Many Worlds Interpretation and act of measuring

[bhobba]

and we can describe what we see as math ...

You can describe it in English?

Thanks
Bill

 

[rootone]

Not in a way that I can be sure to have communicated an idea well.

 

[bhobba]

Not in a way that I can be sure to have communicated an idea well.

Then you are starting to glimpse the issue. Math is a language highly suited to some things and ordinary language to other things. What is chosen depends on the situation. For Brownian motion how do you express the diffusion equation in English? For an experimental set-up to check if its correct how do you express that in math?

Thanks
Bill

 

[craigi]

Hey maybe you can help me on this , but around the 56-57 minute mark, the guy second to the right is arguing that MW intepretation takes out the pure randomness. But that still means pure randomness exists in our universe, since we cannot look into other universes. So technically that means it is still random for us what will happen?

Yes. Although all possibilities are realized in the MWI we only observe the normal probabalisitic outcomes of QM.

An exception occurs when a singnificant fraction of possible outcomes do not contain the observer.

 

[bhobba]

Yes. Although all possibilities are realized in the MWI we only observe the normal probabalisitic outcomes of QM.
An exception occurs when a singnificant fraction of possible outcomes do not contain the observer.

That is incorrect. Its got nothing to do with an observer but for some reason you keep harping on it.

I have Wallaces text on the subject - The Emergent Multiverse. I have read the text - there is no mention of observer. I have looked at the index - there is no mention of observer. But beyond that I understand the interpretation and it has nothing to do with an observer.

As I have explained many times succinctly the interpretation is dead simple. After decoherence you have a mixed state. Each part is interpreted as a world. No observer used, required, or part of the interpretation.

Thanks
Bill

 

[rootone]

This conversation reminds me of Piet Hein, not exactly a mainstream scientist, or philosopher either.
"The universe may be as great as they say. But it wouldn't be missed/ if it didn't exist."
and some other interesting comments.
http://en.wikiquote.org/wiki/Piet_Hein

 

[bhobba]

This conversation reminds me of Piet Hein

As a mathematician and scientist he would understand that modern science isn't particularly concerned with philosophical musings.

Thanks
Bill

 

[rootone]

Yes, but I do like his poetic way of presenting scientific ideas,

Another one.

The road to wisdom? — Well, it's plain
and simple to express:
Err
and err
and err again
but less
and less
and less.

 

[bhobba]

Yes, but I do like his poetic way of presenting scientific ideas,

Highly amusing. But this isn't a forum where such is discussed. Mentioning it as an aside is probably ok - but its not what we are on about here.

Thanks
Bill

 

[rootone]

Fair enough. I am here to learn.

 

[craigi]

That is incorrect. Its got nothing to do with an observer but for some reason you keep harping on it.

I have Wallaces text on the subject - The Emergent Multiverse. I have read the text - there is no mention of observer. I have looked at the index - there is no mention of observer. But beyond that I understand the interpretation and it has nothing to do with an observer.

As I have explained many times succinctly the interpretation is dead simple. After decoherence you have a mixed state. Each part is interpreted as a world. No observer used, required, or part of the interpretation.

Thanks
Bill

Bill,

This is a well documented and a widely understood implication of the interpretation. I have provided reference on this subject matter to you previously. Do I really need to do it again?

The fact that Wallace chooses not to discuss it in the book that you read doesn't imply that it is not relevant and it certainly does not imply that it is incorrect.

There is no observer role in wavefunction collapse in the MWI, since collapse does not take place. However, the observer splits into different worlds upon decoherence. Some worlds may and some worlds may not contain the observer. It is well known that Everett was extremely clear about this. Regardless of whether Wallace wrote about it, I find it very hard to imagine that he doesn't accept it. The fact that you choose not to accept it reflects your own prejudice on interpretational issues.

 

[julcab12]

After decoherence you have a mixed state. Each part is interpreted as a world. No observer used, required, or part of the interpretation.

