The relationship between the quantum and the classical

[geordief]

1 hour ago, studiot said:

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

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 ?

 

No,just trying to keep up with the conversation as much as I can. 

[iNow]

3 hours ago, studiot said:

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

Of course I can, but as you seem not to have reread the exchange as I earlier recommended, and as Markus has already clarified that it was he who made the claim at which you took umbrage, and also since Markus has already further elucidated his intended meaning when making said claim, it seems both moot and unnecessary for me to bother. 

Edited October 18, 2021 by iNow

[studiot]

On 10/18/2021 at 1:55 AM, iNow said:

Of course I can, but as you seem not to have reread the exchange as I earlier recommended,

You have no evidence for what I have or have not read.

So why not just pony up with your explanation ?

 

By the way I seem to be having at least three different discussions with three different members in this thread.

Here, I am responding to your connection of the phrase 'everything is' (as I have already explained) to the my separate discussion with Markus.

On 10/16/2021 at 11:19 AM, studiot said:

On 10/16/2021 at 12:03 AM, 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.

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

With practical exceptions only at the margins, technically everything is 

If you mean that everything is open for debate then I apologize for the misunderstanding.

If you mean the everything is or is not quantised, which is the subject of my discussion with Markus, then please confirm this.

 

I will reply to Markus separately as he seems to have missed some important points or incorrectly dismissed them as unimportant.

 

I sincerely hope you do not mean that everything in Science or even everthing of importance in Science, is quantised, as that impression is easy to dispel by asking for the quantisation of Avogadro's number.

[studiot]

On 10/18/2021 at 12:08 AM, Markus Hanke said:

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.

Yes, let's be clear as it is my turn to respond in our discussion on this.

This discussion is sparate from the one with iNow, who said something different and I am answering separately.

 

You did indeed say this and I quoted you in my original challenge.

You have since amplified your proposition

On 10/18/2021 at 12:08 AM, Markus Hanke said:

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.

 

Yes I have never credited you with saying everything is quantised.

But you do seem to be only considering one aspect of quantum theory and also implying that other aspects are insignificant.

This latter is not the case.

The commutation of observables only occurs directly in Heisenberg matrix theory, from which it arises quite naturally mathematically and leads to quantisation.

But coherence and decoherence do not refer to these variables, but to wave functions which arise in Schrodinger wave theory as partial differential equations in the (quantum) wave function.

The quantisation here again arises quite naturally in the choosing of integer multiples of solutions, but there is no commutation involved and further we obtain the familiar discreteness being talked about.

You are correct in saying that the devil is in the detail, in the guise of the boundary conditions.

 

A further point is that there are many (very) important quantities in Physics which are not quantised, for instance many dimensionless numbers, Avogadro's number, etc

Finally not just for you but a general point, Physics is far from all of Science.

So for instance the biological difference between a coal tit, a blue tit, a great tit and so on, and their classification, has nothing whatsoever to do with quantum theory.

[swansont]

14 minutes ago, studiot said:

A further point is that there are many (very) important quantities in Physics which are not quantised, for instance many dimensionless numbers, Avogadro's number, etc

The claim was not that everything is quantised, and a constant isn’t something that qualifies for consideration 

 

14 minutes ago, studiot said:

Finally not just for you but a general point, Physics is far from all of Science.

We’re discussing a physics topic in the physics section, so it’s reasonable to take as given we’re discussing physics.

 

 

[Markus Hanke]

15 minutes ago, studiot said:

But you do seem to be only considering one aspect of quantum theory and also implying that other aspects are insignificant.

Apologies if that was how it came across, that wasn’t the intention. Of course there are other aspects - besides non-commutative observables there is superposition of states, and the violation of Bell’s theorem which implies there are correlations which are stronger than classically allowed. 

I don’t think these things are wholly independent though.

21 minutes ago, studiot said:

A further point is that there are many (very) important quantities in Physics which are not quantised, for instance many dimensionless numbers, Avogadro's number, etc

Of course. But we are talking about observables of the theory, which these are not.

[iNow]

9 hours ago, studiot said:

If you mean that everything is open for debate then I apologize for the misunderstanding.

Apology accepted

[studiot]

What a pity no one else is prepared to give way even a tiny bit here and accept that someone else has something valid worth stating.

 

Perhaps you are all totally correct.

That would be the day

[iNow]

Perhaps restate the question you feel is being ignored.

[joigus]

On 10/15/2021 at 4:06 AM, geordief said:

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

I think the answers that you got here are more than enough to go by. But let me try and add a very strong inkling that the quantum must be more fundamental. There is a well-known result of formal (mathematical) quantum mechanics called Ehrenfest's theorem, which tells you that whatever evolving quantum configuration reproduces classical physics at least for the expected values. So classical mechanics is somehow in the guts of quantum mechanics. But there is no way that you can get the quantum with all its peculiarities from classical mechanics. The closest you can get to that is very vague, but it does exist. It's called the Hamilton-Jacobi equation. Some people like to say that, had William Rowan Hamilton spent one more sleepless night, he may have surmised something like quantum mechanics was plausible. But I think that's an overstatement. How would he have figured out the need of a fundamental constant like \( \hbar \)?  

