Undergrad Quantum interpretation and classical-quantum distinction

[DesertFox]

Are/were there any quantum interpretation according to which quantum mechanics COMPLETELY cease to apply at the macroscopic scale? If YES, please name it.

I am interested to learn about such interpretation, even if it is inadequate and not widely supported. Of course, the big question remains inevitably unanswered: at what level and how exactly the transition between quantum and classical happens...

 

[PeroK]

Are/were there any quantum interpretation according to which quantum mechanics COMPLETELY cease to apply at the macroscopic scale? If YES, please name it.

There can't be. That's like asking is there a theory of the atom whereby when you put $10^{23}$ atoms together, then the theory does not apply?

What you would need is another theory to add to QM that has negligible effect at smaller scales. In a way, that's what gravity is at the moment. It's too weak to affect the QM model of the atom (although technically there ought to be a gravitational component at work somehow). But, once you put enough elementary particles together it becomes the dominant force/theory.

It's illogical to believe that somehow QM stops applying at some scale. And, in fact, it's not a question of macroscopic scale, per se, as large objects can exhibit quantum behaviour as long at they are prepared with their atoms in a coherent state. E.g.

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

 

[PeterDonis]

Are/were there any quantum interpretation according to which quantum mechanics COMPLETELY cease to apply at the macroscopic scale?

The Copenhagen interpretation sort of says this, since one of its principles is that all experiments and their results must be described in classical terms. That implies that, at the very least, measuring instruments and the results they display cannot be described using QM.

 

[Morbert]

Are/were there any quantum interpretation according to which quantum mechanics COMPLETELY cease to apply at the macroscopic scale? If YES, please name it.

I am interested to learn about such interpretation, even if it is inadequate and not widely supported. Of course, the big question remains inevitably unanswered: at what level and how exactly the transition between quantum and classical happens...

Laboratory QM, as described by Asher Peres et al, says that for any system the physicist applies quantum mechanics to, there must be an external system in which that system is embedded, and to which the physicist completely ceases to apply quantum mechanics. Note though, that this demarcation is determined by the physicist and not nature. Not quite what you were asking for, but probably closer than you think.

 

[atyy]

Also, what @PeroK and @PeterDonis and @Morbert say is true. The Copenhagen interpretation needs an observer. The observer cannot apply QM to itself, but that observer can apply QM to the rest of the universe that it observes.

 

[DesertFox]

The observer cannot apply QM to itself

What do you mean? Could you be, please, more specific?

As far as I understand, Quantum Mechanics is applicable and valid for everything and at all "levels". Let me call that "quantum fundamentalism". It's just that quantum effects get so insignificant in the macroscopic world, that they are negligible. And actually.. it is not the size, but the the number of degrees of freedom.

Anyway, the answers of @PeroK and @PeterDonis are, at least for me, a little bit contradictory: "There can't be" and "The Copenhagen interpretation sort of". But probably it is MY lack of understanding... So, any further explications would be appreciated.

 

[PeroK]

Anyway, the answers of @PeroK and @PeterDonis are, at least for me, a little bit contradictory: "There can't be" and "The Copenhagen interpretation sort of". But probably it is MY lack of understanding... So, any further explications would be appreciated.

However useful the Copenhagen interpretation might be, it isn't logically consistent. When I said it would be "illogical", I forgot that Copenhagen is illogical in this respect.

 

[DesertFox]

However useful the Copenhagen interpretation might be, it isn't logically consistent. When I said it would be "illogical", I forgot that Copenhagen is illogical in this respect.

One can easily hold that we are living in a quantum world (since everything is constituted by atomic and subatomic particles). In that case, however, one must be ready to explain why the macroscopic world appears classical. Any personal views on that?

Of course, one may hope that the decoherence program will provide a solution to the problem. But actually it seems that the decoherence even widens the problem itself.

