Many-worlds true quantum event generator

[rodsika]

Momentum and position eigenstates are axes in Hilbert space, each one forming a different "basis". It's like how in 3D space you can have coordinate system #1 made up of x-y-z axes, as well as coordinate system #2 made up of x'-y'-z' axes pointing in different directions, and you can use either coordinate system to describe points and vectors in that space.

If two observables commute (like position and spin), then one meaning of that is that in terms of the Hilbert space, you can find a set of basis vectors such that each vector is an eigenvector of both those observables. If they don't commute, then an eigenvector of one will be a superposition of different eigenvectors of the other, and vice versa. So, another way of stating the preferred basis problem is that you have to pick a complete set of commuting observables as your basis, if the vectors are position eigenvectors (definite position states) then they won't be momentum eigenvectors (each vector will be a superposition of different momentum states) and vice versa.

A more intuitive physical meaning of commuting vs. not commuting is that that if two observables commute, you can measure one without disturbing the value of the other--if you measure position, then immediately after that (a negligible time interval) measure spin, then immediately after that measure position again, then if the time between measurements is arbitrarily small the second position measurement will be arbitrarily close to the first one (in the limit as the time goes to infinity the change in position goes to zero). But if you measure position, then immediately measure momentum, then immediately measure position again, then no matter how small the time intervals the probability distribution for position will be significantly changed, by an amount which can be calculated from the position/momentum uncertainty relation.

Ok. After continuous reading of many references. I came across this paper by Maximilian Schlosshauer "Decoherence, the measurement problem, and interpretations of quantum mechanics":".. the results thus far suggest that the selected properties are in agreement with our observation: for mesoscopic and macroscopic objects the distance-dependent scattering interaction with surrounding air molecules, photons, etc., will in general give rise to immediate decoherence into spatially localized wave packets and thus select position as the preferred basis. On the other hand, when the environment is comparably "slow," as is frequently the case for microscopic systems, environment-induced superselection will typically yield energy eigenstates as teh preferred states."

So Environmental Superselection is the key to the Prefered Basis Problem. So what's the mystery left to solve? Pls. give an example of the subtle problem, thanks.

Also is this just particulars to Many World? Preferred basis problem also exist in pure Copenhagen and Bohmian Mechanics and other interpretations because Decoherence is the general mechanism that replaces wavefunction collapse.

 

[Fyzix]

1. My hope has some foundation. For example, most of the scientific community agree on Block Time, hence there is nothing special in the moment of time called "NOW". The "NOW" phenomenon is not explained by physics, but is moved the the realm of the yet-to be-explained consciousness.

I don't quite see how this would have anything in common with a consciousness somehow SELECTING to remember a world line that corresponds with Born Rule.
This introduces some sort of "collapse" through consciousness, both of which MWI was supposed to do away with...

2. When I google
Jeffrey A Barrett mwi relativity
I find this thread and few links to Barrett works
what exactly are you talking about
Could you provide a description of that "problem"?

Yes, here you go:

A final problem is that it is unclear how to formulate a splitting-worlds reading of Everett that is compatible with the constraints of special relativity. Suppose one opts for a strong sort of splitting, contrary to what Everett seems to suggest, where there are more physical systems after a typical measurement than before. If this involves somehow the creation of an entirely new universe (a complete copy of spacetime with an event structure, say) then when is the new universe created? One problem is in giving a frame-independent description of the creation event in the original universe, another is in making sense in relativity of an event that creates a new spacetime when all events, including the creation event, are supposed to be characterized by the local features of a particular fixed spacetime.

Those who favor a decoherence account of splitting worlds sometimes seem to imagine some sort of “unzipping” of spacetime that occurs along the forward light cone of the spacetime region that contains the measurement interaction. While decoherence effects can be expected to propagate along the forward light cone of the region that contains the interaction event between the measuring device and the object system, and while there is no problem describing the decoherence effects themselves in a way that is perfectly compatible with relativity, there is a problem in imagining that such a splitting process somehow physically copies the systems involved. A strong picture of spacetime somehow unzipping into connected spacetime regions along the forward light cone of the measurement event, would not be compatible with special relativity insofar as relativity presupposes that all events occur on the stage of Minkowski spacetime. And if we give up this assumption, then it is unclear what the rules are for compatibility with special relativity.

source: http://plato.stanford.edu/entries/qm-everett/#5

 

[rodsika]

https://www.amazon.com/gp/product/0199560560/?tag=pfamazon01-20
*
"Many Worlds?: Everett, Quantum Theory, and Reality"

I wonder if anyone has read the above book and whether it's worth the $84 tag. What other good books about Many Worlds are there that are available and anyone has actually read?
If Many Worlds is possible, then the current movie showing at theaters called "Source Code" can happen too. It's up to this weekend only and will show you the possibilities that can happen in Many Worlds Interpretation.

 

[Fyzix]

From reading a quick summary of this movie called "Source Code" I must say, this movie seems like crap, but other than that...
No, you could not (eventhough all possibitilies would happen in MWI) become someone elses consciousness, that would essentially make YOU disappear and be nonexistant.

Anyway, back to MWI.
It seems from your posts that you do not even possesses a basic grasp of QM, buying a book like that would not be advisable, I've been in touch with some of the authors of the papers in the book and it's quite technical.

It's aimed at people who got understanding of the subject.

Besides, you can find preprints of some of the papers online.

Here is written for the layman review of the book: http://ndpr.nd.edu/review.cfm?id=21669

 

[Dmitry67]

Those who favor a decoherence account of splitting worlds sometimes seem to imagine some sort of “unzipping” of spacetime that occurs along the forward light cone of the spacetime region that contains the measurement interaction. While decoherence effects can be expected to propagate along the forward light cone of the region that contains the interaction event between the measuring device and the object system, and while there is no problem describing the decoherence effects themselves in a way that is perfectly compatible with relativity, there is a problem in imagining that such a splitting process somehow physically copies the systems involved. A strong picture of spacetime somehow unzipping into connected spacetime regions along the forward light cone of the measurement event, would not be compatible with special relativity insofar as relativity presupposes that all events occur on the stage of Minkowski spacetime. And if we give up this assumption, then it is unclear what the rules are for compatibility with special relativity.

Wait, wait... He explicitly admits, that MWI *IS* compatible with relativity! The only problem for him is (bold part) *imagining* this process!

LOL!

It is like saying that "While GR is perfectly consistent with the observations, there is a problem in imagining that spacetime is not Euclidean" :)

 

[JesseM]

Ok. After continuous reading of many references. I came across this paper by Maximilian Schlosshauer "Decoherence, the measurement problem, and interpretations of quantum mechanics":".. the results thus far suggest that the selected properties are in agreement with our observation: for mesoscopic and macroscopic objects the distance-dependent scattering interaction with surrounding air molecules, photons, etc., will in general give rise to immediate decoherence into spatially localized wave packets and thus select position as the preferred basis. On the other hand, when the environment is comparably "slow," as is frequently the case for microscopic systems, environment-induced superselection will typically yield energy eigenstates as teh preferred states."

So Environmental Superselection is the key to the Prefered Basis Problem. So what's the mystery left to solve? Pls. give an example of the subtle problem, thanks.

