Psi-ontic view, macro objects and decoherence

[PuzzledR]

Good evening!

I know, that the questions similar to mine were frequently asked in this forum. Recently I even saw the thread about macro-realism and was satisfied by the answer to it. However, one article made me try to find some clarification.

The link to the article: https://arxiv.org/pdf/1501.05969.pdf

This article states that superpositions cannot be considered as epistemic. This conclusion is not a surprise for me, especially while taking into account the famous PBR-theorem etc. Nevertheless, as far as I remember, PBR-theorem discussed isolated quantum systems, which are not entangled with the others, so everything was quite logical.

But in the article I did not find any information about the conditions under which the quantum system was analyzed. Moreover, the author talked about Schrödinger's cat, which, I suppose, cannot be treated as isolated:

"Schrödinger’s cat is set up to be in a superposition of |dead⟩ and |alive⟩ quantum states. The epistemic real- ist (and probably the cat) would ideally prefer the ontic state of the cat to only ever be one of “dead” or “alive” (viz., only in ontic states accessible to either the |dead⟩ or |alive⟩ quantum states). In that case, the cat’s apparent quantum superposition would be epistemic—there would be nothing ontic about the superposition state. Conversely, if there are ontic states which can only obtain when the cat is in a quantum superposition (and never when the cat is in either quantum |alive⟩ or |dead⟩ states) then the superposition is unambiguously ontic: there are ontological features which correspond to that superposition but not to non-superpositions, so that superposition is real.Obviously quantum superpositions are different from proper mixtures of basis states. The question here is rather whether quantum superpositions over basis states can be understood as probability distributions over some subset of underlying ontic states, where each such ontic state is also accessible by preparing some basis state."

Eventually, the author concluded that superpositions must be real and "the unfortunate cat cannot be put out of its misery." Frankly speaking, I did not get all mathematical stuff, that is why I, of course, make some mistakes in my assumptions. But does the author mean that if one accepts psi-ontic point of view, one should accept that even systems of higher dimensions such as cats must be in superposition even if they are entangled with environment etc. ? Or the author mean that superpositions of large systems can be real in principle, under conditions of being isolated etc. ?

P. S. I understand that you are all tired of such questions, I expect to read in your answers the explanation of distinction between pure and mixed states etc., but I cannot get rid of the thought that all these quite common and fundamental explanations are ruled out by that article. Maybe I have a syndrome of typical amateur who had been frightened by "weird" quantum mechanics and implausible interpretations such as "consciousness causes collapse" and so on. That is why I beg to forgive me!

 

[Mentz114]

In laboratories a super-conducting charge qubit can be held in a superposed state for about 90 $\mu s$ at a temperature of 0.02 K.

Something as big and hot as a cat cannot be in superposition and QT does not say that it can.

 

[PuzzledR]

In laboratories a super-conducting charge qubit can be held in a superposed state for about 90 $\mu s$ at a temperature of 0.02 K.

Something as big and hot as a cat cannot be in superposition and QT does not say that it can.

Thank you for your answer!

Yes, I understand this. But, keeping this in mind, what is the need to even mention cats in such articles? What makes me confused is that the author makes some accent on large systems.

 

[Mentz114]

I don't understand the article or the motivation for it but these words seem to destroy the general validity of their conclusions.

Note that this theorem does not immediately imply
that almost all superpositions are real. However, by
demonstrating that Thm.2’s arguments can be made ro-
bust against error, it suggests that a noise-tolerant ver-
sion of Thm.1 should also be possible. Even so, a noise-
tolerant version of Thm.1 would require the definition of
“epistemic superposition” to be modified, since it is cur-
rently defined in terms of the noise intolerant asymmetric
overlap and is therefore noise intolerant

The subject of superposition is deep and subtle but any arguments about 'dead/alive' cats is absurd.

 

[bhobba]

"Schrödinger’s cat is set up to be in a superposition of |dead⟩ and |alive⟩ quantum states.

That's impossible.

