Is MWI Self-Contradictory and Does Time Travel Need a New Approach?

[Fyzix]

I create this thread to discuss the different objections to MWI.
Hopefully creating some interresting debate.

First I'd like to start with the Born Rule.

What are the views of proponents and opponents of MWI on this?

 

[A. Neumaier]

I create this thread to discuss the different objections to MWI.

My objections are formulated in the entries '' Circularity in Everett's measurement theory'' and ''On the Many-Worlds-Interpretation'' in Chapter A4 of my theoretical physics FAQ at http://arnold-neumaier.at/physfaq/physics-faq.html#everett

Hopefully creating some interresting debate.

First I'd like to start with the Born Rule.

What are the views of proponents and opponents of MWI on this?

You should first provide yourself the information you know rather than ask others to do all the work for you.

 

[Demystifier]

We already had a thread with a similar subject
https://www.physicsforums.com/showthread.php?t=364063
where I already said what I think about it.

 

[Fyzix]

How exactly does your argument differ from the following:

Instead of Schroedinger's Cat, we have Hilary Putnam's Light Bulps.

1 RED and 1 BLUE

10% chance of the red being turned on, 90% chance of the blue on being turned on.
After performing this a hundred times, we get 90 blue ones and 10 red ones, if not QM would have been falsified.

So how does this relate to MWI?
Well in MWI the worlds "split" at each possible outcome, so essentially this becomes a 50-50% probability.
After conducting this experiment which can only have 2 outcomes at a time, we should expect in MWI to have 50 red and 50 blueones.

Now this is the gist of the basic argument against MWI that most physicists and philosophers hold, but from what I gather you hold a slightly different position, yet I can't seem to quite figure out exactly what this is?

 

[Hurkyl]

Well in MWI the worlds "split" at each possible outcome, so essentially this becomes a 50-50% probability.
...
Now this is the gist of the basic argument against MWI that most physicists and philosophers hold,

This is a ridiculously bad argument. It's usually told in joke form, e.g. some form of:

There are two possible outcomes of playing the lottery: either you win, or you won't. Therefore you have a 50% of winning if you play!​

 

[Fyzix]

No...

This is the ridiculous reality demonstrating the falsehood of MWI.

If there will always be 2 outcomes of a certain experiment, ONLY 2 different outcomes.
What happens after conducting the experiment once?

1 blue 1 red

twice?
another 1 blue 1 red world

It's just MWI taken seriously.

It's weird you decide to comment on a thread regarding MWI if you are not aware of the number one argument against it.

 

[Demystifier]

Two points:

1. I partially agree with Hurkyl: if there are only two possible outcomes, it does not necessarily mean that each of them is equally probable.

2. Yet, the example of Hurkyl is not completely fair, because "to loose in lottery" actually corresponds to many possible different outcomes.

MWI people are, of course, aware of both points above (as is Hurkyl, I'm convinced). Essentially, they try to argue that larger |psi|^2 somehow corresponds to a larger number of different possible outcomes - and hence to a larger probability. But they cannot find a convincing argument for that without taking some additional assumptions. This is not necessarily wrong, but additional assumptions make the whole MWI idea less attractive. Not necessarily because these assumptions by themselves are not attractive (it is a matter of taste), but because MWI usually proclaims the smallest number of assumptions as its main virtue with respect to other interpretations. The smallest number of assumptions is indeed a virtue, but with the smallest number of assumptions the Born rule cannot be explained. To explain it one necessarily must introduce additional ones, but then MWI is no longer better than other interpretations, as long as quality of an interpretation is measured by the inverse number of assumptions.

 

[Demystifier]

If there will always be 2 outcomes of a certain experiment, ONLY 2 different outcomes.
What happens after conducting the experiment once?

1 blue 1 red

Oh, now I see where the mistake in your reasoning is. If there are two different outcomes, what happens after conducting the experiment once? Your answer is wrong, and the correct answer is

1 blue OR 1 red

In other words, only one of the outcomes will appear (according to MWI) to one experimentalist, not both of them. In your argument you count the frequencies in the whole multiworld, but it is not what the Born rule is supposed to describe. To count what the Born rule is really supposed to describe (successfully or not), you should count frequencies appearing to one experimentalist only.

