Many Worlds Interpretation and act of measuring

[Ilja]

I have tried to give Wallace a fair chance and started to read his "Emergend multiverse" up to p. 43, where I have found this:

The fact remains that, at present, there are no known ways of explaining the quantitative predictions of relativistic quantum theory other than Everett’s. The Everett interpretation is the only game in town.

Sorry, that's too much.

 

[atyy]

I have tried to give Wallace a fair chance and started to read his "Emergend multiverse" up to p. 43, where I have found this:

Sorry, that's too much.

To give him more chances, does he also say
(1) It's the only game in town, if we consider interpretations without hidden variables
(2) The Everett interpretation maybe doesn't make sense, so maybe there are no games in town?

I think this is what he says in his earlier review of the measurement problem http://arxiv.org/abs/0712.0149.

Edit: I take it back. There he wrongly says "Things are otherwise when we try to solve the measurement problem by modifying the formalism. The plain truth is that there are currently no hidden variable or dynamical-collapse theories which are generally accepted to reproduce the empirical predictions of any interacting quantum field theory. This is a separate matter to the conceptual problems with such strategies, discussed in sections 5 and 6. We do not even have QFT versions of these theories to have conceptual problems with."

However, to his credit, I think he says it is unclear whether MWI makes sense, so maybe there are no games in town: If the Everett interpretation is incoherent for one reason or another (as is probably the mainstream view among philosophers of physics, if not among physicists) then currently we have no realist solutions to the measurement problem.

 

[julcab12]

I don't think that's how collapse theories work

..Well. That's what the author did -- he modified the math. "In ordinary quantum mechanics, the wave function (or state vector) “evolves,” changing over time in a perfectly predictable way. In other words, the odds of different results can change, and you can predict exactly how they will change, up until the time a measurement is made. But several physicists have suggested over the years that the evolution itself can change in a random (or stochastic) way causing it to collapse all by itself".

Its nothing like quantum superposition eg it doesn't require complex numbers that imply interference effects - at least as far as I can see.

They have the same aspect on actuality but differ in complexity which QM clearly has the edge for weirdness, hands-down. But the fact remains. Both produces multiplicity. One obeys deterministic and governed by some odd no. theorem. While the other -- "We tried, but it remains weird. Might as well take it at it is". Like what is mentioned in 352 --It's the only game in town, if we consider interpretations without hidden variables and we can't say anything about hidden variables for now (I am hoping in the future though).
I'm a relationalist and realist most-likely contributed to my discomfort on anything weird/nonlocal, always attempting to make deterministic approach (classical) on things weird. I'll leave with this vid. Skip to 15:00 and study on QM more.
.

 

[bhobba]

Both produces multiplicity

I have zero idea what you mean by multiplicity. Gravitational lensing is an illusion. In QM the principle of superposition is VERY real.

Thanks
Bill

 

[bhobba]

The plain truth is that there are currently no hidden variable or dynamical-collapse theories which are generally accepted to reproduce the empirical predictions of any interacting quantum field theory

That is WRONG and I think Wallace should know better. I know that there is controversy about if BM does, but generally accepted it doesn't is IMHO far too strong - for what its worth I believe it does. But there are others like GRW where I have never heard of any issues being raised. In fact as far as I can tell its the opposite - it generalises quite easily:
http://arxiv.org/pdf/quant-ph/0508230.pdf

But gee Ballentine makes errors in his very well respected book as well. I know Atty marks it down a bit because of that but most don't get too perturbed about it because overwhelminlgly it is excellent.

Thanks
Bill

 

[atyy]

One way to see that BM can reproduce relativistic QFT is that QED itself is not truly relativistic, since it is only an effective theory. If one uses a lattice regularization in finite volume, then QED is just non-relativistic quantum mechanics.

The tricky thing is whether BM can do it for the standard model. Demystifier and Ilya both have suggestions on how to do it, and maybe they are right, but it's tricky for an amateur like me to check, because Demystifer uses the Schroedinger functional, which non-rigourous QFT certainly uses, but I don't know whether it really exists, or whether for the standard model one has to do some regularization. Ilja's suggestion is tricky because if correct it would solve the problem of lattice chiral fermions, but I think the lattice community still has no consensus on whether there is any solution to the problem. To see how controversial lattice chiral fermions are, read these hilarious (to the person not receiving them) reviews: http://blog.sciencenet.cn/blog-1116346-736247.html.

 

[julcab12]

I have zero idea what you mean by multiplicity.