Bill

... I am one of the " Maybe it is true but I'm not convinced -- yet ". Simply because i don't take 'multiplicity-overlaps' very lightly or direct especially on the part where we assign eigenstates on each point -- equivalent of a multiple real state/space; Model wise? Yes! it is helpful as a build up to a theory but I view it as strange -- unless we have sufficient evidence that each part is really a world, -- although i must agree that the interpretation is direct, simplest and workable (at least to some extent). I am more comfortable in viewing the multiple effect or any in particular as distortions of the field. IMO The collapse/interference is real to me while the decoherence is just an effect created by field distortions or any effect causing the collapse to appear decohered. I'm trying to understand the reason behind why we took this 'pointer'/direct approach. Imagine the problem it will cause if the same is used in cosmology. Without that field effect. We could be counting multiple supernova when in fact it is only one.

http://apod.nasa.gov/htmltest/gifcity/nslens_effects.html
http://hubblesite.org/newscenter/archive/releases/2015/08/full/

 

[bhobba]

This is a well documented and a widely understood implication of the interpretation. I have provided reference on this subject matter to you previously. Do I really need to do it again?

Yes you do. I can recall nothing you have posted doing that.

Wallace is an acknowledged expert on the interpretation and his text a definitive reference. If he doesn't include it the VERY strong odds are its a crock. But it goes beyond that. I spent quite a bit of time studying it to understand the interpretation and can say it has nothing to do with observers - nothing.

The issue isn't if the world contains observers - of course if they do they will be split and copied with each observation just like everything is - that's utterly obvious - the issue is if it in anyway affects the interpretation. It doesn't - and obviously so.

However, if, as required by forum rules, you have a peer reviewed physics paper (not a philosophy one) that shows otherwise post it. Also rather than me going through the paper you should be able to present a précis of the argument.

Thanks
Bill

 

[bhobba]

... I am one of the " Maybe it is true but I'm not convinced -- yet ". Simply because i don't take 'multiplicity-overlaps' very lightly or direct especially on the part where we assign eigenstates on each point -- equivalent of a multiple real state/space; Model wisehttp://hubblesite.org/newscenter/archive/releases/2015/08/full/

Before going any further I need to check you understand exactly what is being said. Can you explain to me the difference between a superposition and a mixed state? In general can you explain why a state is a positive operator rather than an element of a vector space?

Thanks
Bill

 

[bhobba]

http://apod.nasa.gov/htmltest/gifcity/nslens_effects.html
http://hubblesite.org/newscenter/archive/releases/2015/08/full/

BTW we are discussing MW not astronomy or cosmology.

Don't be fooled by incorrect comments that the multiverse of the many worlds interpretation is the same as the multiverse in cosmology (eg eternal inflation) - it isn't.

Although there has been speculation linking the two:
http://arxiv.org/abs/1105.3796

Thanks
Bill

 

[julcab12]

Before going any further I need to check you understand exactly what is being said. Can you explain to me the difference between a superposition and a mixed state?

Thanks
Bill

The common understanding is that --Worlds, or branches of the universal wavefunction, split when different components of a quantum superposition "decohere" from each other. Decoherence refers to the loss of coherency or absence of interference effects between the elements of the superposition. For two branches or worlds to interfere with each other all the atoms, subatomic particles, photons and other degrees of freedom in each world have to be in the same state, which usually means they all must be in the same place or significantly overlap in both worlds, simultaneously.

 

[bhobba]

Decoherence refers to the loss of coherency or absence of interference effects between the elements of the superposition.

Not quite - it refers to a superposition being converted to a mixed state - but to understand it you need to know what those terms mean. Since you didn't reply to my query about that I can only assume you haven't come across it before. Unfortunately with this stuff it can only be detailed in the math.

For two branches or worlds to interfere with each other all the atoms, subatomic particles, photons and other degrees of freedom in each world have to be in the same state, which usually means they all must be in the same place or significantly overlap in both worlds, simultaneously.

No. For them to interfere it means if P is the state <bi|P|bj> is not zero for i not equal to j where |bi> is the basis singled out by decoherence. These are called interference terms or off diagonal elements. Decoherence is considered to have occurred if that is so small it's way below detectability and can be taken as zero. This is a genuine issue with MW and decoherence in general. It depends on the non-detectability of off diagonal interference effects which is a technological moving target. The general unstated assumption is they quickly fall so low that it can never be detected. That of course is a debatable assumption and could form the basis of a new thread if anyone wants to pursue it - although it has been discussed a number of times before.

