Bohmian vs. Copenhagen

[NeverEnoughFuel!!]

https://www.quantamagazine.org/20160517-pilot-wave-theory-gains-experimental-support/

A very good text about quantum mechanics... to me as a layman sounds and looks very reasonable... could it be we are so wrong for a such a long time?

[cantab]

Going by that article, it looks like the Bohm interpretation still shares the key shortcoming of the Copenhagen interpretation: What exactly is meant by a 'measurement' or 'observation'? Without really pinning that down, I don't think quantum mechanics can really be complete or sensible.

 

[gogozerg]

I can't remember exactly, but maybe you can help me... How does Bohm's approach deal with delayed choice experiments? What kind of super-pilot-wave?

 

I love quantum mechanics. At first you get into it with the feeling you can find a reasonable explanation, and you end up believing Everett had the most rational mind! (where observation means the observer is simply an object intricated with the observed object, in the context of an infinity of branching universes)

 

Edited November 20, 2016 by gogozerg

[Steel]

Correct me if I'm wrong, but doesn't Bohm's approach also require FTL communication?

 

Also, let's just clear this up: even if one interpretation appears to make more sense to us, doesn't means its any more "correct" than, say, Copenhagen or Feynman. At present, regardless of which of these three interpretations you use, you will get result in the same measurements in experiments. Surely this suggests that actually none of them are truly correct?

Edited November 20, 2016 by Steel

[K^2]

Pilot-wave theory is an archaic attempt to bring Quantum Mechanics in line with human intuition. The notion that nature should be intuitive is idiotic, of course, which is why there is absolutely no sense in any of this.

Modern Field Theory, of which Quantum Mechanics is a branch, is fully deterministic and local. There is no faster-than-light action or any random effects in it. Basically, none of the perceived problems that led to attempts to develop a pilot-wave theory in the first place. Moreover, modern Field Theory has withstood one test after another, while pilot-wave fanatics keep needing to come up with one patch after another to prevent the whole thing from coming apart. Field Theory describes all fundamental forces, yes, including gravity, via gauge symmetries, thereby, bridging the gap between Quantum and Relativity. Pilot-wave models can barely avoid violating Special Relativity. Nobody has a clue how to make them compatible with gravity.

It is possible, in principle, to keep adding features to a pilot-wave description of the world to make it match any Field Theory. Just like it is possible to keep adding epicycles to orbits of celestial bodies to describe any star system. But at some point, any sane person would have to admit that epicycles just don't work, and gravity is a much better explanation. Unfortunately, proponents of pilot-wave lack that sanity. They are much happier with a needlessly complicated theory, one which needs to be patched up after each new discovery, just so long as it doesn't offend their intuition of how the world should work.

To be fair, it should be noted that there is some scientific and mathematical interest in pilot-wave models as cases of duality. In that sense, it is on the same shelf as holographic interpretation or string theory. There are interesting things you can do with it, and some useful knowledge can be gained from juxtaposing it with Field Theory. In short, not all people who spend time on pilot-wave are crackpots. But no serious scientist looks at it as a viable alternative to Standard Model.

 

P.S. Yes, Many Worlds is probably the closest we'll get to an intuitive interpretation. But one important distinction is that Many Worlds and Copenhagen are both interpretations of the same underlying theory. They are not distinguishable by experiment. So you'll never have to make tweaks to one or the other because you got new experimental data. The underlying theory could be in trouble, but not the interpretations. So you really are free to just pick the one you like. Pilot-wave started out as an interpretation, but it only passes that bar in a one-particle case with fixed frame of reference. Once you introduce variable reference frames and multi-particle interaction, pilot-wave actually becomes experimentally distinguishable from Quantum Mechanics, making it a distinct theory. Of course, it's a very bad theory, since it needs patch-work for just about every serious test of Quantum Mechanics.

Edited November 20, 2016 by K^2

[gogozerg]

A recent PBS Space Time episode talking about the subject.

 

 

[todofwar]

On 11/19/2016 at 8:24 PM, gogozerg said:

I love quantum mechanics. 

 

I love thinking about it, but I hate working with it. The equations just never seemed related to anything real, and way too often explanations for physical phenomena boil down to "math says so" like electric dipole allowedness in transitions. Fortunately, density functional theory allows someone like me to skip the math and focus on important things like electronic structure.

No idea about this stuff though. The experiment they describe doesn't seem to prove it either way? They shoot two entangled photons, one goes one way the other goes another, and supposedly the polarization of one reports on the polarization of the other, but only if they measure it after specified amount of time. How does this distinguish the two theories? Seems like this is just testing entanglement? 

[PB666]

On 11/20/2016 at 8:23 AM, K^2 said:

Pilot-wave theory is an archaic attempt to bring Quantum Mechanics in line with human intuition. The notion that nature should be intuitive is idiotic, of course, which is why there is absolutely no sense in any of this.

Modern Field Theory, of which Quantum Mechanics is a branch, is fully deterministic and local. There is no faster-than-light action or any random effects in it. Basically, none of the perceived problems that led to attempts to develop a pilot-wave theory in the first place. Moreover, modern Field Theory has withstood one test after another, while pilot-wave fanatics keep needing to come up with one patch after another to prevent the whole thing from coming apart. Field Theory describes all fundamental forces, yes, including gravity, via gauge symmetries, thereby, bridging the gap between Quantum and Relativity. Pilot-wave models can barely avoid violating Special Relativity. Nobody has a clue how to make them compatible with gravity.

