Are the implications of MWI really this horrifying?

[PeterDonis]

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]

In the MWI, the wave function is what is real; that is "the universe"; and there is just one wave function.

Yes but the issue is that we always find the electron at one single location.

 

[dextercioby]

Is the „split into multiple worlds” : A) observer/ frame dependent and B) only caused by measurement, i.e. in the absence of measurement there is only one world...?

 

[mitchell porter]

The most advanced version of many worlds is the "consistent histories" or "decoherent histories" formalism of Hartle, Gell-Mann, Omnes, etc. Each individual history is specified as a time series of properties, and then the overall formalism allows you to take a set of such histories, and assign them each a probability, given a wavefunction of the universe.

This formalism does not inherently require that "all" possible histories happen. Instead, it has a limit on how similar two of the histories can be: all the histories must "decohere" from each other, for the ensemble of worlds to be well-defined. It has occasionally been suggested that if there is a quantum multiverse, it might consist of a Hartle multiverse ensemble that is as tightly packed as possible, within the constraint of mutual decoherence.

Since the question is worrying about the possible reality of hellish universes in the multiverse... Such worlds probably exist in a maximal Hartle ensemble of worlds, but if they rely on extreme bad luck to take place, they will have a very low probability. 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.)

The essence of the problem of probability in many worlds, is that if all the worlds are equally real, they should each have an equal prior probability of being the world that you are in; and that gives you probabilities inconsistent with the Born rule of quantum probability, which is the rule that actually works empirically. I only know of two ways around this. One is: if there is an uncountable number of quantum worlds, then counting them is problematic, you need a "measure" in the sense of integral calculus, and perhaps the Born probability could be the measure. The other is: if there is a countable number of quantum worlds, perhaps there are multiple copies of the different possible histories, and this is how the worlds can have the unequal prior probabilities required by the Born rule.

 

[PeterDonis]

Yes but the issue is that we always find the electron at one single location.

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.

You might not agree with the MWI; many people don't. But you can't use the fact that we observe a single result for measurements to refute it, since the MWI already accounts for that.

 

[Quantumental]

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.)

These videos makes it very clear that Hartle and his Consistent/Decoherent Histories are just a synonym for Everettian Many Worlds:

1

2

 

[mitchell porter]

These videos makes it very clear that Hartle and his Consistent/Decoherent Histories are just a synonym for Everettian Many Worlds

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.

 

[Demystifier]

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.

Can this probability be computed from the Born rule?

 

[hungrybear]

Thanks, I genuinely appreciate all the replies.

Yes in a sense this is a philosophical issue (as you might guess I'm coming from a philosophical, not a science background, both help us to understand what is real). From what I understand, it's not as simple as "the worlds don't interact so you are not in them" because we are talking about your future.

If you in the present moment split into multiple worlds, then although these selves won't experience each other, from the perspective of you in the present moment wondering what your future will be, it will be all the futures. It's not the case that 'the real you' splits into one world in particular. So it is different from worrying about the suffering of people who aren't you.

You exist in the present moment, you wonder what your future will be. Everything that makes up you, all the particles in your body, split into multiple worlds so your future fate is whatever happens to each self.

This is discussed here:
https://www.quantamagazine.org/why-...quantum-mechanics-has-many-problems-20181018/

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.

It relates to this topic here: https://www.physicsforums.com/threa...many-worlds-everettian-interpretation.616019/

The other question I have is in the Many Worlds Interpretation, what causes the many worlds not to interact with each other? As I understand it, each world is in the same space-time and each particle has the same properties that would previously have caused them to interact but after decoherance they don't interact. Is there a physical / ontological justification for why they don't interact or is it just an assumption based the maths?

Thanks again for any replies!

 

[hutchphd]

Is there a physical / ontological justification for why they don't interact or is it just an assumption based the maths?

You are assuming there is a physical justification for any of it. There is not.

At least with matters of religion one can rely on Pascal's Wager to provide a rational framework.

 

[CoolMint]

Wavefunctions are mathematical entities. How would they 'interact' in the first place?

 

[PeroK]

Wavefunctions are mathematical entities. How would they 'interact' in the first place?

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.

 

[PeterDonis]

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 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.

in the Many Worlds Interpretation, what causes the many worlds not to interact with each other?

The fact that they are decohered.

 

[CoolMint]

How does the fact that probability amplitudes lose coherence lead to the suggestion that they land in... other unobservable universes?

 

[PeroK]

How does the fact that probability amplitudes lose coherence lead to the suggestion that they land in... other unobservable universes?

That question has lost coherence!

Seriously, you need to read up a bit on the MWI, rather than react with astonishment or consternation at every turn this thread takes!

 

[PeterDonis]

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.

 

[PeterDonis]

Seriously, you need to read up a bit on the MWI, rather than react with astonishment or consternation at every turn this thread takes!

@CoolMint, this is excellent advice for you to take.

 

[Fra]

MWI has no observable consequences beyond the standard consequences of quantum mechanics. Hence there is nothing horrifying to worry about.

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,

I never got what is attractive, or the promise of MWI. I can't even related to it conceptually in a constructive way, so I rarely butt into MWI threds, this was an exception. The idea that the world splits, but the observer is only in one of them, which problems does it even solve?

