What has changed since the Copenhagen interpretation?

[martinbn]

It then "just so happens" that $\mathcal{H}$ is isomorphic to $\mathcal{L}^{2}\left(\Sigma\right)$ with $\Sigma$ a space of functions over a hypersurface of a Lorentzian manifold.

How is that?

 

[DarMM]

How is that?

What do you mean?

 

[Demystifier]

How is that?

You must read what he means, not what he writes.

 

[DarMM]

I don't think it matters that the fundamental object is naturally interpreted as alocal. For a theory to be local, it must be possible to write down the states in a local manner, where all the "elements of reality" are confined to a region.

What are those elements of reality in Tipler's paper? In Deutsch's work they are the reduced density matricies, I'm not really sure what they are in Tipler's case.

Tipler presents the evolution of the global state from Alice's perspective and then from Bob's and then says because the evolution has different time orderings from these two perspectives there's nothing nonlocal. To be honest I don't fully understand that, but my first sentence above is more important. What are the local elements of reality in his picture?

Also perhaps relevant: Against Wavefunction Realism (Wallace, 2017). He's saying that you shouldn't take Hilbert space as the fundamental ontology in many worlds, but instead consider the non-fundamental ontologies implied by a specific models. My point in linking this is to show that many worlds does not necessarily imply any specific decomposition.

Of course, but this really reduces to the case of $\mathcal{H} = \mathcal{L}^{2}(Q)$ with $Q$ model dependent. I get and I think agree with Wallace's point, but I don't think it affects this discussion as the state, regardless of how it is represented, seems to have these properties (even if formulated as a functional on a C*-algebra) and you need the environmental decomposition I mentioned earlier, again regardless of $Q$ or even if you don't view the theory through a Hilbert space lens the issue can be reposed in an Algebraic approach, it remains fundamentally the same issue.

Perhaps there is no discussion because like me, he doesn't see how it's an issue.

I meant there seems to be no discussion of his paper by other authors. Everybody who references him does it mostly to say either "Tippler thinks otherwise" or "Here's a many worlds treatment of entanglement", there's no real discussion.

Can you help me understand your objection a little better? Is it about the encoding of that information at the ontic level or about the mechanism which matches up those overlapping systems? I don't know how it's implemented, but I can't see any reason why it would be problematic.

Both, I'm certainly not saying there is a no-go theorem that means it can't be done, but it hasn't been done and I'm not sure it could be done. Very simply, as above, Tipler looks at the global state from Alice/Bob's view. However what you'd really need is what are the actual local degrees of freedom? The global state isn't a valid candidate. What element of reality/degree of freedom exists in Bob's spacetime region where he does the measurement?

Deutsch says the density matrix, is that what Tipler is saying?

 

[DarMM]

You must read what he means, not what he writes.

Let's hope I don't forget what I meant or we'd have a real paradox.

To explain that's the basic form of the Hilbert space in QFT, as space of square integrable functions over fixed time classical fields, with fixed time classical fields being the aforementioned functions on a hypersurface. Though this is being informal, correctly they are tempered distributions over a hypersurface.

 

[martinbn]

What I am confused about is that you say that the Hilbert space is an $L^2$ space over a space of functions. But what is the measure in that space of functions?

 

[DarMM]

What I am confused about is that you say that the Hilbert space is an $L^2$ space over a space of functions. But what is the measure in that space of functions?

Depends on the field theoretic Hamiltonian, unlike non-relativistic QM there isn't a unique (up to Unitary transformations) measure from the Stone-VonNeumann theorem. This is related to the issue of renormalization.

 

[martinbn]

Depends on the field theoretic Hamiltonian, unlike non-relativistic QM there isn't a unique (up to Unitary transformations) measure from the Stone-VonNeumann theorem. This is related to the issue of renormalization.

Any examples?

 

[DarMM]

Any examples?

It's quite technical, the measures are more proven to exist rather than being directly quotable. James Glimm's "Boson fields with the $:\phi^4:$ interaction in three dimensions", Comm. Math. Phys. 10(1) p.1-47, is one of the gentler introductions.

