Many Worlds Interpretation and act of measuring

[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.

 

[TrickyDicky]

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..

It is a fact that QM dynamics doesn't actually follow the basis independence of Hilbert space(there is even a theorem that proves it). The measurement problem(preferred basis) is just a manifestation of this state of affairs. So 't hooft is just acknowledging it.

Applied to something like a spin-1/2 particle,.

The case with spinors lies outside the theorem mentioned above.

 

[Jilang]

The conceptual issue I have with MW is that once you think you have pinned down a state (and established world you are in) the state can often be described as a superposistion of states in another basis (so you don't know which world you are in). How to reconcile this?

 

[bhobba]

The conceptual issue I have with MW is that once you think you have pinned down a state (and established world you are in) the state can often be described as a superposistion of states in another basis (so you don't know which world you are in). How to reconcile this?

That's the so called preferred basis problem. Decoherence resolves that - but for the details you need to consult a text on the matter. THE book of course is the following:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

The measurement problem actually has a number of parts - again the above reference gives the full detail. The part it doesn't explain without further interpretive assumptions is why we get any outcomes at all - which is the modern version of collapse. In MW it's of course trivial.

Thanks
Bill

 

[Derek Potter]

The conceptual issue I have with MW is that once you think you have pinned down a state (and established world you are in) the state can often be described as a superposistion of states in another basis (so you don't know which world you are in). How to reconcile this?

Well you always know which world you are in because a world is defined by the outcome you observe. What I think you mean is you don't know which set of possible outcomes you are in - or more accurately which superposition you are in. But of course you do. The mathematics certainly allows you to choose any basis you fancy but in the real world(s) the measurement basis is determined by the observer.

You choose to measure position or momentum, not both, so you must either lie in a superposition of position worlds or a superposition of momentum worlds.

Alice chooses her polarization angle so she lies in a superposition of horizontal/vertical worlds or a superposition of +45 and -45 degree worlds etc.

Schrodinger, tormenting his long-suffering cat, makes a general "cat" observation but decoherence results in a preferred basis of alive/dead.

Alice's choice may even be made by an automated quantum random number generator. In this case she is already in a superposition of the different possible bases before the actual measurement. The final result is a superposition of superpositions. Fortunately superpositions don't nest heirarchically so the result is just a bigger superposition. Alice is thus in a superposition of all possible measurements and all possible outcomes for each measurement.

 

[Derek Potter]

It is a fact that QM dynamics doesn't actually follow the basis independence of Hilbert space(there is even a theorem that proves it).

Basis independence is also a theorem. Please elucidate and say where QM dynamics fails to meet the criteria for basis independence.