I What is the current status of Many Worlds?

[Jarvis323]

Even Philosophers should recognize this logical fallacy, namely No True Scotsman.

You've missed the mark on this one. I was pointing out the conflation between philosopher and philosophy. This is the mistake you're making now.

Nope. None of those things were given to use by Philosophy. The scientific method, to the extent that term even has a definite meaning, was given to us by scientists. Formalization of computation, logic, and axiomatic systems was given to us by mathematicians. The fact that some of the people who contributed to those things also billed themselves as philosophers does not mean Philosophy gets to take credit for those things.

It sounds like your concern is with which group of people get credit for something, and which identify they place on themselves.

 

[PeroK]

Philosophy has given us the scientific method, formalization of computation, logic, and axiomatic systems in mathematics.

There's no doubt that mathematics and the natural sciences began as part of philosophy - but, that was the problem. Real progress only began when science and mathematics were freed from religion and philosophy.

The great advances in mathematics in the 19th century are all mathematical. There's no great philosophy behind real and complex analysis, differential geometry and vector calculus, for example. It's just hard-nosed mathematics.

To take three concrete examples from the 20th Century: General Relativity, Goedel's Incompleteness Theorem and the Turing machine. Wittgenstein and Popper could not possibly have produced anything so definitive and concrete. Not by doing philosophy.

I know Wittgenstein wrote and thought about mathematics, but because (unlike Einstein, Goedel and Turing) he didn't actually do mathematics, there is nothing left to show for his efforts. He was never going to produce the theorems that settled the question of mathematical decidability, for example. Undecidability would have remained a point of philosophical debate, rather than an established theorem.

That's the difference. Philosophy can never really resolve anything - and, perhaps the endless shifting sands are what philosophers like about it. And, perhaps, that's what makes them generally poor scientists and mathematicians.

 

[Jarvis323]

There's no doubt that mathematics and the natural sciences began as part of philosophy - but, that was the problem. Real progress only began when science and mathematics were freed from religion and philosophy.

The great advances in mathematics in the 19th century are all mathematical. There's no great philosophy behind real and complex analysis, differential geometry and vector calculus, for example. It's just hard-nosed mathematics.

To take three concrete examples from the 20th Century: General Relativity, Goedel's Incompleteness Theorem and the Turing machine. Wittgenstein and Popper could not possibly have produced anything so definitive and concrete. Not by doing philosophy.

I know Wittgenstein wrote and thought about mathematics, but because (unlike Einstein, Goedel and Turing) he didn't actually do mathematics, there is nothing left to show for his efforts. He was never going to produce the theorems that settled the question of mathematical decidability, for example. Undecidability would have remained a point of philosophical debate, rather than an established theorem.

That's the difference. Philosophy can never really resolve anything - and, perhaps the endless shifting sands are what philosophers like about it. And, perhaps, that's what makes them generally poor scientists and mathematicians.

Maybe our disagreement is simply about what philosophy is and what it isn't. I think I may be thinking too literally.

To me, Goedel's Incompleteness Theorem is a work of philosophy. And the Church Turing thesis that underpins the Turing Machine is also in my opinion.

 

[PeterDonis]

I was pointing out the conflation between philosopher and philosophy.

If Philosophy does not mean what philosophers actually do, then it's a meaningless term. Anyone can make it mean whatever they want by just saying that the doings of any "philosopher" who doesn't do what they want Philosophy to mean don't count as Philosophy. That's what you were doing.

It sounds like your concern is with which group of people get credit for something, and which identify they place on themselves.

You're the one who said Philosophy gave us all these great things. That's a claim of credit. I was simply pointing out that it's not a justified claim of credit.

 

[Jarvis323]

If Philosophy does not mean what philosophers actually do, then it's a meaningless term.

This is a highly imprecise and logically problematic statement. But since we aren't philosophers I guess we don't use logic anyways?

That's what you were doing.

This isn't at all what I was doing.

 

[PeterDonis]

Maybe our disagreement is simply about what philosophy is and what it isn't.

Quite possibly, yes. See below.

To me, Goedel's Incompleteness Theorem is a work of philosophy. And the Church Turing thesis that underpins the Turing Machine is also in my opinion.

I have never seen anyone else take this position; everyone else that I'm aware of considers these to be works of mathematics, not philosophy. Of course you are free to adopt your own idiosyncratic definition of "philosophy" for your own use, but that doesn't mean you should expect everyone else to go along with it.

This is a highly imprecise and logically problematic statement. But since we aren't philosophers I guess we don't use logic anyways?

This isn't at all what I was doing.

Evidently we disagree, and it doesn't look like our disagreement will be resolved here. Which is fine, in this particular forum it is understood that many topics discussed will not be amenable to a definite resolution. But, once again, while you can adopt your own definitions of terms for your own use, that doesn't mean you should expect everyone else to use them.

