "beyond a certain mass", quantum theory no longer works. If that were the case, my bucketload of superfluid in the kitchen would stop being superfluid. In my opinion ( there are many other opinions on this!) where you do get into trouble applying quantum mechanics universally is when you try to apply it to the universe all at once. Because then the observer (measuring equipment, not necessarily conscious observer) also is part of the wavefunction, and gets put into superpositions, so how do you ask the questions which have probabilistic answers? As far as I can tell, science needs to separate things into a system it can observe, and something classical outside the system which can ask the questions.

This something outside the system has to be capable of doing measurements. The key identifying feature of a measurement seems to be irreversibility.
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Re: Help with correct description of Everettian QM

#47  Postby aban57 » May 04, 2016 12:19 pm

crank wrote:
Cito di Pense wrote:
crank wrote:So you're saying that Newton trumps QM now? Can you tell me what physicists would say this? The wave function collapsing doesn't mean QM collapses, this is a fignewtonment of your imagination.


Do you want something that you can say 'works everywhere', crank? If that's the case, go with God, top dog in ideological warfare. If you want something that works everywhere, don't use too precise a definition of works.

I made it fairly clear earlier my understanding of this in post #4, where I said
Plus, it's my understanding that QM, meaning the Standard Model IS the theory for everything but the regimes requiring GR. It's not that QM doesn't apply to macro systems, it's just not feasible to do so, and Newtonian mechanics, classical mechanics, is more than adequate. At least is what I've understood for a long time. I'm obviously no physicist.


If this is not right, I'm of course open to correction, as I hope is clear in that quote. As for god, strangely, he's got his TOE shoved all the way up his ass, and can't get it out.


I thought that Schrödinger's cat thought experiment' failure as a point was precisely that quantum mechanics didn't apply outside the scope of infinitely small ?
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Re: Help with correct description of Everettian QM

#48  Postby crank » May 04, 2016 12:24 pm

aban57 wrote:
crank wrote:
Cito di Pense wrote:
crank wrote:So you're saying that Newton trumps QM now? Can you tell me what physicists would say this? The wave function collapsing doesn't mean QM collapses, this is a fignewtonment of your imagination.


Do you want something that you can say 'works everywhere', crank? If that's the case, go with God, top dog in ideological warfare. If you want something that works everywhere, don't use too precise a definition of works.

I made it fairly clear earlier my understanding of this in post #4, where I said
Plus, it's my understanding that QM, meaning the Standard Model IS the theory for everything but the regimes requiring GR. It's not that QM doesn't apply to macro systems, it's just not feasible to do so, and Newtonian mechanics, classical mechanics, is more than adequate. At least is what I've understood for a long time. I'm obviously no physicist.


If this is not right, I'm of course open to correction, as I hope is clear in that quote. As for god, strangely, he's got his TOE shoved all the way up his ass, and can't get it out.


I thought that Schrödinger's cat thought experiment' failure as a point was precisely that quantum mechanics didn't apply outside the scope of infinitely small ?

Who said it failed? Can a thought experiment fail? The question is somewhat what this thread is about.
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Re: Help with correct description of Everettian QM

#49  Postby aban57 » May 04, 2016 12:32 pm

crank wrote: Can a thought experiment fail?


You clearly haven' read this topic :)

A thought experiment is built to prove a point. If it's based on a false premises, like this one, it won't prove its point, so I guess one can say that it failed.
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Re: Help with correct description of Everettian QM

#50  Postby VazScep » May 04, 2016 2:33 pm

Cito di Pense wrote:Do you want something that you can say 'works everywhere', crank? If that's the case, go with God, top dog in ideological warfare. If you want something that works everywhere, don't use too precise a definition of works.
In principle! Details not forthcoming.

I make this complaint when someone mentions the unreasonable effectiveness of mathematics when it comes to describing the universe. It's supposedly effective with unbounded computation and near perfect information of the state of the universe. In other words, its effectiveness, in principle, isn't known, and it isn't very effective at stuff like the weather.
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Re: Help with correct description of Everettian QM

#51  Postby crank » May 04, 2016 3:15 pm

aban57 wrote:
crank wrote: Can a thought experiment fail?


You clearly haven' read this topic :)

A thought experiment is built to prove a point. If it's based on a false premises, like this one, it won't prove its point, so I guess one can say that it failed.

