# What is the current status of Many Worlds?

PeroK
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2020 Award
I wonder who the mother was?

Demystifier
Gold Member
I wonder who the mother was?
Science is not created by sexual reproduction, so the existence of father does not imply the existence of mother.

Aanta, Fervent Freyja, vanhees71 and 1 other person
hutchphd
Homework Helper
I am astonished that William Gilbert appears nowhere in this list (did I miss him?)

vanhees71
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"
I think there seems to be the suggestion of a special status for some systems among other systems and I prefer interpretations concerned with maintaining equivalence of all systems. If so, regarding IGUS, is there a definition that answers questions like whether a system’s status is subject to change? (E.g., were Gell-Man’s frog suffering brain damage and losing calculating skills, would it lose the status to some extent or altogether?) Also, do these articles suggest a knowledge or mind dependent reality?

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I think there seems to be the suggestion of a special status for some systems among other systems and I prefer interpretations concerned with maintaining equivalence of all systems. If so, regarding IGUS, is there a definition that answers questions like whether a system’s status is subject to change? (E.g., were Gell-Man’s frog suffering brain damage and losing calculating skills, would it lose the status to some extent or altogether?) Also, do these articles suggest a knowledge or mind dependent reality?
For clarification, could expand on what you mean by special status for some systems?

From the Gell-Mann, 2011 paper you linked on page 5, section III, “Settleable bets, records, and decoherence”, there is a discussion of (some?) systems that might include frogs, ...”Information gathering and utilizing systems (IGUSes) like ourselves exploit the regularities summa- rized by physical theories to construct schemata, make predictions, and direct behavior [3]. Even a frog aiming to catch a fly can be said to be exploiting a rudimentary classical approximation to quantum mechanics (perhaps hard wired). The frog is in effect betting on these regularities”

From the Gell-Mann, 2011 paper you linked on page 5, section III, “Settleable bets, records, and decoherence”, there is a discussion of (some?) systems that might include frogs, ...”Information gathering and utilizing systems (IGUSes) like ourselves exploit the regularities summa- rized by physical theories to construct schemata, make predictions, and direct behavior [3]. Even a frog aiming to catch a fly can be said to be exploiting a rudimentary classical approximation to quantum mechanics (perhaps hard wired). The frog is in effect betting on these regularities”

I'm not sure if this answers your question but: From the decoherent histories perspective, if we want a quantum theory of our entire universe that includes a description of systems like frogs, we need the triple ##(\rho,H,\{C_\alpha\})##. The terms ##\rho## and ##H## are the initial conditions of our universe and its dynamics respectively. These make up the core of our physical theory. ##\{C_\alpha\}## are a set of mutually exclusive alternative histories of our universe, each assigned a probability computed from ##\rho, H##. There is great freedom in how we construct ##\{C_\alpha\}##, and if we want to discuss frogs, we should construct histories of quasiclassical variables (i.e. averages of conserved quantities over small volumes) that make up systems like frogs. Let's call this set of histories ##\mathcal{F}##. In this framework, we can identify frogs, and the environmental variables they correlate with (like flies, predators, weather, etc). What Gell-mann says is we can understand the behaviour of systems like frogs in terms of bets. E.g. A frog bets (instinctively), that if it does not eat flies, it will die, and our quantum theory could be used to show that this is true. Histories containing frogs that don't eat flies and live a long time will have a probability of almost 0. Our quantum theory supplies the reliable odds that systems like frogs or humans should adhere to for their self-interest.

But these frogs are not special as far as the theory is concerned. An alternative set of histories ##\mathcal{F}'## might not have any quasiclassical variables. No quasiclassical variables, no planets, no rain forests, no frogs etc. This alternative set is not mutually exclusive with ##\mathcal{F}##. It's just that it's not suitable for the purposes of describing systems like frogs. Both ##\mathcal{F}## and ##\mathcal{F}'## are equally valid, and our choice of a set does not elevate that set to some privileged ontic status.

All Gell-Mann's EPE formalism in that paper really does is let us relate ##\mathcal{F}## and ##\mathcal{F}'## by a common refinement of fine-grained histories, each of with are assigned an extended probability by our theory.

