The natural causal explanation is that we are seeing multiple phenomena that exhibit an interference pattern when they are in superposition. — Andrew M

But supposing that "there are multiple, branching universes" certainly doesn't strike me as a "natural causal explanation" that's more reasonable than "this stuff has some unusual characteristics that seems to behave like a wave at times and like a particle at times; we don't completely understand why yet, but these equations work for making predictions about it." Instead, it seems like incoherent fantasy.material.

This is where the realist metaphysics kicks in. What explains the interference pattern in a double-slit experiment? What is actually interfering? — AndrewM

What if probability waves are exactly what they seem - distributions of possibilities? So the patterns will appear along the lines of possibility, but when an object is measured, then they're no longer subject to probability, so the wave "collapses". But really nothing collapses because nothing was there in the first place other than a potentiality.

I think the issue with that, is that so-called 'realism' can't accomodate the notion of a 'real possibility'. It wants to assign existence in terms of a binary value - something either exists or it doesn't. But Heisenberg recognised that on the sub-atomic level, things 'kind of' exist. The parallel, in metaphysics, is the distinction between potential and actual existence - so the observation 'actualises' the potential existence of the object.

Which, I think, is far more like the general approach associated with the Copenhagen Interpretation. I think the issue is that it threatens the notion of 'a fundamental particle'. That is what the 'realist' approach is wanting to preserve - the fundamental separation of observer and observed. And they're prepared to accomodate the absurdities of the infinite branching universe in order to do it.

Take a look at this.

What if probability waves are exactly what they seem - distributions of possibilities? So the patterns will appear along the lines of possibility, but when an object is measured, then they're no longer subject to probability, so the wave "collapses". But really nothing collapses because nothing was there in the first place other than a potentiality.

I think the issue with that, is that so-called 'realism' can't accomodate the notion of a 'real possibility'. It wants to assign existence in terms of a binary value - something either exists or it doesn't. But Heisenberg recognised that on the sub-atomic level, things 'kind of' exist. The parallel, in metaphysics, is the distinction between potential and actual existence - so the observation 'actualises' the potential existence of the object. — Wayfarer

All material existence can be reduced to potential. The concept of energy does this. If this is reality, that all material existence is simply potential, then the appearance of an actual object is a creation of the mind. But I'd say that's a faulty premise, that all material existence is simply potential. If we assume this premise as faulty, we have to question what the reality of "an object" is based in..

But supposing that "there are multiple, branching universes" certainly doesn't strike me as a "natural causal explanation" that's more reasonable than "this stuff has some unusual characteristics that seems to behave like a wave at times and like a particle at times; we don't completely understand why yet, but these equations work for making predictions about it." Instead, it seems like incoherent fantasy.material. — Terrapin Station

If a single photon is fired in the double-slit experiment, the probability that it arrives at any particular position on the back screen is a function of the sum of the paths it could take.

There are really only two options available. Either the paths are real or they are not.

But possible (or potential) paths cannot create real interference patterns. Which leaves us with the first option, whether we like it or not.

(One other option is that QM is false, but I don't think anyone is arguing for that.)

That is what the 'realist' approach is wanting to preserve - the fundamental separation of observer and observed. — Wayfarer

Yes, the main issue is that the realist wants to keep things separate from our talk about those things. So modal language has an epistemic function and does not refer to (kinds-of) things that can exhibit interference patterns.

If a single photon is fired in the double-slit experiment, the probability that it arrives at any particular position on the back screen is a function of the sum of the paths it could take.

There are really only two options available. Either the paths are real or they are not. — Andrew M

You forgot one important option. Is the photon real?

(One other option is that QM is false, but I don't think anyone is arguing for that.) — Andrew M

The option is not that QM is necessarily false, the option is that the interpretation of the photoelectric effect, which inclines people to describe light energy in terms of photons, is not a good interpretation.

Either the paths are real or they are not. — AndrewM

That is how Einstein wanted to see it, but I think that is precisely what is at issue. Or - 'it depends on what you mean by "real" - because we're not dealing with particles at all, and it's mistaken to think of them as being that.

These 'particles' have no definite location until being measured; they're not in one place and don't have an actual trajectory. That is the 'fuzzy' nature of quantum particles. So the interference patterns might really represent the probabilities and nothing more than that; they're not really trails left by a particle, because there really aren't any particles until they're measured.