 

[Jarvis323]

When an architect uses the pythagorean theorum, it seems not controversal that they are doing math.

I don't see why it should be different for philosophy. If a physicist uses logic or inductive reasoning, they are doing philosophy. Maybe it is more complicated, due to connotations about philosophy?

I sense that here people have certain branches of philosophy in mind, e.g. metaphysics.

 

[PeterDonis]

If a physicist uses logic or inductive reasoning, they are doing philosophy.

I think most people would consider logic and inductive reasoning to be general tools applicable to a variety of disciplines, not philosophy. If they are part of any particular discipline, I think most people would say that discipline is mathematics.

 

[PeterDonis]

When an architect uses the pythagorean theorum, it seems not controversal that they are doing math.

No, they are doing architecture, using math as a tool. One can similarly do philosophy using logic and inductive reasoning as a tool. That does not mean logic and inductive reasoning are philosophy, any more than math is architecture.

 

[Jarvis323]

I think most people would consider logic and inductive reasoning to be general tools applicable to a variety of disciplines, not philosophy. If they are part of any particular discipline, I think most people would say that discipline is mathematics.

I guess maybe you're right. I've been taking people's rejection of the use of philosophy in physics as a rejection of the tools of philosophy, or formal methods of reasoning in general, in physics.

It's still not clear to me where the line is between using/doing philisophy, and just using tools that were origionally developed as tools of philosophy. Philosophy is generally the study of discovering truth and knowledge, and its tools are methods of reasoning and analysis. I guess every discipline used to be a branch of philosophy.

I'm still not exactly sure what people are rejecting when they say they reject philosophy in physics. It would help to be more precise.

 

[PeterDonis]

tools that were origionally developed as tools of philosophy.

I don't think logic and inductive reasoning were originally developed as tools of philosophy. If they were originally developed as tools of anything, it was mathematics.

Philosophy, perhaps, has used these tools to try to discover propositions with more generality than any other discipline, which is both a strength and a weakness. It's a strength because the more general a proposition is, the more ways you can use it. It's a weakness because the more general a proposition is, the less it is likely to be able to tell you. In science, in particular, I think the weakness weighs much more than the strength.

 

[Demystifier]

I don't think logic and inductive reasoning were originally developed as tools of philosophy.

They were. https://en.wikipedia.org/wiki/Term_logic

 

[Lord Jestocost]

Remind me - what does ontology mean?

When it is allowed to answer:

Ontology is an easy concept. Maybe, some get mixed up with the term “ontology” because there is a meta-question which questions – so to speak – the basic question of ontology itself. David J. Chalmers puts it in a nutshell in “Ontological Anti-Realism”:

“The basic question of ontology is ‘What exists?’. The basic question of metaontology is: are there objective answers to the basic question of ontology? Here ontological realists say yes, and ontological anti-realists say no.” [bold by LJ]

That’s it. It’s only the meta-question which unsettles physicists who cling with ferocity to the concepts of daily life, which – in the scientific language of physics – may be refined to the concepts of classical physics.

 

[haushofer]

It is? Then what is considered the "modern" formulation of Bohmian mechanics? I thought the whole point of BM was to have a deterministic equation for the time evolution of particle positions, just like in classical mechanics. To do that you need the quantum potential.

No, determinism is not the primary concern of BM; ontology (a realist's form of it) is.

 

[PeterDonis]

They were. https://en.wikipedia.org/wiki/Term_logic

Aristotle may have been the first to explicitly codify these tools, but he wasn't the first to discover or use them. The Greek geometers were already using them (and the Greeks weren't the first to do geometry either), though they did not systematize their use of them until Euclid, who was a rough contemporary of Aristotle.

 

[Sunil]

I think many worlds is not even an interpretation. It is a big confusion. It would be better named "many words". Or, in more detail, "many not well-defined words". The basic objection: If there would be many worlds, then the notion of probability would simply not make sense.

 

[martinbn]

I think many worlds is not even an interpretation. It is a big confusion. It would be better named "many words". Or, in more detail, "many not well-defined words". The basic objection: If there would be many worlds, then the notion of probability would simply not make sense.

That is true for most interpretations. By definition an interpertation is many words added on to the core QM.

 

[gentzen]

I think MWI as a project was completed by Consistent histories.

I am familiar with both MWI and Consistent histories, and I get sad when MWI and Consistent histories are mixed up by people unfamiliar with Consistent histories. (I am not familiar with the Decoherent histories variant by Gell-Mann and Hartle, and I don't get David Wallace's point in section 3.7 Decoherent histories, and cannot see the connection to the Consistent histories framework I am familiar with.)

