Realism and quantum mechanics

Reply to Andrew M I would say that a non-realist account would revolve around the assertion that there are actually no particles as such, and that what manifests as 'particles' are not actually particles. Realism wants to believe that there are particles which exist whether or not the measurement is taken; this is what is thrown into doubt by the double-slit experiment, which is the godfather of all quantum weirdness.

(For the benefit of the audience, here's animated depiction of the experiment):

Reply to Andrew M The Bohm model simplifies everything.

There is a real particle (actually a wave perturbation) that goes through one of the slits. It is being guided by a quantum potential field that has immediate, non-local action (confirmed by the Bell/Aspect and subsequent experiments).

The quantum potential field continues to guide the particle until it hits the screen. Thus, any changes to the slits (delayed choice) will immediately effect the particle even if it has passed the slit.

The Bohm model pretty much describes the universe of quanta. It is real, eliminates all quantum paradoxes, and easy to grasp. It was initially rejected (the deBroglie pilot wave model) because Van Neumann supposedly stated that it was mathematically impossible. Bohm showed her was wrong but to b do this, he had to introduce non-locality in the form of the quantum potential.

Bohm specifically states in his writings that model is causal not deterministic, thereby allowing for an agent of choice. The probabilistic nature of the model is embedded in the quantum potential field which is essentially a process.

We can also have an option that's realist but that admits ignorance: Namely, particles are something real, but we don't really know their nature very well. The model we have of particles being something like a "chunk of stuff" is wrong--or at least what we're taking to be single particles isn't actually a single particle.

Quoting Terrapin Station

We can also have an option that's realist but that admits ignorance: Namely, particles are something real, but we don't really know their nature very well. The model we have of particles being something like a "chunk of stuff" is wrong--or at least what we're taking to be single particles isn't actually a single particle.

In his later writings, de Broglie dropped the notion of a particle and instead referred to it as a wave perturbation.

Here Basil Hiley, a close associate of Bohm, discusses the process nature of the Bohm model. He prefers to avoid wave/particle concepts. Pretty interesting if you are familiar with basic quantum theory. It's nice to hear directly from the source.

https://youtu.be/9gFCj5PPEyw

Quoting Wayfarer

I would say that a non-realist account would revolve around the assertion that there are actually no particles as such, and that what manifests as 'particles' are not actually particles. Realism wants to believe that there are particles which exist whether or not the measurement is taken; this is what is thrown into doubt by the double-slit experiment, which is the godfather of all quantum weirdness.

A realist account need not require that particles are fundamental entities. A particle can be an emergent feature of an underlying field. So, in the double-slit experiment, an initial ripple in an electron field can then propagate through both slits and interfere on the other side. We identify those ripples as electrons.

But, whether fundamental or emergent, the particles are real and behave in a consistent way independent of observation.

BTW, the video describes the basic experiment well. However at the end it claims that the act of observation or measurement changes the behavior of the particle. That is the interpretational claim at issue.

The realist claim is that a particle's behavior is the same independent of observation and measurement. The observed difference is instead due to the manner in which the observer becomes entangled with the experiment and how this changes their vantage point on what they observe.

Quoting Rich

The Bohm model simplifies everything.

I would say the same of the Everett model except more so. As I see it, the Bohm model modifies the quantum picture and tries to provide a picture that conforms more with a classical view (using non-local hidden variables). Whereas the Everettian view interprets quantum mechanics on its own terms without needing to modify the equations, introduce non-locality or posit a quantum potential field.

Quoting Terrapin Station

We can also have an option that's realist but that admits ignorance: Namely, particles are something real, but we don't really know their nature very well. The model we have of particles being something like a "chunk of stuff" is wrong--or at least what we're taking to be single particles isn't actually a single particle.

Yes we can do that and that is the best approach in the absence of a viable alternative. My claim here is that the Everett model does provide the features that we normally associate with a good scientific theory. However it just happens not to be reducible to a classical picture of the world.

Quoting Andrew M

A realist account need not require that particles are fundamental entities.
....
But, whether fundamental or emergent, the particles are real and behave in a consistent way independent of observation.

I think those two points are in conflict. If the particle is said to be 'real and to behave in a consistent way independent of observation', then it's said to be 'a fundamental entity'.

Belief in an 'observer-independent reality' is what Kant means by 'transcendental realism': the transcendental realist says that there is some real entity, over and above what can be observed and measured, independent of the act of measurement.

The transcendental realist therefore represents outer appearances (if their reality is conceded) as things in themselves, which would exist independently of us and our sensibility and thus would also be outside us according to pure concepts of the understanding. (CPR, A369)

Quoting Terrapin Station

particles are something real, but we don't really know their nature very well.

