# Many Worlds or Many Problems?

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

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://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html#everett
Hopefully creating some interresting debate.

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.

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
Staff Emeritus
Gold Member
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!​

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
Gold Member
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
Gold Member
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.

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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).

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.

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

"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.

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.

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.

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
Gold Member
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.
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
Gold Member
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.)

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?

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.

If that's the case, I understand better why you don't accept this argument.

Demystifier
Gold Member
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
Gold Member
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

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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
Gold Member
I'm not really sure how this relates to probability ?!
It doesn't. It relates to the difference between "contradicts" and "cannot explain".

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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