Is the Quantum/Classical Boundary the most important question in Physics?

[Quantum Alchemy]

I have been reading Sean Carroll's recent book called Something Deeply Hidden where he advocates a Many Worlds Interpretation of Quantum Mechanics. I was thinking that the most important question in all of Physics might be is everything quantum or is there a quantum/classical boundary.

If we answer this question, doesn't it explain just about everything? It would mean a mixed state is just a mixture of real probable states that just don't interfere. So classical reality would still be quantum but without interference. So you can have a superposition of Earth's where in 80% of the Earth's Hillary won the election, 20% of the Earth's Trump won the election and we're in one of those 20% of Earth's so that's our local experience.

So looking at Schrodinger's cat it's either the cat is distinctly a live cat until decay/not decay occurs if there's a quantum/classical boundary but if all is quantum then the cat would be in a mixed state of both probable states until information about the cat reached the environment because if all is quantum how can the state of the cat be known prior to a measurement of decay/not decay?

Here's some papers on this topic and they all seem to point to the all is quantum point of view.

Death by experiment for local realism

A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests.

https://www.nature.com/articles/nature15631

Quantum superposition of molecules beyond 25 kDa

Matter-wave interference experiments provide a direct confirmation of the quantum superposition principle, a hallmark of quantum theory, and thereby constrain possible modifications to quantum mechanics1. By increasing the mass of the interfering particles and the macroscopicity of the superposition2, more stringent bounds can be placed on modified quantum theories such as objective collapse models3. Here, we report interference of a molecular library of functionalized oligoporphyrins4 with masses beyond 25,000 Da and consisting of up to 2,000 atoms, by far the heaviest objects shown to exhibit matter-wave interference to date. We demonstrate quantum superposition of these massive particles by measuring interference fringes in a new 2-m-long Talbot–Lau interferometer that permits access to a wide range of particle masses with a large variety of internal states. The molecules in our study have de Broglie wavelengths down to 53 fm, five orders of magnitude smaller than the diameter of the molecules themselves. Our results show excellent agreement with quantum theory and cannot be explained classically. The interference fringes reach more than 90% of the expected visibility and the resulting macroscopicity value of 14.1 represents an order of magnitude increase over previous experiments2.

https://www.nature.com/articles/s41567-019-0663-9

The paper talks about some of the amazing things we could do if we could put large objects like a planet into a superposition of probable states.

Wheeler's delayed-choice gedanken experiment with a single atom

The wave–particle dual nature of light and matter and the fact that the choice of measurement determines which one of these two seemingly incompatible behaviours we observe are examples of the counterintuitive features of quantum mechanics. They are illustrated by Wheeler’s famous ‘delayed-choice’ experiment1, recently demonstrated in a single-photon experiment2. Here, we use a single ultracold metastable helium atom in a Mach–Zehnder interferometer to create an atomic analogue of Wheeler’s original proposal. Our experiment confirms Bohr’s view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes place1. This result is encouraging for current work towards entanglement and Bell’s theorem tests in macroscopic systems of massive particles3.

https://www.nature.com/articles/nphys3343

Here's a recent study where Wigner's Friend was confirmed on quantum scales.

Experimental rejection of observer-independence in the quantum world

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically exposed in Eugene Wigner's eponymous thought experiment where two observers can experience fundamentally different realities. While observer-independence has long remained inaccessible to empirical investigation, recent no-go-theorems construct an extended Wigner's friend scenario with four entangled observers that allows us to put it to the test. In a state-of-the-art 6-photon experiment, we here realize this extended Wigner's friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. This result lends considerable strength to interpretations of quantum theory already set in an observer-dependent framework and demands for revision of those which are not.

https://arxiv.org/abs/1902.05080

Is there any more important question in Physics than this one? Also, what's the evidence that points to a quantum/classical boundary where a cat or a baseball distinctly becomes a cat or a baseball independent of QM? Also, is a pure state and a mixed state essentially the same thing minus interference?

 

[bhobba]

Well this might be better in our new Quantum Foundations sub-forum.

