Realism and quantum mechanics

Andrew M

Realism is a philosophical premise. Perhaps, like breakfast cereals, we can choose a different one. But, given that premise, we can inquire about what quantum mechanics says.

Here is the (bare bones) time evolution of the quantum states in the double-slit experiment.

(2) is a quantum state that is a linear superposition of two component quantum states (2a) and (2b). On multiple runs of this experiment, the particles arriving at the back screen will build up an interference pattern.

Superpositions have a mathematical basis which is part of the quantum formalism and common to all interpretations. The philosophical question is about what a superposition really is.

The Everettian (relative state or Many Worlds) interpretation says that each quantum state in the time evolution physically obtains, including those in superposition. So, in this case, there is a particle travelling through slit A and another particle travelling through slit B at the same time - each one in a mutually isolated spacetime region. Where there are two physically available paths, a particle takes each path. Where two paths can physically merge, they do merge and interfere.

It's a non-intuitive conclusion - a particle is classically considered to have a single trajectory through spacetime, not have two futures and two histories. But, intuitions aside, is there any empirical or logical contradiction here?

Wayfarer

↪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):

Rich

↪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.

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.

Rich

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. — Terrapin Station

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

Rich

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

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. — Wayfarer

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.

Andrew M

The Bohm model simplifies everything. — Rich

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.

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. — Terrapin Station

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.

Wayfarer

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. — Andrew M

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)

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

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.

Metaphysician Undercover

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

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

Andrew M

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'. — Wayfarer

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).

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

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.

Wayfarer

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

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:

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. — Werner Heisenberg

The Debate between Plato and Democritus. Emphasis added.
.

Rich

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. — Andrew M

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).

Rich

The Debate between Plato and Democritus. Emphasis added. — Wayfarer

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.

Metaphysician Undercover

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

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.

Rich

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

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.

Andrew M

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). — Rich

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.

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

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.

Rich

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. — Andrew M

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.

Andrew M

↪Wayfarer

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

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.)

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. — Werner Heisenberg

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.

Andrew M

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. — Rich

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

Rich

— Rich

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

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.

Andrew M

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. — Rich

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.

Are there equations for inter-universe frame of references? — Rich

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

Rich

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). — Andrew M

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

Rich

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

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?

Metaphysician Undercover

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

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?

Andrew M

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

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.

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? — Rich

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.

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? — Metaphysician Undercover

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).

Rich

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". — Andrew M

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

Wayfarer

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. — Andrew M

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.

Andrew M

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

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:

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

Andrew M

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'? — Wayfarer

The initial superposition that expresses the entangled particle pair is:

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

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.

Rich

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

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.

Realism and quantum mechanics

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