Here is why. Now a cat is not just a macro object, but a macro object that in order to be alive needs to be in contact with its environment eg it breaths air, sweats from its pores etc. Even a dead one is hard to imagine apart from its environment - for example if a cat was in outer space it likely its lungs would rupture - but being dead that's not really a concern - so I suppose its possible for a dead cat to be isolated from its environment - but we are considering a superposition of alive and dead cat so the environment must be included. That's the key issue. You see its due to interaction with the environment that gives classical objects their properties, to be specific here a pretty much exact position - its not quite exact - but for all intents and purposes as far as everyday things are concerned it is. Its done by decoherence - but I will not go into how that happens - its a bit technical. If interested here is the book to get:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Anyway think of the cat as composed of small constituent parts - not atomic sized but say a mm square so they are also macro and will have an exact position. So let's compare such parts between a live and dead cat. In a live cat the lungs will expand and contract, the heart beats etc. It does not do that in a dead cat. Those parts will have entirely different positions for alive and dead cats. But since they are macro objects they have exact positions hence cannot be in a superposition of position - its impossible - utterly impossible.

Once you understand that you realize the whole thought experiment is silly. It was not silly when it was proposed because QM was not understood as well as it is now - it raised a genuine issue - namely as Von-Neumann showed the quantum classical cut can be placed anywhere - even outside the box in the thought experiment. But since then a lot of progress has been made in understanding QM and we know its resolution very well - the cat can never be in a superposition. BTW the augment I gave is not the only way to resolve it - there is one based on entanglement with the radioactive source - that leads to the cat being in what's called a mixed state of alive and dead rather than a superposition. But I find after explaining this stuff a lot to beginners the above argument is easier for them to understand.

Regarding superposition's the first thing you have to understand is what a superposition is - its simply that pure quantum states form a vector space. Perhaps the simplest vector space of all is just real numbers so I will use it to explain the issue. Take 6. 6 = 3+3. In quantum parlance 6 is a superposition of 3 and 3. But 6 = 4 +2 so 6 is also a superposition of 4 and 2 - I could go on but I am sure you get the drift - any state is in a superposition of a myriad of other states in a myriad of different ways. I am not a philosophy type but I would say epistemic, ontic etc are not appropriate things to describe such a state of affairs.

I could go on about other stuff in your post and that link - but I think you get the drift - the paper has a number of misconceptions in it so I personally would forget it.

Want to learn QM - I mean the real deal - not pop-sci half truths get these two books:
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

After reading those you too will be able to spot - how to put it - papers of genuine content and those that are somewhat dubious.

Thanks
Bill

 

[PuzzledR]

That's impossible.

Here is why. Now a cat is not just a macro object, but a macro object that in order to be alive needs to be in contact with its environment eg it breaths air, sweats from its pores etc. Even a dead one is hard to imagine apart from its environment - for example if a cat was in outer space it likely its lungs would rupture - but being dead that's not really a concern - so I suppose its possible for a dead cat to be isolated from its environment - but we are considering a superposition of alive and dead cat so the environment must be included. That's the key issue. You see its due to interaction with the environment that gives classical objects their properties, to be specific here a pretty much exact position - its not quite exact - but for all intents and purposes as far as everyday things are concerned it is. Its done by decoherence - but I will not go into how that happens - its a bit technical. If interested here is the book to get:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Anyway think of the cat as composed of small constituent parts - not atomic sized but say a mm square so they are also macro and will have an exact position. So let's compare such parts between a live and dead cat. In a live cat the lungs will expand and contract, the heart beats etc. It does not do that in a dead cat. Those parts will have entirely different positions for alive and dead cats. But since they are macro objects they have exact positions hence cannot be in a superposition of position - its impossible - utterly impossible.