 

[Fyzix]

Nono, ofcourse I don't think the experimentalist will see both.

he splits just like whole world including the experiment.
However,

Pre-experiment you got:

1 Experimenter
A experiment with 2 possible outcomes

After the experiment

You got 2 experimenters each observing one of the 2 outcomes.
They repeat the experiment and this occurs again.

However unless MWI ASSUMES that there is something very special about consciousness that makes the universe somehow put the observers in the universe which would then correspond to the correct probability, they will see 50/50.

It's that simple.

Just draw it on a piece of paper and you will understand exactly what I mean (this thought experiment is Putnam's not mine originally).

 

[Fyzix]

Also, I'm well aware of the fact that Deutsch and Wallace thinks this can be solved by desicion-theoretic approach.

However most disagree strongly with this and there are tons of litterature explaining just why this fails (Adrian Kent, Peter Lewis, Hemmo Meir, Jacques Mallah, David Albert etc. etc. etc.).

However Demystifier, what exactly are the simplest way to show why MWI fails with born rule in your opinion? As I don't really see the difference.

 

[Dmitry67]

"why this fails" is based on faulty assumptions - we had discussed it many times :)

 

[Fyzix]

"why this fails" is based on faulty assumptions - we had discussed it many times :)

I was under the impression that you too realized this problem?
Otherwise, why don't you just fully adopt the Deustch-Wallace approach instead of some mystic consciousness approach?

If by "faulty assumptions" you mean, that there is actually 2 worlds with 2 outcomes after the experiment, then sure.

 

[Dmitry67]

My quote was about other critics you will probably use later (about "splitting the worlds" etc)

I admit that there is a problem with the Born rule. To begin with, I don't know how to even formulate Born rule is the MWI framework. This is why I tend to believe that this is an illusion (like a very special moment 'NOW', which is also not explained by physics). But of course it is not an explanation, rather than a "stub" for a future one.

 

[Fyzix]

My quote was about other critics you will probably use later (about "splitting the worlds" etc)

I admit that there is a problem with the Born rule. To begin with, I don't know how to even formulate Born rule is the MWI framework. This is why I tend to believe that this is an illusion (like a very special moment 'NOW', which is also not explained by physics). But of course it is not an explanation, rather than a "stub" for a future one.

Aha, so have you read and understood Deutsch/Wallace's approach through?

I think it's a bit unfair that you are saying that these arguments we have discussed in PM's are based on false assumptions.

I will have to ask the authors for permission for citing any of them public.
However quite a few "pro-Everettian"s actually feel the exact same way, that you need to add something to the "bare idea" to get a coherent hypothesis.

 

[Dmitry67]

It would be very nice to have quotes. Also, I suggest clearly stating what are we discussing: Born rule or something else - otherwise it could be a mess.

Regarding the Born rule - can anyone formulate Born rule in the MWI framework? Before solving a problem, sometimes it is useful to read the description of the problem.

 

[Demystifier]

However Demystifier, what exactly are the simplest way to show why MWI fails with born rule in your opinion?

I have already answered it several times.

As I don't really see the difference.

The difference is that you think that MWI contradicts Born rule, while I only think that MWI cannot explain the Born rule. Do you see a difference now?

 

[Demystifier]

However unless MWI ASSUMES that there is something very special about consciousness that makes the universe somehow put the observers in the universe which would then correspond to the correct probability, they will see 50/50.

It's that simple.

It may be simple to you, but to me (and not only to me) your argument is totally vague.

Just draw it on a piece of paper and you will understand exactly what I mean (this thought experiment is Putnam's not mine originally).

Again, you should give the source, so that we can judge by ourselves whether the Putnam's argument is really identical to yours. (I would bet it isn't.)

 

[Fyzix]

I have already answered it several times.


The difference is that you think that MWI contradicts Born rule, while I only think that MWI cannot explain the Born rule. Do you see a difference now?

Not really, could you make another simple example?

 

[Fyzix]

It may be simple to you, but to me (and not only to me) your argument is totally vague.


Again, you should give the source, so that we can judge by ourselves whether the Putnam's argument is really identical to yours. (I would bet it isn't.)

Unfortunately the paper isn't free online.
He just gave me the example a long time ago...

Most other physicists I've talked to agree though, that this shows what MWI's main problem is.

Let me ask you this: do you buy the desicion-theoertic approach?
If that's the case, I understand better why you don't accept this argument.

