The Many-Worlds Interpretation of QM

[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