Once you understand that you realize the whole thought experiment is silly. It was not silly when it was proposed because QM was not understood as well as it is now - it raised a genuine issue - namely as Von-Neumann showed the quantum classical cut can be placed anywhere - even outside the box in the thought experiment. But since then a lot of progress has been made in understanding QM and we know its resolution very well - the cat can never be in a superposition. BTW the augment I gave is not the only way to resolve it - there is one based on entanglement with the radioactive source - that leads to the cat being in what's called a mixed state of alive and dead rather than a superposition. But I find after explaining this stuff a lot to beginners the above argument is easier for them to understand.

Regarding superposition's the first thing you have to understand is what a superposition is - its simply that pure quantum states form a vector space. Perhaps the simplest vector space of all is just real numbers so I will use it to explain the issue. Take 6. 6 = 3+3. In quantum parlance 6 is a superposition of 3 and 3. But 6 = 4 +2 so 6 is also a superposition of 4 and 2 - I could go on but I am sure you get the drift - any state is in a superposition of a myriad of other states in a myriad of different ways. I am not a philosophy type but I would say epistemic, ontic etc are not appropriate things to describe such a state of affairs.

I could go on about other stuff in your post and that link - but I think you get the drift - the paper has a number of misconceptions in it so I personally would forget it.

Want to learn QM - I mean the real deal - not pop-sci half truths get these two books:
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

After reading those you too will be able to spot - how to put it - papers of genuine content and those that are somewhat dubious.

Thanks
Bill

Bill, thank you!

I had the same point of view on this problem as you. But when I faced some materials like that and also the works concentrated around, for example, Anthony Leggett's assumptions about macroscopic behavior , I became a little bit confused. For example, I don't understand, why in such researches macroscopic "observables" such as "dead" or "alive" are equivalent to states like "spin up" or "spin down". I suppose, that macroscopic observables are complex and "consist" of many-many "simple" states, and we can't just put them together with standard quantum formalism and prove-as it is usually proved in such papers-that QM is incompatible with define macroscopic states independent from observation.

 

[Lord Jestocost]

Schrödinger’s cat is set up to be in a superposition of |dead⟩ and |alive⟩ quantum states.

That's impossible..

With all respect, where do you get that from: “That’s impossible“. Can you design an appropriate experiment to prove your "That's impossible".

Once you understand that you realize the whole thought experiment is silly. It was not silly when it was proposed because QM was not understood as well as it is now...

I think that the founders of quantum theory had a deeper understanding of quantum theory compared to those who nowadays present a lot of "QT interpretation" and "decoherence" folklore. The cat story was never meant to be a "thought experiment". Schroedinger^* merely wanted to point out - a little bit ironically: "There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks." The fact that many haven't understood and still don't understand what he wanted to say isn't his fault.

^* E. Schroedinger "Die gegenwärtige Situation in der Quantenmechanik", Die Naturwissenschaften, 1935, Volume 23, Issue 48 (translation by J. D. Trimmer in Proc. Am. Philos. Soc. 124, 323 (1980))

 

[bhobba]

With all respect, where do you get that from: “That’s impossible“. Can you design an appropriate experiment to prove your "That's impossible".

Its simple logic. Let's follow it - 1. Here in the classical world small elements of matter - say 1mm in size act in a classical way eg have a definite position, momentum etc. 2. If the cat was in a superposition of alive and dead those small elements with definite position would not have definite position. So - its impossible to be in a superposition of position hence of alive and dead. Of course any state is a superposition of all sorts of other states so of course its in a superposition - but not of alive and dead.

Thanks
Bill

 

[bhobba]

I think that the founders of quantum theory had a deeper understanding of quantum theory compared to those who nowadays present a lot of "QT interpretation" and "decoherence" folklore.

I don't think we will ever agree on this but my view is the exact opposite - the early pioneers, except Dirac, had views that were deeply flawed. I tend to side with Wienberg:
http://physicstoday.scitation.org/doi/full/10.1063/1.2155755

The other mistake that is widely attributed to Einstein is that he was on the wrong side in his famous debate with Niels Bohr over quantum mechanics, starting at the Solvay Congress of 1927 and continuing into the 1930s. In brief, Bohr had presided over the formulation of a “Copenhagen interpretation” of quantum mechanics, in which it is only possible to calculate the probabilities of the various possible outcomes of experiments. Einstein rejected the notion that the laws of physics could deal with probabilities, famously decreeing that God does not play dice with the cosmos. But history gave its verdict against Einstein—quantum mechanics went on from success to success, leaving Einstein on the sidelines.

