View Poll Results: Which QM interpretation do you like
MWI 8 24.24%
MMI 0 0%
Copenhagen? 4 12.12%
Shut up and calculate 9 27.27%
String theory 1 3.03%
M-Theory 1 3.03%
Stochastic models 1 3.03%
LQG 1 3.03%
Other 4 12.12%
None of the above? 4 12.12%
Voters: 33. You may not vote on this poll

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Which interpretation is your favourite?

by Schrodinger's Dog
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Dmitry67
#55
Dec26-09, 03:11 PM
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Quote Quote by Jarle View Post
So the problem is that it is not a realist view, because it does not treat the observer as an object?
Yes, an observer (or measurement device) can not be described using QM. Because if we try to apply Qm rules to systems of atoms called 'measurement devices', they dont collapse the wavefunction. All collapse interpretations associate some magic property with some configurations of atoms.
Astepintime
#56
Dec26-09, 03:12 PM
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Quote Quote by Fredrik View Post

Irrelevant. The MWI is an interpretation of QM, not a new theory, so it doesn't make any predictions that QM doesn't.
Nice discussion guys, but this statement reminds me of why I dropped these discussions twenty years ago! It gets you nowhere! If there are no differences in the predictions then just pick the one you like and be done with it. Until an experiment successfully differentiates between the interpretations it seems that it is just religion or worse metaphysics.
OmCheeto
#57
Dec26-09, 03:21 PM
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String Theory.

But only because I play the violin.

I don't really know anything about any of these theories.
disregardthat
#58
Dec26-09, 06:28 PM
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Quote Quote by Dmitry67 View Post
Yes, an observer (or measurement device) can not be described using QM. Because if we try to apply Qm rules to systems of atoms called 'measurement devices', they dont collapse the wavefunction. All collapse interpretations associate some magic property with some configurations of atoms.
What exactly are these "magic properties" you associate with the collapse phenomena, and why can they not be used?

CI appeals to the non-realist, and one can find it consistent with this view.
PTM19
#59
Dec26-09, 11:15 PM
P: 36
Quote Quote by Fredrik View Post
No, I said that if QM describes what actually happens, the universe will be in a mixed state after a measurement, and none of the terms can be dismissed without reason. But that doesn't mean that we should see mixed states! "We" are states of well-defined memories, and you only have the memory of measuring spin up in the world where the result was "up". See the stuff at the end of my previous post (added in my last edit), but read this correction first: I shouldn't have said that a basis defines a world. I think I should have said that the worlds are the terms in the density matrix, and that every possible way to express the density matrix in terms of a basis is equally valid. So not only are there infinitely many worlds (terms in the density matrix). There are infinitely many ways to describe the universe as consisting of worlds! (One for each basis).

What that stuff at the end of my previous post means is that the basis that decoherence singles out as "special" is precisely the one that describes the universe as consisting of worlds in which the physicist's brain quickly goes into a well-defined state with a memory of only one measurement result. In the MWI, this is a very significant part of the reason why we can't experience superpositions, but it's not the whole story. I think an experience can be described as the formation of a well-defined memory state, and that would imply that your memory is always in a well-defined state of remembering a specific measurement result in the world where that was the result of the measurement.

Edit: See also this post.
I think I get what you mean, but I don't think you understood my criticism since your objection already assumes many worlds.

I'll try to be more explicit. Imagine the time before MWI was developed - the time when there was no such concept as "many worlds" in physics. If someone were to perform the experiment back then the fact that mixed states were not observed should lead him to conclude that QM without collapse does not agree with experimental results.

This is what I meant and I argue that it would have been the right conclusion and that it is not a sound approach to physics to invent unobservable universes to circumvent contradictory experimental results even if that makes the underlying mathematics much more consistent and appealing.