 

[Morbert]

As far as I understand, Quantum Mechanics is applicable and valid for everything and at all "levels". Let me call that "quantum fundamentalism". It's just that quantum effects get so insignificant in the macroscopic world, that they are negligible. And actually.. it is not the size, but the the number of degrees of freedom.

Modern iterations of CI-QM is applicable to any system, but not to all systems.

The distinction: Consider a standard Wigner's friend scenario. Wigner's friend is studying a microscopic system in her macroscopic lab, which is embedded in Wigner's larger lab. Wigner's friend can apply QM to the microscopic system, or the microscopic system + her measurement apparatus, or the microscopic system + her measurement apparatus + her eyes and ears, or the microscopic system + her measurement apparatus + her eyes and ears + her brain stem. In all cases, there must be an external classical system into which outcomes are registered by an observer and compared with predicted probabilities, and she must be a part of this external system to be that observer.

Wigner, on the other hand, is free to apply QM wholly to his friend, her lab, and all systems within, so long as he keeps himself external.

 

[PeroK]

One can easily hold that we are living in a quantum world (since everything is constituted by atomic and subatomic particles). In that case, however, one must be ready to explain why the macroscopic world appears classical. Any personal views on that?

I answered that in a previous post: the law of large numbers and decoherence (among other things). For example, certain classical laws appear in QM as expectation (average) values. Also, if you apply the uncertainty principle to a tennis ball, then the uncertainty is unmeasurably small. There's no mystery why a tennis ball does not behave like an electron: in the simplest terms the answer is quantitative, not necessarily a fundamental difference in behaviour.

If you roll one million dice, you will always get something very close to a total of 3.5 million. Whereas, each die is randomly 1-6. Again, there is no mystery why quantitatively a million dice taken together behave predictably; whereas, a single die is unpredictable.

Of course, one may hope that the decoherence program will provide a solution to the problem. But actually it seems that the decoherence even widens the problem itself.

Your expectation that QM randomness should be measureable in complex systems is quantitatively wrong. Another example is quantum tunneling. An electron tunnels and a tennis ball does not (statistically) not necessarily because of any fundamental difference in behaviour but quantitiatively. The probability of an electron tunneling may be $0.25$, say. And, by the same mathematics and the same QM theory, the probability of a tennis ball tunnelling may be too small to even write down.

You can't ignore this aspect of vanishingly small probabilities of "raw" QM events being observed in complex macroscopic objects.

 

[DesertFox]

by the same mathematics and the same QM theory, the probability of a tennis ball tunnelling may be too small to even write down.

Hm. What do you think. Even if the probability of tunnelling gets so small…

Given that there are so many different objects in the world (visible with the naked eye: from a grain of sand to a ping pong ball and so on) and given that so many different people have lived on the Earth and are living up to now…..

isn’t it likely that someone must have observed tunnelling of a visible object?

 

[Lord Jestocost]

It is uttermost unlikely that someone has ever observed the tunnelling of a visible object. The tunneling probability for objects with a given mass and energy approaching a barrier of a given height and thickness can be evaluated with the help of a 'calculator' on http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/barr.html.

 

[PeroK]

isn’t it likely that someone must have observed tunnelling of a visible object?

No. We're not talking about the "impossible" or "unbelievable" in the sense that a sports commentator might mean it. Which is anything from a bit unlikely to inevitable.

We are talking about probabilities that are so small that, as I said, it's not even possible to write them down. For example, if you did an experiment with a tennis ball (looking for "quantum behaviour") every Planck unit of time (i.e. every $10^{-44}$ seconds) for the duration of the universe (let's say $100$ trillion years), then the probability of it happening in just one of those experiments is still too small to write down.

These numbers matter.

I'll give you a task: write a computer program to simulate the tossing of a fair coin and let it run until you get 50 heads in a row. See how long it takes. In order for a tennis ball to tunnel, you might need at the very least $10^{23}$ heads in a row. You could, if you like, estimate for yourself how long that would take.