[Bohr said] there was no such thing as a particle with a well-defined path. It was this lack of definite trajectory that was behind the appearance of an interference pattern, even though it was particles, one at a time, which had passed through the two-slit set-up, and not waves.

Heisenberg says that the tracks that appear in bubble chambers are not really paths. It seems like the vapour trail left by a jet, but it's not. 'Perhaps we merely saw a series of discrete and ill-defined spots through which the electron had passed. In fact, all we do see in a cloud chamber are individual water droplets which must be certainly much larger than the electron. There was no continuous, unbroken path.'

I think Einstein's view was actually presumptious - he believed that we could know the ultimate nature of things through science. Whereas I find Bohr's and Heisenberg's philosophy much more modest - they recognise the limitations of even the most exact descriptions. 'Heisenberg understood that Einstein...wanted to return to the reality concept of classical physics, or, to use the more general philosophical term, to the ontology of materialism: the believe in an objective real world whose smallest parts exist objectively in the same sense that stones or trees existk independently of whether we observe them or not.' This was, for Heisenberg, a throw-back to the simplistic materialist views that prevailed in the natural sciences of the nineteenth century.

Einstein and Bohr never came to terms over this matter, and they were, after all, two of the greatest scientists of the 20th Century, so it's obviously a really hard question. But insofar as I understand the so-called Copenhagen intepretation, which is probably not much, I'm in agreement with it.

All quotes from Kumar

.

There are really only two options available. Either the paths are real or they are not. — Andrew M

In terms of talking about ontological commitments yes. But you can simply see it as an instrumental way of talking about what's going on, and assume that we don't really understand at all just what photons are like ontologically yet. What's really going on, what photons are really like, might be something that we can't really fathom yet. "Paths" are just a way to relate it to what we can conceive of, what we have experience with, etc.

I also see the idea that we're firing a single photon as an instrumental description, by the way. I don't think that we really know that we're firing single photons, a fortiori because we don't even really know what photons are yet--a "single one" might not even make any sense depending on what they turn out to be. Insofar as physics goes, they're primarily instrumental mathematical models.

Anyway, to understand my stances when it comes to this sort of stuff, just keep in mind that I'm basically a logical positivist in this realm. In my view, what we can say with any ontological commitment is that we're reading meters and computer screens and adjusting instruments and manipulating mathematical constructions and so on. That doesn't mean that I'm a logical positivst wholesale, and I certainly do not agree with them on the idea of meaning, etc., but I focus on what we are actually experiencing contra the stories we come up with via theorizing, via analogies to everyday experience, etc.

Instrumental interpretations don't care about ontological commitments. It's a matter of simply approaching the explanation or account as something that works for what it is, where it doesn't matter if it's a fiction or not.

Ontological commmitment interpretations are the opposite, obviously. One takes the explanation or account to be literally picking out things in the world, just as they are. — Terrapin Station

I am not sure there is a real distinction here, but anyway, I still don't see how MWI can be taken instrumentally in this sense. "The explanation or account as something that works for what it is" - that is the bare-bones QM. It gives us enough to perform measurements, relate them to other measurements, and make predictions. Interpretations, MWI included, go beyond that and make metaphysical commitments - which is what you seem to be shunning.

I'm an instrumentalist on some things, and not on other things. I particularly tend to be an instrumentalist with respect to explanations/theories that are mathematical-only (or primarily), or that are more abstract in received view interpretations. — Terrapin Station

There are no mathematical-only explanations. Mathematics doesn't explain anything: in order for it to be an explanation, a physical theory, it needs to be related to the physical world. I don't understand the distinction that you are trying to make here.

The 'interference patterns' are really patterns left on screens. The question is, what is causing those? The answer is, sub-atomic particles. But if they're particles are fired one-at-a-time, then how can they form an 'interference pattern'? How is a single particle interfering with itself? That seems to be the issue.

But what if the wave function which describes the path really does represent the probable paths of any single particle? Then even if the particles were fired sequentially, they would always behave as though they were part of an ensemble. In other words, the addition of all of their paths would always end up forming an interference pattern. That is something like the 'pilot wave' theory, except the 'pilot wave' is really nothing other than the probability distribution.

Has anyone thought of that explanation? Does it make any sense?

