Problems with Many Worlds Interpretation

[Hurkyl]

Hurkyl,

if I flip a coin and you don't see the result, would you say that the reality of the coin is a probability distribution with roughly equal probabilities for heads and tails? Based on what you said, I don't see how your answer can be anything but "yes".

Yes.

So reality is subjective?

No. This is something that's awkward to discuss because of a lack of good words -- but I'm hoping that since he's using words that can sound like what I mean, that he's thinking what I mean.

For this particular scenario to result as it did, the reality of the coin and environment was that it was already in a mixed state pre-flip. The mixture persisted (as they do in classical mechanics) through the flip, and we supposed that the mixture was typical enough that the final state was close enough to 50-50.

(snip)

I also don't consider this a rejection of QM, as you apparently do. To reject it would be to say that its predictions aren't very accurate (this is certainly not true), or that it doesn't improve our understanding of reality at all. I would say that it does. It just doesn't improve it as much as we want it to.

One of the important features of physical theories is that they give us the language and ideas in terms of which we talk about reality. I do consider rejecting this application of QM as rejecting QM itself.

How much sympathy would you have for an alchemist who teaches his students chemistry, but advises his students that they should not form an idea that materials are made out of molecules and atoms and such; that they are just calculating devices to work out material properties or transmutations or what-not?

 

[Fredrik]

No. This is something that's awkward to discuss because of a lack of good words -- but I'm hoping that since he's using words that can sound like what I mean, that he's thinking what I mean.

I don't understand. If I've seen the result and you haven't, we would use different probability distributions for the coin. So if the probability distribution is reality, then we have different realities. Maybe you were talking about a probability distribution for the state of the entire universe, or at least a big enough part of it to include both of us and the coin.

How much sympathy would you have for an alchemist who teaches his students chemistry, but advises his students that they should not form an idea that materials are made out of molecules and atoms and such; that they are just calculating devices to work out material properties or transmutations or what-not?

Not much, but that's because I know that there's a good theory about "what things are made of". In the case of QM, there's no theory (good or bad) about what's really going on. All we have are non-scientific guesses. It's also possible that we have reached the limits of what the scientific method can do.

 

[Dmitry67]

Just wanted to say Hi, I am back after almost 3 month. I was trying to struggle with the internet addiction, so I tried to be away from the internet this summer. Finally, the victory is on the Internet's side. Anyway, what did I miss, and what I should begin with? :) Just shoked by so many MWI/collapse threads...

 

[Hurkyl]

Maybe you were talking about a probability distribution for the state of the entire universe, or at least a big enough part of it to include both of us and the coin.

Yes. Actually, at that point in the discussion I was happy for it to be just big enough to include the coin.

In the case of QM, there's no theory (good or bad) about what's really going on.

What's wrong with, say, QM? Seriously? Aside from incompatibility with GR, What deficiency does it have other than:

• It doesn't have a unique interpretation, and
• It offends many peoples' sensibilities?

I consider neither of those points relevant.

I suppose I have a big advantage in this matter, that my background includes pure math, computer science, and abstract gaming, so I'm very, very used to the idea of building an understanding based on what something is, rather than upon any preconceived notions of how it should work.

 

[mitchell porter]

I suppose I have a big advantage in this matter, that my background includes pure math, computer science, and abstract gaming, so I'm very, very used to the idea of building an understanding based on what something is, rather than upon any preconceived notions of how it should work.

Hilarious... Hurkyl, if I decide that I will or will not kick you in the balls on the basis of a coin toss, and then I toss a coin and act accordingly, you do not end up in a state of "50% kicked in the balls, 50% not kicked in the balls". At the end, either I left you alone, or you are doubled over in pain.

It is very gauche and aggressive of me to resort to reductio ad kick-in-the-balls, after such a dainty intellectual argument, but the line you are taking is just incredibly absurd if you try to apply it to anything real. Reality consists of a series of definite situations. Maybe there are other realities defined by a different series of situations. But if your model of reality contains no such definiteness anywhere, it is false.

 

[Delta Kilo]

What's wrong with, say, QM? Seriously? Aside from incompatibility with GR, What deficiency does it have other than:

• It doesn't have a unique interpretation, and
• It offends many peoples' sensibilities?

