# Question about Many Worlds branching in Quantum Mechanics

Supposing the Many Worlds interpretation of QM is true... If a branching occurs during what we perceive is a wave function collapse, why would this be perceptible to us as probabilties? Wouldn't we just branch, leaving it just as imperceviable as the passage of time? That is, it just happens. By way of analogy, there is no intermediate process in the passage of time. And this is how I envision the branching of many worlds.

In other words, why would we have any evidence of our own branching?

Demystifier
Gold Member
Supposing the Many Worlds interpretation of QM is true... If a branching occurs during what we perceive is a wave function collapse, why would this be perceptible to us as probabilties? Wouldn't we just branch, leaving it just as imperceviable as the passage of time? That is, it just happens. By way of analogy, there is no intermediate process in the passage of time. And this is how I envision the branching of many worlds.

In other words, why would we have any evidence of our own branching?
Why do you think that passage of time is not perceivable? What do you mean by "intermediate process in the passage of time"?

Why do you think that passage of time is not perceivable? What do you mean by "intermediate process in the passage of time"?

Point is, the supposed branching process is impercievable in the macroscopic world. I know decoherence explains part of that, but it isn't scientifically accounted for why there is a manifestation of the branching process in the microscopic world, when measuring/observing. It is unaccounted for as far as I can tell

Demystifier
Gold Member
the supposed branching process is impercievable in the macroscopic world.
Why?

I know decoherence explains part of that, but it isn't scientifically accounted for why there is a manifestation of the branching process in the microscopic world, when measuring/observing. It is unaccounted for as far as I can tell
It is one thing to claim that it is not clear why the branching has a manifestation, and another to claim that it does not have a manifestation. Which of the two do you actually claim?

It is one thing to claim that it is not clear why the branching has a manifestation, and another to claim that it does not have a manifestation. Which of the two do you actually claim?

The first one. We are assuming for arguments sake that some version of the many worlds is true, that is alternate histories are constantly taking place. There is no accounting for why there is a visible branching in the subatomic world when measuring, instead of just branching right away and no knowledge that it took place.

Demystifier
Gold Member
Perhaps you are asking this: If the wave function branches, then why do we see only one branch and not all of them? Is that your question?

Perhaps you are asking this: If the wave function branches, then why do we see only one branch and not all of them? Is that your question?

I am asking why we see any branching at all, and probabilities assigned, supposing Many Worlds is true. I don't believe a Many Worlds universe would look like the universe we live in. If it was proposed prior to findings of QM, I would be far more receptive to the theory.

Demystifier
Gold Member
I don't believe a Many Worlds universe would look like the universe we live in.
So how do you believe that a many worlds universe would look like?

DrChinese
Gold Member
I am asking why we see any branching at all, and probabilities assigned, supposing Many Worlds is true. I don't believe a Many Worlds universe would look like the universe we live in. If it was proposed prior to findings of QM, I would be far more receptive to the theory.

No, it was proposed about 30 years later, essentially with the intent to explain elements of probabilistic outcomes. If you throw a die repeatedly, and there is branching, you still see the expected statistical outcomes. Except in perhaps a very small number of branches. And even those will eventually evidence the normal patterns.

For instance, a many worlds theory can account for why the value of the universe is largely arbitrary. As you know, there a far more theoretically functional universes than just the one we live in, and this is driving physicists nuts. One is prone to thinking that the universe somehow exists out a mathematical neccesity, but modern physics debunks this old notion.
Many worlds can explain these arbitrary values due to the other universes already existing parallel to ours.

No, it was proposed about 30 years later, essentially with the intent to explain elements of probabilistic outcomes. If you throw a die repeatedly, and there is branching, you still see the expected statistical outcomes. Except in perhaps a very small number of branches. And even those will eventually evidence the normal patterns.

I wrote "if" it was proposed before rather than after QM

So how do you believe that a many worlds universe would look like?

A subatomic world identical to the macroscopic, driven by Newtonian principles.

DrChinese
Gold Member
I wrote "if" it was proposed before rather than after QM

I saw that. What I meant is that MWI was proposed to resolve the some of the collapse issue.

I personally don't see it as much of a solution, but that's me. After all, some collapse is reversible.

I saw that. What I meant is that MWI was proposed to resolve the some of the collapse issue.

I personally don't see it as much of a solution, but that's me. After all, some collapse is reversible.

It is not a bad theory and exemplifies "thinking out of the box". I don't think there was much thought given to multiverses and many worlds prior to Everett.

Demystifier
Gold Member
I am asking why we see any branching at all, and probabilities assigned, supposing Many Worlds is true.
Perhaps you think that first there are many worlds and then branching somehow happens? If that's what you think, that's completely wrong and probably the main source of your confusion. We first have one world, then a branching (more precisely - decoherence) occurs, and then we have many branches which can be thought of as "many worlds".

