Why does nothing happen in MWI?

[Ilja]

I am still not sure whether I understand you. How exactly does classical mechanics specify that a classical theory must specify a particular configuration space? edit - Why shouldn't a theory consider different spaces and develop transforms to go from one to another? In QM, polarization states do not have a unique "configuration space", i.e. basis. Yet it's easy to say *this* state (suitably prepared) is a|A>+ b|B> in one basis and |c|C>+d|D> in another. Why is it non-physical to talk of the photon state as being different things in different configuration spaces? I don't see why you should reject MWI as unphysical because it accommodates talking about circular polarization and linear polarization at the same time.

I think this is the simplest, and, therefore, preferable way - to specify one particular configuration space, one particular evolution equation and so on.

Your proposal to allow for different configuration spaces, which may be transformed into each other, but leave all physical observables unchanged, is a variant, which appears automatically, in a quite natural way, if one observes that all the observable things are not sufficient to specify the configuration space completely. For a realist (and, let's not forget, MWI claims to be a realistic interpretation, thus, justified or not, it has to follow the general principles of realistic theories) this is an unimportant complication caused by the unfortunate fact that we cannot observe everything. But this is unimportant, because, if we would have the ability to observe everything, we would have no problem to identify the correct choice. And, moreover, it would not be problematic at all if we make the wrong choice - the observables would be the same.

So, note, this allowance for different but equivalent descriptions does not change any physical prediction - we allow these different description only because these observable things are not changed.

MWI is rejected because the Nirvana frame does not reproduce the visible changing universe, but predicts an obviously different, statical universe. Or, in my variant, because different choices of the operator q lead to different physics, even if the Hamilton operator h is the same.

 

[Demystifier]

What do you think is inelegant about it?

It is mathematically and physically natural to consider the same vector represented in different basis as the same object. So it is un-natural and in-elegant not to consider so.

 

[Quantumental]

Happy to see this topic getting some serious discussion, as it deserves.
My question for you Derek Potter with regards to the Many-Many worlds you seem to be a proponent of, is how you get the probabilities correct?

 

[mfb]

We don't want to insist on a preferred basis/frame unless it's forced upon us.

Exactly. Something happens in most bases/frames. Only very special ones have a trivial time-evolution.

 

[atyy]

Happy to see this topic getting some serious discussion, as it deserves.
My question for you Derek Potter with regards to the Many-Many worlds you seem to be a proponent of, is how you get the probabilities correct?

I think one way to see the probabilities are right is to use Bohmian Mechanics as a form of MWI. So if BM is right, MWI is right. To paraphrase some MWI proponents: MWI is simply BM with no worlds picked out :p

So the only debate seems to be over what counts as "extra structure". For example, how is the factorization problem seen to be solved in BM? The factorization problem is: how do we know that what is "observable" is independent of the definition of subsystems? BM solves this by constructing a finest subsystem, and saying that all systems are made up of are composed from these finest subsystems, and we have strong heuristic arguments and we can see explicitly (although I don't know a general proof) that we recover common sense reality in all test cases known to date.

But how about crazy factorizations? Can they solve the nonlocality problem? In fact, amazingly to me, BM again shows how MWI solves the nonlocality problem - by denying that the Bell inequalities are violated at spacelike separation - making obvious what the Many-Minds people have been saying - they are brains in a vat! http://arxiv.org/abs/1112.2034: "Finally note that our result that different observers may live in different branches of the wave function is very similar to the many-world interpretation [16, 17], briefly discussed in Sec. 2.2. Yet, there is one crucial difference. In the many-world interpretation, there is a copy of each observer in any of the branches. In our solipsistic interpretation, for each observer there is only one copy living in only one of the branches."

Returning to "extra structure" - are all factorizations, no matter how crazy, discontinuous etc ok? If they are not, and something like BM's factorization are needed to rule those out, then one might argue that BMW (Bohmian Many Worlds) does have extra structure. If the crazy non-workable factorizations are harmless or obviously crazy, then one may say that's an obvious assumption, so it isn't "extra".

