Undergrad Is Entanglement Properly Defined in Bohr's Idea of Complementarity?

[bhobba]

I was reading an interesting article on the Bohr/Einstein debates. Interestingly it got most things right including the real reason Einstein thought QM incomplete - its not the probabilistic nature - it was that it didn't gel with his intuition on how nature works having features like entanglement - in fact the article goes further and argues his objections were all really about entanglement - but that is not what I want to talk about here.

But it gave in Bohr's own words his idea of complementarity:

'Now, the quantum postulate implies that any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected. Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomenon or to the agencies of observation. . . .This situation has far-reaching consequences. On one hand, the definition of the state of a physical system, as ordinarily understood, claims the elimination of all external disturbances. But in that case, according to the quantum postulate, any observation will be impossible, and, above all, the concepts of space and time lose their immediate sense. On the other hand, if in order to make observation possible we permit certain interactions with suitable agencies of measurement, not belonging to the system, an unambiguous definition of the state of the system is naturally no longer possible, and there can be no question of causality in the ordinary sense of the word. The very nature of the quantum theory thus forces us to regard the space-time co-ordination and the claim of causality, the union of which characterizes the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and definition respectively. (Bohr 1928, 54-55)'

Further the author says 'These few words have given rise to much confusion in the secondary literature'. Golly gee - much of it makes no sense to me at all eg 'Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomenon or to the agencies of observation' Really - I thought that was interpretation dependent. Now I realize this was 1928, Dirac's transformation theory was only published in 1927 and exactly what it means has certainly not been fully worked out yet. But that's my issue - how can even as great a scientist like Bohr know that until it has been studied fully? And without an actual proof, even after years of study, you can't be sure.

I won't be participating much in this tread but I am curios what others think.

Thanks
Bill

 

[Demystifier]

how can even as great a scientist like Bohr know that until it has been studied fully? And without an actual proof, even after years of study, you can't be sure.

Bohr just expressed his opinion, based more on his philosophy and intuition than on proved facts.

 

[martinbn]

Which article?

 

[bhobba]

Revisiting the Einstein-Bohr Dialogue available from Academia.edu.

Thanks
Bill

 

[vanhees71]

When I read Bohr, I'm always reminded that Lichtenberg's famous saying: "If you hit your head with a book and it sounds hollow, it's not always due to the book." can also turned against the author by just changing one word: "If you hit your head with a book and it sounds hollow, it's not always due to the head."

 

[vanhees71]

Revisiting the Einstein-Bohr Dialogue available from Academia.edu.

Thanks
Bill

Could you give the full bibliographic data? I don't want to register for academia.edu again. When I once did first of all it spammed me like crazy with even crazier philosophical papers, then obviously scanned my coauthors, I've written papers with, and spammed them too. So I'll never ever register with them again!

 

[bhobba]

Could you give the full bibliographic data? I don't want to register for academia.edu again. When I once did first of all it spammed me like crazy with even crazier philosophical papers, then obviously scanned my coauthors, I've written papers with, and spammed them too. So I'll never ever register with them again!

Same here, but not having any co-authors or anything that bit is not an issue so I just put up with it. And some of the stuff is, like you say, just crazy - I will not disrespect philosophers by calling them that - legit philosophers could not be that crazy. The good news is it only takes a moment to surface read a paper and see its rubbish - but others probably do not have my free time.

Here is a direct link:
https://www3.nd.edu/~dhoward1/Revisiting the Einstein-Bohr Dialogue.pdf
Thanks
Bill

 

[DarMM]

First note that Bohr's views in that 1928 essay are a good bit different from the final development of his views in the 1940s.

'Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomenon or to the agencies of observation' Really - I thought that was interpretation dependent. Now I realize this was 1928, Dirac's transformation theory was only published in 1927

Let's say in an expression for the probability of some event today represented most generally by a POVM element $E$, we have:
$$Tr\left(\rho E\right)$$
That as such is part of the basic core of QM.

All Bohr is saying is that $E$ represents not a property of the microscopic system, but refers to a device-system event. Thus QM does not deal with the device or the system "independently", but of interaction events between the two.