Also his book with Arthur Jaffe, Quantum Physics: A Functional Integral Point of View.

Though his paper derives their existence more directly.

Some properties of the measure and the closest to an easily quotable example is given in:
Reed, M. & Rosen, L. "Support properties of the free measure for Boson fields" Commun. Math. Phys. (1974) 36: 123

The case here being the measure for the free field.

 

[atyy]

As I argue in http://de.arxiv.org/abs/1703.08341 , MWI is neither local nor non-local. It is alocal.

I guess you would say MWI is a-nonlocal?

 

[Auto-Didact]

Not much.

Great, saves me the time of reading those again!

Note that the fractal nature in the Abbott & Wise case is caused by measurement. On the other hand, unmeasured BM trajectories do not have a fractal nature.

I see. On a related note, see this thread.

The currently strongest particle accelerator (LHC) sees nothing beyond the Standard Model, but my theory predicts that a much much stronger accelerator should see new particles with Lorentz non-invariant cross sections.

Any specific energy scale in mind? And if so, is that some physically derived scale or just an ad hoc guess?

 

[Demystifier]

Any specific energy scale in mind? And if so, is that some physically derived scale or just an ad hoc guess?

It's ad hoc, but my first guess would be Planck scale. However, one terminological notion is in order. In the context of violated Lorentz invariance, I would not talk about energy scale. I would talk about length scale or its inverse 3-momentum scale.

 

[Auto-Didact]

It's ad hoc, but my first guess would be Planck scale.

Okay. Last question for now: I probably missed it, but does your model, being fundamentally non-relativistic, have any strong explicit predictions about the existence, modification or non-existence of zitterbewegung? I ask mainly due to the arguments made in Hestenes 1990 that zitterbewegung need not be regarded as a purely relativistic phenomenon, instead amenable to a geometric algebra reinterpretation consistent with the Madelung reformulation of the SE.

However, one terminological notion is in order. In the context of violated Lorentz invariance, I would not talk about energy scale. I would talk about length scale or its inverse 3-momentum scale.

Ha, of course. As Poincaré said: We must use language, and our language is necessarily steeped in preconceived ideas.

 

[akvadrako]

I meant there seems to be no discussion of his paper by other authors. Everybody who references him does it mostly to say either "Tippler thinks otherwise" or "Here's a many worlds treatment of entanglement", there's no real discussion.

Ah, I agree with that. The original Deutsch-Hayden paper received a fair amount of citations, but after his latest reply to the most common criticisms there has been very little discussion. I'm not sure if it's because researchers aren't interested in the locality of many worlds or they don't know where to take it from there.

What are those elements of reality in Tipler's paper?

I like this question :) There are four "elements of reality" or local wave packets after Alice and Bob's measurements. One is Alice's copy of the "global state from Alice's perspective" when she measures q=0 at pos x=1, time t=1 and environment E, along with the other combinations:

Alice0(q=0,x=1,t=1,E) & Alice1(q=1,x=1,t=1,E)
Bob0(q=0,x=2,t=1,E) & Bob1(q=1,x=2,t=1,E)

After the measurement Bob moves to Alice and the wave packets evolve to x=1, t=2. The non-orthogonal wave packets overlap because now both have x=1, creating:

AB0(q=0,x=1,t=2,E) & AB1(q=1,x=1,t=2,E)

We need a degree of freedom to store q, but this is something Alice and Bob both can do, for example by writing it down - it's encoded in the degrees of freedom on the paper. It's not a global state, but local to that wave packet: even Alice0 and Alice1 disagree about the value. The majority of the local state (E) contains a rough copy of the world - everything that wave packet "knows" about its environment. If you are talking about Alice's experience, that knowledge must be encoded in the relations between atoms in the brain.

If you take a very simple system with limited degrees of freedom, the world it resides in will also be much simpler. Say Alice's lab doesn't have any spare degrees of freedom so they decide to erase the measurement of q completely; then there should be just one Alice system, overlapping with both Bob0 and Bob1. If you restrict Alice even further so she doesn't even know if it's Bob or Buster coming to meet her, she'll encounter both of them. So any local degree of freedom will suffice, but it actually does need to be recorded.