//edit//So I suppose what is happening is, that when they're fired sequentially, they're still behaving as though they're an ensemble. It's as if Time has been taken out of the equation, so that the sequence of particles behaves as though they're being fired together instead of being chronologically separated.//end//

You forgot one important option. Is the photon real? — Metaphysician Undercover

Yes - see What’s a Photon, and How Do We Know they Exist?

But even if you disagree, firing single electrons will also produce an interference pattern. In fact the double-slit experiment has been performed with molecules comprising 810 atoms.

These 'particles' have no definite location until being measured; they're not in one place and don't have an actual trajectory. That is the 'fuzzy' nature of quantum particles. So the interference patterns might really represent the probabilities and nothing more than that; they're not really trails left by a particle, because there really aren't any particles until they're measured. — Wayfarer

The particles are real before they are measured. But they never have a precise position and momentum at the same time, either before or after measurement. A position measurement just picks out a more precise position at the expense of spreading out the momenta (or vice versa, for a momentum measurement).

This can be observed in the single-slit experiment where the slit is narrow enough that the position is precise when the particle goes through the slit, which means the momenta is spread out, thus resulting in a wide spread of photons (again in an interference pattern) on the back screen.

Note that the narrow slit constitutes a position measurement. We know that individually fired particles that reached the back screen all went through the slit, yet they still build up an interference pattern. So there is nothing special about measurement that changes the nature of the particle.

The particles are real before they are measured.

Not according to Neils Bohr; which is part of what is at issue, isn't it?

In terms of talking about ontological commitments yes. But you can simply see it as an instrumental way of talking about what's going on, and assume that we don't really understand at all just what photons are like ontologically yet. What's really going on, what photons are really like, might be something that we can't really fathom yet. "Paths" are just a way to relate it to what we can conceive of, what we have experience with, etc. — Terrapin Station

You don't need to know what photons are really like. QM applies to any quantum system whether it be photons, electrons, or more complex systems like 810-atom molecules and, conceivably, Schrodinger's Cat.

Scientific theories are meant to be ontological commitments, which means they can be tested (and potentially falsified).

Not according to Neils Bohr; which is part of what is at issue, isn't it? — Wayfarer

Yes, but I'm pointing out that there is no need to question the reality of particles prior to measurement.

According to whom?

Do you know about the Bohr-Einstein debates?

According to whomever looks at the issue. Are there logical or empirical inconsistencies in the realist view?

The Einstein-Bohr debates is what this book is about, from which I have been quoting.

Neils Bohr says there are no sub-atomic particles until an experiment is performed which elicits them as a response to that particular experimental set-up. This is an aspect of his famous 'wave-particle duality' argument. It is part of a set of theories, of which the uncertainty principle and 'entanglement' are other aspects.

The reason Einstein couldn't accept that quantum mechanics was complete, is because he believed that there simply must be objects that are not 'mind-dependent' in the way that Bohr was suggesting. They had many fierce debates, usually in the form of mind experiments which Einstein posed, which he hoped would show that QM must be incomplete in some sense. Every one of those challenges was met by Neils Bohr.

The EPR paper was one of these 'thought experiments' - it was that paper that led to the Bell's Inequality paper which was published in 1964 (many scientists will say that it is one of the greatest scientific papers in history). And it was that paper that formed the basis of the Alain Aspect experiments which in the early 1980's empirically demonstrated the entanglement of remote paired particles, thereby showing that the EPR paper was wrong. It was the final nail in the coffin of Einstein's realist philosophy, pending something world-shatteing coming along.

So the debate between Bohr and Einstein was very much between Einstein's classical scientific realism, and the interpretations of Bohr, Heisenberg, and others, which comprised the 'Copenhagen Interpretation'. But that is not anything like a philosophical school or principle, in fact Heisenberg, Bohr, Born, Pauli and Schrodinger, all had great differences of opinion between themselves. But Heisenberg and Bohr, in particular, were very critical of Einstein's realism (as per the quotes a few posts back). So the 'Copenhagen Interpretation' (a term not coined until the 1950's by the way) was just shorthand for 'the kinds of ideas Bohr and Heisenberg talked about.'

The upshot is, that so-called 'sub-atomic particles' aren't simply whizzing around, waiting to be measured. They're not 'out there somewhere'. Until they're measured, they can't be said to exist - as Heisenberg points out, words such as 'exist' have a certain set of meanings, which can't be unequivocally applied to photons and electrons. That is why QM is so perplexing - what were thought to be the 'ultimate building blocks' turn out to be more like mathematical ghosts.