Rudolf Haag, Berthold-Georg Englert, Julian Schwinger and Asher Peres make the same points in their textbooks. As Asher Peres has pointed out in his Foundational papers from the 1980s it's very hard to take POVMs in general in any other way and he gives arguments against taking even PVMs any other way as well. See his paper "Einstein, Godel, Bohr" for a simple example.

 

[Lord Jestocost]

But it gave in Bohr's own words his idea of complementarity:

Maybe, the following might help to understand Bohr's reasoning:
"Causality and Space-time Description" http://www1.kcn.ne.jp/~h-uchii/Bohr/complementarity.html

 

[bhobba]

Maybe, the following might help to understand Bohr's reasoning:
"Causality and Space-time Description" http://www1.kcn.ne.jp/~h-uchii/Bohr/complementarity.html

Nice paper. I think 'Whether or not you can understand what Bohr is trying to get at, entirely depends on whether you question this statement. Why can Bohr say this, and what is the reason for this?' captures my view pretty well. Bohr had an issue with verbal communication in that he was a well known mumbler, and had a definite philosophical bent, compared to say Dirac. This is not to say he could not be understood, I firmly believe his good friend Einstein understood him, so obviously he could be understood, but even Heisenberg had to be corrected by Bohr about the uncertainty principle.

And it is fair to say he did change his position a bit until about 1940 or so.

These days many say Decoherent Histories is Copenhagen done right - not necessarily my view - I just understand it better than I do Bohr.

Thanks
Bill

 

[vanhees71]

I think it was not only Bohr's mumbling, but it's also in his writing that it's pretty unclear what he wanted to say. The advantage with Dirac is that he is crystal clear because he uses an adequate amount of mathematics. At least in his book Dirac is, however, clearly a Copenhagian of the Heisenberg flavor, i.e., introducing explicitly the collapse postulate.

I think concerning the Bohr-Einstein debate this paper, you've recently quoted, by Howard gets the issue pretty clear.

Howard, Don. "Revisiting the Einstein—Bohr Dialogue." Iyyun: The Jerusalem Philosophical Quarterly / עיון: רבעון פילוסופי 56 (2007): 57-90.
http://www.jstor.org/stable/23354465.

Einstein's real quibble was not so much about probabilities and indeterminism but about inseparability. Einstein also didn't like the EPR paper, because he thought it was not so clearly written as he would wished it, and he wrote a much more lucid paper in German about it:

A. Einstein, Dialectica 2, 320 (1948)
https://doi.org/10.1111/j.1746-8361.1948.tb00704.x

 

[DarMM]

My experience with Bohr is that he doesn't lay out his terms in advance, which causes problems because he uses words with very specific meanings.

For example by "physical" he often means strictly "in a theory of physics" and by "foundational" or "basic" he often means what we call "axiomatic". So you can have sentences like (I'm parodying here) "Dogs don't exist in a basic physical sense", where he means "Dogs don't enter the theory as a foundational axiom or mathematical component".

In his explicit sense this is true, but anybody reading would think he meant there are no dogs.

 

[EPR]

I was reading an interesting article on the Bohr/Einstein debates. Interestingly it got most things right including the real reason Einstein thought QM incomplete - its not the probabilistic nature - it was that it didn't gel with his intuition on how nature works having features like entanglement - in fact the article goes further and argues his objections were all really about entanglement - but that is not what I want to talk about here.

But it gave in Bohr's own words his idea of complementarity:

'Now, the quantum postulate implies that any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected. Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomenon or to the agencies of observation. . . .This situation has far-reaching consequences. On one hand, the definition of the state of a physical system, as ordinarily understood, claims the elimination of all external disturbances. But in that case, according to the quantum postulate, any observation will be impossible, and, above all, the concepts of space and time lose their immediate sense. On the other hand, if in order to make observation possible we permit certain interactions with suitable agencies of measurement, not belonging to the system, an unambiguous definition of the state of the system is naturally no longer possible, and there can be no question of causality in the ordinary sense of the word. The very nature of the quantum theory thus forces us to regard the space-time co-ordination and the claim of causality, the union of which characterizes the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and definition respectively. (Bohr 1928, 54-55)'

Further the author says 'These few words have given rise to much confusion in the secondary literature'. Golly gee - much of it makes no sense to me at all eg 'Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomenon or to the agencies of observation' Really - I thought that was interpretation dependent. Now I realize this was 1928, Dirac's transformation theory was only published in 1927 and exactly what it means has certainly not been fully worked out yet. But that's my issue - how can even as great a scientist like Bohr know that until it has been studied fully? And without an actual proof, even after years of study, you can't be sure.