*now*, Demystifier and gentzen
Demystifier
Gold Member
our choice of a set does not elevate that set to some privileged ontic status.
My (and not only my) main problem with CH interpretation is that it is not clear what does elevate some set to a privileged ontic status. And if nothing does (or if only measurement does), then is CH interpretation any more ontic/realistic than the "standard" Copenhagen interpretation?

is not clear what does elevate some set to a privileged ontic status
I would say that Roland Omnès simply accepts that CH is an incomplete interpretation. The advantage over the "standard" Copenhagen is a greater logical clarity, not being more ontic per se. But if you would decide to arbitrarily declare some set of histories (for example those based on particle positions) to be more ontic than others, then the resulting interpretation would still be consistent, and be more ontic than Cohenhagen.

Aanta and Demystifier
My (and not only my) main problem with CH interpretation is that it is not clear what does elevate some set to a privileged ontic status. And if nothing does (or if only measurement does), then is CH interpretation any more ontic/realistic than the "standard" Copenhagen interpretation?
That's the beauty. None are ever elevated. The use of one set over another is a matter of which set is fit for purpose, rather than which set is real. Quoting Robert Griffiths

"The choice of family is up to the physicist, and will generally be made on the grounds of utility, e.g., how to model a particular experimental situation with apparatus put together by a competent experimentalist. This choice has no influence upon what really goes on in the world, and alternative choices applied to the same set of data and conclusions will yield consistent results. The quantum world is of such a nature that it can be described in distinct ways which (in general) cannot be combined into a single all-encompassing description"

I would say that Roland Omnès simply accepts that CH is an incomplete interpretation. The advantage over the "standard" Copenhagen is a greater logical clarity, not being more ontic per se. But if you would decide to arbitrarily declare some set of histories (for example those based on particle positions) to be more ontic than others, then the resulting interpretation would still be consistent, and be more ontic than Cohenhagen.
Would you have a source re/ any thorough discussion of the completeness of CH by Omnes. I'm quite familiar with his early work ("Interp. of QM" and "Undestanding QM") but I have not read Quantum Philosophy.

Demystifier
Would you have a source re/ any thorough discussion of the completeness of CH by Omnes. I'm quite familiar with his early work ("Interp. of QM" and "Undestanding QM") but I have not read Quantum Philosophy.
"Interp. of QM" was too technical for me, but I am familiar with "Understanding QM" (I didn't read it cover to cover, but I did read many parts and had the impression that I understood those parts). Much later I read "Quantum Philosophy" cover to cover. Two surprising claims from "Understanding QM" no longer occurred, but I will have to lookup specifics later at home.

In "Quantum Philosophy," R. Omnès is pretty clear that he is convinced that there is only a single world, even if CH doesn’t explain it, and cannot even disprove Many-Worlds. His defense is to admit that there is still a disagreement left between Reality and quantum theory, but that it would be hubris to expect otherwise. At least that is how I interpret his words on page 214:
… have reproached quantum physics for not explaining the existence of a unique state of events. It is true that quantum theory does not offer any mechanism or suggestion in that respect. This is, they say, the indelible sign of a flaw in the theory, … Those critics wish at all costs to see the universe conform to a mathematical law, down to the minutest details, and they certainly have reason to be frustrated.

I embrace, almost with prostration, the opposite thesis, the one proclaiming how marvelous, how wonderful it is to see the efforts of human beings to understand reality produce a theory fitting it so closely that they only disagree at the extreme confines. They must eventually diverge, though; otherwise Reality would lose its nature proper and identify itself with the timeless forms of the kingdom of signs, frozen in its own interpretation. No, science’s inability to account for the uniqueness of facts is not a flaw of some provisional theory; it is, on the contrary, the glaring mark of an unprecedented triumph. Never before has humanity gone so far in the conquest of principles reaching into the heart and the essence of things, but that are not the things themselves.

Demystifier
Gold Member
That's the beauty. None are ever elevated. The use of one set over another is a matter of which set is fit for purpose, rather than which set is real. Quoting Robert Griffiths

"The choice of family is up to the physicist, and will generally be made on the grounds of utility, e.g., how to model a particular experimental situation with apparatus put together by a competent experimentalist. This choice has no influence upon what really goes on in the world, and alternative choices applied to the same set of data and conclusions will yield consistent results. The quantum world is of such a nature that it can be described in distinct ways which (in general) cannot be combined into a single all-encompassing description"
Inspired by this any by what I already knew about CS, let me try explain the motivation for CH in my own words:

CH is not about what really goes on in the world. Instead, it is about what we can cay about the world, if we require that the things we say obey the following rules:
- It is logically consistent and precise. (Unlike some vague variants of Copenhagen and statistical ensemble interpretations.)
- We don't need any additional mathematical objects except those which are already there in standard QM. (Unlike Bohmian trajectories or GRW modifications of the Schrodinger equation.)
- There is no a priori preferred basis in the Hilbert space.
- Measurement and observation do not play any fundamental roles.
- There are no many worlds.