Thanks
Bill

 

[julcab12]

BTW we are discussing MW not astronomy or cosmology.

Don't be fooled by incorrect comments that the multiverse of the many worlds interpretation is the same as the multiverse in cosmology (eg eternal inflation) - it isn't.

Although there has been speculation linking the two:
http://arxiv.org/abs/1105.3796

Thanks
Bill

-- I'm perfectly aware of that and understand the incompatibility (Besides, I've been reading and contemplating it for years now). I just wanted to understand the QM's perspective on multiplicity and why should be taken or designated as a direct pointer's view. I have no luck in googling the subject so I'm asking an experts opinion. Technically, why should we consider a representation of 'A' as real-multi space operator

.

The link i provided is just supplement on the principle involved that is apparent in nature -- that a multiplicity can also be interpreted as distortions causing it to appear multiple. -- Oh, I'll read the link soon.

 

[bhobba]

Technically, why should we consider a representation of 'A' as real-multi space operator

.

I have zero idea what you mean by a real-multi space operator.

Thanks
Bill

 

[julcab12]

ut to understand it you need to know what those terms mean. Since you didn't reply to my query about that I can only assume you haven't come across it before. Unfortunately with this stuff it can only be detailed in the math.

I only have the general idea. Can you please provide a link on the subject? Much appreciated! I only have this link

http://farside.ph.utexas.edu/teaching/qmech/Quantum/node40.html.. But there are no explanation on why it is pointing to a multiples. Well It assumes that"

acts a multiple of the original wavefunction (Simply bec. it is looking to be that way- multiples). These special wavefunctions are called eigenstates, and the multiples are called eigenvalues. -- Meaning we assign a variable that act as a multiple and not as far as explaining why it should be the case.

No. For them to interfere it means if P is the state <bi|P|bj> is not zero for i not equal to j where |bi> is the basis singled out by decoherence. These are called interference terms or off diagonal elements. Decoherence is considered to have occurred if that is so small it's way below detectability and can be taken as zero. This is a genuine issue with MW and decoherence in general. It depends on the non-detectability of off diagonal interference effects which is a technological moving target

Can you provide me a link to elaborate this claim. Thanks!

 

[julcab12]

I have zero idea what you mean by a real-multi space operator.

Thanks
Bill

Consider a general real-space operator

. When this operator acts on a general wavefunction

the result is usually a wavefunction with a completely different shape. However, there are certain special wavefunctions which are such that when

acts on them the result is just a multiple of the original wavefunction. These special wavefunctions are called eigenstates, and the multiples are called eigenvalues...

 

[bhobba]

Can you provide me a link to elaborate this claim. Thanks!

Its very basic. The textbook that explains with just a first course in QM (which usually doesn't cover it) it is the following:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

The following also covers it, but not in the same depth:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill

 

[bhobba]

Consider a general real-space operator

. When this operator acts on a general wavefunction

the result is usually a wavefunction with a completely different shape. However, there are certain special wavefunctions which are such that when

acts on them the result is just a multiple of the original wavefunction. These special wavefunctions are called eigenstates, and the multiples are called eigenvalues...

Yes - your point being?

Here is even more detail on exactly what's going on (see post 137):
https://www.physicsforums.com/threads/the-born-rule-in-many-worlds.763139/page-7

It is a foundational axiom of QM, in fact the foundational axiom because Born's Rule can be derived from it, that the outcomes of observations can be mapped to a POVM which leads to what you said above, as the above link explains.

In MW what happens is you have a mixed state ∑pi |bi><bi| after decoherence. The interpretation is each |bi><bi| is a separate world. MW also has a decision theory argument that shows the pi is the probability of experiencing the world with |bi><bi|. Note - despite what Gragi says that probability has nothing to do if an observer is actually in the world or not any more than if you have a machine that throws a dice the probability changes depending on if someone observes it or not.

Thanks
Bill

 

[stevendaryl]

That is incorrect. Its got nothing to do with an observer but for some reason you keep harping on it.

I'm sure that what I'm about to say is exactly on-topic, but to me, observers are relevant in MWI in the following sense:

• MWI is a completely deterministic theory.
• Yet, from the subjective view of an observer, the universe seems non-deterministic.

So observers come into play simply because of the need to explain why things appear the way they do. Without observers, there is no reason to introduce probability into the theory at all.