I won't be participating much in this tread but I am curios what others think.

Thanks
Bill

Hardly surprising given that the eye's retina and visual cortext were designed to make sense of the world(location, distance, size) by 'particles', not by delocalised waves. Without the classical context, no perception would ever take place. Our classical perception is almost too perfect - it took 1 million years of evolution to notice the hidden intricacies of the world.

 

[A. Neumaier]

Hardly surprising given that the eye's retina and visual cortex were designed to make sense of the world(location, distance, size) by 'particles', not by delocalised waves.

The eye is commonly analyzed not in terms of particles but in terms of geometric optics involving arbitrary rays, not particles. Eye and brain make sense of the world not by particles but by the motion of extended objects with manifold shapes.

 

[EPR]

The eye is commonly analyzed not in terms of particles but in terms of geometric optics involving arbitrary rays, not particles. Eye and brain make sense of the world not by particles but by the motion of extended objects with manifold shapes.

Rays of what?

Spell it out. Are you saying that the eye's retina uses electromagnetic frequencies of the electromagnetic field to determine location, etc.? Because it makes NO sense.
Photons are ALWAYS absorbed as single particles. No exceptions.

Rods in photoreceptor cells can respond to single photons, while the retina and the visual cortext respond to signals made up of about 5-6 single photons.

https://en.wikipedia.org/wiki/Photoreceptor_cell
http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

Eye and brain make sense of the world not by particles but by the motion of extended objects with manifold shapes.

It makes no sense if you leave out the critical term 'via photons'.

 

[A. Neumaier]

Rays of what?

Rays of graphite on a piece of paper when discussing how lenses produce images. They are imagined lines (in geometric optics, a much used approximation of Maxwell's equations) along which light travels - everywhere simultaneously. The functioning of the eye was understood long before the discovery of quantum mechanics!

Rods in photoreceptor cells can respond to single photons, while the retina and the visual cortex respond to signals made up of about 5-6 single photons.

A photocell responds to classical light by emitting occasionally an electron, as the derivation of the photoelectric effect (e.g., in the quantum optics book by Mandel and Wolf) shows. Thus these discrete events (which the retina can individually resolve under suitable circumstances) are independent of the notion of photons. It is usually talked about in terms of photons. However, these photons are not particles but detection events!

 

[EPR]

Rays of graphite on a piece of paper when discussing how lenses produce images.

I never talked of lenses but of the structure of the eye. Detection events detect photons striking the retina. This is a very basic fact.

I would say a third of the people here come to argue just for the sake of the argument. If i said something obvious like the Earth is round, someone would argue that if you treated the Earth as a 4-dimensional worldline or a quantum field...
Photons are ALWAYS detected as single particles. Period. This ends the 'argument' how the retina transforms detection events(of single "particles' aka photons).

I am not aware what rays of graphite means. Can you elaborate?

 

[vanhees71]

The eye is commonly analyzed not in terms of particles but in terms of geometric optics involving arbitrary rays, not particles. Eye and brain make sense of the world not by particles but by the motion of extended objects with manifold shapes.

However, if there's anything "particle like" in electromagnetism it's the Eikonal equation, i.e., "ray optics".

I think the "point particle" is indeed a stranger in relativistic physics even in the classical (i.e., non-quantum) realm. According to relativity the world is really best described by fields and local field equations.

 

[vanhees71]

I never talked of lenses but of the structure of the eye. Detection events detect photons striking the retina. This is a very basic fact.

