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| {{Use dmy dates|date=June 2013}}
| | I'm Bess and I live in Marseille. <br>I'm interested in Japanese Studies, Book collecting and Danish art. I like to travel and watching Grey's Anatomy.<br><br>Feel free to visit my site [http://issuu.com/likeablehijacke50/docs/140822395453efcad200413 gopro filmpjes] |
| {{Quantum mechanics|cTopic=[[Interpretation of quantum mechanics|Interpretations]]}}
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| The '''de Broglie–Bohm theory''', also known as the '''[[pilot wave|pilot-wave]] theory''', '''Bohmian mechanics''', the '''Bohm or Bohm's interpretation''', and the '''causal interpretation''', is an [[Interpretations of quantum mechanics|interpretation]] of [[Quantum mechanics|quantum theory]]. In addition to a [[wavefunction]] on the space of all possible configurations, it also includes an actual configuration, even when unobserved. The evolution over time of the configuration (that is, of the positions of all particles or the configuration of all fields) is defined by the wave function via a [[#Guiding equation|guiding equation]]. The evolution of the wave function over time is given by [[Schrödinger Equation|Schrödinger's equation]]. The theory is named after [[Louis de Broglie]] (1892–1987) and [[David Bohm]] (1917–1992).
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| The de Broglie–Bohm theory is explicitly [[Principle of locality|nonlocal]]: the velocity of any one particle depends on the value of the guiding equation, which depends on the whole configuration of the universe. Because the [[standard model of particle physics|known laws of physics]] are all local, and because nonlocal interactions combined with [[special relativity|relativity]] lead to causal paradoxes, many physicists find this unacceptable.
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| The theory is [[Deterministic system|deterministic]]. Most (but not all) variants of the theory that support [[special relativity]] require a preferred frame. Variants which include spin and curved spaces are known. It can be modified to include [[quantum field theory]]. [[Bell's theorem]] was inspired by Bell's discovery of the work of David Bohm and his subsequent wondering if the obvious nonlocality of the theory could be eliminated.
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| The theory results in a measurement formalism, analogous to thermodynamics for classical mechanics, which yields the standard quantum formalism generally associated with the [[Copenhagen interpretation]]. The measurement problem is resolved by this theory since the outcome of an experiment is registered by the configuration of the particles of the experimental apparatus after the experiment is completed. The familiar wavefunction collapse of standard quantum mechanics emerges from an analysis of subsystems and the [[quantum equilibrium hypothesis]].
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| There are several equivalent [[#Derivations|mathematical formulations]] of the theory and it is known by a number of different [[#History|names]]. The de Broglie wave has a macroscopical analogy termed [[Faraday wave]].<ref>John W. M. Bush: Quantum mechanics writ large - http://www.tcm.phy.cam.ac.uk/~mdt26/tti_talks/deBB_10/bush_tti2010.pdf</ref>
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| ==Overview==
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| De Broglie–Bohm theory is based on the following postulates:
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| * There is a configuration <math>q</math> of the universe, described by coordinates <math>q^k</math>, which is an element of the configuration space <math>Q</math>. The configuration space is different for different versions of pilot wave theory. For example, this may be the space of positions <math>\mathbf{Q}_k</math> of <math>N</math> particles, or, in case of field theory, the space of field configurations <math>\phi(x)</math>. The configuration evolves (for spin=0) according to the guiding equation
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| :<math>m_k\frac{d q^k}{dt} (t) = \hbar \nabla_k \operatorname{Im} \ln \psi(q,t) = \hbar \operatorname{Im}\left(\frac{\nabla_k \psi}{\psi} \right) (q, t) = \frac{m_k \bold{j}_k}{\psi^*\psi} = \mathrm{Re}\left ( \frac{\bold{\hat{P}}_k\Psi}{\Psi} \right ) </math>.
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| Where <math>\bold{j}</math> is the [[probability current]] or probability flux and <math>\bold{\hat{P}}</math> is the [[momentum operator]]. Here, <math>\psi(q,t)</math> is the standard complex-valued wavefunction known from quantum theory, which evolves according to Schrödinger's equation
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| :<math>i\hbar\frac{\partial}{\partial t}\psi(q,t)=-\sum_{i=1}^{N}\frac{\hbar^2}{2m_i}\nabla_i^2\psi(q,t) + V(q)\psi(q,t)</math>
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| This already completes the specification of the theory for any quantum theory with Hamilton operator of type <math>H=\sum \frac{1}{2m_i}\hat{p}_i^2 + V(\hat{q})</math>.
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| * The configuration is distributed according to <math>|\psi(q,t)|^2</math> at some moment of time <math>t</math>, and this consequently holds for all times. Such a state is named quantum equilibrium. With quantum equilibrium, this theory agrees with the results of standard quantum mechanics.
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| ===Two-slit experiment===
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| [[File:doppelspalt.svg|thumb|The Bohmian trajectories for an electron going through the two-slit experiment. A similar pattern was also extrapolated from [[weak measurement]]s of single photons.<ref>http://www.sciencemag.org/content/332/6034/1170.figures-only - Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer</ref>]]
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| The [[double-slit experiment]] is an illustration of [[wave-particle duality]]. In it, a beam of particles (such as electrons) travels through a barrier with two slits removed. If one puts a detector screen on the other side, the pattern of detected particles shows interference fringes characteristic of waves; however, the detector screen responds to particles. The system exhibits behaviour of both waves (interference patterns) and particles (dots on the screen).
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| If we modify this experiment so that one slit is closed, no interference pattern is observed. Thus, the state of both slits affects the final results. We can also arrange to have a minimally invasive detector at one of the slits to detect which slit the particle went through. When we do that, the interference pattern disappears.
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| The [[Copenhagen interpretation]] states that the particles are not localised in space until they are detected, so that, if there is not any detector on the slits, there is no matter of fact about which slit the particle has passed through. If one slit has a detector on it, then the wavefunction collapses due to that detection.
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| In de Broglie–Bohm theory, the wavefunction travels through both slits, but each particle has a well-defined trajectory that passes through exactly one of the slits. The final position of the particle on the detector screen and the slit through which the particle passes is determined by the initial position of the particle. Such initial position is not knowable or controllable by the experimenter, so there is an appearance of randomness in the pattern of detection. The wave function interferes with itself and guides the particles in such a way that the particles avoid the regions in which the interference is destructive and are attracted to the regions in which the interference is constructive, resulting in the interference pattern on the detector screen.
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| To explain the behavior when the particle is detected to go through one slit, one needs to appreciate the role of the conditional wavefunction and how it results in the collapse of the wavefunction; this is explained below. The basic idea is that the environment registering the detection effectively separates the two wave packets in configuration space.
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| ==The theory==
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| ===The ontology===
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| The [[ontology]] of de Broglie-Bohm theory consists of a configuration <math>q(t)\in Q</math> of the universe and a pilot wave <math>\psi(q,t)\in\mathbb{C}</math>. The configuration space <math>Q</math> can be chosen differently, as in classical mechanics and standard quantum mechanics.
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| Thus, the ontology of pilot wave theory contains as the trajectory <math>q(t)\in Q</math> we know from classical mechanics, as the wave function <math>\psi(q,t)\in\mathbb{C}</math> of quantum theory. So, at every moment of time there exists not only a wave function, but also a well-defined configuration of the whole universe. The correspondence to our experiences is made by the identification of the configuration of our brain with some part of the configuration of the whole universe <math>q(t)\in Q</math>, as in classical mechanics.
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| While the ontology of classical mechanics is part of the ontology of de Broglie–Bohm theory, the dynamics are very different. In classical mechanics, the accelerations of the particles are imparted directly by forces, which exist in physical three-dimensional space. In de Broglie–Bohm theory, the velocities of the particles are given by the wavefunction, which exists in a 3N-dimensional configuration space, where N corresponds to the number of particles in the system;<ref>{{cite book|author=David Bohm|title=Causality and Chance in Modern Physics|year=1957|publisher=Routledge & Kegan Paul and D. Van Nostrand|isbn=0-8122-1002-6}}, p. 117.</ref> Bohm hypothesized that each particle has a "complex and subtle inner structure" that provides the capacity to react to the information provided by the wavefunction.<ref>D. Bohm and B. Hiley: ''The undivided universe: An ontological interpretation of quantum theory'', p. 37.</ref> Also, unlike in classical mechanics, physical properties (e.g., mass, charge) are spread out over the wavefunction in de Broglie-Bohm theory, not localized at the position of the particle.<ref>H. R. Brown, C. Dewdney and G. Horton: "Bohm particles and their detection in the light of neutron interferometry", ''Foundations of Physics'', 1995, Volume 25, Number 2, pp. 329-347.</ref><ref>J. Anandan, "The Quantum Measurement Problem and the Possible Role of the Gravitational Field", ''Foundations of Physics'', March 1999, Volume 29, Issue 3, pp 333-348.</ref>
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| The wavefunction itself, and not the particles, determines the dynamical evolution of the system: the particles do not act back onto the wave function. As Bohm and Hiley worded it, "the Schrodinger equation for the quantum field does not have sources, nor does it have any other way by which the field could be directly affected by the condition of the particles [...] the quantum theory can be understood completely in terms of the assumption that the quantum field has no sources or other forms of dependence on the particles".<ref>D. Bohm and B. Hiley: ''The undivided universe: An ontological interpretation of quantum theory'', [http://books.google.de/books?id=vt9XKjc4WAQC&pg=PA24 p. 24]</ref> P. Holland considers this lack of reciprocal action of particles and wave function to be one "[a]mong the many nonclassical properties exhibited by this theory".<ref>Peter R. Holland: ''The Quantum Theory of Motion: An Account of the De Broglie-Bohm Causal Interpretation of Quantum Mechanics'', Cambridge University Press, Cambridge (first published 25 June 1993), ISBN 0-521-35404-8 hardback, ISBN 0-521-48543-6 paperback, transferred to digital printing 2004, Chapter I. section (7) "There is no reciprocal action of the particle on the wave", [http://books.google.com/books?id=BsEfVBzToRMC&pg=PA26 p. 26]</ref> It should be noted however that Holland has later called this a merely ''apparent'' lack of back reaction, due to the incompleteness of the description.<ref>* P. Holland: ''Hamiltonian theory of wave and particle in quantum mechanics II: Hamilton-Jacobi theory and particle back-reaction'', Nuovo Cimento B 116, 2001, pp. 1143-1172, [http://users.ox.ac.uk/~gree0579/index_files/NuovoCimento2.pdf#page=31 full text preprint p. 31])</ref>
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| In what follows below, we will give the setup for one particle moving in <math>\mathbb{R}^3</math> followed by the setup for <math>N</math> particles moving in 3 dimensions. In the first instance, configuration space and real space are the same while in the second, real space is still <math>\mathbb{R}^3</math>, but configuration space becomes <math>\mathbb{R}^{3N}</math>. While the particle positions themselves are in real space, the velocity field and wavefunction are on configuration space which is how particles are entangled with each other in this theory.
