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The relationship between the quantum and the classical


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Could  it be said that there is a causal relationship between the way things work at the quantum level and the way they work  at the macro level?

 

Would it be something of a one way street?(ie is the quantum more fundamental and the classical  more derivative?)

 

Is causal the wrong word, might "emerge from"  be closer?

 

I am fishing here, but are there any (maybe many?) phenomena that could be described as both quantum and classical or is that just a bad way to look at it in the first place?

 

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7 hours ago, geordief said:

Could  it be said that there is a causal relationship between the way things work at the quantum level and the way they work  at the macro level?

Can you give an example? 

7 hours ago, geordief said:

Would it be something of a one way street?(ie is the quantum more fundamental and the classical  more derivative?)

There is an argument that QM is more fundamental; classical is what you get in the limit as you move from small scales to large.

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7 hours ago, geordief said:

Could  it be said that there is a causal relationship between the way things work at the quantum level and the way they work  at the macro level?

 

Would it be something of a one way street?(ie is the quantum more fundamental and the classical  more derivative?)

 

Is causal the wrong word, might "emerge from"  be closer?

 

I am fishing here, but are there any (maybe many?) phenomena that could be described as both quantum and classical or is that just a bad way to look at it in the first place?

 

 

I think you need to cast some more bread upon the waters or fish with a finer net.

The scope of your proposition seems too wide to me.

There are lots of different reasons why analysing the behaviour of a large aggregate may introduce additional features non existant or negligable in small ones.

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51 minutes ago, swansont said:

Can you give an example?

Just what I can't do.It feels to me that I may be   "arguing" from  the general rather than to advance from anything specific.

 

As @studiot says ,there could be different ways in which the macro operates differently from the micro.

 

At present my feeling is that the macro could be the statistical outcome of  the micro but that seems to be just a part of the picture,if correct.

If entanglement can operate on millions of particles then that might argue against my "statistical" idea.

 

 

 

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1 hour ago, studiot said:

There are lots of different reasons why analysing the behaviour of a large aggregate may introduce additional features non existant or negligable in small ones.

I agree. This is true classically - e.g. thermodynamics. 

 

45 minutes ago, geordief said:

Just what I can't do.It feels to me that I may be   "arguing" from  the general rather than to advance from anything specific.

The things that pop to mind are the ways QM differs from classical, even though underlying concepts are similar. 

If you look at bound states, the energies are quantized. But in macroscopic examples, the energy states can’t be distinguished so it looks like a continuum, which is our classical experience. But you seem to be asking for cases where quantum actually causes the classical behavior.

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4 minutes ago, swansont said:

But you seem to be asking for cases where quantum actually causes the classical behavior.

Yes. a behaviour that could be classed as quantum that would be solely responsible for (or would metamorphose into) a behaviour that could only be classed as classical.

 

I don't think entanglement would do that. There seems no limit ,outside practicalities  to how extensive it can be made to  behave.

 

I expect it would be possible to find classical behaviour that would be analagous to quantum behaviour but that would likely  mean very little.  

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3 hours ago, geordief said:

As @studiot says ,there could be different ways in which the macro operates differently from the micro.

Exactly why I suggested you narrow down the question.

Swansont clearly had similar ideas in asking for specific examples.

To be general about your question I would say that too much distinction is made between 'quantum' and 'classical'.

A system has certain behaviours which are driven by or influenced by specifc properties.

Those specific properties may have no influence at all on other behaviours of the system where you would need to invoke other different properties as appropriate.

Sometimes these properties will be quantum in origin, sometimes classical and sometimes the boundaries will be blurred.
There is no reason preventing that both quantum and classical properties being involved.

Two perfectly respectable classical objects, a hydrogen atom and an iodine atom, approach each other, a respectable classical action.

But if that approach is too close then a quantum property is brought into play and the atoms 'bond' together.
Classical macro Chemistry is based on this.
In the macro world, more complicated macro unions can arise with quantum properties.
Chemists call these Ligands.

Conversely Physicists distinguish metals and semiconductors, whose behaviour relies heavily on the quantum properties of the constituents.
So you can have your electronic devices.

 

 

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34 minutes ago, studiot said:

Sometimes these properties will be quantum in origin, sometimes classical and sometimes the boundaries will be blurred.
There is no reason preventing that both quantum and classical properties being involved.

Agree.

There are “semi-classical” models for some phenomena 

https://en.wikipedia.org/wiki/Semiclassical_physics

One needs to keep in mind that the approach is to use the best model that applies to the problem. e.g. if QM works best, you use QM. If you’re in between, then you use the hybrid approach 

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Fundamentally, the world is quantum, irrespective of the size of the system you are looking at.

The problem is just that superposition of states can persist only as long as no observation takes place, which means as long as the system does not interact with its environment (‘decoherence’). And the larger the size of a system, the harder it is to keep it isolated, and thus the more rapidly it decoheres. That’s the only reason why you never see quantum effects in the macroscopic world. There is no sharp ontological boundary between quantum and classical - the distinction is a relational one.

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11 hours ago, Markus Hanke said:

Fundamentally, the world is quantum, irrespective of the size of the system you are looking at.

That is still a matter of open debate in the scientific world.

 

 

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1 hour ago, studiot said:

That is still a matter of open debate in the scientific world.

With practical exceptions only at the margins, technically everything is 

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16 hours ago, Markus Hanke said:

Fundamentally, the world is quantum, irrespective of the size of the system you are looking at.