The global wavefunction then becomes the union of all local wave packets. This is how it's decomposed into local elements of reality and why Wallace's paper is relevant.

One of the aspects I'm unclear about above is how nature knows that Alice's q and Bob's q are the same basis, which is why I mentioned the shared reference frame before.

 

[Demystifier]

Okay. Last question for now: I probably missed it, but does your model, being fundamentally non-relativistic, have any strong explicit predictions about the existence, modification or non-existence of zitterbewegung? I ask mainly due to the arguments made in Hestenes 1990 that zitterbewegung need not be regarded as a purely relativistic phenomenon, instead amenable to a geometric algebra reinterpretation consistent with the Madelung reformulation of the SE.

For non-relativistic Bohmian treatment of spin see e.g. http://de.arxiv.org/abs/1305.1280 .
As you can see on Fig. 2, typical trajectories do not exhibit zitterbewegung.

 

[DarMM]

The global wavefunction then becomes the union of all local wave packets. This is how it's decomposed into local elements of reality and why Wallace's paper is relevant.

Firstly thanks for the post. I still don't fully understand, perhaps something just hasn't clicked yet. It's basically that I'd like to know what "global state from Alice's perspective" is mathematically. I'll elaborate.

From reading Wallace's paper I see that regarding local degrees of freedom he is essentially discussing his work with Timpson (and ideas like it) in:
Wallace, D., and C.G. Timpson. 2010. Quantum mechanics on spacetime I: Spacetime state realism. The British Journal for the Philosophy of Science 61(4): 697–727.

Where he also says, like Deutsch, that what are real are the reduced density matrices associated with a region, or (if one wishes to be abstract) the state on the local algebra of that region formed by restricting the global state (which is ultimately a non-pure state on the local algebra and thus a mixed state). Do you think that Tipler's local degree's of freedom are basically this or fundamentally different? If so, do you take the Deutsch-Wallace's or Tipler's view of what the local elements of reality are?

 

[akvadrako]

Do you think that Tipler's local degree's of freedom are basically this or fundamentally different? If so, do you take the Deutsch-Wallace's or Tipler's view of what the local elements of reality are?

In the latest Deutsch-Hayden paper (the vindication one), he says the factual information of each qubit or sub-network is encoded in its Heisenberg observables. A typical Schrödinger-picture density matrix is not enough because it misses some of the entanglement correlations with other sub-networks.

In Tipler's 2014 paper he is using pure states, it seems, though it's less clear. Each sub-system (Alice0, Bob1, ...) has its own pure-state view of the universe.

I think both approaches contain the same information. In contrast, the Wallace-Timpson view is explicitly non-local, though oddly they reference a companion paper that doesn't exist which is supposed to talk about the locality in the Deutsch-Hayden picture: Quantum mechanics on spacetime II: Quantum gauge freedom.

 

[DarMM]

Okay so I've read all these papers and the literature around them. Quite a morass I must say.

From the point of view of Algebraic QFT, one has the algebra of observables $\mathcal{A}$ and a state $\rho$ on that algebra. It is simply a fact that for a pure state it's restriction to the algebra of a region, i.e. $\rho|_{\mathcal{A}(\mathcal{O})}$, is going to be a mixed state.

At this point you can say that only pure states are ontic objects, like standard Everett MWI, in which case you have to accept that the element of reality is alocal, in a sense "outside of spacetime".

Or alternatively you can say that density matricies are real ontic objects, which allows $\rho|_{\mathcal{A}(\mathcal{O})}$ to be an element of reality, this is what Wallace and Timpson do in their Spacetime realism interpretation (an alternate form of MWI). However this to me is pretty odd, as the density matricies carry classical Kolmolgorov probaility, standard "ignorance" based probability. It seems strange to view this as an element of reality, as the density operator at a point describes statistics of experiments even in a $\psi$-ontic view!

