James Higgo I know you're not a fan of Everett's interpretation... but can't we MWI-philes use Occam's razor to get rid of theories which posit a collapse, or hidden variables, as both inventions are surplus to requirements? It seems simpler and more elegant to say that Schroedinger's equations hold always.

 Victor Stenger We have had a lot of discussion of MWI on the list recently, as well as the other notion of parallel universes that comes from cosmology and may not (or may?) be related. I wholly agree that MWI was a vast improvement by doing away with collapse. It is not the formalism I objected to in The Unconscious Quantum. I welcomed it's doing away with collapse and the associated mysticism of quantum consciousness. It was the mysticism attached to MWI that I objected to - and there is a lot of that too. That's why I praised "post Everett" quantum mechanics, which comes under various names: alternate histories, consistent histories, decoherence.

 James Higgo You might argue that I need to posit parallel universes, which is just as bad as a wave function collapse, or a hidden nonlocal variable, or time reversibility. But actually I don't. That's simply a consequence of the equation. I don't need to posit it. That's reality. That argument would be like saying I have to say that a metal bar is shorter when it's travelling towards me if I want to argue for general relativity.

 Victor Stenger But, I noticed you still had the observer affect the outcome in your little article, which seem like you have combined the worst of both Copenhagen and MWI. Squires does something similar, with the "channel selector" being human consciousness. This strikes me as doubly uneconomical (and you, an economist!), an objection I also have to Bohm.

 James Higgo I'm right behind you in wanting to be rid of mysticism. As Arthur C Clarke said, any sufficiently advanced technology is indistinguishable from magic. On the Quantum Immortality idea: no, there is no channel selection. You find yourself in branches where you are alive because you don't find yourself anywhere when you're dead - anthropic principle stuff. The issue that I am most concerned about at the moment is gradual loss of consciousness, eg Alzheimer's. Max Tegmark raised this too in his last e-mail. But I have made "various assumptions about the underlying reality" and I cling to the belief that once we understand consciousness better, this paradox will be seen as a category mistake.

 Bob Drake When I studied quantum, the Copenhagen Interpretation was the popular reality. While this reality is rather strange, the even stranger (my opinion - specifically when considering Occam's razor) Everett's MWI (which might be called the Infinity Worlds Interpretation), seems to be in vogue not only on this list but others as well. Is there any reason for this?

 Ed Weinmann Bob, The MWI, and its offspring (such as "consistent histories interpretation") have the virtue of bringing what is a completely inexplicable, _ad hoc_ "wavefunction collapse" (under Copenhagen) into the same mathematical and logical framework as the rest of QM. Frank Tipler, in his book _The Physics of Immortality_, comments that many physical cosmologists feel that they are *forced*, mathematically, to accept the MWI (p. 172) and demonstrates why in an appendix to the book (appendix "I", page 483). What is called the "Copenhagen Interpretation" is really a refusal to proceed any further than the experimental data, on which QM is based, warrant. It is a kind of agnosticism as to "what's really out there". Bohr's famous statement went something like "we can only know, and should only state that we can confidently know, whatever those experimental results, that can reliably be reproduced, tell us". Such results tell us nothing of many worlds or of hidden variables (the MWI's chief competitor). But they also contribute nothing towards the solution of the quantum measurement paradox ("Schrodinger's Cat") . So it seems to me that if you don't care, personally, about resolving the paradox, then Copenhagen will do fine. But if you *do* care- and *I* certainly do- then something better than Copenhagen is wanted. However, I could see where a "bible-reading Christian", as you describe yourself, could happily shrug off Copenhagen, but become quite uneasy with MWI. The bible would provide you with one kind of solution to the measurement problem... which will probably boil down to "God the hidden variable"... whereas the MWI provides quite a different one. The advantage of the MWI over hidden variable interpretations is that it requires no new physics, or physical theory. Such theories must hypothesise the existence of the hidden variable, and there is currently no experimental evidence for any such variable. The MWI depends on no such experimental evidence, nor could ever actually be furnished for it, as I see the situation. Hence while it could never be verified, as a hidden variable theory might be, it is not really a theory, for which such verification could be required, either. It is an ontological speculation, recommended to us merely for the purpose of resolving the measurement paradox. (This is how the MWI avoids Occam's Razor... it requires no new physics, absent any evidence therefor, and hence does not "stick up", epistemologically speaking, so as to be "shaved off" by the "Razor".) However, such ontological speculation *does* have practical consequences, to the degree that one accepts it... and this is why I don't think that discussing it is a waste of time comparable to "how many angels can dance on the head of a pin?". (This question was originally used by the mediaeval schoolmen to show the implications of angels being entities of no material species... the answer was, of course, "an infinite number". But this answer has significance only for those who believe in angels, whereas the MWI has significance for most moral questions commonly discussed today.)