Would you agree?

I'm not sure if this answers your question but: From the decoherent histories perspective, if we want a quantum theory of our entire universe that includes a description of systems like frogs, we need the triple ##(\rho,H,\{C_\alpha\})##. The terms ##\rho## and ##H## are the initial conditions of our universe and its dynamics respectively. These make up the core of our physical theory. ##\{C_\alpha\}## are a set of mutually exclusive alternative histories of our universe, each assigned a probability computed from ##\rho, H##. There is great freedom in how we construct ##\{C_\alpha\}##, and if we want to discuss frogs, we should construct histories of quasiclassical variables (i.e. averages of conserved quantities over small volumes) that make up systems like frogs. Let's call this set of histories ##\mathcal{F}##. In this framework, we can identify frogs, and the environmental variables they correlate with (like flies, predators, weather, etc). What Gell-mann says is we can understand the behaviour of systems like frogs in terms of bets. E.g. A frog bets (instinctively), that if it does not eat flies, it will die, and our quantum theory could be used to show that this is true. Histories containing frogs that don't eat flies and live a long time will have a probability of almost 0. Our quantum theory supplies the reliable odds that systems like frogs or humans should adhere to for their self-interest.

But these frogs are not special as far as the theory is concerned. An alternative set of histories ##\mathcal{F}'## might not have any quasiclassical variables. No quasiclassical variables, no planets, no rain forests, no frogs etc. This alternative set is not mutually exclusive with ##\mathcal{F}##. It's just that it's not suitable for the purposes of describing systems like frogs. Both ##\mathcal{F}## and ##\mathcal{F}'## are equally valid, and our choice of a set does not elevate that set to some privileged ontic status.

All Gell-Mann's EPE formalism in that paper really does is let us relate ##\mathcal{F}## and ##\mathcal{F}'## by a common refinement of fine-grained histories, each of with are assigned an extended probability by our theory.
This helps a lot, thank you

Inspired by this any by what I already knew about CS, let me try explain the motivation for CH in my own words:

CH is not about what really goes on in the world. Instead, it is about what we can cay about the world, if we require that the things we say obey the following rules:
- It is logically consistent and precise. (Unlike some vague variants of Copenhagen and statistical ensemble interpretations.)
- We don't need any additional mathematical objects except those which are already there in standard QM. (Unlike Bohmian trajectories or GRW modifications of the Schrodinger equation.)
- There is no a priori preferred basis in the Hilbert space.
- Measurement and observation do not play any fundamental roles.
- There are no many worlds.

Would you agree?
Yes I think those statements are fair. The only one that might need to be sharpened a little is "CH is not about what really goes on in the world. Instead, it is about what we can cay about the world". If by this we mean that no specific ontology is an essential premise of, or determined by, the formalism then yes (even if some founders like Griffiths put forth an ontology). CH, as a minimum project, generalises the procedure for computing reliable probabilities and logical inferences so that it can be applied to closed systems without recourse to some external environmental context.

gentzen and Demystifier
Demystifier
Gold Member
... (even if some founders like Griffiths put forth an ontology). CH, as a minimum project ...
So by "minimum project" you mean CH which does not put forth an ontology, am I right? Or are there also other ways how CH can be non-minimal?

It sounds to me like CH is then by default just a more elaborate "Shut up and calculate using these tools" rather than an actual interpretation of QM

It sounds to me like CH is then by default just a more elaborate "Shut up and calculate using these tools" rather than an actual interpretation of QM
Would you also say that Copenhagen is not an actual interpretation of QM? And what is wrong with "... calculate ..."? All those calculations felt quite positive to me on my first encounter with CH, because they gave me a nice illusion of understanding:
In 1998, I had to endure a QM 1 course at university, and I didn’t manage to connect at all to QM. ... I couldn’t create a picture or film in my head of how to use this to describe nature. ... Occasional attempts to read material discussing interpretation of QM failed quite early, I couldn’t penetrate into the material and words at all. Around 2005, I read (or rather browsed) “Understanding Quantum Mechanics” by Roland Omnès, and it was the first time that I felt that the material was presented in a way that I would understand it, if I invested the time to work through it. It felt like “let me calculate and explain” as opposed to “don’t ask questions, nobody understands QM anyway”.