Quote Quote by Fredrik View Post
Irrelevant. The MWI is an interpretation of QM, not a new theory, so it doesn't make any predictions that QM doesn't.
It looks like you dismissed my examples without even considering them, the situation in both cases is analogous to MWI. I'll first quote those examples to make it easier to refer to them and then explain one in more detail.

Quote Quote by PTM19 View Post
Imagine for a moment that someone else say a chemist developed a nice hypothesis with beautiful math which explained most but not all experimental results and which could be modified to explain all but at a cost of spoiling the mathematical picture. What would you say if said chemist postulated existence of additional completely unobservable molecules everywhere so that they can provide what is needed to make the hypothesis agree with all experiments?

Or say a doctor developed a very nice symmetry hypothesis which explained the placement of many organs but when confronted with the fact that some organs are not placed symmetrically instead of modifying the hypothesis he insisted on postulating additional invisible organs which perfectly fulfill the symmetry.
I argue that both examples are exactly analogous to MWI. Let's focus on the doctor, I will modify it a bit to further stress the analogy. So, let's say that over the years doctors developed a symmetry theory which correctly explains placement of most organs - this is analogous to QM (both are theories whose predictions agree with experiments in most cases). There is however a problem, some organs like hearth are not placed symmetrically - this is an analogy to measurement problem. One way to deal with wrong placement problem is to amend the symmetry theory but that will spoil the mathematical beauty and consistency by ruining the perfect symmetry - this solution would be analogous to adding wavefunction collapse. Now however one doctor develops a radical solution - he postulates existence of additional unobservable organs which are placed in such a way that symmetry is preserved - for example he claims we all have an unobservable second hearth on the right side of the chest - this novel interpretation of symmetry theory of organ placement is analogous to MWI. Notice that additional postulated organs are unobservable and therefore the theory makes no new experimental predictions - exactly like MWI. Also I want to stress that just as it feels unnatural to add collapse which "artificially" selects one state over the other it is also unnatural to break symmetry and "artificially" place hearth on the left side of the body when the right side would have been just as good from mathematical point of view.

I consider the above example to be a perfect analogy to MWI, the case of a chemist can also be made into a perfect analogy, what's more I believe that by postulating additional unobservable entities almost any problem can be "solved" in similar manner. To me this is not science unless there is a way to experimentally verify such "solutions."

Now, I suspect you would find a proposition that we all have unobservable additional organs, which preserve body symmetry, ridiculous and this is precisely how I see MWI and for exactly the same reasons.

Quote Quote by Fredrik View Post
The result of the experiment isn't all that actually happens. What about the actual experiment? Didn't that happen? Didn't something happen to the silver atom as it passed through the inhomogeneous magnetic field of the Stern-Gerlach apparatus?

The difference between the two types of interpretations is huge. The first kind is what turns a mathematical model into a theory. (I define a "theory" as "a set of statements that associates a probability with each possible result of each experiment in some set of experiments". Note that a theory doesn't have to describe any aspect of reality between state preparation and measurement, not even approximately. It just has to make predictions about results of experiments). The second kind of interpretation turns the theory into a (possibly completely incorrect) description of what actually happens. The first kind of interpretation is a part of the definition of QM, but an "interpretation of QM" is always of the second kind.
Ok, I know what you mean, so to clarify I object to the second kind of interpretation, specifically the part which invokes unobservable parallel universes.

Quote Quote by Fredrik View Post
Do you often consider something proved wrong the moment you discover that it has features that rubs your emotions the wrong way?
No, I tried to explain my reasons, they don't involve emotions, it's more a matter of skepticism and conviction that ideas which are not falsifiable are not science. Besides extraordinary claims require extraordinary evidence.

Quote Quote by Fredrik View Post
I'm undecided on that, since the ensemble interpretation strongly suggests that there's an underlying hidden-variable theory, with variables weird enough to avoid the conclusion of Bell's theorem.
Yes, it does leave room for a better theory but any such theory would not be part of ensemble interpretation and therefore should not affect whether ensemble is preferred based on Ockham's Razor principle.