But you are the Consistent histories expert here, so if you acknowledge that there is that connection to MWI, then I guess it is really there, and it is my fault that I don't get it. From my perspective, Consistent histories seems to work with the statistical operator, and a "reasonably complete" mathematical description. MWI seems to insist on a pure wavefunction, doesn't use a "more complete" mathematical description compared to CI, and instead seems to invoke philosophical arguments for getting a "reasonably complete" description of the observed phenomenology.

Many people tried to get somewhat familiar with MWI. Finding people familiar with even the most basic notions of Consistent histories seems difficult. Understanding the basic notion of Consistent histories seems easy to me, compared to understanding the philosophy behind MWI. More fundamental, Robert Griffith or Roland Omnès don't seem to claim that Consistent histories is a complete interpretation of quantum theory, but MWI proponents seem to make exactly that claim with respect to MWI.

 

[Morbert]

I am familiar with both MWI and Consistent histories, and I get sad when MWI and Consistent histories are mixed up by people unfamiliar with Consistent histories. (I am not familiar with the Decoherent histories variant by Gell-Mann and Hartle, and I don't get David Wallace's point in section 3.7 Decoherent histories, and cannot see the connection to the Consistent histories framework I am familiar with.)

But you are the Consistent histories expert here, so if you acknowledge that there is that connection to MWI, then I guess it is really there, and it is my fault that I don't get it. From my perspective, Consistent histories seems to work with the statistical operator, and a "reasonably complete" mathematical description. MWI seems to insist on a pure wavefunction, doesn't use a "more complete" mathematical description compared to CI, and instead seems to invoke philosophical arguments for getting a "reasonably complete" description of the observed phenomenology.

Many people tried to get somewhat familiar with MWI. Finding people familiar with even the most basic notions of Consistent histories seems difficult. Understanding the basic notion of Consistent histories seems easy to me, compared to understanding the philosophy behind MWI. More fundamental, Robert Griffith or Roland Omnès don't seem to claim that Consistent histories is a complete interpretation of quantum theory, but MWI proponents seem to make exactly that claim with respect to MWI.

The sentiment that consistent histories is continuous with many-worlds and in a sense completes it is expressed most explicitly by Hartle ( https://philpapers.org/rec/SAUMWE-3 )

"These conferences marked 50 years of Everett’s formulation of quantum theory. But they were only a year away from marking 25 years of the decoherent (or consistent) histories quantum theory that can be viewed as an extension and to some extent a completion of Everett’s work"

At the same time MW proponents see consistent histories research as in service of many-worlds. It provides machinery for exploring branching, assigning them probabilities, quantifying decoherence etc.

MW proponents regularly balk at consistent histories and have made plenty of objections, so it's certainly a hill that needs to be defended. I acknowledge the qualitative distinctions between the MW and CI camps.

 

[Quantumental]

Isn't Consistent Histories just MWI with more coherent decoherence math? I've always struggled to separate the two. Sure in CH/DH you can postulate that post-decoherence only one subset is 'real', but that does not seem to be derived from any of its math. In fact, I am not sure why they are even categorized as different. I don't see any bigger difference between CH/DH and MWI than I do with say the Oxford interpretation of Everett, classical Everett or Everett interpreted by Lev Vaidman for instance

 

[Quantumental]

As far as I know there is no way to derive Born's rule in MWI, so the theory cannot make any predictions. There is no way to ascribe any meaning for probabilities in MWI.

This I do agree with and in many ways were the inspiration for the title of this post; even among the most ardent proponents of MWI there is fierce disagreement on this topic. Decision-Theoretic approach by David Deutsch and David Wallace vs Lev Vaidman's view for instance, another one that comes to mind is Zurek's Envariance approach.

Some conclude that this renders MWI not only wrong, but 'not even wrong', since you literally can't call it science, but it could still be true, it would simply mean that we would have to accept that we are living in a universe where probabilities make sense and that there's infinite other versions of us where the Born Rule is violated repeatedly. This 'incoherence' problem would usually turn people off in the same way that Solipsism does: "Sure, it could be true, but it makes no testable predictions and its consequences mean that there is no science or logic to follow." which does not tend to sit well with either experimentalists or theoreticians. For some reason though, with MWI's fanfare over the past 20 years, this 'elegance' only reinforces their stance. It's fascinating.

 

[gentzen]

Isn't Consistent Histories just MWI with more coherent decoherence math? I've always struggled to separate the two.

You try to tease me? Let me check: Can you tell me the basic notion(s) of Consistent histories?

 

[Quantumental]

You try to tease me? Let me check: Can you tell me the basic notion(s) of Consistent histories?

Unless you're injecting a 'slight of hand' indeterministic element, how does CH derive a singular clear history which represents reality?

 

[gentzen]

Unless you're injecting a 'slight of hand' indeterministic element, how does CH derive a singular clear history which represents reality?

No, that is not related to the basic notion(s) of CH. Want to try again? By the way, CH does not claim to be a deterministic interpretation, and its indeterminstic elements are not 'slight of hand'.