If we accept BMW, then one way to answer the question of extra structure is: what is the widest class of BM that is possible? What hidden variables are allowed or not allowed? What dynamics are allowed or not allowed? If the hidden variables or dynamics that are not allowed is non-trivial, then one can argue that these constitute "extra structure". The issue is commented on by http://arxiv.org/abs/quant-ph/9704021 and http://arxiv.org/abs/1112.2034.

 

[Derek Potter]

Happy to see this topic getting some serious discussion, as it deserves.
My question for you Derek Potter with regards to the Many-Many worlds you seem to be a proponent of, is how you get the probabilities correct?

I am not sure what you are asking. Probabilities emerge in MMWI in exactly the same way as they do in MWI. Of course the probabilities in one factorization are not the same as those in another - they do not even refer to the same (edited -) set of possible outcomes.

 

[bhobba]

is how you get the probabilities correct?

Gleason's Theorem. Contextuality does not make sense in MW.

Added Later:
In particular its the requirement for neutrality and equivalence.

All Wallaces proof is is a different route to the same result with the same assumptions ie a rational agent would naturally require neutrality and equivalence so that's why he does it.

Even Later Addition:
I managed to dig up the following that examines it:
http://users.ox.ac.uk/~mert2255/papers/pitei.pdf

Note - and this is VERY important:
Just because something is controversial does not mean its been proven wrong. It simply means issues need to be sorted out before final judgement is passed. That some do not agree with the approach MW adherents take does not prove them wrong - that is a much higher standard than some have issues with it.

Thanks
Bill

 

[atyy]

Another way to see how MWI saves BM and CI with a real collapse from Bell's theorem is that if we have many worlds, we can simply assume that all Lorentz ether frames are real, so that the theory has full Lorentz invariance. So BMW and CMW are something like spontaneously broken Lorentz invariance.

 

[Derek Potter]

Happy to see this topic getting some serious discussion, as it deserves.
My question for you Derek Potter with regards to the Many-Many worlds you seem to be a proponent of, is how you get the probabilities correct?

Ahem, just because I seem to be a proponent of something doesn't mean it's wrong.

 

[Demystifier]

Ahem, just because I seem to be a proponent of something does not mean it's wrong.

Just because MWI is not (even) wrong does not mean that one does not need to answer how the correct probabilities emerge from it.

 

[Derek Potter]

Just because MWI is not wrong does not mean that one does not need to answer how the correct probabilities emerge from it.

The question posed by Quantumental was about MMWI, not MWI.

For MWI, I think bhobba's answer - before he added to it :) - is sufficient: Gleason's Theorem.

There is no controversy that the BR gives the correct probabilities. All the argument is about how to justify applying a theorem about probability measures to a model which denies probability. But that's not what Quantumental asked about.

 

[bhobba]

The question posed by Quantumental was about MMWI, not MWI.

It was about MWI.

The issue is, as I posted, neutrality and equivalence, which is based on reasonableness arguments. To understand it you need to read the link I gave where a number of alternative ways of assigning probability are explored and shown to not be reasonable, and only a measure that is basis independent is what a rational agent would say is valid. Via Gleason that, basically (there are a few other assumptions like the strong superposition principle), means you have the Born rule.

The key point to understand is within the axioms assumed by MW adherents (ie its use of probability based on decision theoretic axioms) Born's rule does follow. The argument is if that's a valid way to define it within this context. It's logically equivalent to the Kolmogorov axioms so can't be faulted on logic alone. Basically its an argument about the meaning of probability - which believe it or not - is what most interpretations of QM is actually about:
http://math.ucr.edu/home/baez/bayes.html

Thanks
Bill

 

[Demystifier]

... only a measure that is basis independent is what a rational agent would say is valid

In classical phenomena, the probability measure is often not basis independent. Consider, for instance, the probability measure of coin flipping.

 

[Derek Potter]

It was about MWI.

"My question for you Derek Potter with regards to the Many-Many worlds..."
I assume, perhaps rashly, that when someone says "Many-Many worlds", they do mean "Many-Many worlds" and not merely "Many worlds". Regrettably I cannot cite a theorem that people mean what they say.

 

[Derek Potter]

In classical phenomena, the probability measure is often not basis independent. Consider, for instance, the probability measure of coin flipping.

Could you expand on that please?

 

[Demystifier]

Could you expand on that please?