I consider neither of those points relevant.

Whats wrong you say? You're joking, right?
How about having two different incompatible laws for the same thing and no clear explanation why and how and at which point exactly one transitions into another?
I'm talking about unitary evolutuion vs. collapse obviously.

 

[K^2]

That's why we have MWI. It explains you when the transition happens.

 

[Demystifier]

Whats wrong you say? You're joking, right?
How about having two different incompatible laws for the same thing and no clear explanation why and how and at which point exactly one transitions into another?
I'm talking about unitary evolutuion vs. collapse obviously.

I think Hurkyl meant that collapse does not belong to QM itself, but to interpretations of QM.

 

[Fredrik]

What's wrong with, say, QM? Seriously?

I think I have answered that. QM tells us how to associate a unique probability with each experimentally verifiable statement, and it does that extremely well, but it doesn't define a description of a possible reality, at least not one that resembles the reality we live in. Descriptions of possible realities are defined by non-scientific assumptions that people like to add on top of QM.

If you think that QM defines a description of a possible reality, then please explain what that description says. Can you do it without making additional assumptions? If not, I would say that you're talking about a description defined by those assumptions, not a description defined by QM.

 

[Fredrik]

Just wanted to say Hi, I am back after almost 3 month. I was trying to struggle with the internet addiction, so I tried to be away from the internet this summer. Finally, the victory is on the Internet's side. Anyway, what did I miss, and what I should begin with? :) Just shoked by so many MWI/collapse threads...

Welcome back. I don't think you missed anything important (if you left after we switched to a new LaTeX system). Yes, there have been a lot of threads about "what causes collapse" recently. I'm a bit fed up with that topic, so I didn't even open those threads.

 

[BruceW]

Hilarious... Hurkyl, if I decide that I will or will not kick you in the balls on the basis of a coin toss, and then I toss a coin and act accordingly, you do not end up in a state of "50% kicked in the balls, 50% not kicked in the balls". At the end, either I left you alone, or you are doubled over in pain.

This is what happens in MWI. The state would be something like |kicked> + |not kicked>

 

[BruceW]

Just wanted to say Hi, I am back after almost 3 month. I was trying to struggle with the internet addiction, so I tried to be away from the internet this summer. Finally, the victory is on the Internet's side. Anyway, what did I miss, and what I should begin with? :) Just shoked by so many MWI/collapse threads...

I said on page 2 that there has not been any experiments so far to distinguish between the possible interpretations of QM. And I didn't get any disagreements, so maybe that's one thing to take away from this discussion.

Although, I am not looking through all the recent journals. If anyone has seen some new expeiments which disprove any of the interpretations, I'd find that interesting.

 

[Ken G]

Let's start with any different predictions, we'll need that even before there can be experiments. Can anyone suggest a single predicted outcome that comes out any different in any of the interpretations?

 

[Fredrik]

Let's start with any different predictions, we'll need that even before there can be experiments. Can anyone suggest a single predicted outcome that comes out any different in any of the interpretations?

If even one such experiment exists, this "interpretation" would by definition (at least by my definitions) be a theory, not an interpretation.

 

[xts]

Can anyone suggest a single predicted outcome that comes out any different in any of the interpretations?

There is a famous Tegmark's "quantum suicide" experiment. But it may convince only you...
In MWI every suicide attempt must fail - you always live in a world, where Russian rulette gun misfired.
Unfortunately this experiment may convince only you - other people live in worlds in which they cry at your funerals.

But we shouldn't take this argument just as a joke. Actually that is a pretty strong argument putting together the collapse/measurement/many-world-forking and consciousness.

 

[K^2]

While Quantum Suicide only works in MWI, MWI does not guarantee that Quantum Suicide works. There is still a lot of debate on that one. So even if you were absolutely dead set on proving MWI, that might not be the way to go.

 

[xts]

While Quantum Suicide only works in MWI, MWI does not guarantee that Quantum Suicide works.

Why? If only my suicide attempt is not 100% effective (well, even as all patrons are loaded, the one still may misfire...), my consciousness must survive!