 James Higgo Deutsch also argues: **where was it factorised** when pointing out that Shor's algorithm will allow you to factorise 400-digit (say) number which would take any classical computer pretty well forever to calculate - i.e. be intractable. The answer is in e500 parallel universes interfering through the quantum computer apparatus.

 Ed Weinmann But one could also ask - and this is what Vic Stenger would ask, I think - "when was it factorised?" The answer would be "in nowhen". If it were factorised in "nowhen"- in reversible, quantum level time- then it would take much, much less time than a classical computer to solve the problem, perhaps eliminating the intractability. This way you don't need the parallel universes interfering with each other in the quantum computer, which has been designed to remain in a coherent state for the duration of the computation... you just need the computer to operate in quantum-level time, which is "reversible" from our point of view in the QCD. So this is no evidence, either, for the existence of the many worlds... because you can just substitute the question of "when was it factorised?" for Deutsch' "where was it factorised?". The tale we will tell ourselves depends on the question we will ask. Ask "where?" and you get "in the many worlds". Ask "when?" and you get "in reversible time". Now try the questions "why am I here?" and "what does it all mean?". :)

 James Higgo Irritating of Vic to have come up with a much less elegant, but seemingly undisprovable alternative to MWI. I'll just choose to explore the implications of MWI, were it 'true'.

 Victor Stenger Whaddiya mean, "much less elegant"? Seriously. What is so elegant about MWI. You can pursue the immortality implications of MWI all you want, of course, just as Henry Stapp and others pursue the psychic phenomena implications quantum consciousness and Bohmians pursue the holistic implications of the quantum potential. I will put my money that none of these bizarre schemes will ever lead to anything and that he world is as simple as it can be, and that to me means as elegant as it can be. All these interference effects, whatever the interpretation, occur just for quantum systems, that is, at the fundamental particle level. This is usually microscopic, but can include macroscopic systems like laser beam. So a large object like a cat is, even in MWI, is still an incoherent mixture. In MWI, the particles may flit from universe to universe, and there may be a version of Schrödinger's cat in each universe, but that version will just be a statistical mixture of its different particles. Probably not that much different as a whole, although alive in half and dead in the other half – when the nucleus that kills it decays, not when someone later looks. In other words, do not make the mistake, which I am sure is wrong, of assuming a cat or a human has a wave function or Hilbert space state vector. It does not. It's not a pure quantum state. It is described in QM by a density matrix, which is basically a table of probabilities which, in this case, has negligible off-diagonal elements and so negligible probability for quantum effects for the system as a whole. Apply the Everett formalism to diagonal density matrices and you will see this. Rainer can do this.

 James Higgo A stinging rebuke! To compare me with Stapp or the Bohmians! The immortality idea is completely non-mystical. If, in MWI, anything that is possible exists, then a billion-year-old Vic exists. That's simple. If MWI is true, then that 'immortality' is true. It's not a bizarre scheme. However I will concede that the idea that you will *definitely* end up as that old Vic is bizarre. MWI just feels right, and solves every philosophical paradox I've ever grappled with - the source, in my view, of the elegance - whereas all this backwards-in-time stuff seems contrived. On your second point, I only invoke Stapp in the way he shows that a single calcium ion could make the difference between one macroscopic brain state and another. I agree that everything else he says seems like rubbish. The cat, of course, does not have a wave function; nothing has a wave function. the 'wave function' is one of several mathematical contrivances which happen to model the way in which parallel universes branch off.