But no that is not the main question. We already know there is no boundary - everything is quantum. Different interpretations define in QM terms what a observation is. Some like Consistent/Decoherent Histories do not even really have observations in the usual sense - they emerge of course - but it is based on a history which is simply a series of projection operators:
http://quantum.phys.cmu.edu/CHS/histories.html
Then there is the interpretation I formally favor - an observation is simply when decoherence occurs and a superposition becomes a mixed state. The issue here is how an improper mixed state becomes a proper one or even if its an issue worth worrying about.
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf
Thanks
Bill

 

[DarMM]

Note that even Bohr thought everything was quantum, in the sense that everything is ultimately described by quantum mechanics. He didn't think there was a literal fundamentally classical world.

an observation is simply when decoherence occurs and a superposition becomes a mixed state. The issue here is how an improper mixed state becomes a proper one or even if its an issue worth worrying about

I don't think there is an issue here. In QFT there is no meaningful difference between a proper and improper mixture due to the Reeh-Schlieder theorem. There are just mixtures. Quantum Mechanical states restricted to the algebra of macroscopic properties can simply be taken to describe ignorance since the interference terms turn out to be operationally meaningless (as Asher Peres says "There are no superobservers").

Some like Consistent/Decoherent Histories do not even really have observations in the usual sense - they emerge of course - but it is based on a history which is simply a series of projection operators

This is just to expand on this point.

There is still the fact that not only do we not know which history will occur, but also which family that history will belong to. The former point is just a basic fact of any stochastic theory, but the latter is a reflection of Kochen-Specker contextuality and is the truly odd point. Thus the theory neither tells us how a history is selected or how the family containing the history is selected. In an actual experiment we choose the family with our actual choice of observation equipment, that choice seemingly being outside the theory.

In essence there is sort of two layers of indeterminism, indeterminism of the events and indeterminism of the set from which those events are drawn. However for macroscopic properties the latter form of indeterminism vanishes (all events belong to one Boolean algebra of macroproperties) and we are free to consider the remaining event indeterminism as ignorance.

 

[atyy]

Note that even Bohr thought everything was quantum, in the sense that everything is ultimately described by quantum mechanics. He didn't think there was a literal fundamentally classical world.

How could everything be quantum in Bohr's view? Didn't he think that nothing was quantum? "There is no quantum world". So if everything is quantum, and there is no quantum world, then it would mean that there is no world.

 

[atyy]

Also, what's the evidence that points to a quantum/classical boundary where a cat or a baseball distinctly becomes a cat or a baseball independent of QM?

The classical/quantum boundary is needed to do quantum mechanics. It is also absurd. This is basically the measurement problem, and yes, I do believe it is one of the most important problems in physics.

 

[DarMM]

How could everything be quantum in Bohr's view? Didn't he think that nothing was quantum? "There is no quantum world". So if everything is quantum, and there is no quantum world, then it would mean that there is no world.

There's a few things here.

First we don't know if Bohr ever said something like "There is no quantum world". It is attributed to him by Aage Petersen in his 1963 essay:
"The Philosophy of Neils Bohr" Bull. At. Sci. 19, 8.
It doesn't appear in his own writings, which is odd for somebody who felt it crucial to communicate what he felt were the most important points of the theory, writing and rewriting essays, such as Bohr.

However some of Bohr's contemporaries, such as Victor Weisskopf, doubt he actually would have said this. Others like Rudolf Peierls believe he did. However in all cases the quote is generally taken to mean that quantum mechanics does not present a pictorial or humanly comprehensible view of the subatomic world. It's not at all that there is "nothing down there" or that "nothing is quantum", just that QM doesn't give you a "visualizable world". For a brief intro to this see David Mermin's essay:
"What's wrong with this quantum world" Physics Today 57, 2, 10 (2004) .

In fact I don't think I've ever heard anybody take it to mean "nothing is quantum". Most papers in quantum foundations quoting it just seem to take it as meaning QM is not representational, e.g. the quantum state does not represent a real physical wave and so on.

In his actual writings though Bohr has:

In the system to which the quantum mechanical formalism is applied, it is of course possible to include any intermediate auxiliary agency employed in the measuring process.
...
Since, however, all those properties of such agencies which, according to the aim of the measurement, have to be compared with corresponding properties of the object, must be described on classical lines, their quantum mechanical treatment will for this purpose be essentially equivalent with a classical description. The question of eventually including such agencies within the system under investigation is thus purely a matter of practical convenience...

Of course this is typical Bohr-ese in how it is written, but we see clearly "quantum mechanical treatment will for this purpose be essentially equivalent with a classical description" and placing them on the quantum or classical side is "purely a matter of practical convenience".

These two papers analyse the writings of Bohr and other Copenhagen writers from a historical perspective. One thing they both mention is that he thought the formalism could be applied to anything, i.e. there are no fundamentally classical systems:
https://arxiv.org/abs/0804.1609https://arxiv.org/abs/1603.00342

 

[atyy]

Of course this is typical Bohr-ese in how it is written, but we see clearly "quantum mechanical treatment will for this purpose be essentially equivalent with a classical description" and placing them on the quantum or classical side is "purely a matter of practical convenience".

