Is All Motion Discrete or Continuous in QM?

[erobz]

We were discussing how much weight it would take to stop the mechanism from rotating in this thread:

https://www.physicsforums.com/threa...-down-with-known-torque.1016470/#post-6646777

I wondered if there were actually a range of weights that would stop it from rotating $[ m, \infty )$ because of the discrete nature of QM. But I realllllly don't know how things are modeled in QM.

I picture it as an electron jumping an orbit: The hanging mass receives a certain discrete amount of energy ( like latent energy) before it makes a discrete jump in space?

 

[PeterDonis]

I wondered if there were actually a range of weights that would stop it from rotating because of the discrete nature of QM.

The scenario you described in the other thread is classical, not quantum. There is no useful way to apply QM to it.

 

[erobz]

The scenario you described in the other thread is classical, not quantum. There is no useful way to apply QM to it.

Surely, I'm not the only person that is bothered by that? Everything in Classical Mechanics is just a collection of Quantum Mechanical things.

 

[PeterDonis]

Surely, I'm not the only person that is bothered by that?

As a matter of philosophy, perhaps. As a matter of physics, we use classical physics for macroscopic objects. That's because classical physics works for macroscopic objects: we don't observe them making "quantum jumps" or having quantum interference effects or any other quantum phenomena.

 

[erobz]

Well, if Classical Mechanics is about systems of Quantum Mechanical objects, then surely there is a point where the two are indistinguishable, $2, 3, 5, 100, ...10^9$ objects? Where does "Quantum behavior" disappear?

 

[PeterDonis]

Well, if Classical Mechanics is about systems of Quantum Mechanical objects, then surely there is a point where the two are indistinguishable, $2, 3, 5, 100, ...10^9$ objects? Where does "Quantum behavior" disappear?

Nobody has a theoretical answer to this. Experimentally it depends on which particular quantum behavior you are looking at. I think double slit experiments have shown interference with molecules consisting of hundreds of atoms. The Josephson effect I believe has been observed in SQUIDs consisting of something like a trillion atoms. But that's still many orders of magnitude smaller than macroscopic objects.

It's possible that as our experimental techniques become more sensitive, we will see quantum effects in macroscopic objects. Or we will discover some kind of new physics that shows a clear break point between quantum and classical behavior. Until we can test this experimentally, anything anyone says about this is speculation.

 

[PeterDonis]

Where does "Quantum behavior" disappear?

We had a recent thread on this topic:

https://www.physicsforums.com/threa...on-and-classical-quantum-distinction.1016085/

There are also older threads about it; it's a perennial question whenever QM is discussed.

 

[vanhees71]

As a matter of philosophy, perhaps. As a matter of physics, we use classical physics for macroscopic objects. That's because classical physics works for macroscopic objects: we don't observe them making "quantum jumps" or having quantum interference effects or any other quantum phenomena.

The only quantum phenomenon we observe is that the matter around us is stable, which is an important prerequisite for the applicability of classical physics ;-)).

 

[PeroK]

Surely, I'm not the only person that is bothered by that? Everything in Classical Mechanics is just a collection of Quantum Mechanical things.

If you imagine a classical model of the atom, then you still have the same question: at what scale does an object stop being a collection of atoms moving frantically in thermodynamic equilibrium and transition to an object seeming at rest?

 

[erobz]

If you imagine a classical model of the atom, then you still have the same question: at what scale does an object stop being a collection of atoms moving frantically in thermodynamic equilibrium and transition to an object seeming at rest?

But the jiggling of the atoms in a classical model ( in a lattice system ) is none the less a quantum effect? Maybe your point is there are other transitional points between CM and QM that we arrive at before looking at the transition to systems of subatomic particles... that we commonly take for granted also? Systems of atoms jiggling would be the transition point I was thinking about. I realize that QM deals more with systems of subatomic particles.

 

[PeroK]

But the jiggling of the atoms in a classical model ( in a lattice system ) is none the less a quantum effect? Maybe your point is there are other transitional points between CM and QM that we arrive at before looking at the transition to systems of atoms, subatomic particles... that we commonly take for granted also?

My point was that, in general, the model of the object a whole and the model of the object in terms of its constituent parts may be dissimilar.

For example the rules for how an IT system works at the overall functional level, e.g. open a post, reply to a post, bear no relation to the rules of how things work at the hardware level. Not to mention at several intermediate software levels.

And, indeed, there is another dissociation between IT hardware functions and the underlying QM.

 

[erobz]

My point was that, in general, the model of the object a whole and the model of the object in terms of its constituent parts may be dissimilar.

For example the rules for how an IT system works at the overall functional level, e.g. open a post, reply to a post, bear no relation to the rules of how things work at the hardware level. Not to mention at several intermediate software levels.

And, indeed, there is another dissociation between IT hardware functions and the underlying QM.

Thats fine, they work well given within their focus, but the lack of continuity could be an artifact of an incomplete understanding. I think that is why some are searching for a "Theory of Everything" that covers the small and the tall?

 

[PeroK]

Thats fine, they work well given within their focus, but the lack of continuity could be an artifact of an incomplete understanding. I think that is why some are searching for a "Theory of Everything" that covers the small and the tall?

You're never going to explain an IT functional specification in terms of QM. The "theory of everything" will not be a theory of everything in the sense that you imply. There's a joke about this in the TV series The Big Bang Theory. Sheldon thinks that because he is an expert in fundamental physics he knows everything. But, he doesn't know how to drive a car.

That's fundamental misunderstanding of the nature of human knowledge. The "theory of everything" in elementary physics will not tell you how to drive a car, or explain Python syntax, for example.

 

[Nugatory]

But the jiggling of the atoms in a classical model ( in a lattice system ) is none the less a quantum effect?

No, it is classical. We have very small objects, namely atoms, with non-zero mechanical energy (sum of kinetic and potential energy) in a potential well. They jiggle.

 

[erobz]

The "theory of everything" will not be a theory of everything in the sense that you imply.

The sense that I got for a "Theory of Everything" was that physicist were after a new framework that encompassed GR and QM, with classical mechanics as simplification in between. I wasn't thinking about Information Theory ( but I'm not sure why it should be thought of as special, but then again I don't even really know what it is).

 

[erobz]

No, it is classical. We have very small objects, namely atoms, with non-zero mechanical energy (sum of kinetic and potential energy) in a potential well. They jiggle.

So temperature is not a part of QM?

EDIT:
What I mean is temperature is not a quantum mechanical in nature?

 

[Drakkith]

So temperature is not a part of QM?

EDIT:
What I mean is temperature is not a quantum mechanical in nature?

It is, but you asked about the classical model.

 

[Nugatory]

So temperature is not a part of QM?

EDIT:
What I mean is temperature is not a quantum mechanical in nature?

It is, but there is more to temperature than just the jiggling of atoms.

 

[berkeman]

It is, but there is more to temperature than just the jiggling of atoms.

Gotta' love B-level threads...

 

[bob012345]

It is, but there is more to temperature than just the jiggling of atoms.

Can you elaborate a bit please?