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| [[#Extensions|Extensions]] to this theory include spin and more complicated configuration spaces.
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| We use variations of <math>\mathbf{Q}</math> for particle positions while <math>\psi</math> represents the complex-valued wavefunction on configuration space.
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| ===Guiding equation===
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| For a spinless single particle moving in <math>\mathbb{R}^3</math>, the particle's velocity is given
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| :<math>\frac{d \mathbf{Q}}{dt} (t) = \frac{\hbar}{m} \operatorname{Im} \left(\frac{\nabla \psi}{\psi} \right) (\mathbf{Q}, t)</math>.
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| For many particles, we label them as <math>\mathbf{Q}_k</math> for the <math>k</math>th particle and their velocities are given by
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| :<math>\frac{d \mathbf{Q}_k}{dt} (t) = \frac{\hbar}{m_k} \operatorname{Im} \left(\frac{\nabla_k \psi}{\psi} \right) (\mathbf{Q}_1, \mathbf{Q}_2, \ldots, \mathbf{Q}_N, t)</math>.
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| The main fact to notice is that this velocity field depends on the actual positions of all of the <math>N</math> particles in the universe. As explained below, in most experimental situations, the influence of all of those particles can be encapsulated into an effective wavefunction for a subsystem of the universe.
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| ===Schrödinger's equation===
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| The one particle Schrödinger equation governs the time evolution of a complex-valued wavefunction on <math>\mathbb{R}^3</math>. The equation represents a quantized version of the total energy of a classical system evolving under a real-valued potential function <math>V</math> on <math>\mathbb{R}^3</math>:
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| :<math>i\hbar\frac{\partial}{\partial t}\psi=-\frac{\hbar^2}{2m}\nabla^2\psi + V\psi</math>
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| For many particles, the equation is the same except that <math>\psi</math> and <math>V</math> are now on configuration space, <math>\mathbb{R}^{3N}</math>.
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| :<math>i\hbar\frac{\partial}{\partial t}\psi=-\sum_{k=1}^{N}\frac{\hbar^2}{2m_k}\nabla_k^2\psi + V\psi</math>
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| This is the same wavefunction of conventional quantum mechanics.
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| ===Relation to the Born Rule===
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| In Bohm's original papers [Bohm 1952], he discusses how de Broglie–Bohm theory results in the usual measurement results of quantum mechanics. The main idea is that this is true if the positions of the particles satisfy the statistical distribution given by <math>|\psi|^2</math>. And that distribution is guaranteed to be true for all time by the guiding equation if the initial distribution of the particles satisfies <math>|\psi|^2</math>.
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| For a given experiment, we can postulate this as being true and verify experimentally that it does indeed hold true, as it does. But, as argued in Dürr et al.,<ref name="dgz92">Dürr, D., Goldstein, S., and Zanghì, N., [http://arxiv.org/abs/quant-ph/0308039 "Quantum Equilibrium and the Origin of Absolute Uncertainty"], Journal of Statistical Physics 67: 843–907, 1992.</ref> one needs to argue that this distribution for subsystems is typical. They argue that <math>|\psi|^2</math> by virtue of its equivariance under the dynamical evolution of the system, is the appropriate measure of typicality for [[initial condition]]s of the positions of the particles. They then prove that the vast majority of possible initial configurations will give rise to statistics obeying the [[Born rule]] (i.e., <math>|\psi|^2</math>) for measurement outcomes. In summary, in a universe governed by the de Broglie–Bohm dynamics, Born rule behavior is typical.
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| The situation is thus analogous to the situation in classical statistical physics. A low entropy initial condition will, with overwhelmingly high probability, evolve into a higher entropy state: behavior consistent with the second law of thermodynamics is typical. There are, of course, anomalous initial conditions which would give rise to violations of the second law. However, absent some very detailed evidence supporting the actual realization of one of those special initial conditions, it would be quite unreasonable to expect anything but the actually observed uniform increase of entropy. Similarly, in the de Broglie–Bohm theory, there are anomalous initial conditions which would produce measurement statistics in violation of the Born rule (i.e., in conflict with the predictions of standard quantum theory). But the typicality theorem shows that, absent some particular reason to believe one of those special initial conditions was in fact realized, Born rule behavior is what one should expect.
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| It is in that qualified sense that the Born rule is, for the de Broglie–Bohm theory, a theorem rather than (as in ordinary quantum theory) an additional postulate.
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| It can also be shown that a distribution of particles that is ''not'' distributed according to the Born rule (that is, a distribution 'out of quantum equilibrium') and evolving under the de Broglie-Bohm dynamics is overwhelmingly likely to evolve dynamically into a state distributed as <math>|\psi|^2</math>. See, for example Ref.
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| .<ref>Towler, M.D., Russell, N.J., Valentini A., pbs.,[http://arxiv.org/abs/1103.1589 "Timescales for dynamical relaxation to the Born rule"] quant-ph/11031589</ref> A video of the electron density in a 2D box evolving under this process is available [http://www.tcm.phy.cam.ac.uk/~mdt26/raw_movie.gif here].
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| ===The conditional wave function of a subsystem===
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| In the formulation of the De Broglie–Bohm theory, there is only a wave function for the entire universe (which always evolves by the Schrödinger equation). However, once the theory is formulated, it is convenient to introduce a notion of wave function also for subsystems of the universe. Let us write the wave function of the universe as <math>\psi(t,q^{\mathrm I},q^{\mathrm{II}})</math>, where <math>q^{\mathrm I}</math> denotes the configuration variables associated to some subsystem (I) of the universe and <math>q^{\mathrm{II}}</math> denotes the remaining configuration variables. Denote, respectively, by <math>Q^{\mathrm I}(t)</math> and by <math>Q^{\mathrm{II}}(t)</math> the actual configuration of subsystem (I) and of the rest of the universe. For simplicity, we consider here only the spinless case. The ''conditional wave function'' of subsystem (I) is defined by:
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| :<math>\psi^{\mathrm I}(t,q^{\mathrm I})=\psi(t,q^{\mathrm I},Q^{\mathrm{II}}(t)). \,</math>
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| It follows immediately from the fact that <math>Q(t)=(Q^{\mathrm I}(t),Q^{\mathrm{II}}(t))</math> satisfies the guiding equation that also the configuration <math>Q^{\mathrm I}(t)</math> satisfies a guiding equation identical to the one presented in the formulation of the theory, with the universal wave function <math>\psi</math> replaced with the conditional wave function <math>\psi^{\mathrm I}</math>. Also, the fact that <math>Q(t)</math> is random with [[Probability density function|probability density]] given by the square modulus of <math>\psi(t,\cdot)</math> implies that the [[Conditional probability density function|conditional probability density]] of <math>Q^{\mathrm I}(t)</math> given <math>Q^{\mathrm{II}}(t)</math> is given by the square modulus of the (normalized) conditional wave function <math>\psi^{\mathrm I}(t,\cdot)</math> (in the terminology of Dürr et al.<ref>Quantum Equilibrium and the Origin of Absolute Uncertainty, D. Dürr, S. Goldstein and N. Zanghì, Journal of Statistical Physics 67, 843-907 (1992), http://arxiv.org/abs/quant-ph/0308039.</ref> this fact is called the ''fundamental conditional probability formula'').
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| Unlike the universal wave function, the conditional wave function of a subsystem does not always evolve by the Schrödinger equation, but in many situations it does. For instance, if the universal wave function factors as:
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| :<math>\psi(t,q^{\mathrm I},q^{\mathrm{II}})=\psi^{\mathrm I}(t,q^{\mathrm I})\psi^{\mathrm{II}}(t,q^{\mathrm{II}}) \,</math>
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| then the conditional wave function of subsystem (I) is (up to an irrelevant scalar factor) equal to <math>\psi^{\mathrm I}</math> (this is what Standard Quantum Theory would regard as the wave function of subsystem (I)). If, in addition, the Hamiltonian does not contain an interaction term between subsystems (I) and (II) then <math>\psi^{\mathrm I}</math> does satisfy a Schrödinger equation. More generally, assume that the universal wave function <math>\psi</math> can be written in the form:
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| :<math>\psi(t,q^{\mathrm I},q^{\mathrm{II}})=\psi^{\mathrm I}(t,q^{\mathrm I})\psi^{\mathrm{II}}(t,q^{\mathrm{II}})+\phi(t,q^{\mathrm I},q^{\mathrm{II}}), \,</math>
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| where <math>\phi</math> solves Schrödinger equation and <math>\phi(t,q^{\mathrm I},Q^{\mathrm{II}}(t))=0</math> for all <math>t</math> and <math>q^{\mathrm I}</math>. Then, again, the conditional wave function of subsystem (I) is (up to an irrelevant scalar factor) equal to <math>\psi^{\mathrm I}</math> and if the Hamiltonian does not contain an interaction term between subsystems (I) and (II), <math>\psi^{\mathrm I}</math> satisfies a Schrödinger equation.