Edited October 21, 2021 by joigus

[Tutoroot]

Familiar formulations of classical and quantum mechanics are shown to follow from a general theory of mechanics based on pure states with an intrinsic probability structure.

[exchemist]

44 minutes ago, Tutoroot said:

Familiar formulations of classical and quantum mechanics are shown to follow from a general theory of mechanics based on pure states with an intrinsic probability structure.

How remarkably uninformative. Are you a 'bot? 

[AbstractDreamer]

Why do we only categorise only two perspectives of reality, "micro" and "macro".   Is that because of the difference between QM and GR?  If we can say decoherence makes observations lose their quantum properties (not sure if we can say this), can we also say a fully coherent observable is "beyond the limit of QM"?

Why is there not three (or more) mechanics?  If there was a third, I would guess it would lie beyond the macro mechanics, "super-macro" rather than beneath the micro, "sub-micro".  It would have to describe phenomena that is beyond the limits of GR, such as the local referenced limit of speed and the apparent superluminal recession speeds at very large distances.  

Is it possible we need a third set of mechanics to describe observable physics at the super-macro scale, but still within our observable or future observable envelope.   Or is the scale of all observations complete with just two mechanics?

 

[exchemist]

5 minutes ago, AbstractDreamer said:

Why do we only categorise only two perspectives of reality, "micro" and "macro".   Is that because of the difference between QM and GR?  If we can say decoherence makes observations lose their quantum properties (not sure if we can say this), can we also say a fully coherent observable is "beyond the limit of QM"?

Why is there not three (or more) mechanics?  If there was a third, I would guess it would lie beyond the macro mechanics, "super-macro" rather than beneath the micro, "sub-micro".  It would have to describe phenomena that is beyond the limits of GR, such as the local referenced limit of speed and the apparent superluminal recession speeds at very large distances.  

Is it possible we need a third set of mechanics to describe observable physics at the super-macro scale, but still within our observable or future observable envelope.   Or is the scale of all observations complete with just two mechanics?

 

For that wouldn't you need evidence of phenomena that can't be predicted by existing physics? Though I suppose dark energy might be a candidate.

But there are plenty of different kinds of mechanics: quantum, statistical, Newtonian, Hamiltonian........

 

[Lorentz Jr]

12 minutes ago, AbstractDreamer said:

can we also say a fully coherent observable is "beyond the limit of QM"?

Yes, that seems to be the case. See the measurement problem. No one currently knows what happens in measurements, and the averaging out of quantum uncertainty in macroscopic objects has led to some bizarre interpretations of QM. My favorite candidate for bridging the gap is the Diósi–Penrose model, which says quantum states of macroscopic objects keep getting collapsed by their own gravity.

Edited March 1, 2023 by Lorentz Jr

[AbstractDreamer]

11 minutes ago, exchemist said:

For that wouldn't you need evidence of phenomena that can't be predicted by existing physics? Though I suppose dark energy might be a candidate.

But there are plenty of different kinds of mechanics: quantum, statistical, Newtonian, Hamiltonian........

 

Yes, I had dark energy or space expansion phenomenon in mind.

Perhaps "mechanics" is confusing the issue.  I had in mind the concept of "micro" vs "macro", or "quantum" vs "classical" as in the OP, when thinking about categories of which there are only two and not three.  The introduction of the word "mechanics" open up the number of categories to any number really.

[normanbatesfig]

I'd say there's definitely a connection between the quantum and macro level. It's almost like the macro level is built on the foundation of the quantum level. It's hard to say if it's a causal relationship, but it's safe to say that the two are related in some way. I'm sure there are phenomena that are both quantum and classical, but it really depends on how you look at it.

[swansont]

48 minutes ago, normanbatesfig said:

I'd say there's definitely a connection between the quantum and macro level. It's almost like the macro level is built on the foundation of the quantum level. It's hard to say if it's a causal relationship, but it's safe to say that the two are related in some way. I'm sure there are phenomena that are both quantum and classical, but it really depends on how you look at it.

The reason QM was not discovered until later on in the timeline of physics is because this connection becomes tenuous as one moves from micro to macro.

[exchemist]

4 hours ago, normanbatesfig said:

I'd say there's definitely a connection between the quantum and macro level. It's almost like the macro level is built on the foundation of the quantum level. It's hard to say if it's a causal relationship, but it's safe to say that the two are related in some way. I'm sure there are phenomena that are both quantum and classical, but it really depends on how you look at it.

The macro is certainly built on the quantum level micro. Much of macro level behaviour is emergent from quantum level behaviour via such things as statistical mechanics.

[Markus Hanke]

14 hours ago, AbstractDreamer said:

Why is there not three (or more) mechanics?

“Mechanics” probably isn’t a good word here, but there are at least three levels - there’s the classical domain of the familiar Newtonian and Einsteinian physics; there’s quantum mechanics that concerns itself with the evolution of quantum system where the number of particles involved does not change; and then there’s quantum field theory, which provides the best currently known description of elementary particles, their properties and interactions.