 

[Demystifier]

Not really, could you make another simple example?

Sure.

The assumption that there is life outside the planet Earth cannot explain why humans have 2 legs. Yet, that assumption does not contradict the fact that humans have 2 legs.

 

[Demystifier]

Let me ask you this: do you buy the desicion-theoertic approach?

Yes and no. I accept that from the assumptions taken in this approach the Born rule can be derived. Yet, I do not accept the assumptions themselves.

More precisely, I do not accept one of these assumptions, not because I can prove that this assumption is wrong, but because this assumption is too artificial and taken ONLY for the purpose of getting the Born rule. This is nicely explained in
http://xxx.lanl.gov/abs/0808.2415

 

[Fyzix]

Sure.

The assumption that there is life outside the planet Earth cannot explain why humans have 2 legs. Yet, that assumption does not contradict the fact that humans have 2 legs.

I'm not really sure how this relates to probability ?!
Maybe I'm missing something obvious in your anology.

 

[Demystifier]

I'm not really sure how this relates to probability ?!

It doesn't. It relates to the difference between "contradicts" and "cannot explain".

 

[Fyzix]

So your main born rule argument is that MWI can't explain where it came from?

However you disagree with the majority that says MWI has evn deeper problems in regards to Born Rule?

 

[Demystifier]

So your main born rule argument is that MWI can't explain where it came from?

However you disagree with the majority that says MWI has evn deeper problems in regards to Born Rule?

Yes.

 

[Fyzix]

How exactly does Bohm explain where it came from though?

 

[Demystifier]

I'm not really sure how this relates to probability ?!

Here is a more useful analogy:

Assume that we have 2 tunnels (a red tunnel and a blue tunnel), and that some sort of consciousness is present in both of them. Assume that we know almost nothing about the nature of that consciousness. (Are there additional beings in tunnels which are conscious about tunels? Or are tunnels themselves self-conscious? We simply don't know.) All we know is that each consciousness is conscious about one tunnel only.

Now we ask the following question: What is the probability that a consciousness is in the red tunnel?

If Fyzix were right, then the answer would be unique: the probability is 50%. This, indeed, is not a stupid answer at all. It is the most reasonable guess. Yet, it's not the only possibility. In particular, as we know almost nothing about the nature of consciousness, various possibilities are conceivable. For example, maybe the red tunnel contains a larger number of conscious beings, so it is more likely that a randomly chosen consciousness is from the red tunnel. Or maybe ... well, use your imagination. The point is that, from the assumptions given, we cannot exclude a possibility that the probability of the red tunnel is different from 50%.

 

[Demystifier]

How exactly does Bohm explain where it came from though?

It explains it dynamically. It starts from an arbitrary probability distribution of particle positions at initial time and explores its evolution. It turns out that, in most cases, after some time the distribution reaches an equilibrium distribution which turns out to be equal to the Born-rule one. Moreover, ones the system reaches the equilibrium, it stays there forever. More generally, ones the system reaches a close-to-equilibrium distribution, it stays close to that most of the time.

 

[Fyzix]

Here is a more useful analogy:

Assume that we have 2 tunnels (a red tunnel and a blue tunnel), and that some sort of consciousness is present in both of them. Assume that we know almost nothing about the nature of that consciousness. (Are there additional beings in tunnels which are conscious about tunels? Or are tunnels themselves self-conscious? We simply don't know.) All we know is that each consciousness is conscious about one tunnel only.

Now we ask the following question: What is the probability that a consciousness is in the red tunnel?

If Fyzix were right, then the answer would be unique: the probability is 50%. This, indeed, is not a stupid answer at all. It is the most reasonable guess. Yet, it's not the only possibility. In particular, as we know almost nothing about the nature of consciousness, various possibilities are conceivable. For example, maybe the red tunnel contains a larger number of conscious beings, so it is more likely that a randomly chosen consciousness is from the red tunnel. Or maybe ... well, use your imagination. The point is that, from the assumptions given, we cannot exclude a possibility that the probability of the red tunnel is different from 50%.

Well, then we are over in a different realm I think.
It seems you are suggesting that if the probability is way higher in the red tunnel versus the blue one, there may be more observers there.
This would suggset that the world does not branch into 2 worlds, but way more for the red one than the blue one at each initial experiment.
This is a completely new assumption and is very hard to back up.
How are you to say "this is enough to make 1 world, this isn't enough, this is enough to make 50,7 worlds" etc. what the **** is a .7 world? A half alive world?

