Now my point, Andrew M. Appeals to identity do not support the real existence of the apple. "Identity" claims, asserts, or presupposes existence, but what we need here is the principles by which such a claim of existence is justified. Then we can apply these principles in an attempt to justify the existence of the particle, as an identified existent. — Metaphysician Undercover

Asking for justification for the existence of the apple is misplaced here. The thing on the table that we can publicly point to is what we mean when we talk about apples. And part of what we mean is that they exist. (As opposed, say, to our talk about unicorns - we can only point to pictures of unicorns.)

This just is the Aristotelian approach that you mention. There really is an apple there and it has a particular form such that we can point to it and identify it. How the apple appears to us depends both on the apple and on the surrounding environment, including the body and brain of the observer and the interacting light particles.

So here's the problem. The logical system at work here is set up with the premise that the existence of the object is justified if, or, "the object exists if", it has contextual relations with other objects (relativity). So any mathematics used will produce conclusions from this premise. If we desire to assume a "Cosmos", "universe", or "world", to objectify such relationships, and validate the existence of any particular object, that very premise, will not allow that the assumed "world" has existence except in relation to other worlds. — Metaphysician Undercover

I agree that is the problem. A scientific theory such as QM is an explanation of the world, not a mere formalism. And it is testable on that basis. We can plug in particles (or, in principle, apples) and compare what the theory predicts with our subsequent observations.

When one refers to "the apple", that individual is referring to a particular instance of temporal continuity in which the similitude of an apple is of the essence. In order that one can refer to 'the apple", it is necessary that this similitude appears for a duration of time. What constitutes the "existence" of that apple is that this similitude persists through a duration of time. If the similitude seemed to flash upon the scene for a simple yoctosecond of time, then was gone, we could hardly assign "existence" to the apple. "Existence" requires that the described thing has a temporal duration — Metaphysician Undercover

The apple sitting on the table is the same apple that I pick up and take a bite out of a few seconds later. That is what is meant by identity.

It doesn't matter that the apple's atoms may be replaced by other atoms of the same kind over time or that the apple's appearance changes. Those aspects are not what our ordinary notions of identity and existence refer to.

Hang on Woz, I think you've crossed threads. — Wayfarer

Quantum tunnelling between threads. It happens...

You're claiming that the only possible starting point for meaningful discussion, is the premise that things exist in the intuitive, common sense notion of "things exist". And you want to maintain this premise, while introducing the QM premise that things do not "have a precise position and momentum at the same time". Do you not see that this QM premise contradicts the common sense notion of "exists"? When there is contradiction, we have an either-or situation. — Metaphysician Undercover

It doesn't contradict it. This just comes down to Wittgenstein's private language argument which, as I recall, you reject.

The term "existence" has a public referent. We point to an apple and say that that is what we mean by something existing. Even though we update our knowledge about apples from time to time, we are still referring to the same ordinary, familiar, existing apples that we were before.

Do you AndrewM, recognize that there is a fundamental incompatibility between the premise that there is just one continuity, and the premise that there is multiple continuities? These two premises are incompatible, contradictory. — Metaphysician Undercover

My apple, at the moment, may have a well-defined position. So it therefore will be in a superposition of momenta. This just means it's not a classical object, it's a quantum object.

There is only a contradiction if I say there is both an apple and not an apple at the same time and in the same respect, which is not what I'm saying here.

Of course, as you know, Everett's theory doesn't make any of those assumptions let alone declare they are principles of reality. — tom

Thanks Tom - great list!

The whole point your long and detailed answer skips over is the 'm' in mwi. — Wayfarer

The point is that a superposition constitutes many states and they are all necessary for the wave function to evolve.

The instrinsically grotesque nature of there being 'many worlds' is skipped over by the advocates; like, the strangeness of the idea that we're all part of an infinite 'hall of mirrors' is being skipped over, on account of the fact that it is 'mathematically convenient'. Don't you see how strange that is? — Wayfarer

Yes it seems strange because we intuitively think we live in a classical world. But we don't, we live in a quantum world. So the key to resolving that strangeness is to think of things (like particles, trees, cats, humans) as quantum systems, not classical systems.

