Quantum potential

The quantum potential or quantum potentiality is a central concept of the de Broglie–Bohm formulation of quantum mechanics, introduced by David Bohm in 1952.

Initially presented under the name quantum-mechanical potential, subsequently quantum potential, it was later elaborated upon by Bohm and Basil Hiley in its interpretation as an information potential which acts on a quantum particle. It is also referred to as quantum potential energy, Bohm potential, quantum Bohm potential or Bohm quantum potential.

Quantum potential

<math> \quad Q = - \frac{\hbar^2}{2m} \frac{\nabla^2 R}{R}</math>

In the framework of the de Broglie–Bohm theory, the quantum potential is a term within the Schrödinger equation which acts to guide the movement of quantum particles. The quantum potential approach introduced by Bohm<ref name="bohm-1952-I">Bohm, David (1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables" I". Physical Review. 85 (2): 166–179. Bibcode:1952PhRv...85..166B. doi:10.1103/PhysRev.85.166. (full text Archived 2012-10-18 at the Wayback Machine)</ref><ref name="bohm-1952-II">Bohm, David (1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables", II". Physical Review. 85 (2): 180–193. Bibcode:1952PhRv...85..180B. doi:10.1103/PhysRev.85.180. (full text Archived 2012-10-18 at the Wayback Machine)</ref> provides a physically less fundamental exposition of the idea presented by Louis de Broglie: de Broglie had postulated in 1925 that the relativistic wave function defined on spacetime represents a pilot wave which guides a quantum particle, represented as an oscillating peak in the wave field, but he had subsequently abandoned his approach because he was unable to derive the guidance equation for the particle from a non-linear wave equation. The seminal articles of Bohm in 1952 introduced the quantum potential and included answers to the objections which had been raised against the pilot wave theory.

The Bohm quantum potential is closely linked with the results of other approaches, in particular relating to work by Erwin Madelung of 1927 and to work by Carl Friedrich von Weizsäcker of 1935.

Building on the interpretation of the quantum theory introduced by Bohm in 1952, David Bohm and Basil Hiley in 1975 presented how the concept of a quantum potential leads to the notion of an "unbroken wholeness of the entire universe", proposing that the fundamental new quality introduced by quantum physics is nonlocality.<ref>D. Bohm, B. J. Hiley: On the intuitive understanding of nonlocality as implied by quantum theory, Foundations of Physics, Volume 5, Number 1, pp. 93-109, 1975, doi:10.1007/BF01100319 (abstract)</ref>

Quantum potential as part of the Schrödinger equation

The Schrödinger equation

<math>

i \hbar \frac{\partial \psi}{\partial t} = \left( - \frac{\hbar^2}{2m} \nabla^2 +V \right)\psi \quad </math> is re-written using the polar form for the wave function <math>\psi = R \exp(i S / \hbar)</math> with real-valued functions <math>R</math> and <math>S</math>, where <math>R</math> is the amplitude (absolute value) of the wave function <math>\psi</math>, and <math>S/\hbar</math> its phase. This yields two equations: from the imaginary and real part of the Schrödinger equation follow the continuity equation and the quantum Hamilton–Jacobi equation respectively.<ref name="bohm-1952-I"/><ref>David Bohm, Basil Hiley: The Undivided Universe: An Ontological Interpretation of Quantum Theory, Routledge, 1993, ISBN 0-415-06588-7, therein Chapter 3.1. The main points of the causal interpretation, p. 22–23.</ref>

Continuity equation

The imaginary part of the Schrödinger equation in polar form yields

<math>

\frac{\partial R}{\partial t} = -\frac{1}{2m} \left[ R \nabla^2 S + 2 \nabla R \cdot \nabla S \right], </math> which, provided <math>\rho = R^2</math>, can be interpreted as the continuity equation <math> \partial \rho / \partial t + \nabla \cdot( \rho v) =0</math> for the probability density <math>\rho</math> and the velocity field <math> v = \frac{1}{m}\nabla S </math>

Quantum Hamilton–Jacobi equation

The real part of the Schrödinger equation in polar form yields a modified Hamilton–Jacobi equation

