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PeterDonis
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In the MWI, the wave function is what is real; that is "the universe"; and there is just one wave function.CoolMint said:How can just one universe be real in the MWI?
In the MWI, the wave function is what is real; that is "the universe"; and there is just one wave function.CoolMint said:How can just one universe be real in the MWI?
Yes but the issue is that we always find the electron at one single location.PeterDonis said:In the MWI, the wave function is what is real; that is "the universe"; and there is just one wave function.
Not according to the MWI. According to the MWI, all possible measurement results occur. The branches of the wave function in which they occur are decohered so they can't interfere with or communicate with each other. But all of the branches are real according to the MWI.CoolMint said:Yes but the issue is that we always find the electron at one single location.
mitchell porter said:And this brings us to one of the recurring problems for the many worlds interpretation - what it means for one world to have a lower probability than other worlds, if all worlds are equally real. (Hartle himself, as far as I know, does not regard his formalism as a theory of many actual worlds, instead it's a way to get a probability for one world, without having an external observer.)
I looked for Hartle's own opinion on the reality of other worlds, and found on page 9 here, the statement that "words like ‘all the other histories are equally real’ can be dispensed with without affecting the experimental implications of the theory". In the same paper, in a section on "reality", he talks about the different realities of schizophrenics and UFOlogists, and also about consensus among "information-gathering-and-using systems", and "realms" within the universal wavefunction. It's almost as if he thinks, not that there are "worlds" within the multiverse, but rather "realms" defined by agreement on observations among multiple observers - but I didn't have the patience to confirm that he has actually committed himself to such a peculiar ontology.Quantumental said:These videos makes it very clear that Hartle and his Consistent/Decoherent Histories are just a synonym for Everettian Many Worlds
Can this probability be computed from the Born rule?martinbn said:I think what believers in MWI should worry about is that there is a chance they happened to be in an universe in which the MWI is wrong.
You are assuming there is a physical justification for any of it. There is not.hungrybear said:Is there a physical / ontological justification for why they don't interact or is it just an assumption based the maths?
In MWI, there is only the one universal wavefunction. It has branches that represent the different worlds. The branches do interfere but always destructively, so that hybrid branches have vanishing probabilites.CoolMint said:Wavefunctions are mathematical entities. How would they 'interact' in the first place?
The part that I bolded in the quote above is actually the case if the MWI is true. The terms in the wave function that correspond to "different worlds" are already there before the "splitting" (which, as I think I've already pointed out in this thread, is a misleading term in this context). All that happens when "the world splits" is that you (the quantum system that is your body and brain) interact with something (some other quantum system) whose state is not an eigenstate of the "you making an observation" observable. This interaction entangles the "you" quantum system with the other quantum system. But it doesn't change what degrees of freedom are in either system; it doesn't make "copies" of either system. It just entangles them. As I pointed out in an earlier post, time evolution in the MWI is always unitary, and unitary evolution can't create or destroy anything.hungrybear said:It doesn't make sense to say that you are only one of those versions of yourself when the world splits, unless all along there were multiple yous and multiple worlds and they only become separate by having different events in them, rather than literally splitting from one single reality.
The fact that they are decohered.hungrybear said:in the Many Worlds Interpretation, what causes the many worlds not to interact with each other?
That question has lost coherence!CoolMint said:How does the fact that probability amplitudes lose coherence lead to the suggestion that they land in... other unobservable universes?
"Lead to the suggestion" is incorrect. The correct statement is that decoherence is the same thing as "landing in other unobservable universes". Decoherence of different branches of the wave function is what defines the different "worlds" in the MWI.CoolMint said:How does the fact that probability amplitudes lose coherence lead to the suggestion that they land in... other unobservable universes?
@CoolMint, this is excellent advice for you to take.PeroK said:Seriously, you need to read up a bit on the MWI, rather than react with astonishment or consternation at every turn this thread takes!