According to Heisenberg, they are not 'particles'. 'Particles' are metaphors, taken from everyday experience. What they are is 'what is described by the equation', which is all we know.

When I was a young physics student I once asked a professor: ‘What’s an electron?’ His answer stunned me. ‘An electron,’ he said, ‘is that to which we attribute the properties of the electron.’

Adam Frank.

Quoting Andrew M

A realist account need not require that particles are fundamental entities. A particle can be an emergent feature of an underlying field.

A field is a mathematical construct. So what type of realism are you talking about, Platonic realism?

Quoting Wayfarer

I think those two points are in conflict. If the particle is said to be 'real and to behave in a consistent way independent of observation', then it's said to be 'a fundamental entity'.

I mean it in the sense that tables, trees and tigers are emergent not fundamental entities. We don't require that they be identified in our fundamental physical equations. They are nonetheless real and exist independently of observation and measurement (per realism, anyway).

Quoting Metaphysician Undercover

A field is a mathematical construct. So what type of realism are you talking about, Platonic realism?

No. It's a way of describing physical phenomena, just as we might describe a car wheel as a circle. A quantized field can be visualized as a mattress with many springs. So an electron that is emitted need not be intrinsically the same electron that hits the back screen. Instead they would be distinct excitations of the electron field.

Quoting Andrew M

They are nonetheless real and exist independently of observation and measurement (per realism, anyway).

Which begs the question, i.e. assumes what it has set out to prove. Because it is known that sub-atomic particles do not exist in the same sense as do chairs, tables and tigers. In speaking of the development of atomic theory, he said:

Werner Heisenberg:The concept of the atom had proved exceptionally fruitful in the explanation of chemical bonding and the physical behavior of gases. It was found, however, that the particles called 'atoms' by the chemist were composed of still smaller units. But these smaller units, the electrons, followed by the atomic nuclei and finally the elementary particles, protons and neutrons, also still seemed to be atoms from the standpoint of the materialist philosophy. The fact that, at least indirectly, one can actually see a single elementary particle—in a cloud chamber, say, or a bubble chamber—supports the view that the smallest units of matter are real physical objects, existing in the same sense that stones or flowers do.

But the inherent difficulties of the materialist theory of the atom, which had become apparent even in the ancient discussions about smallest particles, have also appeared very clearly in the development of physics during the present century.

This difficulty relates to the question whether the smallest units are ordinary physical objects, whether they exist in the same way as stones or flowers [or tables, trees or tigers]. Here, the development of quantum theory some forty years ago has created a complete change in the situation. The mathematically formulated laws of quantum theory show clearly that our ordinary intuitive concepts cannot be unambiguously applied to the smallest particles. All the words or concepts we use to describe ordinary physical objects, such as position, velocity, color, size, and so on, become indefinite and problematic if we try to use then of elementary particles. I cannot enter here into the details of this problem, which has been discussed so frequently in recent years. But it is important to realize that, while the behavior of the smallest particles cannot be unambiguously described in ordinary language, the language of mathematics is still adequate for a clear-cut account of what is going on.

The Debate between Plato and Democritus. Emphasis added.
.

Quoting Andrew M

I would say the same of the Everett model except more so. As I see it, the Bohm model modifies the quantum picture and tries to provide a picture that conforms more with a classical view (using non-local hidden variables). Whereas the Everettian view interprets quantum mechanics on its own terms without needing to modify the equations, introduce non-locality or posit a quantum potential field.

My head spins when I consider the many world concepts that someone had to buy into with the Everett model (endless parallel universes) where everything that is not happening here is happening there and vice versa in some sort of manner which befuddles me. For me, it's just wild and I never seriously considered it, especially since the Bohm model is so tight and was the model that encouraged Bell to develop his theorem on non-locality that has been repeatedly verified in experiments at the macro and micro level. (Bell himself was an advocate for the Bohm model).

Quoting Wayfarer

The Debate between Plato and Democritus. Emphasis added.

Hiley put it quite eloquently when he says we have to think of the universe at both the micro and macro level as processes and not things. This viewpoint echos those of Bergson and Whitehead. The mind is constantly involved with processes and interacts as a process. It it's all entangled like a reconstructive wave interacting with holographic waves. There is no independent thing just continuous flow and interactions.

Quoting Andrew M

. It's a way of describing physical phenomena, just as we might describe a car wheel as a circle.