Thanks
Bill

Single entity appears in superposition. Superposition has a special case in QM. But I'm talking general here.

. Gravitational lensing is an illusion.

.

Literal...At first glance!? No. You can't really tell. Until you factor it out; did the math and so on . Key thing here is that GR lensing is a real 'effect'. What it produces are actual illusions/phenomenon.

Ok. I don't mind superposition being intrinsic in QM especially the proposed behavior of superposed particle having each a unique state in a system. I also understand why it is so and taken very literal -- It remains consistent through experiments so far shown in Stern Gerlach experiment. However, we have conflicting realities (QM and classical). The only advantage of MWI is that it can accommodate both realities with the expense of huge constraint. I'm still optimistic on some unitary solution though

 

[Ilja]

Ilja's suggestion is tricky because if correct it would solve the problem of lattice chiral fermions, but I think the lattice community still has no consensus on whether there is any solution to the problem.

My approach in http://arxiv.org/abs/0908.0591 is, of course, an approach using the e-word (or proposing an ether interpretation) and therefore should be rejected as anathema by every believing relativist, and AFAIK the lattice community follows this rule.

Then, it is not exactly solving the problem with lattice chiral fermions as posed: It puts only pairs of Dirac fermions on the lattice (certainly a progress in comparison with staggered fermions, which appear in groups of four doublers, so one can use them to interpret electroweak pairs - but at the cost of relativistic symmetry, moreover, at the cost of discretization only in space, not in time).

And, even worse, the chiral gauge fields do not have an exact lattice gauge symmetry. I think this is an advantage - last but not least, it explains why the chiral gauge fields have masses. But it clearly does not fit into what the chiral lattice community looks for. There should be exact gauge symmetry, to be broken by explicit symmetry breaking, because this is the renormalizable mainstream way to do things.

I look at all this from the point of view of effective field theory. Which tells me that there is not much reason to care about non-renormalizable terms and so on. The point is that all non-renormalizable terms decrease much faster for large distances than the renormalizable ones. Thus, one can leave the problem of caring for renormalizability to the large distance limit, which manages this automatically. Thus, I do not see a problem with starting at the fundamental level with discretizations of expressions which in itself would define non-renormalizable theories, like non-gauge-invariant gauge theories, even with anomalies.

 

[TrickyDicky]

Something I find really odd about MWI is that if the main motivation for this interpretation is to avoid collapse and the measurement problem, why would it enlarge the problem to the whole universe, when the problem was initially of h-bar size, which made the issue of macroscopic superposition irrelevant since our everyday familiar objects are much bigger than h-bar size. It's like trying to solve the measurement problem making it muh larger and visible than already was.

 

[Felipe Osorio]

That looks like a misunderstanding of Schroedinger's Cat.

You will find many threads on this forum discussing that thought experiment. The point though is in the standard Copenhagen interpretation QM is a theory about observations that occur in an assumed common sense classical world. In Schroedinger's Cat that observation occurs at the particle detector - everything is common sense classical after that. The purpose of the thought experiment was to show, while its obvious where you should put the observation, the theory doesn't force you to do that - in fact it says nothing about it. Then we have the issue of how does a theory explain the classical world when its assumed in the first place.

A lot of progress has been made in resolving those issues. If you are interested in the modern view the following, at the lay level, is a good source:
https://www.amazon.com/Understanding-Quantum-Mechanics-Roland-Omnès/dp/0691004358
Very picturesque. I don't know what the first is trying to depict, but the second one looks like Many Worlds. It's an interpretation and as such may or may not be true - but until there is a way to experimentally test it there is no way of telling. There are tons of other interpretations as well and they are all in the same boat.

Thanks
Bill

In your interpretation, what constitutes an "observer" that collapses the state? Are you saying the Schrodinger's thought experiment cannot be transitioned from the quantum level (particle detector) to the classical?

 

[bhobba]

In your interpretation, what constitutes an "observer" that collapses the state?

Collapse is not part of the quantum formalism - some interpretations have it - some don't. My personal one doesn't (its a slight variant of the following):
http://en.wikipedia.org/wiki/Ensemble_interpretation

But I wasn't really speaking about a specific interpretation - more the modern view that applies to most interpretations.

Are you saying the Schrodinger's thought experiment cannot be transitioned from the quantum level (particle detector) to the classical?

I am saying, almost by definition, superposition can't apply to classical objects like a cat. Its utterly impossible for it to be in a superposition of dead and alive.

