# I Many Worlds Interpretation existence

#### cosmik debris

This morning I got up and went to the bathroom. I believe the bathroom did exist all over night as part of "external reality"; it did not "become real" by being observed or used in the morning. Newton's equations of motion, conservation of energy etc. describing or predicting the continuously existence of the bathroom, and the bathroom itself have - for me - more than a pure epistemic meaning.
As you get older you will visit the bathroom at more points during your sleep cycle verifying the continuing existence of the bathroom. This may have some connection with the Quantum Zeno effect.

Cheers

#### Lord Jestocost

I would not call this "hidden variable interpretation" but "agnostic interpretation". There are identical members in the ensemble behaving differently when being observed. A hidden variable theory would try to present some reason / mechanism / property / ... that would cause this difference. The minimalistic interpretation doesn't do this: there is no such mechanism, and one simply does not ask for such a mechanism.
In quantum mechanics, there is a fundamental difference between a “pure ensemble” and a “mixed ensemble”. It seems that the minimalistic interpretation secretly conceives a “pure ensemble” as some kind of “mixed ensemble” (“...there are identical members in the ensemble behaving differently when being observed...”). That’s why the minimalistic interpretation isn’t a true "agnostic" one. Therefore, why not asking for a mechanism which might be responsible for the different behavior of the ensemble’s members when being observed? Physics is about such questions! One can always stop talking (“…one simply does not ask for such a mechanism…“) to avoid to answer such questions, but that’s not the way to get a coherent physical formulation of a point of view.

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#### tom.stoer

Of course you may ask these questions, but I think then you are no longer in-line with the advocates of the minimalistic interpretation who decided not to ask them.

#### Boing3000

Gold Member
Which means there can't be "absolute time" or "absolute simultaneity", as you were claiming before. Are you now retracting those claims?
There is no "claim" by defining absolute simultaneity by synchronized clock readings. I still don't understand your problem with that. In the context of one "thing" (an entangled state) evolution, it is hardly surprising. What really bothers you is that "thing" is at two place. What bothers you is non-locality.
I am not either going to retract that time is absolutely measured by clock, proper clock. It would be nice if you could stop pretend to contradict what the entire edifice of relativity stands on.

I don't think this is correct; measurements on the two entangled electrons should still commute.
It may not be, only an experiment could sort it out. It could very well assert that non-locality doesn't exist but only some other kind of spookiness (super determinism, or any other unrealistic proposition)

#### Demystifier

2018 Award
It's worth pointing out that lattice theories are defined in Euclidean space
They can also be defined in Minkowski space. But Euclidean space makes some numerical computations (like extraction of the ground-state energy) faster.

#### vanhees71

Gold Member
This is a possible consequence, but not necessarily a logical one. It simply denies the idea to let science provide realistic (ontic) explanations. Of course yours is one possible philosophical position, but there are others. They may not be yours, but they do not go away.
Perhaps, it's not a logical but rather an empirical consequence. As Einstein has suggested, one should look at what (theoretical) physicists do rather than listen to their words, and if you look at what physicists do with QT concerning describing and "understanding" their experimental findings is to use the formalism of QT with the minimal statistical interpretation.

#### vanhees71

Gold Member
They can also be defined in Minkowski space. But Euclidean space makes some numerical computations (like extraction of the ground-state energy) faster.
I'd say "possible" instead of "faster". A full lattice quantum QFT with real time would be a breakthrough at least for non-perturbative first-princile QCD simulations!

#### Lord Jestocost

As respects the "many world interpretation": Can someone tell me the preferred basis that correctly describes the universe and all the branching worlds within it or does all that hinge again on my conscious mind and its preferences?

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#### tom.stoer

Perhaps, it's not a logical but rather an empirical consequence. As Einstein has suggested, one should look at what (theoretical) physicists do rather than listen to their words, and if you look at what physicists do with QT concerning describing and "understanding" their experimental findings is to use the formalism of QT with the minimal statistical interpretation.
But as you may know the same Einstein was absolutely not satisfied with "what theoretical physicists did with QM" :-)

I know that I cannot convince you to go beyond this minimalistic approach - and I'll respect this, of course. But at the same time I am asking you to respect other physicists who are interested in a realistic / ontic interpretation of QM and therefore do go beyond this minimalistic interpretation.