I don't really have a detailed understanding of decoherence and its relation to the MWI, so take what I say with a large grain of salt, but from what I've read if you have the wavefunction for some quantum subsystem and a larger "environment" which interacts thermally with it, then if you calculate the wavefunction of the whole system and use it to figure out what happens to the component of the wavefunction dealing only with the small subsystem alone (not the environment), then interaction with the environment tends to drive the wavefunction of the subsystem into something close to a mixed state, a classical statistical ensemble of different eigenstates in some basis determined "naturally" by the decoherence process, rather than a "pure state" which is just a single quantum state vector that will be a superposition of different eigenstates, different from a statistical ensemble of them (in a statistical ensemble there is no interference, you're free to imagine it's definitely in one of the eigenstates and you're just uncertain about which it is). One problem here is that it only works for the wavefunction of the subsystem, the full wavefunction of the whole system composed of both the subsystem and its "environment" does not approach a mixed state (so this suggests some difficulties in using decoherence to explain how the whole universe ends up as a collection of "worlds", since the universe has no external environment...I don't really understand the "decoherent histories" approach though, maybe it has something to say about this issue). The other problem is that while the subsystem approaches something very close to a mixed state via decoherence, it doesn't do so exactly, the interference terms don't quite go to zero. Here's a quote on this from one of the most prominent MWI advocates today, David Deutsch, quoted on p. 332 of the book Minds, Machines and the Multiverse:

From the point of view of the interpretation of quantum mechanics, I think decoherence is almost completely unimportant. That's because decoherence is a quantitative matter. The interference phenomena never completely vanish; they only decrease exponentially until you can't be bothered to measure them anymore. The question of what the [interference] terms mean is still there, even if the coefficient in front of them is very small. It's like being a little pregnant. Those terms, however small, raise the same problem. If the argument is supposed to be that superpositions occur at a microscopic level but not to macroscopic objects, that's a bit like saying that you believe your bank is honest at the level of pennies but is cheating you at the level of pounds. It just doesn't make sense. It can't be that there are multiple universes at the levels of atoms but only a single universe at the level of cats.

There's also a good discussion of how decoherence relates to Everett interpretations in section 4.3 of this Stanford Encyclopedia article...on the specific subject of the preferred basis problem, they say:

The most useful application of decoherence to Everett, however, seems to be in the context of the problem of the preferred basis. Decoherence seems to yield a (maybe partial) solution to the problem, in that it naturally identifies a class of ‘preferred’ states (not necessarily an orthonormal basis!), and even allows to reidentify them over time, so that one can identify ‘worlds’ with the trajectories defined by decoherence (or more abstractly with decoherent histories).[21] If part of the aim of Everett is to interpret quantum mechanics without introducing extra structure, in particular without postulating the existence of some preferred basis, then one will try to identify structure that is already present in the wave function at the level of components (see e.g., Wallace, 2003a). In this sense, decoherence is an ideal candidate for identifying the relevant components.

A justification for this identification can then be variously given by suggesting that a ‘world’ should be a temporally extended structure and thus reidentification over time will be a necessary condition for identifying worlds, or similarly by suggesting that in order for observers to evolve there must be stable records of past events (Saunders 1993, and the unpublished Gell-Mann & Hartle 1994 (see the Other Internet Resources section below), or that observers must be able to access robust states, preferably through the existence of redundant information in the environment (Zurek's ‘existential interpretation’, 1998).

In alternative to some global notion of ‘world’, one can look at the components of the (mixed) state of a (local) system, either from the point of view that the different components defined by decoherence will separately affect (different components of the state of) another system, or from the point of view that they will separately underlie the conscious experience (if any) of the system. The former sits well with Everett's (1957) original notion of relative state, and with the relational interpretation of Everett preferred by Saunders (e.g., 1993) and, it would seem, Zurek (1998). The latter leads directly to the idea of many-minds interpretations (see the entry on Everett's relative-state interpretation and the website on ‘A Many-Minds Interpretation of Quantum Theory’ referenced in the Other Internet Resources). If one assumes that mentality can be associated only with certain decohering structures of great complexity, this might have the advantage of further reducing the remaining ambiguity about the preferred ‘basis’.

The idea of many minds was suggested early on by Zeh (2000; also 1995, p. 24). As Zeh puts it, von Neumann's motivation for introducing collapse was to save what he called psycho-physical parallelism (arguably supervenience of the mental on the physical: only one mental state is experienced, so there should be only one corresponding component in the physical state). In a decohering no-collapse universe one can instead introduce a new psycho-physical parallelism, in which individual minds supervene on each non-interfering component in the physical state. Zeh indeed suggests that, given decoherence, this is the most natural interpretation of quantum mechanics.[22]

Also is this just particulars to Many World? Preferred basis problem also exist in pure Copenhagen and Bohmian Mechanics and other interpretations because Decoherence is the general mechanism that replaces wavefunction collapse.

In Copenhagen the choice of what to measure determines what basis the quantum state will "collapse" onto a basis vector of. In Bohmian mechanics it's assumed at the outset that position has a special role, all particles have definite positions at all times and the way the positions evolve is determined by the "pilot wave", all other types of measurements are derived from the positions of "pointers" of measuring devices (see this article for more on Bohmian mechanics).

 

[Dmitry67]

The other problem is that while the subsystem approaches something very close to a mixed state via decoherence, it doesn't do so exactly, the interference terms don't quite go to zero.

I don't understand why it is important. I've heard that these terms vanish to about 10^-20 in nanoseconds, when we deal with big macroscopic systems.

The probability of having any influence of such terms is lower that to observe an violation of the second law of thermodynamics. In practice we assume that 2nd law is right, even is not "exactly" right. But who cares? The same for decoherence...

 

[JesseM]

I don't understand why it is important. I've heard that these terms vanish to about 10^-20 in nanoseconds, when we deal with big macroscopic systems.

The probability of having any influence of such terms is lower that to observe an violation of the second law of thermodynamics. In practice we assume that 2nd law is right, even is not "exactly" right. But who cares? The same for decoherence...

As I understand it, it's not a matter of them having any "influence", it's more that their presence makes the notion of dividing the system's wavefunction into distinct "worlds" with distinct probabilities somewhat ill-defined conceptually.

 

[Fyzix]

Wait, wait... He explicitly admits, that MWI *IS* compatible with relativity! The only problem for him is (bold part) *imagining* this process!

LOL!

It is like saying that "While GR is perfectly consistent with the observations, there is a problem in imagining that spacetime is not Euclidean" :)

I have to ask... Are you stupid?
Read again, then reply...

 

[Dmitry67]

there is no problem describing the decoherence effects themselves in a way that is perfectly compatible with relativity, there is a problem in imagining that such a splitting process somehow physically copies the systems involved

How else the above can be interpreted?

 

[JesseM]

I have to ask... Are you stupid?
Read again, then reply...

Dmitry67's comment seems accurate to me, Schlosshauer's main criticism is that he personally finds it counterintuitive that systems would constantly be "copied" (and I think he's taking 'copying' too literally, it's just a metaphor for the different elements in the superposition), not that this would be incompatible with any known physical principles: "there is a problem in imagining that such a splitting process somehow physically copies the systems involved." His other criticism is that "A strong picture of spacetime somehow unzipping into connected spacetime regions along the forward light cone of the measurement event, would not be compatible with special relativity insofar as relativity presupposes that all events occur on the stage of Minkowski spacetime", but this is a strawman since the MWI does not offer such a "strong picture" picture of spacetime "unzipping", it says that there are superpositions of different macroscopic states in the same spacetime (though this would become trickier if we tried to incorporate different curvatures of spacetime in general relativity...without a theory of quantum gravity, what the MWI says about spacetime curvature is bound to be speculative though)

Incidentally, on the subject of how MWI advocates argue that it preserves locality, see the references in [post=1647627]this post[/post].

 

[rodsika]

JesseM,

1. In a double slit experiment.. how many worlds are there.. just 2 corresponding to the 2 slit choices or thousands corresponding to the numerous dots in the interference patterns?

2. Supposed there are 2 worlds corresponding to each slit or 1000 worlds from the dots or other similar quantum divergence. Would ALL events in the world outside the device be identical after 100 years.. I mean.. say the experiment were done in Germany.. would all the movements and behavior of every person and events in America and elsewhere be identical in the 2 worlds forever.. or are histories random such that after 100 years.. America conquered the Arabs in one world, and the Arabs occupied American in the second world (with the initial difference among them just slits in the double slit experiment) or Jerry has 2 sons in one world and 3 sons in the second world? Or is everything absolutely identical?

3. Say there are billions and billions of galaxies. And you do a single double slit experiment. Would the billions and billions of galaxies in the 2 world exist after you do the experiment versus not existing if you don't do it? I think this is what is too much in Many Worlds. Just doing a double split experiments can produce billions and billions of galaxies and if the bubble inflation is true that each bubble produce new Big Bang, then a single double slit experiments can literally produce billions of big bangs in the 2nd copy world. This is near absurdity.. maybe it's just easier to believe in Santa Clause.. :)

But you mentioned ealier in the thread "There aren't really clearly-differentiated "worlds" as I understand it, just a single wavefunction for the entire universe which can be seen (in the same way as any normal quantum wavefunction) as a superposition of different position states, a superposition of different momentum states, etc."