Thanks
Bill

 

[RUTA]

If the Everett interpretation is incoherent for one reason or another (as is probably the mainstream view among philosophers of physics, if not among physicists) then currently we have no realist solutions to the measurement problem.

I'm not sure what you mean by "no realist solutions to the MP." Would you clarify please?

 

[atyy]

I'm not sure what you mean by "no realist solutions to the MP." Would you clarify please?

There are on realist solutions if only the Everett interpretation is acceptable, since the Everett interpretation is probably not acceptable. (BTW, I did not say that, that was a quote from Wallace.)

 

[RUTA]

There are on realist solutions if only the Everett interpretation is acceptable, since the Everett interpretation is probably not acceptable. (BTW, I did not say that, that was a quote from Wallace.)

I have a realist psi-epistemic interpretation that solves the MP trivially (no surprise for a psi-epistemic account). I understand that a realist psi-epistemic account is rare, so I was wondering if this quote was only referring to psi-ontic approaches.

 

[atyy]

I have a realist psi-epistemic interpretation that solves the MP trivially (no surprise for a psi-epistemic account). I understand that a realist psi-epistemic account is rare, so I was wondering if this quote was only referring to psi-ontic approaches.

The quote excludes all interpretations by assumption except the Everett interpretation.

 

[Quantumental]

There are on realist solutions if only the Everett interpretation is acceptable, since the Everett interpretation is probably not acceptable. (BTW, I did not say that, that was a quote from Wallace.)

When did Wallace say so ? He's religiously committed to MWI

 

[atyy]

When did Wallace say so ? He's religiously committed to MWI

http://arxiv.org/abs/0712.0149

"I presented an alternative strategy for justifying subjective uncertainty in Wallace (2005) (and more briefly in Wallace (2006a)). My proposal is that we are led to subjective uncertainty by considerations in the philosophy of language: namely, if we ask how we would analyse the semantics of a community of language-users in a constantly branching universe, we conclude that claims like “X might happen” come out true if X happens in some but not all branches. If the Subjective Uncertainty program can be made to work, it avoids the epistemological problem of the Fission Program, for it aims to recover the quantum algorithm itself (and not just to account for its empirical success.) It remains controversial, however, whether subjective uncertainty really makes sense. For further discussion of subjective uncertainty and identity across branching, see Greaves (2004), Saunders and Wallace (2007), Wallace (2006a) and Lewis (2007)."

"Sometimes it is easy to forget how grave a problem the ‘measurement problem’ actually is. One can too-easily slip into a mindset where there is one theory — quantum mechanics — and a myriad empirically-equivalent interpretations of that theory. Sometimes, indeed, it can seem that the discussion is carried out on largely aesthetic grounds: do I find this theory’s stochasticity more distressing than that interpretation’s ontological excesses or the other theory’s violation of action-reaction?

The truth is very different. ..."

 

[PZIG98]

So is it to say that every time an event happens in the Universe a new universe is created for all possible probabilities? Where would all that energy come from? There can't be a set number of universes as they would become so different from one and other that new events would never coincide... Oh look, I just created another Universe... :)

 

[stevendaryl]

So is it to say that every time an event happens in the Universe a new universe is created for all possible probabilities? Where would all that energy come from? There can't be a set number of universes as they would become so different from one and other that new events would never coincide... Oh look, I just created another Universe... :)

If you have a state $|\psi\rangle$ that is a superposition of two different energy eigenstates, $|\psi \rangle = \alpha |\psi_1\rangle + \beta |\psi_2\rangle$, where $|\psi_1\rangle$ has energy $E_1$ and $|\psi_2\rangle$ has energy $E_2$, then the expectation value of the energy of the superposition is: $\langle E \rangle = |\alpha|^2 E_1 + |\beta|^2 E_2$. This will be an energy between $E_1$ and $E_2$. So the energy doesn't go up as the number of "branches" goes up--it's always less than the maximum energy of anyone branch.

 

[PZIG98]

Are you saying that total energy, E, is the sum of the energy in all of the branches? Would this mean that as more an more branches are created that each branch has to give some energy? Or, do all of the branches already exist and have energy?

The later I can't get me head around... Can't get my head around most of it to be honest... Never the less, If the "Worlds" already exist as they evolve would they not become more and more different from each other? If the cat dies on one world, where does the cat come from in that world if the experimenter from the living-cat world decides to do the experiment a second time?

I, at this point, in my uneducated mind, tend to like the guiding wave idea the best.