 

[andrew s 1905]

No. We're not talking about the "impossible" or "unbelievable" in the sense that a sports commentator might mean it. Which is anything from a bit unlikely to inevitable.

We are talking about probabilities that are so small that, as I said, it's not even possible to write them down. For example, if you did an experiment with a tennis ball (looking for "quantum behaviour") every Planck unit of time (i.e. every $10^{-44}$ seconds) for the duration of the universe (let's say $100$ trillion years), then the probability of it happening in just one of those experiments is still too small to write down.

These numbers matter.

I'll give you a task: write a computer program to simulate the tossing of a fair coin and let it run until you get 50 heads in a row. See how long it takes. In order for a tennis ball to tunnel, you might need at the very least $10^{23}$ heads in a row. You could, if you like, estimate for yourself how long that would take.

Then do you accept there can't be any experimental proof that QM applies at these scales as they are unobservable? Hence it is just an assertion that it applies. Regards Andrew

 

[DesertFox]

No. We're not talking about the "impossible" or "unbelievable" in the sense that a sports commentator might mean it. Which is anything from a bit unlikely to inevitable.

We are talking about probabilities that are so small that, as I said, it's not even possible to write them down. For example, if you did an experiment with a tennis ball (looking for "quantum behaviour") every Planck unit of time (i.e. every $10^{-44}$ seconds) for the duration of the universe (let's say $100$ trillion years), then the probability of it happening in just one of those experiments is still too small to write down.

These numbers matter.

I'll give you a task: write a computer program to simulate the tossing of a fair coin and let it run until you get 50 heads in a row. See how long it takes. In order for a tennis ball to tunnel, you might need at the very least $10^{23}$ heads in a row. You could, if you like, estimate for yourself how long that would take.

These numbers matter, for sure.

However, we are talking about so many people, so many situations and so many visible objects. These numbers are too big to write down, too. Of course, there is not an available on-line calculator for them.

That’s why I don’t get how come your “no” is so categorical...

 

[PeroK]

Then do you accept there can't be any experimental proof that QM applies at these scales as they are unobservable? Hence it is just an assertion that it applies. Regards Andrew

1) There is already a link to quantum behaviour in controlled macroscopic objects. It's not the scale, it's the degrees of freedom.

2) QM is what predicts the behaviour of macroscopic objects in terms of chemistry and states of matter. For example, QM provides the only explanation for why some objects are solid. You need both QM and CM to explain macroscopic objects. When you ask why we think that macroscopic objects obey QM, then I would ask you: if they do not, then how do you explain the chemistry of macroscopic objects?

3) It's not an assertion. It's an assumption there is not another layer of physical laws (that do not originate in the theory of elementary particles). Or, at least, there is no evidence for such laws or what they might be. Note that gravity is, to some extent, an exception to this. There is not yet an adequate explanation of how gravity emerges from elementary interactions. But, also, there is no adqeuate theory that says how it could emerge in any other way. If gravity does not emerge from elementary interactions, then how does it emerge? That must be the default position at this stage.

4) It's clear that raw QM behaviour is practically unobservable in complex macroscopic objects. This can be easily calculated. E.g. the UP, tunneling, diffraction etc. It's one thing to accept that as a limitation of experimental physics. It's quite another thing to demand that it must be observable. This is the point the OP has reached: a belief that if QM applies to complex macroscopic systems, then that must manifest itself in complex macroscopic objects behaving (more or less) like elementary particles. That demand is not supported by the mathematics of QM which shows that such phenomena as the UP, tunneling and diffraction are too small to be observed in the case of a tennis ball. Only by ignoring the mathematics of QM can you come to any other conclusion.

 

[PeroK]

Here's an interesting analogy. Take two snooker balls and roll one past the other a certain distance away: perhaps the width of a snooker ball between them . There is no observable gravitational attraction between the balls.

Now, do the same with planets (to the same scale). One planet passing by an equally sized planet with a gap of about a planet's width between them. Now, there is considerable gravitational attraction.

