Oh my god, I have almost 3000 likes!!! :love:

Oh my god, I have almost 3000 likes!!! :love: — Kenosha Kid

Amateur. I've got 4.6k...overnight. Looking forward to another few k today from my spontaneous fanbase.

Well deserved, sir.

Pluralism: "The cat is dead" is true for Wigner's friend but not for Wigner. = "The cat is dead [is true] for Wigner's friend"

Yes, I think this is what I meant. The first is relativism, the second pluralism, and they are equivalent. As I said, I encountered this first in a discussion on moral relativism versus objectivity, including pluralism, and I understood how the latter isn't just the former insisting it's the latter. — Kenosha Kid

No. Both of your examples are simple pluralism. And second, saying 'true' of what ontologically 'is' is always redundant verbiage in your realism.

In addition to pluralism, relativism whether ontological [dead] or logical [true] requires a second higher level functor which establishes a dependency relation. Observation y depends on the value of x. It's two-dimensional, not a simple is/is-not here and there.

What do you mean by encode?

Simply that everything physical that happens is encoded in the wavefunction. There is no additional physical mechanism such as probabilistic collapse. In that sense, MWI is taking the physics first.

Observation y depends on the value of x. — magritte

As in:

"The cat is dead" is true for Wigner's friend but not for Wigner. — Kenosha Kid

Note the quotes: the statement is "The cat is dead". The truth value of that depends on who you ask.

What do you mean by encode? — Manuel

that what exists are actually instructions, i.e. 'manifest electron here'. :wink:

"The cat is dead". The truth value of that depends on who you ask. — Kenosha Kid

There is an interesting thing about that cat. Wigner's cat and his friend's cat are obviously not the same cat, as one is alive and the other is dead. Ontologically speaking, this is the only correct answer. People who claim that the there is but one cat are suffering from confusion. To expand, there are as many cats as there are observers.

one is alive and the other is dead. — magritte

My understanding is that Bohr would never have agreed with any of this. Talk about the state of anything when it is not being observed is empty words. We're just continually trying to create a stake in 'what is really there' but it is simply imaginative conjecture. It does not exist, but also does not not exist.

Talk about the state of anything when it is not being observed is empty words. — Wayfarer

Sorry about that! I'm only trying to make a point to Kenosha Kid there.
I agree with you about QM. Exists is only meaningful to a philosopher and is an empty word for QM theory. For example, electrons can't philosophically exist because of their lack of 'substance', being only a bundle of instantaneously measurable ephemeral properties.

:up:

Anti-realist-idealists discussing fundamental physical science. :rofl:

Talk about the state of anything when it is not being observed is empty words. — Wayfarer

Why? Do you really want to elevate yourself to a condition of existence? Universalize self-dependence? :brow: Let's talk about Mars.

we've been here before...

People who claim that the there is but one cat are suffering from — magritte

quantum mechanics.

I'm not going to argue the science, but both the alive and dead cat evolve from the pre-experiment cat continuously. No matter is added to make another cat, for instance. So I think you need a more complete answer before you tell anyone they're confused ;)

, still wondering ...

Talk about the state of anything when it is not being observed is empty words. — Wayfarer

you really want to elevate yourself to a condition of existence? Universalize self-dependence? :brow: Let's talk about Mars.

We can easily and meaningfully talk about Mars onlookers or not.

Well, remember, Einstein once asked, I think in exasperation, 'Doesn’t the moon continue to exist when we're not looking at it?' I'm sure he meant it rhetorically, the implication being 'of course it does!' But what caused him to ask? I think it was the discovery of the role of observation in quantum physics, as per Wheeler's remark in this quote. Einstein was committed to scientific realism, he believed that the world existed independently of any of our judgements about it, and that the uncertainty principle showed only that quantum theory must be incomplete. That is the basic gist of the long debate between Bohr and Einstein. The two books, Quantum, Manjit Kumar, and Uncertainty, David Lindley are about this (the first is better).

My intepretation is that scientific realism is a methodological step, not a philosophical principle, and that enormous confusion ensues from not seeing that. Scientific realism begins by assuming the independent reality of the world. But what this stance doesn't allow for is precisely 'the role of the observer', and it was the discovery of quantum mechanics that threw this into relief. The universe doesn't 'exist anyway', regardless of whether humans are in it, because the meaning of 'to exist' relies on the implicit layers of judgement and ordering which the mind brings to the picture (which is where Kant is crucial.) This doesn't mean that the observer literally creates the Universe, but that the mind provides the framework within which any judgement about what exists or doesn't exist is made. So the usual assumption that the Universe exists sans any observer is actually a kind of projection - the idea of the serene early universe that pre-existed the advent of h. sapiens is still mind-dependent, in that crucial sense. This is why Kant stresses that he is both empirical realist and transcendental idealist. It's not an either-or proposition. It's also why I said that reality absent any observer does not exist, but also does not not exist.