 

[stevendaryl]

Are you saying that total energy, E, is the sum of the energy in all of the branches?

No, I specifically said that it wasn't. The total energy is a weighted average over all branches, so it's always less than the maximum energy of all the branches.

 

[PZIG98]

Do all of the branches exist or are new branches evolving all of the time... Or, do branches collapse to make room for new? Seems to me, (perhaps because I am ignorant) that the total energy has to stay the same...

 

[bhobba]

Do all of the branches exist or are new branches evolving all of the time... Or, do branches collapse to make room for new? Seems to me, (perhaps because I am ignorant) that the total energy has to stay the same...

They are being created all the time. And sometimes they do merge because decoherence can in simple cases be undone. There is also some theoretical arguments that suggest if you wait long enough any world will eventually merge.

Thanks
Bill

 

[Derek Potter]

That's precisely what MW says is not going on. The mixed state after decoherence is ∑pi |bi><bi|. Being a mixed state its no longer in superposition. Each |bi><bi| is interpreted as a world.
Bill

That would be an improper mixed state. The global state remains in superposition. Unless you add a collapse postulate to force it to become a proper mixed state. In which case the "many" part of "many worlds" refers to many possibilities, not many worlds.

Except for the preferred basis and born rule problems that has yet to be solved ;p It' really not as mathetmatically beautiful when you look at the contrived attempts at solving these problems that come from David Wallace, Max Tegmark, David Deutsch, Sean Carroll. They add so many axioms that it's really no more elegant than Bohm

Your formulation pretty well shows that MW and conventional QM are bound to produce the same predictions, the difference being that while the conventional view asserts the projection postulate in terms of the intrinsic probability of an observation, MW asserts it as the measure of probability of finding yourself in a particular world. In both models, the BR has to have the familiar amplitude-squared form if classical probability is to emerge from vector amplitudes but why nature should be built this way is not explained either in the conventional theory or in MW. The elegance of MW is not that it instantly solves the BR and preferred basis problems but that it does away with the physical process of state reduction. The infamous Shut Up And Calculate interpretation also does away with them at the cost of introducing an imperative as an axiom...

 

[Derek Potter]

Do all of the branches exist or are new branches evolving all of the time... Or, do branches collapse to make room for new? Seems to me, (perhaps because I am ignorant) that the total energy has to stay the same...

The total energy does remain the same. Although MW is sometimes depicted as separate universes peeling apart, this idea was repudiated by Everett and is not implied by the formalism. On the contrary the whole point is that a single system evolves into a non-classical state which can be broken down into a sum of multiple incompatible states. The twist is that if the observer is included in the system then within each state the observer thinks he/she is the only one (unless he/she has read Everett). So in that sense there are multiple phenomenal worlds. Postulating multiple universes is just science fiction.

 

[bhobba]

That would be an improper mixed state. The global state remains in superposition. Unless you add a collapse postulate to force it to become a proper mixed state. In which case the "many" part of "many worlds" refers to many possibilities, not many worlds.

By definition that's not many worlds. If you want to object to it on semantic grounds and call it many possibilities - go ahead - but such is rather pointless.

Thanks
Bill

 

[Derek Potter]

By definition that's not many worlds. If you want to object to it on semantic grounds and call it many possibilities - go ahead - but such is rather pointless.

No, I was pointing out that you appeared to be contradicting yourself.

 

[Derek Potter]

Something I find really odd about MWI is that if the main motivation for this interpretation is to avoid collapse and the measurement problem, why would it enlarge the problem to the whole universe, when the problem was initially of h-bar size, which made the issue of macroscopic superposition irrelevant since our everyday familiar objects are much bigger than h-bar size. It's like trying to solve the measurement problem making it muh larger and visible than already was.

An h-bar sized event can trigger a nuclear bomb and, no doubt, a Big-Bang. Size isn't everything.

 

[naima]

I just would like to remind that T'hooft interpretation
is at the extreme opposite to MWI
In MWI interpretation you have a world for each possible choice.
In the automaton interpretation there is no free will no possible choice only one world with no branches!

 

[stevendaryl]

I just would like to remind that T'hooft interpretation
is at the extreme opposite to MWI
In MWI interpretation you have a world for each possible choice.
In the automaton interpretation there is no free will no possible choice only one world with no branches!

I was certainly intrigued by the T'hooft interpretation when I first read about it, but there are a few weird things about it.