But if you don't do the double slit experiment. You won't produce 2 worlds. So thinking that there is a single wavefunction for the entire universe doesn't help much. Because if you do or don't do the double slit experiment, you can affect whether there is one world or 2 world in your double slit experiment.

 

[JesseM]

JesseM,

1. In a double slit experiment.. how many worlds are there.. just 2 corresponding to the 2 slit choices or thousands corresponding to the numerous dots in the interference patterns?

I already told you I don't think there'd be a clearly-defined set of "worlds" in the most common version of the MWI. If you just use "world" as a sort of shorthand for aspects of the superposition that would be visibly different to observers like ourselves, I suppose however many distinct ways the particle can hit the screen that your detector will differentiate between (both all the different spatial positions it could be detected at, and all the different times the detection could happen), that would be the number of "worlds" defined directly by the detection-event, although things like random vibrations in the particles making up the equipment could (I would think) also lead to other macro-differences due to the butterfly effect.

2. Supposed there are 2 worlds corresponding to each slit or 1000 worlds from the dots or other similar quantum divergence. Would ALL events in the world outside the device be identical after 100 years.

No, I already said in post #3 that it's not just quantum experiments that lead to multiple outcomes but the quantum nature of all physical systems, and I know you read that post because you responded in post #6, so I don't really understand why you would ask this question.

or are histories random such that after 100 years.. America conquered the Arabs in one world, and the Arabs occupied American in the second world (with the initial difference among them just slits in the double slit experiment) or Jerry has 2 sons in one world and 3 sons in the second world?

Sure. Forget the MWI for a second, suppose we had a mega-computer that could simulate the evolution of the wavefunction of an isolated Solar system for 100 years, starting from a state where the positions of all the particles were fairly narrowly defined (not necessarily a position eigenstate since we may not want huge uncertainty in their momenta either), so at a macroscopic level we could say we were starting with a single "world", do you doubt that after 100 years the calculated wavefunction would be a superposition which would assign significant amplitudes to position eigenstates with very different configurations of particles on the simulated Earth, say an eigenstate where Jerry had 2 sons and another where he had 3? Consider my comment about Schroedinger's cat in post #26:

You're talking about the Copenhagen interpretation, not the MWI. But we can still discuss the issue of chaotic systems in this context, as long as we are willing to have a thought-experiment like Schroedinger's cat where a large macroscopic system can remain totally isolated for a while, until it is finally observed and "collapses". My assertion would be that if the cat's brain is sufficiently chaotic for sensitive dependence on initial conditions to apply (plausible given how many nonlinear effects there are in brains), then even if the experiment is not specifically designed so the cat lives or dies based on the decay of a radioactive particle, it would still be true that if enough time is left between the moment the cat is sealed in the box and the moment it's opened, then at the moment before the box is opened and the cat's wavefunction is collapsed, according to QM the cat would be in a superposition of macroscopically distinct states, like "sitting in North corner", "sitting in South corner", "walking in the middle of the box", "sleeping in the middle", etc.

Do you disagree that this is what standard non-MWI QM would predict about the evolution of the wavefunction of any chaotic macroscopic system, if we could do the calculation?

3. Say there are billions and billions of galaxies. And you do a single double slit experiment. Would the billions and billions of galaxies in the 2 world exist after you do the experiment versus not existing if you don't do it? I think this is what is too much in Many Worlds. Just doing a double split experiments can produce billions and billions of galaxies and if the bubble inflation is true that each bubble produce new Big Bang, then a single double slit experiments can literally produce billions of big bangs in the 2nd copy world. This is near absurdity.. maybe it's just easier to believe in Santa Clause.. :)

I don't know what you mean by "produce billions and billions of galaxies", both before and after the experiment there's a single universal wavefunction which can be broken down into a superposition of vast number of position eigenstates (or whatever basis you like) each of which features different configurations of particles in galaxies. Do you find the quantum rules for wavefunction evolution to be "absurdity"?

But you mentioned ealier in the thread "There aren't really clearly-differentiated "worlds" as I understand it, just a single wavefunction for the entire universe which can be seen (in the same way as any normal quantum wavefunction) as a superposition of different position states, a superposition of different momentum states, etc."

But if you don't do the double slit experiment. You won't produce 2 worlds.

Again I don't know what you mean by "produce". If you don't do the double slit experiment the particles that would have been used in the experiment are still around and in a superposition of different possible states, so if we just define "worlds" loosely as macroscopically distinguishable possibilities, it's not clear that the choice of whether to use those particles in a specific experiment or just leave them lying around in a junk heap makes any difference to the total number of "worlds".

 

[rodsika]

I already told you I don't think there'd be a clearly-defined set of "worlds" in the most common version of the MWI. If you just use "world" as a sort of shorthand for aspects of the superposition that would be visibly different to observers like ourselves, I suppose however many distinct ways the particle can hit the screen that your detector will differentiate between (both all the different spatial positions it could be detected at, and all the different times the detection could happen), that would be the number of "worlds" defined directly by the detection-event, although things like random vibrations in the particles making up the equipment could (I would think) also lead to other macro-differences due to the butterfly effect.

No, I already said in post #3 that it's not just quantum experiments that lead to multiple outcomes but the quantum nature of all physical systems, and I know you read that post because you responded in post #6, so I don't really understand why you would ask this question.

Sure. Forget the MWI for a second, suppose we had a mega-computer that could simulate the evolution of the wavefunction of an isolated Solar system for 100 years, starting from a state where the positions of all the particles were fairly narrowly defined (not necessarily a position eigenstate since we may not want huge uncertainty in their momenta either), so at a macroscopic level we could say we were starting with a single "world", do you doubt that after 100 years the calculated wavefunction would be a superposition which would assign significant amplitudes to position eigenstates with very different configurations of particles on the simulated Earth, say an eigenstate where Jerry had 2 sons and another where he had 3? Consider my comment about Schroedinger's cat in post #26:

Do you disagree that this is what standard non-MWI QM would predict about the evolution of the wavefunction of any chaotic macroscopic system, if we could do the calculation?

I don't know what you mean by "produce billions and billions of galaxies", both before and after the experiment there's a single universal wavefunction which can be broken down into a superposition of vast number of position eigenstates (or whatever basis you like) each of which features different configurations of particles in galaxies. Do you find the quantum rules for wavefunction evolution to be "absurdity"?

Again I don't know what you mean by "produce". If you don't do the double slit experiment the particles that would have been used in the experiment are still around and in a superposition of different possible states, so if we just define "worlds" loosely as macroscopically distinguishable possibilities, it's not clear that the choice of whether to use those particles in a specific experiment or just leave them lying around in a junk heap makes any difference to the total number of "worlds".

In the above questions. I'm asking in the context of DeWitt version of MWI where splitting occurs. In Jim Al-khalili Quantum: Guide to the Perplexed. It is mentioned in page 146 (where he is describing Many Worlds):"The basic idea is the following: When a quantum system is faced with a choice of alternatives such as a particle going through one of two or more slits then, rather than the wave function entering a superposition, we think of it, and the Whole Universe along with it, as splitting into a number of realities equal to the number of options available. These different worlds/universes/branches will be identical to each other apart from the different option chosen by the particle: in one universe it has gone through the upper slit, in the other it has gone through the lower slit. The universes overlap, only in that region where interference is taking place, until decoherence sets in. This then causes them to separate into non-interacting independent realities."

Note Jim Al-Khalili is a theoretical physicist. He seems to be holding the DeWitt view that splitting really occurs. He never mentions your version. So there seems to be two versions of Many Worlds, Everett original and DeWitt splitting? If so, it is not wrong to think in terms of DeWitt, isn't it? Then my questions above has to do with DeWitt version. Pls re-answer them in the context of DeWitt version just as Jim described. I'll read more of Everett original version as I reflect on your words about it. But Dewitt version is distinct from it. Thanks.