I had already run a quick thought experiment after sampling some parts of that thread to decide whether I'd want to read up on the posts and participate. It told me the effort would be immense, and the benefit nil, so I haven't bothered. Which illustrates what I think 'thought experiments' really are, which is why it's one of Dan Dennett's 'thinking tools' [I think it is, pretty sure, but ????], something to aid in thinking about, analyzing, a situation, problem, puzzle, etc. Not being a real experiment, it can't really prove anything. It can be used to argue a point, as an aid informing some part of the argument, but the real failure is the argument, not the 'experiment'. This isn't worth arguing about. I'm explaining how I think of them, maybe you have a different perspective, there being no 'fact' to be decided or argued.
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Re: Help with correct description of Everettian QM

#52  Postby DavidMcC » May 09, 2016 1:25 pm

twistor59 wrote:
Cito di Pense wrote:

This probably isn't being said as well as it can be said, but David has a point: Proposing to do quantum calculations on a system best described in classical terms is only done to prove a point, and at that, it isn't practical. Since you're an engineer, this makes sense to you, whether you'll admit it or not.

Saying that "QM applies everywhere", while it currently can be assumed, is a throwaway line nonetheless. As we review it, there's no point to saying something applies everywhere if we can't apply it everywhere, except in our overactive imaginations. That's filosofeezing, for ya.


Well David's sentence was manifestly wrong, and if you want to investigate the places "where quantum mechanics applies", I don't think it'll be the case that "beyond a certain mass", quantum theory no longer works. If that were the case, my bucketload of superfluid in the kitchen would stop being superfluid. ...

If you read my posts in this thread more carefully, you will notice that I have already dealt with large-scale quantum effects, such as superfluidity and superconductivity - they have a critical temperature to become thermodynamically possible, OK?
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Re: Help with correct description of Everettian QM

#53  Postby twistor59 » May 15, 2016 1:40 pm

DavidMcC wrote:
If you read my posts in this thread more carefully, you will notice that I have already dealt with large-scale quantum effects, such as superfluidity and superconductivity - they have a critical temperature to become thermodynamically possible, OK?


You haven't "dealt" with anything. You claimed:

Re the bolded bit: wrong. If the coherence length is shorter than the distance between particles, then no wavefunction applies, and it is a classical system. OK?
EDIT: Also, the coherence length goes down as the mass goes up.


With reasonable sized systems (i.e. ones not taking up a significantly large fraction of the entire universe) the quantum models apply. You'd be a dick to try to apply them because it would be too complex, but apply they do. There is no critical mass at which quantum mechanics suddenly ceases to apply. OK?
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Re: Help with correct description of Everettian QM

#54  Postby DavidMcC » May 17, 2016 12:27 pm

twistor59 wrote:
DavidMcC wrote:
If you read my posts in this thread more carefully, you will notice that I have already dealt with large-scale quantum effects, such as superfluidity and superconductivity - they have a critical temperature to become thermodynamically possible, OK?


You haven't "dealt" with anything. You claimed:

Re the bolded bit: wrong. If the coherence length is shorter than the distance between particles, then no wavefunction applies, and it is a classical system. OK?
EDIT: Also, the coherence length goes down as the mass goes up.


With reasonable sized systems (i.e. ones not taking up a significantly large fraction of the entire universe) the quantum models apply. You'd be a dick to try to apply them because it would be too complex, but apply they do. There is no critical mass at which quantum mechanics suddenly ceases to apply. OK?

Depends what you mean by "apply". The wavefunction has two aspects: amplitude and phase. In MOST macroscopic systems, phase becomes irrelevant, because it is scrambled (the wavelengths being so short). That is what I mean by QM not applying.
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Re: Help with correct description of Everettian QM

#55  Postby John Platko » Nov 08, 2017 2:52 pm

Here's a good documentary on Everett's life and his work as his son tries to understand more about his dad.