I would say a third of the people here come to argue just for the sake of the argument. If i said something obvious like the Earth is round, someone would argue that if you treated the Earth as a 4-dimensional worldline or a quantum field...
Photons are ALWAYS detected as single particles. Period. This ends the 'argument' how the retina transforms detection events(of single "particles' aka photons).

I am not aware what rays of graphite means. Can you elaborate?

Single photons are detected as single photons, not particles. Photons have nothing particle like. It's described by QED and is defined as an asymptotic free Fock state.

 

[A. Neumaier]

However, if there's anything "particle like" in electromagnetism it's the Eikonal equation, i.e., "ray optics".

But the particle analogy is somewhat poor since in ray optics, the rays are everywhere. Hence they form a continuum, not a set of paths traced out by countably many individual light particles.

I am not aware what rays of graphite means. Can you elaborate?

They are drawn with an ordinary pencil, in place of the infinitely many rays figuring in the theory of ray optics. The latter are composed of nothing since they are objects of pure thought and have no material substrate.

Thus to answer your question ''Rays of what?'' I needed to materialize them. Or should I have answered ''rays of light''?

 

[vanhees71]

Hm, the solutions of the Hamilton-Jacobi differential equation are also in some sense "everywhere" but still describing trajectories of particles. O course, I don't say that light has any particle-like features. To the contrary, I think from a puristic point of view relativistic physics makes only sense as field theory, both in the classical and the quantum realm.

I only wonder, why it takes obviously more than 100 years to get rid of an old-fashioned obsolete view on quantum theory, known as "old quantum mechanics", which was always taken as an unsatisfactory state of affairs concerning the necessity of "quantum laws" for the description of matter and radiation. The solution is modern QT, and this makes "wave-particle dualism" completely obsolete. It's also questionable, whether Bohr's somewhat unsharp idea of "complementarity" helps a lot in understanding quantum mechanics. All there is is a probabilitistic description of what's observable in nature, and there's no need for vague notions like it.

 

[EPR]

Single photons are detected as single photons, not particles. Photons have nothing particle like. It's described by QED and is defined as an asymptotic free Fock state.

There are massless particles.

In particle physics, a massless particle is an elementary particle whose invariant mass is zero. The two known massless particles are both gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force).https://en.m.wikipedia.org/wiki/Massless_particleMy point was that this observed 'particle' like nature of light(electromagnetic radiation) is what makes perception possible(aka classicality).

 

[EPR]

Hm, the solutions of the Hamilton-Jacobi differential equation are also in some sense "everywhere" but still describing trajectories of particles. O course, I don't say that light has any particle-like features. To the contrary, I think from a puristic point of view relativistic physics makes only sense as field theory, both in the classical and the quantum realm.

I only wonder, why it takes obviously more than 100 years to get rid of an old-fashioned obsolete view on quantum theory, known as "old quantum mechanics", which was always taken as an unsatisfactory state of affairs concerning the necessity of "quantum laws" for the description of matter and radiation. The solution is modern QT, and this makes "wave-particle dualism" completely obsolete. It's also questionable, whether Bohr's somewhat unsharp idea of "complementarity" helps a lot in understanding quantum mechanics. All there is is a probabilitistic description of what's observable in nature, and there's no need for vague notions like it.

Obsolete picture of the world but still, that's how the eye works on an intuitive macro level.

I guess it's easier to visualise what is really happening via the assumption that there are 'particles'. Though this picture is certainly wrong as it's both old and inaccurate, lots of things(all?) appear to work as if there are particles.

 

[vanhees71]

A photon, described by a massless spin-1 quantum field, has not even a position operator in the usual sense. It has quite little in common with macroscopic "particle-like objects". The most comprehensive theory of matter today is relativistic QFT, i.e., (almost) all things appear to work "as if" there are quantum fields.

 

[EPR]

Relativistic quantum fields and superpositions, brain(eyes + visual cortex), classical perception('particles'). Somewhere along the chain lies the measurement problem and the issue of Bohr's complimentarity.
The whole world in 1 sentence as far as is known today in a simplified way. A biological problem, it seems(pending further reasearch).