I'll be grateful for any references to the debate on quantum suicide!

 

[Fyzix]

Why? If only my suicide attempt is not 100% effective (well, even as all patrons are loaded, the one still may misfire...), my consciousness must survive!

I'll be grateful for any references to the debate on quantum suicide!

No... A consciousness will survive, will it be you? not guaranteed at all.

For simplicity's sake say your suicide attempt can have 10 outcomes, 9 of which you will infact die and 1 where you will live.
Nothing will make the 9 consciousnesses that cease to exist JUMP over into the single branch that their "twin" will survive in.Jacques Mallah has written a lot on this: http://arxiv.org/ftp/arxiv/papers/0902/0902.0187.pdf

 

[JordanL]

No... A consciousness will survive, will it be you? not guaranteed at all.

For simplicity's sake say your suicide attempt can have 10 outcomes, 9 of which you will infact die and 1 where you will live.
Nothing will make the 9 consciousnesses that cease to exist JUMP over into the single branch that their "twin" will survive in.


Jacques Mallah has written a lot on this: http://arxiv.org/ftp/arxiv/papers/0902/0902.0187.pdf

When a death occurs, the experience of everyone else would be of that person dying, while that person would continue living from their subjective experience. However their "jumping" between worlds would indicate that consciousness transcends reality and knowledge, or any particular version of it. In order for this to make sense, at least from what I understand, awareness has to be more fundamental to existence than any other concept, including logic or realism, as consciousness would represent something which can exist both independent of and within anything that can be described within our Universe.

In this way, awareness must be something upon which everything else is based, instead of the other way around, if jumping were to occur. If the Quantum Suicide experiment were true, we would have found a plausible cause for the existence of the universe: so that (at least) we could experience it.

The Quantum Suicide experiment has less to do with MWI and more to do with meaning and existence, in my opinion. And in that way is interesting to ponder, but also not helpful for understanding the differences between CI and MWI.

 

[BruceW]

This quantum suicide thought experiment doesn't actually provide a way of discerning between MWI and CI, right?

 

[JordanL]

This quantum suicide thought experiment doesn't actually provide a way of discerning between MWI and CI, right?

It provides a way to help explain conceptual differences, but does not discern them or define them.

 

[Hurkyl]

In MWI every suicide attempt must fail - you always live in a world, where Russian rulette gun misfired.

This is a level slip -- it's confusing the frog's eye and the bird's eye view. No matter how much you want to pay attention to

P(You live) > 0​

you can't get around the fact that

P(You live | You died) = 0.​

You point out there are "worlds" where people live to attend your funeral. In those same "worlds" you get to attend your funeral dead.

 

[Dmitry67]

Yes, it is a confusion of 2 meanings of "YOU" - YOU as a worldline (frog) and YOU as a tree (Bird). These 2 meanings are different in MWI. "Observer" has 2 meanings as well in MWI.

 

[Hurkyl]

It is very gauche and aggressive of me to resort to reductio ad kick-in-the-balls, after such a dainty intellectual argument, but the line you are taking is just incredibly absurd if you try to apply it to anything real. Reality consists of a series of definite situations.

Yes it is. What does your proposal accomplish other than to make you look like a fool? The variable "X or not X" has only one outcome -- "true". Checking for that result has no power to distinguish between any alternatives at all, let alone the ones under consideration.

 

[mitchell porter]

Hilarious... Hurkyl, if I decide that I will or will not kick you in the balls on the basis of a coin toss, and then I toss a coin and act accordingly, you do not end up in a state of "50% kicked in the balls, 50% not kicked in the balls". At the end, either I left you alone, or you are doubled over in pain.

It is very gauche and aggressive of me to resort to reductio ad kick-in-the-balls, after such a dainty intellectual argument, but the line you are taking is just incredibly absurd if you try to apply it to anything real. Reality consists of a series of definite situations. Maybe there are other realities defined by a different series of situations. But if your model of reality contains no such definiteness anywhere, it is false.

Yes it is. What does your proposal accomplish other than to make you look like a fool?

It is supposed to make you remember the existence of pain, which in turn is supposed to make you realize that reality has definite properties.