<math>

\frac{\partial S}{\partial t} = - \left[ \frac{\left|\nabla S\right|^2}{2m} + V + Q \right], </math> also referred to as quantum Hamilton–Jacobi equation.<ref>David Bohm, Basil Hiley: The Undivided Universe: An Ontological Interpretation of Quantum Theory, Routledge, 1993, ISBN 0-415-06588-7, also as cited in: B. J. Hiley and R. E. Callaghan: Clifford Algebras and the Dirac-Bohm Quantum Hamilton-Jacobi Equation, Foundations of Physics, January 2012, Volume 42, Issue 1, pp 192-208 (published online 20 May 2011), doi:10.1007/s10701-011-9558-z (abstract, 2010 preprint by B. Hiley)</ref> It differs from the classical Hamilton–Jacobi equation only by the term

<math>Q = - \frac{\hbar^2}{2m} \frac{\nabla^2 R}{R}.</math>

This term <math>Q</math>, called quantum potential, thus depends on the curvature of the amplitude of the wave function.<ref>See for ex. Robert E. Wyatt, Eric R. Bittner: Quantum wave packet dynamics with trajectories: Implementation with adaptive Lagrangian grids of the amplitude of the wave function, Journal of Chemical Physics, vol. 113, no. 20, 22 November 2000, p. 8898 Archived 2011-10-02 at the Wayback Machine</ref><ref>See also: Pilot wave#Mathematical formulation for a single particle</ref>

In the limit <math>\hbar \to 0</math>, the function <math>S</math> is a solution of the (classical) Hamilton–Jacobi equation;<ref name="bohm-1952-I"/> therefore, the function <math>S</math> is also called the Hamilton–Jacobi function, or action, extended to quantum physics.

Properties

Bohm trajectories under the influence of the quantum potential, at the example of an electron going through the two-slit experiment.

Hiley emphasised several aspects<ref name="teleportation-P7">B. J. Hiley: Active Information and Teleportation, p. 7; appeared in: Epistemological and Experimental Perspectives on Quantum Physics, D. Greenberger et al. (eds.), pages 113-126, Kluwer, Netherlands, 1999</ref> that regard the quantum potential of a quantum particle:

• it is derived mathematically from the real part of the Schrödinger equation under polar decomposition of the wave function,<ref>B.J. Hiley: From the Heisenberg picture to Bohm: A New Perspective on Active Information and it Relation to Shannon Information, pp. 2 and 5. Published in: A. Khrennikov (ed.): Proc. Conf. Quantum Theory: reconsideration of foundations, pp. 141–162, Vaxjö University Press, Sweden, 2002</ref> is not derived from a Hamiltonian<ref name="information-quantum-theory-and-the-brain-P207"/> or other external source, and could be said to be involved in a self-organising process involving a basic underlying field;
• it does not change if <math>R</math> is multiplied by a constant, as this term is also present in the denominator, so that <math>Q</math> is independent of the magnitude of <math>\psi</math> and thus of field intensity; therefore, the quantum potential fulfils a precondition for nonlocality: it need not fall off as distance increases;
• it carries information about the whole experimental arrangement in which the particle finds itself.

In 1979, Hiley and his co-workers Philippidis and Dewdney presented a full calculation on the explanation of the two-slit experiment in terms of Bohmian trajectories that arise for each particle moving under the influence of the quantum potential, resulting in the well-known interference patterns.<ref>C. Philippidis, C. Dewdney, B. J. Hiley: Quantum interference and the quantum potential, Il nuovo cimento B, vol. 52, no. 1, 1979, pp.15-28, doi:10.1007/BF02743566</ref>

Schematic of double-slit experiment in which Aharonov–Bohm effect can be observed: electrons pass through two slits, interfering at an observation screen, and the interference pattern undergoes a shift when a magnetic field B is turned on in the cylindrical solenoid.