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| The fact that the conditional wave function of a subsystem does not always evolve by the Schrödinger equation is related to the fact that the usual collapse rule of Standard Quantum Theory emerges from the Bohmian formalism when one considers conditional wave functions of subsystems.
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| ==Extensions==
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| ===Spin===
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| To incorporate spin, the wavefunction becomes complex-vector valued. The value space is called spin space; for a [[spin-½]] particle, spin space can be taken to be <math>\mathbb{C}^2</math>. The guiding equation is modified by taking inner products in spin space to reduce the complex vectors to complex numbers. The Schrödinger equation is modified by adding a Pauli spin term.
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| :<math>\frac{d \mathbf{Q}_k}{dt} (t) = \frac{\hbar}{m_k} Im \left(\frac{(\psi,D_k \psi)}{(\psi,\psi)} \right) (\mathbf{Q}_1, \mathbf{Q}_2, \ldots, \mathbf{Q}_N, t)</math>
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| :<math>i\hbar\frac{\partial}{\partial t}\psi = \left(-\sum_{k=1}^{N}\frac{\hbar^2}{2m_k}D_k^2 + V - \sum_{k=1}^{N} \mu_k \mathbf{S}_{k}/{S}_{k} \cdot \mathbf{B}(\mathbf{q}_k) \right) \psi </math>
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| where <math>\mu_k</math> is the magnetic moment of the <math>k</math>th particle, <math>\mathbf{S}_{k}</math> is the appropriate spin operator acting in the <math>k</math>th particle's spin space, <math>{S}_{k}</math> is spin of the particle (<math>{S}_{k} = 1/2</math> for electron),
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| :<math>D_k=\nabla_k-\frac{ie_k}{c\hbar}\mathbf{A}(\mathbf{q}_k)</math>,
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| <math>\mathbf{B}</math> and <math>\mathbf{A}</math> are, respectively, the magnetic field and the vector potential in <math>\mathbb{R}^{3}</math> (all other functions are fully on configuration space), <math>e_k</math> is the charge of the <math>k</math>th particle, and <math>(\cdot,\cdot)</math> is the inner product in spin space <math>\mathbb{C}^d</math>,
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| :<math>(\phi,\psi) = \sum_{s=1}^d \phi_s^* \psi_s.</math>
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| For an example of a spin space, a system consisting of two spin 1/2 particle and one spin 1 particle has a wavefunctions of the form
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| :<math>\psi: \mathbb{R}^{9}\times \mathbb{R} \to \mathbb{C}^{2}\otimes \mathbb{C}^{2} \otimes \mathbb{C}^{3}</math>.
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| That is, its spin space is a 12 dimensional space.
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| ===Curved space===
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| To extend de Broglie–Bohm theory to curved space ([[Riemannian manifolds]] in mathematical parlance), one simply notes that all of the elements of these equations make sense, such as gradients and Laplacians. Thus, we use equations that have the same form as above. Topological and boundary conditions may apply in supplementing the evolution of Schrödinger's equation.
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| For a de Broglie–Bohm theory on curved space with spin, the spin space becomes a [[vector bundle]] over configuration space and the potential in Schrödinger's equation becomes a local self-adjoint operator acting on that space.<ref>Dürr, D., Goldstein, S., Taylor, J., Tumulka, R., and Zanghì, N., J. [http://arxiv.org/abs/quant-ph/0506173 "Quantum Mechanics in Multiply-Connected Spaces"], Phys. A: Math. Theor. 40, 2997–3031 (2007)</ref>
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| ===Quantum field theory===
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| In Dürr et al.,<ref name="dgtz04">Dürr, D., Goldstein, S., Tumulka, R., and Zanghì, N., 2004, [http://arxiv.org/abs/quant-ph/0303156 "Bohmian Mechanics and Quantum Field Theory"], Phys. Rev. Lett. 93: 090402:1–4.</ref><ref>Dürr, D., Tumulka, R., and Zanghì, N., J. Phys. A: Math. Gen. 38, R1–R43 (2005), [http://arxiv.org/abs/quant-ph/0407116v1 quant-ph/0407116]</ref> the authors describe an extension of de Broglie–Bohm theory for handling [[creation and annihilation operators]], which they refer to as "Bell-type quantum field theories". The basic idea is that configuration space becomes the (disjoint) space of all possible configurations of any number of particles. For part of the time, the system evolves deterministically under the guiding equation with a fixed number of particles. But under a stochastic process, particles may be created and annihilated. The distribution of creation events is dictated by the wavefunction. The wavefunction itself is evolving at all times over the full multi-particle configuration space.
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| Hrvoje Nikolić <ref name="nikolicqft">Nikolic, H. 2010 [http://arxiv.org/abs/0904.2287 "QFT as pilot-wave theory of particle creation and destruction"], Int. J. Mod. Phys. A 25, 1477 (2010)</ref> introduces a purely deterministic de Broglie–Bohm theory of particle creation and destruction, according to which particle trajectories are continuous, but particle detectors behave as if particles have been created or destroyed even when a true creation or destruction of particles does not take place.
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| ===Exploiting nonlocality===
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| [[Antony Valentini]]<ref>Valentini, A., 1991, "Signal-Locality, Uncertainty and the Subquantum H-Theorem. II," Physics Letters A 158: 1–8.</ref> has extended the de Broglie–Bohm theory to include signal nonlocality that would allow entanglement to be used as a stand-alone communication channel without a secondary classical "key" signal to "unlock" the message encoded in the entanglement. This violates orthodox quantum theory but it has the virtue that it makes the parallel universes of the [[chaotic inflation theory]] observable in principle.
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| Unlike de Broglie–Bohm theory, Valentini's theory has the wavefunction evolution also depend on the ontological variables. This introduces an instability, a feedback loop that pushes the hidden variables out of "sub-quantal heat death". The resulting theory becomes nonlinear and non-unitary.
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| ===Relativity===
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| Pilot wave theory is explicitly nonlocal. As a consequence, most relativistic variants of pilot wave theory need a [[foliation]] of space-time. While this is in conflict with the standard interpretation of relativity, the preferred foliation, if unobservable, does not lead to any empirical conflicts with relativity.
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| The relation between nonlocality and preferred foliation can be better understood as follows. In de Broglie–Bohm theory, nonlocality manifests as the fact that the velocity and acceleration of one particle depends on the instantaneous positions of all other particles. On the other hand, in the theory of relativity the concept of instantaneousness does not have an invariant meaning. Thus, to define particle trajectories, one needs an additional rule that defines which space-time points should be considered instantaneous. The simplest way to achieve this is to introduce a preferred foliation of space-time by hand, such that each hypersurface of the foliation defines a hypersurface of equal time. However, this way (which explicitly breaks the relativistic covariance) is not the only way. It is also possible that a rule which defines instantaneousness is [[Contingency (philosophy)|contingent]], by emerging dynamically from relativistic covariant laws combined with particular initial conditions. In this way, the need for a preferred foliation can be avoided and relativistic covariance can be saved.
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| There has been work in developing relativistic versions of de Broglie–Bohm theory. See Bohm and Hiley: The Undivided Universe, and [http://xxx.lanl.gov/abs/quant-ph/0208185], [http://xxx.lanl.gov/abs/quant-ph/0302152], and references therein. Another approach is given in the work of Dürr et al.<ref>Dürr, D., Goldstein, S., Münch-Berndl, K., and Zanghì, N., 1999, [http://arxiv.org/abs/quant-ph/9801070 "Hypersurface Bohm-Dirac Models"], Phys. Rev. A 60: 2729–2736.</ref> in which they use Bohm-Dirac models and a Lorentz-invariant foliation of space-time.