My emphasis on this point is not only b/c I am interested in these discussions.

My emphasis is due to the fact that the intention of Everett et al. can be understood in its entirety only if one understands their philosophical position; with a positivist or instrumentalist attitude their relative state approach is irrelevant or even waste. So one must not criticize their approach based on an instrumentalist point of view, that misses the point. There are two options: either one accepts their philosophical position (at least as a valid intellectual approach), then one can criticize their results in terms of physics; or one does not accept their position at all, then one should critize their position in terms of philosophical, but not the logical consequences.

For the latter discussion this forum is the wrong place. Therefore we should come to an agreement that for a discussion of the many worlds interpretation one should accept the philosophical position from which this interpretation emerged, and we should then discuss the physical consequences and obstacles, e.g. how can a probabilistic interpretation emerge? what would be a physically reasonable axiom which has to be added to the undisputable subset (Hilbert space, unitary time evolution)?

#### tom.stoer

As respects the "many world interpretation": Can someone tell me the preferred basis that correctly describes the universe and all the branching worlds within it or does all that hinge again on my conscious mind and its preferences?
The preferred basis is the basis that is selected by a measurement device. So if one measures the energy of a quantum object the preferred basis will be one that is "peaked" at the energy eigenstates.

We will check for some introductory papers.

#### Lord Jestocost

To my mind, as an experimental physicist, every interpretation of quantum theory is as good/bad as every other interpretation as long as it is incapable to predict physical consequences on the basis of which its truthfulness can be proven. All adherents of all interpretations should understand that these interpretations are to a large extent based on personal psychological predispositions. It makes no sense trying to pick other interpretations into pieces (this reminds of religious disputes); a scientific approach means to think of or to provide experimental consequences that might “prove” that one’s own interpretation might be closer to the truth.

#### Boing3000

Gold Member
and we should then discuss the physical consequences and obstacles
I am thinking of a Schrodinger's cat type of situation. If I can understand that in the Hilbert space, can cohabit various orthogonal'ized versions of the cat, in various state of alive, rotten, or dying. If the Hilbert space is still that real "space" that can be projected in any "true" 3+1 dimension, I don't think any physical consequence could be "tested", aside that a dead cat would lie more "down" the box. That mean that across universes, there would be more gravity "down the box" if the likeliness of dead cats were bigger...(even if the cat is alive).

I know that QM don't pretend to account for gravity, but I think it is a logical obstacle for MWI. For example if I propose to boost the cat's thought experiment.
Let's say the radioactive decay trigger the engine of a 10 tons rocket that will or not fall onto a neutron start hovering 10 ton below the black hole limit.
Do a pure MWI like Hilbert space can still describe uniformly all universes ? Aren't there in-reconcilable "coordinate" holes in some of them ?

A simpler way would be to ask if it is even possible to talk of "universe" and not only a very small subset of it (like a cat's box) ?

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#### tom.stoer

A scientific approach means to think of or to provide experimental consequences that might “prove” that one’s own interpretation might be closer to the truth.
Yes - but a scientific approach is not restricted to experimental consequences, it also aims for reasonable theoretical explanations.

#### Lord Jestocost

The preferred basis is the basis that is selected by a measurement device. So if one measures the energy of a quantum object the preferred basis will be one that is "peaked" at the energy eigenstates.

We will check for some introductory papers.
That means, I know when using a certain measurement device that every outcome - after selecting the preferred basis - will actually occur (according to MWI). Why do I then always have the "feeling" that the results of an intended measurement are uncertain? What branches at the end? An “objective universe” (where I exist) or my "ideational realization" or "ideational construction" of something which I conceive as "universe"?

What even Everett didn't grasp: There are no means to re-establish Einstein's comfortable and deterministic classical world.

Edit: What Everett didn't also understand: You are sitting in some arbitrary branch an think that your thinking within this branch is of any relevance for an overall understanding.

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#### AlexCaledin

All adherents of all interpretations should understand that these interpretations are to a large extent based on personal psychological predispositions.
- but it's probably only Niels Bohr, whose predisposition was to avoid every sort of wishful thinking . . .