It's not that observers necessarily involve any different physics than any other system, it's just that they are the systems that have a point of view that needs explaining.

 

[stevendaryl]

Not quite - it refers to a superposition being converted to a mixed state - but to understand it you need to know what those terms mean.

At this point, I'm not sure what objections are legitimate, and what objections are quibbling. But in MWI, there is never a conversion to a mixed state. Instead, the wave function is FAPP (for all practical purposes) in a mixed state if it is impossible to observe interference between the different elements of the superposition. Mathematically, this means something like: the density matrix becomes that of a mixed state after "environmental degrees of freedom" are traced out.

 

[bhobba]

So observers come into play simply because of the need to explain why things appear the way they do.

I can't follow that at all.

Probabilities enter due to a decision theory rational agent argument - this is a Bayesian view - namely what would a rational agent assign the probability to be where its a subjective thing. Its got nothing to do if observers are in the world or not.

Thanks
Bill

 

[stevendaryl]

I can't follow that at all.

Probabilities enter due to a decision theory rational agent argument

And some people use the word "observer" instead of "decision theory rational agent". The notion of a "decision theory rational agent" is a way to formalize those aspects of an "observer" that are relevant to reasoning about probability.

 

[bhobba]

And some people use the word "observer" instead of "decision theory rational agent". The notion of a "decision theory rational agent" is a way to formalize those aspects of an "observer" that are relevant to reasoning about probability.

If that's what they mean then they should state it. However its obvious that is not affected by if there are actual observers in the world or not. In particular the following is false - 'An exception occurs when a singnificant fraction of possible outcomes do not contain the observer.' That a rational agent decides on a particular probability has nothing to do if there are observers in the world or not, or even if the rational agent is in the world or not. Its simply what the rational agent would conclude if they were experiencing it. Whether they actually do is obviously irrelevant any more than it would be relevant if a machine flipped a coin and a rational agent decided it had a 50-50 chance of being heads or tales.

Thanks
Bill

 

[julcab12]

The interpretation is each |bi><bi| is a separate world

Of course. You can create a model dependent on that axiom simple because it is essential within the demand of the framework. But it can be utterly indistinguishable whether each individual system is in an eigenstate of some multiple hilbert spaces , or each individual system is in a superposition state with huge constraint like MWI. I'm not saying i ignore the superposition. Almost everything I've read about QM invovles a crude formalism of separation and interpretation -- "That is how the system literally looks like. It appears multiple, let's assign each multiple state to be real create worlds pragmatic to ours. Let's make it independent to any specific interpretation and built something within that framework -- It works!". Ok here's an image of 4 supernovas in single frame must be 4 distinct supernovas. Actually we can create a model within that premise and can still work..Well, Lensing will tell you it's not the case anyways..

 

[stevendaryl]

If that's what they mean then they should state it. However its obvious that is not affected by if there are actual observers in the world or not.

By "actual observers" do you mean human beings (or intelligent aliens, or whatever)? I certainly agree with that; you can substitute a mechanical device for a human, and get the same "appearance" of probability. Or do you mean that the decision-theoretic notion of probability doesn't require any rational agents to exist, only that IF they existed, THEN they would do ... (whatever).

In particular the following is false - 'An exception occurs when a singnificant fraction of possible outcomes do not contain the observer.' That a rational agent decides on a particular probability has nothing to do if there are observers in the world or not, or even if the rational agent is in the world or not. Its simply what the rational agent would conclude if they were experiencing it. Whether they actually do is obviously irrelevant any more than it would be relevant if a machine flipped a coin and a rational agent decided it had a 50-50 chance of being heads or tales.

Okay, so you do mean the counterfactual: if such agent did exist, then it would behave in such and such a way.

 

[jimmylegss]

If MW is purely deterministic, what determines what will happen in which universe? So If something can go left or right, what will determine which thing will happen to the universe that I am in now?

The whole MW thing seems to me they put another layer in between that mechanism, to kind of put a distance between us and that whole pure randomness thing?

For example it might still be completely random what will happen in which universe? Unless you can observe this mechanism, it does not really do anything about the whole pure randomness problem at all. You just moved it around a bit.