Yes, but taking forms of Copenhagen where the classical side is real, and the quantum side is not necessarily real, then nothing is quantum is quite a reasonable interpretation, unless one believes that there is no reality. Then everything is quantum.

 

[bhobba]

First we don't know if Bohr ever said something like "There is no quantum world". or classical side is "purely a matter of practical convenience".

After being spammed a lot by academia.edu (I am sure you can unsubscribe or something similar - its a legit site) because it was so cheap I decided to subscribe. They send me link after link to modern papers on the philosophy/history of science in QM. Some get the physics not quite right, but, a bit surprisingly to me, they are mostly correct. The reason likely is most authors have both degrees in physics and philosophy/history of science. The favorite topic right now seems Bohr's view on QM, his idea of complementary, his disagreements with Heisenberg and of course Einstein - that sort of thing.

The outcome of me reading these papers, and what I already knew from other sources, is:

1. Bohr was a mumbler so understanding what he said required some effort.
2. His ideas were subtle and deep. In fact I would say verging on incomprehensible - I certainly do not understand him - and the number of papers with differing views shows that perhaps even experts in it are confuse. That said I am certain he did have a comprehensible view because his good friend Einstein by all accounts did understand him - but simply disagreed. When Einstein finally reached the conclusion QM is correct (but incomplete) he subscribed to the Ensemble interpretation, bypassing a lot of Bohr's concerns. People think Bohr won the Einstein/Bohr debates - from our modern viewpoint I, and quite a few of the papers I read are not so sure. Personally I think Einstein won. The issue I have with Einstein is he didn't participate in QM developments - he could have, and things would have progressed faster. But I think he was jaded from his debates with Bohr.
3. These days we have many interpret ions that bypass the issues Bohr and Heisenberg thought were so important. Weinberg gives an overview here:
http://quantum.phys.unm.edu/466-17/QuantumMechanicsWeinberg.pdf
Note, he is coming to the view, like Einstein, QM may be incomplete. My view is it definitely is because the standard model and gravity (yes an effective Quantum Theory of Gravity valid to about the Plank Scale exists: https://arxiv.org/abs/1209.3511) are all effective - only valid to about the Plank scale. To me it looks likely something different may be going on at the Plank Scale and that may not be based on QM - but who knows - I certainly do not.

Anyway to the OP, yes everything is quantum. We nowadays have interpretations like decoherent histories where such is the case. But there is a lot of baggage from the past that if you get caught up in it could lead you to an academic career doing just that. Personally to me its interesting, but I do not think the modern view of QM is as impenetrable as that. Some people like the history/philosophy of science, but apart from reading the papers from such every now and then it's not really my bag.

One thing tht needs to be empasised is mathematically we now understand QM very well - its an example of what is called a generalised probability model:
https://arxiv.org/pdf/1402.6562.pdf
Note that one of Weinberg's main issues why is the world probabilistic - it is - but why? In thinking about this, its wise to keep in mind any theory must assume things. You can ask why is that true. A perfectly legit question and scientifically researching it again a perfectly good area of research. However if you answer it, advance human knowledge and get a Nobel prize etc, you still have things you assumed - so in one sense where have you ended up because you now have the new assumptions to answer. Science is really never ending and getting hung up on a particular assumption is basically a personal thing - in the end you will still have to accept something. Dirac and Heisenberg discussed this possibly in relation to a tenant of Copenhagen - A system is completely described by a wave function ψ, representing an observer's subjective knowledge of the system (the word completely is the issue here)
http://philsci-archive.pitt.edu/1614/1/Open_or_Closed-preprint.pdf.

Thanks
Bill

 

[bahamagreen]

Looks like two boundaries separating three "logical space" worlds?

The quantum (inferred through the classical)
The classical (inferred through the mind)
The mind

 

[romsofia]

A good (non technical) article is: https://www.nature.com/articles/351357a0

I enjoy the line at the end: "But if a classical record is what matters, a quantum superposition of viable and lethal sequence of base pairs in DNA is as absurd as a cat that is both dead and alive. Should one seek to to learn more about Schrodigner's cat by studying her kittens?"

 

[DarMM]

Yes, but taking forms of Copenhagen where the classical side is real, and the quantum side is not necessarily real, then nothing is quantum is quite a reasonable interpretation, unless one believes that there is no reality. Then everything is quantum.