Maybe I'm misunderstanding you; but Fredrik asked you directly, if he flips a coin, what is the state of that coin. You could have said that it's actually heads, or actually tails, and you don't know which; you could even have said that it's actually heads in one world, and actually tails in another world; but instead you said that the actual state is a probability distribution.

Your subsequent brag (about how your conceptual facility with mathematics and computers has equipped you to accept things as they are, in all their counterintuitiveness) reinforces my initial suspicion that there's something seriously lacking in your concept of what physics is about. Physics, including quantum mechanics, is supposed to describe the real world of stars and planets and living things. It is not just an abstraction living in your imagination or on your hard drive. One of the features of the real world is that specific things happen.

I have, I hope, been fairly clear about how I think quantum mechanics should be interpreted. Observables are real, wavefunctions are not, they just provide probabilities for the behavior of observables; that is how QM is meant to be used. I have so far failed to discern any similarly clear position from you. You objected when I said that wavefunctions aren't real, but your subsequent remarks have been entirely formal in character, and do not indicate where you think QM connects to reality. Well, there was the nonsensical affirmation that a probability distribution over the possible outcomes of a coin toss is the actual state of the coin. This is where the kick in the balls comes in. It is supposed to make you understand that things actually happen.

 

[Hurkyl]

you could even have said that it's actually heads in one world, and actually tails in another world; but instead you said that the actual state is a probability distribution.

Yes. (Aside: you do notice that, in classical mechanics "probability distribution over N states" is essentially identical to "N worlds (with weights that sum to 1)" right?)

Well, there was the nonsensical affirmation that a probability distribution over the possible outcomes of a coin toss is the actual state of the coin.

I maintain there is no experiment that can possibly be done to distinguish between definite outcomes and indefinite outcomes.

If there is no experiment that can distinguish between possibilities, there is no physical content in the assertion that reality has definite outcomes rather than indefinite outcomes.

If there is no physical content to the assertion that reality has definite outcomes rather than indefinite outcomes, then the claim that you're talking about reality and I'm talking about nonsense is just you blowing smoke.


Nothing QM specific is relevant to anything you've said in this post. You are familiar with probability distributions on classical state spaces, right? I challenge you to find an experiment that can distinguish between reality is a single point of phase space versus reality is a probability distribution on phase space.

 

[mitchell porter]

Yes. (Aside: you do notice that, in classical mechanics "probability distribution over N states" is essentially identical to "N worlds (with weights that sum to 1)" right?)

If you mean "possible worlds" then I think I accept the equivalence.

I maintain there is no experiment that can possibly be done to distinguish between definite outcomes and indefinite outcomes.

If there is no experiment that can distinguish between possibilities, there is no physical content in the assertion that reality has definite outcomes rather than indefinite outcomes.

If there is no physical content to the assertion that reality has definite outcomes rather than indefinite outcomes, then the claim that you're talking about reality and I'm talking about nonsense is just you blowing smoke.

Every second of your waking life tells you that at least one "definite outcome" exists. This whole idea of an "indefinite outcome" is a contradiction. Either something is happening or it isn't happening.

Many worlds makes sense as a belief in "many definite outcomes". It then founders on the problem of reproducing the Born probabilities, but at least there's a definite hypothesis.

 

[Hurkyl]

If you mean "possible worlds" then I think I accept the equivalence.

I don't know what you're trying to connote with "possible". My best guess is that you're using it to mean "reality is in one of these worlds", which is very much not what I'm talking about.

Every second of your waking life tells you that at least one "definite outcome" exists. This whole idea of an "indefinite outcome" is a contradiction. Either something is happening or it isn't happening.

Show me an experiment that can tell the difference. Anything at all. The criterion you're suggesting is to observe:

P(X or not X) = 1​

or possibly

P(X and not X) = 0​

but neither criterion differentiates between X being a definite value that is either "true" or "false" and X being a random variable with sample space {"true", "false"}.

Let me repeat that for emphasis. In an indefinite outcome interpretation of classical mechanics, if X is any proposition tested by experiment, then you are guaranteed to observe either X or not X. "X or not X" is definitely true, whether X is definite or not.

 

[Demystifier]

There is a famous Tegmark's "quantum suicide" experiment. But it may convince only you...