A subatomic world identical to the macroscopic, driven by Newtonian principles.
I have no idea why do you think so.

So in my opinion, Many Worlds leads to a

"In logic, reductio ad absurdum (Latin for "reduction to absurdity"; also argumentum ad absurdum, "argument to absurdity") is a form of argument which attempts either to disprove a statement by showing it inevitably leads to a ridiculous, absurd, or impractical conclusion, "

Chris Miller
Perhaps you think that first there are many worlds and then branching somehow happens? If that's what you think, that's completely wrong and probably the main source of your confusion.

Not really. It is equally unaccounted for why we are privy to witnessing the branching.

Demystifier
Gold Member
For instance, a many worlds theory can account for why the value of the universe is largely arbitrary. As you know, there a far more theoretically functional universes than just the one we live in, and this is driving physicists nuts. One is prone to thinking that the universe somehow exists out a mathematical neccesity, but modern physics debunks this old notion.
Many worlds can explain these arbitrary values due to the other universes already existing parallel to ours.
Aaaa, I think I finally see the source of your confusion. You are confusing multiverse with many worlds. Those are different things!!!!

The source cited in this post is not acceptable under the Physics Forums rules
I have no idea why do you think so.

Hehe, well... My contention is that the citizens of a many worlds reality would NOT be privy the manifestation of the branching at any scales of observation. The reason I believe that is because it is an unneccesary component of the theory, and one which is unnaccounted for. You could say that I am applying Ockhams Razor.

Demystifier
Gold Member
Multiverse = Many Worlds, Say Physicists
Two of the most bizarre ideas in modern physics are different sides of the same coin, say string theorists

https://www.technologyreview.com/s/424073/multiverse-many-worlds-say-physicists/
So are you actually trying to say that you disagree with this interpretation? I do too, as do most other physicists. Many worlds and multiverse are considered different by almost all physicists. Even the authors of the idea above did not take this idea seriously in their subsequent works.

bhobba
So are you actually trying to say that you disagree with this interpretation? I do too, as do most other physicists. Many worlds and multiverse are considered different by almost all physicists. Even the authors of the idea above did not take this idea seriously in their subsequent works.

I am not suggesting that they are one and the same, but they are compatible models that could very well be the same thing.

So are you actually trying to say that you disagree with this interpretation?

Given what we know about quantum mechanics, yes.

bhobba
Mentor
So in my opinion, Many Worlds leads to a Reductio ad absurdum

I think you need to understand what it really is saying first.

Murray Gell-Mann explains it well:

What MW calls a world is almost certainly not what you think it is - it not specified that clearly.

Thanks
Bill

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I think you need to understand what it really is saying first.

Murray Gell-Mann explains it well:

What MW calls a world is almost certainly not what you think it is - it not specified that clearly.

Thanks
Bill

I'm not saying that the many worlds theory leads to reductio ad absurdum by default. The theory is consistent but probably not an accurate description of the universe. Quantum Mechanics casts serious doubt on the MW postulates, for the reason I gave.

bhobba
Mentor
I'm not saying that the many worlds theory leads to reductio ad absurdum by default. The theory is consistent but probably not an accurate description of the universe. Quantum Mechanics casts serious doubt on the MW postulates, for the reason I gave.

Well I will detail QM's postilutes

1. For every observation there exists an observable O whose eigenvalues are the possible outcomes of an observation.
2. The average of those outcomes E(O) = trace (OS) where S is a positive operator of unit trace by definition called the state of the system.

Now in what way does MW contradict them, bearing in mind of course, as explained by Murray Gell-Mann it's virtually the same, except for some semantics about what you call a history, as Decoherent Histories. You prove MW wrong and you have done the same for Decoherent Histories.

Thanks
Bill

The man in the clip seems to suggest that the probability distribution is a function of the theory, rather than just some sideffect. This makes the theories plausability even more remote than I originally perceived it to be. I would be very amused if the universe operates in this way.

I think people are so befuddled by the probability distribution that they desperately conjure up anything. There are less freakish explanations as to why we are stuck with probabilities.

bhobba
Mentor
The man in the clip seems to suggest that the probability distribution is a function of the theory, rather than just some sideffect. This makes the theories plausability even more remote than I originally perceived it to be. I would be very amused if the universe operates in this way. I think people are so befuddled by the probability distribution that they desperately conjure up anything. There are less freakish explanations as to why we are stuck with probabilities.

Do you know who Murray Gell-Mann is? Most would take what he says a bit more seriously and think through it a bit more carefully, asking for clarification on any point not clear.

Anyway that's not what he is saying, but as to the role of probabilities in QM, there is nothing befuddled about it - we know a lot about why it's that way:
https://en.wikipedia.org/wiki/Gleason's_theorem

You don't like MW, and many agree, but the situation is more nuanced than you seem to think it is.