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#### PeterDonis

Mentor
There is no "claim" by defining absolute simultaneity by synchronized clock readings.
The term "absolute simultaneity" already has a definition--you can't just decide to use your own instead of the standard one.

What really bothers you is that "thing" is at two place. What bothers you is non-locality.
I have said nothing whatever about non-locality in response to you. I'm just responding to your incorrect claims about "absolute simultaneity". You are claiming that, if I start out two synchronized clocks at the same event and then let them separate, spatially separated events where they both read the same proper time provide "absolute simultaneity". That's not correct. (Also see further comment below.)

I am not either going to retract that time is absolutely measured by clock, proper clock. It would be nice if you could stop pretend to contradict what the entire edifice of relativity stands on.
The word "absolutely" is wrong. What "the entire edifice of relativity stands on" is that proper time is measured by clocks. But proper time is not "absolute"--it's just arc length along a timelike worldline. Such an arc length is invariant, yes--it's the same regardless of our choice of coordinates. But "invariant" is not "absolute". It's just "invariant".

It may not be, only an experiment could sort it out.
The prediction of QM is quite clear: the measurements should commute. (Provided that the electrons are not allowed to interact further when they come back together--further interaction would change their joint quantum state.) Given the initial entanglement and no further interaction, the prediction for the measurement results is the same regardless of where in spacetime the measurement events are--they can be spacelike separated, timelike separated, null separated, or even the same event.

A further comment: you seem to think that the proper time of the two electrons once they separate has something to do with determining the measurement results. It doesn't. Electron spin states are stationary, so they don't change with time; that means that, given the initial entanglement, it doesn't matter how much proper time elapses for each electron before a measurement on it is made. The QM prediction for the measurement results remains the same.

#### vanhees71

Gold Member
To my mind, as an experimental physicist, every interpretation of quantum theory is as good/bad as every other interpretation as long as it is incapable to predict physical consequences on the basis of which its truthfulness can be proven. All adherents of all interpretations should understand that these interpretations are to a large extent based on personal psychological predispositions. It makes no sense trying to pick other interpretations into pieces (this reminds of religious disputes); a scientific approach means to think of or to provide experimental consequences that might “prove” that one’s own interpretation might be closer to the truth.
Well, there's however one constraint also from a physical (and not philosophical) point of view: An interpretation must not contradict the very model/theory it is claiming to interpret. The assumption of an instantaneous collapse, e.g., contradicts the very foundation relativistic local QFT is built upon and thus should be rejected as a valid interpretation of relativistic QFT (including the very successful Standard Model, how incomplete it indeed might be).

#### tom.stoer

That means, I know when using a certain measurement device that every outcome - after selecting the preferred basis - will actually occur (according to MWI). Why do I then always have the "feeling" that the results of an intended measurement are uncertain?
What exactly is uncertain and when?

Before the measurement it is uncertain in which branch "Jestocost" will be in after the measurement. But according to MWI Jestocost is branching as well, so all branches are populated by one Jestocost.

(strange wording; did I get what you are asking for?)

What branches at the end?
All of them.

In a typical Stern-Gerlach-experiment measuring spin w.r.t. z-direction it will look like

$$|s_x = +1\rangle \,\otimes\, |\text{initial pointer}\rangle \,\otimes\, |\text{initial Jestocost}\rangle \,\to\, |s_z = +1\rangle \,\otimes\, |\text{pointer indicating +1}\rangle \,\otimes\, |\text{Jestocost observing +1}\rangle + |s_z = -1\rangle \,\otimes\, |\text{pointer indicating -1}\rangle \,\otimes\, |\text{Jestocost observing -1}\rangle$$

where I neglected several other subsystems like the environment.

What even Everett didn't grasp: There are no means to re-establish Einstein's comfortable and deterministic classical world.
He did!

The overall time evolution is perfectly deterministic (but of course quantum), whereas from the "intra-branch" or frog perspective there is an apparent collaps and an apparent probabilistic evolution.

Unfortunately Everett was not able to discuss with Einstein.

What Everett didn't also understand: You are sitting in some arbitrary branch an think that your thinking within this branch is of any relevance for an overall understanding.
He did!

Every branch will be populated by one Jestocost.