For coin flipping, the basis {head,tail} is the preferred basis. (The probability of head+tail or head-tail is always zero.) Since there is a preferred basis, the probability measure is not basis independent.

That's the rough idea, for a more formal treatment see e.g.
http://lanl.arxiv.org/abs/quant-ph/0609163 [Found.Phys.37:1563-1611,2007]
Secs. 5.2 and 5.3.

 

[bhobba]

In classical phenomena, the probability measure is often not basis independent. Consider, for instance, the probability measure of coin flipping.

Sure. But from the axioms of decision theory in MW it's the only one that makes sense.

Consider for example the obvious one of assigning equal probability to each outcome of a two outcome observation. Then consider a combined observation with three outcomes where one of the outcomes feeds into another two outcome observation. What is each outcome - is it 1/3 1/3 1/3 or 1/2 1/4 1/4? Rationally you can't decide so you reject it. Its that sort of thing - only one assignment seems to be free of issues like that - the Born rule.

Thanks
Bill

 

[maline]

It is mathematically and physically natural to consider the same vector represented in different basis as the same object. So it is un-natural and in-elegant not to consider so.

Please help me understand: how can it matter whether we "consider" the representations to be the same object? If there is one valid basis according to which the vector contains us and our classical observations, then isn't that fact a property of the overall vector?

 

[Demystifier]

But from the axioms of decision theory in MW it's the only one that makes sense.

Yes, but those axioms are chosen in such a way to get what one wants to get. This is nicely explained in
http://lanl.arxiv.org/abs/0808.2415
especially in 2) at page 4.

 

[bhobba]

Yes, but those axioms are chosen in such a way to get what one wants to get.

That's part of the argument about it isn't it. They look pretty natural to me though.

As it says, and I pointed out above:
'The ‘Equivalence’ assumption is what does the real work, and it is the main point of controversy with the approach.'

Thanks
Bill

 

[Demystifier]

That's part of the argument about it isn't it. They look pretty natural to me though.

Well, they don't look natural to me. See e.g.
https://www.physicsforums.com/threads/why-mwi-cannot-explain-the-born-rule.364063/
especially posts #1 and #2.

 

[Demystifier]

Please help me understand: how can it matter whether we "consider" the representations to be the same object? If there is one valid basis according to which the vector contains us and our classical observations, then isn't that fact a property of the overall vector?

Let me use an analogy from everyday life. Consider a rough piece of stone. A skillful sculptor can remove a part of the stone material from it to get a beautiful sculpture. Thus, referring to the removed part as "garbage", we can write
sculpture=stone-garbage
or equivalently
stone=sculpture+garbage
But of course, from the same piece of stone the sculptor may choose to make a different sculpture, called "sculpture2", so we can also write
stone=sculpture2+garbage2

Now I am asking you: Is it the fact that the overall stone contains the sculpture? And is it also the fact that the same overall stone contains the sculpture2? And do you get the analogy?

 

[bhobba]

Well, they don't look natural to me.

The equivalence principle, informally is (page 172 of Wallace's text):
'If two acts assign the same weight to each reward the agent must be indifferent to them'

That seems pretty reasonable to me.

But its validity isn't based on how reasonable it is - the formal statement of it is proved section 5.7 page 182 from the decision theoretic foundations his method is formally based on. I have read the book - its pretty dense with theorem proof, theorem proof etc, and while I didn't go though them with a fine tooth comb the math looks tight to me. And I haven't seen anyone challenge it on those grounds so I suspect it's valid. The issue isn't the math - its if you accept a decision theory approach is a valid way to proceed.

Thanks
Bill

 

[Demystifier]

The equivalence principle, informally is (page 172 of Wallace's text):
'If two acts assign the same weight to each reward the agent must be indifferent to them'

That seems pretty reasonable to me.

Here is a counterexample from the classical world. Consider two human twins, one weighting 70 kg and another 80 kg. (The second one eats more, so weights more.) Is it reasonable to conclude that the second twin is therefore more probable than the first twin? And what does it even mean, especially from the point of view of the twins themselves?

 

[bhobba]

Is it reasonable to assume that the second twin is therefore more probable than the first twin?

