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Possible Duplicate:
Why do people still talk about bohmian mechanics/hidden variables

I've heard of De Broglie–Bohm theory a.k.a causal interpretation of quantum theory. The predictions match accurately with with the nondeterministic quantum theory.

As a philosophy buff this one seems just like the classical universe. No funny new-age religion style gimmicks.

So why don't people use this interpretation instead of the Copenhagen "magic" interpretation?

P.S. I'm not an expert. I possess only superficial knowledge of quantum physics.

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marked as duplicate by Luboš Motl, dmckee Apr 18 '11 at 23:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Suffice to say that De Broglie–Bohm theory and Special Relativity are on collision course. $\endgroup$ – Qmechanic Apr 18 '11 at 9:35
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    $\begingroup$ Real quantum physics is neither new-age religion style nor "magic". It is woo-doctors and crackpots such as Deepak Chopra who abuse quantum physics for selling their new age myths that make quantum physics appear so otherworldly. Also, classical physics means physics of medium sized bodies at low velocities. There is no justifiable reason why this should remain true "all the way down" to the atomic and "all the way up" to the cosmological scale. So physicists don't have a problem with quantum theory not being classical. $\endgroup$ – Lagerbaer Apr 18 '11 at 14:55
  • $\begingroup$ This question, like the earlier one, asks for a philosophical discussion: a mode on discourse for which the Stack Exchange engine is singularly ill-designed. $\endgroup$ – dmckee Apr 18 '11 at 23:48
  • $\begingroup$ @dmckee Clarification: as OP of earlier question, I had asked a plain question to correct my understanding from a reading. It did not ask for a philosophical discussion, and what happened later was really beyond me. $\endgroup$ – yayu Apr 19 '11 at 16:26
  • $\begingroup$ @dmckee I've flaggd your comment as I consider philosopher a not-very appraising term, short of an insult to a physics student, and plainly wrong in the context. Moreover, last question was closed after I had flagged it. After reading the answers, I realized that the topic was not as simplistic as I thought. I also suspect that I was the only one who asked for moderator attention on it. Asking for philosophical discussion is the last thing I would do. $\endgroup$ – yayu Apr 20 '11 at 0:15
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So why don't people use this interpretation instead of the Copenhagen "magic" interpretation?

People don't use this interpretation, because it is not useful. On the experimental front it doesn't aid in prediction of experiments, and on the theoretical front it doesn't aid in advancing theoretical techniques or understanding of newly found phenomenology.

A fundamental assumption of Bohmian mechanics is that the unknown particle position when we do an experiment, has an ensemble probability density the same as that predicted by the wavefunction in the usual interpretation. This is necessary for Bohmian mechanics to reproduce the Copenhagen approach. So even in Bohmian mechanics, the results for experiments can only be given probabilistically, and the wavefunction encodes these probabilities. So when calculating experimental predictions, adding additional pieces to the state function do not help, and just are not useful.

Even if we wanted to calculate using Bohmian mechanics, even some simple things become quite needlessly complicated. For example even if you somehow knew the particle positions and the wavefunction, you still wouldn't know how to update the positions unless you somehow also knew what coordinate system is defining the absolute simultaneity ... and of course this frame isn't measureable. And even if you knew this as well somehow, things like spin are now not clearly defined. Spin in Bohmian mechanics is purely contextual, which means to predict the outcome of an experiment (even if you knew all the things previously mentioned) you'd now need to know precise details of the entire measuring device and simulate that as well. So it should be clear why people use the simpler calculational interpretations instead of Bohmian mechanics.

Furthermore, it has not been useful in advancing our knowledge. Bohmian mechanics comes from ordinary non-relativistic quantum mechanics. A large portion of our advances in theoretical physics have come from understanding how to appropriately use and apply non-trivial symmetries. For example Lorentz symmetry allows us to use certain representation for particles, and also allows many terms in a general Lagrangian to be thrown out. Bohmian mechanics instead obscures this right from the start.

Instead of bringing us the spin-statistics theorem, or advancing to relativistic quantum mechanics and to quantum field theory, Bohmian mechanics confuses instead of clarifying. To keep it up to date with current understanding, people would need to keep updating it and playing 'catch-up'.