It is possible, in principle, to keep adding features to a pilot-wave description of the world to make it match any Field Theory. Just like it is possible to keep adding epicycles to orbits of celestial bodies to describe any star system. But at some point, any sane person would have to admit that epicycles just don't work, and gravity is a much better explanation. Unfortunately, proponents of pilot-wave lack that sanity. They are much happier with a needlessly complicated theory, one which needs to be patched up after each new discovery, just so long as it doesn't offend their intuition of how the world should work.

To be fair, it should be noted that there is some scientific and mathematical interest in pilot-wave models as cases of duality. In that sense, it is on the same shelf as holographic interpretation or string theory. There are interesting things you can do with it, and some useful knowledge can be gained from juxtaposing it with Field Theory. In short, not all people who spend time on pilot-wave are crackpots. But no serious scientist looks at it as a viable alternative to Standard Model.

 

P.S. Yes, Many Worlds is probably the closest we'll get to an intuitive interpretation. But one important distinction is that Many Worlds and Copenhagen are both interpretations of the same underlying theory. They are not distinguishable by experiment. So you'll never have to make tweaks to one or the other because you got new experimental data. The underlying theory could be in trouble, but not the interpretations. So you really are free to just pick the one you like. Pilot-wave started out as an interpretation, but it only passes that bar in a one-particle case with fixed frame of reference. Once you introduce variable reference frames and multi-particle interaction, pilot-wave actually becomes experimentally distinguishable from Quantum Mechanics, making it a distinct theory. Of course, it's a very bad theory, since it needs patch-work for just about every serious test of Quantum Mechanics.

This comes at the problem from a MWI bias. I should point out that copenhagen is the most conservative, and basically grants that there are unsolvables (defining itself as incomplete). MWI creates a mess in its wake that is unneccesary. And pilot wave theory does not know how to reconcile quantum space-time into space-time (which BTW is problematic for all interpretations). The problem I think is in field character, that we confuse consistent experimental behavior with actual proof. The essense of resolution is at the fine scale space-time (or to be more specific at the local level quantum space-time) the 'construct' in which these fields both traverse and compose is not known (you can cap that and put explanation points behind it to fuel its importance). As one scientist remarked, we should stop guessing.

 

[cantab]

Does quantum field theory resolve the measurement problem then? And if so how?

[K^2]

On 12/2/2016 at 6:09 AM, PB666 said:

This comes at the problem from a MWI bias. I should point out that copenhagen is the most conservative, and basically grants that there are unsolvables (defining itself as incomplete). MWI creates a mess in its wake that is unneccesary. And pilot wave theory does not know how to reconcile quantum space-time into space-time (which BTW is problematic for all interpretations). The problem I think is in field character, that we confuse consistent experimental behavior with actual proof. The essense of resolution is at the fine scale space-time (or to be more specific at the local level quantum space-time) the 'construct' in which these fields both traverse and compose is not known (you can cap that and put explanation points behind it to fuel its importance). As one scientist remarked, we should stop guessing.

 

MWI and Copenhagen are interpretations. You can pick either one you like. If you feel Copenhagen offends fewer of your sensibilities, you can take that one. I like MWI because it eliminates illusion of randomness in QM, and that bothers me more than infinite universes.

The important part is that you cannot conduct an experiment to distinguish between Copenhagen and MWI by definition. When I say it's matter of preference, it really is. Neither is provable. And the only way to disprove Copenhagen or MWI is to disprove the entirety of QM.

More importantly, a corollary of above is taht any sound conclusion under one interpretation applies to the other interpretation. As far as measurable quantities are concerned, at least. Which can be a wonderful shortcut. You can prove the No-communication Theorem using just Copenhagen. And it takes a bit of work. Or you can pose the problem in MWI and wonder how people got confused in the first place. On the other hand, some problems are way easier with Copenhagen. Explaining Quantum Zeno under MWI is a total pain. It requires introducing a Hamiltonian responsible for the measurement and showing that under that Hamiltonian the eigen states are stationary. Good homework problem for Ph.D. students, bad times for anyone else. But under Copenhagen, it's absolutely obvious. So take your pick for a favorite, and I strongly recommend understanding both for practical purposes.

Pilot Wave is different. It actually requires a different theory, which can be distinguished from QM. Of course, every experiment so far has fallen onto the side of QM, which lead PW proponents to keep adding features to try and bring it to parity. Which, granted, doesn't automatically disprove the entire approach, but it makes it very dubious at best.

As for space-time characteristics, this would have been a good argument back in Michelson Morely experiment, if you tried to argue that we might not need Special Relativity, but we moved on quite a bit past that. Relativity effects are no longer considered a curious feature needing an explanation. They are a direct consequence of the underlying symmetries, proof for which comes from absolutely every corner. Special Relativity, General Relativity, conservation of energy and momentum itself comes from the same principles. Saying, "We might simply not understand how it works," while isn't technically wrong, is not very productive. Until we find an actual smoking gun in the theory, like if EM drive turns out to actually operate at above photon-drive efficiency, I say we assume these things are fundamentally correct. In which case, Pilot Wave is still broken.

On 12/2/2016 at 2:41 PM, cantab said:

Does quantum field theory resolve the measurement problem then? And if so how?

No, as measurement problem is part of interpretation, not theory. In aforementioned MWI, there is no measurement problem, but it's the same quantum field theory.