This has been an instructive debate. — Wayfarer

Indeed - I've enjoyed the discussion.

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? — Metaphysician Undercover

It's not an either-or. MW is just the ordinary language interpretation of QM.

That doesn't imply that things will therefore exist in the way that we might intuitively think. Who knew that things wouldn't have a precise position and momentum at the same time? They still exist, but we've learned new things about them.

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.

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).

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.

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.

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

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.

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).

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.

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.

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.

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.)

I would have thought that the 'many paths' are not phenomena. They're inferences. But one cannot see 'the other paths', by definition - they're what's in the 'other worlds'. We only see one path - so that is the only 'phenomenon' being observed, the rest is inference. — Wayfarer

This is where the realist metaphysics kicks in. What explains the interference pattern in a double-slit experiment? What is actually interfering? What we are seeing is the interference of many paths. It is like we are standing on a road and seeing the road fork in front of us. But instead of the two roads being side-by-side, the roads are superimposed on each other like a superimposed photograph.

Consider the bent-stick illusion. We seem to see a bent stick when it is partly submerged in water. But we seem to see a straight stick when it is out of the water. Are we seeing a stick that has a mysterious straight-bentness nature that depends on how we observe it? Is the scientist's job just to record the observables and shut-up-and-calculate or do we expect that there is a natural causal explanation for why the stick appears differently under different circumstances?

Similarly, are we seeing phenomena that have a mysterious wave-particle nature that depends on how we observe it? The natural causal explanation is that we are seeing multiple phenomena that exhibit an interference pattern when they are in superposition.

So, for the umpteenth time, 'many worlds' means 'many worlds'? Yes or no? — Wayfarer

Yes. In the Schrodinger's Cat thought experiment, there are two cats, both of them equally real. One is alive (and wondering why it is in a box) and one is dead.

Andrew, 'Worlds' doesn't begin with 'M', does it. The question was, what does "M" stand for? And obviously the answer is 'many' - as in 'many worlds'. That is what it means, it refers to branching or splitting or multiple universes. — Wayfarer

Sorry I misread and I've edited my earlier comment.

Now, if it doesn't mean that, then there's nothing to debate. If the 'many' in 'many worlds' is simply an hypothetical mathematical construct or device, then it's back to 'shut up and calculate'. But the controversy is about the notion that it really does say there are many or parallel realms. — Wayfarer

People simply use different terminology to refer to the same phenomena. What is being referred to is the many paths, or branches, that can be in superposition.

And, yes, the realists say those branches are real.

Obviously it stands for 'many worlds'. It refers to the many paths (or branches) that can be in superposition.

only for 'weak' mwi; for the strong version there really are countless separate or parallel universes. And that is metaphysics, — Wayfarer

I'm guessing by 'weak' mwi, you mean instrumentalism. An instrumentalist would say that they don't know what summing over paths really means or, more strongly, that it doesn't mean anything. But it works extremely well for predicting phenomena, so we should use it.

But why does the math work? The straightforward realist explanation is that the world really is as the math describes it. There are paths that split off and interfere.

BTW, MW doesn't imply parallel universes, but instead paths (branches) within the universe. And these paths are observed in quantum interference experiments.

The metaphysics is realism, which brings us back to Einstein and Bohr.

I think 'we don't know' is the superior answer. Physics is getting hopelessly entangled in pseudo-metaphysics, Everett's being the most egregious example. A dose of humility and a sense of the limitations of science might be preferable. — Wayfarer

By all means keep an open mind. Nonetheless, Everett's is the only theory that explains why we see quantum interference effects. So it is the theory to beat.

Under MW, entanglement is not just a (statistical) correlation. "Correlation" is the wrong word. It is the anti-explanatory word used by anti-realists to avoid any questions about why their algorithm works, particularly as the algorithm does not apply to reality, but rather what can be said about reality.