All this familiar story is true, but it leaves out an irony. Bohr’s version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wavefunction (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from?

Thanks
Bill

 

[Lord Jestocost]

Its simple logic.

No, you are using simple "classical" logic based upon "Tertium non datur".

 

[Gigaz]

Ah, the cat :D

I would say it can be done to have a cat in the |dead>+|alive> state, maybe not today but in the far future. One must keep in mind though that the decomposing cat stores detailed information about the time of death, so the state which is measured in the end, which results from an interference of the dead and the living cat, must be one where the body of the cat is annihilated beyond repair.

 

[PuzzledR]

Ah, the cat :D

I would say it can be done to have a cat in the |dead>+|alive> state, maybe not today but in the far future. One must keep in mind though that the decomposing cat stores detailed information about the time of death, so the state which is measured in the end, which results from an interference of the dead and the living cat, must be one where the body of the cat is annihilated beyond repair.

You have mentioned interference and I recalled one thesis by Anthony Leggett. There is the link to the material: http://people.physics.illinois.edu/leggett/FP_IQC 051310.pdf

I have attached the page where the thesis is:

There is stated:"If decoherence implies no interference, then both superposition and mixed state give the same predictions". Does that mean that the system can be in such superposition without "showing" interference?

 

[AlexCaledin]

... The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong:..

- is the Peres book wrong, too??
Peres says what seems quite reasonable (chapter 12, page 423) ,

Quantum theory is not an objective description of physical reality. It only predicts the probability of occurrence of stochastic macroscopic events, following specified preparation procedures, such as, for instance, the triggering of particle detectors following the collision of a specified beam with a given target. These theoretical predictions rely on conceptual models which involve elusive microscopic objects, such as electrons, photons, etc. The latter are usually considered as real things, but they occasionally display extraordinary nonlocal properties, quite at variance with intuitive classical realism, as we have seen in Chapter 6. The essential difference between these two classes of objects, microscopic and macroscopic, is not a matter of size, but of the completeness of their description: all the degrees of freedom of a quantum system are taken into account in its Hamiltonian. On the other hand, only a very small fraction of the degrees of freedom of a macroscopic system are explicitly used in its description.

Quantum Theory: Concepts and Methods (Fundamental Theories of Physics)

 

[Lord Jestocost]

There is stated:"If decoherence implies no interference, then both superposition and mixed state give the same predictions". Does that mean that the system can be in such superposition without "showing" interference?

The elimination of coherence doesn't imply that a quantum mechanical "AND" can be replaced by a classical "OR". A superposition remains a superposition when consequently applying the quantum mechanical formalism, viz. there is nothing within the calculational framework of QM to "convert" a superposition into a statistical mixture.

 

[PuzzledR]

The elimination of coherence doesn't imply that a quantum mechanical "AND" can be replaced by a classical "OR". A superposition remains a superposition when consequently applying the quantum mechanical formalism, viz. there is nothing within the calculational framework of QM to "convert" a superposition into a statistical mixture.

Interesting! Sorry for silly questions, but if we extrapolate this to the double-slit experiment, does this mean that when we do not observe interference pattern, the photon can still be in the superposition of going through one or another slit?

 

[Lord Jestocost]

...but if we extrapolate this to the double-slit experiment, does this mean that when we do not observe interference pattern, the photon can still be in the superposition of going through one or another slit?

For me, quantum mechanics is nothing but a set of calculational recipes designed and used to predict and describe the probabilities of various directly observed macroscopic outcomes (observations). For me, the wave function is merely an intellectual tool corresponding to nothing in the - as we call it - "real world". I have thus no idea how the entity "photon" – if it "exists" at all in the "classical" sense – behaves before the act of observation. I think it always behaves like itself, but the answer we get depends on the question we put. Here, I follow Wheeler: "No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon".