 Victor Stenger You should take that as a complement! Stapp is and Bohm was a brilliant physicist.

 >JH The immortality idea is completely non-mystical.

VS: I said it was bizarre, not mystical. I agree that MWI is not mystical like Stapp and Bohm can be.

 >JH If, in MWI, anything that is possible exists, then a billion-year-old Vic exists.

VS: Yeah, but the chances of this universe's me being transported to that universe are so small that I'd better live my life not counting on it.

 >JH That's simple. If MWI is true, then that 'immortality' is true. It's not a bizarre scheme. However I will concede that the idea that you will *definitely* end up as that old Vic is bizarre. MWI just feels right, and solves every philosophical paradox I've ever grappled with - the source, in my view, of the elegance - whereas all this backwards-in-time stuff seems contrived.

VS: Ah, the argument from feelings. That and two bucks will get you a cup of coffee.

 >JH On your second point, I only invoke Stapp in the way he shows that a single calcium ion could make the difference between one macroscopic brain state and another. I agree that everything else he says seems like rubbish. The cat, of course, does not have a wave function; nothing has a wave function. the 'wave function' is one of several mathematical contrivances which happen to model the way in which parallel universes branch off.

VS: Then you are not talking about Everett MWI but a more general notion of many universes.

James Higgo You're right all the way down. I'm being imprecise in use of the MWI, but what short moniker do you prefer for 'parallel universes, decoherence, consistent histories'? I'm working on a project to catalogue a bunch of paradoxes - e.g. from Ayer's Problems of Philosophy' and reinterpreting them in the light of neo-MWI, Copenhagen and your wacky time-reversible system. I'll see which interpretation banishes the most paradoxes and draw my own conclusions.

On Plaga's point, if vacuum decay could be proved to be a regular occurrence, would you take that as firm evidence for neo-MWI and QTI?

Ricardo Aler Mur Why?. The nice thing about time-symmetry is that it is already there!. Anti-particles being equivalent to particles going backwards in time, dynamic laws being symmetric, asymmetry itself not being required by any experimental data!. I wonder why nobody thought of exploring the consequences of time-symmetry before experiments showed that classical physics did not work. Why, s/he might have just found QM! Currently, for me it's either time-simmetry or total puzzlement. I just can't believe MWI ...

Ed Weimann Why "irritating"? and why not explore the implications of Vic Stenger's TRI, as well? I think you may find that they have much the same implications, for practical purposes. In the one case, everything possible *will* happen, in some world(s). In the other, everything possible already *has* happened, though not apparent to us, in our non-reversible time. Our non-reversible temporal perspective "hides" the quantum-level reality from us, just as the other worlds are hidden from the one observer, due to his relative state. Either view blows all the traditional ethics, "natural law"-based reasoning, teleology, and eschatology away. This is all to the best, if it gets the naturalist-supernaturalist debate off poor old "Darwininsm" and *on* to the significance of modern physical theory, where it belongs.

James Higgo Vic, you have made your position very clear: "I am taking Feynman's, and other's, ideas and attempting to carry them into a metaphysics. Thus the arrows are assumed to be associated with the reality of individual particles". As you know, I'm not a mystic. But the Buddhists have a saying that 'maya' or the illusion of reality, is like 'mistaking the map for the territory'. Feynman diagrams are a wonderful thing, in that they let economists get a grip on what's going on in atomic physics. But do you have anything more than a hunch that they are better representative of 'underlying reality' than a matrix or a wavefunction?