I don't think this is what they have in mind with the desicion-theoretic approach at all..

 

[Fyzix]

It explains it dynamically. It starts from an arbitrary probability distribution of particle positions at initial time and explores its evolution. It turns out that, in most cases, after some time the distribution reaches an equilibrium distribution which turns out to be equal to the Born-rule one. Moreover, ones the system reaches the equilibrium, it stays there forever. More generally, ones the system reaches a close-to-equilibrium distribution, it stays close to that most of the time.

Ah, thanks.
However why can't MWI ever reach this equilibrium?

 

[Demystifier]

This would suggset that the world does not branch into 2 worlds ...

No, this would only suggest that consciousness is not described by MWI.

I don't think this is what they have in mind with the desicion-theoretic approach at all..

You are right. But both approaches (my example with consciousness and the decision-theoretic approach) have something in common: Both have SOME additional assumptions (not spelled out in the minimal set of axioms common to all variants of MWI). That's why I say that MWI could be right (or wrong), but it can't be complete.

 

[Demystifier]

Ah, thanks.
However why can't MWI ever reach this equilibrium?

Because it does not have the particles the distribution of which could have this (or any other) distribution. Of course, you can add particles to MWI as an additional axiom (as I repeated a million times, any variant of MWI needs some additional axioms/assumptions) and then explain the Born rule easily, but once you do that you cease to be an orthodox MWI believer and become a member of an unorthodox MWI church, the members of which like to call themselves Bohmians.

 

[A. Neumaier]

It explains it dynamically. It starts from an arbitrary probability distribution of particle positions at initial time and explores its evolution. It turns out that, in most cases,

... only in most cases of the few thousand simulations done so far, for extremely simple (but not too simple) systems. As we had seen in another discussion

If the Boltzmann's H-therorem were any evidence for the universe being in global equilibrium then we would observe this global equilibrium - which means we wouldn't exist, contradiction.

Thus the Boltzmann's H-therorem is no evidence at all evidence for reaching global equilibrium, and because you agreed to the first part of my statement, you have also no evidence for that the state of the universe reached quantum equilibrium in BM.

there is not the slightest trace of a proof that it does so in most cases realized in Nature.

after some time the distribution reaches an equilibrium distribution which turns out to be equal to the Born-rule one.

 

[Hurkyl]

Regarding the Born rule - can anyone formulate Born rule in the MWI framework? Before solving a problem, sometimes it is useful to read the description of the problem.

The literal statement is sort of a trivial one; the Born rule as it relates to propensity is already built-in to the linear algebra used by quantum mechanics. e.g. in the relationship between density matrices and kets, or in the partial trace.

The more interesting question is can we connect to other interesting things. Here's one possible way to cast probabilities as being frequentist probabilities in a decoherence-based interpretation.

e.g. let v_1 be a pure quantum state of a qubit. As usual in QM, we use v_2 = v_1 \otimes v_1 to describe a system that contains two independent copies of v_1, and so forth -- v_n = v_{n-1} \otimes v_1 is a state that describes n independent copies of a qubit in state v_1.

Let S be an observable on a qubit with basis vectors A and B.

Now, let T_{n, p, \epsilon} be the operator that acts on n-qubit states with eigenvalues 0 and 1, whose action on basis states (relative to S) is multiplication by:

• 1, if the proportion of A's in the basis state is in the interval (p -\epsilon, p+\epsilon)
• 0 otherwise

(So T represents an experiment to detect if the proportion of n trials is near p)

With this, there is a projection P (a partial trace) from n-qubit states to (possibly impure) 1-qubit states that discards everything except the one bit of information related to T.

One can now state the frequentist probability as saying the claim "a measurement of v_1 gives A with probability p" is the claim that

\lim_{n \to \infty} P\left( T_{n, p, \epsilon} v_n \right)
converges to the eigenstate |1>.

(For simplicity in the above, I've written states as kets when possible -- but I'm not really working in the Hilbert space of kets)​

 

[Fyzix]

No, this would only suggest that consciousness is not described by MWI.

