The Many-Worlds Interpretation of QM

[Fredrik]

So, You mean that "electron being in 2 places at one" DOESN'T nescesseraly mean "universe splitted" and other interpretations of that are available?

No one knows for sure if the electron really is at two places at once, or what that would even mean. What we know is that the wavefunction has peaks at both places. We don't know how to interpret that.

 

[Alexis1304]

Thanks, folks
Another question arises
I think this article "Nothing happens in the Universe of the Everett Interpretation' was already discussed here

http://arxiv.org/pdf/1210.8447v1.pdf

The article itself is critical towards MWI, but starts that "why MWI is attractive" and mentions that with decoherence entangelment and brancing is "totally natural".
If branching is totally naturall, then why all other discussions?Why no everybody just accept MWI and the case closed?After all, the branching is natural, thanks to decoherence...

 

[Nugatory]

If branching is totally natural, then why all other discussions?Why no everybody just accept MWI and the case closed?After all, the branching is natural, thanks to decoherence...

De [STRIKE]gustibus [/STRIKE]interpretationes non est disputandum.

What's natural to one person isn't necessarily natural to another... try feeding brussels sprouts to a four-year-old if you don't believe me.

 

[Alexis1304]

De [STRIKE]gustibus [/STRIKE]interpretationes non est disputandum.

What's natural to one person isn't necessarily natural to another... try feeding brussels sprouts to a four-year-old if you don't believe me.

Not sure this is good analogy,though.As I learned, branching is natural thanks to decoherence.It just may mean differnt things,perhaps

 

[Fredrik]

...with decoherence entangelment and brancing is "totally natural".
If branching is totally naturall, then why all other discussions?Why no everybody just accept MWI and the case closed?After all, the branching is natural, thanks to decoherence...

The MWI is perhaps the simplest and most straightforward way to interpret QM as a description of what is actually happening, but there are other interpretations, and also the possibility that QM simply can't tell us what's actually happening. This has been discussed earlier in the thread.

Not sure this is good analogy,though.As I learned, branching is natural thanks to decoherence.It just may mean differnt things,perhaps

"Branching is natural thanks to decoherence"...I'm not sure were you learned that, but it's not that simple. See the discussion earlier in the thread.

 

[Alexis1304]

Hi Fredrik,
thanks for the reply,
Well,as I told I got that - "branching is natural thanks to decoherence- ironically in the article that overall is critical to mwi, but starts why mwi is attractive- "Nothing happens in the Universe of the Everett Interpretation' was already discussed here

http://arxiv.org/pdf/1210.8447v1.pdf

I just found that this article was posted here on previous pages,as claiming that preferral basis is not solved in mwi. ironically, the conclusion in the article in the opinion of the author is that if mwi is taken alone it is "empty picture". only in combination with copehnagen it makes sense, like they 2 complement each other. by the way I sent e-mail to the author and got interesting response - maybe it is good idea to post his full response here. In his opinion neither Copehnage nor MWI is "complete" ot "true" ,and whatever You said - "the possibility that QM simply can't tell us what's actually happening." - he is in full agreement with that.

By the way folks, did you discuss already the lates book by Steven Weinberg "lectures on quantum mechanics"? I'm not physicists, so was reading only the chapter about interpretations, and for me was pretty technical. But I got his overall conclusion, seems rather radical...

Thanks,
Alexis

 

[Quotidian]

However I recently came across some writings by Murray Gell-Mann on emergence that solved the issue to my satisfaction without invoking such a weird idea.

The idea that 'number is real' is really not such a weird idea when you think it through. Heisenberg once gave a lecture on 'the relationship between Plato and Democritus'. Plato represents mathematical realism and classical idealism - the notion that ideas and number are real in their own right - whereas Democritus was one of the early proponents of atoms.

Heisenberg comes down in favour of Plato on the grounds that the most fundamental elements of existence can only be satisfactorily conceived in mathematical terms, and that they don't 'exist' in the way that 'stones and flowers' exist. Although he acknowledges that the ancients couldn't, and didn't, understand many fundamental things that science has subsequently discovered, he says that such discoveries have vindicated Plato's 'ideas' over Democritus' 'atoms'.

The way I have started to conceptualize the idea is that the nature of reality itself can be understood in terms of relationships rather than entities. When I say 'relationships', this refers to the fundamental ratios and regularities which seem to govern nature on many levels (among other things). For instance there's the 'six constants' that Martin Rees talks of in his 'http://amzn.com/0465036732 . But the intuition of the existence of 'rational relationships' actually goes back to Pythagoras and is the very basis of the idea of 'rationality' and reason itself. It seems to me that 'natural law' operates on a deeper level of reality. But the problem is, empiricism doesn't recognize 'levels'. In its view, things either exist or they don't exist. Whereas the mathematical/rationalist view is that natural and logical laws, and the like, actually dictate what happens in reality, even though they are obviously not available to empirical examination. We can examine the effects of the laws, but the laws themselves are, as it were, sewn into the fabric of the Cosmos.