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| Initially, it had been considered impossible to set out a description of photon trajectories in the de Broglie–Bohm theory in view of the difficulties of describing bosons relativistically.<ref name="ghose-1996">Partha Ghose: [http://www.springerlink.com/content/yq03611746404204/fulltext.pdf ''Relativistic quantum mechanics of spin-0 and spin-1 bosons''], Foundations of Physics, vol. 26, no. 11, pp. 1441-1455, 1996, {{doi|10.1007/BF02272366}}</ref> In 1996, [[Partha Ghose]] had presented a relativistic quantum mechanical description of spin-0 and spin-1 bosons starting from the [[Duffin–Kemmer–Petiau equation]], setting out Bohmian trajectories for massive bosons and for massless bosons (and therefore [[photon]]s).<ref name="ghose-1996"/> In 2001, [[Jean-Pierre Vigier]] emphasized the importance of deriving a well-defined description of light in terms of particle trajectories in the framework of either the Bohmian mechanics or the Nelson stochastic mechanics.<ref>Nicola Cufaro Petroni, Jean-Pierre Vigier: ''Remarks on Observed Superluminal Light Propagation'', Foundations of Physics Letters, vol. 14, no. 4, pp. 395-400, {{doi|10.1023/A:1012321402475}}, therein: section ''3. Conclusions'', page 399</ref> The same year, Ghose worked out Bohmian photon trajectories for specific cases.<ref>Partha Ghose, A.S. Majumdar, S. Guhab, J. Sau: [http://web.mit.edu/saikat/www/research/files/Bohmian-traj_PLA2001.pdf ''Bohmian trajectories for photons''], Physics Letters A 290 (2001), pp. 205–213, 10 November 2001</ref> Subsequent [[weak measurement]] experiments yielded trajectories which coincide with the predicted trajectories.<ref>Sacha Kocsis, Sylvain Ravets, Boris Braverman, Krister Shalm, Aephraim M. Steinberg: Observing the trajectories of a single photon using weak measurement, 19th Australian Institute of Physics (AIP) Congress, 2010 [http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf]</ref><ref>Sacha Kocsis, Boris Braverman, Sylvain Ravets, Martin J. Stevens, Richard P. Mirin, L. Krister Shalm, Aephraim M. Steinberg: ''Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer'', Science, vol. 332 no. 6034 pp. :1170-1173, 3 June 2011, {{doi|10.1126/science.1202218}} ([http://www.sciencemag.org/content/332/6034/1170.abstract abstract])</ref>
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| Nikolić has proposed a Lorentz-covariant formulation of the Bohmian interpretation of many-particle wave functions.<ref>Hrvoje Nikolić: ''[http://www.springerlink.com/content/y23q534285n7170l/ Relativistic Quantum Mechanics and the Bohmian Interpretation]'', Foundations of Physics Letters, vol. 18, no. 6, November 2005, pp. 549-561, {{doi|10.1007/s10702-005-1128-1}}</ref> He has developed a generalized relativistic-invariant probabilistic interpretation of quantum theory,<ref name="nikolicqft"/><ref>Hrvoje Nikolić: ''[http://xxx.lanl.gov/abs/0811.1905 Time in relativistic and nonrelativistic quantum mechanics]'', arXiv:0811/0811.1905v2 (submitted 12 November 2008 (v1), revised 12 Jan 2009)</ref><ref>Hrvoje Nikolić: ''[http://arxiv.org/abs/1002.3226 Making nonlocal reality compatible with relativity]'', {{arxiv|1002.3226v2}} [quant-ph] (submitted on 17 Feb 2010, version of 31 May 2010)</ref> in which <math>|\psi|^2</math> is no longer a probability density in space, but a probability density in space-time. He uses this generalized probabilistic interpretation to formulate a relativistic-covariant version of de Broglie–Bohm theory without introducing a preferred foliation of space-time. His work also covers the extension of the Bohmian interpretation to a quantization of fields and strings.<ref>Hrvoje Nikolić: ''[http://iopscience.iop.org/1742-6596/67/1/012035/pdf/jpconf7_67_012035.pdf Bohmian mechanics in relativistic quantum mechanics, quantum field theory and string theory]'', 2007 J. Phys.: Conf. Ser. 67 012035</ref>
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| ==Results==
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| Below are some highlights of the results that arise out of an analysis of de Broglie–Bohm theory. Experimental results agree with all of the standard predictions of quantum mechanics in so far as the latter has predictions. However, while standard quantum mechanics is limited to discussing the results of 'measurements', de Broglie–Bohm theory is a theory which governs the dynamics of a system without the intervention of outside observers (p. 117 in Bell<ref name="bell" />).
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| The basis for agreement with standard quantum mechanics is that the particles are distributed according to <math>|\psi|^2</math>. This is a statement of observer ignorance, but it can be proven<ref name="dgz92" /> that for a universe governed by this theory, this will typically be the case. There is apparent collapse of the wave function governing subsystems of the universe, but there is no collapse of the universal wavefunction.
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| ===Measuring spin and polarization===
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| According to ordinary quantum theory, it is not possible to measure the [[Spin (physics)|spin]] or [[Polarization (waves)|polarization]] of a particle directly; instead, the component in one direction is measured; the outcome from a single particle may be 1, meaning that the particle is aligned with the measuring apparatus, or -1, meaning that it is aligned the opposite way. For an ensemble of particles, if we expect the particles to be aligned, the results are all 1. If we expect them to be aligned oppositely, the results are all -1. For other alignments, we expect some results to be 1 and some to be -1 with a probability that depends on the expected alignment. For a full explanation of this, see the [[Stern-Gerlach Experiment]].
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| In de Broglie–Bohm theory, the results of a spin experiment cannot be analyzed without some knowledge of the experimental setup. It is possible<ref>Albert, D. Z., 1992, Quantum Mechanics and Experience, Cambridge, MA: Harvard University Press</ref> to modify the setup so that the trajectory of the particle is unaffected, but that the particle with one setup registers as spin up while in the other setup it registers as spin down. Thus, for the de Broglie–Bohm theory, the particle's spin is not an intrinsic property of the particle—instead spin is, so to speak, in the wave function of the particle in relation to the particular device being used to measure the spin. This is an illustration of what is sometimes referred to as contextuality, and is related to naive realism about operators.<ref>Daumer, M., Dürr, D., Goldstein, S., and Zanghì, N., 1997, [http://arxiv.org/abs/quant-ph/9601013 "Naive Realism About Operators"], Erkenntnis 45: 379–397.</ref>
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| ===Measurements, the quantum formalism, and observer independence===
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| De Broglie–Bohm theory gives the same results as quantum mechanics. It treats the wavefunction as a fundamental object in the theory as the wavefunction describes how the particles move. This means that no experiment can distinguish between the two theories. This section outlines the ideas as to how the standard quantum formalism arises out of quantum mechanics. References include Bohm's original 1952 paper and Dürr et al.<ref name="dgz92" />
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| ====Collapse of the wavefunction====
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| De Broglie–Bohm theory is a theory that applies primarily to the whole universe. That is, there is a single wavefunction governing the motion of all of the particles in the universe according to the guiding equation. Theoretically, the motion of one particle depends on the positions of all of the other particles in the universe. In some situations, such as in experimental systems, we can represent the system itself in terms of a de Broglie–Bohm theory in which the wavefunction of the system is obtained by conditioning on the environment of the system. Thus, the system can be analyzed with Schrödinger's equation and the guiding equation, with an initial <math>|\psi|^2</math> distribution for the particles in the system (see the section on [[#The conditional wave function of a subsystem|the conditional wave function of a subsystem]] for details).
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| It requires a special setup for the conditional wavefunction of a system to obey a quantum evolution. When a system interacts with its environment, such as through a measurement, the conditional wavefunction of the system evolves in a different way. The evolution of the universal wavefunction can become such that the wavefunction of the system appears to be in a superposition of distinct states. But if the environment has recorded the results of the experiment, then using the actual Bohmian configuration of the environment to condition on, the conditional wavefunction collapses to just one alternative, the one corresponding with the measurement results.
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| [[wavefunction collapse|Collapse]] of the universal wavefunction never occurs in de Broglie–Bohm theory. Its entire evolution is governed by Schrödinger's equation and the particles' evolutions are governed by the guiding equation. Collapse only occurs in a phenomenological way for systems that seem to follow their own Schrödinger's equation. As this is an effective description of the system, it is a matter of choice as to what to define the experimental system to include and this will affect when "collapse" occurs.
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| ====Operators as observables====
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| In the standard quantum formalism, measuring observables is generally thought of as measuring operators on the Hilbert space. For example, measuring position is considered to be a measurement of the position operator. This relationship between physical measurements and Hilbert space operators is, for standard quantum mechanics, an additional axiom of the theory. The de Broglie–Bohm theory, by contrast, requires no such measurement axioms (and measurement as such is not a dynamically distinct or special sub-category of physical processes in the theory). In particular, the usual operators-as-observables formalism is, for de Broglie–Bohm theory, a theorem.<ref>Dürr, D., Goldstein, S., and Zanghì, N., [http://arxiv.org/abs/quant-ph/0308038 "Quantum Equilibrium and the Role of Operators as Observables in Quantum Theory"] Journal of Statistical Physics 116, 959–1055 (2004)</ref> A major point of the analysis is that many of the measurements of the observables do not correspond to properties of the particles; they are (as in the case of spin discussed above) measurements of the wavefunction.
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| In the history of de Broglie–Bohm theory, the proponents have often had to deal with claims that this theory is impossible. Such arguments are generally based on inappropriate analysis of operators as observables. If one believes that spin measurements are indeed measuring the spin of a particle that existed prior to the measurement, then one does reach contradictions. De Broglie–Bohm theory deals with this by noting that spin is not a feature of the particle, but rather that of the wavefunction. As such, it only has a definite outcome once the experimental apparatus is chosen. Once that is taken into account, the impossibility theorems become irrelevant.
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| There have also been claims that experiments reject the Bohm trajectories
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| [http://arxiv.org/abs/quant-ph/0206196] in favor of the standard QM lines. But as shown in [http://arxiv.org/abs/quant-ph/0108038] and [http://arxiv.org/abs/quant-ph/0305131], such experiments cited above only disprove a misinterpretation of the de Broglie–Bohm theory, not the theory itself.
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| There are also objections to this theory based on what it says about particular situations usually involving eigenstates of an operator. For example, the ground state of hydrogen is a real wavefunction. According to the guiding equation, this means that the electron is at rest when in this state. Nevertheless, it is distributed according to <math>|\psi|^2</math> and no contradiction to experimental results is possible to detect.
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| Operators as observables leads many to believe that many operators are equivalent. De Broglie–Bohm theory, from this perspective, chooses the position observable as a favored observable rather than, say, the momentum observable. Again, the link to the position observable is a consequence of the dynamics. The motivation for de Broglie–Bohm theory is to describe a system of particles. This implies that the goal of the theory is to describe the positions of those particles at all times. Other observables do not have this compelling ontological status. Having definite positions explains having definite results such as flashes on a detector screen. Other observables would not lead to that conclusion, but there need not be any problem in defining a mathematical theory for other observables; see Hyman et al.<ref>Hyman, Ross et al [http://www.iop.org/EJ/abstract/0305-4470/37/44/L02 Bohmian mechanics with discrete operators], J. Phys. A: Math. Gen. 37 L547–L558, 2004</ref> for an exploration of the fact that a probability density and probability current can be defined for any set of commuting operators.