Quote Quote by Fredrik View Post
When you have some time to spare, you might want to check out David Mermin's "Ithaca" interpretation. That one is really "correlations without correlata".

http://arxiv.org/abs/quant-ph/9801057
http://arxiv.org/abs/quant-ph/9609013
Ok, I'll have a look, thanks for the links.
Fredrik
#60
Dec27-09, 06:55 PM
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Quote Quote by PTM19 View Post
I think I get what you mean, but I don't think you understood my criticism since your objection already assumes many worlds.
You claimed that the MWI predicts that we should see many worlds. I'm saying it doesn't. The only way to argue for either of these two positions is to examine what the MWI actually says.

Quote Quote by PTM19 View Post
Imagine the time before MWI was developed - the time when there was no such concept as "many worlds" in physics. If someone were to perform the experiment back then the fact that mixed states were not observed should lead him to conclude that QM without collapse does not agree with experimental results.
I don't know how you define this "QM without collapse", but if it "doesn't agree with experimental results" then it doesn't make the same predictions as QM, which means that it isn't the MWI. So your objection against the MWI appears to be that QM can be modified to something undefined that makes incorrect predictions. That obviously doesn't qualify as an argument against the MWI.

Quote Quote by PTM19 View Post
I argue that both examples are exactly analogous to MWI
And I argue that they're not. To prove that conclusively, I'd have to spend a lot more time extracting information from you about your fictional theory of organ symmetry than I'm willing to do, so I'll just point out one obvious difference: The fictional theory doesn't explain why the unobservable organs are unobservable.

Quote Quote by PTM19 View Post
I consider the above example to be a perfect analogy to MWI, the case of a chemist can also be made into a perfect analogy, what's more I believe that by postulating additional unobservable entities almost any problem can be "solved" in similar manner. To me this is not science unless there is a way to experimentally verify such "solutions."
It's possible that it was originally intended to be an explanation of why QM is a probabilistic theory (I haven't read Everett), but it certainly isn't such an explanation. It doesn't solve any problems. It's just the minimal realistic interpretation of QM.

Quote Quote by PTM19 View Post
Ok, I know what you mean, so to clarify I object to the second kind of interpretation, specifically the part which invokes unobservable parallel universes.
If we reject all interpretations of the second kind, we can't ever claim that a theory describes reality. We can't even say that "the Earth is spherical" is an approximately correct statement. "The Earth is spherical" would be nothing more than a meaningless string of text that we can use to obtain predictions according to a specific set of rules.

In this particular case, we all feel that it's obvious that the theory not only makes accurate predictions, but also describes reality. In other words, we all feel that it's obvious that the simplest possible interpretation (of the second kind) is correct. (Approximately, of course). Most of us also think that it makes sense to say that Newtonian mechanics describes reality. We can think of it as an approximate description of (some aspect of) the universe we live in, or as an exact description of a fictional universe, but it's certainly a description of something.

Things get a bit more weird when we start talking about special and general relativity. There's a theory involving an aether and a preferred reference frame that makes the same predictions as SR. We can think of these as two different but equivalent theories, or as two formulations of the same theory. They make the same predictions, but describe things differently. There's also a version of GR that treats spacetime as flat, and measurement devices as being distorted by influence from matter. This theory makes the same predictions as GR, but describes things differently.

At this point in time, no one expected that we would ever have to settle for less. Everyone just assumed that a good theory had to be an approximate description of reality. They would even define the word "theory" to mean exactly that. People like Bohr, Heisenberg and Schrödinger, certaintly weren't looking for an algorithm that tells us how to calculate probabilities of possibilities. They were looking for a description of what actually happens at the atomic level.

It must have been quite a shock for the physicists of the time to find a theory that's so weird it doesn't have an obvious interpretation of the second kind. Physicists have always been trying to find the truth about what things really are like, and without an interpretation of the second kind, one can question whether the theory has improved our understanding of how the world works at all! Imagine that the theory "the world is spherical" had been stated in the form of some text that you can't even understand, along with instructions about how to use that text to find predictions. This is how QM must have seemed to the physicists at the time.