All that kind of thinking is traditionalist, but it was part of the tradition that enabled science to develop in the first place. (I am firmly of the view that the reason that modern science developed in Western Culture was because of Pythagoras.) However the radical empiricism of the modern period has cut itself off from all that and even rebelled against it. That, I think, is why you find such ideas as 'mathematical realism' to be 'weird'. They are simply out of step with the zeitgeist.

 

[bhobba]

That, I think, is why you find such ideas as 'mathematical realism' to be 'weird'. They are simply out of step with the zeitgeist.

I am a mathematical realist and I would say most people of a mathematical physics bent like I am are as well. I, and I suspect those of a similar bent, and Penrose is definitely of that bent, believe the reality is the math. However Penrose goes further and believes in the literal existence of a Platonic realm our mathematical intuition is somehow in touch with and it is that realm that really determines 'reality'. That is the idea that is viewed as a bit kooky and when I held it freely admitted that - I held it because it resolved the issues in Wigners essay. Now I believe it is simply a result of emergence and the phenomena of self similarity where we find the deeper we go the same thing seems to repeat with perhaps a small variation. This is one of the results of the effective field theory approach to physics and is a deep insight.

Thanks
Bill

 

[bhobba]

I just found that this article was posted here on previous pages,as claiming that preferral basis is not solved in mwi.

Frederic and I have discussed that one. The skinny is the MWI contains decoherence as any interpretation of QM must that conforms its formalism. The consensus is that decoherence does solve the preferred basis problem and that is the view presented in standard textbooks on the subject such as the book by Schlosshauer that is my bible:


The key point that the naysayers attack is the ability to arbitrarily break a system into any subsystems you like. I think most physicists think that assumption is fine but, to the best of my knowledge, key theorems are lacking proving its true.

While it is generally accepted it solves the basis problem decoherence only gives the appearance of wavefunction collapse. It is a matter of opinion, and the particular interpretation you hold to, if that solves the measurement problem

Thanks
Bill

 

[Fredrik]

The consensus is that decoherence does solve the preferred basis problem and that is the view presented in standard textbooks on the subject such as the book by Schlosshauer that is my bible:


The key point that the naysayers attack is the ability to arbitrarily break a system into any subsystems you like. I think most physicists think that assumption is fine but, to the best of my knowledge, key theorems are lacking proving its true.

The consensus is that given a decomposition into subsystems, decoherence selects a basis. But as far as I know, there's no consensus about the basis being independent of the decomposition. I see no reason to think that it is. I expect the bases corresponding to different decompositions to be extremely different.

So I don't see any reason to claim that the "preferred basis problem" has been solved. However, I have never thought of it as an actual problem. It's a problem only if you insist on thinking that at any instant, there's exactly one way to view the universe as consisting of classical worlds. It's not a problem if you're OK with thinking that at any instant, there are many inequivalent ways to view the universe as consisting of classical worlds.

I don't know if anyone is attacking the ability to arbitrarily break up the universe into subystems. I know I'm not.

 

[Quotidian]

I am a mathematical realist and I would say most people of a mathematical physics bent like I am are as well. I, and I suspect those of a similar bent, and Penrose is definitely of that bent, believe the reality is the math. However Penrose goes further and believes in the literal existence of a Platonic realm our mathematical intuition is somehow in touch with and it is that realm that really determines 'reality'. That is the idea that is viewed as a bit kooky and when I held it freely admitted that - I held it because it resolved the issues in Wigners essay. Now I believe it is simply a result of emergence and the phenomena of self similarity where we find the deeper we go the same thing seems to repeat with perhaps a small variation. This is one of the results of the effective field theory approach to physics and is a deep insight.

Thanks
Bill

I have just now ordered Emperor's New Mind, finally, having read quite a bit about it over the years. I am interested by Penrose, but I think he might be a bit difficult for a non-mathematical thinker.

I too believe in 'that realm' but as I tried to explain, it is a mistake to conceptualize it as being 'somewhere'. It is simply the most general attributes and characteristics of 'this realm'. There is a way of conceptualizing the Platonic ideals as 'real possibilities' - that is, the constraints and characteristics of a system (in this case, the Universe) that determine the kinds of things that might happen. (See Meaning and the Problem of Universals. This essay makes reference to QM.) This actually chimes in well with the probabalistic nature of QM, in my view. The world, overall, consists of 'probability waves' which are neither entirely random nor entirely determined, and where intentionality has a role in actualizing possibilities.