Also the shift of the interference pattern which occurs in presence of a magnetic field in the Aharonov–Bohm effect could be explained as arising from the quantum potential.<ref>C. Philippidis, D. Bohm, R. D. Kaye: The Aharonov-Bohm effect and the quantum potential, Il nuovo cimento B, vol. 71, no. 1, pp. 75-88, 1982, doi:10.1007/BF02721695</ref>

Relation to the measurement process

The collapse of the wave function of the Copenhagen interpretation of quantum theory is explained in the quantum potential approach by the demonstration that, after a measurement, "all the packets of the multi-dimensional wave function that do not correspond to the actual result of measurement have no effect on the particle" from then on.<ref>Basil J. Hiley: The role of the quantum potential. In: G. Tarozzi, Alwyn Van der Merwe: Open questions in quantum physics: invited papers on the foundations of microphysics, Springer, 1985, pages 237 ff., therein page 239</ref> Bohm and Hiley pointed out that

‘the quantum potential can develop unstable bifurcation points, which separate classes of particle trajectories according to the "channels" into which they eventually enter and within which they stay. This explains how measurement is possible without "collapse" of the wave function, and how all sorts of quantum processes, such as transitions between states, fusion of two states into one and fission of one system into two, are able to take place without the need for a human observer.’<ref>D. Bohm, B. J. Hiley, P. N. Kaloyerou: An ontological basis for the quantum theory, Physics Reports (Review section of Physics Letters), volume 144, number 6, pp. 323–348, 1987 (abstract)</ref>

Measurement then "involves a participatory transformation in which both the system under observation and the observing apparatus undergo a mutual participation so that the trajectories behave in a correlated manner, becoming correlated and separated into different, non-overlapping sets (which we call ‘channels’)".<ref>B. J. Hiley: The conceptual structure of the Bohm interpretation of quantum mechanics, In: K. V. Laurikainen, C. Montonen, K. Sunnarborg (eds.): Symposium on the Foundations of Modern Physics 1994 – 70 years of Matter Waves, Editions Frontières, pp. 99–118, ISBN 2-86332-169-2, p. 106</ref>

Quantum potential of an n-particle system

The Schrödinger wave function of a many-particle quantum system cannot be represented in ordinary three-dimensional space. Rather, it is represented in configuration space, with three dimensions per particle. A single point in configuration space thus represents the configuration of the entire n-particle system as a whole.

A two-particle wave function <math>\psi(\mathbf{r_1},\mathbf{r_2},\,t)</math> of identical particles of mass <math>m</math> has the quantum potential<ref name="teleportation-P10">B. J. Hiley: Active Information and Teleportation, p. 10; appeared in: Epistemological and Experimental Perspectives on Quantum Physics, D. Greenberger et al. (eds.), pages 113-126, Kluwer, Netherlands, 1999</ref>

<math> Q(\mathbf{r_1},\mathbf{r_2},\,t) = - \frac{\hbar^2}{2m} \frac{(\nabla_1^2 + \nabla_2^2) R(\mathbf{r_1},\mathbf{r_2},\,t)}{R(\mathbf{r_1},\mathbf{r_2},\,t)} </math>

where <math>\nabla_1^2</math> and <math>\nabla_2^2</math> refer to particle 1 and particle 2 respectively. This expression generalizes in straightforward manner to <math>n</math> particles:

<math>

Q(\mathbf{r_1},...,\mathbf{r_n},\,t) = -\frac{\hbar^2}{2 R(\mathbf{r_1},...,\mathbf{r_n},\,t) } \sum_{i=1}^{n} \frac{\nabla_i^2}{m_i} R(\mathbf{r_1},...,\mathbf{r_n},\,t) </math>

In case the wave function of two or more particles is separable, then the system's total quantum potential becomes the sum of the quantum potentials of the two particles. Exact separability is extremely unphysical given that interactions between the system and its environment destroy the factorization; however, a wave function that is a superposition of several wave functions of approximately disjoint support will factorize approximately.<ref>See for instance Detlef Dürr et al: Quantum equilibrium and the origin of absolute uncertainty, arXiv:quant-ph/0308039v1 6 August 2003, p. 23 ff.</ref>

Derivation for a separable quantum system

That the wave function is separable means that <math>\psi</math> factorizes in the form <math>\psi(\mathbf{r_1},\mathbf{r_2},\,t) = \psi_A(\mathbf{r_1},\,t) \psi_B(\mathbf{r_2},\,t) </math>. Then it follows that also <math>R</math> factorizes, and the system's total quantum potential becomes the sum of the quantum potentials of the two particles.<ref>David Bohm, Basil Hiley: The Undivided Universe: An Ontological Interpretation of Quantum Theory, Routledge, 1993, ISBN 0-415-06588-7, transferred to digital printing 2005, therein Chapter 4.1. The ontological interpretation of the many-body system, p. 59</ref>