 

[PeroK]

On a point of English spelling, it's "sleight of hand".

https://www.dictionary.com/browse/sleight-of-hand

 

[AndreiB]

Can you tell me the basic notion(s) of Consistent histories?

It seems to me that CH looks like a hidden variable theory (the measured values are "there" before measurement and measurements reveal them). This allows them to provide an explanation for EPR correlations. But then they add the idea that all frameworks are equally valid. I cannot understand this. If I measure the Z-spin, in what sense is the X-framework valid? The X-framework does not model the experiment being done and says nothing about what was observed.

 

[AndreiB]

This I do agree with and in many ways were the inspiration for the title of this post; even among the most ardent proponents of MWI there is fierce disagreement on this topic. Decision-Theoretic approach by David Deutsch and David Wallace vs Lev Vaidman's view for instance, another one that comes to mind is Zurek's Envariance approach.

I think the decision-theoretic approach is circular. It assumes that rational agents exist. I see no reason to grant them this assumption. It might be that in a world described by MWI rational agents cannot exist.

Vaidman introduces a "measure of existence". I don't understand the meaning of this. Are the copies in a world with a smaller amplitude less real? Do they develop a sloppy memory so that they "forget" their past measurements? This should not happen since the copies are supposed to be identical in all respects except for the observed measurement outcome.

I am not familiar with Zurek's Envariance approach. I'll try to look at it.

For some reason though, with MWI's fanfare over the past 20 years, this 'elegance' only reinforces their stance. It's fascinating.

Yeah, I agree. Even Weinberg was for some reason compelled to some extent by MWI. It's hard to understand why.

 

[gentzen]

But then they add the idea that all frameworks are equally valid. I cannot understand this. If I measure the Z-spin, in what sense is the X-framework valid? The X-framework does not model the experiment being done and says nothing about what was observed.

Nice, somebody who is somewhat familiar with CH. In fact, the answer from Omnès to your question about "in what sense the X-framework is valid" is quite in line with your comment. I once quoted from “Quantum Philosophy” by Roland Omnès parts of the replies to such objections:

Some of them will lead to the same conclusions and they are just as good. Others will be useless, not necessarily wrong but only idle talk of no consequence. Why bother? Asking questions about the existence of useless histories amounts to performing calculations that are of no help in solving a problem. They belong in the waste-paper basket.

 

[Morbert]

It seems to me that CH looks like a hidden variable theory (the measured values are "there" before measurement and measurements reveal them).

CH as a formalism is itself open to different interpretations. A realist interpretation is most commonly put forth by Robert Griffiths ( https://arxiv.org/abs/quant-ph/0001093 ), but an antirealist interpretation, where histories are only useful building blocks for constructing reliable logical relations between different experimental outcomes at different times, is also possible and probably a lot less contentious.

Griffiths's realist interpretation is still distinct from a standard hidden variable theory in important ways.
i) In CH, all variables are accessible by experiment, and all variables are the typical variables of standard QM/QFT (we don't need an additional state space where some physical state, distinct from the quantum state, resides). If we want to reveal spin-z, we measure spin-z. If we want to reveal spin-x, we measure spin-x. We cannot measure both spin-x and spin-z, but there is no subspace corresponding to "both spin-x and spin-z". It's not that it is hidden. There is nothing there to measure, as there is no framework that gives sense to "both spin-x and spin-z"
ii) The realism in CH is local ( https://arxiv.org/abs/0908.2914 ), while your standard hidden variable theory is nonlocal .

Gell-Mann has published an extended probability formalism which has an immediate realist interpretation ( https://arxiv.org/abs/1106.0767 ), and does invite comparison with hidden variables:
"Is this in effect a hidden variable theory? There are no variables involved beyond the usual quantum fields of sum-over-histories quantum theory — the {q(t)}. However their fine-grained values are not completely accessible to experiment or observation and therefore partially hidden"

 

[Fervent Freyja]

They were. https://en.wikipedia.org/wiki/Term_logic

I know you probably already know this. But for those who don’t: Plato first introduced the idea of using universals in his attempts to understand reality. His idea was to find something in nature that had unchanging properties. He believed that finding an unchanging property of nature was the only place that we could start if we wanted real knowledge about our reality. His idea of constants and universals was passed onto physics later. That’s what philosophy does and is supposed to do- it hands ideas over to the sciences and humbly accepts that it will never receive credit for it.

Also, Bertrand Russell and Alfred Whitehead were almost successful in describing much of mathematics using logic alone. The father of logic himself considered logic a branch of philosophy. There are probably a thousand more examples of where philosophy handed over ideas to the sciences. I am too tired to even try to argue this any further right now.

Edit: Descartes invented analytical geometry and introduced skepticism as an essential part of the scientific method. He is considered the father of philosophy.