First, it will not "convince" only me, but also all my neighbors living in the branch in which I survive each time.

Second, I doubt that it will really convince anybody; it could also be that I was just lucky to survive each time, or that there is some secret mechanism providing my survival in a deterministic manner (for example, perhaps someone found a way to control the initial conditions of Bohmian hidden variables).

 

[mitchell porter]

I don't know what you're trying to connote with "possible".

I don't want to presuppose the actuality of all the worlds appearing in the probability distribution.

Every second of your waking life tells you that at least one "definite outcome" exists. This whole idea of an "indefinite outcome" is a contradiction. Either something is happening or it isn't happening.

Show me an experiment that can tell the difference. Anything at all.

The criterion you're suggesting is to observe:

P(X or not X) = 1​

or possibly

P(X and not X) = 0​

but neither criterion differentiates between X being a definite value that is either "true" or "false" and X being a random variable with sample space {"true", "false"}.

Let me repeat that for emphasis. In an indefinite outcome interpretation of classical mechanics, if X is any proposition tested by experiment, then you are guaranteed to observe either X or not X. "X or not X" is definitely true, whether X is definite or not.

The radical insanity of your position makes it difficult to rebut. Or rather, it is easy to rebut, but it is difficult to make you acknowledge the rebuttal. I can't say exactly what your problem is, but it has something to do with misuse of formal concepts like "random variable".

First you ask for an experiment that can tell the difference between a definite outcome and an indefinite outcome. I repeat that an indefinite outcome is an incoherent concept, a logical contradiction. The experiment has an outcome, or it doesn't. If it has an outcome, it is necessarily a definite outcome. There is no such thing as an experiment with an "indefinite outcome" in the sense you are discussing.

Your distinction between

X being a definite value that is either "true" or "false" and X being a random variable with sample space {"true", "false"}

appears to be a confusion of levels. I'll just quote Wikipedia:

In probability and statistics, a random variable or stochastic variable is, roughly speaking, a variable whose value results from a measurement on some type of random process. Formally, it is a function from a probability space, typically to the real numbers, which is measurable.

It's as if you're saying "Your experiment can't distinguish between the possibility that X is a value of a function, and the possibility that X is a function."

I am not sure about the following attempt at cognitive debugging, I can only guess at the details of the thought process which causes you to say what you do; but your remark may be arising from an inconsistent use of the concept "random variable". Either you are talking about it as a mathematical concept, or you are using it to refer to a physical quantity. If you are talking about an experiment, then your random variable is a physical quantity, in which case it necessarily satisfies your first option - 'a definite value that is either "true" or "false"'. The "function from a probability space", etc, specifies the mathematical formalism we use to describe the physical variable, and it says something about the properties of the possible values of the physical variable (their probabilities).

 

[Fredrik]

First, it will not "convince" only me, but also all my neighbors living in the branch in which I survive each time.

If that's true, there's no need to have the experiment involve a suicide. Just fire the gun into a barrel of water, or throw away the gun and just measure the spin of a bunch of silver atoms. But does getting the result "up" the first 20 times really make the CI less likely to be true than the MWI? I don't see how, and apparently neither does Tegmark. What he said (I just checked) is that

• a quantum suicide is the only experiment he can think of that allows us to distinguish between MWI and Copenhagen.
• it will only convince the person in front of the gun, not the person behind it.
• the probability that the person in front of the gun will hear a click (indicating that the gun isn't going to fire) is 1.

 

[Hurkyl]

Let P be the phase space of a universe-describing physical theory. Any theory will work here (even hypothetical yet-to-be-discovered theories)

Suppose we've solved the measurement problem for P -- for a configuration x, we are able to ask the question

E(x) = "does the configuration x include a person named mitchell porter that flipped a coin?"​

Similarly,

H(x) = "does the configuration x include a person named mitchell porter that flipped a coin that turned up heads?"
T(x) = "does the configuration x include a person named mitchell porter that flipped a coin that turned up tails?"
​

where E, H, and T take values in the boolean algebra {true, false}. And, for example in our hypothetical universe, we have a logical identity E(x) ==> (H(x) or T(x)) expressing the fact that if a configuration includes a coin-flipping experiment, then it either comes out heads or it comes out tails.