Rather, under MW, the physical mechanism by which the right branches encounter each other - in order to preserve conservation laws etc - is termed "information flow" or something similar. The process was fully worked out here: http://xxx.lanl.gov/abs/quant-ph/9906007 — tom

Yes, it's not merely statistical, it has a causal basis. My main point is that there is nothing mysterious about entangled particles under MW - it just means that they are on the same world branch. And if the observer becomes entangled with them, then they will also be on that world branch.

Thanks - that's pretty close to what I thought, but I don't understand your first point, 'an observation entangles the observer with objects on a particular world branch'. I had thought 'entanglement' was what Einstein meant when he spoke of 'spooky action at a distance'. What does 'entangling an observer with objects' mean? How does that manifest? — Wayfarer

Under MWI, entanglement is just correlation. In terms of the EPR experiment, there will be a pair of correlated opposite-spin particles on one branch and another pair on another branch and these branches are in superposition. When Alice observes the first particle, a process of decoherence occurs whereby Alice becomes correlated with the pair of particles on one branch (and similarly on the other branch). There is no spooky action at a distance because there is no action happening between the particles at all.

You say that MWI is 'highly explanatory' - it may be, but at the cost of assuming an infinite number of parallel universes. — Wayfarer

Not necessarily infinite. But, yes, a lot. The branching isn't assumed. It's just the natural interpretation. That's what the summing over paths is about. The difficulty is in coming up with a coherent interpretation that omits the other branches. If the other branches aren't real, then what causes the interference effects?

Under MWI, observations do have causal consequences. An observation entangles the observer with objects on a particular world branch.

The real objection, I think, which is the objection that Einstein had, is that the Copenhagen Interpretation entails a rejection of realism. That is, does the world, in some sense, depend on the mind? Does God play dice? Does the moon only exist when you look at it? Is there spooky action at a distance?

Under MWI, these are all trivially answered in the negative. It is an interpretation that is highly explanatory. Whereas the Copenhagen Interpretation is not well-defined and does not explain why quantum interference effects occur.

All quantum interference experiments are evidence of Many Worlds. — Tom

@Tom - could I put the question to you: what problem is the 'many worlds' interpretation a solution for? Why is it necessary to invoke 'many worlds'? — Wayfarer

'Many worlds' is a solution for why we observe quantum interference patterns.

When a single photon is fired in the double-slit experiment, the probability that it will be detected at any particular position on the back screen can be calculated by summing over all the possible photon paths to that position.

In MWI, those photon paths are not merely possible, but real. That is, each path is traversed by a photon in a separate world branch. When a photon is observed on the back screen, that is the result of world branches combining in superposition which causes an interference effect.

The difficulty for other interpretations is how to explain those interference effects if the paths are not real. How can merely possible photon paths cause a real interference effect?

Quite a few philosophically minded folk would love to do away with the subjective/objective distinction. — Marchesk

In ordinary usage, the terms "objective" and "subjective" refer to judgements that we think do or don't meet normative epistemological standards. That is, is the person considering the evidence or merely expressing their opinion on the matter?

For example, compare "historians try to be objective and impartial" with "his views are highly subjective".

The problems arise, I think, when either that ordinary distinction is disputed (e.g., radical skepticism, subjectivism), or when it is applied to something other than judgements (e.g., dualistic phenomena, worlds, viewpoints).

It just came up as part of a discussion with John where he is arguing (as I understand him) that some things in the world, such as relations between things, are conceptual and thus imply intentionality.

My argument is that relations between things are instead abstractions. That does not, by itself, imply intentionality. But conceptualizing those relations does.

The concept of a tiger is part of our mental map. The tiger is not - it is part of the territory. The concept is not the referent.

I'm saying that tigers don't depend on anyone's concept of them. That is realism.

They are real. But our (conceptual) map is not part of the (physical) territory.

If the territory is utterly a-conceptual then how could our conceptual judgements bear any relation to it whatsoever? — John

Because we differentiate features of the territory. But differentiating them is a cognitive and conceptual process.

The territory has its rivers and hills. But there is no map until we create one.