 

[JesseM]

In the above questions. I'm asking in the context of DeWitt version of MWI where splitting occurs. In Jim Al-khalili Quantum: Guide to the Perplexed. It is mentioned in page 146 (where he is describing Many Worlds):"The basic idea is the following: When a quantum system is faced with a choice of alternatives such as a particle going through one of two or more slits then, rather than the wave function entering a superposition, we think of it, and the Whole Universe along with it, as splitting into a number of realities equal to the number of options available. These different worlds/universes/branches will be identical to each other apart from the different option chosen by the particle: in one universe it has gone through the upper slit, in the other it has gone through the lower slit. The universes overlap, only in that region where interference is taking place, until decoherence sets in. This then causes them to separate into non-interacting independent realities."

Does Al-khalili say specifically that he is talking about DeWitt's version of the MWI and not other versions? As I said in [post=3236959]post #28[/post], I thought DeWitt's version was just about picking a preferred basis and saying every possible eigenstate in that basis with nonzero amplitude would be a separate "world", so the whole issue of decoherence should have nothing to do with the number of worlds, whereas Al-khalili seems to say decoherence determines the number of distinct worlds in the quote above (this seems more in line with the idea I referred to earlier that we can only talk about 'worlds' in an approximate sense, with different worlds being differentiable at a macroscopic coarse-grained level, which I think is the same idea discussed in this section of an online Everett FAQ).

Note Jim Al-Khalili is a theoretical physicist.

In post #28 I quoted from a Stanford Encyclopedia article as a basis for my understanding of DeWitt, which was also written by a philosopher of science, http://www.lps.uci.edu/barrett/ , and the other Stanford Encyclopedia article I quoted in the same post was written by a physicist (Lev Vaidman) and said in section 6:

Barrett uses the name "MWI" for the splitting worlds view publicized by De Witt 1970. This approach has been justly criticized: it has both some kind of collapse (an irreversible splitting of worlds in a preferred basis) and the multitude of worlds.

The level of discussion in those articles is typically a bit more rigorous than what you find in a typical pop physics book aimed at a broad audience (does Al-khalili talk about the preferred basis problem, for example?) If Al-Khalili says he is talking specifically about DeWitt's version and not some other version, please quote the section where he says so.

 

[rodsika]

Does Al-khalili say specifically that he is talking about DeWitt's version of the MWI and not other versions? As I said in [post=3236959]post #28[/post], I thought DeWitt's version was just about picking a preferred basis and saying every possible eigenstate in that basis with nonzero amplitude would be a separate "world", so the whole issue of decoherence should have nothing to do with the number of worlds, whereas Al-khalili seems to say decoherence determines the number of distinct worlds in the quote above (this seems more in line with the idea I referred to earlier that we can only talk about 'worlds' in an approximate sense, with different worlds being differentiable at a macroscopic coarse-grained level, which I think is the same idea discussed in this section of an online Everett FAQ).

In post #28 I quoted from a Stanford Encyclopedia article as a basis for my understanding of DeWitt, which was also written by a philosopher of science, http://www.lps.uci.edu/barrett/ , and the other Stanford Encyclopedia article I quoted in the same post was written by a physicist (Lev Vaidman) and said in section 6:

The level of discussion in those articles is typically a bit more rigorous than what you find in a typical pop physics book aimed at a broad audience (does Al-khalili talk about the preferred basis problem, for example?) If Al-Khalili says he is talking specifically about DeWitt's version and not some other version, please quote the section where he says so.

No. Jim Al-Khalili didn't emphasize what he mentioned is DeWitt's version. He just mixed them up. He said:

"Back to Everett. His interpretation has since spawned a number of variants. While his original idea is now known as the many-worlds interpretation, there is also the multiverse interpretation, the many-histories interpretation and the many-minds interpretation. I quick like the first, do not really understand the second, and am not at all keen on the third"

Looking at the Index. Dewitt is only mentioned in one sentence where he mentioned:
"Interest in his work was revived in the late 1960s by Bryce DeWitt who coined the term 'many-worlds interpretation'."

So I guess Jim Al-Khalili just mixed Everett with Dewitt and make them one. I think he can be forgiven because the book is pop-sci account of the quantum and Many worlds is just mentioned in two page. He didnt mention about preferred basis problem. Maybe he doesnt
even know what it is. The book has so many colorful illustrations. In the one describing Many Worlds. He wrote something beside the illustration I still can't fully understand after years of reading it. He said:

"The many-worlds explanation: all possible realities co-exist. The atom goes through a different slit in each universe and the two universes overlap only at the level of the single atom. In each universe, the atom feels the presence of its parallel self which has gone through the other slit. The superposition, and hence interference, is the result of superposition of universes."

Let's put it in the context of Dewitt Splitting, There is some sense I can't understand.
Let's analyze this step by step. As the emitter sends off the atom. It begins to split into two atoms in each world. Now does each atom still behave as a wave? I wonder what the above means by the atom feeling the presence of its parallel self and interfering. We know the empty part of the interference at the detector is where there is 180 degrees out of phase in the wave from each slit. Now replacing it with atoms. In what sense can the atoms feel the other's presence and know they are 180 degrees out of phase and cause destructive interference? Unless the atom is still behaving as wave and it is the wave that interferes? If not. Does it mean when the atoms are in the same space, destructive inference is what results and both of the atoms shifted their positions from the null area? Also both atoms are supposed to be in their own worlds. How can they still feel each other's presence?*

Again we are talking about the version of Dewitt Splitting. But I think I prefer Everett original version as Dewitt Splitting is so ridicuous as one atom in your body can spaw billions of big bangs but I have to understand Dewitt's first before fully concentrating on Everett version which I think is the one you like and prefer.

 

[JesseM]

So I guess Jim Al-Khalili just mixed Everett with Dewitt and make them one.

Why do you think he mixed them up? Again, the description of "splitting" you gave didn't sound like DeWitt's version at all, since DeWitt's version shouldn't depend on decoherence for splitting.

He wrote something beside the illustration I still can't fully understand after years of reading it. He said:

"The many-worlds explanation: all possible realities co-exist. The atom goes through a different slit in each universe and the two universes overlap only at the level of the single atom. In each universe, the atom feels the presence of its parallel self which has gone through the other slit. The superposition, and hence interference, is the result of superposition of universes."

Let's put it in the context of Dewitt Splitting, There is some sense I can't understand.
Let's analyze this step by step. As the emitter sends off the atom. It begins to split into two atoms in each world.

Why two? I would think in DeWitt's version, if the preferred basis vectors are position eigenvectors, the number of "worlds" would become infinite immediately after emission since there is a continuous infinity of distinct possible positions for the atom and the wavefunction assigns each one some nonzero amplitude (though perhaps in relativistic QM it would only assign nonzero amplitude to positions which were inside the future light cone of the event of the emitter sending the atom). I suppose in quantum gravity space might be quantized so the range of possible positions wouldn't be truly infinite, still very large. And it's also possible the preferred basis wouldn't be position eigenvectors, but in that case I would think in DeWitt's version you couldn't talk about "worlds" where the particle has a precise position at any time (perhaps each basis vector would at least confine the particle's position to a narrow possible range).

 

[rodsika]

Why do you think he mixed them up? Again, the description of "splitting" you gave didn't sound like DeWitt's version at all, since DeWitt's version shouldn't depend on decoherence for splitting.

Why two? I would think in DeWitt's version, if the preferred basis vectors are position eigenvectors, the number of "worlds" would become infinite immediately after emission since there is a continuous infinity of distinct possible positions for the atom and the wavefunction assigns each one some nonzero amplitude (though perhaps in relativistic QM it would only assign nonzero amplitude to positions which were inside the future light cone of the event of the emitter sending the atom). I suppose in quantum gravity space might be quantized so the range of possible positions wouldn't be truly infinite, still very large. And it's also possible the preferred basis wouldn't be position eigenvectors, but in that case I would think in DeWitt's version you couldn't talk about "worlds" where the particle has a precise position at any time (perhaps each basis vector would at least confine the particle's position to a narrow possible range).