There is a big difference here. "Circle" is what we attribute to the wheel, it is a property of a wheel, the shape of the wheel. The wheel is circular. A "field" is not attributed to anything. It is not the property of anything, nor is it itself a thing. It is not a description of a physical phenomenon. A mathematical equation for example, is not a description of a physical phenomenon, though it may be applied toward understanding described physical phenomena.

One might argue that the field is a property of space-time, but to do this is to reify space-time, and I would argue that this is a mistake. In reality, space and time are distinct, and are not one united thing. Furthermore, space and time are not physical phenomena. So even if you attribute the field to space-time, this is not a description of a physical phenomenon, because space-time is not a physical phenomenon.

The "field" functions as a mathematical equation which is applied to the described phenomena.

Quoting Metaphysician Undercover

A "field" is not attributed to anything. It is not the property of anything, nor is it itself a thing.

In Bohm's conception of the universe as a holomovement process the Implicate/Explicate order, the field would be a manifestation of this movement. Things appear to be moving about towards each other or away, because of the holomovement. Similarly electrons may appear to be jumping from states to states as they move in and out of the Implicate/Explicate order as a wave might.

There is full continuity, with allowances for the appearance of discontinuity, in this model, while preserving the essential elements of process and motion in the universe.

Quoting Rich

For me, it's just wild and I never seriously considered it, especially since the Bohm model is so tight and was the model that encouraged Bell to develop his theorem on non-locality that has been repeatedly verified in experiments at the macro and micro level. (Bell himself was an advocate for the Bohm model).

What Bell proved was that hidden variables and locality were incompatible. The Bohm model accepts hidden variables and rejects locality. Whereas the Everett model accepts locality and rejects hidden variables. Which makes it a more natural fit with Einstein's special theory of relativity.

Locality is an uncomplicated principle compared to hidden variables. Per locality, an object can only be directly influenced by its immediate surroundings.

Quoting Metaphysician Undercover

The "field" functions as a mathematical equation which is applied to the described phenomena.

Yes and, similarly, the equation of the circle is x²+y²=r². When we say that a car wheel is circular, we are describing the car wheel in mathematical terms.

Quoting Andrew M

What Bell proved was that hidden variables and locality were incompatible. The Bohm model accepts hidden variables and rejects locality. Whereas the Everett model accepts locality and rejects hidden variables. Which makes it a more natural fit with Einstein's special theory of relativity.

Yes, but not-locality has been experimentally observed which is why Bell preferred Bohm's model. Plus it gets around the awkwardness of a never-ending multitudes of universes interacting with each other in a presumably super-non-local manner unless of course it can be shown that different universes preserve locality.

Reply to Wayfarer

Werner Heisenberg:The mathematically formulated laws of quantum theory show clearly that our ordinary intuitive concepts cannot be unambiguously applied to the smallest particles.

Heisenberg was correct. That is, the idea of a classical state does not apply to the smallest particles.

Schrodinger took this a step further with his thought experiment. Not only does the classical state not apply to the smallest particles, it does not apply to stones or flowers either. Or cats. The notion of a classical state is untenable for any physical system of any size. (This is the implication of the Heisenberg Uncertainty Principle.)

Werner Heisenberg:But it is important to realize that, while the behavior of the smallest particles cannot be unambiguously described in ordinary language, the language of mathematics is still adequate for a clear-cut account of what is going on.

Yes, the mathematics gives a clear-cut account of what is going on. So we need to add a word to our ordinary language vocabulary, which is "superposition".

We never directly observe a system in superposition. We only ever measure systems in one specific state or another (a particle at slit A or a particle at slit B). But, in quantum mechanics, it is valid for a system to be in a superposition state. Thus, on the premise of realism, quantum mechanics is telling us something about the nature of the world that we are unable to directly observe.

Quoting Rich

Yes, but not-locality has been experimentally observed which is why Bell preferred Bohm's model. Plus it gets around the awkwardness of a never-ending multitudes of universes interacting with each other in a presumably super-non-local manner.

Non-locality hasn't been experimentally observed. That is an interpretational claim. The Everett model explains EPR-style experiments in a local manner.

Quoting Andrew M

— Rich

Non-locality hasn't been experimentally observed. That is an interpretational claim. The Everett model explains EPR-style experiments in a local manner.

Let's put it this way, the experiments that have been designed to test noon-locality gave confirmed non-locality, in the same manner any scientific experiment is interpreted. I'm totally on board with subjectivity all over science.

In any case, in would be interesting to ponder how sprouting universes for into Relativity. Are there equations for inter-universe frame of references? I don't think the Lorentz transformations can handle measurements between multiple universes. I guess that is something that Everett proponents will have to work on.

Quoting Rich

Let's put it this way, the experiments that have been designed to test noon-locality gave confirmed non-locality, in the same manner any scientific experiment is interpreted.