Rainer Plaga I thought about your point that you don't need experimental evidence in favour of MWI, that it's (undisputed) elegance is sufficient. In the end I find this point of view (which seems close to the one of Max) sterile, it can hamper progress. Imagine people would have been content in 1890 with the ``elegance'' of Boltzmann's indirect thermodynamical evidence in favour of atoms. The wish to find direct evidence in favour of single atoms was an important driving force in the early days of quantum physics. In other words: I'm convinced that direct evidence for MWI will lead to a qualitatively new understanding of the quantum world. I doubt that purely theoretical or philosophical work on the MWI will ever lead to this, the problems are too complicated. Counterexample to a theorem ``direct experimental evidence against alternatives to the MWI'' can't be found (like one Max seems to have in mind: ``Copenhagen always leads to the same phenomenology''). There exist parameter regions in the Standard Model of particle physics where the vacuum is metastable. Single quantum events, which raise the energy density (e.g. particle collisions) then lead to the decay of our vacuum to a more stable form: this would kill humanity. Normally this is taken as evidence that the SM does not have such parameters. In the MWI this argument does not hold of course. Each vacuum decay has only a certain probability, so there are always surviving humanities. (This is very close to my ``atom bomb'' alteration of Max's suicide test of MWI). In other words: if future research on the SM would prove that the parameters are such that the vacuum is metastable, this would be direct (and non Byzantine or macabre) evidence for the MWI. Of course it might well be that such parameters are not found, however any general theorem about the untestability of MWI is doubtful.

 James Higgo I have to concede that you are right, and we must hope that an experiment such as the one you proposed could be undertaken to prove the validity of MWI. That doesn't mean that the exploration of the consequences of MWI is not a useful pursuit here and now. On the latter point, my quantum theory of immortality would make vacuum decays, no matter how common, imperceptible to us as we continue in those universes in which there has been no decay. This is an interesting avenue to pursue.

 Rainer Plaga Hello James. We seem to be MWI proponents who agree in all major points! This is something extremely rare!

James Higgo <to Vic Stenger> MWI just feels right, and solves every philosophical paradox I've ever grappled with - the source, in my view, of the elegance - whereas all this backwards-in-time stuff seems contrived. On your second point, I only invoke Stapp in the way he shows that a single calcium ion could make the difference between one macroscopic brain state and another. I agree that everything else he says seems like rubbish. The cat, of course, does not have a wave function; nothing has a wave function. the 'wave function' is one of several mathematical contrivances which happen to model the way in which parallel universes branch off.

Rainer Plaga 1. Wavefunction. in orthodox QM there are pure states and mixtures. Now (as is described in great detail e.g. in Gottfried's book on QM ``Quantum Mechanics'' Vol.1, Benjamin) the unitary evolution given by the Schr. Eq. can never take a state into mixture, but only into an ``approximate'' mixture, where two components (cat alive\dead) of the initial state lost coherence to a nearly perfect degree (via environment induced decoherence). If this happens, it is OK ``for all practical purposes'' to call it an incoherent mixture like Vic does. However *within the MWI a la Everett* (*not* in the orthodox interpretation where the reduction of the wavefunction creates a true mixture) the two cats together and a large part of their environment are still in a pure state. In the MWI there is only unitary evolution of a universal wavefunction and that simply cannot create true mixtures. According to the MWI there was an initial wavefunction of the universe at creation time (a pure state) and that remains for all times (even if we ``inside the game'' see only parts of this wavefunction, which are very well described by mixtures) A direct consequence is, that the cats continue to interfere in general at all times. Only, after a very short time, the interference is so small that it becomes unnoticeable. This is also Deutsch's standpoint, as I discussed with him. My aim was to find a special situation in which this interference remains measurable in some way.

I strongly disagree when you write ``in the MWI the wavefunction is only a math. contrivance''(this sounds like Copenhagen). Everett called his interpretation originally the ``wave interpretation'' because in the MWI the wavefunction is the basic physical reality. MWI is based on the idea that the wavefunction and the Schr. eq. is all there is, in naive realistic sense. The talk about parallel universes etc. is a secondary (if unavoidable) consequence.

2. Elegance. I agree with Vic that exploring consequences like immortality will convince nobody that MWI is true (and your insistence on a ``billion'' years irritates me, how do you know *for sure* the maximal lifetime is not a few hundred years or even less?). We have to accept that there are people who, e.g. like hidden variables more. At the moment it's a matter of taste, sorry. Up to now, the ``statistical interpretation'' (Einstein, Ballentine) gives identical results in all experiments and has less metaphysical baggage than MWI. How can you blame anybody to prefer this standpoint?