Wait. When you speak about "worlds", are you talking about parallel universes where there is a duplicate milky way and everything in the universe, or just in the measurement setting? Anyway. This is the complete statements of Jim's to get the whole context across:

"The basic idea is the following: When a quantum system is faced with a choice of alternatives such as a particle going through one of two or more slits then, rather than the wave function entering a superposition, we think of it, and the Whole Universe along with it, as splitting into a number of realities equal to the number of options available. These different worlds/universes/branches will be identical to each other apart from the different option chosen by the particle: in one universe it has gone through the upper slit, in the other it has gone through the lower slit. The universes overlap, only in that region where interference is taking place, until decoherence sets in. This then causes them to separate into non-interacting independent realities. That is it. There is no measurement process any more and the wavefunction never needs to 'collapse'. Schrodinger's cat will end up dead in one universe and alive in the other. We, as observers, will also split and so only ever see the outcome of our branch. But there will be other copies of us in parallel universe for whom the alternative outcomes are realized."

So you see. Any atomic process in our body can spawn new worlds.. meaning new universes. This is what I mean when I said that just doing one double slit experiment can create a duplicate of everything with a duplicate United States, duplicate solar system, dupicate galaxy, duplicate billions of galaxies and duplicate universe. This is possible because if there are two copies of us in parallel universe as Jim said, then there is duplicate Universes with billions of galaxies. Since atomic processes occur naturally in our body atoms and everything. Then in one second billions of universe and billions of galaxies are spawned. This is why I can't accept it all these years. Now you are saying this is not the correct idea and Jim is just hyperbolating or exagerating when he says there are other copies of us in parallel universes?

 

[JesseM]

Wait. When you speak about "worlds", are you talking about parallel universes where there is a duplicate milky way and everything in the universe, or just in the measurement setting?

In DeWitt's version as I'm understanding it there'd be no restriction to measuring instruments, you take the wavefunction of the entire universe, use a preferred basis to express it as a vast sum of basis vectors (if the preferred basis is position, each vector in the sum would be a distinct configuration of positions for the entirety of all the particles in the universe), and treat each of these basis vectors as a distinct "world".

"The basic idea is the following: When a quantum system is faced with a choice of alternatives such as a particle going through one of two or more slits then, rather than the wave function entering a superposition, we think of it, and the Whole Universe along with it, as splitting into a number of realities equal to the number of options available. These different worlds/universes/branches will be identical to each other apart from the different option chosen by the particle: in one universe it has gone through the upper slit, in the other it has gone through the lower slit. The universes overlap, only in that region where interference is taking place, until decoherence sets in. This then causes them to separate into non-interacting independent realities. That is it. There is no measurement process any more and the wavefunction never needs to 'collapse'. Schrodinger's cat will end up dead in one universe and alive in the other. We, as observers, will also split and so only ever see the outcome of our branch. But there will be other copies of us in parallel universe for whom the alternative outcomes are realized."

Again, the fact that he is talking about decoherence being needed to differentiate worlds suggests to me he is not talking about DeWitt's version, which just seems to treat each basis vector as a distinct "world" regardless of decoherence. Did you read this entry of the Everett FAQ I linked to earlier? It seems like it's talking about the same sort of idea as in the quote above. In both cases I think the "worlds" are only approximate, there are no clear precise criteria for when a sufficient "amount" of decoherence has occurred for a "split", though after sufficient time the interference terms will have become so tiny that we can ignore them "for all practical purposes" (a commonly-used phrase in papers about the measurement problem thanks to John Bell's abbreviation "FAPP" in his 1990 paper Against Measurement).

Now you are saying this is not the correct idea and Jim is just hyperbolating or exagerating when he says there are other copies of us in parallel universes?

I'm saying the "parallel universes" aren't differentiated by any totally clear-cut criteria, they're just an approximate way about talking about different elements of the single universal wavefunction, but certainly the universal wavefunction contains a superposition of different versions of the Earth (different position eigenstates for all the particles making up the Earth and its inhabitants, say) with different versions of you that have recorded different experiences in their brains and in other records they leave such as internet postings.

 

[JesseM]

Did Everett mention specifically that there aren't really clearly-differentiated worlds??

I don't think he specifically said that, but I also don't think he specifically said there are clearly-differentiated worlds, which has led to a lot of different interpretations of what he meant, see the Stanford Encyclopedia page on Everett's formulation of QM. This bit of section 5 may be relevant:

One problem is simply interpretational: it is unclear that Everett himself had a physical splitting of observers in mind. Perhaps Everett's most careful explanation of how pure wave mechanics accounts for determinate experience is found in a footnote to his extended presentation.

At this point we encounter a language difficulty. Whereas before the observation we had a single observer state afterwards there were a number of different states for the observer, all occurring in a superposition. Each of these separate states is a state for an observer, so that we can speak of the different observers described by the different states. On the other hand, the same physical system is involved, and from this viewpoint it is the same observer, which is in different states for the different elements of the superposition (i. e., has had different experiences in the separate elements of the superposition). In this situation we shall use the singular when we wish to emphasize that a single physical system is involved, and the plural when we wish to emphasize the different experiences for the separate elements of the superposition. (1973, 68 footnote)

On the face of it, this seems perfectly clear. There is exactly one post-measurement physical observer and multiple classical states simultaneously obtain for the one physical observer. It remains to explain how it is possible for multiple, classically incompatible, states each to obtain determinately for a single physical system in a way that does not require physical splitting. But it does seem, at least here, that for Everett there is no physical splitting of observers or any other physical systems.

Also, looking on google books at p. 158 of a biography of Everett, I see he emphasized the reversibility of the evolution of the universal wavefunction, suggesting he didn't believe in any irreversible "splits" between worlds:

Linking entropy, information, and probability in his long thesis, Everett showed that the universal wavefunction is intrinsically reversible (time can flow backwards, broken eggs reverse trajectories to reunite yolk and shell). But for us, the motion through time appears to be irreversible, said Everett:

Macroscopically irreversible phenomena are common to both classical and quantum mechanics, since they arise from our incomplete information concerning a system, not from any intrinsic behavior of the system.

And in the handwritten draft:

Thus the apparent irreversibility of natural processes is understood also as a subjective phenomena, relative to observers who lose information in an essential manner, still within a determinate framework which is overall reversible (in which total information is conserved).

So, even though the universal wave function allows information to be transferred to a scientist who does not stand outside the system observed, his perspective is limited:

These are, therefore, fundamental restrictions to the knowledge that an observer can obtain about the state of the universe...Any single observer can therefore possesses knowledge only of relative state function (relative to his state) of any systems, which is in any case all that is of any importance to him.

I think the emphasis on defining observations relative to the state of observers also suggests he wasn't thinking of complete "worlds", just of a universal superposition which contains different versions of any given observer having different experiences. This fits pretty well with the version of the many-worlds interpretation discussed in this Stanford Encyclopedia article, where for example the author writes:

Another concept (considered in some approaches as the basic one, e.g., in Saunders 1995) is a relative, or perspectival, world defined for every physical system and every one of its states (provided it is a state of non-zero probability): I will call it a centered world. This concept is useful when a world is centered on a perceptual state of a sentient being. In this world, all objects which the sentient being perceives have definite states, but objects that are not under her observation might be in a superposition of different (classical) states. The advantage of a centered world is that it does not split due to a quantum phenomenon in a distant galaxy, while the advantage of our definition is that we can consider a world without specifying a center, and in particular our usual language is just as useful for describing worlds at times when there were no sentient beings.

The concept of "world" in the MWI belongs to part (ii) of the theory, i.e., it is not a rigorously defined mathematical entity, but a term defined by us (sentient beings) in describing our experience. When we refer to the "definite classically described state" of, say, a cat, it means that the position and the state (alive, dead, smiling, etc.) of the cat is maximally specified according to our ability to distinguish between the alternatives and that this specification corresponds to a classical picture, e.g., no superpositions of dead and alive cats are allowed in a single world.[2]

Also see sections 3.4 and 6.2 which talk more about this observer-centered perspective on the MWI, which seems to me to be closest to what Everett had in mind.