What the experiments have tested for and confirmed is that the measurements of two entangled particles separated by large distances conform to the predictions of quantum mechanics. That is, if Alice measures spin-up then Bob measures spin-down regardless of the distance separating them. The Copenhagen, Bohm and Everett interpretations all agree about the results of the experiment. What they don't agree on is whether they entail non-locality (action at a distance).

On the Everett model, the results do not entail non-locality. They instead entail a linear superposition of states as described by the wave function. That is, one state where Alice measures spin-up and Bob measures spin-down and a second state where Alice measures spin-down and Bob measures spin-up.

Quoting Rich

Are there equations for inter-universe frame of references?

It's really one universe with quantum states in superposition as described by the wave function. The relativistic wave equation is the Dirac equation.

Quoting Andrew M

What the experiments have tested for and confirmed is that the measurements of two entangled particles separated by large distances conform to the predictions of quantum mechanics. That is, if Alice measures spin-up then Bob measures spin-down regardless of the distance separating them. The Copenhagen, Bohm and Everett interpretations all agree about the results of the experiment. What they don't agree on is whether they entail non-locality (action at a distance).

http://www.sciencemag.org/news/2017/06/china-s-quantum-satellite-achieves-spooky-action-record-distance

https://scholar.google.com/scholar?q=quantum+nonlocality+experiments&hl=en&as_sdt=0&as_vis=1&oi=scholart&sa=X&ved=0ahUKEwj97M_ro_zUAhVJNT4KHbVsAS0QgQMIGzAA

Quoting Andrew M

It's really one universe with quantum states in superposition as described by the wave function. The relativistic wave equation is the Dirac equation.

Ok. It's 'many-worlds" or never-ending branches (multi-verses?) that are interacting with each other (manifestation in one world creates another) non-locally totally entangled (one depended he upon the outcome of the other?). Now, how does Einstein's theory apply to all of these branches whatever they may be?

Quoting Andrew M

When we say that a car wheel is circular, we are describing the car wheel in mathematical terms.

The wheel is what is circular, it is described by "circular", so the wheel is what we claim to be real. Now what is it which is described by the "field"? What is the real thing which "field" is attributed to, as the property of?

Quoting Rich

http://www.sciencemag.org/news/2017/06/china-s-quantum-satellite-achieves-spooky-action-record-distance

Yes, that was the kind of experiment I was referring to. What the results demonstrate is a violation of Bell's inequalities, not "spooky action at a distance".

The Everett model is a local counterfactually-indefinite theory. Such a theory is not ruled out by Bell's Theorem and so is compatible with experiments that demonstrate a violation of Bell's inequalities.

Quoting Rich

Ok. It's 'many-worlds" or never-ending branches (multi-verses?) that are interacting with each other (manifestation in one world creates another) non-locally totally entangled (one depended he upon the outcome of the other?). Now, how does Einstein's theory apply to all of these branches whatever they may be?

The Everett model is local - things only influence their immediate surroundings. Entangled particle pairs do not act or communicate at a distance, their measurement correlation is instead an artifact of being in the same relative quantum state (or branch). The quantum states are all there evolving according to quantum and relativistic laws (e.g., the Dirac equation). However we observe just the relative quantum state (or branch) we are entangled with.

Quoting Metaphysician Undercover

The wheel is what is circular, it is described by "circular", so the wheel is what we claim to be real. Now what is it which is described by the "field"? What is the real thing which "field" is attributed to, as the property of?

In Quantum Field Theory, as far as I know, a field is itself regarded as a real physical thing (which can be visualized as a mattress with springs).

Quoting Andrew M

Yes, that was the kind of experiment I was referring to. What the results demonstrate is a violation of Bell's inequalities, not "spooky action at a distance".

Ok. A violation of Bell's Inequalities which is designed to test .....??

Quoting Andrew M

Entangled particle pairs do not act or communicate at a distance, their measurement correlation is instead an artifact of being in the same relative quantum state (or branch). The quantum states are all there evolving according to quantum and relativistic laws (e.g., the Dirac equation). However we observe just the relative quantum state (or branch) we are entangled with.

How does that obviate the apparent fact of 'action at a distance'? If the measurement of this particle here, fixes the spin of that particle over there, isn't that still 'action at a distance'?

I also question whether fields are physical. Obviously they have effects on physical things, but whether they themselves are physical is, I think, not known.

Quoting Rich

Ok. A violation of Bell's Inequalities which is designed to test .....??