Thus, I have no idea what it means to say "the photon can still be in the superposition of going through one or another slit". In case I do not observe an interference pattern, I would simply modify the corresponding time-dependent Schroedinger equation. However, in case one has a double-slit experiment, the appropriate wave function will always be a superpostion.

 

[Mentz114]

Interesting! Sorry for silly questions, but if we extrapolate this to the double-slit experiment, does this mean that when we do not observe interference pattern, the photon can still be in the superposition of going through one or another slit?

If the phase difference between the two parts is $\pi/2$ then - yes.

 

[bhobba]

- is the Peres book wrong, too??
Quantum theory is not an objective description of physical reality. It only predicts the probability of occurrence of stochastic macroscopic events, following specified preparation procedures, such as, for instance, the triggering of particle detectors following the collision of a specified beam with a given target. These theoretical predictions rely on conceptual models which involve elusive microscopic objects, such as electrons, photons, etc. The latter are usually considered as real things, but they occasionally display extraordinary nonlocal properties, quite at variance with intuitive classical realism, as we have seen in Chapter 6. The essential difference between these two classes of objects, microscopic and macroscopic, is not a matter of size, but of the completeness of their description: all the degrees of freedom of a quantum system are taken into account in its Hamiltonian. On the other hand, only a very small fraction of the degrees of freedom of a macroscopic system are explicitly used in its description.

Its not that its wrong with a capital WRONG - its that it's much more nuanced than that. The formalism of QM is silent on if its a theory about objective reality - even assuming you can define what objective reality is and get everyone to agree on it. I will, for the purpose of this post, call objective reality classical theories like EM or classical mechanics. We have interpretions like that (eg BM) and others that are not (eg the ensemble interpretation). Also the person that wrote it has got some facts incorrect - QFT has no non local properties - electrons etc are not non-local. He doesn't actually understand Bell properly - but he is not alone in that. The reason I mention QFT and not standard QM, again this is not appreciated even by even some experts, standard QM is actually derived from the Galilean transformations, the same as holds in Classical Mechanics - you can find it in chapter 3 of Ballentine. But the Galilean transformations are inherently non-local - classical mechanics is inherently non-local. The great physicist, Lev landau, wrote a classic on Classical Mechanics that IMHO is THE book on it - simply called Mechanics. Its very beautiful and I could, as I sometimes do when I talk about that book post reactions to it such as 'Read this book as a test: if you love it, you really love the beautiful science of Physics.' Why? While terse it explains things others don't - such as right at its foundations Classical physics is non-local and even why mass exists and is positive.

Just as a comment the best way to grasp QM is not by reading and deconstructing things like the above, its to slowly and carefully go though the actual physics.

It will take time, but if you want to understand modern physics, the actual why not just solving problems, I suggest the following in this order:

1, Quick Calculus
https://www.amazon.com/dp/0471827223/?tag=pfamazon01-20
2. Susskinds theoretical minimum books:
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20
https://www.amazon.com/dp/0465093345/?tag=pfamazon01-20
3. Landau's book mentioned previously
https://www.amazon.com/dp/0750628960/?tag=pfamazon01-20
4. Physics From Symmetry
http://physicsfromsymmetry.com/