 

[JesseM]

How many percentage of physicists do you think believe in clearly differentiated world than those who do not?

No idea.

For this idea of clearly differentiated world. Can a Universal Wavefunction able to produce a billion duplicate universes spawned by say atomic processes in the atoms of the bodies of an ant? But what would happen if a second ant spawed another billion duplicate universes. And millions of ants spawning billions and billions and billions of duplicate universes. Do they fall under one Universal Wavefunction common to all the ants? Or does each ant has its own Universal Wavefunction it can split separate from others? This concept is for those who believe in clearly differentiated worlds. And scientists like Nick Herbert believes in this.

"Universal wavefunction" means the wavefunction of the entire universe considered as a single system, it doesn't make any sense to talk about different parts of the universe having their "own Universal Wavefunction".

 

[rodsika]

In DeWitt's version as I'm understanding it there'd be no restriction to measuring instruments, you take the wavefunction of the entire universe, use a preferred basis to express it as a vast sum of basis vectors (if the preferred basis is position, each vector in the sum would be a distinct configuration of positions for the entirety of all the particles in the universe), and treat each of these basis vectors as a distinct "world".

Again, the fact that he is talking about decoherence being needed to differentiate worlds suggests to me he is not talking about DeWitt's version, which just seems to treat each basis vector as a distinct "world" regardless of decoherence. Did you read this entry of the Everett FAQ I linked to earlier? It seems like it's talking about the same sort of idea as in the quote above. In both cases I think the "worlds" are only approximate, there are no clear precise criteria for when a sufficient "amount" of decoherence has occurred for a "split", though after sufficient time the interference terms will have become so tiny that we can ignore them "for all practical purposes" (a commonly-used phrase in papers about the measurement problem thanks to John Bell's abbreviation "FAPP" in his 1990 paper Against Measurement).

I think Decoherence applied MWI is more natural than needing special preferred basis. Have you read this paper by Maximilian Schlosshauer called "Decoherence, the measurement problem, and interpretations of quantum mechanics"?

http://arxiv.org/abs/quant-ph/0312059

It says (page 23):

"There are several advantages in a decoherence-related approach to selecting the preferred Everett bases: First, no a priori existence of a preferred basis needs to be postulated, but
instead the preferred basis arises naturally from the physical criterion of robustness. Second, the selection will be likely to yield empirical adequacy, since the decoherence program is derived solely from the well-confirmed Schr¨odinger dynamics (modulo the possibility that robustness may not be the universally valid criterion). Lastly, the decohered components of the wave function evolve in such a way that they can be reidentified over
time (forming "trajectories" in the preferred state spaces) and thus can be used to define stable, temporally extended Everett branches. Similarly, such trajectories can be used to ensure robust observer record states and/or environmental states that make information about the state of the system of interest widely accessible to observers (see, for example, Zurek’s "existential interpretation," outlined in Sec. IV.C.3 below)."

I'll read the paper in its entirely tomorrow as well as check out Maximilian book in the library. I can't believe that watching the movie Source Code to give me relaxation just ignites my interests in the Many Worlds and the ensuing headache this study can cause. :( :)

 

[JesseM]

I think Decoherence applied MWI is more natural than needing special preferred basis. Have you read this paper by Maximilian Schlosshauer called "Decoherence, the measurement problem, and interpretations of quantum mechanics"?

http://arxiv.org/abs/quant-ph/0312059

Haven't read it, but looks like an excellent resource, thanks. Skimming it over, I see that on p. 24 he discusses the concept that decoherence gives only an approximate basis, and therefore only an approximate decomposition of the universal wavefunction into "worlds", though he doesn't see this as a problem:

The approach of using environment-induced superselection and decoherence to define the Everett branches has also been critized on grounds of being “conceptually approximate,” since the stability criterion generally leads only to an approximate specification of a preferred basis and therefore cannot give an “exact” definition of the Everett branches (see, for example, the comments of Kent, 1990; Zeh, 1973, and also the well-known “anti-FAPP” position of Bell, 1982). Wallace (2003a, pp. 90–91) has argued against such an objection as

(. . . ) arising from a view implicit in much discussion of Everett-style interpretations: that certain concepts and objects in quantum mechanics must either enter the theory formally in its axiomatic structure, or be regarded as illusion. (. . . ) [Instead] the emergence of a classical world from quantum mechanics is to be understood in terms of the emergence from the theory of certain sorts of structures and patterns, and . . . this means that we have no need (as well as no hope!) of the precision which Kent [in his (1990) critique] and others (. . . ) demand.

Accordingly, in view of our argument in Sec. II.B.3 for considering subjective solutions to the measurement problem as sufficient, there is no a priori reason to doubt that an “approximate” criterion for the selection of the preferred basis can give a meaningful definition of the Everett branches—one that is empirically adequate and that accounts for our experiences.

 

[Dmitry67]

Interesting. Many of the articles about MWI were written in pre-decoherence era. I believe, before the discovery of the decoherence MWI looked very artificial. Decoherence had created a solid ground for it. Were scientists thinking that Everett was crazy in 195x-196x?

 

[ExecNight]

Just guessing but you would need the whole energy of our universe to create an alternative one right?

 

[Dmitry67]

Just guessing but you would need the whole energy of our universe to create an alternative one right?

No
http://www.hedweb.com/manworld.htm#ockham's

Q22 Does many-worlds violate conservation of energy?
First, the law conservation of energy is based on observations within each world. All observations within each world are consistent with conservation of energy, therefore energy is conserved.
Second, and more precisely, conservation of energy, in QM, is formulated in terms of weighted averages or expectation values. Conservation of energy is expressed by saying that the time derivative of the expected energy of a closed system vanishes. This statement can be scaled up to include the whole universe. Each world has an approximate energy, but the energy of the total wavefunction, or any subset of, involves summing over each world, weighted with its probability measure. This weighted sum is a constant. So energy is conserved within each world and also across the totality of worlds.

One way of viewing this result - that observed conserved quantities are conserved across the totality of worlds - is to note that new worlds are not created by the action of the wave equation, rather existing worlds are split into successively "thinner" and "thinner" slices, if we view the probability densities as "thickness".

 

[ExecNight]

So there is a thick arrow of space-time. And this is only getting sliced by probabilities.

Any thought experiment about what happens when split two probabilities again connect with each other after a certain amount of time then? Be it 10 years or 10 nanoseconds i don't know. 10 years seem absurd but well just for brainstorming.

 

[rodsika]

I already told you I don't think there'd be a clearly-defined set of "worlds" in the most common version of the MWI. If you just use "world" as a sort of shorthand for aspects of the superposition that would be visibly different to observers like ourselves, I suppose however many distinct ways the particle can hit the screen that your detector will differentiate between (both all the different spatial positions it could be detected at, and all the different times the detection could happen), that would be the number of "worlds" defined directly by the detection-event, although things like random vibrations in the particles making up the equipment could (I would think) also lead to other macro-differences due to the butterfly effect.

No, I already said in post #3 that it's not just quantum experiments that lead to multiple outcomes but the quantum nature of all physical systems, and I know you read that post because you responded in post #6, so I don't really understand why you would ask this question.

Sure. Forget the MWI for a second, suppose we had a mega-computer that could simulate the evolution of the wavefunction of an isolated Solar system for 100 years, starting from a state where the positions of all the particles were fairly narrowly defined (not necessarily a position eigenstate since we may not want huge uncertainty in their momenta either), so at a macroscopic level we could say we were starting with a single "world", do you doubt that after 100 years the calculated wavefunction would be a superposition which would assign significant amplitudes to position eigenstates with very different configurations of particles on the simulated Earth, say an eigenstate where Jerry had 2 sons and another where he had 3? Consider my comment about Schroedinger's cat in post #26:

Do you disagree that this is what standard non-MWI QM would predict about the evolution of the wavefunction of any chaotic macroscopic system, if we could do the calculation?

