Wave-particle duality of atoms and molecules

[votingmachine]

This seems analogous to asking at what low speed relativity is not valid. There may be no way to measure the deviation from Newtonian calculations at a low speed but that doesn’t make relativity invalid.

The wavelength of a large slow object may be immeasurable, but that doesn’t prove QM does not apply. Or that at some threshold the rules change. At some point between small fast things and large slow things it is reasonable to ignore QM. That is different than the theory fading away.

 

[vanhees71]

Sufficiently large systems may no longer be well-defined in terms of a fixed set of elementary particles; or, at least in terms that QT could make sense of them completely. For example, trying to establish the UP for a car would require defining and monitoring the precise set of particles that constitute the car to a level of detail that even theoretically may make no sense. It's not a time-independent set of particles the way a specific molecule is. That seems to me to be more than a technical challenge. It's not clear to me what an experiment to detect an interference pattern for Formula One cars would even look like.

Of course, and that's why the classical approximation is usually sufficient to describe a car. The relevant macroscopic observables are coarse-graining over all these minute microscopic details that are of course neither describable nor relevant for the description of the car, but that doesn't make QT invalid only because all the particles are forming a car, i.e., a macroscopic object.

 

[DesertFox]

that's just a technical challenge not a fundamental limit beyond which QT might get invalid.

The "technical challenge" sets the limit beyond which we can't say meaningfully if Quantum Theory is still valid or gets invalid.

Wittgenstein: “Whereof one cannot speak, thereof one must be silent.”

 

[DesertFox]

The wavelength of a large slow object may be immeasurable, but that doesn’t prove QM does not apply. Or that at some threshold the rules change. At some point between small fast things and large slow things it is reasonable to ignore QM. That is different than the theory fading away.

Excuse me for the off-topic parallel, but you sound a little bit like a theologian: "God is transcendent (supernatural), so by definition you can't empirically prove his existence. Now, atheists, prove that God does NOT exist!"

Some more food for thought:

1) Of course, "reasonable to ignore" and "the theory fading away" are different. They are just different choices of linguistic framework. Nothing more.

2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK

 

[CoolMint]

2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK

You seem to want a number. Your number is 7 nm. This is the size where quantum effects start to dominate. How do I know? Ask the microprocessor industry. They have dealt with these issues for a decade now with gates leaking via quantum tunneling below this threshold.

So how big is 7 nm? About a few dozen water molecules across.
For comparision's sake - most viruses are 70-200 nm in size.

 

[PeroK]

2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK

You have the same issue in classical mechanics. When you drop a bouncy ball onto a hard floor its momentum reverses. So, what happens to conservation of momentum?

The theory says that the Earth's momentum changes by the same amount in the opposite direction. But, that is untestable as it's impossible to isolate the Earth from other impacts or the effects of other gravitaional bodies in the solar system; not to mention actually identifying and monitoring the motion of the Earth's centre of mass.

Instead, we test conservation of momentum where it can be tested and infer that the law holds even where it is impossible to verify directly.

 

[vanhees71]

The "technical challenge" sets the limit beyond which we can't say meaningfully if Quantum Theory is still valid or gets invalid.

Wittgenstein: “Whereof one cannot speak, thereof one must be silent.”

How do you come to this conclusion? Condensed-matter physics is very successful in describing the properties of all kinds of macroscopic objects, using quantum theory. In fact already the stability of matter around us were completely incomprehensible without quantum theory.

 

[PeroK]

2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK

Another point worth making is that a theory does not necessarily depend on its fundamentals being tested directly in all circumstances. Instead, these fundamentals are raised to the status of postulates and the theory and its predictions are developed from there. The point being that if the fundamentals are wrong in some way then some false prediction will emerge from the theory. In that sense, QT does not does not hinge on being able to prove, for example, the UP for all macroscopic objects.

Instead, as mentioned above, QT explains the properties of macroscopic objects - by explaining fundamental chemistry and states of matter etc. And the theory is tested, for example, in the development of modern electronics and more recently quantum computers. Those are the real tests of the theory.

Likewise, there are experiments like the Muon g2 experiment that do probe the limits of our current understanding of QT and may posssibly lead to an extension or revision of the theory.

The question of whether billiard balls may or may not produce an interference pattern, for example, is something of a wild goose chase in terms of the experimental physics. That's probably not where an insight into the limitations of the theory is likely to be found.

 

[vanhees71]

Another point worth making is that a theory does not necessarily depend on its fundamentals being tested directly in all circumstances. Instead, these fundamentals are raised to the status of postulates and the theory and its predictions are developed from there. The point being that if the fundamentals are wrong in some way then some false prediction will emerge from the theory. In that sense, QT does not does not hinge on being able to prove, for example, the UP for all macroscopic objects.

But it's very simple to prove the UP for all macroscopic objects since the uncertainties of observable due to, e.g., thermal motion, usually are much larger than the bounds given by QT. This is not surprising since the thermal-equilibrium state is just a special case of a quantum state, described by the corresponding statistical operators (given by the various "ensembles", microcanonical, canonical, and grand canonical).

Instead, as mentioned above, QT explains the properties of macroscopic objects - by explaining fundamental chemistry and states of matter etc. And the theory is tested, for example, in the development of modern electronics and more recently quantum computers. Those are the real tests of the theory.

Likewise, there are experiments like the Muon g2 experiment that do probe the limits of our current understanding of QT and may posssibly lead to an extension or revision of the theory.

I don't think that if this or another experiment confirms finally really some deviations from the predictions of the Standard Model implies that the fundamentals of Q(F)T are wrong. It rather hopefully hints to "new physics beyond the Standard Model", e.g., the existence of other than the known elementary particles, preferably hinting at the solution of the question, what the "Dark Matter" is make of needed to describe many astronomical and cosmological observations within the "Standard Model of Cosmology".

The question of whether billiard balls may or may not produce an interference pattern, for example, is something of a wild goose chase in terms of the experimental physics. That's probably not where an insight into the limitations of the theory is likely to be found.

That's for sure true ;-).