My main reason to prefer MWI so strongly is not that it ``explains all paradoxes'' (though it certainly achieves that). This is achieved e.g. by the statistical int. as well. Rather it is that it can give us back a local realistic description without giving the observer a special mystical status (Copenhagen) or to renounce forever to understand the measurement process in detail (statistical) or to invent new formalism without any exp. Basis for it (hidden var.). One might ask: how do you know there is a naïve realistic description? We don't know. However to stop to try to find it would be a complete `Waffenstreckung'' (surrender of arms) of physics (Schroedinger). Perhaps you begin to understand why I'm longing so much for real evidence for MWI.

 Vic Stenger Rainer Plaga cc'd the [above] message to avoid-l, but it was rejected since he is not a subscriber and I allow only subscriber messages (I may change that - and friend Rainer is most welcome to comment). The original was to James Higgo. Anyway, here's what he sent, along with a few comments of mine. Comment (1): This is a very clear explanation which shows a fundamental idea of MWI, that the universe itself has a wavefunction. I still think discreteness gets us out of the bind where the off diagonal elements of the cat's density matrix are very small, rational numbers. Comment (2): Good reasons. Now consider time reversibility (at the atomic level).

 James Higgo I defer to Vic and Rainer. I have finally been forced to go back to Vic's book and take time-reversibility more seriously.

 Ed Weinmann One question I need to clear away before responding to your last missive. According to MWI, at a shearpoint (if you remain comfortable with that term) do we get two (somehow) new universes, or do we get the original one plus another "parallel" to it?

James Higgo Deutsch says, we get a very, very large number of different universes. It doesn't make sense to think of 'getting the same one' as a universe is a snapshot in time; it does not extend forwards or backwards in time.

Vic Stenger This has to be wrong, since there is no absolute time. How doe MWI come to grips with relativity? How can the universal wave function collapse simultaneously throughout the universe when the notion of simultaneity is meaningless?

James Higgo In Deutsch MWI there is no absolute time; time is the relationship between universes. It is, if you like, the path we have followed, zig-zagging on our little personal timeline between universes. It is purely subjective, as is everything in the infinite block universe. The notion of simultaneity is not meaningless, it is merely relative.

Vic Stenger OK, I mean absolute simultaneity. This is a distinction I make in AR. "Objectively real" things must be invariants--not relative. So I was using meaning in that sense, ontologically.

James Higgo Ah. If objectively real things must be invariants and not relative then the only thing I admit is objectively real is everything. The universal wavefunction doesn't 'collapse' - we just find ourselves 'connected' with 'forward in our time' universes where it has collapsed. There's nothing implausible about this. When spill a glass of milk I suddenly find myself connected with forward-in-time universes in which there is spilt milk.

[missing discussion about knights turning into petunia plants]

John Mazetier Thank you, Ed and Higgo (who, I am sure, bears scant relation to a petunia plant, though for all I know, may be knighted!). HIGGO says: "How do we get a full-blown secondary world? In precisely the same way we got the full-blown primary world. In an infinite universe it is bound to exist. And the pair of temporally-related worlds under discussion is bound to exist. Of course, the 'shearpoint' does not cause me to turn into a petunia but I am sure that *one* of the very many worlds 'temporally subsequent' to this one, everything is the same except where I was there is now a petunia. Worlds do not evolve. Evolution is a function of the relationship we see. It is subjective. But we only 'exist' where these relationships exist. In an infinite multiverse there is a chain of 'snapshots' from amoeba to petunia, but you need to string them together in the right way. Fortunately the sentient beings who are able to do this, such as you, can only exists in just such a string of snapshots."

John replies: As I have suspected, this makes MWI sound like an elaborate way of describing a quantum to quasi-classical divide, to use Gell-Man's terms, where the fine grain description of the quantum realm trumps the coarser grain quasi-classical descriptions. I envision a roiling sea of quantum events, a combination of random and precisely determined micro events, supporting (summed over by) a cruder realm of macro events. MWI envisions a roiling sea of worlds, or shearpoints (take your pick) which trumps the coarser grain "string of snapshots". Or is this just fascile (mis)equivalence?

 HIGGO says: "On your second point, you have assessed, accurately, that time is a subjective phenomenon. It is not a property of the multiverse, but only of we 'creatures in time'. 'Continuity' just means whizzing through lots of shearpoints (one every planck-time) and us not noticing any change."