After absorbing that you will be in a much better position to sort this stuff out. You seem to think its about dialectic arguments. It isn't. Its written in the language of math which is rather nuanced converted to English. Like I mentioned before with the case of Gauss vs Kant. Kant was a philosopher supreme - but his tools were not sufficient to distill the truth about geometry - he thought he had, and millions thought he had as well - but they were wrong. Gauss was equally as eminent mathematician as Kant a philosopher - but he was able to find the truth - the truth being a big surprise - namely there is nothing special about euclidean geometry a-priorii - non-euclidean geometries are just as valid and in fact logically as consistent or not consistent as ordinary geometry. This was the start of the path that lead to GR. That's another characteristic of math as a tool - it seems to be ever increasing in knowledge, breath and scope, but philosophy just seems to rehash the same stuff - in pretty prose - I can't write essays of the standard they do - but nothing actually new seems to come from it. Sure you get Nietzsche that had some different ideas (but what was it one of my math professors said - pointless really), Wittgenstein who went toe to toe with Turing on what math is (people thought Wittgenstein won that debate but later more came to realize Turing was correct - math is more than just a convention) and Ayn Rand whose ideas are harder to describe and debunk. But after a while you realize with Ayn Rand she has not learned the central lesson of science - observation trumps theory - she has a problem with novelty. Reading what her disciples say about QM sometimes actually makes me mad the misunderstandings they have, despite I had a lot of time for her in my youth. They for example say the sum over histories approach of Feynman proves they are mad and have along with many other intellectuals are now semi mystical. They didn't get it was just a pictorial heuristic suggested by the math - it doesn't mean literally quantum objects take all paths - in fact while not being observed we have zero idea what quantum objects are doing at all. Yet rather than just accept the obvious - we are all mad and somehow under the spell of the irrationalist views she thinks has gripped the world. Irrational rubbish abounds - but she reads more into it than it actually means.
.
Fundamental truths about the physical world are written in the language of math - its only by using that can its nature be revealed - and its more surprising, subtle and nuanced than those of a dialectic bent can imagine.

Thanks
Bill

 

[PuzzledR]

If the phase difference between the two parts is $\pi/2$ then - yes.

So, is there actually a need to test whether macroscopic objects can show interference or not? For example, if we make macroscopic object interfere it can mean that either it is in superposition only in the conditions where it interferes or it is also in superposition even under influence of decoherence.

 

[Lord Jestocost]

So, is there actually a need to test whether macroscopic objects can show interference or not? For example, if we make macroscopic object interfere it can mean that either it is in superposition only in the conditions where it interferes or it is also in superposition even under influence of decoherence.

To say it again: When you assume that the initial state function of the system which you are studying is a superposition, its evolution over time will always be characterized by a superposition according to the calculational rules of QM. You can feed in as much of “decoherence” as you want, a superposition remains a superposition as long as you correctly use the mathematics and understand what you are doing, viz. there is no way to “convert” a superposition into a “mixed state” in the sense of a statistical mixture. What some decoherence proponents sometimes call classical-like probabilities are still probabilities of what will be observed, and have nothing in common with true classical probabilities of what supposedly actually exist. That’s the essence of Schroedinger’s cat fable.

To use again Schroedinger's words: "There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks."

Now, to quote A. J. Leggett, we encounter a paradox: “Basically, the quantum measurement paradox is that most interpretations of QM at the microscopic level do not allow definite outcomes to be realized, whereas at the level of our human consciousness it seems a matter of direct experience that such outcomes occur (indeed, it seems so difficult to imagine what it would be like for the world to be otherwise that I suspect that Immanuel Kant, had he had occasion to consider the problem, would have classified our knowledge of this state of affairs as ‘synthetic a priori’)”.

 

[Mentz114]

So, is there actually a need to test whether macroscopic objects can show interference or not? For example, if we make macroscopic object interfere it can mean that either it is in superposition only in the conditions where it interferes or it is also in superposition even under influence of decoherence.

As far as the rules of QT come into this I agree with jestocost. But the rules of QT are there to make sure that there are no predictions that will break a more important set of rules - the symmetries that govern the dynamics of the system.

 

[PuzzledR]

Oh, now it starts to become more clear for me. I express gratitude to everyone who had taken part in this conversation! You are all so smart and, if it is appropriate to say, brave! Thank you! :)

 

[bhobba]

So, is there actually a need to test whether macroscopic objects can show interference or not?

We know they can - look up buckyballs.

But for it to do it, it must be isolated from an environment - that's the key.

The larger the object - the harder it is to do - but technological progress is never-ending so expect the objects that show quantum effects to get bigger and bigger.