Deutsch seems to take an even more radical approach, saying the ontic object associated to a region is $\mathcal{A}(\mathcal{O})$, the algebra itself and not the quantum state! This cannot be considered Everett MWI, but a new interpretation, as it is algebra-ontic not $\psi$-ontic like MWI. Now the evolution of $\mathcal{A}(\mathcal{O})$ and properties of regional algebras in general are local, so this is an advantage, but this is a completely undeveloped view. There's no measurement theory for example. It'll take a lot of work to show equivalence with standard QM. How does decoherence work? How does the algebra "effectively" become a function algebra in order to be classical? Will that be multiple function algebras or not, i.e. will there be more than one world?

As for Tipler, I have no idea what kind of mathematical object "Alice's pure state view" is and how it connects with $\rho|_{\mathcal{A}(\mathcal{O})}$, the entire paper is very vague to me and I don't see how it is demonstrating anything beyond no signalling.

 

[ftr]

Quite a morass I must say.

But isn't all this about some mathematical formalism but does not answer the real important question as to where is the electron in the hydrogen atom at some point in time.

 

[DarMM]

But isn't all this about some mathematical formalism but does not answer the real important question as to where is the electron in the hydrogen atom at some point in time.

No, Many Worlds attempts to explain what is actually going on. Although "where the electron is in the hydrogen atom at some point in time" might not have a valid answer depending on the interpretation. In some interpretations there is no electron in the hydrogen atom.

 

[ftr]

In some interpretations there is no electron in the hydrogen atom.

wow, I knew some "models" hinted at that but not an "interpretation", which interpretation is that?

 

[DarMM]

wow, I knew some "models" hinted at that but not an "interpretation", which interpretation is that?

Relational Block World so far as I understand it, some takes on QBism for interpretations of QM.

In QFT many would take this view even ignoring the issue of interpreting QM in general, since an electron number operator isn't defined on hydrogen states in the Hilbert space of two flavor QED or whatever QFT you are using that contains hydrogen.

This is a general feature of QFT where particle number isn't defined on the interacting Hilbert space.

 

[akvadrako]

Okay so I've read all these papers and the literature around them. Quite a morass I must say.

Thanks for the followup and such a clear post.

From the point of view of Algebraic QFT, one has the algebra of observables $\mathcal{A}$ and a state $\rho$ on that algebra. It is simply a fact that for a pure state it's restriction to the algebra of a region, i.e. $\rho|_{\mathcal{A}(\mathcal{O})}$, is going to be a mixed state.

At this point you can say that only pure states are ontic objects, like standard Everett MWI, in which case you have to accept that the element of reality is alocal, in a sense "outside of spacetime".

When taking the global wavefunction and restricting it to a spacetime region, you'll definitely have a mixed state. At the very least because multiple worlds will be occupying that region. But I don't see why that makes anything alocal; a mixed state is multiple pure states, so if pure states are ontic surely multiple pure states are too.

More practically, if you are restricting your view to a single region (say Alice's lab) and only considering a single world, then you should have a pure state again.

Deutsch seems to take an even more radical approach, saying the ontic object associated to a region is $\mathcal{A}(\mathcal{O})$, the algebra itself and not the quantum state! This cannot be considered Everett MWI, but a new interpretation, as it is algebra-ontic not $\psi$-ontic like MWI.

Hum, this is really beyond my understanding, but it directly contradicts a claim he reiterated at few times; that they are equivalent. I suppose one way out is to assume the algebra is encoded in $\psi$ itself. I'm not even sure what you mean by algebra - is that just the observables and the state?

 

[DarMM]

Hum, this is really beyond my understanding, but it directly contradicts a claim he reiterated at few times; that they are equivalent. I suppose one way out is to assume the algebra is encoded in ψ\psi itself. I'm not even sure what you mean by algebra - is that just the observables and the state?

Just the observables (not the state). See the last section of the vindication paper about "algebra-stuff". He says they'll be equivalent, maybe they are, but I think a theory with only observables as physically real needs a much more justification.

When taking the global wavefunction and restricting it to a spacetime region, you'll definitely have a mixed state. At the very least because multiple worlds will be occupying that region. But I don't see why that makes anything alocal; a mixed state is multiple pure states, so if pure states are ontic surely multiple pure states are too.