I don't fully understand this, nor have I ever read a view like this. Do you have a link to a paper discussing it? I've never really read of quantum systems "not being real" or that "nothing is quantum". The other view you mention that there is no reality and everything is quantum, again I genuinely don't understand. You call it a form of Copenhagen, but it's certainly not what most of the early workers on QM thought, whose view is this?

 

[DarMM]

That said I am certain he did have a comprehensible view because his good friend Einstein by all accounts did understand him - but simply disagreed

In his biography of Bohr, Abraham Pais states he found Bohr's papers on the meaning and interpretation of QM easy to understand. So I suspect he explained enough to these people in private that his papers made sense. However to the majority of us who obviously did not know him personally it's very difficult to get exactly what he is saying. I think the rough idea is pretty clear, but the details are virtually impossible to recover and probably not worth the effort.

People think Bohr won the Einstein/Bohr debates - from our modern viewpoint I, and quite a few of the papers I read are not so sure. Personally I think Einstein won

In what sense do you mean that he won? In other words, what view(s) of Einstein turned out to be correct/valid? Particularly as opposed to Bohr.

My view is it definitely is because the standard model and gravity (yes an effective Quantum Theory of Gravity valid to about the Plank Scale exists: https://arxiv.org/abs/1209.3511) are all effective

When you say the Standard Model is effective do you mean:
(a) Requires a cut off to be well defined
(b) Becomes inaccurate at a certain length scale

 

[atyy]

I don't fully understand this, nor have I ever read a view like this. Do you have a link to a paper discussing it? I've never really read of quantum systems "not being real" or that "nothing is quantum". The other view you mention that there is no reality and everything is quantum, again I genuinely don't understand. You call it a form of Copenhagen, but it's certainly not what most of the early workers on QM thought, whose view is this?

I don't see it as different from your reading that "Most papers in quantum foundations quoting it just seem to take it as meaning QM is not representational, e.g. the quantum state does not represent a real physical wave and so on."

Also, in the Copenhagen interpretation, it doesn't make sense to say that everything is quantum when, quantum physics is not about what "is".

Finally, quantum physics in the orthodox interpretation requires a classical/quantum cut - in line with its operational nature and the possibility that the quantum state is not real. The cut means that it is not possible to say that everything is quantum.

For an example in a paper, see https://arxiv.org/abs/quant-ph/0509061.

 

[DarMM]

I don't see it as different from your reading that

Sorry I'm getting very confused here. I was speaking about how Bohr viewed everything as quantum in the sense that quantum theory can be applied to anything. That isn't changed by the quantum state not being representational. And it certainly doesn't mean the systems to which you apply quantum theory aren't real.

Also, it doesn't make sense to say that everything is quantum when, quantum physics is not about what "is".

I think it's a common enough phrase for describing that you can apply quantum theory to anything. Anything is quantum = You can use quantum theory to describe anything, nothing fundamentally requires classical mechanics.

Finally, quantum physics in the orthodox interpretation requires a classical/quantum cut - in line with its operational nature and the possibility that the quantum state is not real. The cut means that it is not possible to say that everything is quantum.

The cut doesn't contradict that quantum theory can be applied to anything. It means that in a given application some system must be treated as classical.

I would say Boolean is more accurate since we don't literally require it to obey classical mechanics. Ultimately this is nothing more than some system must represent the choice of history that I mentioned above. QM tells us neither which event or history occurs or which set of events/family it is drawn from, thus some system must select the family or set. In our experiments this is the measuring device, which then picks out a Boolean algebra of observables giving us a well defined sample space from which events may be drawn.

However that's in a given application, no system must be taken as Boolean fundamentally or cannot have quantum probability applied to it. An aspect not known to Bohr and other early researchers is that QM itself tells us that macroscopic degrees of freedom are Boolean/obey classical probability theory.

 

[Quantum Alchemy]

Finally, quantum physics in the orthodox interpretation requires a classical/quantum cut - in line with its operational nature and the possibility that the quantum state is not real. The cut means that it is not possible to say that everything is quantum.

What?

Where's is this cut exactly?

Quantum Mechanics gives us our modern world. Many technologies wouldn't exists if the quantum state wasn't real. Google and IBM wouldn't be on the verge of quantum supremacy if the quantum state wasn't real.

I understand that people have a problem with the notion that the classical world might not be objective reality because they want the universe to conform to their common sense view of the world, but science doesn't work that way.

The problem with this is, there's no way to know which measurement will occur prior to a measurement occurring. I know this was unsettling to Einstein and people like Smolin but experiment after experiment has shown this is the case and I've listed some in the OP so I won't list them again.