Note that a theory that assigns non-trivial probabilities (not always 0 or 1) to possible results of experiments doesn't even satisfy the condition of falsifiability. If the probability of getting a specific result is 99.9999%, you can still get a different result a million times or more. So to bring statistical, non-descriptive theories like QM into the world of science, we even had to drop the requirement of falsifiability from the definition of science, and replace it with a related concept which I think of as "statistical falsifiability". (I think it's obvious what I mean by that, so I won't explain it). Can you imagine changing the definition of science to get a new theory to be considered science? This what dowsers, homeopaths and remote viewers occasionally try to get us to do. (I'm sure that scientists had their share of encounters with those fools back then too).

With all that in mind, it's certainly not surprising that people have been trying to find ways to interpret QM as a description of something. Some people think that it's just a waste of time because experiments can't distinguish between interpretations. "It isn't science". I think this is a bizarre attitude (even though the "it isn't science" bit is technically correct). If we find a bunch of logically consistent interpretations, there's at least a chance that we have found the correct description of the universe. And even if they're all wrong, they might still improve our understanding of the theory, and therefore our understanding of the real world. Even an incorrect description might make it easier to for us to solve problems in QM, by giving us a way to visualize what's actually happening in a fictional universe where experiments have the same results as in ours. There's also the possibility that one of the interpretations will be easier to modify into a new theory than the others, and if that's the case, working on that interpretation has been science all along. It just wasn't possible to see it before the job was done. I don't think many of those people would have that attitude if they understood the things I'm talking about in this post.

There are also people who think that it would be a waste of time to try to interpret QM, because QM can't be interpreted. This I can understand. I was in that camp myself until I understood the MWI well enough to see that it isn't as crazy as it seems. I still don't think that QM can tell us what actually happens in our universe, but I find it fascinating that it might, and even if I knew that it doesn't, I would still be interested in finding out if it can describe anything at all (a possible universe).

There are two more things that I find really annoying when I read what other people are saying about interpretations. The first is the belief that QM obviously can be interpreted. This is just wishful thinking. Mermin displays this belief in the articles I linked to. Penrose does it indirectly in his books by repeatedly claiming that the measurement problem is a huge problem in physics. (It isn't a problem at all for the ensemble interpretation, but it's a problem for some realistic interpretations). The second is the attitude that interpretations don't need proper definitions. It's hard to even find two people who have the same thing in mind when they mention the name of a specific interpretation.
RUTA
#61
Dec27-09, 09:24 PM
P: 647
I don't worry about the multiplicity of interpretations for QM, under determination is unavoidable in physics as Fredrik pointed out about SR and GR. I'll happily live with such multiplicity as long as each ontology is consistent with all of our theories of physics. We don't have that situation now because violations of Bell's inequality imply causal and/or constitutive non-locality while GR is local on both counts. This incongruity prompted Smolin to write (The Trouble with Physics, 2006, p 9), "This is probably the most serious problem facing modern science," and (p 10), “The problem of quantum mechanics is unlikely to be solved in isolation; instead, the solution will probably emerge as we make progress on the greater effort to unify physics.” That remains to be seen of course, but those of us in foundations who subscribe to this attitude are hoping to find clues to unification by considering various interpretations/ontologies for QM. The study of QM interpretations serves as a basis for the study of unification in that sense.
PTM19
#62
Dec28-09, 08:28 AM
P: 36
Fredrik, I really appreciate your feedback and I understand your position but I don't think we can get much further with this discussion. I think my position should be more or less clear by now, I simply don't find the arguments for MWI convincing for reasons I already stated. To me the doctor analogy holds, you claim it doesn't say why organs are unobservable - does MWI say why parallel universes are unobservable? In any case organs can be in a parallel universe.