It was Wittgenstein who said "the world is the totality of facts, not of things". I take that to mean the world is the totality of states of affairs, not the totality of epistemic facts. The totality of states of affairs constitutes the total nexus of relations between things, and things themselves are also, unless they be some kind of posited, but really incomprehensible, atomic simple, further complexes of relations. Relations are not physical, but rather conceptual, which leads to the conclusion that the world must be, at bottom, not merely brute non-conceptual entities, if the idea of such entities even makes any actual sense, but also the conceptual relations between them. — John

OK, I'll accept the totality of facts in this sense for the sake of argument. And I agree that there were relationships between things (say, the dinosaurs and the earth).

So we have abstractions of matter. But the abstractions are not something separate from, or additional to, the matter. They are ways of looking at matter. However it still requires a cognitive process to actually look at and conceptualize matter in that way.

No, I wouldn't - I don't see what purpose it would serve. I would just say that dinosaurs existed.

I see those definitions as language on holiday. They are an attempt to elucidate a sense of a term that serves no useful function and that no-one, except philosophers, cares about.

The existence of dinosaurs is not eliminably conceptual. But the words in that sentence are. The conceptual map is not the territory.

So, what was the fact prior to its discovery? — John

As Willow points out, there wasn't one. I'm just following standard usage here - the OED defines a fact as "a thing that is known or proved to be true".

We don't call things facts unless we think they are established knowledge. Aliens may well exist, but we wouldn't say it was a fact, we would say it was a conjecture or an opinion.

I see that Wikipedia defines a fact as "something that has really occurred or is actually the case". In that sense, we could say that it was a fact back then that dinosaurs existed. But all we're saying is that dinosaurs existed back then. Is this sense of the term actually serving any useful function?

I hope I can butt in to ask, one puzzle for me in this area is why some terms are deemed 'mental' and some 'physical' and where the border falls. For instance, a description of a chemical compound, while arguably an abstraction over more primitive physical terms, is deemed physical, but words like 'thought' are deemed mental. I presume the one is vertically constitutive of and the other is just supervenient on the physical, but I'm not clear. To describe someone's character I might call them 'hot-blooded' or 'cold-hearted' but these are understood to be mental descriptions. — mcdoodle

I think they have a pragmatic origin. We understand other human beings to be intentional creatures and we develop mental language around that. That works well for humans, somewhat less well for animals and bacteria, and pretty badly for trees and particles. Coming from the other direction, we find commonalities between humans, animals, bacteria, trees and particles that we develop physical language for. They are logically distinct categories and the terms we use depends on the purposes at hand.

I'm interested for instance in the practising medical scientist's use of terms. In dealing with pain in a phantom limb, for instance, the patient's belief seems central, and we have no idea what the physical equivalent of their belief in their limb is. So the working scientist has to engage in methodological dualism. And yet a different, theoretical scientist argues that this 'belief' is non-primary, even though they can offer no empirical model of explanation. — mcdoodle

A practicing medical scientist is interested in what works. So if using psychological language brings about the desired physical changes, then it makes sense to do that. Further research could be conducted to find out what is physically happening. I wouldn't say the belief is non-primary. It's just language at a different level of abstraction that may or may not be appropriate. But the philosophical point here is that nothing in addition to, or contrary to, the physical is going on.

Anyway, my working definition of mind is 'that which cognises differences'. — Wayfarer

Yes. And your definition is the flip side of behaviorism - the idea that it is the body that does things.

What I'm trying to suggest is an alternative to the idea that it is either the mind or body that does things. It is instead human beings that do things. Human beings have minds and bodies, but mind and body are two logically different categories. It is not, per dualism, the mind that is the subject of experience and the body that is the object of experience. Instead a human being is both the subject and the object of experience.

So, for you there are no unknown facts about the universe that are yet to be discovered? Such as for example, whether some particular distant galaxy has a black hole at its center or is some very precise number of light-years across, or contains exactly so many stars? — John

Right. If we find out those things, then we will have discovered new facts about the world, not existing facts.