The thing I don't get with 'emergence' is how it provides an explanatory principle. The point about reductionism, even though I am generally opposed to it, is that it provides a explanandum which accounts for a wide range of explanans. (Hope I have that the right way around.) Whereas, all emergence seems to say is that novel properties seem to emerge from the combination of substances, that couldn't be predicted on the basis of the attributes of the substances themselves. What does that 'explain', exactly?

 

[kashan123999]

Selection bias favors MWI in the press because it's cool-sounding and fun to write about.

Spot on

 

[bhobba]

The consensus is that given a decomposition into subsystems, decoherence selects a basis. But as far as I know, there's no consensus about the basis being independent of the decomposition. I see no reason to think that it is. I expect the bases corresponding to different decompositions to be extremely different.

Yea that's the key point - the physics should be independent of the decomposition and if the basis problem was truly solved it would and should give the same basis regardless - but key theorems are lacking proving it is. I know there is a lot of active research into decoherence so hopefully the issue will be resolved one way or the other a bit further down the line. If its not true then an explanation needs to be given on why a particular decomposition is singled out.

Thanks
Bill

 

[bhobba]

The thing I don't get with 'emergence' is how it provides an explanatory principle.

It's not emergence per-se that solves the issues in Wigners essay - its the self similarity of that emergence. What we find is that as we move up in a hierarchy in terms of fundamental physical systems, from string theory, to the electro-weak theory, to QED, to standard QM, to the laws of chemistry, to the laws of biochemistry, to the laws of biology and on and on things tend to repeat and the same concepts such as pi occur over and over. That is the explanation for the issues raised in Wigners essay - its the self similarity of each emergent level. This was one of the key lessons of effective field theory - and IMHO, and Murray Gell-Mann's opinion, a very deep and important result.

You can read about it here:
http://www.csicop.org/si/show/gell-mann_reality_is_out_there_._._._and_itrsquos_beautiful/

I have to say however we are getting way off the topic of this thread and really if you want to pursue issues like this you should start a new one.

Thanks
Bill

 

[Fredrik]

Yea that's the key point - the physics should be independent of the decomposition and if the basis problem was truly solved it would and should give the same basis regardless - but key theorems are lacking proving it is. I know there is a lot of active research into decoherence so hopefully the issue will be resolved one way or the other a bit further down the line. If its not true then an explanation needs to be given on why a particular decomposition is singled out.

You seem to think that the only options are:

1. The basis is independent of the decomposition.
2. The basis depends on the decomposition, but there's a preferred decomposition.
3. There is no MWI.

I think it's extremely likely that 1 and 2 are both false. But I don't think that this is a problem for the idea that the universe consists of many worlds. It's only a problem for the idea that it consists of only one set of worlds.

 

[bhobba]

You seem to think that the only options are:

1. The basis is independent of the decomposition.
2. The basis depends on the decomposition, but there's a preferred decomposition.
3. There is no MWI.

I think it's extremely likely that 1 and 2 are both false. But I don't think that this is a problem for the idea that the universe consists of many worlds. It's only a problem for the idea that it consists of only one set of worlds.

1 and 2 may be false. Its part of the so called factorization problem and not a lot of work seems to have been done on it. The consensus from the literature I have read is its not an issue researches seem to worry about. Schlosshauer, for example, in his writings doesn't even mention it. It only seems to get any real traction around here where people bring up the few papers that makes a big deal about it. The reason for that may be because while a lot of work doesn't seem to have been done on it some work has and that seems to indicate 1 is true eg:
http://arxiv.org/pdf/quant-ph/9910004v1.pdf

My opinion is more research is required but I personally believe 1 is true.

I do not believe in MWI - that's an opinion - opinions are like bums - everyone has one - it doesn't make it true. MWI is very elegant and beautiful and may indeed be true - I just don't like it - which really doesn't mean anything.

Thanks
Bill

 

[stevendaryl]

I do not believe in MWI - that's an opinion - opinions are like bums - everyone has one - it doesn't make it true. MWI is very elegant and beautiful and may indeed be true - I just don't like it - which really doesn't mean anything.

I don't know what it really means to believe or not believe in MWI. A related issue that I think is more meaningful is: Can there be a superposition of macroscopic objects? (The universe is the limiting case of a macroscopic object.) The formalism of quantum mechanics doesn't give us any reason to believe that there is a maximum size for objects that can be treated quantum-mechanically.