If you do the mathematics, then you see that despite the equivalent scale in each case, the actual size and mass of the system makes all the difference. It's the same same theory of gravity and the same mathematics in each case but very different quantitative outcomes.

We do not conclude from this that gravity somehow does not apply to small objects. But, if you ignore the mathematics and simply look at the theory of gravity qualitatively, then you might expect the same gravitational deflection in each case, and come to some erroneous conclusion about the scales at which gravity applies.

 

[DesertFox]

Here's an interesting analogy. Take two snooker balls and roll one past the other a certain distance away: perhaps the width of a snooker ball between them . There is no observable gravitational attraction between the balls.

Now, do the same with planets (to the same scale). One planet passing by an equally sized planet with a gap of about a planet's width between them. Now, there is considerable gravitational attraction.

If you do the mathematics, then you see that despite the equivalent scale in each case, the actual size and mass of the system makes all the difference. It's the same same theory of gravity and the same mathematics in each case but very different quantitative outcomes.

We do not conclude from this that gravity somehow does not apply to small objects. But, if you ignore the mathematics and simply look at the theory of gravity qualitatively, then you might expect the same gravitational deflection in each case, and come to some erroneous conclusion about the scales at which gravity applies.

The analogy is actually evading the real problem; I don't find it enough appropriate.

We talk about (1) very small probability (too small to write down) and (2) a great deal of… let’s say it “tries” (too many to write down)...

How can we claim categorically that “nobody has ever noticed such a thing!”…. I don't get it.

 

[PeroK]

I don't get it.

That may be true. And, in general, is the problem in studying physics without mathematics. Numbers are important. Why is the sky blue, when all wavelengths of light scatter? Unless and until you run the numbers, you have nothing really.

 

[DesertFox]

That may be true. And, in general, is the problem in studying physics without mathematics. Numbers are important. Why is the sky blue, when all wavelengths of light scatter? Unless and until you run the numbers, you have nothing really.

Although numbers are undeniably important, we didn't ever bothered to run the numbers of all that people and objects (as the context of my question supposes). Just a simple "no".

 

[PeroK]

Although numbers are undeniably important, we didn't ever bothered to run the numbers of all that people and objects (as the context of my question supposes). Just a simple "no".

Write the program I suggested, then we can talk:

write a computer program to simulate the tossing of a fair coin and let it run until you get 50 heads in a row. See how long it takes.

 

[DesertFox]

Write the program I suggested, then we can talk:

I can't...

Anyway.

So, if the universe don't cease to exist and if it keeps on function the way it does for enternity (lets imagine it for the sake of my question)... then somebody will see tunnelling of a visible object for sure. Right?

 

[andrew s 1905]

...
4) It's clear that raw QM behaviour is practically unobservable in complex macroscopic objects. This can be easily calculated. E.g. the UP, tunneling, diffraction etc. It's one thing to accept that as a limitation of experimental physics. It's quite another thing to demand that it must be observable...

...Only by ignoring the mathematics of QM can you come to any other conclusion.

Demanding observability seems to me to be a fundamental requirement of physics and what differentiates it from pure mathematics.

I am quite happy with the emergence of the macroscopic (high degree if freedom) world from the microscopic (low degree QM) but if if were as clear cut as you propose I think the measurement problem would have been solved. The emergence of classical behaviour from QM is I believe still an open question.
Regards Andrew

 

[PeroK]

I can't...

Then you have to take my word for it that even something like that is practically impossible. Although, to be honest, perhaps 100 heads would be better. I can run the numbers later if you want.

Anyway. If the Universe don't cease to exist and if it keeps on function like now... for, let's say for the sake of my imaginary question, infinity... Somebody will see tunnelling of a visible object for sure. Right?

There's an important difference between pure mathematics and physics. In particular, the second law of thermodynamics prevents truly unlimited experiments for "infinity".