Hence, this whole debate. I wouldn't think you would propose infinite branching universes unless you had a real need to do so.

Scientific theories are meant to be ontological commitments, which means they can be tested (and potentially falsified). — Andrew M

That I certainly don't agree with. I agree with the tested/potentially falsified part, but that doesn't mean that theories etc. are not read instrumentally, and I neither agree that (a) something makes it the case that theories etc. are meant to be read as ontological commitments nor that (b) most scientists read theories etc. as ontological commitments rather than instrumentally.

Scientific theories are meant to be ontological commitments, which means they can be tested (and potentially falsified). — Andrew M

To paraphrase... On making observations of fossils, paleontologists developed a theory of dinosaurs to explain fossils. They made an ontological commitment to dinosaurs - i.e. the theory *IS* that dinosaurs really existed, had real behaviour, and occasionally really died in such a way that their remains are preserved. The theory of fossils is actually a theory of dinosaurs, which no one will ever observe.

But there are alternative theories - how about that fossils only come into existence when consciously observed? Thus fossils aren't evidence of dinosaurs, but rather evidence of those particular acts of observation. Another theory is that dinosaurs are such strange animals that conventional logic does not apply to them. Or, how about the theory that it is meaningless to ask if dinosaurs were real or just a useful fiction?

None of those alternative theories are empirically distinguishable from the rational theory of fossils, yet we manage to reject them. Not so in QM unfortunately.

But even if you disagree, firing single electrons will also produce an interference pattern. In fact the double-slit experiment has been performed with molecules comprising 810 atoms. — Andrew M

But in my post, I questioned the existence of all objects, so referring to molecules doesn't change anything. It is likely that the appearance of an object is something which is created by the mind. If this is the case, then we have to ask, what is it about the world which our minds interpret as objects. In the most simple, fundamental form, an object is the appearance of a temporal continuity of stability, something which stays the same, or can be described by laws of inertia, for a period of time.

The mind sees thing in terms of objects, which is a temporal continuity of the same, such that a change to the same, must be accounted for, by causation. The mind "chooses" aspects of reality which demonstrate a continuity of sameness, and produces the appearance of objects. We can challenge this, is it real this temporal continuity of the same, and if so, what are those aspects of reality which the mind chooses to seize upon, and create the appearance of objects.

If we remove this assumed temporal continuity of the same, then we have no support for the appearance of objects. In order to validate the existence of objects now, we must determine what produces the appearance of a temporal continuity of the same, what produces the appearance of stability in time. It is this appearance of stability which produces our conclusions that something should be like this, or should be like that. But if this is simply taken for granted, then we have no understanding of the reasons why it should be like that. Therefore it is necessary to determine what causes temporal continuity, the sameness from one moment to the next, in order to validate the real existence of any object.

Again, what tends to make the difference there is to what extent we're talking about observables rather than abstractions including strictly talking about mathematics. That's not binary consideration. There are degrees or levels of remove from direct observables.

You describe yourself as a Positivist, i.e. you hold the view that all statements apart from those describing or predicting measurements are meaningless. Why do you care if photons exist, if the question is strictly meaningless? I think I covered that particular dinosaur theory.

Your theory of meaninglessness, along with the theories of the inapplicability of reason, and the theory of consciousness-induced-creation, are all generic ways of denying anything. You can even use them to deny that quantum theory is true.

You complain about abstractions and mathematics. Given a simple experiment such as the photo-electric effect, what mathematics and abstractions particularly trouble you?

On making observations of fossils, paleontologists developed a theory of dinosaurs to explain fossils. — Tom

The analogy is not apt. The fossil history of life, whilst having some puzzles, is a story of pretty straight-forward linear development. The exploration of the sub-atomic realm is not at all like that. A more likely analogy would be, finding a fossil which had absolutely no explanation - like finding a raptor fossil with a rabbit fossil inside it. Then you would have some explaining to do.

None of those alternative theories are empirically distinguishable from the rational theory of fossils, yet we manage to reject them. Not so in QM unfortunately. — tom

Excellent analogies!

Per your comment Wayfarer, the issue is that realism serves us well in the straightforward cases and the alternatives obviously fail us - the scientific enterprise is built on that realization. So why, when the going gets tough, should we abandon what has proven to work and switch to the alternatives? It seems completely predictable that it will cause confusion.