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| ====Hidden variables====
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| De Broglie–Bohm theory is often referred to as a "hidden variable" theory. The alleged applicability of the term "hidden variable" comes from the fact that the particles postulated by Bohmian mechanics do not influence the evolution of the wavefunction. The argument is that, because adding particles does not have an effect on the wavefunction's evolution, such particles must not have effects at all and are, thus, unobservable, since they cannot have an effect on observers. There is no analogue of [[Newton's third law]] in this theory. The idea is supposed to be that, since particles cannot influence the wavefunction, and it is the wavefunction that determines measurement predictions through the Born rule, the particles are superfluous and unobservable.
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| Bohm and Hiley have stated that they found their own choice of terms of an "interpretation in terms of hidden variables" to be too restrictive. In particular, a particle is not actually hidden but rather "is what is most directly manifested in an observation", even if position and momentum of a particle cannot be observed with arbitrary precision.<ref>David Bohm, Basil Hiley: ''The Undivided Universe: An Ontological Interpretation of Quantum Theory'', edition published in the Taylor & Francis e-library 2009 (first edition Routledge, 1993), ISBN 0-203-98038-7, [http://books.google.com/books?id=vt9XKjc4WAQC&pg=PA2 p. 2]</ref> Put in simpler words, the particles postulated by the de Broglie–Bohm theory are anything but "hidden" variables: they are what the objects we see in everyday experience are made of; it is the wavefunction itself which is "hidden" in the sense of being invisible and not-directly-observable.
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| Even a whole particle trajectory can be measured by a [[weak measurement]]. Such a measured trajectory coincides with the de Broglie–Bohm trajectory. In this sense, de Broglie–Bohm trajectories are not hidden variables. Or at least they are not more hidden than the wave function, in the sense that both can only be experimentally determined through a large number of measurements on an ensemble of equally prepared systems.
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| ===Heisenberg's uncertainty principle===
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| The [[Heisenberg uncertainty principle]] states that when two complementary measurements are made, there is a limit to the product of their accuracy. As an example, if one measures the position with an accuracy of <math>\Delta x</math>, and the momentum with an accuracy of <math>\Delta p</math>, then <math>\Delta x\Delta p\gtrsim h.</math> If we make further measurements in order to get more information, we disturb the system and change the trajectory into a new one depending on the measurement setup; therefore, the measurement results are still subject to Heisenberg's uncertainty relation.
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| In de Broglie–Bohm theory, there is always a matter of fact about the position and momentum of a particle. Each particle has a well-defined trajectory. Observers have limited knowledge as to what this trajectory is (and thus of the position and momentum). It is the lack of knowledge of the particle's trajectory that accounts for the uncertainty relation. What one can know about a particle at any given time is described by the wavefunction. Since the uncertainty relation can be derived from the wavefunction in other interpretations of quantum mechanics, it can be likewise derived (in the [[Epistemology|epistemic]] sense mentioned above), on the de Broglie–Bohm theory.
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| To put the statement differently, the particles' positions are only known statistically. As in [[classical mechanics]], successive observations of the particles' positions refine the experimenter's knowledge of the particles' [[initial conditions]]. Thus, with succeeding observations, the initial conditions become more and more restricted. This formalism is consistent with the normal use of the Schrödinger equation.
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| For the derivation of the uncertainty relation, see [[Heisenberg uncertainty principle]], noting that it describes it from the viewpoint of the [[Copenhagen interpretation]].
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| ===Quantum entanglement, Einstein-Podolsky-Rosen paradox, Bell's theorem, and nonlocality===
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| De Broglie–Bohm theory highlighted the issue of [[Quantum nonlocality|nonlocality]]: it inspired [[John Stewart Bell]] to prove his now-famous [[Bell's theorem|theorem]],<ref name="Bell 1964">{{cite journal | author = Bell J. S. | year = 1964 | title = On the Einstein Podolsky Rosen Paradox | url = http://www.drchinese.com/David/Bell_Compact.pdf | format = PDF | journal = Physics | volume = 1 | issue = | page = 195 }}</ref> which in turn led to the [[Bell test experiments]].
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| In the [[EPR paradox|Einstein–Podolsky–Rosen paradox]], the authors describe a thought-experiment one could perform on a pair of particles that have interacted, the results of which they interpreted as indicating that quantum mechanics is an incomplete theory.<ref>{{cite journal |last=Einstein |last2=Podolsky |last3=Rosen |title=Can Quantum Mechanical Description of Physical Reality Be Considered Complete? |journal=[[Physical Review|Phys. Rev.]] |volume=47 |issue=10 |pages=777–780 |year=1935 |doi=10.1103/PhysRev.47.777 |bibcode = 1935PhRv...47..777E }}</ref>
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| Decades later [[John Stewart Bell|John Bell]] proved [[Bell's theorem]] (see p. 14 in Bell<ref name="bell">{{cite book |last=Bell |first=John S. |title=Speakable and Unspeakable in Quantum Mechanics |location= |publisher=Cambridge University Press |year=1987 |isbn=0521334950 }}</ref>), in which he showed that, if they are to agree with the empirical predictions of quantum mechanics, all such "hidden-variable" completions of quantum mechanics must either be nonlocal (as the Bohm interpretation is) or give up the assumption that experiments produce unique results (see [[counterfactual definiteness]] and [[many-worlds interpretation]]). In particular, Bell proved that any local theory with unique results must make empirical predictions satisfying a statistical constraint called "Bell's inequality".
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| [[Alain Aspect]] performed a series of [[Bell test experiments]] that test Bell's inequality using an EPR-type setup. Aspect's results show experimentally that Bell's inequality is in fact violated—meaning that the relevant quantum mechanical predictions are correct. In these Bell test experiments, entangled pairs of particles are created; the particles are separated, traveling to remote measuring apparatus. The orientation of the measuring apparatus can be changed while the particles are in flight, demonstrating the apparent nonlocality of the effect.
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| The de Broglie–Bohm theory makes the same (empirically correct) predictions for the Bell test experiments as ordinary quantum mechanics. It is able to do this because it is manifestly nonlocal. It is often criticized or rejected based on this; Bell's attitude was: "It is a merit of the de Broglie–Bohm version to bring this [nonlocality] out so explicitly that it cannot be ignored."<ref>Bell, page 115</ref>
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| The de Broglie–Bohm theory describes the physics in the Bell test experiments as follows: to understand the evolution of the particles, we need to set up a wave equation for both particles; the orientation of the apparatus affects the wavefunction. The particles in the experiment follow the guidance of the wavefunction. It is the wavefunction that carries the faster-than-light effect of changing the orientation of the apparatus. An analysis of exactly what kind of nonlocality is present and how it is compatible with relativity can be found in Maudlin.<ref>{{cite book |last=Maudlin |first=T. |year=1994 |title=Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics |location=Cambridge, MA |publisher=Blackwell |isbn=0631186093 }}</ref> Note that in Bell's work, and in more detail in Maudlin's work, it is shown that the nonlocality does not allow for signaling at speeds faster than light.
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| ===Classical limit===
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| Bohm's formulation of de Broglie–Bohm theory in terms of a classical-looking version has the merits that the emergence of classical behavior seems to follow immediately for any situation in which the quantum potential is negligible, as noted by Bohm in 1952. Modern methods of [[decoherence]] are relevant to an analysis of this limit. See Allori et al.<ref>Allori, V., Dürr, D., Goldstein, S., and Zanghì, N., 2002, [http://arxiv.org/abs/quant-ph/0112005 "Seven Steps Towards the Classical World"], Journal of Optics B 4: 482–488.</ref> for steps towards a rigorous analysis.
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| ===Quantum trajectory method===
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| Work by [[Robert E. Wyatt]] in the early 2000s attempted to use the Bohm "particles" as an adaptive mesh that follows the actual trajectory of a quantum state in time and space. In the "quantum trajectory" method, one samples the quantum wavefunction with a mesh of quadrature points. One then evolves the quadrature points in time according to the Bohm equations of motion. At each time-step, one then re-synthesizes the wavefunction from the points, recomputes the quantum forces, and continues the calculation. (QuickTime movies of this for H+H<sub>2</sub> reactive scattering can be found on [http://research.cm.utexas.edu/rwyatt/movies/qtm/index.html the Wyatt group] web-site at UT Austin.)
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| This approach has been adapted, extended, and used by a number of researchers in the Chemical Physics community as a way to compute semi-classical and quasi-classical molecular dynamics. A recent (2007) issue of the [http://pubs.acs.org/toc/jpcafh/111/41 Journal of Physical Chemistry A] was dedicated to Prof. Wyatt and his work on "Computational Bohmian Dynamics".
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| [[Eric R. Bittner]]'s [http://k2.chem.uh.edu group] at the [[University of Houston]] has advanced a statistical variant of this approach that uses Bayesian sampling technique to sample the quantum density and compute the quantum potential on a structureless mesh of points. This technique was recently used to estimate quantum effects in the heat-capacity of small clusters Ne<sub>n</sub> for n~100.
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| There remain difficulties using the Bohmian approach, mostly associated with the formation of singularities in the quantum potential due to nodes in the
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| quantum wavefunction. In general, nodes forming due to interference effects lead to the case where
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| <math>R^{-1}\nabla^2R\rightarrow\infty.</math>
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| This results in an infinite force on the sample particles forcing them to move away from the node and often crossing the path of other sample points (which violates single-valuedness). Various schemes have been developed to overcome this; however, no general solution has yet emerged.
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| These methods, as does Bohm's Hamilton-Jacobi formulation, do not apply to situations in which the full dynamics of spin need to be taken into account.