<math>

Q(\mathbf{r_1},\mathbf{r_2},\,t) = - \frac{\hbar^2}{2m} (\frac{\nabla_1^2 R_A(\mathbf{r_1},\,t)}{R_A(\mathbf{r_1},\,t)} + \frac{\nabla_2^2 R_B(\mathbf{r_2},\,t)}{R_B(\mathbf{r_2},\,t)}) = Q_A(\mathbf{r_1},\,t) + Q_B(\mathbf{r_2},\,t) </math> In case the wave function is separable, that is, if <math>\psi</math> factorizes in the form <math>\psi(\mathbf{r_1},\mathbf{r_2},\,t) = \psi_A(\mathbf{r_1},\,t) \psi_B(\mathbf{r_2},\,t) </math>, the two one-particle systems behave independently. More generally, the quantum potential of an <math>n</math>-particle system with separable wave function is the sum of <math>n</math> quantum potentials, separating the system into <math>n</math> independent one-particle systems.<ref>D. Bohm, B. J. Hiley, P. N. Kaloyerou: An ontological basis for the quantum theory, Physics Reports (Review section of Physics Letters), volume 144, number 6, pp. 323–348, 1987 (p. 351, eq. (12)<--page=31 p. 351 is not(!) a typo--></ref>

Formulation in terms of probability density

Quantum potential in terms of the probability density function

Bohm, as well as other physicists after him, have sought to provide evidence that the Born rule linking <math>R</math> to the probability density function

<math>\rho = R^2 \quad</math>

can be understood, in a pilot wave formulation, as not representing a basic law, but rather a theorem (called quantum equilibrium hypothesis) which applies when a quantum equilibrium is reached during the course of the time development under the Schrödinger equation. With Born's rule, and straightforward application of the chain and product rules

<math>\nabla^2 \sqrt \rho = \nabla \nabla \rho^{1/2} = \nabla \left(\frac{1}{2} \rho^{-1/2} \nabla \rho\right) = \frac{1}{2} \left[ \left(\nabla \rho^{-1/2}\right) \nabla \rho + \rho^{-1/2} \nabla^2 \rho \right]</math>

the quantum potential, expressed in terms of the probability density function, becomes:<ref>See for example the Introduction section of: Fernando Ogiba: Phenomenological derivation of the Schrödinger equation Archived 2011-10-11 at the Wayback Machine, Progress in Physics (indicated date: October 2011, but retrieved online earlier: July 31, 2011)</ref>

<math> Q = - \frac{\hbar^2}{2m} \frac{\nabla^2 \sqrt{\rho}}{\sqrt{\rho}} = - \frac{\hbar^2}{4m} \left[ \frac{\nabla^2 \rho}{\rho} - \frac{1}{2} \frac{(\nabla \rho)^2}{\rho^2} \right]</math>

Quantum force

The quantum force <math>F_Q = - \nabla Q</math>, expressed in terms of the probability distribution, amounts to:<ref name="maddox-bittner-2003">Jeremy B. Maddox, Eric R. Bittner: Estimating Bohm’s quantum force using Bayesian statistics Archived 2011-11-20 at the Wayback Machine, Journal of Chemical Physics, October 2003, vol. 119, no. 13, p. 6465–6474, therein p. 6472, eq.(38)</ref>

<math>F_Q = \frac{\hbar^2}{4m} \left[ \frac{\nabla (\nabla^2\rho)}{\rho} - \frac{ \nabla (\nabla \rho \cdot \nabla \rho) }{ 2\rho^2 } - \left( \frac{\nabla^2 \rho}{\rho} - \frac{ \nabla \rho \cdot \nabla \rho }{ \rho^2 } \right) \frac{\nabla\rho}{\rho} \right]</math>

Formulation in configuration space and in momentum space, as the result of projections

M. R. Brown and B. Hiley showed that, as alternative to its formulation terms of configuration space (<math>x</math>-space), the quantum potential can also be formulated in terms of momentum space (<math>p</math>-space).<ref>M. R. Brown: The quantum potential: the breakdown of classical symplectic symmetry and the energy of localisation and dispersion, arXiv.org (submitted on 6 Mar 1997, version of 5 Feb 2002, retrieved 24 July 2011) (abstract)</ref><ref name="brown-hiley">M. R. Brown, B. J. Hiley: Schrodinger revisited: an algebraic approach, arXiv.org (submitted 4 May 2000, version of 19 July 2004, retrieved June 3, 2011) (abstract)</ref>