But in my post, I questioned the existence of all objects, so referring to molecules doesn't change anything. — Metaphysician Undercover

OK, but then QM would not be applicable to anything since it only applies to things that exist.

While of course there are philosophical issues here, the fact is that most people reasonably do think that many things exist and also think that standard scientific explanations are applicable to those things. So that really needs to be the starting point for any meaningful discussion.

That I certainly don't agree with. I agree with the tested/potentially falsified part, but that doesn't mean that theories etc. are not read instrumentally, and I neither agree that (a) something makes it the case that theories etc. are meant to be read as ontological commitments nor that (b) most scientists read theories etc. as ontological commitments rather than instrumentally. — Terrapin Station

I agree that a theory can be used instrumentally, whether or not it is true, as is done with Newtonian gravity. But a theory provides an explanation of the world, which is why we consider it to be true or false (or reserve judgement if we're not sure).

The EPR paper was one of these 'thought experiments' - it was that paper that led to the Bell's Inequality paper which was published in 1964 (many scientists will say that it is one of the greatest scientific papers in history). And it was that paper that formed the basis of the Alain Aspect experiments which in the early 1980's empirically demonstrated the entanglement of remote paired particles, thereby showing that the EPR paper was wrong. It was the final nail in the coffin of Einstein's realist philosophy, pending something world-shatteing coming along. — Wayfarer

Some might say that the world-shattering thing was Everett's relative state formulation which was published in 1957 - two years after Einstein's death. It preserves realism, locality and causality. EPR-style entanglement and Bell's Theorem are non-issues under this interpretation. (Bell's Theorem proves that local hidden-variable theories are not possible which is why pilot wave theories have to be non-local.)

Hence, this whole debate. I wouldn't think you would propose infinite branching universes unless you had a real need to do so. — Wayfarer

That's the irony. Many-Worlds does not propose infinite (or finite) branching universes. The branching is already integral to QM. The Copenhagen Interpretation has to add a postulate to QM to prune the branches it doesn't want, which is the famous wave function collapse. Here is an example to demonstrate how it works.

Suppose there is a particle in a superposition of spin-up and spin-down. Alice has a device that can measure the particle spin and display the result. Before she makes the measurement, there is a 50% probability of the device measuring spin-up and a 50% probability of it measuring spin-down. The wave function initially includes the particle in superposition, and also the external environment which includes Alice and the device.

Now Alice measures the particle spin. The wave function evolves to a superposition of (particle is spin-up and the device measures spin-up and Alice reads "spin-up" on the device) and (particle is spin-down and the device measures spin-down and Alice reads "spin-down" on the device).

That just is the relative state formulation, or Many-Worlds. The wave function does not collapse, it continues to evolve. Whereas Copenhagen prunes the branch of the superposition that doesn't match what Alice reads on the device, which is just one state in this example.

So Many-Worlds is just the straightforward meaning of QM. Whereas Copenhagen is QM plus a collapse postulate that otherwise appears nowhere in QM. Unfortunately, as well as failing to explain why Alice observes one definite spin, it also introduces various paradoxes, such as non-local EPR entanglement, that simply don't exist under Many-Worlds.

These are the kinds of considerations that motivate the Many-Worlds Interpretation.

OK, but then QM would not be applicable to anything since it only applies to things that exist.

While of course there are philosophical issues here, the fact is that most people reasonably do think that many things exist and also think that standard scientific explanations are applicable to those things. So that really needs to be the starting point for any meaningful discussion. — Andrew M

I think the issues with QM, especially the MWI, indicate quite clearly that things do not exist in the same way "that most reasonable people" think that they exist. Therefore your claim that this "really needs to be the starting point for any meaningful discussion" is completely unjustified. In fact, that claim only demonstrates your ontological prejudice.

How can you take MWI seriously, yet at the same time, claim that the existence of things, as most reasonable people use "existence", needs to be the starting point for any meaningful discussion? The two are contradictory. Either we take QM, and MWI seriously, as a premise, to see what conclusions may be produced, and forget about the "existence" which most reasonable people refer to, or we take the "existence" which most reasonable people refer to, and forget about QM and MWI.

So, which do you choose? Do you want to discuss MWI, or do you want to adhere to the "existence" which most reasonable people refer to?