Now, construct a new physical theory whose phase space is PxP (i.e. a configuration of the new theory is a pair of configurations of the old theory). The laws of physics operate pointwise (so the time evolution of (y,z) can be computed by evolving y and z individually according to the old theory, then pairing them back together).

(PxP could be replaced with other interesting things, like a product of more copies of P, or by the space of probability distributions on P, or by a space of arbitrary distributions on P -- everything behaves in essentially same way, so I've used PxP for simplicity)

The solution to the measurement problem works the same way -- with E, H, T, or any other proposition that asserts something about what goes on in the universe taking values in the boolean algebra { (true, true), (true, false), (false, true), (false, false) }.

And in this new theory, the logical identity E(x) ==> (H(x) or T(x)) for any configuration x (= (y,z)) in the phase space PxP.


All of the physical laws in the theory I described with phase space PxP have exactly the same form as the physical laws on the original theory with phase space P. All of our propositions about what happens in the universe decompose into a pair describing what happens on each component.


The difference between the original theory and the new theory are completely imperceptible to anything or anyone described by the universe. If it is possible to talk about whether the state of the universe includes an experiment and what outcome occurred, then it is impossible to use this experiment to distinguish between the two theories described above.

And if no experiment can distinguish, then there is no scientific basis for insisting that one does a better job of describing reality than the other.

I don't want to presuppose the actuality of all the worlds appearing in the probability distribution.

Whether you want to presuppose something shouldn't affect your ability to acknowledge it equivalent or otherwise indistinguishable to something else.

 

[kith]

it will only convince the person in front of the gun, not the person behind it.

I haven't read the paper(s), but doesn't the answer to the question 'who will be convinced?' also depend on the branch, in which the surviving experimentalist ends up? There will be a number of branches where he survives, and in some the spectators will be convinced of MWI, and in some not. Depending on the unlikelihood of his survival, on the question whether the spectators are aware of his experiment, etc.

So although it is only the person who commits the suicides who can conduct the experiment, the question 'how likely will others be convinced of MWI?' should be subject to the experimental setup.

 

[Fyzix]

First, it will not "convince" only me, but also all my neighbors living in the branch in which I survive each time.

Second, I doubt that it will really convince anybody; it could also be that I was just lucky to survive each time, or that there is some secret mechanism providing my survival in a deterministic manner (for example, perhaps someone found a way to control the initial conditions of Bohmian hidden variables).

Yeah and not to mention: if MWI had been true it would have to somehow obey Born Rule, so whether it would be many worlds or only one world the probability of surviving would have to be the same, so it wouldn't be a proof of anything to you.

In a single universe the chance of you surviving a shot to the head would be identical to the probability in a functioning model of MWI which could somehow make sense of Born Rule.

QS is just a fallacy anyway, even if MWI miraculously made sense you would not get immortality with it.
Like I started earlier in the thread, it just says that "a" version of you survives, not that "you" survive.
If you are on one of the branches where you die, you die just like a single universe model, what difference does it make that a twin of you survived? Your consciousness doesn't "jump" into this parallel universe like some sort of magical soul.

I'm actually pretty astonished that someone like Tegmark considered this view to ever be legit.

 

[Fredrik]

I haven't read the paper(s),

In case you or someone would want to, this is the one: http://arxiv.org/abs/quant-ph/9709032. It's right at the end.

but doesn't the answer to the question 'who will be convinced?' also depend on the branch, in which the surviving experimentalist ends up? There will be a number of branches where he survives, and in some the spectators will be convinced of MWI, and in some not. Depending on the unlikelihood of his survival, on the question whether the spectators are aware of his experiment, etc.

I don't think we need to complicate things by considering spectators that don't have all the information. The experimenter's assistant knows exactly what the experiment is about, and the question is whether he will be convinced by a low probability result. As I said in my previous post, I don't see why a low probability result would favor a MWI over the CI, and neither does Tegmark.

Tegmark's argument is that the suicide experiment will convince the person who's trying to kill himself, because the MWI says that he/she is certain to survive. (That's his argument, not mine).