I understand that some may prefer to believe QM does indeed describe reality no matter where it leads them in hope they can learn something about reality from interpretations like MWI. Personally however I believe we have a much better chance to understand reality if we accept ensemble interpretation and the fact that QM does not describe reality and instead concentrate on searching for an underlying hidden variable theory which does. Unfortunately it is my impression that not many people are going that route and I suspect this may be the reason why there has been hardly any progress in physics lately.
Fredrik
#63
Dec28-09, 05:33 PM
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Quote Quote by PTM19 View Post
does MWI say why parallel universes are unobservable?
Yes, the version of the MWI that I'm talking about does that. (I tried to explain how). But it's hard to find two people who mean the same thing when they say "MWI", so you won't have any problems finding a version that doesn't.
rewebster
#64
Jan6-10, 11:31 AM
P: 880
Quote Quote by PTM19 View Post
Fredrik, I really appreciate your feedback and I understand your position but I don't think we can get much further with this discussion. I think my position should be more or less clear by now, I simply don't find the arguments for MWI convincing for reasons I already stated. To me the doctor analogy holds, you claim it doesn't say why organs are unobservable - does MWI say why parallel universes are unobservable? In any case organs can be in a parallel universe.

I understand that some may prefer to believe QM does indeed describe reality no matter where it leads them in hope they can learn something about reality from interpretations like MWI. Personally however I believe we have a much better chance to understand reality if we accept ensemble interpretation and the fact that QM does not describe reality and instead concentrate on searching for an underlying hidden variable theory which does. Unfortunately it is my impression that not many people are going that route and I suspect this may be the reason why there has been hardly any progress in physics lately.
I agree
RUTA
#65
Jan6-10, 05:49 PM
P: 647
Quote Quote by PTM19 View Post
I understand that some may prefer to believe QM does indeed describe reality no matter where it leads them in hope they can learn something about reality from interpretations like MWI. Personally however I believe we have a much better chance to understand reality if we accept ensemble interpretation and the fact that QM does not describe reality and instead concentrate on searching for an underlying hidden variable theory which does. Unfortunately it is my impression that not many people are going that route and I suspect this may be the reason why there has been hardly any progress in physics lately.
Check out where the interpretation called Relational Blockworld (published in 2008 in Foundations of Physics and Studies in History & Philosophy of Modern Physics) suggests physics should go (http://xxx.lanl.gov/abs/0908.4348). That interpretation suggests a definite path for unification and quantum gravity based on constitutive non-locality as the correction for GR. We're trying to solve the equations for (discrete) tensor CFT now to see precisely where the theory differs from GR. This is an example of how the study of QM interpretations can lead to new ideas for physics.
rewebster
#66
Jan6-10, 10:07 PM
P: 880
Quote Quote by RUTA View Post
Check out where the interpretation called Relational Blockworld (published in 2008 in Foundations of Physics and Studies in History & Philosophy of Modern Physics) suggests physics should go (http://xxx.lanl.gov/abs/0908.4348). That interpretation suggests a definite path for unification and quantum gravity based on constitutive non-locality as the correction for GR. We're trying to solve the equations for (discrete) tensor CFT now to see precisely where the theory differs from GR. This is an example of how the study of QM interpretations can lead to new ideas for physics.
I just read the thread:

"The Fatal Flaw in Every Techno Show on TV

Let's Enhance!"

http://physicsforums.com/showthread.php?t=367648

for some reason, I got the same feeling when reading this post

"Let's Enhance!"
RUTA
#67
Jan7-10, 08:00 AM
P: 647
Quote Quote by rewebster View Post
I just read the thread:

"The Fatal Flaw in Every Techno Show on TV

Let's Enhance!"

http://physicsforums.com/showthread.php?t=367648

for some reason, I got the same feeling when reading this post

"Let's Enhance!"
We're still waiting for the referee reports, but if you've found a "fatal flaw" in the paper, let us know and we'll withdraw it.


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