Editing De Broglie–Bohm theory (section)

== Similarities with the many-worlds interpretation ==
Kim Joris Boström has proposed a non-relativistic quantum mechanical theory that combines elements of de Broglie-Bohm mechanics and [[Hugh Everett III|Everett]]'s many-worlds. In particular, the unreal many-worlds interpretation of Hawking and Weinberg is similar to the Bohmian concept of unreal empty branch worlds:

{{Blockquote|The second issue with Bohmian mechanics may, at first sight, appear rather harmless, but which on a closer look develops considerable destructive power: the issue of empty branches. These are the components of the post-measurement state that do not guide any particles because they do not have the actual configuration ''q'' in their support. At first sight, the empty branches do not appear problematic but on the contrary very helpful as they enable the theory to explain unique outcomes of measurements. Also, they seem to explain why there is an effective "collapse of the wavefunction", as in ordinary quantum mechanics. On a closer view, though, one must admit that these empty branches do not actually disappear. As the wavefunction is taken to describe a really existing field, all their branches really exist and will evolve forever by the Schrödinger dynamics, no matter how many of them will become empty in the course of the evolution. Every branch of the global wavefunction potentially describes a complete world which is, according to Bohm's ontology, only a possible world that would be the actual world if only it were filled with particles, and which is in every respect identical to a corresponding world in Everett's theory. Only one branch at a time is occupied by particles, thereby representing the actual world, while all other branches, though really existing as part of a really existing wavefunction, are empty and thus contain some sort of "zombie worlds" with planets, oceans, trees, cities, cars and people who talk like us and behave like us, but who do not actually exist. Now, if the Everettian theory may be accused of ontological extravagance, then Bohmian mechanics could be accused of ontological wastefulness. On top of the ontology of empty branches comes the additional ontology of particle positions that are, on account of the quantum equilibrium hypothesis, forever unknown to the observer. Yet, the actual configuration is never needed for the calculation of the statistical predictions in experimental reality, for these can be obtained by mere wavefunction algebra. From this perspective, Bohmian mechanics may appear as a wasteful and redundant theory. I think it is considerations like these that are the biggest obstacle in the way of a general acceptance of Bohmian mechanics.<ref>{{Cite arXiv |eprint = 1208.5632|last1 = Valentini|first1 = Antony|title = Combining Bohm and Everett: Axiomatics for a Standalone Quantum Mechanics|last2 = Westman|first2 = Hans|class = quant-ph|year = 2012}}</ref>}}

Many authors have expressed critical views of de Broglie–Bohm theory by comparing it to Everett's many-worlds approach. Many (but not all) proponents of de Broglie–Bohm theory (such as Bohm and Bell) interpret the universal wavefunction as physically real. According to some supporters of Everett's theory, if the (never collapsing) wavefunction is taken to be physically real, then it is natural to interpret the theory as having the same many worlds as Everett's theory. In the Everettian view the role of the Bohmian particle is to act as a "pointer", tagging, or selecting, just one branch of the [[universal wavefunction]] (the assumption that this branch indicates which ''wave packet'' determines the observed result of a given experiment is called the "result assumption"<ref name=BrownWallace />); the other branches are designated "empty" and implicitly assumed by Bohm to be devoid of conscious observers.<ref name=BrownWallace /> [[H. Dieter Zeh]] comments on these "empty" branches:<ref>[[Daniel Dennett]] (2000). ''With a little help from my friends.'' In D. Ross, A. Brook, and D. Thompson (Eds.), ''Dennett's Philosophy: a comprehensive assessment.'' MIT Press/Bradford, {{ISBN|0-262-68117-X}}.</ref>
{{cquote|It is usually overlooked that Bohm's theory contains the same "many worlds" of dynamically separate branches as the Everett interpretation (now regarded as "empty" wave components), since it is based on precisely the same ... [[universal wavefunction|global wave function]] ...}}
[[David Deutsch]] has expressed the same point more "acerbically":<ref name=BrownWallace /><ref>{{cite journal | last1 = Deutsch | first1 = David | author-link = David Deutsch | year = 1996 | title = Comment on Lockwood | journal = British Journal for the Philosophy of Science | volume = 47 | issue = 2| pages = 222–228 | doi = 10.1093/bjps/47.2.222 }}</ref>
{{cquote|Pilot-wave theories are parallel-universe theories in a state of chronic denial.}}

This conclusion has been challenged by Detlef Dürr and Justin Lazarovici:
<blockquote>The Bohmian, of course, cannot accept this argument. For her, it is decidedly the particle configuration in three-dimensional space and not the wave function on the abstract configuration space that constitutes a world (or rather, the world). Instead, she will accuse the Everettian of not having local beables (in Bell's sense) in her theory, that is, the ontological variables that refer to localized entities in three-dimensional space or four-dimensional spacetime. The many worlds of her theory thus merely appear as a grotesque consequence of this omission.<ref>{{cite book |last1=Dürr |first1=Detlef |last2=Lazarovici |first2=Justin | author-link = Detlef Dürr and Justin Lazarovici |title=Understanding Quantum Mechanics: The World According to Modern Quantum Foundations|publisher= Springer | date = 2022 |isbn = 978-3-030-40067-5}}</ref></blockquote>