How can anything classical know what a measurement will be before it's measured?

Is there any evidence of a mechanism that knows the outcome of a measurement prior to a measurement occurring? Now, if there's a wave function of the universe and you had knowledge of this entire system, maybe you can know which measurement will occur but this would imply the wave function is real.

What about the growing field of quantum biology? How can you have photosynthesis or a sense of smell if quantum mechanics isn't real?

 

[atyy]

The cut doesn't contradict that quantum theory can be applied to anything. It means that in a given application some system must be treated as classical.

But the everything is "quantum" only in the larger sense of orthodox quantum mechanics. It is not "quantum" in the sense that the classical-quantum cut can be removed. [Also, I don't necessarily agree that quantum theory can be applied to everything, since it is not obvious how something can be both real and not real.]

I would say Boolean is more accurate since we don't literally require it to obey classical mechanics.

Of course. "Classical" is standard shorthand for "real" or "macroscopic". In relativistic quantum theory, we only require it to obey the reality conception of classical relativity, and not the full laws of any particular relativistic theory.

 

[atyy]

Where's is this cut exactly?

It is unclear. I am tempted to say with von Neumann that the cut is in the observer's head. The cut is of course absurd, but standard quantum mechanics is formulated with a cut, which is subjective, just like notions of measurement and measurement outcome.

Removing the cut is the same problem as trying to remove the observer or measurement as fundamental. Bohmian Mechanics and the Many-Worlds Interpretation are two attempts to remove the cut.

I recommend Bell's article about the measurement problem: https://m.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf

 

[Quantum Alchemy]

Removing the cut is the same problem as trying to remove the observer or measurement as fundamental. Bohmian Mechanics and the Many-Worlds Interpretation are two attempts to remove the cut.

You can simply say that the Observer collapses the wave function locally and has a subjective experience.

Say you have 2 Professors in a lab. One in room A and the other in room B. They're both carrying out the double slit experiment. They have their atomic clocks and will start at 1 PM. At 1:07, the Professor in room A carries out a measurement and finds the electron going through the right slit. The Professor in room B didn't carry out a measurement and he still sees an interference pattern. This is just 2 different local experiences, in one Professor A measures the electron going through the right slit at 1:07 PM and Professor B has a different local reality where he sees an interference pattern at 1:07.

There was a recent experiment that confirmed Wigner's Friend on a quantum scale and it showed that 2 observers can measure different outcomes for the same event.

This is also the reason why you have all of these interpretations. People can't accept QM in a classical world so they need to interpret it. So you can have 10 interpretations and the people who believe these interpretations will all have evidence that says their interpretation is the correct one. Interpretations can be insightful but they're philosophical and a matter of taste.

I think we're essentially saying the same thing. I'm just talking about an objective quantum/classical cut vs a subjective one which might point to a quantum mind/brain ala Penrose or Fisher.

 

[atyy]

Here's a recent study where Wigner's Friend was confirmed on quantum scales.

Experimental rejection of observer-independence in the quantum world

https://arxiv.org/abs/1902.05080

There was a recent experiment that confirmed Wigner's Friend on a quantum scale and it showed that 2 observers can measure different outcomes for the same event.

That reminds me of another paper I haven't read:
https://arxiv.org/abs/1907.05607
Edit: The experimental paper https://arxiv.org/abs/1902.05080 by Proietti and colleagues that you linked to is discussed by https://arxiv.org/abs/1907.05607. It says the experiments are based on a paper by Brukner, which however, has been analyzed by Healey https://arxiv.org/abs/1807.00421.

"However, as discussed by Healey in [6], Brukner’s argument relied on an additional claim, as mentioned above,namely, that OIF in the Wigner’s friend scenario entails the following (which Brukner calls a postulate): that there is a matter of fact about the results of all measurements, even ones Wigner chose not to perform. Assuming this postulate is equivalent to assuming that all possible measurement outcomes are predetermined by hidden variables — a very strong assumption. The claim that this postulate follows from the OIF assumption is not justified in Brukner’s paper, which places the no-go theorem put forward by him into jeopardy."

"In this work, we have provided the first rigorous proof of the theorem stated in [2] [Brukner]. That is, we have proven that the joint assumption of observer-independent facts, locality and freedom of choice (which we call Local Friendliness) is incompatible with the empirical predictions of quantum mechanics if one observer (a “superobserver”) can manipulate the quantum state ascribed to another observer (a “friend”). "

 

[bhobba]

You can simply say that the Observer collapses the wave function locally and has a subjective experience

All this talk of an observer makes zero sense to me. Exactly what is an observer? Does it have to be conscious - what ever that is - is a computer that passes the Turing Test conscious?