John relies: I am sorry, but I do not find this logically tenable. While I agree that time may be considered subjective in the sense of being *relative to a point of view*, it is our point of view and we are in the (multi)universe. It is not wholly arbitrary in the sense of being anything one would like it to be. As for "whizzing through lots of shearpoints", how can one zip through shearpoints if there is no time? But regardless of this, I think there is still a major problem with moving through the branchings at all:  

ED says: "…I think your problem may lie in your choice of terms..."shearpoint". It is more a branching, than a shearing. It is true that, once a world has branched off, observers in *its* relative state cannot go back to the fork, and either continue on down to the trunk, or else turn off exploring other branches. However this is not a "shearing" such as would require a "full blown secondary world" at each branching. The observers in the relative states of the "secondary world" will have shared memories with our (primary) world, up to the branching point, after which their memories would begin to diverge from ours....in the Schrodinger's Cat paradox, for example, *they* saw the cat dead, and *we* saw it alive, subsequent to which someone in the secondary world joked about its being dead, whereas someone in our world joked about the cat's "nine lives", and so on...this is how the events, or contents, of the two worlds would begin to diverge. The continuity would be the shared history prior to the branching that separated them. Calling "the world after t + x" the tertiary world, it would share a the history of the primary world, up to the branching of the secondary world, and the history of the secondary world, up to the branching off of the tertiary world (itself). So each of the worlds- the primary, the secondary, and the tertiary- has a complete history, parts of which it shares in common with the other worlds, parts of which are peculiar to itself."

John replies: But the picture arising from the "basic rules" of MWI is not one of vivid, elaborate worlds branching off common trunks, but of interconnected nodes of quantum events with nary a whisper of reality between them. From what I have read so far, the focus on the "no leakage" rule is linked to an image of extant parallel worlds, between which nothing can transfer. While I find this a muddled picture on other accounts, it certainly misdirects attention from my main point about continuity: if no transfer of matter, energy or information is possible between worlds, then there is no "shared history" to branch off from! This was the thought behind my question about what kind of events are "allowable" between T and T + x. All we have as starting points to define any character for the intervals between quantum probabilistic events is whatever these events cast into this virtual world that lasts--if Higgo is right--only Planck time! (And we haven't even considered just what framework to use in defining "between". Space? Time? Space/time? Relative to what?) The Many Worlds are empty. So far, when thinking about MWI, this egg feels scrambled. What am I missing?

James Higgo Actually I am related to a rather pretty Dahlia, which you will find out if you do a search on the web. From John’s comments below it seems that he is not happy with the quantum concept at all: that things are quantized, with nothing in between. If this is the objection, then it is not MWI that you should be wrestling with, but the very notion of quantum theory. The points raised below are very intersesting - I just have time to defend my opinion that time is subjective: I can't help slipping in and out of subjective everyday language when talking about this concept. 'To whizz' is a function of time and therefore is obviously subjective. However time is not what *anyone* would like it to be: this is true only for Archimedes, who cannot exist because the multiverse is all-inclusive.

Ed Weinmann Yes, it does accomplish this, to the extent that is resolves the Schroedinger's Cat paradox. The states of "dead" and "alive", superposed at the quantum level, separate out in different worlds at the quasi-classical level. The transition, instead of requiring a wavefunction collapse, is accomplished by the branching off of new worlds, which can be as real (or as nominal a mathematical device) as you wish them to be.

 John Mazetier I envision a roiling sea of quantum events, a combination of random and precisely determined micro events, supporting (summed over by) a cruder realm of macro events. This is the CHI (consistent histories interpretation) of Gell-Mann, Hartle, Zurek, Griffiths, _et al_. MWI nvisions a roiling sea of worlds, or shearpoints (take your pick) which trumps the coarser grain "string of snapshots". Or is this just facile (mis)equivalence?