So by "minimum project" you mean CH which does not put forth an ontology, am I right? Or are there also other ways how CH can be non-minimal?
Yes, in this convo, by minimum I just mean the development of the formalism without commitment to an ontology. So e.g. if we take a typical case of a microscopic system ##s## and a measuring device ##M## that measures observable ##A = \sum a_i \Pi_{a_i}## with pointer states ##\{M_i\}##, the conventional approach would be to write down a Hilbert space for the microscopic system, and to compute the probability of a measurement result ##a_i## as $$p(a_i) = \mathrm{Tr}\left[\Pi_{a_i}(t)\rho_s\right]$$A consistent historian would be perfectly comfortable expanding the Hilbert space to include the measurement device and computing the probability for the result ##a_i## and immediately after, the pointer outcome ##M_i## $$p(M_i\land a_i) = \mathrm{Tr}\left[\Pi_{M_i}(t+\delta t)\Pi_{a_i}(t)\rho_s\otimes\rho_M\Pi_{a_i}(t)\right]$$They could then go further and show that ##p(a_i) = p(M_i) = p(M_i\land a_i)## which establishes the logical relation ##a_i\iff M_i## which identifies a measurement scenario in the interaction between ##M## and ##s##.

What is normally an implicit measurement context is made explicit by the consistent historian. They can do all this without committing to ##a_i## as a real property that exists before (or after) measurement. They can limit their ontic commitment to ##M## if they like.

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Demystifier
Gold Member
What is normally an implicit measurement context is made explicit by the consistent historian. They can do all this without committing to ##a_i## as a real property that exists before (or after) measurement. They can limit their ontic commitment to ##M## if they like.
That's very much similar to my solipsistic hidden variables, where one can choose objects for which to postulate the ontic Bohm-like trajectories: everything including the measured system, or the apparatus but not the measured system, or perhaps only the state of brain responsible for consciousness. And of course, all this has roots in von Neumann theory of measurement where one has similar freedom where to put the collapse postulate.

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Morbert
A consistent historian would be perfectly comfortable expanding the Hilbert space to include the measurement device and computing the probability for the result ##a_i## and immediately after, the pointer outcome ##M_i##
##p(M_i\land a_i)=\ldots##
The formula for ##p(M_i\land a_i)=\ldots## looks wrong: ##\Pi_{a_i}(t)## appears two times in the formula, and the rightmost occurence feels like a typo to me.

The formula for ##p(M_i\land a_i)=\ldots## looks wrong: ##\Pi_{a_i}(t)## appears two times in the formula, and the rightmost occurence feels like a typo to me.
If I work in an explicit history space ##\mathcal{H}_t\otimes\mathcal{H}_{t+\delta t}## the rightmost term is not necessary. But otherwise, histories made up of chains of projectors like ##\Pi_{M_i}(t+\delta t)\Pi_{a_i}(t)## are not necessarily projectors themselves and so it is good practice to avoid any time-ordering commutation issues. This is why you will see authors like Omnes (in "Understanding QM") write the probability of a history like so

as opposed to just ##p(a) = \mathrm{Tr}(C_a E_0)##

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Minnesota Joe
Gold Member
Such a view is rather obsolete. The quantum potential is as obsolete as e.g. relativistic mass in modern formulation of special relativity. Wave function is fundamental but not ontic, in the same sense in which Hamiltonian in classical mechanics is fundamental but not ontic.
I find it really difficult to understand how the configuration space wavefunction could be real (not that my difficulties are an argument for anything), so I was intrigued to find this paper by Norsen, Marian, and Oriols (2015, Synthese, or https://arxiv.org/abs/1410.3676) that sounds a little like your description. They argue that the single particle conditional wave functions are real but the configuration space wave function is not; instead, it plays a role analogous to the role the Hamiltonian plays in classical mechanics (the part similar to what you wrote). Is this the kind of approach you are hinting at or something else?

Thanks,

Joe

Demystifier
PAllen
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.
Well, most universities, at least when I went, offered formal logic as part of philosophy. That was how some geeks got to take a “math” course for a humanities distribution requirement. And many of the most eminent figures in the history of logic (e.g. Quine), certainly considered themselves philosophers.

Demystifier
Demystifier