The problem is just that superposition of states can persist only as long as no observation takes place, which means as long as the system does not interact with its environment (‘decoherence’). And the larger the size of a system, the harder it is to keep it isolated, and thus the more rapidly it decoheres. That’s the only reason why you never see quantum effects in the macroscopic world. There is no sharp ontological boundary between quantum and classical - the distinction is a relational one.

Or the effects are too small to notice. People diffract walking through a door, but the reason we don’t notice is not solely decoherence.

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7 hours ago, swansont said:

Or the effects are too small to notice. People diffract walking through a door, but the reason we don’t notice is not solely decoherence.

Fair point, I shouldn’t have said “only” reason.

12 hours ago, studiot said:

That is still a matter of open debate in the scientific world.

 

 

Is it? How would one resolve the classical<>quantum transition other than via decoherence? 

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13 hours ago, studiot said:

That is still a matter of open debate in the scientific world.

 

 

Yes.   To say the world is essentially quantum is to assert scientific realism,  a philosophic view that many interpretations of QM reject.  I am more comfortable saying the world is essentially discrete packets, without any ontological assertions about superpositions, wavefunctions,  or cats.   

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12 hours ago, studiot said:

And your justification for this statement is ?

The fact that scientific knowledge is always provisional. It doesn’t deal with universal or objective truths chiseled into stone tablets. It deals instead with our current best models of the universe, but those models are forever subject to change as we learn new things.

Hence, everything in science… except perhaps for some rare exceptions at the margins… is open for debate. 

Edited by iNow
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14 hours ago, iNow said:

The fact that scientific knowledge is always provisional. It doesn’t deal with universal or objective truths chiseled into stone tablets. It deals instead with our current best models of the universe, but those models are forever subject to change as we learn new things.

Hence, everything in science… except perhaps for some rare exceptions at the margins… is open for debate. 

Except that you did not do this but made the bold claim that everything is quantised, in reply to my observation that quantisation of some quantities is still under debate.

 

On 10/16/2021 at 1:10 PM, iNow said:

With practical exceptions only at the margins, technically everything is 

Here is my reason for claiming that debate for space and time.

A recent technical book from Cambridge University

majid1.jpg.b3682a94e0f07e70264c3de6c411a0cb.jpg

 

Conversely I asked for yours claiming no debate for everything.

 

15 hours ago, TheVat said:

Yes.   To say the world is essentially quantum is to assert scientific realism,  a philosophic view that many interpretations of QM reject.  I am more comfortable saying the world is essentially discrete packets, without any ontological assertions about superpositions, wavefunctions,  or cats. 

 

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2 hours ago, studiot said:

Except that you did not do this but made the bold claim that everything is quantised, in reply to my observation that quantisation of some quantities is still under debate.

Since iNow made no such claim, this is pointless.

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2 hours ago, studiot said:

you did not do this but made the bold claim that everything is quantised

Please reread our exchange. You have misunderstood me. 

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3 hours ago, studiot said:

Here is my reason for claiming that debate for space and time.

A recent technical book from Cambridge University

What does it say about the way quantum phenomena transition to classical phenomena ?(if I have caught the gist of what you are saying)

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2 hours ago, geordief said:

What does it say about the way quantum phenomena transition to classical phenomena ?(if I have caught the gist of what you are saying)

Again, it would be so much easier to discuss specific examples.

 

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4 minutes ago, swansont said:

Again, it would be so much easier to discuss specific examples.

 

I would like to hear what Studiot has to say about the book ,whether specific examples or just how it illustrates the point he was making.

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3 hours ago, iNow said:

Please reread our exchange. You have misunderstood me. 

Surely, if I have misunderstood, you can explain your point in more detail so I can understand ?

1 hour ago, geordief said:

I would like to hear what Studiot has to say about the book ,whether specific examples or just how it illustrates the point he was making.

The flysheet explains the scope of the book, which encapsulates the ongoing debate about the granularity (quantisation) or continuity of space and time and spacetime.

Do you need more ?

flysheet.jpg.197dd3d7c94050cd14fba3cb85ffcdb2.jpg

 

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6 hours ago, studiot said:

Except that you did not do this but made the bold claim that everything is quantised,

I think clarification is in order. Firstly, I was the one who posted this, not iNow. Secondly, I said only that ‘the world is essentially quantum’ - a statement I do stand by. What I meant by this, in the context of the discussion, is that there is no ontological difference between the macroscopic and the microscopic world; what is different is only the degree by which these domains are subject to decoherence; the larger the system, the more quickly it will generally decohere. This is why you don’t normally observe interference effects in the macro world. Classical mechanics is thus an effective description in the macro domain. But do remember that there is no law forbidding such effects - if you can find a way to delay decoherence for long enough, even macro objects would in principle exhibit such behaviour, at least for a short time. This is an engineering problem, not one of fundamental physics.

I don’t believe that this is really in contention.

None of this implies that ‘everything is quantised’, which clearly it is not. Even in standard QM the spectrum of hermitian operators is continuous until you impose boundary conditions on your evolution equation - it’s only this that yields discreteness of observables. Quantum-ness does not necessarily equal discreteness; it means primarily that there are pairs of observables that do not commute.

And yes, I fully agree that quantisation of spacetime is very much under debate. I did not mean to imply otherwise.

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