More practically, if you are restricting your view to a single region (say Alice's lab) and only considering a single world, then you should have a pure state again.

Firstly I just want to separate two things, I'm saying the claim that the global state is ontic is an alocal ontology. The view where the density matricies are ontic, not the global state (as Wallace and Timpson do), is not alocal as mentioned in #168.

The global pure state is alocal, that just follows from the fact that it is a state on the entire observable algebra $\mathcal{A}(\mathcal{M}^4)$, with $\mathcal{M}^{4}$ the spacetime and obtains nonseperable relations between spacelike regions. If this is ontic, then you have ontic alocality. That's not to say the theory is signalling, which is a separate issue. This is what motivates Wallace and Timpson to not have the global state as the ontic object, i.e. to reject wavefunction realism.

Secondly, Wallace and Timpson avoid this by saying that what's actually ontic are the local density matrices. This would make things local, but it's a very strange ontology. Density matrices contain normal Kolmolgorov probabilities and imbuing that with the meaning of multiple worlds seems odd, as you don't do that in any other application of Kolmolgorov probabilities. Also I'd have several questions about the ontology, what's the ontic difference between Kolmolgorov mixing and non-Kolmolgorov superposition, since both now exist? Also the density matrix normally only has statistical meaning, what is its non-statistical meaning?
Also note that proper and improper mixtures are the same ontic situation under this picture. It seems odd to me that there really is no difference between one electron of an entangled pair (improper mixed state) and an electron fired at random from a silver oven (proper mixed state). In this picture they are identical because the classical uncertainty in the latter case has ontic status.

 

[akvadrako]

Just the observables (not the state). See the last section of the vindication paper about "algebra-stuff". He says they'll be equivalent, maybe they are, but I think a theory with only observables as physically real needs a much more justification.

How does this require more than the well-known equivalence between the Heisenberg and Schrödinger pictures, given the algebra stuff is the Heisenberg observables?

Firstly I just want to separate two things, I'm saying the claim that the global state is ontic is an alocal ontology.

I did see how they were separate points, though for now I'm ignoring the Wallace/Timpson picture; it's not very compelling. Let me see if I understand what you are saying about the global pure state. You're saying that because when it's defined on an algebra without spacetime it's alocal by definition and when defined on an algebra with spacetime $(\mathcal{M}^4)$ as a primitive element that it contains global properties and becomes non-local.

This isn't the definition of *local I'm using. A theory is local if it possibly can be reformulated in terms of separate regions, so that actions on separated regions don't effect each other. And for the example of many worlds, those separate regions are located in configuration space (which includes spacetime dimensions).

Ignoring the algebra stuff part, Deutsch's paper shows you don't need to think about spacetime at all. If you show a theory is qubit-local, along with the universality of quantum computers and the fact they can be built in our spacetime, that demonstrates there is a spacetime picture available.

So to show many worlds is local, it would be enough to show that non-interacting qubits can't effect each other. Maybe there is a more intuitive way than Deutsch's paper. What defines a qubit but it's correlation with other qubits? So if we have a bunch of qubits in Bob's lab can Alice effect their correlations at all? Well, the amount of entanglement between her lab and Bob's lab stays the same. She can cause Bob's qubits to appear entangled from her point of view, but the information defining that entanglement is necessarily encoded in her lab. I'll think more about this.

 

[DarMM]

How does this require more than the well-known equivalence between the Heisenberg and Schrödinger pictures, given the algebra stuff is the Heisenberg observables?

Firstly I will say note they are only that equivalent in QM, in QFT the Schrödinger picture can require additional renormalizations, so they aren't unitarily equivalent as the Schrödinger picture will have a slightly different Hamiltonian due to the new counter terms.

Deutsch's algebra perspective moves beyond the equivalence of the Schrödinger and Heisenberg picture and has nothing to do with it really. That's just moving time evolution between the state and observables. He is staying that the quantum state could ultimately be eliminated from the theory as it has no ontic existence. When a measuring device and a particle interact, that is purely an interaction between the device's algebra-stuff and the particle's algebra-stuff, the state doesn't exist. It's only useful to convey constraints on measuring the algebra (i.e. extracting a real number from an algebra element) when you don't know the device algebra details, i.e. the state is epistemic.