Put aside some time to look at this lecture by Michel Bitbol on 'Bohr's complementarity and Kant's epistemology'.

If this was just a story about the friend telling Wigner that the measurement has been done by, say, sending a photon, ignoring everything else, even that the measurement was a quantum one, would you say that this process of sending a photon from one system to another didn't entangle the two systems? — Kenosha Kid

I would. For Wigner, the photon is separable from the superposed lab. From page 3 of Brukner's paper (where the above photon is system M):

The novelty of Deutsch’s proposal [10] lies in the possibility for Wigner to acquire direct knowledge on whether the friend has observed a definite outcome upon her measurement or not without revealing what outcome she has observed. The friend could open the laboratory in a manner that allowed communication (e.g., a specific message written on a piece of paper) to be passed outside to Wigner, keeping all other degrees of freedom fully isolated, as illustrated in Figure 1. Obviously, it is of central importance that the message does not contain any information concerning the specific observed outcome (which would destroy the coherence of state (1)), but merely an indication of the kind: “I have observed a definite outcome” or “I have not observed a definite outcome”. If the message is encoded in the state of system M, the overall state is:

$\small | \Phi \rangle _{S F M} = \frac{1}{\sqrt{2}} (| z + \rangle _{S} | F_{z +} \rangle _{F} + | z - \rangle _{S} | F_{z -} \rangle _{F}) | I've \space observed \space a \space definite \space outcome \rangle {_M}$ (2)

since the state of the message is factorized out from the total state (I leave the option for the message “I have not observed a definite outcome” out, as it conflicts with our experience of the situation that we refer to as measurement and it also can be used to violate the bound on quantum state discrimination [8]). — A No-Go Theorem for Observer-Independent Facts - Caslav Brukner

Interestingly, Deutsch was originally using the Wigner's friend thought experiment to distinguish an objective collapse version of the Copenhagen Interpretation from Many Worlds. He says,

In Section 8 I describe a thought experiment whose main purpose is to show how the conventional and Everett interpretations are in principle experimentally distinguishable. — Quantum Theory as a Universal Physical Theory - David Deutsch, 1985

Also of interest, at the end of section 8 (p37) Deutsch uses the term "merge":

The interference phenomenon seen by our observer at the end of the experiment requires the presence of both spin values, though he accurately remembers having known at a previous time that only one of them was present. He must infer that there was more than one copy of himself (and the atom) in existence at that time, and that these copies merged to form his present self.

He must infer that there was more than one copy of himself (and the atom) in existence at that time, and that these copies merged to form his present self.

I'm sorry, but I just find this really creepy. And I still would like to know what Deutsch would be obliged to admit if it were shown it could not be true. I mean, what's he frightened of?

"Once we have granted that any physical theory is essentially only a model for the world of experience,” Everett concluded in the unedited version of his dissertation, “we must renounce all hope of finding anything like the correct theory ... simply because the totality of experience is never accessible to us.” — The Many Worlds of Hugh Everett

I'm sorry, but I just find this really creepy. And I still would like to know what Deutsch would be obliged to admit if it were shown it could not be true. I mean, what's he frightened of? — Wayfarer

I don't know what his motives are beyond thinking that the Everett interpretation is correct. In the above paper, Deutsch described a method for experimentally distinguishing between the Everett interpretation and a particular version of the Copenhagen interpretation. So he's making his own position subject to falsification. His thought experiment has been the basis for the recent experiments discussed in this thread, and has lead to progress in the area of quantum foundations.

I think he explains his motivation in the introduction as a response to:

their [Physicists] attempts to see in the very inadequacy of the conventional interpretation of quantum theory a deep physical principle have often led physicists to adopt obscurantist, mystical, positivist, psychical, and other irrational world views. Undermining, as it thus does, the view that it is the task of physics to seek a systematic understanding of a real, objectively existing world, the widespread acceptance of the conventional interpretation cannot but have impeded the growth of knowledge in physics.

I note the posit of a 'real, objectively existing world'. Presumably this is not regarded as an axiom? It would seem a philosophical pre-supposition, at least.

I will try and read this paper, but it's difficult without real ability to read mathematical physics. I suppose that's part of the territory.

I note the posit of a 'real, objectively existing world'. Presumably this is not regarded as an axiom? It would seem a philosophical pre-supposition, at least. — Wayfarer

Realism is a philosophical presupposition. Within that, the terms "intrinsic realism" and "participatory realism" have been proposed (see Table 1 in Interpretations of quantum theory: A map of madness) which differentiates Many Worlds from Copenhagen.

I'm not a fan of the "objectively existing" qualifier (with it's Cartesian implications). It seems enough to say that the task of physics is to seek a systematic understanding of the world. And we bring our philosophical presuppositions to that task.