I like to imagine ...
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Re: Help with correct description of Everettian QM

#56  Postby DavidMcC » Nov 15, 2017 6:07 pm

crank wrote:
DavidMcC wrote:
crank wrote:
aban57 wrote:back on topic, the first thing I thought when I read your post was this :



I didn't go deep into the QM field, but I knew that. I'm still trying to figure out how scientists of that level can overlook that. Or am I missing something ? :scratch:

DavidMcC is way overstating this. QM applies everywhere, it's just not practical at the macroscopic levels. Really more like impossible, there are on the order of 1027 atoms in everyday objects, the equations/solutions needed for fairly simple interactions of a few electrons, photons, protons, etc, can get too intractable very quickly, 1027 particles interacting and forget about it. Newton does quite nicely. But, the macro systems are still obeying QM, it is still the theory underlying pretty much everything, it just won't make nice with relativity. Your smart phone is one of the very few things normal human beings interact with that actually require both theories to work, QM for all the electronics, and GR for the GPS.

Re the bolded bit: wrong. If the coherence length is shorter than the distance between particles, then no wavefunction applies, and it is a classical system. OK?
EDIT: Also, the coherence length goes down as the mass goes up.


So you're saying that Newton trumps QM now? Can you tell me what physicists would say this? The wave function collapsing doesn't mean QM collapses, this is a fignewtonment of your imagination.

[Edited to add quotes, this was reply to DMcC, sorry for any confusion]

V. late reply (sorry - I missed this part until now);
No, I'm not saying that "Newton trumps QM", or that wavefunction collapse means QM collapse! :roll:
These are just rather silly straw man-type misinterpretations by you.
And, yes, the coherence length in the CM frame is useful for determining whether QM will produce significantly different results from classical mechanics.
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Re: Help with correct description of Everettian QM

#57  Postby DavidMcC » Nov 15, 2017 6:18 pm

twistor59 wrote:
DavidMcC wrote:
crank wrote:
aban57 wrote:back on topic, the first thing I thought when I read your post was this :



I didn't go deep into the QM field, but I knew that. I'm still trying to figure out how scientists of that level can overlook that. Or am I missing something ? :scratch:

DavidMcC is way overstating this. QM applies everywhere, it's just not practical at the macroscopic levels.

Re the bolded bit: wrong. If the coherence length is shorter than the distance between particles, then no wavefunction applies, and it is a classical system. OK?
EDIT: Also, the coherence length goes down as the mass goes up.


No, that doesn't make sense. To even say "the coherence length is shorter than the distance between the particles", there has to be a wavefunction to take the coherence length of!

Of course there is, but, if that length turns out to be way shorter than the particle separation, it will effectively be an incoherent system if thermal fluctuations in energies are taken into account. Then, wavefunction phase angles are scrambled, leading to addition of intensities rather than additon of amplitudes. This, in turn, suppresses quantum coherence effects, and Newton is good enough.
OK?
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Re: Help with correct description of Everettian QM

#58  Postby newolder » Jan 25, 2018 6:41 pm

twistor59 wrote:
crank wrote:
But,while you're shutting up and calculating, you're not extremely curious? I'm not knowledgeable enough to be passionate about any of the given choices, but still passionately curious about it all. I would bet most on 'none of the above', that whatever they figure out is going to be weirder than anything they're thinking, and simpler somehow.


Yes, I do have some curiosity: I imagine the final answer will involve something we don't know about yet. Something to do with spacetime and information maybe? I don't really know! I find trying to understand the parts of physics that are already well established to be challenging enough for the moment. :lol:


An essay at FQXi by Sean Carroll and Ashmeet Singh seems to make progress along the lines you suggest... I could be wrong as I haven't read it all yet but I keep hearing talk of 'emergent spaces' and/or 'emergent spacetimes' these days related to the entropy of the fields under study. All very interesting and leading towards matter transports? Who can tell? "Beam me sideways, Scotty!"
Essay Abstract

To the best of our current understanding, quantum mechanics is part of the most fundamental picture of the universe. It is natural to ask how pure and minimal this fundamental quantum description can be. The simplest quantum ontology is that of the Everett or Many-Worlds interpretation, based on a vector in Hilbert space and a Hamiltonian. Typically one also relies on some classical structure, such as space and local configuration variables within it, which then gets promoted to an algebra of preferred observables. We argue that even such an algebra is unnecessary, and the most basic description of the world is given by the spectrum of the Hamiltonian (a list of energy eigenvalues) and the components of some particular vector in Hilbert space. Everything else – including space and fields propagating on it – is emergent from these minimal elements.

FQXi source
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