One is that it suggests the "basis independence" of QM is an illusion. Underneath the appearance of the superpositions, there is actually a "true" state variable that always has a definite value. Applied to something like a spin-1/2 particle, that would be like saying that $S_z$ always has a definite value, $\pm \frac{1}{2}$, but that $S_x$ and $S_y$. That's a little strange--why would there seem to be rotational symmetry, if there was so much difference between the different spatial directions? I suppose it's sort of like Bohmian mechanics, which gives a preference to position over momentum.

The second weird thing is just the weirdness of any superdeterministic theory. It's not free-will--I don't care about that. It's that superdeterminism necessarily involves weird coincidences. In an EPR-type experiment testing Bell's inequalities, Alice may decide the setting of her detector based on a baseball score, which can be affected by a bee buzzing around a batter at a crucial moment, which might only happen because a spider didn't eat the bee, which might happen because a bird ate the spider, etc. The reason that Alice chose one setting over another may depend on the rest of world. If superdeterminism is to be the explanation for EPR correlations, it would seem to mean that Bob's detector somehow takes into account the whole rest of the world.

On the other hand, absolutely any chain of events looks weird when you run it backwards. Superdeterminism only doubles the weirdness, by making weird coincidences show up running time forward or backward.

 

[Rajkovic]

http://motls.blogspot.com.br/2014/07/many-worlds-pseudoscience-again.html
This article took all my doubts, It's pseudoscience...(If It's not pseudoscience, it's not really science either)

and BTW, schrodinger's cat to me is too much misunderstood, there is no magic.. bgaede explained it very well on youtube, he shows how simple is this experiment..

Now I can move on.. no magic here, Universe remains the same, and there is only ONE universe. which is the one we live in!

 

[stevendaryl]

http://motls.blogspot.com.br/2014/07/many-worlds-pseudoscience-again.html
This article took all my doubts, It's pseudoscience...(If It's not pseudoscience, it's not really science either)

and BTW, schrodinger's cat to me is too much misunderstood, there is no magic.. bgaede explained it very well on youtube, he shows how simple is this experiment..

Now I can move on.. no magic here, Universe remains the same, and there is only ONE universe. which is the one we live in!

That article is just one person venting. There is no science or coherent argument there, and what he's saying is wrong.

The fact that Lubos Motl speaks contemptuously about something doesn't really hold any weight. He's a smart enough guy, but completely incapable of being fair to ideas he doesn't agree with.

 

[stevendaryl]

That article is just one person venting.

With a second read, I find Lubos' even more incoherent. In discussing Schrodinger's cat, he's actually reproducing the same arguments that lead to many-worlds. He seems to assume that there IS a superposition of a live cat and a dead cat, according to QM, but

So the "dead cat" and the "alive cat" are two intermediate histories whose mutual interference may in principle affect the probabilities of later measurements just like the mutual interference of "slit A" and "slit B". Just like we can't assume that the particle in the double slit experiment picked one of the possible slits (and intermediate histories), we can't assume it about the cat.

...

The interference patterns are too fine or average out in the real measurements. Equivalently, the aforementioned operators "not commuting" with the operator of the "livelihood of the cat" are extremely unnatural and hard to operationally measure. The classical reasoning is therefore OK

So Lubos is saying that because the interference between "dead cat" and "live cat" is not practically measurable, we can, as an approximation, act as if classical reasoning is correct, that the cat is either dead or alive, we just don't know which. But that's EXACTLY the point of view of MWI. He's arguing FOR MWI, it appears to me. He just doesn't like the layman's description of it in terms of "splitting" of worlds. But that's always been an oversimplification. From the very beginning, Everett proposed smooth evolution of the wavefunction; no discontinuous "splits". The main feature that makes something MWI (to me) is the possibility of macroscopic superpositions, and Lubos accepts this as an inevitable consequence of QM.

I don't get his rant, at all.

 

[stevendaryl]

I don't get his rant, at all.

In general, I feel like I want to take a bath after reading anything written by Lubos Motl. In the comments, there were very reasonable counter-arguments presented by Ron Maimon, and Lubos just attacked him, rather than address what he had to say.

 

[Derek Potter]

The second weird thing is just the weirdness of any superdeterministic theory. It's not free-will--I don't care about that. It's that superdeterminism necessarily involves weird coincidences.

They are only weird if we accept probability theory. If there is no randomness in nature then our world with all its fine-tuned EPR coincidences is just as likely as one with classical correlation or, come to that, one where the trees migrate to the North Pole every year and the parked cars hang motionless upside-down in mid-air outside our houses. Just a thought.