That just is the test. What the violation of Bell's inequalities means is that, at most, only one of the following can be true:

• [1] Locality, or[2] Counterfactual definiteness

If 2 is false (as is asserted by the Everett model), then the violation of Bell's inequalities do not demonstrate non-locality.

Quoting Wayfarer

How does that obviate the apparent fact of 'action at a distance'? If the measurement of this particle here, fixes the spin of that particle over there, isn't that still 'action at a distance'?

The initial superposition that expresses the entangled particle pair is:

• [1a] particle A is spin-up and particle B is spin-down +[1b] particle A is spin-down and particle B is spin-up

When Alice measures the spin of particle A she becomes entangled with the superposition, which evolves to:

• [2a] Alice measured particle A as spin-up and particle A is spin-up and particle B is spin-down + [2b] Alice measured particle A as spin-down and particle A is spin-down and particle B is spin-up

Within each quantum state, Alice knows what she measured and so can deduce what the other particle's spin in that same quantum state must be. But no action at particle B occurred, so it doesn't matter how far away particle B is before or after Alice's measurement.

Quoting Andrew M

If 2 is false (as is asserted by the Everett model), then the violation of Bell's inequalities do not demonstrate non-locality.

Of course, one can buy into an infinite number of universes to avoid non-locality. But then, what is being observed in all of these experiments? Bohm would say the quantum potential acting at a distance.

Quoting Andrew M

2a] Alice measured particle A as spin-up and particle A is spin-up and particle B is spin-down +
[2b] Alice measured particle A as spin-down and particle A is spin-down and particle B is spin-up

So - two 'Alices'?

Quoting Wayfarer

So - two 'Alices'?

Yes, that's the result of the unitary evolution of the quantum state per the Schrodinger equation.

Quoting Andrew M

Yes.

Thank you. Perhaps one of them has indeed gone through the looking glass.

Quoting Andrew M

In Quantum Field Theory, as far as I know, a field is itself regarded as a real physical thing (which can be visualized as a mattress with springs).

Any physicist who regards a field as a real thing, has got a very strange ontology. It cannot be visualized as a mattress with springs, because numerous fields can occupy the same place, and mattresses can't do that.

As I said before, to think of it as something real requires that it is the property of a thing, and the only thing which it can be a property of is space-time. But this is to reify space and time. How is space-time a substance?

Quoting Wayfarer

So - two 'Alices'?

Quoting Andrew M

Yes, that's the result of the unitary evolution of the quantum state according to the Schrodinger equation.

Quoting Wayfarer

Thank you. Perhaps one of them has indeed gone through the looking glass.

You didn't understand MW enough to know that Alice is in both of them? Argument from incredulity? One of the main points of MW is to do away with action at a distance.

Quoting noAxioms

You didn't understand MW enough to know that Alice is in both of them? Argument from incredulity? One of the main points of MW is to do away with action at a distance.

Yes, this it's how far scientists are willing to go to preserve locality. They'll even present a never ending, every growing universe of multi-verses. How many of me does there exist? As many choices I've made in my life so far, moment by moment and combined with the choices being made by others?? It boggles the mind to imagine such a multi-universe. It totally destroys the notion of realism whatever ones notion of realism must be. Alice has indeed come to life.

In the meantime, non-locality as predicted by the Bohm model has been experimently verified as interpreted by those scientists who are working on the problem.

Quoting Rich

It totally destroys the notion of realism whatever ones notion of realism must be.

It does at least make a hash of ones dualistic notions of personal identity.
I mist-stated the position. There is an Alice in both of them. I initially said that Alice is in both of them. The reference to an objective identity doesn't work.
Quoting Rich

How many of me does there exist?

One still.

I'm not asserting the position here, just being amused at the boggling. It solves the problem of effects happening well before their causes, something that is not mind boggling, but still a violation of a lot of principles of physics. So I ask you to say what the difference in experience would be if MW turned out to be how things are objectively? Surely there must be a difference if it is so implausible.

Quoting noAxioms

I'm not asserting the position here, just being amused at the boggling. It solves the problem of effects happening well before their causes, something that is not mind boggling, but still a violation of a lot of principles of physics. So I ask you to say what the difference in experience would be if MW turned out to be how things are objectively? Surely there must be a difference if it is so implausible.

The difference lies in our understanding of the nature of the universe and what that might entail spiritually, metaphysically and physical practicality. No one could have predicted the effects that the introduction of quantum physics would have on our lives, in all of these dimensions, when we moved from the mechanical universe of Newton to the quantum universe. Entanglement at a distance, if it is a real phenomenon, would probably have a profound impact on all of our lives.

Quoting noAxioms

You didn't understand MW enough to know that Alice is in both of them?