Maybe one possible consequence is that thinking too much about it, may give you weird ideas that may not add explanatory yet occupy your attention for better use,A. Neumaier said:MWI has no observable consequences beyond the standard consequences of quantum mechanics. Hence there is nothing horrifying to worry about.
I have. Thanks anyway.PeterDonis said:@CoolMint, this is excellent advice for you to take.
Thanks for both your thoughtful replies Peter. On the above point, isn't it the case that all classical interactions rely on quantum interactions, it's just that some quantum outcomes are extremely statically unlikely? So if every series of quantum outcomes occurs with 100% probability in MWI, it would lead to most conceivable scenarios? This seems like what David Deutsch is talking about when he says most worlds imagined by science fiction exist.PeterDonis said:No. One "branch" of the wave function will only include the plane crashing if some event with quantum uncertainty has a nonzero probability of making the plane crash. MWI proponents often wave their hands and assume that every event you can possibly imagine has some nonzero probability, but that is not at all obvious when you actually look at the math.
Let's assume that MWI is not right and there is only one planet Earth in any meaningful sense. Now, let's look at the history of human beings from 1900, say - at least according to mainstream historical record. Assume we can assess everyone's life and make some overall judgement on how good or bad or terrible people's lives have been. A certain number of people - perhaps more than anyone would like to admit - have come to some truly horrific end.hungrybear said:Summary:: The implications of MWI theory seem to me to be horrifying for each us individually. Am I getting something wrong?
It means that with 100% certainty each of us sitting here reading this forum will experience the most distressing and painful possible outcomes. Yes it will be split parts of us but it seems we must think of these split selves as us or the alternative is that we cease to exist and new copies of us are created, which also isn't great.
I really genuinely appreciate any help with understanding this as it scares the hell out of me to be honest.
That's the current belief, yes. However, we don't have a theory of quantum gravity, so there is one interaction, gravity, that we don't have a theory with which to back this belief up. For the other three fundamental interactions (strong, weak, electromagnetic), we do have such a theory, the Standard Model of particle physics, which is a quantum field theory that includes all three of those interactions.hungrybear said:isn't it the case that all classical interactions rely on quantum interactions
Yes.hungrybear said:it's just that some quantum outcomes are extremely statically unlikely?
Not necessarily, because "conceivable" is not the same as "has a nonzero probability amplitude in the wave function". MWI proponents often talk as though those two things are the same, but they're not.hungrybear said:if every series of quantum outcomes occurs with 100% probability in MWI, it would lead to most conceivable scenarios?
Yes I guess my meaning is less altruistic than that! The overall proportion of suffering in the world won't change, but what I wonder is, if MWI is true, will I personally suffer all the terrible possibilities I'd be statistically unlikely to suffer if there is just one world? If the particles that make up me in this present moment split into many different worlds in which all possibilities are realized, from the perspective of me in the present moment, the future looks pretty terrifying.PeroK said:The extra branches, therefore, don't necessarily change the overall proportion of suffering against a good life - they may simply multiply everything up. MWI in that respect is no worse or better than the non-MWI single world. It's just more of the same. On average, 25,000 people will be starving to death every day in each of these branches. In a few, perhaps, hunger will have been eradicated; in others, inequality will be much worse than it is in our world.
The rest of the worlds - the ones with the really weird stuff - will be statistically dominated by the more normal worlds.
I would, therefore, refer back to my original point: the horrors of MWI are the same horrors that are already with us.
This is false. Unitary evolution can't do this.hungrybear said:1. There was one version of me, but the experiment literally causes the particles that make up me (and the rest of the world) to split into two
This is closer to what MWI says.hungrybear said:2. There have always been two versions of me who have been experiencing the same things until this experiment. After the experiment one will go on to learn that the electron was spin up, and one will find out it is spin down.
Yes, I guess, some sub-system that is recognisable as you in some sense will suffer everything with the appropriate probability or weighting. It's not entirely clear how MWI translates into anything meaningful in terms of human experience.hungrybear said:Yes I guess my meaning is less altruistic than that! The overall proportion of suffering in the world won't change, but what I wonder is, if MWI is true, will I personally suffer all the terrible possibilities I'd be statistically unlikely to suffer if there is just one world?