What do you mean by more probable? If its more probable of existing then the augment is tautological. You assume they exist and have those properties - those properties have nothing to do with them existing in the first place - you have assumed it to begin with.

I think people are getting a bit confused about what's going on here.

In MW there is no way you can decide what world you as a rational agent will experience. That immediately raises all sorts of issues such as is what a rational agent would experience a valid way to proceed. If you assume that, is a decision theoretic approach using the formal axioms of decision theory as used by actuaries and other risk professionals valid? Once you reach that point then we have axioms and tight theorem proof consequences. At that point it looks pretty unassailable.

If you want to challenge it you need to challenge it before it reaches the theorem proof stage.

My take is it all looks reasonable to me - but if you find it unpalatable - I have no issues. I personally do not hold to MW - but for other reasons.

Thanks
Bill

 

[Demystifier]

What do you mean by more probable?

I don't know, you tell me by applying the decision theory or whatever you use to explain probability in MWI. My point is that the twins (which both exist) are analogous to the two branches of the wave function in MWI.

 

[Demystifier]

I personally do not hold to MW - but for other reasons.

What reasons?

 

[bhobba]

I don't know, you tell me by applying the decision theory or whatever you use to explain probability in MWI. My point is that the twins (which both exist) are analogous to the two branches of the wave function in MWI.

If you want to challenge this here is how to do it:

1. You can't have a theory where probabilities are determined by rational agents. Science is about objective truth - not what rational agents would conclude.
2. Even if you accept point one why do you use the axioms of decision theory? Science isn't a betting strategy like actuaries use to decide the costs their clients would incur for a particular action.

They, or similar criticisms are where its vulnerable - once it reaches where, say the equivalence principle is used, then you have all these formal proofs to support it - it must be attacked prior to that.

If you go down that path I am with you - I may not agree - but its a reasonable criticism. Others I have seen are IMHO missing the point.

Thanks
Bill

 

[bhobba]

What reasons?

Its this exponential dilution of energy - in any normal process we would say it quickly decays to zero - but not here. Its a bit too weird for me.

However the specific question was asked - how is the Born Rule justified. Its justified on decision theoretic grounds.

Thanks
Bill

 

[Jimster41]

Here is a counterexample from the classical world. Consider two human twins, one weighting 70 kg and another 80 kg. (The second one eats more, so weights more.) Is it reasonable to assume that the second twin is therefore more probable than the first twin? And what does it even mean, especially from the point of view of the twins themselves?

I'm still trying to read this thread. Great discussion.
The "onion" structure of AdS/CFT (my favorite) does seem to imply hidden dimension(s), and so in some sense says the world as we experience is not sufficient to explain itself... That said, if ever there was navel staring... extending that somewhat unsurprising result to the idea that no observer is privileged (because how could we be?) and so all observers exist (because we exist), and so all realities exist (because ours exists)... wow.

So, I hat MWI because it just sounds like infinity = everything = nothing.

Personally, I'd just like to see some concrete (viscerally convincing) evidence of something like AdS/CFT.

irregardless...

The equivalence principle, informally is (page 172 of Wallace's text):
'If two acts assign the same weight to each reward the agent must be indifferent to them'

This sounds like a line from an evolutionary dynamics text. So I'm not following the literal interpretation vis-a-vis the heavier vs. lighter twin. And I'm missing the sequitur. We are agents (it is an unfortunate fact - not even axiomatic) and we have a preferred basis, and from it our reality is formed, and in that we must act (for where-else) according to the fitness function (the weighting of action) it presents.

The Work Fluctuation Theorem, seems to describe exactly the decision engine we inhabit, I think.

To repeat myself more precisely (I hope), I would just like to know totally how that works, Quantum mechanically, whether or not it is one and the same as the emergence of space-time from some larger context and process - a context and process with clear as well as surprising (like maybe non-local) structure. This understanding, if it could be held at gut level, could place experience inside a real thing (that may be made of many things) - upon which information is inscribed. A thing we can know is not equivalent to us even though it contains us, because it is connected to things we, at least while here, are not (like non-local dimension), and because it behaves according to rules that do not apply to us (like superposition), at least while we are here - That is a feeling that might alleviate some fear of oblivion.

I want to be the cat. That would be better. Or at least I want to believe I may be the cat.