Only?
So you are saying that this only suggests that you have to make the additional assumption that consciousness is infact not part of the wavefunction?
I would say this is worse than copenhagen!

You are right. But both approaches (my example with consciousness and the decision-theoretic approach) have something in common: Both have SOME additional assumptions (not spelled out in the minimal set of axioms common to all variants of MWI). That's why I say that MWI could be right (or wrong), but it can't be complete.

When you say "it can be right or wrong, but it can't be complete", what exactly do you mean?
I understand that you regard Bohm as a "MWI"-ish interpretation, so I gues this is what you mean by "could be right", but that the bare theory can't be right?

 

[Hurkyl]

Many world supporters.. so what do you think is the most promising approach to solve the "Measure" problem in Many Worlds

Note that a "solution" for MWI is a solution for pretty much any interpretation. (Since MWI is a theory of wavefunctions undergoing unitary evolution, a feature that appears in most interpretations of quantum mechanics)

IMO a large portion of the problem is just psychological -- that people have trouble accepting a solution that is observationally indistinguishable from definite outcomes without actually having definite outcomes.

We don't have to dive into quantum mechanics to wrap our heads around the idea of indefinite outcomes -- the idea already exists in terms of a statistical ensemble. An observer "inside" the ensemble cannot infer any information about the ensemble, including whether or not it's a trivial ensemble with just one component. So all that's left, psychologically, is to wrap your head around the idea of an ensemble really being the state of reality, rather than being a collection of (possibly hypothetical) subsystems of reality.


Once you can accept indefinite outcomes, the potential solution space to the measurement problem becomes much larger -- e.g. subsystems in mixed states become a good substitute for the notion of a physical collapse. Now, the question becomes:

1. Does unitary evolution cause subsystems to transition into mixed states compatible with the observation of apparent collapse?
2. Can we effectively study mixtures? (e.g. by decomposing into individual components and studying those, or maybe thermodynamically)

Aside: classically, it is justifiable to reject this line of thought due to Occam's razor, because the components perfectly fail to interact. But when applied in QM, the razor only really implies the need for a theory of quantum thermodynamics.

 

[Fyzix]

Hurkyl: no it's not psychological at all.
It may be for some layman down the street, but not for scientists and philosophers.

If you partake in this discussion you will see the born/probability problems.
Not to mention in the bare theory of MWI you don't get determinate outcomes at all...

You can't count branches in it, because there is no determinate way to say "here starts world X".

From your naive optimism that MWI has no problems, I guess you are a MWI proponent?

 

[Hurkyl]

Hurkyl: no it's not psychological at all.
It may be for some layman down the street, but not for scientists and philosophers.

Education does not free one from biased viewpoints. There's even a famous quote:

Science progresses one death at a time​

Not to mention in the bare theory of MWI you don't get determinate outcomes at all...

This fact is central to my point -- I think you've severely misunderstood what I was saying.

 

[Fyzix]

no, from what I've seen you are advocating for MWI / born rule.
yesterday you claimed that my example of why branch counting doesn't work was the same as playing lotto and thinking it was 50-50 if you win or not.
If I'm mistaken, please elaborate.

 

[Demystifier]

When you say "it can be right or wrong, but it can't be complete", what exactly do you mean?
I understand that you regard Bohm as a "MWI"-ish interpretation, so I gues this is what you mean by "could be right", but that the bare theory can't be right?

Yes.

 

[Fyzix]

Demystifier: You said something about MWI not being able to explain consciousness?
What did you mean?

 

[Demystifier]

Demystifier: You said something about MWI not being able to explain consciousness?
What did you mean?

I mean that no current theory in physics can explain consciousness.

 

[Fyzix]

Ok, but let's assume consciousness isn't any mysterious at all.
Functionalism, that's it. A system that is aware, nothing more, nothingless.

Do you then see how it is incoherent to say there are 2 outcomes, a observer in both, but one is higher probability?

 

[Hurkyl]

no, from what I've seen you are advocating for MWI / born rule.
yesterday you claimed that my example of why branch counting doesn't work was the same as playing lotto and thinking it was 50-50 if you win or not.
If I'm mistaken, please elaborate.

In a statistical mixture, the different components don't have to be equally weighted. This is true even in classical statistical mechanics -- a probability distribution on configuration space doesn't have to be uniform, or even resemble uniformity.