Some people find it interesting to try to answer 'is this possible?' type questions by trying to find if Bohmian mechanics can be adjusted or fixed to somehow create a Bohmian quantum field theory. I have yet to see a convincing relativistic formulation. It is a bit worrisome that there are also researchers (who publish at least, so its not complete fringe) that instead feel Bohmian mechanics gives the correct picture. At some point it often crosses into metaphysics. So physicists also tend to avoid Bohmian mechanics, as dicussions of it can often become controversial but rarely ever useful.

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    $\begingroup$ I'd say the pilot wave theory is much easier to comprehend. That in and of itself is useful. And a new way of thinking about quantum mechanics may lead people to new developments faster. Like they say, the definition of madness is continuing to do the same thing and expecting a different result. $\endgroup$ – B T Jan 23 '15 at 7:05
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    $\begingroup$ Experimental nonlocal and surreal Bohmian trajectories... 5 years after the question: Bohmian Mechanics takes a giant step forward... (my paraphrase) advances.sciencemag.org/content/2/2/e1501466 $\endgroup$ – Cyberchipz Jan 12 '17 at 16:39
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    $\begingroup$ Bell's Theorem wasn't useful? "De Broglie–Bohm theory highlighted the issue of nonlocality: it inspired John Stewart Bell to prove his now-famous theorem,[55] which in turn led to the Bell test experiments." en.wikipedia.org/wiki/… $\endgroup$ – dylan oliver Mar 27 '17 at 22:18
  • $\begingroup$ @BT Pilot wave theory is easy for laymen to think they comprehend if you make some pretty graphics and put new-age music behind it. It's appealing because it tells people "hey, QM isn't actually weird and you don't have to put any effort into understanding it, it's actually just like classical mechanics with pretty pictures". Of course, the fine print is that the pilot wave itself behaves extremely strangely, stranger than anything in ordinary QM, and that makes pilot wave useless for actually doing anything, conceptually or practically. $\endgroup$ – knzhou May 27 '18 at 9:36
  • $\begingroup$ @knzhou I haven't heard anyone claim that Pilot Wave theory behaves at all differently from normal QM. My understanding is that the outcomes of pilot wave theory predictions match QM exactly. $\endgroup$ – B T Jun 8 '18 at 23:33
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First, it is a mistake to think that DBB theory is classical. Momenta and positions are still not simultaneously defined, Heiseinberg's uncertainty principle still holds. Second, DBB theory only clarifies the Copenhagen interpretation by providing it with an ontology. A couple of reasons why it is not liked:

  1. It's thought to be cheap, i.e. the way positions are added to complete the theory.
  2. It's non-local, while most people argue that quantum mechanics is local.

Here is one of the latest extensive threads about the subject which in the end got closed because it was too argumentative.

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  • $\begingroup$ Thanks for correcting me. Wikipedia says DBB theory says "It is the lack of knowledge of the particle's trajectory that accounts for the [Heisenberg's] uncertainty relation.". Thanks for the thread, I will read it. It seems hard to digest. $\endgroup$ – louzer Apr 18 '11 at 8:42
  • $\begingroup$ I think that is not true. That would be akin to saying that the lack of knowledge of the particle's trajectories is the reason for Hiroshima. If we knew al the particle trajectories, there would have been no Hiroshima. Which is silly. Likewise, the HUP is a consequence of the wave function guiding the particles. Even if one knew the positions of all the particles, we still would have HUP. However, it is true that DBB theory makes a preparation assumption on the initial probability distribution of the particles to retrieve the usual results of quantum mechanics. But, that is normal QM fare. $\endgroup$ – Raskolnikov Apr 18 '11 at 8:48
  • $\begingroup$ +1, specifically for the non-local part. To my POV, the opposition is not because QM is supposed to be local, but to the fact that this nonlocality pretty much cancels out the supposed intuitiveness brought by this theory. This basically reintroduces wavefunction collapse... $\endgroup$ – Frédéric Grosshans Apr 18 '11 at 16:07
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    $\begingroup$ @Raskolnikov: "Momenta and positions are still not simultaneously defined, Heiseinberg's uncertainty principle still holds." The thing is that no truly physical theory can simultaneously define both momentum and position, simply because momentum for a point does not exist. Momentum is mass times velocity, and velocity is a measure of movement, which means it is a measure of change of position, which requires two points. By definition. So it is not peculiar to CI or Heisenberg. It is just simple mechanics. $\endgroup$ – bright magus Nov 10 '14 at 13:29
  • $\begingroup$ @bright magus: If you say so. See you, I'm off throwing away all my books about classical phase space. $\endgroup$ – Raskolnikov Nov 18 '14 at 7:12

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