This does make Deutsch's view local as the evolution of the algebra is provably locally, most generally in algebraic QFT where regional algebra are even called "local algebras". However it is typically viewed that the state is nonlocal, nobody questions the locality of the observable algebra. Another way of putting it is that the dynamics are thought to be local, but the states are not. Deustch avoids this by dropping the ontic status of the quantum state and thus all his ontic elements are local.

However the state not being ontic, but just an epistemic constraint, and the algebra being is very different from MWI or indeed any interpretation of QM, it's not just the Heisenberg picture. It's also a very undeveloped view.

If you retain the state as ontic, then Deutsch's proof is just a (interesting) Heisenberg picture demonstration of no-signalling, not locality.

You're saying that because when it's defined on an algebra without spacetime it's alocal by definition and when defined on an algebra with spacetime $\mathcal{M}^4$ as a primitive element that it contains global properties and becomes non-local.

The algebra doesn't have the spacetime as an element, it's a sheaf over the spacetime (and always a sheaf over a spacetime, can't be without it). The state is then nonlocal as it doesn't factorise across the algebras of regions.

This isn't the definition of *local I'm using. A theory is local if it possibly can be reformulated in terms of separate regions, so that actions on separated regions don't effect each other.

Before we continue and I think this might be core to the whole thing, I would add:
"can be reformulated in terms of ontic elements in separate regions, so that actions on separated regions don't effect each other".

If the restrictions to regions that don't effect each other are necessarily epistemic, then any locality demonstrations would only be non-signalling demonstrations, agreed?

 

[akvadrako]

Deutsch's algebra perspective moves beyond the equivalence of the Schrödinger and Heisenberg picture and has nothing to do with it really. That's just moving time evolution between the state and observables. He is staying that the quantum state could ultimately be eliminated from the theory as it has no ontic existence.

It doesn't seem like that's quite the argument...

It is true that such states – including |0> itself – like the laws of motion, the axioms of quantum mechanics, and the global topology of spacetime, are all elements of reality that do not have locations, and are ‘non-local’ in that sense – a sense in which all theories in physics are ‘non-local’. But they also have in common that they cannot be altered. Being universal constants, they are ‘independent of what is done to S1', and therefore their existence does not violate Einstein’s criterion of locality. To avoid confusion, they should be called ‘global’, not ‘non-local’.​

So if all the time evolution is moved to the observables and the state never changes, then the state doesn't provide any way for distant systems to effect each other. Would you call something like the constant global state, the laws of physics or other constants epistemic?

If you retain the state as ontic, then Deutsch's proof is just a (interesting) Heisenberg picture demonstration of no-signalling, not locality.

It's more than just no-signaling, it's Einstein's criterion of locality. Even taking the state as ontic, it's saying that all the ontic elements in a region are unaffected by events in distant regions. The local algebra can't be changed (by proof) and the state can't be changed (by definition).

The algebra doesn't have the spacetime as an element, it's a sheaf over the spacetime (and always a sheaf over a spacetime, can't be without it). The state is then nonlocal as it doesn't factorise across the algebras of regions.

That does sound more precise that what I said and basically what I was thinking. And I see how it looks non-local, but that doesn't mean the situation it's representing is non-local. I know that's not a very clear way to phrase it, but I'm saying it could be a consequence of, for example, ignoring some information via the factorization. In this case, isn't it considering the overlap of all the worlds at each spacetime point? That is a very strange thing to do. Take worlds and regions $W_1\{r_1=\uparrow,r_2=\uparrow\}, W_2\{r_1=\downarrow,r_2=\downarrow\}$ and factor by region, then throw away the world labels. It take a perfectly sensible situation and gives you nonsense: $r_1\{\uparrow,\downarrow\} \times r_2\{\uparrow,\downarrow\}$.

Before we continue and I think this might be core to the whole thing, I would add:
"can be reformulated in terms of ontic elements in separate regions, so that actions on separated regions don't effect each other".