There's another problem, which is how would you know? If you hit enough tennis balls with one racket, then eventually the strings break and the ball will go through. Does that count as "tunnelling"? Or, perhaps, two freak gusts of wind blow the ball around the racket. Does that count? Or, the experimenter or equipmemt fails and the ball is fumbled around the racket?

One problem is that each of these events may happen billions or trillions of times more often than what you might consider genuine quantum tunnelling. So, you end up with the event you want practically happening, but not for QM reasons.

For example, if I check my bike shed every morning then eventually my bike will be gone. But not because it has quantum tunnelled somewhere else, but because it's been stolen.

By imagining that if QM is correct, then macroscopic objects such as tennis balls and bicycles must behave observably like electrons, you are fundamentally misunderstanding QM. This is not something that it is necessary to debate.

 

[andrew s 1905]

@PeroK one could turn your argument round. One could have applied it to Newtonian mechanic and gravity and argued it were universally true and just because the very small or fast were experimentally inaccessible ( at that time) the maths still applied. Of course it was shown not to be the case.

Regards Andrew

 

[DesertFox]

Then you have to take my word for it that even something like that is practically impossible. Although, to be honest, perhaps 100 heads would be better. I can run the numbers later if you want.

There's an important difference between pure mathematics and physics. In particular, the second law of thermodynamics prevents truly unlimited experiments for "infinity".

There's another problem, which is how would you know? If you hit enough tennis balls with one racket, then eventually the strings break and the ball will go through. Does that count as "tunnelling"? Or, perhaps, two freak gusts of wind blow the ball around the racket. Does that count? Or, the experimenter or equipmemt fails and the ball is fumbled around the racket?

One problem is that each of these events may happen billions or trillions of times more often than what you might consider genuine quantum tunnelling. So, you end up with the event you want practically happening, but not for QM reasons.

For example, if I check my bike shed every morning then eventually my bike will be gone. But not because it has quantum tunnelled somewhere else, but because it's been stolen.

By imagining that if QM is correct, then macroscopic objects such as tennis balls and bicycles must behave observably like electrons, you are fundamentally misunderstanding QM. This is not something that it is necessary to debate.

I constructed a question that is (maybe) unnecessary to debate, but it is still (in principle) a valid question.

Of course, I am concerned only with
genuine quantum tunnelling Of a visible object.

And once again: the number of visible objects and people is crazy big... Just like the probbility of tunnelling of a visible object is crazy low.

The imaginary part of my question: ignore the the second law of thermodynamics. And we can obtain (at least imaginary) eternity, which is essential for my imaginary context.

IN SUMMA: I think my question remains practically untouched.

 

[PeroK]

@PeroK one could turn your argument round. One could have applied it to Newtonian mechanic and gravity and argued it were universally true and just because the very small or fast were experimentally inaccessible ( at that time) the maths still applied. Of course it was shown not to be the case.

Regards Andrew

I don't buy that argument. What you are arguing for is that there are laws of physics that do not apply to elementary particles, but do apply to collections of elementary particles. The main problem I would say is that appears not to be necessary.

This thread is motivated by a misunderstanding of QM. I.e. that if QM is correct, then we should have obvious QM behaviour of tennis balls, say. And, that we don't suggests that there must be more than QM.

Once we remove this misunderstanding, where is your motivation for additional laws of physics? And, what phenomena are they designed to explain?

 

[PeterDonis]

However useful the Copenhagen interpretation might be, it isn't logically consistent.

Why not?

 

[PeterDonis]

Why not?

Your answer appears to be:

It's illogical to believe that somehow QM stops applying at some scale.

I disagree. It is perfectly logically consistent to have two sets of laws of physics for different domains, with some kind of rule about what happens at the boundary. It might not suit your expectations, and it might not be what you believe, but that's not the same as it being logically inconsistent. Logical inconsistency is an extremely strong claim.

 

[DesertFox]

Why not?

Copenhagen interpretation is completely consistent in terms of Modern logic.

"It isn't logically consistent".. That sounds (at least to me) more like an articulation of purely personal taste or just a loose word choice.