Ed Weinmann The difference would be that in the original MWI, the quantum events are not summed over in our world, to form our world... nor in any world. They form the branches along which new worlds begin to develop, and diverge in their contents from the primary worlds. Gell-Mann & Co. stepped back from the ontological claims of the original MWI by having our world be formed by the summing over of fine grain quantum events, rather than having these events start up new worlds. This way, our world is the one and only world that needs to be considered "the real world", in the CHI. The advantage here is that counterfactuality is preserved, and existential crises are avoided. The disadvantage is that we give up the complete solution of the Schrodinger's Cat paradox... the ghost of the *other* cat remains, to haunt us, forever... so in fact the CHI does not quite succeed in preserving classical counterfactuality, but rather exchanges it for counterprobability. I think here is our postmodern cosmology. :)

John Mazetier But the picture arising from the "basic rules" of MWI is not one of vivid, elaborate worlds branching off common trunks, but of interconnected nodes of quantum events with nary a whisper of reality between them. How so? the trunk -> branch picture is exactly how I conceive of MWI, the originating event of our universe (the quantum fluctuation of the primordial vacuum which led to inflation -> big bang) being the very base of the trunk. There can be nothing "between" quantum events. The world(s) is(are) composed of them. From what I have read so far, the focus on the "no leakage" rule is linked to an image of extant parallel worlds, between which nothing can transfer.

Ed Weinmann No. It is related to the first principles of QM, itself, in that observers may be in one of several relative states, but cannot be "between" those states. The observers' states, themselves, are quantized, and form (in the MWI) integral elements of the quantum mechanical theory, itself... and this has been part of its great appeal.

John Mazetier While I find this a muddled picture on other accounts, it certainly misdirects attention from my main point about continuity: if no transfer of matter, energy or information is possible between worlds, then there is no "shared history" to branch off from! This was the thought behind my question about what kind of events are "allowable" between T and T + x. All we have as starting points to define any character for the intervals between quantum probabilistic events is whatever these events cast into this virtual world that lasts--if Higgo is right--only Planck time!

Ed Weinmann Again, there are no "intervals between" quantum events. There are only the events. This *is* an ontological claim of QM, itself, and not of any interpretation thereof, and is very well experimentally verified. It demolishes entirely the old classical world. "All the choir of heaven, and all the furniture of earth" are nothing but probabilities. It is one hell of a lot to deal with.

 John Mazetier (And we haven't even considered just what framework to use in defining "between". Space? Time? Space/time? Relative to what?) The Many Worlds are empty.

 Ed WeinmannNo framework is possible, for there is no "between". And *any* world contains, and consists of nothing but, quantum events. These events form histories that can be viewed as more or less consistent with each other, forming the quasi-classical domain of our commons senses, or as branching off into the many worlds.

John Mazetier said earlier: So far, when thinking about MWI, this egg feels scrambled. What am I missing?

Ed Weinmann I don't think you quite accept the cosmological implications of standard, experimentally-confirmed QM. They are nothing less than shattering, as regards the older cosmologies, with their nice clear counterfactual certainties.

John Mazetier 2) How can the worlds be at once "composed of" quantum events while also branching from them?

James Higgo The 'branching from them' is the problem. The branches are, again, a subjective feature. Not real. No roots.

John Mazetier 3) If the branches are sets of quantum probabilistic events, and no information may be exchanged between worlds, how can memories be shared from branch to branch?

James Higgo - They can't. However there are certain sets of branches in which 'prior' worlds do bear some relationship to 'subsequent' ones - in an infinite multiverse thsi must be so - and it is in these branches that we 'exist'.

John Mazetier For me, this last point is a kicker. I see no way to salvage the branching metaphor, even if we expand our imagistic toolkit to include more than the three- or four-dimensions most easily presumed. What does an infinity of dimensions do for us? And this is supposed to be parsimonious?

James Higgo Why bring dimensions into it?

John Metzier No, I have just made another poor show at asking my questions. I was not objecting to QM, though I have reservations about it (don't most of us?). I was trying to suss out the relationship between quantum probabilistic events and the worlds of MWI, given the constraints required by the theory. It was not making sense to me. I am more comfortable with CHI, in as far as I understand it. But it is too soon for me to have a useful. fully fledged and defendable opinion about MWI. I have since had a conversation with a friend who is in the field, and he suggested a way that may surpass my objections. I am working on it. Thanks for the kind and thoughtful responses.