Nowadays with technology way ahead of what the QM founders had available we see some very strange things indeed:
http://www.nature.com/news/2010/100317/full/news.2010.130.html

As the link says:
"The environment is this huge, complex thing," says Cleland. "It's that interaction with this incredibly complex system that makes the quantum coherence vanish."

Only when that's removed do quantum effects manifest here in the macro-world.

Thanks
Bill

 

[PuzzledR]

We know they can - look up buckyballs.

But for it to do it, it must be isolated from an environment - that's the key.

The larger the object - the harder it is to do - but technological progress is never-ending so expect the objects that show quantum effects to get bigger and bigger.

Nowadays with technology way ahead of what the QM founders had available we see some very strange things indeed:
http://www.nature.com/news/2010/100317/full/news.2010.130.html

As the link says:
"The environment is this huge, complex thing," says Cleland. "It's that interaction with this incredibly complex system that makes the quantum coherence vanish."

Only when that's removed do quantum effects manifest here in the macro-world.

Thanks
Bill

Bill, I understand this. Of course, the coherence vanishes due to interaction with environment. But that does not mean that superposition stops to exist.

 

[bhobba]

Bill, I understand this. Of course, the coherence vanishes due to interaction with environment. But that does not mean that superposition stops to exist.

Superposition of whatj? Of position? - yes it is interaction with the environment that generally removes superposition's of position.

Thanks
Bill

 

[PuzzledR]

Superposition of whatj? Of position? - yes it is interaction with the environment that generally removes superposition's of position.

Thanks
Bill

I mean superposition of any eigenvalues.

 

[vanhees71]

Eigenvalues cannot be in superposition, only eigenvectors...

 

[Lord Jestocost]

Superposition of whatj? Of position? - yes it is interaction with the environment that generally removes superposition's of position.

There seems to be some misunderstanding regarding superpositions.

As respects to the double-slit experiment:

Even in case you don't see a clear interference pattern, you are never allowed to say: The "particle" has traveled either through slit#1 or through slit#2 by assuming some "classical" probabilities. The "particle" must always be "described" as a quantum mechanical superposition when analyzing the experiment; when – maybe due to some phase randomization – the quantum probabilities will at the end behave numerically the same as classical probabilities, there remains – conceptually - a fundamental difference: those classical-like probabilities are still probabilities of what will be observed, and have nothing in common with true classical probabilities of what supposedly actually exists. Thus, even if - at the end - the "interference" pattern looks somehow like "50% of the particles have passed slit#1 and 50% of the particles have passed slit#2", such a classical thinking would be completely wrong. Such a statement is only allowed when you design your experimental setup in such a way that you can follow the paths of the particles.

 

[bhobba]

The "particle" must always be "described" as a quantum mechanical superposition when analyzing the experiment;

I gave it my like because it does make some valid points.

But again the question is a superposition of what?

Consider just one slit.- just behind the slit its in a state of exact position - its not in a superposition of position. However it is in a superposition of momentum which is why you can't predict where it will hit the screen.

If you have two slits then its in a superposition of the position of slit 1 and slit 2 so when you look at the momentum you get interference effects.

I know Vanhees doesn't like the following analysis believing correctly it has issues, but its better that the usual wave particle duality stuff:
https://arxiv.org/abs/quant-ph/0703126

Generally things here in the macro-world being in contact with the environment is in a state of nearly exact position - obviously not exact because then it will have a very inexact momentum - but the errors in both position and momentum is so small for all practical purposes both are exact. Why that is is technical but it has to do with the radial nature of most interactions.

Thanks
Bill

 

[vanhees71]

Well, this is indeed a bit misleading. If you want to observe interference effects you must have a quite precise momentum rather than precise position of the particles reaching the slits. Spoken in wave-mechanical language, the wave packet must be broad enough to overlap more or less evenly both slits in the double-slit experiment to show interference effects. If you instead determine the position of the particles so precisely that, when they reach the slits, you know through which slit they go, you won't see interference effects. It's a nice example for what Bohr, a bit nebulously, called "complementarity", which is made comprehensible and precise by the uncertainty relation for incompatible observables (in this case spatial position and momentum of the incoming particles).