If the restrictions to regions that don't effect each other are necessarily epistemic, then any locality demonstrations would only be non-signalling demonstrations, agreed?

I'm having trouble parsing that question, so let me try by stating my own understanding of non-signaling and locality.

• non-signaling — observer A cannot communicate any information to distant observer B. This also implies system A can't do anything which will have measurable consequences on system B.
• locality — also called separability. Given a composite system A+B, the most complete description possible of the composite system can be written in terms of the most complete description possible of A & B separately. Also, as long as A & B don't interact, events at A can have no effect on the description of B.

This probably also gives a clue about my understanding of the terms ontic and epistemic. Ontic is the most complete description possible. It doesn't mean it's a fundamental description, but within the framework you're using, there is nothing you can learn which will increase your knowledge of the system. Thus it will give you maximum predictive power. Epistemic would then be about what partial knowledge, by say throwing aware a dimension to simplify a problem.

 

[DarMM]

It doesn't seem like that's quite the argument...

The state is a constraint in Deutsch's theory, a relation between the objects, it's an element of the theory in the same sense that the action is in classical mechanics, a constraint among the ontic objects. This is what he seems to be arguing to me when he says only the algebra exists, the state merely is a "law of physics" for them, but it's not an ontic element of the theory, i.e. in Classical Mechanics you could move between the Hamiltonian and the Lagrangian as ways of expressing the constraints on the ontic elements (the particles), but the ontic elements are just the particles.

At least that is how his argument seems to me.

So if all the time evolution is moved to the observables and the state never changes, then the state doesn't provide any way for distant systems to effect each other. Would you call something like the constant global state, the laws of physics or other constants epistemic?

I don't think the state is the same as "a law of physics" in Deutsch's paper, even though Deutsch claims that what it is. Basically I'm just not taking the claim that it's exactly the same as a law of physics at face value because Deutsch says so. If it were I would be unjustified in my claim, but again that's the issue here.

In Deutsch's view the state is a "law of physics" in a sense, as it expresses a constraint among the actual ontic elements, the algebra constituents. However what is the constraint? The average real number you will extract from an algebra element when you measure it (Deutsch himself says this). That is an epistemic concept to me as it attaches to repeated experimental runs. This is exactly the same point Timpson makes here: https://arxiv.org/abs/quant-ph/0312155

I agree with the vindication paper overall and he doesn't seem to deny Timpson on this point alone, i.e. the meaning of the state is the expected real number you'd extract from an algebra element.

Now maybe it is ontic, maybe you can show that interaction among algebra elements leads to Boolean subalgebra worlds splitting into ones where the element is effectively a real number and averaged across worlds that value agrees with the one you'd get from the state, so the state just encodes something you can prove about the evolution of the algebra. However that would need to be shown.

So I agree that it is a global constant, but it seems to be an epistemic one.

Deutsch's proof can be taken as proof of locality for an interpretation where the world is made of C*-algebra elements (that's pretty cool, it makes objects way more complex than the usual picture) and there is a primitive global notion of the average experimental value associated with each element. The latter seems to introduce measurement as a primitive (like Copenhagen).

To remove measurement as a primitive you'd need to show that average value represents a shorthand for interactions among a microscopic algebra and a large scale algebra.

I'm having trouble parsing that question...
This probably also gives a clue about my understanding of the terms ontic and epistemic. Ontic is the most complete description possible

Probably my bad phrasing. Let me be clearer, in your view, in a subregion of spacetime $\mathcal{A}$, what mathematical objects are the ontic elements that don't interact with similar elements at another spacetime region $B$? The local density matrices? The algebra elements?

 

[DarMM]

A way of seeing this is that even in a modestly sized region of spacetime a typical algebra element will be millions of dimensions in size. The state then associates a single real number with each element. I would expect that to be shown from interactions between the system algebra and the device algebra, having an ontic global "average value extracted from algebra element" is a bit strange to me.