To me, the implication of decoherence is that macroscopic superpositions very rapidly "infect" the entire universe. If you could somehow produce a cat in a superposition of a dead cat and a live cat, very rapidly, you would have a superposition of a world in which the cat is dead, and a world in which the cat is alive. After that, it's not the cat that's in a superposition, but the entire world. But once you have superpositions of the entire world, it seems that you have MWI. It seems inevitable to me. That's why it seems strange for people to talk about MWI as extravagant, having unobservable entities. It does, but they aren't put in by hand, they seem to me to be consequences of quantum mechanics. There is nothing you have to add to quantum mechanics to get superpositions of possible worlds. To the contrary, you have to add something to quantum mechanics (hidden variables, for example) to prevent such macroscopic superpositions.

 

[mfb]

Can there be a superposition of macroscopic objects? (The universe is the limiting case of a macroscopic object.) The formalism of quantum mechanics doesn't give us any reason to believe that there is a maximum size for objects that can be treated quantum-mechanically.

Superposition was shown with macroscopic (~50µm) cantilevers - large enough to see it by eye (but not during the measurement, of course).

To me, the implication of decoherence is that macroscopic superpositions very rapidly "infect" the entire universe. If you could somehow produce a cat in a superposition of a dead cat and a live cat, very rapidly, you would have a superposition of a world in which the cat is dead, and a world in which the cat is alive.

A good isolation from the environment is crucial for those experiments.

After that, it's not the cat that's in a superposition, but the entire world. But once you have superpositions of the entire world, it seems that you have MWI. It seems inevitable to me. That's why it seems strange for people to talk about MWI as extravagant, having unobservable entities. It does, but they aren't put in by hand, they seem to me to be consequences of quantum mechanics. There is nothing you have to add to quantum mechanics to get superpositions of possible worlds. To the contrary, you have to add something to quantum mechanics (hidden variables, for example) to prevent such macroscopic superpositions.

Very true.

 

[Fredrik]

1 and 2 may be false. Its part of the so called factorization problem and not a lot of work seems to have been done on it. The consensus from the literature I have read is its not an issue researches seem to worry about. Schlosshauer, for example, in his writings doesn't even mention it. It only seems to get any real traction around here where people bring up the few papers that makes a big deal about it. The reason for that may be because while a lot of work doesn't seem to have been done on it some work has and that seems to indicate 1 is true eg:
http://arxiv.org/pdf/quant-ph/9910004v1.pdf

My opinion is more research is required but I personally believe 1 is true.

I haven't made any attempt to prove that 1 is false, but I have to say that to I have a hard time imagining anything that sounds less likely to be true, that I don't already know for sure is false.

Suppose that decomposition 1 is "the silver atom in this Stern-Gerlach experiment + everything else" and that decomposition 2 is "an antiproton about to enter Earth's atmosphere + everything else". These are very different decompositions, with very different interaction Hamiltonians. So why should those interaction Hamiltonians be diagonalized by the same basis?

I do not believe in MWI - that's an opinion - opinions are like bums - everyone has one - it doesn't make it true. MWI is very elegant and beautiful and may indeed be true - I just don't like it - which really doesn't mean anything.

I kind of like it, but at the same time, I think QM looks too much like a toy theory that someone invented just to prove that it's possible to define a theory of physics that assigns non-trivial probabilities to results of experiments. So I don't know what to think about whether it's true. I think it's an interesting idea, but so many things are possible, including that there are many worlds, but they have nothing to do with the worlds of QM.

Right now I just want to make sure that the MWI is at least wrong. I'm not entirely convinced that it even makes sense. I used to think that it doesn't, but now that I understand that there may be more than one correct way to describe the universe as consisting of many classical worlds, I think it probably does.

 

[bhobba]

I don't know what it really means to believe or not believe in MWI.

Good point. I believe in it as a valid and very beautiful interpretation - but the implication of these many different worlds constantly being created and coexisting doesn't sit well - that's it - that's all.

Thanks
Bill

 

[bhobba]

Suppose that decomposition 1 is "the silver atom in this Stern-Gerlach experiment + everything else" and that decomposition 2 is "an antiproton about to enter Earth's atmosphere + everything else". These are very different decompositions, with very different interaction Hamiltonians. So why should those interaction Hamiltonians be diagonalized by the same basis?

Hey man - beats me - which is why I think more research needs to be done. The paper I linked to showed for the model they analysed - at least from my reading of it anyway - that was the case - the basis was singled out regardless. So I think the question is open. And its good people on this forum are raising the issue - it doesn't seem to be getting much traction elsewhere - my standard bible on Decoherence doesn't even mention it.

Thanks
Bill