 

[bhobba]

By "actual observers" do you mean human beings (or intelligent aliens, or whatever)?

Well here is where we run into semantic difficulties with QM and that this observer business was even introduced in the first place. Cragi wasn't forthcoming in detailing exactly what he means but by observer I assume he means rational observer. Its utterly obvious if a rational observer is there or not the probability of an outcome does not change. In fact the decision theoretical argument used contains nothing of that nature.

Thanks
Bill

 

[stevendaryl]

If MW is purely deterministic, what determines what will happen in which universe? So If something can go left or right, what will determine which thing will happen to the universe that I am in now?

Nothing determines it.Initially, you have one universe. Then after a random event, the universe splits into two universes, one in which the something went left, and another in which the something went right.

Of course, the language of "splitting" is not completely accurate.

 

[bhobba]

ISo If something can go left or right, what will determine which thing will happen to the universe that I am in now?

One world will be left, the other right. The issue is which will a rational agent, if they were there, experience. All you can do is give a probability. It may seem the rational choice is 50-50. But if you analyse it carefully it turns out to have problems - the detail can be found on page 149 of Wallaces reference. In fact the only one that works properly is one that is basis independent so Gleason applies and you get the Born rule. However for the detail you need to consult the reference.

Thanks
Bill

 

[craigi]

Yes you do. I can recall nothing you have posted doing that.

Wallace is an acknowledged expert on the interpretation and his text a definitive reference. If he doesn't include it the VERY strong odds are its a crock. But it goes beyond that. I spent quite a bit of time studying it to understand the interpretation and can say it has nothing to do with observers - nothing.

The issue isn't if the world contains observers - of course if they do they will be split and copied with each observation just like everything is - that's utterly obvious - the issue is if it in anyway affects the interpretation. It doesn't - and obviously so.

However, if, as required by forum rules, you have a peer reviewed physics paper (not a philosophy one) that shows otherwise post it. Also rather than me going through the paper you should be able to present a précis of the argument.

Thanks
Bill

I don't have a copy of Wallace's book at hand, but I suggest that you read again his final chapter entitled, "A Cornucpia of Everettian Consqeuences", particularly, "10.2 Exotic Consqeuences of Quantum Probability", "10.2.1. Cosmoloigical Probabilities and Anthropic Reasoning" and "10.2.2. Quantum Russian Roulette". I have no doubt that it will make my comments crystal clear to you.

Tegmark, another highly respected MWI expert, discusses a significant difference for an observer, under the MWI compared to other interpretations, in this paper:
The Interpretation of Quantum Mechanics: Many Worlds or Many Words?
http://arxiv.org/abs/quant-ph/9709032

It's a short paper and not very technical. I won't summarise it, because I'd like you to read it all, since you obviously have an interest in the MWI and it also addresses your "too weird" objection in direct terms, amongst other things that you really should read.

I trust that this now clears this all up, but if you do still have questions, when you have read the final chapter of Wallace's book and the paper by Tegmark, then please get back to me.

BTW we are discussing MW not astronomy or cosmology.

Don't be fooled by incorrect comments that the multiverse of the many worlds interpretation is the same as the multiverse in cosmology (eg eternal inflation) - it isn't.

Although there has been speculation linking the two:
http://arxiv.org/abs/1105.3796

Thanks
Bill

Tegmark also offers an interpretation of QM which makes Everetts worlds real beyond our cosmological horizon, linking different types of multiverse:
Born in an Infinite Universe: a Cosmological Interpretation of Quantum Mechanics
http://arxiv.org/abs/1008.1066

By "actual observers" do you mean human beings (or intelligent aliens, or whatever)? I certainly agree with that; you can substitute a mechanical device for a human, and get the same "appearance" of probability. Or do you mean that the decision-theoretic notion of probability doesn't require any rational agents to exist, only that IF they existed, THEN they would do ... (whatever).

It really doesn't matter what form the observer or observation device takes, since if in a significant proportion of the worlds, it doesn't exist then no such observation can be made. The observer cannot find itself in those worlds. The probability of an observer making an observation, where the observer does not exist is zero. This is distinctly different from other interpretations.

To me, this seems like obvious stuff and I'm suprised that there's so much debate about it. It's most notable by the fact that Everett believed that the interpretation rendered him immortal.