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| ===Occam's razor criticism===
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| Both [[Hugh Everett III]] and Bohm treated the wavefunction as a [[Scientific realism|physically real]] [[Field (physics)|field]]. Everett's [[many-worlds interpretation]] is an attempt to demonstrate that the [[wavefunction]] alone is sufficient to account for all our observations. When we see the particle detectors flash or hear the click of a [[Geiger counter]] then Everett's theory interprets this as our ''wavefunction'' responding to changes in the detector's ''wavefunction'', which is responding in turn to the passage of another ''wavefunction'' (which we think of as a "particle", but is actually just another [[wave-packet]]).<ref name=BrownWallace>[[Harvey Brown (philosopher)|Harvey R Brown]] and David Wallace, ''Solving the measurement problem: de Broglie-Bohm loses out to Everett'', ''Foundations of Physics'' 35 (2005), pp. 517-540. [http://philsci-archive.pitt.edu/archive/00001659/01/Cushing.pdf] Abstract: "The quantum theory of de Broglie and Bohm solves the measurement problem, but the hypothetical corpuscles play no role in the argument. The solution finds a more natural home in the Everett interpretation."</ref> No particle (in the Bohm sense of having a defined position and velocity) exists, according to that theory. For this reason Everett sometimes referred to his own [[many worlds interpretation|many-worlds approach]] as the "pure wave theory". Talking of Bohm's 1952 approach, Everett says: {{cquote|Our main criticism of this view is on the grounds of simplicity - if one desires to hold the view that <math>\psi</math> is a real field then the associated particle is superfluous since, as we have endeavored to illustrate, the pure wave theory is itself satisfactory.<ref>See section VI of Everett's thesis:[http://www.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf ''Theory of the Universal Wavefunction''], pp 3-140 of [[Bryce Seligman DeWitt]], [[R. Neill Graham]], eds, ''The Many-Worlds Interpretation of Quantum Mechanics'', Princeton Series in Physics, [[Princeton University Press]] (1973), ISBN 0-691-08131-X</ref>}}
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| In the Everettian view, then, the Bohm particles are superfluous entities, similar to, and equally as unnecessary as, for example, the [[luminiferous ether]], which was found to be unnecessary in [[special relativity]]. This argument of Everett's is sometimes called the "redundancy argument", since the superfluous particles are redundant in the sense of [[Occam's razor]].<ref name=callender>[http://philosophy.ucsd.edu/faculty/ccallender/The%20Redundancy%20Argument%20Against%20Bohmian%20Mechanics.doc Craig Callender, "The Redundancy Argument Against Bohmian Mechanics"]</ref>
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| Many authors have expressed critical views of the de Broglie-Bohm theory, by comparing it to Everett's many worlds approach. Many (but not all) proponents of the de Broglie-Bohm theory (such as Bohm and Bell) interpret the universal wave function as physically real. According to some supporters of Everett's theory, if the (never collapsing) wave function 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 Bohm 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:{{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]] . . .<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>}}
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| [[David Deutsch]] has expressed the same point more "acerbically":<ref name=BrownWallace/>
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| {{cquote|pilot-wave theories are parallel-universe theories in a state of chronic denial.<ref>[[David Deutsch]], Comment on Lockwood. ''British Journal for the Philosophy of Science'' 47, 222228, 1996</ref>}}
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| According to [[Harvey Brown (philosopher)|Brown]] & Wallace<ref name="BrownWallace"/> the de Broglie-Bohm particles play no role in the solution of the measurement problem. These authors claim<ref name=BrownWallace/> that the "result assumption" (see above) is inconsistent with the view that there is no measurement problem in the predictable outcome (i.e. single-outcome) case. These authors also claim<ref name=BrownWallace/> that a standard tacit assumption of the de Broglie-Bohm theory (that an observer becomes aware of configurations of particles of ordinary objects by means of correlations between such configurations and the configuration of the particles in the observer's brain) is unreasonable. This conclusion has been challenged by [[Antony Valentini|Valentini]]<ref>Valentini A., [http://arxiv.org/abs/0811.0810 "De Broglie-Bohm pilot wave theory: many worlds in denial?"] 'Many Worlds? Everett, Quantum Theory, and Reality', eds. S. Saunders et al. (Oxford University Press, 2010), pp. 476--509</ref> who argues that the entirety of such objections arises from a failure to interpret de Broglie-Bohm theory on its own terms.
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| According to [[Peter R. Holland]], in a wider Hamiltonian framework, theories can be formulated in which particles ''do'' act back on the wave function.<ref>P. Holland, "Hamiltonian Theory of Wave and Particle in Quantum Mechanics I, II", Nuovo Cimento B 116, 1043, 1143 (2001) [http://users.ox.ac.uk/~gree0579/index_files/NuovoCimento2.pdf online]</ref>
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| ==Derivations==
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| De Broglie–Bohm theory has been derived many times and in many ways. Below are six derivations all of which are very different and lead to different ways of understanding and extending this theory.
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| * [[Schrödinger equation#Derivation|Schrödinger's equation]] can be derived by using [[Photoelectric effect|Einstein's light quanta hypothesis]]: <math>E = \hbar \omega \;</math> and [[Matter wave|de Broglie's hypothesis]]: <math>\mathbf{p} = \hbar \mathbf{k}\;</math>.
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| :The guiding equation can be derived in a similar fashion. We assume a plane wave: <math>\psi(\mathbf{x},t) = Ae^{i(\mathbf{k}\cdot\mathbf{x}- \omega t)}</math>. Notice that <math>i\mathbf{k}= \nabla\psi /\psi</math>. Assuming that <math>\mathbf{p} = m \mathbf{v}</math> for the particle's actual velocity, we have that <math>\mathbf{v}= \frac{\hbar}{m} Im \left(\frac{\nabla\psi}{\psi}\right) </math>. Thus, we have the guiding equation.
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| :Notice that this derivation does not use Schrödinger's equation.
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| * Preserving the density under the time evolution is another method of derivation. This is the method that Bell cites. It is this method which generalizes to many possible alternative theories. The starting point is the [[continuity equation]] <math>-\frac{\partial \rho}{\partial t} = \nabla \cdot (\rho v^{\psi})</math> for the density <math>\rho=|\psi|^2</math>. This equation describes a probability flow along a current. We take the velocity field associated with this current as the velocity field whose integral curves yield the motion of the particle.
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| * A method applicable for particles without spin is to do a polar decomposition of the wavefunction and transform Schrödinger's equation into two coupled equations: the continuity equation from above and the Hamilton–Jacobi equation. This is the method used by Bohm in 1952. The decomposition and equations are as follows:
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| :Decomposition: <math>\psi(\mathbf{x},t) = R(\mathbf{x},t)e^{i S(\mathbf{x},t) / \hbar}.</math> Note <math>R^2(\mathbf{x},t)</math> corresponds to the probability density <math>\rho (\mathbf{x},t) = |\psi (\mathbf{x},t)|^2</math>.
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| :Continuity Equation: <math>-\frac{\partial \rho(\mathbf{x},t)}{\partial t} = \nabla \cdot \left(\rho (\mathbf{x},t)\frac{\nabla S(\mathbf{x},t)}{m}\right)</math>
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| :Hamilton–Jacobi Equation: <math>\frac{\partial S(\mathbf{x},t)}{\partial t} = -\left[ V + \frac{1}{2m}(\nabla S(\mathbf{x},t))^2 -\frac{\hbar ^2}{2m} \frac{\nabla ^2R(\mathbf{x},t)}{R(\mathbf{x},t)} \right]. </math>
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| :The Hamilton–Jacobi equation is the equation derived from a Newtonian system with potential <math>V-\frac{\hbar ^2}{2m} \frac{\nabla ^2 R}{R}</math> and velocity field <math>\frac{\nabla S}{m}.</math> The potential <math>V</math> is the classical potential that appears in Schrödinger's equation and the other term involving <math>R</math> is the [[quantum potential]], terminology introduced by Bohm.
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| :This leads to viewing the quantum theory as particles moving under the classical force modified by a quantum force. However, unlike standard [[Newtonian mechanics]], the initial velocity field is already specified by <math>\frac{\nabla S}{m}</math> which is a symptom of this being a first-order theory, not a second-order theory.
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| * A fourth derivation was given by Dürr et al.<ref name="dgz92" /> In their derivation, they derive the velocity field by demanding the appropriate transformation properties given by the various symmetries that Schrödinger's equation satisfies, once the wavefunction is suitably transformed. The guiding equation is what emerges from that analysis.
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| * A fifth derivation, given by Dürr et al.<ref name="dgtz04" /> is appropriate for generalization to quantum field theory and the Dirac equation. The idea is that a velocity field can also be understood as a first order differential operator acting on functions. Thus, if we know how it acts on functions, we know what it is. Then given the Hamiltonian operator <math>H</math>, the equation to satisfy for all functions <math>f</math> (with associated multiplication operator <math>\hat{f}</math>) is
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| :<math>(v(f))(q) = \mathrm{Re} \frac{(\psi, \frac{i}{\hbar} [H,\hat f] \psi)}{(\psi,\psi)}(q)</math> where <math>(v,w)</math> is the local Hermitian inner product on the value space of the wavefunction.
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| :This formulation allows for stochastic theories such as the creation and annihilation of particles.
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| * A further derivation has been given by Peter R. Holland, on which he bases the entire work presented in his quantum physics textbook ''The Quantum Theory of Motion'', a main reference book on the de Broglie–Bohm theory. It is based on three basic postulates and an additional fourth postulate that links the wave function to measurement probabilities:<ref>Peter R. Holland: ''The quantum theory of motion'', Cambridge University Press, 1993 (re-printed 2000, transferred to digital printing 2004), ISBN 0-521-48543-6, p. 66 ff.</ref>
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| :1. A physical system consists in a spatiotemporally propagating wave and a point particle guided by it;
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| :2. The wave is described mathematically by a solution <math>\psi</math> to Schrödinger's wave equation;
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| :3. The particle motion is described by a solution to <math>\mathbf{\dot x}(t) = [\nabla S (\mathbf{x}(t),t))]/m</math> in dependence on initial condition <math>\mathbf{x}(t=0)</math>, with <math>S</math> the phase of <math>\psi</math>.