#### tom.stoer

Well, there's however one constraint also from a physical (and not philosophical) point of view: An interpretation must not contradict the very model/theory it is claiming to interpret. The assumption of an instantaneous collapse, e.g., contradicts the very foundation relativistic local QFT is built upon and thus should be rejected as a valid interpretation of relativistic QFT (including the very successful Standard Model, how incomplete it indeed might be).
The instantaneous collapse is problematic if and only if you combine it with an ontic interpretation, i.e. that "there is an external reality" and that "in this external reality something really collapses instantaneously".

The collapse is harmless if the collapse is applied to a mathematical representation of your (!) knowledge about the system; then this collapse is nothing else but a reset of your (!) knowledge after performing and observing an experiment. Then the collapse is strictly local; nothing "out there" really collapses b/cr your (!)knowledge is not "out there".

The collapse according to Everett might be problematic if one combines a quantum system with a classical background spacetime. If it's possible to unify quantum system and spacetime into one Hilbert space than this problem goes away (other problems survive, and new problems will enter the stage, e.g. the problem of time evolution in a quantum gravity theory).

#### Lord Jestocost

The overall time evolution is perfectly deterministic (but of course quantum), whereas from the "intra-branch" or frog perspective there is an apparent collaps and an apparent probabilistic evolution.
Every branch will be populated by one Jestocost.
Thus, all my current thinking results always from my current "intra-branch" or frog perspective. Or does my mind not belong to the deterministically evolving world Everett has in mind.

#### tom.stoer

Thus, all my current thinking results always from my current "intra-branch" or frog perspective. Or does my mind not belong to the deterministically evolving world Everett has in mind.

In the sense of microscopic (quantum) objects and processes it (your mind) undergoes a branching as well, therefore it (your mind) always has this frog perspective as well. In the sense of some "non-physical entity" quantum mechanics can't tell you anything about your mind.

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#### AlexCaledin

... In the sense of some "non-physical entity" quanten mechanics can't tell you anything about your mind.
- but Feynman tells/hints (in the "Lectures") that the physicist's mind (whatever it may be) is a little part(icipant) of Nature's growing knowledge/choice.

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#### Lord Jestocost

The overall time evolution is perfectly deterministic (but of course quantum), whereas from the "intra-branch" or frog perspective there is an apparent collaps and an apparent probabilistic evolution.
Let's try it again (as German is my mother language, it's really difficult to express my thoughts in English):

The overall time evolution is perfectly deterministic and that must apply to me, as a conscious observer, too (I assume that a conscious observer is part of the world Everett has in mind, so the conscious observer cannot simply jump out of it). So, why should there be - from my "intra-branch" or frog perspective - an apparent collaps and an apparent probabilistic evolution? To my mind, here seems to be a severe logical inconsistency.

#### PeterDonis

Mentor
The overall time evolution is perfectly deterministic and that must apply to me, as a conscious observer, too
You're misstating this. A correct statement is: The overall time evolution is perfectly deterministic and that must apply to the physical process that underlie the existence of me, as a conscious observer, too. But according to the MWI, the mapping between the physical process--the unitary evolution of the wave function--and you, the conscious observer--the actual experience you have of observing a particular measurement result--is not one-to-one. You are implicitly assuming that it is, and that assumption is incompatible with the MWI. According to the MWI, one unitary evolution can correspond to many "conscious observers" in the sense of particular experiences of particular measurement results.

#### tom.stoer

why should there be - from my "intra-branch" or frog perspective - an apparent collaps and an apparent probabilistic evolution? To my mind, here seems to be a severe logical inconsistency.
The (your) perspective before the measurement is

$$|s_x = +1\rangle \,\otimes\, |\text{initial pointer}\rangle \,\otimes\, |\text{initial Jestocost}\rangle$$

whereas after the measurement there are two mutually invisible "intra-branch" perspectives; one of these (yours) perspectives reads

$$|s_z = +1\rangle \,\otimes\, |\text{pointer indicating +1}\rangle \,\otimes\, |\text{Jestocost observing +1}\rangle$$

So for "Jestocost observing +1" it seems as if the original state has collapsed to one component in which "+1" is realized.

"Mutually invisible" means that this branch structure which appears due to decoherence remains stable w.r.t. time evolution and - of course - mutually orthogonal such that no interference can occurre anymore.

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"Many Worlds Interpretation existence"

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