 

[bhobba]

Well, this is indeed a bit misleading. If you want to observe interference effects you must have a quite precise momentum rather than precise position of the particles reaching the slits. Spoken in wave-mechanical language, the wave packet must be broad enough to overlap more or less evenly both slits in the double-slit experiment to show interference effects. If you instead determine the position of the particles so precisely that, when they reach the slits, you know through which slit they go, you won't see interference effects. It's a nice example for what Bohr, a bit nebulously, called "complementarity", which is made comprehensible and precise by the uncertainty relation for incompatible observables (in this case spatial position and momentum of the incoming particles).

Yes, yes, of course - that's the state before the slits. But just after the slits its in a superposition of the position of slit 1 and slit 2. That's why you get the interference pattern.

Thanks
Bill

 

[Lord Jestocost]

But again the question is a superposition of what?

A superposition is not a superposition of „what“ when the „what“ is thought as some underlying stuff. Any direct connotation with some underlying stuff produces immediately nonsense like “wave-particle duality”. A superposition is an intellectual tool representing nothing else than our quantum ignorance. To quote Schroedinger (Schroedinger to Sommerfeld, 1931): “Quantum mechanics forbids statements about the object. It deals only with the object-subject relation.”

 

[PuzzledR]

A superposition is not a superposition of „what“ when the „what“ is thought as some underlying stuff. Any direct connotation with some underlying stuff produces immediately nonsense like “wave-particle duality”. A superposition is an intellectual tool representing nothing else than our quantum ignorance. To quote Schroedinger (Schroedinger to Sommerfeld, 1931): “Quantum mechanics forbids statements about the object. It deals only with the object-subject relation.”

Oh, I so hope that you are right, because all these PBR-like theorems about ontic states of superposition just terrify me, and I can't get rid of some existential fears. /Sorry, I know that PF is not for those who has depression because of physics :)/

 

[bhobba]

A superposition is not a superposition of „what“ when the „what“ is thought as some underlying stuff. Any direct connotation with some underlying stuff produces immediately nonsense like “wave-particle duality”. A superposition is an intellectual tool representing nothing else than our quantum ignorance. To quote Schroedinger (Schroedinger to Sommerfeld, 1931): “Quantum mechanics forbids statements about the object. It deals only with the object-subject relation.”

Yes yes - all true - I am using loose language.

Thanks
Bill

 

[fanieh]

I don't want to create a new thread about Collapse so let me put it here as the topic is related...
I couldn't reply to this thread https://www.physicsforums.com/threads/how-does-qft-handle-non-locality.849972/page-3

atyy wrote in message number 58:

"When A measures u, then the state will immediately collapse to |uu>, so B will measure u with certainty. But can B tell that A made a measurement? He cannot, because if A always measures before B, A will collapse the state to |uu> half the time and to |dd> the other half of the time. But if A measures after B, then B will measure u half the time and d half the time. So although taking collapse as reality will violate relativistic causality as something real, collapse does not lead to any superluminal communication. This is why collapse is consistent with "no superluminal signalling".

The quantum correlations are a subset of no-signalling, and the relativistic causality correlations are a subset of the quantum correlations. Quantum mechanics including collapse violates relativistic causality as something real, but it does not violate no signaling"

Does this mean it is perfectly all right to treat wave function collapse as objective as long as it doesn't violate no signaling meaning "no superluminal signalling" and yet it is ok to violate relativistic causality where causality means measurements at spacelike separated events must commute (meaning their results cannot depend on the order in which they are performed). So the probabilistic nature of the collapse can violate causality yet doesn't violate "no superluminal signaling"..

In other words. It is ok to violate causality as long as there is no information transferred? What is the consensus about this? Is this how the justification for those who want to treat collapse as objective?