Explaining irony is pointless, isn't it. I was trying to drive home the point of the inherent implausibility of the so-called 'Everett Interpretation', but I guess if you're willing to entertain the notion of [ x ] Alice's, then it's never going to work.

Quoting Rich

They'll even present a never ending, every growing universe of multi-verses.

The extent of my knowledge of such matters is bounded by New Scientist and other such sources, but one thing I do know is that 'multiverses' and 'parallel universes' are two different beasts. The 'multiverse conjecture' is associated with string theory, and consists of the idea that 'our universe' is but one instance of an unthinkably large number of 'universes' which are theoretically predictable on the basis of string theory (the number being larger than all the atoms in 'our universe'). But the rub is, 'other universes' are forever outside our 'event horizon', i.e. even if they exist, there's no way to know. (This is the subject of a large and ongoing controversy between George Ellis (against) and Sean Carroll (for)).

Parallel worlds, on the other hand, does fall out of the Everett conjecture. That is the idea that every possible outcome of a measurement is real in some parallel universe. It implies that everything that could happen, does happen, in one or another of these universes.

Personally, I am glad not to be a scientist in these times - if I was I would steer well clear of these subjects.

Reply to Wayfarer I've read journals where they have referred to Everett's Many Worlds as Multi-verses, but it is off no matter, because it is impossible to describe or imagine such a reality whether it be I've or a million words.

Reply to Rich Even though it's obviously an abstruse topic, 'many worlds' and 'multiverses' are actually different ideas, that's what I'm saying.

Incidentally, in my universe, Everett holds no sway. X-)

Reply to Wayfarer OK. Thanks.

Quoting Rich

Of course, one can buy into an infinite number of universes to avoid non-locality. But then, what is being observed in all of these experiments? Bohm would say the quantum potential acting at a distance.

What is being observed is the correlation due to the initial preparation of the entangled pair of particles. If they have been prepared in a state where they have opposite spins, then that's the way they stay regardless of the distance they travel apart.

It's a philosophical choice. Local action or spooky action.

Quoting Wayfarer

Thank you. Perhaps one of them has indeed gone through the looking glass.

"Who in the world am I? Ah, that's the great puzzle!" - Alice

Quoting Metaphysician Undercover

Any physicist who regards a field as a real thing, has got a very strange ontology. It cannot be visualized as a mattress with springs, because numerous fields can occupy the same place, and mattresses can't do that.

Fair enough. See https://xkcd.com/895/

Quoting Andrew M

It's a philosophical choice. Local action or spooky action.

I would put it a little differently. The choice is:

A) Experimentally verified entanglement at a distance (non-locality)

or

B) An a never ending,, growing number of unverifiable, unknowable universes.

I think this is a much better way to describe what is actually being presented.

Quoting Rich

B) An a never ending,, growing number of unverifiable, unknowable universes.

If true, then the same would apply to the Bohmian pilot wave.

To summarize:

• [1] The Everett model - realism plus quantum mechanics (the relative states of the wave function equally exist, also called "worlds" or "branches").[2] The Bohm model - realism plus a classical modification of quantum mechanics (non-local, incompatible with special relativity and quantum field theory).[3] The Copenhagen model - quantum mechanics with a wave function collapse postulate (inherently random, wave-particle duality, paradoxes including EPR, Schrodinger's cat).

Quoting Andrew M

If true, then the same would apply to the Bohmian pilot wave.

Absolutely. All in one universe so we can actually explore the phenomenon.

https://www.newscientist.com/article/2078251-quantum-weirdness-may-hide-an-orderly-reality-after-all/

Quoting Andrew M

[3] The Copenhagen model - quantum mechanics with a wave function collapse postulate (inherently random, wave-particle duality, paradoxes including EPR, Schrodinger's cat).

There is no 'Copenhagen model'. It is not a model or even a scientific hypothesis. The 'Copenhagen interpretation' refers merely to the kinds of things that Bohr, Heisenberg, and Pauli said about the philosophical implications of quantum physics during the 1920's and 30's. The term 'copenhagen interpretation' wasn't even coined until the 1950's.

I think it can be stated that the phenomenon of quantum entanglement undermines scientific realism, but again that's a philosophical observation. The science of the issue is simply that this is something that happens as indicated by a set of measurements.

Werner Heisenberg: On the whole the book contains nothing that is not to be found in previous publications, particularly in the investigations of Bohr. The purpose of the book seems to me to be fulfilled if it contributes somewhat to the diffusion of that 'Kopenhagener Geist der Quantentheorie' [i.e., Copenhagen spirit of quantum theory] if I may so express myself, which has directed the entire development of modern atomic physics.