I don't know what you mean by "produce billions and billions of galaxies", both before and after the experiment there's a single universal wavefunction which can be broken down into a superposition of vast number of position eigenstates (or whatever basis you like) each of which features different configurations of particles in galaxies. Do you find the quantum rules for wavefunction evolution to be "absurdity"?

Again I don't know what you mean by "produce". If you don't do the double slit experiment the particles that would have been used in the experiment are still around and in a superposition of different possible states, so if we just define "worlds" loosely as macroscopically distinguishable possibilities, it's not clear that the choice of whether to use those particles in a specific experiment or just leave them lying around in a junk heap makes any difference to the total number of "worlds".

I've been thinking about this over and over again all day and re-reading the thread and other references. I think knowing what are branches and worlds is vitally important and we seem to have different ideas here. First.. based on a website you shared:

"Q19 Do worlds differentiate or split?
Can we regard the separate worlds that result from a measurement-like interaction (See "What is a measurement?") as having previous existed distinctly and merely differentiated, rather than the interaction as having split one world into many? This is definitely not permissible in many-worlds or any theory of quantum theory consistent with experiment. Worlds do not exist in a quantum superposition independently of each other before they decohere or split. The splitting is a physical process, grounded in the dynamical evolution of the wave vector, not a matter of philosophical, linguistic or mental convenience (see "Why do worlds split?" and "When do worlds split?") If you try to treat the worlds as pre-existing and separate then the maths and probabilistic behaviour all comes out wrong."

Anyway. I'm still a bit confused by what constitute a world. Let me give an example.

1. When the atoms in my body produce choices.. diverging of worlds occur.. so are those worlds only in my body atom vicinity.. or do they travel or expand and to what extend?

2. In the above site you gave... splitting is mentioned which means no longer in superposition.. but splitting doesn't necessary mean there are not clearly differentiated worlds as you emphasized. It means other components of the superposition forming branches and diverging. Now say the atoms in my body is diverging into "worlds". How long before me as a person diverge into two copies or branches and how far will it encompass?

3. Now most important. Supposed I'm diverging into 2 copies or branches/worlds.. and you are also diverging into two or more copies. And supposed we are going to meet physically tomorrow at Madison Square Garden.. which one of our bodies/branches are going to meet? Is this what you mean there are no clearly defined worlds.. because it's mixed up? Any of our branches can cross each other? something like that? But how could this be.. how do worlds merge or blend into each other? This is the consequence if branches don't form parallel world but intermingle. This is the part I don't understand quite well.

My belief previously in 10 years of my study of quantum mechanics is that when worlds split, a separate parallel universe forms that extend to the cosmos.. but this seems to be wrong idea that I'm still analyzing. And this is the main source of confusion in Many Worlds. If you know of web sites that have illustrations of how worlds blend or merge and split and all those going on simultaneously. Pls. share it as I want to imagine how it all happen. Many thanks.

 

[JesseM]

Anyway. I'm still a bit confused by what constitute a world. Let me give an example.

1. When the atoms in my body produce choices.. diverging of worlds occur.. so are those worlds only in my body atom vicinity.. or do they travel or expand and to what extend?

2. In the above site you gave... splitting is mentioned which means no longer in superposition.. but splitting doesn't necessary mean there are not clearly differentiated worlds as you emphasized. It means other components of the superposition forming branches and diverging. Now say the atoms in my body is diverging into "worlds". How long before me as a person diverge into two copies or branches and how far will it encompass?

3. Now most important. Supposed I'm diverging into 2 copies or branches/worlds.. and you are also diverging into two or more copies. And supposed we are going to meet physically tomorrow at Madison Square Garden.. which one of our bodies/branches are going to meet? Is this what you mean there are no clearly defined worlds.. because it's mixed up? Any of our branches can cross each other? something like that? But how could this be.. how do worlds merge or blend into each other? This is the consequence if branches don't form parallel world but intermingle. This is the part I don't understand quite well.

My belief previously in 10 years of my study of quantum mechanics is that when worlds split, a separate parallel universe forms that extend to the cosmos.. but this seems to be wrong idea that I'm still analyzing. And this is the main source of confusion in Many Worlds. If you know of web sites that have illustrations of how worlds blend or merge and split and all those going on simultaneously. Pls. share it as I want to imagine how it all happen. Many thanks.

Well, again a "split" based on decoherence can only be an approximate matter, but the FAQ I linked to suggests that when one object "splits" due to decoherence, other objects around it "split" along the future light cone of the first object's splitting, due to actual causal influences from one version or another of the first object. From Q12, Is Many-Worlds a local theory?:

Macrostates descriptions of objects evolve in a local fashion. Worlds split as the macrostate description divides inside the light cone of the triggering event. Thus the splitting is a local process, transmitted causally at light or sub-light speeds. (See "Does the EPR experiment prohibit locality?" and "When do worlds split?")

 

[rodsika]

Well, again a "split" based on decoherence can only be an approximate matter, but the FAQ I linked to suggests that when one object "splits" due to decoherence, other objects around it "split" along the future light cone of the first object's splitting, due to actual causal influences from one version or another of the first object. From Q12, Is Many-Worlds a local theory?:

So in split caused by decoherence, the newly created branch in my atoms would form its own world that includes another version of you there.. this means the new branch would not be able to blend into this world and meet you here.. right?
It is stuff like this that makes it hard to believe in Many worlds. A single Ant has billions of atoms in his bodies and any random process can create branches that would include another version of Earth and the galaxies and the universe. This is absurdity to the max. How can you accept this thought?

Are you saying that split not by decoherence but by DeWitt Preferred basis won't cause the branch creation of an entire universe?
But Decoherence is tied up with Many Worlds. This is because if wave function doesn't collapse. Then one have Decoherence. Or are you saying that collapse and decoherence can both not exist, but how could that it.. they are the only possible occurences to wave function. Since Many Worlds don't have collapse, it automatically have Decoherence. No?

 

[JesseM]

So in split caused by decoherence, the newly created branch in my atoms would form its own world that includes another version of you there.. this means the new branch would not be able to blend into this world and meet you here.. right?

I think that's right, although as I said I don't understand the technical details. My understanding is that if you "split" into multiple versions, then a short time after, causal influences between you and me will cause a corresponding "split" in me so that after that point it's in some sense predetermined which version of me will encounter which version of you from that original split (though there will of course be plenty of subsequent splits in both of us)

It is stuff like this that makes it hard to believe in Many worlds. A single Ant has billions of atoms in his bodies and any random process can create branches that would include another version of Earth and the galaxies and the universe. This is absurdity to the max. How can you accept this thought?

Counter-intuitiveness is not really much of an argument--it may seem strange, but it doesn't lead to any predictions which conflict with our ordinary experience, and there are plenty of other aspects of modern physics that are also pretty counter-intuitive like the notion of "curved spacetime". Also, I've often seen MWI advocates use quantum computing as a way of supporting this picture--quantum computers would be able to perform certain kinds of calculations quickly that might take an ordinary computer billions of years or something, this is easier to understand conceptually in a picture where the computer effectively splits the task up into a vast number of different "worlds" and then merges them again (a quantum computer would be designed so that it would not include any thermodynamically irreversible 'splits' in its operations).

Are you saying that split not by decoherence but by DeWitt Preferred basis won't cause the branch creation of an entire universe?

I think the DeWitt version would involve a vast number of different "worlds" at any given moment, but I'm not sure about how temporal evolution works in this picture--how you would justify saying that basis vector A at time t0 is part of the "same history" as basis vector B at a later time t1. I suppose you could look at whether the records in B are consistent with A, but it seems like that would only be approximate, there wouldn't be any definitive way to decide whether B was a possible future of A or if B was more like a future of a different "world" that had already diverged from A's "world" prior to t0. So I don't know how the concept of "splitting" would apply in DeWitt's version, but I think the number of "worlds" at any given time should be defined merely by the number of vectors assigned a nonzero amplitude in the preferred basis, I don't see why decoherence would enter into it.

But Decoherence is tied up with Many Worlds. This is because if wave function doesn't collapse. Then one have Decoherence.