I hold to the ignorance ensemble interpretation personally, but no interpretation is any more valid than any other. All have something to like and equally dislike. The choice is made purely on personal preference.

You made the claim - 'Quantum Mechanics casts serious doubt on the MW postulates, for the reason I gave.'

I gave the postulates of QM and asked you to simply back it up.

Thanks
Bill

Well I will detail QM's postilutes

1. For every observation there exists an observable O whose eigenvalues are the possible outcomes of an observation.
2. The average of those outcomes E(O) = trace (OS) where S is a positive operator of unit trace by definition called the state of the system.
Do those axioms include the bit about the outcomes of the operator will occur one at a time with probabilities determined by the eigenvalues of the operatior ?
I understand the MW ditches that by asserting that every outcome exists in a branch.

bhobba
Mentor
Do those axioms include the bit about the outcomes of the operator will occur one at a time with probabilities determined by the eigenvalues of the operatior ? I understand the MW ditches that by asserting that every outcome exists in a branch.

Of course it does - its in axiom 2.

What you are probably talking about is MW also has a decision theory proof of axiom 2. That is controversial. I prefer to stick with Gleason. There was a thread a while back looking at the various proofs - but it is at an advanced level not really suitable here in a beginners thread. But to be fair I spoke about things like eigenvalues etc that are I level, not B level to flesh out the OP's logic that MW is logically flawed. That is a very strong statement that can only be discussed at the proper technical level. I do not really like doing that - but then again like I said it is a rather strong statement. You can do a search on the thread if you like - but beware it is at the A level.

Murray Gell-Mann explains that the worlds in MW are not really what the usual conception of a world is - Everett, Wallace and others are talking of something less concrete, although one can, if one wishes think of it that way. But what he points out is to what gain - except confusing people. And when thought of in what he thinks of as the correct way it becomes more of a semantic issue if it's any different to the Decoherent Histories approach he, Hartle and Feynman (just before he died) developed.

Please, please, I know its a pain but do look at Murray's video. Murray of course, along with Feynman were the two great physicists at Caltech. Nobody else there could really keep up with them - Murray could only converse at his level with Feynman and initially they were very good friends - you would find them talking all the time with most other physicists just sitting there amazed at how freely the ideas and concepts flowed. Both, like one of the other great physicists of the time, Lev Landau, were so at ease with the physics behind the math it was literally frightening. But Murray and Feynman became a bit estranged later due to differences in style which you can Google. When Feynman died Murray was quite saddened, and was to give a eulogy at his wake. He never turned up and some thought was it because of his estrangement. It wasn't - he was dressed, ready to leave, when FBI agents detained him about a matter to do with possibly stolen Mayan artifacts - that was one of the differences between he and Feynman - Murray was interested in cultural things like that while Feynman was more along the lines of - I am just a kid from Far Rockaway that has seen through the ways of you city slickers. Not germane to the issue here of course, just an interesting aside on Gell-Mann and why what he says in the video should be taken seriously.

Just to finish - there in no logical contradiction in MW - but I would prefer the OP to understand that himself rather than me explaining why.

Thanks
Bill

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Of course it does - its in axiom 2.
[clipped to save space]
Thank you, Bill. Just checking.

atyy
Do you know who Murray Gell-Mann is? Most would take what he says a bit more seriously and think through it a bit more carefully, asking for clarification on any point not clear.

Well, then take negative probabilities seriously: https://arxiv.org/abs/1106.0767

bhobba
when you compute the probability (<a + <b)(a> + b>) = aa* + bb* + ab* + a*b
the last sum may be negative. calling it a negative probability is a question of words
but it is not an exotic thing.

stevendaryl
Staff Emeritus
I saw that. What I meant is that MWI was proposed to resolve the some of the collapse issue.

I personally don't see it as much of a solution, but that's me. After all, some collapse is reversible.

I don't understand the connection with the last sentence with the topic. Does reversibility of collapse say something about MW, or does it say something about human's fickleness in embracing and rejecting interpretations of quantum mechanics?

bhobba
Mentor
Well, then take negative probabilities seriously: https://arxiv.org/abs/1106.0767

I had a conversation with my probability professor about that one. It occurs as an intermediate calculational result in a number of advanced areas - but is thought to be just that - results of calculations that cant be interpreted as usual. I gave him Feynman's famous article on it and explained when it first occurred in QM it turned out it was not really negative probabilities but positive probabilities of antimatter - although I do not think Gell-Mann is talking about something that easily resolved - not that it didn't take a while to sort it out as far as antimatter and the Klein-Gordon equation was concerned.

Decoherent Histories is not a fully developed interpretation - its quite ambitious in its aims so maybe whats going on with negative probabilities will eventually be clearer.

Thanks
Bill