/Fredrik

 

[CoolMint]

@CoolMint, this is excellent advice for you to take.

I have. Thanks anyway.

It strikes me that most other interpretations keep shy of making outlandish unverifiable claims.
The mathematical framework doesn't say or hint that there are other parallel universes. I guess I am a disbeliever. I will stop posting to this thread for parsimony reasons - I must have created a billion new universes just by participating in this thread and the resultant interactions.

 

[hungrybear]

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.

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.

 

[PeroK]

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.

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.

Very optimistically, perhaps, and very simplistically, we are going to say that things are 99% good and 1% bad here on Earth. For example, according to the UN 25,000 people die from hunger every day. That's nine million people every year.

https://en.wikipedia.org/wiki/Starvation

Now, let's assume that MWI is correct. Note that in general, the vast majority of "worlds" have no planet Earth at all; and, the vast majority of those don't have you in it. But, let's focus on the ones that, say, start from the day you were born.

Statistically, most worlds will follow a similar pattern to the one we are familiar with. In fact, the variations (although QM in origin) could be understood by looking at the probabilities associated with higher-level processes; namely, social, political and religious movements that determine, for example, how many wars there are and how many human beings starve to death, or are tortured to death by other human beings.

We have, therefore, a core set of branches or worlds that must look very like our world. They may represent a proportion extremely close to the total number. Overall, therefore, we are still talking about a 99% good and 1% bad experience for a very large (perhaps infinite) number of sub-systems that can be identified as a human being in some way. Even if the suffering is nominally spread out a bit.

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.

 

[PeterDonis]

isn't it the case that all classical interactions rely on quantum interactions

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.

it's just that some quantum outcomes are extremely statically unlikely?

Yes.

if every series of quantum outcomes occurs with 100% probability in MWI, it would lead to most conceivable scenarios?

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]

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.

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.

That's why I use the flight example. In a single universe view my chances of being in a plane crash are about 1 in 10,000,000. If I split into all possible futures, the possibility seems to be about 100% certain. The Quantum Suicide experiment suggests a future in which you can expect to survive a quantum suicide attempt – leaving aside the problems with this thought experiment, MWI also seems to imply you can expect a future in which you will always experience the worst possible suffering, among other outcomes.

Let me try to explain my main question in two scenarios:

Someone in a lab measures the spin on an electron

Does MWI mean:

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, one version of me would go on to learn that the electron was spin up, one would find out it is spin down.

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. The particles making up the different versions of me do not literally split, they just take different routes to different futures.

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.

(Genuinely hope I’m not annoying the hell out of everyone , I just want to understand more and I don’t have the technical ability to follow the literature on this – I also think some of the literature is quite philosophically confused!).

 

[PeterDonis]

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 false. Unitary evolution can't do this.

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.

This is closer to what MWI says.

 

[PeroK]

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?

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.

 

[akvadrako]

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.

I don't think they are philosophically different, at least in terms of what you should expect to experience.

Each version of you will only experience one timeline in both the splitting and the diverging descriptions. If this wasn't true, you would already be experiencing something different. So it's a matter of degree how altruistic it is to care about those other "yous" vs other people in the current world, because once the split has happened, you are different beings.

It's hard to explain this in normal language, but there is a classical analogy which I think captures the essence of it pretty well. Imagine we had perfect cloning machines. Each night you enter the machine, the original body is removed, and say a million perfect copies wake up in different rooms which will never interact until the end of the universe. Most of those copies are giving a very typical room, some are given bad rooms, and some good rooms.

From then on they live their own lives – they are not the same person. So even though there is no randomness involved from an objective (god or bird's eye) viewpoint, there is something like a random expectation from a subjective viewpoint, just like in non-MWI theories. In either case, over repeated iterations, to maximize your expected payout, the best strategy is to place bets in accordance with expecting to become one of the typical copies.

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.

 

[PeterDonis]

Imagine we had perfect cloning machines.

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]

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.

Are you saying that the versions of each "future you" in the different branches cannot be perfect copies?

 

[PeterDonis]

Are you saying that the versions of each "future you" in the different branches cannot be perfect copies?

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.

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.

 

[akvadrako]

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.

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.

And better not to say copies, but perhaps more future versions. 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, even assuming a diverging view.

 

[PeterDonis]

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

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.

 

[Moes]

These two scenarios are very philosophically different

You are getting into many philosophical questions, many which we already are confused about. It’s not even clear whether your two scenarios are at all philosophically different.

If there are two identical versions of you are there two of you or one?

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.

I don’t think it makes any more sense to expect to become only one of the copies.

If the first version is true, philosophy has a lot of work to do on what MWI means for individuals

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?

Before we have answers to all these questions I think it’s pointless to ask specifically about MWI.

 

[PeterDonis]

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?

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.

Before we have answers to all these questions I think it’s pointless to ask specifically about MWI.

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]

since you are basically saying the thread is pointless

Sorry if this is what it sounded like I was saying, its not at all what I was trying to say.

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 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.

 

[akvadrako]

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.

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.