 

[bhobba]

Einstein's real quibble was not so much about probabilities and indeterminism but about inseparability. Einstein also didn't like the EPR paper, because he thought it was not so clearly written as he would wished it, and he wrote a much more lucid paper in German about it:

A. Einstein, Dialectica 2, 320 (1948)
https://doi.org/10.1111/j.1746-8361.1948.tb00704.x

Yes, Einstein went through a number of phases in his views. I think his rock bottom issue was what the dickens is going on with entanglement. Even now people get confused about it, not understanding once two particles are entangled you can't speak of the particles having independent existence at all. This is semantically difficult to even talk about and gets some people into a lot of semantic problems. This idea that particles can simply 'not exist' anymore would be an anathema to someone like Einstein. There must be more to it in his view of the world. Not an unreasonable idea at all. Bohr's answer, and even the answer I would give, is not at all satisfying even though I think its true. Its the very essence of quantum weirdness.:
https://arxiv.org/abs/0911.0695
You simply can't have other desirable properties without it.

Thanks
Bill

 

[sysprog]

Could you give the full bibliographic data? I don't want to register for academia.edu again. When I once did first of all it spammed me like crazy with even crazier philosophical papers, then obviously scanned my coauthors, I've written papers with, and spammed them too. So I'll never ever register with them again!

That entity did stuff like that to me too.

 

[vanhees71]

Yes, Einstein went through a number of phases in his views. I think his rock bottom issue was what the dickens is going on with entanglement. Even now people get confused about it, not understanding once two particles are entangled you can't speak of the particles having independent existence at all. This is semantically difficult to even talk about and gets some people into a lot of semantic problems. This idea that particles can simply 'not exist' anymore would be an anathema to someone like Einstein. There must be more to it in his view of the world. Not an unreasonable idea at all. Bohr's answer, and even the answer I would give, is not at all satisfying even though I think its true. Its the very essence of quantum weirdness.:
https://arxiv.org/abs/0911.0695
You simply can't have other desirable properties without it.

Thanks
Bill

I only think that it's not quantum theory is weird but our belief that the world should be describable by our classical intuitions in all microscopic details. Physics has told us that this is nothing else than human hubris. Nature rather behaves as described by QT than by classical physics, which is an approximation applicable to the coarse grained description of macroscopic many-body systems.

 

[EPR]

Einstein was a naive realist and was wrong when he said "Physics is nothing but a refinement of common-sense thinking"

Yet, some of his conteporaries pushing for a deeper layer of reality can't be dismissed easily.

Why is this experienced "classical" reality so imposing, so incredibly detailed and consistent, so comprehensible? Coincidence? Hardly.

What lies beneath it all?
A deeper, objectively real reality? What is the "classical" reality a reflection of? How can it be so that we get these extraordinarily convincing images and impressions of classical reality, if there isn't such anywhere?
Could this seemingly classical reality be a reflection of an objectively real reality we currently have no access to... with realism in all scales?(note to moderation - this is not a statement but a question). Your input is welcome. Espagnat, D.Deutsch, Bohm and others tried to follow a similar deeper metaphysical route.
In Plato's cave allegory(Plato being a real giant of human thinking) the prisoners observed shadows of real men although not having access to the actual mechanics of shadows formation, they were led to consider they were objectively real.

 

[vanhees71]

I think it's simply the fact that in everyday life we deal with macroscopic systems, and our senses have evolved to help us survive given this macroscopic environment. It's simply unnecessary and "uneconomic" to resolve all microscopic details with our senses. That's why we don't perceive all these details directly with our senses, and quantum statistics tells us that macroscopic bodies almost always behave according to classical physics, as far as the effective macroscopic degrees of freedom we preceive with our senses trained to do so by evolution. That's why physics as an empirical science developed the way it did, i.e., starting with what we call classical physics, which deals with everyday macroscopic situations very well.

For the same reason humans first discovered Euclidean geometry, because in his little radius of direct exprience space is well described as a Euclidean affine space (because the gravitational field around us is pretty weak). Also non-relativistic physics is almost always sufficient to describe everyday situations. That's why we discovered Newtonian mechanics first and only later learned through the investigation of light and electromagnetism that this is only an approximation to the better relativistic description.