Sure, even in the Copenhagen interpretation you could have decoherence if you could keep a sufficiently complex system consisting of both a subsystem and its "environment" in isolation for a little while, so there'd be no external system to "collapse" it (like Schroedinger's cat, or a simulation on a large quantum computer). I'm not saying there isn't such a thing as decoherence in the DeWitt version, just that I don't think it has anything to do with the number of "worlds" or their contents at any given time.

 

[yoda jedi]

So there seems to be two versions of Many Worlds, Everett original and DeWitt splitting? If so, it is not wrong to think in terms of DeWitt, isn't it? Then my questions above has to do with DeWitt version. Pls re-answer them in the context of DeWitt version just as Jim described. I'll read more of Everett original version as I reflect on your words about it. But Dewitt version is distinct from it. Thanks.

MWI is realist, deterministic and local theory.
there are various versions of MWI, DeWitt, Deustch, Zurek, Polley etc

 

[rodsika]

I think that's right, although as I said I don't understand the technical details. My understanding is that if you "split" into multiple versions, then a short time after, causal influences between you and me will cause a corresponding "split" in me so that after that point it's in some sense predetermined which version of me will encounter which version of you from that original split (though there will of course be plenty of subsequent splits in both of us)

Counter-intuitiveness is not really much of an argument--it may seem strange, but it doesn't lead to any predictions which conflict with our ordinary experience, and there are plenty of other aspects of modern physics that are also pretty counter-intuitive like the notion of "curved spacetime". Also, I've often seen MWI advocates use quantum computing as a way of supporting this picture--quantum computers would be able to perform certain kinds of calculations quickly that might take an ordinary computer billions of years or something, this is easier to understand conceptually in a picture where the computer effectively splits the task up into a vast number of different "worlds" and then merges them again (a quantum computer would be designed so that it would not include any thermodynamically irreversible 'splits' in its operations).

I think the DeWitt version would involve a vast number of different "worlds" at any given moment, but I'm not sure about how temporal evolution works in this picture--how you would justify saying that basis vector A at time t0 is part of the "same history" as basis vector B at a later time t1. I suppose you could look at whether the records in B are consistent with A, but it seems like that would only be approximate, there wouldn't be any definitive way to decide whether B was a possible future of A or if B was more like a future of a different "world" that had already diverged from A's "world" prior to t0. So I don't know how the concept of "splitting" would apply in DeWitt's version, but I think the number of "worlds" at any given time should be defined merely by the number of vectors assigned a nonzero amplitude in the preferred basis, I don't see why decoherence would enter into it.

Sure, even in the Copenhagen interpretation you could have decoherence if you could keep a sufficiently complex system consisting of both a subsystem and its "environment" in isolation for a little while, so there'd be no external system to "collapse" it (like Schroedinger's cat, or a simulation on a large quantum computer). I'm not saying there isn't such a thing as decoherence in the DeWitt version, just that I don't think it has anything to do with the number of "worlds" or their contents at any given time.

We must know the tecnnical details so we can know whether the whole idea can be refuted by nonsolvable conflicts.

Anyway. If I split into multiple versions. You website says "Thus the splitting is a local process, transmitted causally at light or sub-light speeds." so it means my other copy (a branch) would have its reality expanding at the speed of light. Supposed you are in Alpha Centauri 4.37 light years away.* It would take 4.37 years for the branch to include you?* Or instantaneously? Supposed halfway you got a transgender operation and became a female. Would my branch included you as the new female or the original male?* Previously I thought Alpha Centauri is copied instantaneously but now I think the branch expanding has to travel as the speed or sublight speed.

Well. The whole idea is beyond science fiction that is why I have to go back to interpretations where Many worlds don't exist. Let's go back to pure Copenhagen.

In the buckyball with 430 atoms sent as a quantum particle in the double slit. Supposed you are the buckyball and sent off. Would you still feel the same where you either go to the left or right slit.. or would you experience superposition?* This wouldn't require any Many Worlds. But it's not hard to imagine you can experience superposition, isn't it. So I guess if I were the buckyball, I'd experience superposition or like a ghost. Why is this not possible. When object is in coherence, there is no classical state.

Let's go to the Schroedinger Cat. Supposed it could be isolated in a box (let's ignore gravity for now). Then the Schroedinger Cat is in pure state. This means it's entire body is in coherence, right? Or can pure state occur without Coherence? Anyway. Supposed the cat is in coherence, then we can assume that whenever an object is in coherence, it loses all classical state or positions. So the cat should experience superposition itself. This would avoid any Many Worlds. If one can accept the multiciplicity of worlds as default. Why not accept that superposition is also default mode. Meaning classical positions are abnormal things. Superposition is the norm. This would remove the need of Many Worlds.

You may argue that the entire universe is in pure state. Why do we not experience Superposition. Let's pretend for now that there is something out there that observe us, let's say God which collapse the universe wave function. If God removed his influenced on us. Then we will all experience superposition. Why not? But ignore this paragraph and arguments for now. Let's just deal with the Schroedinger Cat in the paragraphs prior to this. Many thanks.

 

[ExecNight]

Noting about what happens when,

There are two probabilities, A and B.
Both A and B have the same outcome C after 10 nanoseconds.
It is a closed system and doesn't affect the universe at all.

How does this work? Are they still split?

Or when we make an experiment to observe how C happened do we find both A and B as the answer?

 

[JesseM]

In the buckyball with 430 atoms sent as a quantum particle in the double slit. Supposed you are the buckyball and sent off. Would you still feel the same where you either go to the left or right slit.. or would you experience superposition?* This wouldn't require any Many Worlds. But it's not hard to imagine you can experience superposition, isn't it. So I guess if I were the buckyball, I'd experience superposition or like a ghost. Why is this not possible. When object is in coherence, there is no classical state.

Let's go to the Schroedinger Cat. Supposed it could be isolated in a box (let's ignore gravity for now). Then the Schroedinger Cat is in pure state. This means it's entire body is in coherence, right? Or can pure state occur without Coherence? Anyway. Supposed the cat is in coherence, then we can assume that whenever an object is in coherence, it loses all classical state or positions. So the cat should experience superposition itself. This would avoid any Many Worlds. If one can accept the multiciplicity of worlds as default. Why not accept that superposition is also default mode. Meaning classical positions are abnormal things. Superposition is the norm. This would remove the need of Many Worlds.

You may argue that the entire universe is in pure state. Why do we not experience Superposition. Let's pretend for now that there is something out there that observe us, let's say God which collapse the universe wave function. If God removed his influenced on us. Then we will all experience superposition. Why not? But ignore this paragraph and arguments for now. Let's just deal with the Schroedinger Cat in the paragraphs prior to this. Many thanks.

This is getting rather philosophical, but my question here would be, what does it mean to "experience" superposition? Suppose we replace Schroedinger's cat with an intelligent being capable of communication (perhaps a person, but slightly more realistically it could be an A.I. running on a quantum computer), and instead of the random radioactive decay either killing them or letting them live, the outcome of the decay just determines which of two hidden photographs will be uncovered and shown to this being. The subject of the photos isn't known in advance to the being and they could be absolutely anything, perhaps one is a photo of a painting of George Washington and the other is a photo of a duck. So would "experiencing" superposition of (left photo uncovered, right photo remains hidden) and (right photo uncovered, left photo remains hidden) involve being aware of what was in both photos at once? The problem is, suppose we ask this being to then write down a story about whatever it is he has seen...obviously we'll get a superposition of stories, and significant amplitude will be assigned to both stories involving George Washington and stories involving ducks, but the amplitude assigned to stories that actually involve George Washington interacting with a duck will be totally negligible (perhaps not exactly zero since a person who just sees a picture of George Washington might by chance happen to write a story which also involves a duck and vice versa, but the amplitude to "George Washington interacts with a duck" stories shouldn't be any less negligible than "George Washington interacts with a tiger" or any other random animal). So, if you claim that this individual has "experienced" a superposition of George Washington and a duck, it seems like you have to say that somehow the individual can't act on this composite knowledge when writing a story (or superposition of stories), which seems to indicate a radically dualistic view of the relation between their "experience" and the actual behavior caused by their physical brain.