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| :The fourth postulate is subsidiary yet consistent with the first three:
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| :4. The probability <math>\rho (\mathbf{x}(t))</math> to find the particle in the differential volume <math>d^3 x</math> at time t equals <math>|\psi(\mathbf{x}(t))|^2</math>.
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| ==History==
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| De Broglie–Bohm theory has a history of different formulations and names. In this section, each stage is given a name and a main reference.
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| ===Pilot-wave theory===
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| [[Louis de Broglie|Dr. de Broglie]] presented his pilot wave theory at the 1927 Solvay Conference,<ref name="Solvay Conference 1928">Solvay Conference, 1928, Electrons et Photons: Rapports et Descussions du Cinquieme Conseil de Physique tenu a Bruxelles du 24 au 29 October 1927 sous les auspices de l'Institut International Physique Solvay</ref> after close collaboration with Schrödinger, who developed his wave equation for de Broglie's theory. At the end of the presentation, [[Wolfgang Pauli]] pointed out that it was not compatible with a semi-classical technique Fermi had previously adopted in the case of inelastic scattering. Contrary to a popular legend, de Broglie actually gave the correct rebuttal that the particular technique could not be generalized for Pauli's purpose, although the audience might have been lost in the technical details and de Broglie's mild mannerism left the impression that Pauli's objection was valid. He was eventually persuaded to abandon this theory nonetheless in 1932 due to both the Copenhagen school's more successful P.R. efforts and his own inability to understand [[quantum decoherence]]. Also in 1932, [[John von Neumann]] published a paper,<ref>von Neumann J. 1932 Mathematische Grundlagen der Quantenmechanik</ref> that was widely (and erroneously<ref>{{cite journal |title=Von Neumann's 'No Hidden Variables' Proof: A Re-Appraisal|year=2010 |last1=Bub |first1=Jeffrey |journal=Foundations of Physics |volume=40|issue=9–10|pages=1333–1340|bibcode = 2010FoPh...40.1333B |doi = 10.1007/s10701-010-9480-9 |arxiv = 1006.0499 }}</ref>) believed to prove that all hidden-variable theories are impossible. This sealed the fate of de Broglie's theory for the next two decades.
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| De Broglie's theory already applies to multiple spin-less particles, but lacks an adequate theory of measurement as no one understood [[quantum decoherence]] at the time. An analysis of de Broglie's presentation is given in Bacciagaluppi et al.<ref>Bacciagaluppi, G., and Valentini, A., Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference</ref><ref>See the brief summary by Towler, M., [http://www.tcm.phy.cam.ac.uk/~mdt26/PWT/lectures/bohm7.pdf "Pilot wave theory, Bohmian metaphysics, and the foundations of quantum mechanics"]</ref>
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| Around this time [[Erwin Madelung]] also developed a hydrodynamic version of Schrödinger's equation which is incorrectly considered as a basis for the density current derivation of the de Broglie–Bohm theory.<ref>{{cite journal |last=Madelung |first=E. |title=Quantentheorie in hydrodynamischer Form |journal=[[Zeitschrift für Physik|Zeit. f. Phys.]] |volume=40 |year=1927 |issue=3–4 |pages=322–326 |doi=10.1007/BF01400372 |bibcode = 1927ZPhy...40..322M }}</ref> The [[Madelung equations]], being quantum [[Euler equations (fluid dynamics)]], differ philosophically from the de Broglie–Bohm mechanics<ref>Tsekov, R. (2009) [http://arxiv.org/abs/0904.0723 Bohmian Mechanics versus Madelung Quantum Hydrodynamics] arXiv 0904.0723</ref> and are the basis of the [[hydrodynamic interpretation of quantum mechanics]].
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| [[Peter R. Holland]] has pointed out that, earlier in 1927, [[Albert Einstein|Einstein]] had actually submitted a preprint with a similar proposal but, not convinced, had withdrawn it before publication.<ref>Peter Holland: ''What's wrong with Einstein's 1927 hidden-variable interpretation of quantum mechanics?'', Foundations of Physics (2004), vol. 35, no. 2, p. 177–196, {{doi|10.1007/s10701-004-1940-7}}, arXiv: quant-ph/0401017, [http://arxiv.org/ftp/quant-ph/papers/0401/0401017.pdf#page=1 p. 1]</ref> According to Holland, failure to appreciate key points of the de Broglie–Bohm theory has led to confusion, the key point being "that the trajectories of a many-body quantum system are correlated not because the particles exert a direct force on one another (''à la'' Coulomb) but because all are acted upon by an entity – mathematically described by the wavefunction or functions of it – that lies beyond them."<ref>Peter Holland: ''What's wrong with Einstein's 1927 hidden-variable interpretation of quantum mechanics?'', Foundations of Physics (2004), vol. 35, no. 2, p. 177–196, {{doi|10.1007/s10701-004-1940-7}}, arXiv: quant-ph/0401017, [http://arxiv.org/ftp/quant-ph/papers/0401/0401017.pdf#page=14 p. 14]</ref> This entity is the [[quantum potential]].
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| After publishing a popular textbook on Quantum Mechanics which adhered entirely to the Copenhagen orthodoxy, Bohm was persuaded by Einstein to take a critical look at von Neumann's theorem. The result was 'A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables" I and II' [Bohm 1952]. It was an independent origination of the pilot wave theory, and extended it to incorporate a consistent theory of measurement, and to address a criticism of Pauli that de Broglie did not properly respond to; it is taken to be deterministic (though Bohm hinted in the original papers that there should be disturbances to this, in the way [[Brownian motion]] disturbs Newtonian mechanics). This stage is known as the ''de Broglie–Bohm Theory'' in Bell's work [Bell 1987] and is the basis for 'The Quantum Theory of Motion' [Holland 1993].
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| This stage applies to multiple particles, and is deterministic.
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| The de Broglie–Bohm theory is an example of a [[hidden variables theory]]. Bohm originally hoped that hidden variables could provide a [[principle of locality|local]], [[causal]], [[objectivity (philosophy)|objective]] description that would resolve or eliminate many of the paradoxes of quantum mechanics, such as [[Schrödinger's cat]], the [[measurement problem]] and the collapse of the wavefunction. However, [[Bell's theorem]] complicates this hope, as it demonstrates that there can be no local hidden variable theory that is compatible with the predictions of quantum mechanics. The Bohmian interpretation is [[causal]] but not [[principle of locality|local]].
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| Bohm's paper was largely ignored or panned by other physicists. [[Albert Einstein]] did not consider it a satisfactory answer to the quantum nonlocality question, calling it "too cheap".,<ref>(Letter of 12 May 1952 from Einstein to Max Born, in ''The Born–Einstein Letters'', Macmillan, 1971, p. 192.</ref> and [[Werner Heisenberg]] considered it a "superfluous 'ideological superstructure' ".<ref>Werner Heisenberg, ''Physics and Philosophy'' (1958), p. 133.</ref> [[Wolfgang Pauli]], who had been unconvinced by de Broglie in 1927, conceded to Bohm as follows:
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| <blockquote>I just received your long letter of 20th November, and I also have studied more thoroughly the details of your paper. I do not see any longer the possibility of any logical contradiction as long as your results agree completely with those of the usual wave mechanics and as long as no means is given to measure the values of your hidden parameters both in the measuring apparatus and in the observe [sic] system. As far as the whole matter stands now, your ‘extra wave-mechanical predictions’ are still a check, which cannot be cashed.<ref>Pauli to Bohm, 3 December 1951, in Wolfgang Pauli, ''Scientific Correspondence'', Vol IV – Part I, [ed. by Karl von Meyenn], (Berlin, 1996), pp. 436-441.</ref></blockquote>
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| When Bohm's theory was presented at the Institute for Advanced Study in Princeton, many of the objections were [[ad hominem]], focusing on Bohm's sympathy with communists as exemplified by his refusal to give testimony to the House Un-American Activities Committee.{{Citation needed|date=July 2013}}
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| Eventually [[John Stewart Bell|John Bell]] began to defend the theory. In "Speakable and Unspeakable in Quantum Mechanics" [Bell 1987], several of the papers refer to hidden variables theories (which include Bohm's).
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| ===Bohmian mechanics===
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| This term is used to describe the same theory, but with an emphasis on the notion of current flow, which is determined on the basis of the [[quantum equilibrium hypothesis]] that the probability follows the [[Born rule]]. The term "Bohmian mechanics" is also often used to include most of the further extensions past the spin-less version of Bohm. While de Broglie–Bohm theory has Lagrangians and Hamilton-Jacobi equations as a primary focus and backdrop, with the icon of the quantum potential, Bohmian mechanics considers the continuity equation as primary and has the guiding equation as its icon. They are mathematically equivalent in so far as the Hamilton-Jacobi formulation applies, i.e., spin-less particles. The papers of Dürr et al. popularized the term.
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| All of non-relativistic quantum mechanics can be fully accounted for in this theory.
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| ===Causal interpretation and ontological interpretation===
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| Bohm developed his original ideas, calling them the ''Causal Interpretation''. Later he felt that ''causal'' sounded too much like ''deterministic'' and preferred to call his theory the ''Ontological Interpretation''. The main reference is 'The Undivided Universe' [Bohm, Hiley 1993].
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| This stage covers work by Bohm and in collaboration with [[Jean-Pierre Vigier]] and [[Basil Hiley]]. Bohm is clear that this theory is non-deterministic (the work with Hiley includes a stochastic theory). As such, this theory is not, strictly speaking, a formulation of the de Broglie–Bohm theory. However, it deserves mention here because the term "Bohm Interpretation" is ambiguous between this theory and the de Broglie–Bohm theory.