I don't think they are philosophically different, at least in terms of what you should expect to experience.hungrybear said:These two scenarios are very philosophically different, because in version two I can only ever experience one timeline, but in version one, I am continually literally splitting and experiencing every possible future. If the first version is true, philosophy has a lot of work to do on what MWI means for individuals. The second one doesn’t have any huge consequences other being interesting for the imagination.
Note that, by the quantum no-cloning theorem, this is not possible. So this aspect of the analogy does not carry over to the quantum case. That should be kept in mind when interpreting it.akvadrako said:Imagine we had perfect cloning machines.
PeterDonis said:Note that, by the quantum no-cloning theorem, this is not possible. So this aspect of the analogy does not carry over to the quantum case. That should be kept in mind when interpreting it.
In the sense of your classical analogy, they can't be perfect copies. In quantum terms, "you" are entangled with what you are observing, and each "future you" is the "you" degrees of freedom in one particular branch of the entangled wave function. So strictly speaking, none of the "future yous" have a definite state at all; only the overall entangled wave function is in a definite state. (Note that this is one of the issues with the MWI, explaining how we experience definite outcomes even though no individual branch of the wave function has a definite state at all.) Certainly no "copying" is going on in the quantum wave function; as I have already said, unitary evolution can't do that.akvadrako said:Are you saying that the versions of each "future you" in the different branches cannot be perfect copies?
Indeed, better not to say they are perfect; but a clone that's good enough so it can reasonably think it's a future you.PeterDonis said:Even if we assume there is some way of getting around that difficulty, each "future you" experiences a different result of whatever measurement we're talking about, so they aren't perfect copies in that sense either.
As long as you're willing to ignore or hand-wave the fact that none of the branches individually has a definite state at all, yes.akvadrako said:Even though there is no copying going on, it's the orthogonal differences between the different branches that let's us say the branching tends to give us more branches in the future compared to the past
hungrybear said:These two scenarios are very philosophically different
I don’t think it makes any more sense to expect to become only one of the copies.akvadrako said:Expecting to become all of the copies does not make sense – it just comes about from a limitation of our language in describing such a situation.
Even without MWI philosophy has a lot of work to do. Why should you care about your future self/selves? Even if you do care about it why should you care about your own future self more than you care about anyone else’s? What makes you the same you today and tomorrow?hungrybear said:If the first version is true, philosophy has a lot of work to do on what MWI means for individuals
Even if philosophy hasn't come up with what it believes to be good answers to all these questions (and I'm not sure all philosophers would agree with that--I think some think they, at least, do have good answers to them), our ordinary everyday view of things has reasonable pragmatic answers to them. Given that, it's reasonble to ask what, if any, impact the MWI has on those reasonable pragmatic answers. That is what I take the OP to be doing in this thread.Moes said:Even without MWI philosophy has a lot of work to do. Why should you care about your future self/selves? Even if you do care about it why should you care about your own future self more than you care about anyone else’s? What makes you the same you today and tomorrow?
Since this is your opinion, please do not post further in this thread, since you are basically saying the thread is pointless. Others may disagree, and they are the ones who should be posting here.Moes said:Before we have answers to all these questions I think it’s pointless to ask specifically about MWI.
Sorry if this is what it sounded like I was saying, its not at all what I was trying to say.PeterDonis said:since you are basically saying the thread is pointless
I agree. My opinion is, like others already explained in this thread, that there is basically no impact that MWI has on those reasonable pragmatic answers.PeterDonis said:our ordinary everyday view of things has reasonable pragmatic answers to them. Given that, it's reasonble to ask what, if any, impact the MWI has on those reasonable pragmatic answers
I am willing to do that since I would say branching is just an approximate and convenient description; only the universe as a whole is independently consistent.PeterDonis said:As long as you're willing to ignore or hand-wave the fact that none of the branches individually has a definite state at all, yes.