Foreword to The Physical Principles of the Quantum Theory, published in 1930.

The wikipedia entry goes on:

The term 'Copenhagen interpretation' suggests something more than just a spirit, such as some definite set of rules for interpreting the mathematical formalism of quantum mechanics, presumably dating back to the 1920s.However, no such text exists, apart from some informal popular lectures by Bohr and Heisenberg, which contradict each other on several important issues. It appears that the particular term, with its more definite sense, was coined by Heisenberg in the 1950s,[3] while criticizing alternate "interpretations" (e.g., David Bohm's[4]) that had been developed.[5]

Emphasis added.

Quoting Rich

Absolutely. All in one universe so we can actually explore the phenomenon.

Agreed. For example, see the interaction-free measurements such as the Elitzur–Vaidman bomb tester.

Quoting Wayfarer

I think it can be stated that the phenomenon of quantum entanglement undermines scientific realism, but again that's a philosophical observation.

I think that's true for classical scientific realism. But, of course, quantum entanglement has a straightforward realist explanation on the Everett model as outlined earlier in this thread.

Quoting noAxioms

I mist-stated the position. There is an Alice in both of them. I initially said that Alice is in both of them. The reference to an objective identity doesn't work.

It's interesting to see what the math says here. A quantum state can itself be in a linear superposition of quantum states which, for a two-state system, can be represented as psi = psi_1 + psi_2.

So, in the double-slit experiment, we can refer to the (absolute) quantum system that is the particle that is emitted and arrives at the back screen at a location predicted by the interference pattern. We can also refer to the two (relative) quantum systems that are the two distinct particles that go through each slit. So there are three quantum systems, each with its own quantum state, and each with distinct and real identities.

Quoting Andrew M

But, of course, quantum entanglement has a straightforward realist explanation on the Everett model as outlined earlier in this thread.

And, as I responded, the fact that it implies 'two Alices' means that, as far as I am concerned, it is not realistic.

Quoting Wayfarer

And, as I responded, the fact that it implies 'two Alices' means that, as far as I am concerned, it is not realistic.

Heliocentrism was once considered unrealistic. Same with 4D spacetime. Reality doesn't seem too concerned with what we consider realistic.

Quoting Andrew M

Heliocentrism was once considered unrealistic.

You don't seem to grasp how much more radical the notion that there are many worlds, or parallel universes, is, than that of heliocentrism, which had actually already been proposed by some ancient asteronomers. But I have learned that if you can accept the idea that there are parallel universes, which myself and many others thinks is an absurd idea, then clearly there is no line of argument that can be used against it. By definition it is not a scientific concept. as it can't be falsified. There is, therefore, no point in discussing it further.

I agree with the idea that the concept of "particle", or "wave" is not the reality. There is no real wave-particle duality it's just mathematically convenient to think one way or another when that is what works.

The interpretation that comes closest to reality, for me, is Carlo Rovelli's RQM (https://en.wikipedia.org/wiki/Relational_quantum_mechanics). For him, the underlying structure is composed of quantum states. Wave-function collapse occurs when quantum states interact and human consciousness is just one of those quantum states, so the observer problem disappears. It's the most "real" interpretation I've come across.

Quoting Wayfarer

But I have learned that if you can accept the idea that there are parallel universes, which myself and many others thinks is an absurd idea, then clearly there is no line of argument that can be used against it.

The multiplicity is inherent in the mathematics of quantum mechanics - its not something that can just be ignored as if it weren't there. That's why the Everett model just is unitary quantum mechanics and is falsifiable on that basis (for example, it would be falsified if a physical collapse mechanism were discovered). And it is why the Bohm model, while also realist, is necessarily a different physical model requiring non-local action (with the multiplicity relegated to the pilot wave).

Quoting daldai

The interpretation that comes closest to reality, for me, is Carlo Rovelli's RQM

Great to hear another perspective.

The way I would characterize RQM is as quantum mechanics with a relativist premise. That is, RQM defines reality in terms of the interaction between systems. So, in the double-slit experiment, it would be real for the particle/apparatus system that the particle has gone through a particular slit. But it would also be real for an independent observer that the particle is in a superposition of going through both slits.

According to RQM, this is not a contradiction since no comparison between observables can be made until the observer and particle/apparatus system have interacted. If they do interact, then the observer will find that they agree that the particle has gone through a particular slit and so reality is then defined for the entire observer/particle/apparatus system.

Is that how you understand RQM?

Quoting Andrew M

The multiplicity is inherent in the mathematics of quantum mechanics - its not something that can just be ignored as if it weren't there

But again, what is the motivation behind the mathematics? What is the problem that the maths is trying to solve? Why go to the bother?