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| An in-depth analysis of possible interpretations of Bohm's model of 1952 was given in 1996 by [[philosopher of science]] [[Arthur Fine]].<ref>A. Fine: ''On the interpretation of Bohmian mechanics'', in: J. T. Cushing, A. Fine, S. Goldstein (Eds.): ''Bohmian mechanics and quantum theory: an appraisal'', Springer, 1996, pp. 231−250</ref>
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| ==See also==
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| *[[David Bohm]]
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| *[[Faraday wave]]
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| *[[Interpretation of quantum mechanics]]
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| *[[Madelung equations]]
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| *[[Local hidden variable theory]]
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| *[[Quantum mechanics]]
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| *[[Pilot wave]]
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| ==Notes==
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| {{Reflist|30em}}
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| ==References==
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| {{Refbegin}}
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| * {{Cite journal| doi = 10.1038/scientificamerican0594-58| first = David Z. | last = Albert | title = Bohm's Alternative to Quantum Mechanics | journal = Scientific American |date=May 1994 | volume= 270 | pages = 58–67 | issue=5}}
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| * {{Cite journal| first = G. D. | last = Barbosa | coauthors = N. Pinto-Neto | title = A Bohmian Interpretation for Noncommutative Scalar Field Theory and Quantum Mechanics | journal = Physical Review D | volume = 69 | year = 2004 | pages = 065014 | id = | doi = 10.1103/PhysRevD.69.065014 | arxiv=hep-th/0304105|bibcode = 2004PhRvD..69f5014B }}
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| * {{Cite journal| first = David | last = Bohm | title = A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables" I | journal = Physical Review | year = 1952 | volume = 85 | pages = 166–179 | doi = 10.1103/PhysRev.85.166|bibcode = 1952PhRv...85..166B }} ([https://www.nd.edu/~dhoward1/Bohm%20HV-I%20Phys%20Rev%201952.pdf full text])
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| * {{Cite journal| first = David | last = Bohm | title = A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables", II | journal = Physical Review | year = 1952 | volume = 85 | pages = 180–193 | doi = 10.1103/PhysRev.85.180|bibcode = 1952PhRv...85..180B }} ([https://www.nd.edu/~dhoward1/Bohm%20HV-II%20Phys%20Rev%201952.pdf full text])
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| * {{Cite journal| first = David | last = Bohm | title = A new theory of the relationship of mind and matter | url = http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/bohm_mind_matter_1990.pdf | year = 1990 | journal = Philosophical Psychology | volume = 3 | issue = 2 | pages = 271–286 | doi = 10.1080/09515089008573004}}
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| * {{Cite book| first = David | last = Bohm | coauthors = B.J. Hiley | title = The Undivided Universe: An ontological interpretation of quantum theory | location = London | publisher = Routledge | year =1993 | isbn = 0-415-12185-X}}
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| * {{Cite journal| first = Detlef | last = Dürr | coauthors = Sheldon Goldstein, Roderich Tumulka and Nino Zanghì | url = http://www.math.rutgers.edu/~oldstein/papers/bohmech.pdf |format=PDF| title = Bohmian Mechanics |date=December 2004 | volume = 93 | issue = 9 | pages = 090402 | issn = 0031-9007 | pmid = 15447078 | journal = Physical review letters | doi=10.1103/PhysRevLett.93.090402|arxiv = quant-ph/0303156 |bibcode = 2004PhRvL..93i0402D }}
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| * {{Cite journal| first = Sheldon | last = Goldstein | url = http://plato.stanford.edu/entries/qm-bohm/ | title = Bohmian Mechanics | year = 2001 | journal = [[Stanford Encyclopedia of Philosophy]]}}
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| * {{Cite journal| first = Michael J.W. | last = Hall | title = Incompleteness of trajectory-based interpretations of quantum mechanics | year = 2004 | id = | doi = 10.1088/0305-4470/37/40/015 | journal = Journal of Physics a Mathematical and General | volume = 37 | pages = 9549 | arxiv=quant-ph/0406054|bibcode = 2004JPhA...37.9549H }} (Demonstrates incompleteness of the Bohm interpretation in the face of fractal, differentialble-nowhere wavefunctions.)
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| * {{Cite book| first = Peter R. | last = Holland | title = The Quantum Theory of Motion : An Account of the de Broglie–Bohm Causal Interpretation of Quantum Mechanics | location = Cambridge | publisher = Cambridge University Press | year = 1993 | isbn = 0-521-48543-6}}
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| * {{Cite journal| first = H. | last = Nikolic | title = Relativistic quantum mechanics and the Bohmian interpretation | year = 2004 | id = | doi = 10.1007/s10702-005-1128-1 | journal = Foundations of Physics Letters | volume = 18 | pages = 549 | arxiv=quant-ph/0406173|bibcode = 2005FoPhL..18..549N }}
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| * {{Cite journal| first = Oliver | last = Passon | title = Why isn't every physicist a Bohmian? | year =2004 | id = | arxiv=quant-ph/0412119|bibcode = 2004quant.ph.12119P }}
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| * {{Cite journal| first = A. S. | last = Sanz | coauthors = F. Borondo | title = A Bohmian view on quantum decoherence | year = 2003 | id = | doi = 10.1140/epjd/e2007-00191-8 | journal = European Physical Journal D | volume = 44 | pages = 319 | arxiv=quant-ph/0310096|bibcode = 2007EPJD...44..319S }}
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| * {{Cite journal| first = A.S. | last = Sanz | doi = 10.1088/0305-4470/38/26/013 | title = A Bohmian approach to quantum fractals | year = 2005 | journal = J. Phys. A: Math. Gen. | volume = 38 | pages = 319|arxiv = quant-ph/0412050 |bibcode = 2005JPhA...38.6037S }} (Describes a Bohmian resolution to the dilemma posed by non-differentiable wavefunctions.)
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| * {{Cite book| first = Mark P. | last = Silverman | title = And Yet It Moves: Strange Systems and Subtle Questions in Physics | location = Cambridge | publisher = Cambridge University Press | year = 1993 | isbn = 0-521-44631-7}}
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| * {{Cite web| first = Ray F. | last = Streater | authorlink = Ray Streater | url = http://www.mth.kcl.ac.uk/~streater/lostcauses.html#XI | title = Bohmian mechanics is a "lost cause" | year = 2003 | accessdate = 2006-06-25}}
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| * {{Cite journal| first = Antony | last = Valentini | authorlink = Antony Valentini | coauthors = Hans Westman | title = Dynamical Origin of Quantum Probabilities | year = 2004 | id = | arxiv=quant-ph/0403034|bibcode = 2005RSPSA.461..253V |doi = 10.1098/rspa.2004.1394 }}
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| *[http://xstructure.inr.ac.ru/x-bin/theme3.py?level=1&index1=-139823 Bohmian mechanics on arxiv.org]
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| {{Refend}}
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| ==Further reading==
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| * [[John S. Bell]]: ''Speakable and Unspeakable in Quantum Mechanics: Collected Papers on Quantum Philosophy'', Cambridge University Press, 2004, ISBN 0-521-81862-1
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| * [[David Bohm]], [[Basil Hiley]]: ''The Undivided Universe: An Ontological Interpretation of Quantum Theory'', Routledge Chapman & Hall, 1993, ISBN 0-415-06588-7
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| * Detlef Dürr, Sheldon Goldstein, Nino Zanghì: ''Quantum Physics Without Quantum Philosophy'', Springer, 2012, ISBN 978-3-642-30690-7
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| * Detlef Dürr, Stefan Teufel: ''Bohmian Mechanics: The Physics and Mathematics of Quantum Theory'', Springer, 2009, ISBN 978-3-540-89343-1
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| * [[Peter R. Holland]]: ''The quantum theory of motion'', Cambridge University Press, 1993 (re-printed 2000, transferred to digital printing 2004), ISBN 0-521-48543-6
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| ==External links==
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| {{Wikiversity|Making sense of quantum mechanics}}
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| *[http://plato.stanford.edu/entries/qm-bohm "Bohmian Mechanics" (Stanford Encyclopedia of Philosophy)]
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| *[http://www.youtube.com/playlist?list=PL7LbfRoKBR5OpRjt8toBOmzqGjH7zaM1m Videos answering frequently asked questions about Bohmian Mechanics]
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| *[http://www.bohmian-mechanics.net "Bohmian-Mechanics.net"], the homepage of the international research network on Bohmian Mechanics that was started by D. Dürr, S. Goldstein and N. Zanghì.
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| * [http://www.mathematik.uni-muenchen.de/~bohmmech/ Workgroup Bohmian Mechanics at LMU Munich (D. Dürr)]
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| * [http://bohm-mechanics.uibk.ac.at/ Bohmian Mechanics Group at University of Innsbruck (G. Grübl)]
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| *[http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html "Pilot waves, Bohmian metaphysics, and the foundations of quantum mechanics"], lecture course on de Broglie-Bohm theory by [[Mike Towler]], Cambridge University.
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| *[http://www.vallico.net/tti/deBB_10/conference.html "21st-century directions in de Broglie-Bohm theory and beyond"], August 2010 international conference on de Broglie-Bohm theory. Site contains slides for all the talks - the latest cutting-edge deBB research.
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| *[http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf "Observing the Trajectories of a Single Photon Using Weak Measurement"]
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| *[http://www.physicsforums.com/blog.php?b=3077 "Bohmian trajectories are no longer 'hidden variables'"]
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| {{DEFAULTSORT:Bohm Interpretation}}
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| [[Category:Interpretations of quantum mechanics]]
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| [[Category:Quantum measurement]]
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