What the issue is, is measurement (which is used interchangeably with observation but does not semantically raise the issue of observer which is problematical), needs to be defined in terms of QM. At the start of course we leave it 'up in the air' so to speak as a primitive of the theory. But when we interpret it physically we need to be more exact. The usual two answers are when decoherence occurs as in the ensemble interpretation incorporating decoherence. Interestingly Ballentine, the big supporter of the Ensemble interpretation, doesn't believe decoherence has anything to do with interpretations - I think most people respectfully disagree. The other is to replace the concept of observation/measurement with the concept of history and have observation emerge from that - that's what decoherent histories does.

Thanks
Bill

 

[bhobba]

It is unclear.

Von-Neumann proved it could be anywhere - he chose consciousness because it was the only place that was different. As much as I greatly admire Von-Neumann, in fact he is a hero of mine, this, along with his 'proof' of no hidden variables, makes me 'wince'. Great polymath, in fact perhaps the greatest that ever lived, but perfect he was not. But after reading the book Einsteins Mistakes he is not the only one. Both of course had the ability to penetrate problems in an almost uncanny manner, the difference is I do not think anyone did that better than Einstein, but then again he was not one of the greatest mathematicians that ever lived - many, correctly IMHO, have him in the top 10 or 20. I have him in my top 10 along with Ramanujan and Noether. Still mathematical ability, in physics, is not as important as being at ease with the substance behind the equations. Anyway I digress. The other possibility didn't seem to occur to anyone in those early days, that it doesn't have to be placed anywhere.

Thanks
Bill

 

[bhobba]

In his biography of Bohr, Abraham Pais states he found Bohr's papers on the meaning and interpretation of QM easy to understand. So I suspect he explained enough to these people in private that his papers made sense. However to the majority of us who obviously did not know him personally it's very difficult to get exactly what he is saying. I think the rough idea is pretty clear, but the details are virtually impossible to recover and probably not worth the effort.

Yes. Since Einstein was his friend he had the advantage of simply asking him on points that were not clear. Modern scholars do not have that advantage and must decipher it from his writings. After reading a few of these it confirms its definitely not my bag.

In what sense do you mean that he won? In other words, what view(s) of Einstein turned out to be correct/valid? Particularly as opposed to Bohr.

That I think QM is incomplete. Sorry for not being clear - to Einstein at the end he was not that worried about probabilities coming into it - after all he made fundamental contributions to statistical physics - he was more worried by entanglement which we now know is a fundamental property that distinguishes QM from ordinary probability:
https://arxiv.org/abs/0911.0695
Again we see Einsteins remarkable ability to penetrate a problem and get to its essence, although in this case it took a while. He thought entanglement led to inconsistencies. We now know it is simply inconsistent with local realism, but that it is fundamental to what makes QM, well, QM, has only recently been clarified.

When you say the Standard Model is effective do you mean:
(a) Requires a cut off to be well defined
(b) Becomes inaccurate at a certain length scale

Probably (b). The triviality issue and the appearance of things like the Landau pole. I can't be sure though, as you know these issues are still under active investigation, but I think most physicists do not trust the standard model at about the Plank scale. I am not that sure about (a) these days as I come to understand zeta function regularization more:
https://www.imperial.ac.uk/media/im.../dissertations/2009/Nicolas-Robles-Thesis.pdf
It is equivalent to dimensional regularization, so does have a cutoff. But with Zeta function regularization there is still a cutoff of course but it is removed by analytic continuation - is it really a cutoff? In other words divergent series or integrals all depend on the definition one uses to calculate them and extension via analytic continuation looks like it perhaps 'bypasses' the cutoff. But that one needs its own thread.

Thanks
Bill

 

[Quantum Alchemy]

All this talk of an observer makes zero sense to me. Exactly what is an observer? Does it have to be conscious - what ever that is - is a computer that passes the Turing Test conscious?

An observer can extract information about the state of a system. There's strong observers and weak observers.

A weak observer would be a measuring device which stores information like which slit the particle went through but it does nothing with this information. It's just stored information that needs to be extracted.

A strong observer would be the human brain. It not only stores information but it can write books and publish papers about it. It can build technologies with it. It doesn't need to be extracted because the brain or the mind if you support the notion of a quantum mind, can extract this information.