Quoting Wayfarer

But again, what is the motivation behind the mathematics? What is the problem that the maths is trying to solve? Why go to the bother?

The problem is to predict the behavior of particle systems which classical mechanics cannot do. What distinguishes quantum mechanics from classical mechanics is that a quantum state can be a linear superposition (i.e., multiplicity) of component quantum states. Take the multiplicity out and you're back to classical mechanics.

Much is made of the experiment’s into Bell’s inequalities and their implication for Einstein.

https://plato.stanford.edu/entries/bell-theorem/

Einstein, to speak very roughly, appeared to plump for realism - that things really did have defined properties even when they weren’t being observed or interacting with other systems.

With loopholes of the Bell equalities being closed in recent years we often paint Einstein as being wrong-headed about realism.

I suspect, however, that there are too many assumptions being smuggled in about causation when it is not clear people have thought deeply about the subject.

The implication for so-called ‘entangled’ particles seems to be that the detection of a particle here can cause the spin of a particle on the other side of the galaxy. This is NOT to say the detection of a particle here can cause you to KNOW the spin of a particle on the other side of the galaxy, which would have been a more easily explainable fact.

If all we mean by ‘causation’ is simply that the words ‘detection of a particle’ don’t give you information about the spin of a particular particle, and that the words ‘the spin of a particle’ don’t give you information about the detection of a particle, then the result IS that the detection of a particle would cause the spin of a particle.

Now that’s a particular hobby horse of mine, you may disagree that’s what causation is. Yet it is not settled in science on what causation is, so who is to to say that’s NOT what causation is?

Do your preferred understandings of causation explain the Bell experiments? Or do they simply tell us the universe is non deterministic after all?

Quoting invizzy

understandings of causation

My own views on causation, in response to the issue you raise, is that it's a pattern:

1. sunlight + water + soil + seeds [math]\to[/math] germination of seeds

2. Ditto

3. Ditto

.
.
.

This pattern consists of antecedent conditions (sun + water + soil + seeds) and consequents, here one (germination of seeds). As (just) patterns in nature, causation is as simple as that.

Reply to Agent Smith

Interesting, so some sort of regularity theory (like Hume?) is what I think you're describing?

From memory there are reasons why a naive version of regularity theory is out of favour. One might be that things can sometimes cause other things when they are genuine 'one offs'. One might think the Big Bang caused the universe to expand or something like that. Can your idea cover that type of causation too?

Reply to invizzy

The Big Bang isn't, to my knowledge, a cause as much as it's the purported beginning (of our universe), oui mon ami?

As for singleton events, like for example this: 2, the question of pattern and therefore cause is moot.

Reply to Agent Smith

I think that’s one where intuitions differ. I might not say the Big Bang caused the universe, but it surely seems correct to say the Big Bang caused the universe to expand.

I’d love to hear what others think!

What if something novel happens though. What if man made climate change causes human civilization to end? Surely plausible that this could happen. But nothing like that has happened before. Wouldn’t we still use ‘cause’?

A good question!

Quoting invizzy

The implication for so-called ‘entangled’ particles seems to be that the detection of a particle here can cause the spin of a particle on the other side of the galaxy.

That the particles are entangled only means that there will be a correlation between the two spin measurements. As physicist Asher Peres noted, "Bell’s theorem does not imply the existence of any nonlocality in quantum theory itself. In particular relativistic quantum field theory is manifestly local." (longer quote here).

Quoting invizzy

This is NOT to say the detection of a particle here can cause you to KNOW the spin of a particle on the other side of the galaxy, which would have been a more easily explainable fact.

Assuming the other particle is measured in the same basis as your particle, you will know what the other particle's spin will be.

Reply to Andrew M

Sorry I should have been clearer! I was assuming realism. In that case isn’t it true that Bell’s theory implies non-locality? (And also the fact that the measurement of spin in one place cause the spin in another?)

Quoting invizzy

I was assuming realism. In that case isn’t it true that Bell’s theory implies non-locality? (And also the fact that the measurement of spin in one place cause the spin in another?)

Basically, yes. Superdeterminism is the one exception - it is local and real in Bell's sense, and instead rejects statistical independence.

Reply to Andrew M

I’m just looking up superdeterminism and trying to get my mind around it. Does the idea imply that causes somehow change things from one thing to another?

Quoting invizzy

I’m just looking up superdeterminism and trying to get my mind around it. Does the idea imply that causes somehow change things from one thing to another?

No, it just means that the choice of measurement settings and the measurement outcomes are predetermined.