The new wrinkles that relative states add that I can think of off the top of my head are:
Wrinkle 1:
The decomposition into a statistical mixture of states is not unique -- e.g. the following three qubit states are identical:

• 50% weight on |X+> and 50% weight on |X->
• 50% weight on |Y+> and 50% weight on |Y->
• 50% weight on |Z+> and 50% weight on |Z->

and there are other more esoteric ways to achieve the decomposition.

(note: the qubit state (1/\sqrt{2}) |X+\rangle + (1/\sqrt{2}) |X-\rangle is very very different from the state mentioned above)
Wrinkle 2:
A decomposition of the state of a subsystem into a statistical mixture is not eternal -- the different components can be a coherent superposition in the entire state, and thus interfere and what-not.

 

[Hurkyl]

For fun, here's a more complicated decomposition.

Let V be the vector (0, 3/5, 4/5), and W be the vector (0, -3/5, 4/5)

Then, the following states are identical:

• 50% weight on |V+> and 50% weight on |W+>
• 90% weight on |Z+> and 10% weight on |Z->
• 80% weight on |Z+> and 50% weight on |X+> and 50% weight on |X->
• 25% weight on |W+> and 15% weight on |Y+> and 60% weight on |Z+>

This particular post has nothing to do with MWI -- that each of these statistical mixtures describe the same quantum state of a qubit is pure "shut up and calculate" quantum mechanics.

 

[Fyzix]

I must admit I understood nothing of what you just said :P
I'm sure someone else will and comment on it tho
But could you explain it a little less technical?

 

[Hurkyl]

I must admit I understood nothing of what you just said :P
I'm sure someone else will and comment on it tho
But could you explain it a little less technical?

A Pointa
The non-technical short version is "MWI studies wavefunctions evolving unitarily". Everything MWI studies is already there in "shut up and calculate" quantum mechanics. Among all approaches to quantum mechanics, MWI is the most interested in fully understanding the wavefunction.

An Exercise
Try to imagine classical statistical mechanics, but rather than thinking of a probability distribution as referring to a collection of separate experiments, think of the probability distribution as being what is actually real. e.g. after one trial of your red-blue experiment, 10% of reality is "there was one red outcome" and 90% of reality is "there was one blue outcome". Just one reality, but that reality is a distribution among many real configurations.

This is a purely classical thought exercise -- no quantum involved.

 

[Fyzix]

Yes I'm aware that everything in MWI is already present in shut up and calculate, but this rings true for every interpretation.

I don't see how your thought exercise shows Putnam's argument wrong.
If you are going to insist on 2 REAL outcomes, you instantly have 50/50.
if there is only 2 observers and 2 outcomes after and both stem from the same "Original", there is simply NO other way to get around this argument.

 

[Hurkyl]

Yes I'm aware that everything in MWI is already present in shut up and calculate, but this rings true for every interpretation.

Not really. The various Copenhagen variants assume that a collapse occurs, with probabilities computed via shut-up and calculate. Bohmian mechanics adds a collection of bohmian particles to the state of the system.

I don't see how your thought exercise shows Putnam's argument wrong.

Putnam's argument is wrong because of the lottery thing. The "number of worlds" -- if that phrase even makes sense -- has absolutely no reason to coincide with any probability that is supposed to have bearing on reality.

My thought exercise has nothing to do with whether or not Putnam's argument is wrong. It's to give you a better chance of really understanding the idea of indefinite outcomes. It's very easy for people suppress all critical thought towards an argument against something they dislike (in this case, your dislike of MWI). It's also very easy to obfuscate the issue if you are trying to learn two or three new things at once, rather than just one thing at once. (I'm assuming classical mechanics is not a "new thing" to you)

 

[Fyzix]

First: this had nothing to do with me disliking MWI, I dislike MWI for technical reasons, not some aesthetical reason.
Stop assuming so much, it's so typical of MWI'ers to assume I guess...
I'm detecting a trend.

I guess I'll just take Putnam's argument and my own logic over your twisted logic.
You have demonstrated twice that you don't understand the argument by stating "it's the same as believing lotto is 50/50".

Well yea, if the lottery only has 2 balls and you get one, I get one, but I have 90% chance of getting the ball, but you only got 10% of getting the ball, yet both of us have a 100% chance of getting the ball.
Are you starting to see the error of your thinking?