If we have a region $\mathcal{O}$, then the subalgebra of a system $\mathcal{E} \subset \mathcal{A}(\mathcal{O})$ and that of a device $\mathcal{F} \subset \mathcal{A}(\mathcal{O})$ apparently interact in some way that embed $G \in \mathcal{E}$ not in its true algebraic form but as a collection of real numbers in $\mathcal{F}$, each real number being a "world" and $\mathcal{F}$ gains the effective form of a boolean algebra (device decoheres). Then the average value attached, weighted by the manner $\mathcal{F}$ turns Boolean is $\rho(G)$.

This needs to be shown, also this is obviously not just trivially the same thing as Everettian Many Worlds where the state is real.

 

[akvadrako]

So I agree that it is a global constant, but it seems to be an epistemic one.

I can see now how the state is epistemic. It seems like indexical information, akin to the particle location in dBB. Timpson's mentions this:

Now the qi(t) capture the effects of our sequence of unitary operations for all initial states. Thus their time evolution can be said to depict the histories of the entire set of possible worlds; whilst the world from amongst these that is realized is determined by which initial state is chosen. However, when we move to the ontological view, the very same structure (the sequence of time evolving qi(t)) only represents a single world, as the choice of initial state is a fixed part of the formalism. What seems like it can represent a range of possible worlds, we are to suppose, can only represent a single one; and conversely, the structure being used to describe a single world in the ontological Deutsch-Hayden picture is one we know in fact to be adequate to describe a whole set of possible worlds in quantum mechanics. Thus the Deutsch-Hayden picture, taken ontologically, would seem to be extremely, perhaps implausibly, extravagant in the structure it uses to depict a single world.​

This would indeed imply the state is arbitrarily chosen. If we are talking about the universal wavefunction, then the basis is completely arbitrary, so an initial state like $|0\rangle$ would be quite unnatural. Though when talking about specific situations like our pocket universe, then it seems like the state should be at least partially specified; at least all the dimensions that have affected measurement results.

Well, this changes my view. Now I understand that what their analysis is showing is that a fixed state leads to locality. And the fixed state corresponds to one world-line. I will have to re-read some of the Deutsch-Hayden papers with this in mind. Perhaps what they're saying isn't that the state is fixed, just constant.

I would expect that to be shown from interactions between the system algebra and the device algebra, having an ontic global "average value extracted from algebra element" is a bit strange to me.

Does this average value effect the evolution of the system? It can't change the algebra or the state. It looks like it should be given by the Born rule and be a measure of world density.

Probably my bad phrasing. Let me be clearer, in your view, in a subregion of spacetime $\mathcal{A}$, what mathematical objects are the ontic elements that don't interact with similar elements at another spacetime region $B$? The local density matrices? The algebra elements?

My answer will be basically the same as before, but I'll try again. In my view, the local information must include both the algebra and the state vector, though either can be considered constant. If we are only considering one copy (one world) of $\mathcal{A}$, then all the local qubits must be entangled, $\mathcal{A}$ is pure, containing all of $\mathcal{A}$'s information about $B$. If we ignore some local information then $\mathcal{A}$'s view of $B$ would be mixed, but that's not an ontic operation.

However, due to self-locating uncertainty, we should consider all worlds containing $\mathcal{A}$. I suppose that makes it a mixed state, but then we aren't really anymore talking about just that one universe with $\mathcal{A}$ and $B$. And if we consider the global wavefunction, I don't know if it begins to look like a pure state again.

There is something else important about my view that I'm not sure I need to clarify. The quantum description exists on the boundary between systems and is symmetric, $\mathcal{A} \rightarrow \psi \leftarrow B$. The boundary doesn't contain complete information about the other system, so they will evolve in different but overlapping worlds. So the description of region $\mathcal{A}$ from $\mathcal{C}$'s viewpoint is different than from $B$'s. $B$'s model of $\mathcal{A}$ is actually stored in $B$'s region (or the B/A boundary).

The ontic description of $\mathcal{A}$ would then be the union of all other viewpoints. The important part is normally we don't consider all viewpoints from all worlds (though I suspect that's $\mathcal{A}$'s experience) but just $\mathcal{C}$'s view of them, which is where that important global state comes from.