In a study reported in the February 26 issue of Nature (Vol. 391, pp. 871-874), researchers at the Weizmann Institute of Science have now conducted a highly controlled experiment demonstrating how a beam of electrons is affected by the act of being observed. The experiment revealed that the greater the amount of "watching," the greater the observer's influence on what actually takes place.

The research team headed by Prof. Mordehai Heiblum, included Ph.D. student Eyal Buks, Dr. Ralph Schuster, Dr. Diana Mahalu and Dr. Vladimir Umansky. The scientists, members of the Condensed Matter Physics Department, work at the Institute's Joseph H. and Belle R. Braun Center for Submicron Research.

To demonstrate this, Weizmann Institute researchers built a tiny device measuring less than one micron in size, which had a barrier with two openings. They then sent a current of electrons towards the barrier. The "observer" in this experiment wasn't human. Institute scientists used for this purpose a tiny but sophisticated electronic detector that can spot passing electrons. The quantum "observer's" capacity to detect electrons could be altered by changing its electrical conductivity, or the strength of the current passing through it.

Apart from "observing," or detecting, the electrons, the detector had no effect on the current. Yet the scientists found that the very presence of the detector-"observer" near one of the openings caused changes in the interference pattern of the electron waves passing through the openings of the barrier. In fact, this effect was dependent on the "amount" of the observation: when the "observer's" capacity to detect electrons increased, in other words, when the level of the observation went up, the interference weakened; in contrast, when its capacity to detect electrons was reduced, in other words, when the observation slackened, the interference increased.

Thus, by controlling the properties of the quantum observer the scientists managed to control the extent of its influence on the electrons' behavior. The theoretical basis for this phenomenon was developed several years ago by a number of physicists, including Dr. Adi Stern and Prof. Yoseph Imry of the Weizmann Institute of Science, together with Prof. Yakir Aharonov of Tel Aviv University. The new experimental work was initiated following discussions with Weizmann Institute's Prof. Shmuel Gurvitz, and its results have already attracted the interest of theoretical physicists around the world and are being studied, among others, by Prof. Yehoshua Levinson of the Weizmann Institute.

https://www.sciencedaily.com/releases/1998/02/980227055013.htm

So an observer seems to reduce interference and extract information about a system. There's human observers and non human observers. Human observers seem to be aware of the information we're extracting and we write books and publish papers about it. Why are we aware of it, the honest answer is we don't know.

 

[DarMM]

But the everything is "quantum" only in the larger sense of orthodox quantum mechanics. It is not "quantum" in the sense that the classical-quantum cut can be removed.

Sorry, again I don't understand. What do you mean "in the larger sense of orthodox quantum mechanics".

[Also, I don't necessarily agree that quantum theory can be applied to everything, since it is not obvious how something can be both real and not real.]

Again and this could be my failing, I don't know what you mean by "real" here or what this problem of real and not real is.

My understanding of modern QM is as follows:

We are studying a microscopic system. QM is a generalised probability theory. We find that neither the events nor the set they are drawn from are determined. We choose a device which selects a single Boolean frame/sample space of events. Our measurement result is then randomly drawn from this set.

However the device itself may be treated by quantum theory where initially it would seem a second device will select a Boolean frame for measurements upon it. However modern decoherence theory shows that in fact within QM itself macro-properties form a Boolean space of events, so a second device actually is not necessary.

To me we have the device clearly being treated by QM here, what is this problem of real and not real?

 

[bhobba]

An observer can extract information about the state of a system. There's strong observers and weak observers.
A weak observer would be a measuring device which stores information like which slit the particle went through but it does nothing with this information. It's just stored information that needs to be extracted

A few stray photons from the CBMR is enough to give a dust particle a definite position - would you count those photons as an observer? I am not saying photons are or are not observers - I have my view of course - but right now I am just trying to flesh out your view. So far I see nothing wrong with it, its just not how I would look at it.

Thanks
Bill

 

[EPR]

A few stray photons from the CBMR is enough to give a dust particle a definite position - would you count those photons as an observer? I am not saying photons are or are not observers - I have my view of course - but right now I am just trying to flesh out your view. So far I see nothing wrong with it, its just not how I would look at it.

Thanks
Bill

A few collapsed photons from the CBMR is enough to...

But everything in reality is in a state of superposition since by definition it's the essence of the quantum world. So you can't use 'collapse' to prove 'collapse' of the wavefunction. Even FAPP.

 

[DarMM]

A few collapsed photons from the CBMR is enough to...

But everything in reality is in a state of superposition since by definition it's the essence of the quantum world. So you can't use 'collapse' to prove 'collapse' of the wavefunction. Even FAPP.