Thanks. I relate more to Type II, for what it’s worth.

I noticed Deutsch’s reference to the ‘lacuna’ in some of the standard interpretations in his opening remarks. Philip Ball is no fan of many worlds. He notes:

As DeWitt put it: ‘every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies’. Recall that this profusion is deemed necessary only because we don’t yet understand wavefunction collapse. It’s a way of avoiding the mathematical ungainliness of that lacuna. — Phillip Ball, Too Many Worlds

Which is just how I see it. Again, for what it’s worth.

Anti-realist-idealists discussing fundamental physical science. :rofl: — 180 Proof

Philip Ball is no fan of many worlds. He notes:

As DeWitt put it: ‘every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies’. Recall that this profusion is deemed necessary only because we don’t yet understand wavefunction collapse. It’s a way of avoiding the mathematical ungainliness of that lacuna.
— Phillip Ball, Too Many Worlds

Which is just how I see it. Again, for what it’s worth. — Wayfarer

Good quote.

Only in this world. In many other worlds we’re all over the place.

That's what infinity in mathematics gives us, ungainly lacunas.

Yes, this is what I meant regarding separability. In reality, it's not that clean: if you have two atoms, say, correlated by exchange of a photon, you cannot evolve them independently: it's a single many-body wavefunction describing the whole system, and the exchange and correlation parts of that are not trivial.

The formalism above necessarily neglects the fact that Wigner and his friend are entangled anyway. Previously an argument for this was the classical limit. However if we're in the regime of seeing macroscopic bodies in superposition (if!), we're not in the classical limit anymore and that approximation doesn't obviously follow.

What we should see in MWI is each branch evolving independently as if it were the whole universe. As I mentioned above to Pfhorrest, this result, if consistent with scale, leads to odd contradictions in which the friend can indirectly observe his other branches.

Yes, this is what I meant regarding separability. In reality, it's not that clean: if you have two atoms, say, correlated by exchange of a photon, you cannot evolve them independently: it's a single many-body wavefunction describing the whole system, and the exchange and correlation parts of that are not trivial. — Kenosha Kid

Yes, it's just a thought experiment that shows the implications of QM at a macroscopic level.

The formalism above necessarily neglects the fact that Wigner and his friend are entangled anyway. — Kenosha Kid

But this is the point at issue. In the thought experiment as described in Deutsch's paper (and assumed in Brukner's paper), Wigner and his friend are not entangled and Wigner demonstrates this with an interference experiment. See below.

What we should see in MWI is each branch evolving independently as if it were the whole universe. — Kenosha Kid

Each friend's branch evolves independently. But Wigner, per MWI (and unitary QM), can in principle undo the friend's measurement and apply a Hadamard to the spin state (i.e., H((|spin up> + |spin down>)/sqrt(2)) = |spin up>), resulting in a single branch again. The friend would have no memory of what measurement she made [1] but she would have the message saying that she had made a definite measurement. That is, the final state would be:

|Wigner>|friend>|spin up>|I've observed a definite outcome>

The above is what MWI (and unitary QM) predicts. Whereas on an objective collapse version of the CI, the spin state after applying the Hadamard would be |spin up> with 50% probability only. That is, the final state [2] would be:

|Wigner>|friend>(|spin up> +/- |spin down>)/sqrt(2)|I've observed a definite outcome>

Hence the two interpretations (theories, really) are experimentally distinguishable.

--

[1] This is similar to a photon emerging from the second beam splitter of an MZI - it has no encoded information about which interferometer path it travelled along.

[2] For anyone wanting to check the math, if Wigner were correlated with the spin up state, then applying a Hadamard to that state puts it into superposition: H(|spin up>) = (|spin up> + |spin down>)/sqrt(2). Alternatively, if Wigner were correlated with the spin down state, then H(|spin down>) = (|spin up> - |spin down>)/sqrt(2). Either way, there's only a 50% probability of Wigner subsequently measuring the spin up state.

|Wigner>|friend>|spin up>|I've observed a definite outcome>

The above is what MWI (and unitary QM) predicts. — Andrew M

As someone whose job it was to calculate exact solutions to the many-body wave equation for simple, symmetric systems, I can tell you that is not true. Communication between two subsystems like this does not permit the factorisation of those subsystems. Wigner would have been communicating with an already branched friend so would be entangled within-branch. This is what MWI would predict too.

If there was truly zero entanglement prior to and throughout the experiment until Wigner made his own measurement, then he ought to see interference effects as Deutsche originally intended. However by communicating with his friend, e.g. by exchange of photons or electrons, directly or indirectly, after his friend had branched, he would see no such interference effects. It just isn't possible to separate Wigner out of the wavefunction the way you think we can.