As @bhobba has said quantum theory is a generalized probability theory. Sometimes a linear sum of states represents a type of statistics that doesn't appear in classical/Kolmogorov probability theory. However sometimes a state of the form:
$$|\psi\rangle = \frac{1}{\sqrt{2}}\left(|a\rangle + |b\rangle\right)$$
simply represents classical "ignorance" based probability. Such as when $|a\rangle$ and $|b\rangle$ belong to different superselection sectors.

To rephrase what @bhobba is saying, a few stray CMB photons turn out to be enough to put different states of a dust mote in different superselection sectors.

 

[atyy]

Sorry, again I don't understand. What do you mean "in the larger sense of orthodox quantum mechanics".

Even if we accept that anything can be described by quantum mechanics, quantum mechanics itself has a classical-quantum cut. So saying that anything can be described by quantum mechanics does not remove the classical-quantum cut.

Again and this could be my failing, I don't know what you mean by "real" here or what this problem of real and not real is.

My understanding of modern QM is as follows:

We are studying a microscopic system. QM is a generalised probability theory. We find that neither the events nor the set they are drawn from are determined. We choose a device which selects a single Boolean frame/sample space of events. Our measurement result is then randomly drawn from this set.

However the device itself may be treated by quantum theory where initially it would seem a second device will select a Boolean frame for measurements upon it. However modern decoherence theory shows that in fact within QM itself macro-properties form a Boolean space of events, so a second device actually is not necessary.

To me we have the device clearly being treated by QM here, what is this problem of real and not real?

That is not correct. The second classical device is still required. If what you are saying is correct, then Many-Worlds or some interpretation where the entire universe is described by a unitarily evolving quantum state would be already considered viable without controversy.

 

[pinball1970]

I have been reading Sean Carroll's recent book called Something Deeply Hidden where he advocates a Many Worlds Interpretation of Quantum Mechanics. I was thinking that the most important question in all of Physics might be is everything quantum or is there a quantum/classical boundary.

I am two threads and half a book behind on this question so I will check back in when I have completed the book.

 

[phinds]

Is the Quantum/Classical Boundary the most important question in Physics

Personally, I think the most important question is why QM and GR don't play well together. Clearly one or the other or both need modification and so far no one has been able to do it.

 

[Lord Jestocost]

1. Bohr was a mumbler so understanding what he said required some effort.
2. His ideas were subtle and deep. In fact I would say verging on incomprehensible - I certainly do not understand him - and the number of papers with differing views shows that perhaps even experts in it are confuse.

It is not all that difficult to understand Bohr when taking an instrumentalist’s point of view.

 

[EPR]

As @bhobba has said quantum theory is a generalized probability theory. Sometimes a linear sum of states represents a type of statistics that doesn't appear in classical/Kolmogorov probability theory. However sometimes a state of the form:
$$|\psi\rangle = \frac{1}{\sqrt{2}}\left(|a\rangle + |b\rangle\right)$$
simply represents classical "ignorance" based probability. Such as when $|a\rangle$ and $|b\rangle$ belong to different superselection sectors.

But when is that 'sometimes'? Under what scenario and conditions?

 

[DarMM]

When is that 'sometimes'? Under what scenario and conditions?

Whenever there is a superselection rule.

 

[EPR]

This is circular reasoning - whenever classical environment-enduced superselection takes place, classical states appear.

 

[DarMM]

Even if we accept that anything can be described by quantum mechanics, quantum mechanics itself has a classical-quantum cut. So saying that anything can be described by quantum mechanics does not remove the classical-quantum cut.

What has this got to do with my original point about Bohr considering anything to be capable of being described by quantum theory?
In a given application something must be taken to select the Boolean event space, but in another application it may be the system under consideration. Thus nothing requires a classical treatment.

That is not correct. The second classical device is still required. If what you are saying is correct, then Many-Worlds or some interpretation where the entire universe is described by a unitarily evolving quantum state would be already considered viable without controversy.

Perhaps I didn't explain myself well. I'm saying that the different states of the macroscopic device belong to different superselection sectors and thus any probability due to a superposition of their states is simply classical ignorance. Thus for such degrees of freedom unitary evolution is just classical stochastic evolution concerning classical probabilities. This is nothing as such to do with Many Worlds.

EDIT: Maybe another way to say it. When Von Neumann formulated his "chain" in the 1930s at each stage in the chain we had the $n$th device necessary to select the Boolean frame choosen for the event space of the $(n - 1)$th device. However more modern study has shown that for actual devices there is only one event space, thus another device is not necessary to select out the Boolean algebra of events.