In line with David Bohm's approach, Basil Hiley and mathematician Maurice de Gosson showed that the quantum potential can be seen as a consequence of a projection of an underlying structure, more specifically of a non-commutative algebraic structure, onto a subspace such as ordinary space (<math>x</math>-space). In algebraic terms, the quantum potential can be seen as arising from the relation between implicate and explicate orders: if a non-commutative algebra is employed to describe the non-commutative structure of the quantum formalism, it turns out that it is impossible to define an underlying space, but that rather "shadow spaces" (homomorphic spaces) can be constructed and that in so doing the quantum potential appears.<ref name="brown-hiley"/><ref>Maurice A. de Gosson: "The Principles of Newtonian and Quantum Mechanics – The Need for Planck's Constant, h", Imperial College Press, World Scientific Publishing, 2001, ISBN 1-86094-274-1</ref><ref name="hiley-reappraisal-bohm-2005">B. J. Hiley: Non-commutative quantum geometry: A reappraisal of the Bohm approach to quantum theory, in: A. Elitzur et al. (eds.): Quo vadis quantum mechanics, Springer, 2005, ISBN 3-540-22188-3, p. 299–324</ref><ref name="non-commutative-2005">B.J. Hiley: Non-Commutative Quantum Geometry: A Reappraisal of the Bohm Approach to Quantum Theory. In: Avshalom C. Elitzur, Shahar Dolev, Nancy Kolenda (eds.): Quo Vadis Quantum Mechanics? The Frontiers Collection, 2005, pp. 299-324, doi:10.1007/3-540-26669-0_16 (abstract, preprint)</ref><ref>B.J. Hiley: Phase space description of quantum mechanics and non-commutative geometry: Wigner–Moyal and Bohm in a wider context, In: Theo M. Nieuwenhuizen et al (eds.): Beyond the quantum, World Scientific Publishing, 2007, ISBN 978-981-277-117-9, pp. 203–211, therein p. 204</ref> The quantum potential approach can be seen as a way to construct the shadow spaces.<ref name="hiley-reappraisal-bohm-2005"/> The quantum potential thus results as a distortion due to the projection of the underlying space into <math>x</math>-space, in similar manner as a Mercator projection inevitably results in a distortion in a geographical map.<ref name="hiley-anpa-23-2001">Basil J. Hiley: Towards a Dynamics of Moments: The Role of Algebraic Deformation and Inequivalent Vacuum States, published in: Correlations ed. K. G. Bowden, Proc. ANPA 23, 104-134, 2001 (PDF)</ref><ref name="hiley-callaghan-2010-A">B. J. Hiley, R. E. Callaghan: The Clifford Algebra approach to Quantum Mechanics A: The Schroedinger and Pauli Particles, arXiv.org (submitted on 17 Nov 2010 - abstract)</ref> There exists complete symmetry between the <math>x</math>-representation, and the quantum potential as it appears in configuration space can be seen as arising from the dispersion of the momentum <math>p</math>-representation.<ref name="hiley-phase">B. Hiley: Phase space description of quantum mechanics and non-commutative geometry: Wigner-Moyal and Bohm in a wider context, in: Th. M. Nieuwenhuizen et al. (eds.): Beyond the Quantum, World Scientific, 2007, ISBN 978-981-277-117-9, p. 203–211, therein: p. 207 ff.</ref>

The approach has been applied to extended phase space,<ref name="hiley-phase"/><ref>S. Nasiri: Quantum potential and symmetries in extended phase space, SIGMA 2 (2006), 062, quant-ph/0511125</ref> also in terms of a Duffin–Kemmer–Petiau algebra approach.<ref>Marco Cezar B. Fernandes, J. David M. Vianna: On the Generalized Phase Space Approach to Duffin–Kemmer–Petiau Particles, Brazilian Journal of Physics, vol. 28, no. 4. December 1998, doi:10.1590/S0103-97331998000400024</ref><ref>M.C.B. Fernandes, J.D.M. Vianna: On the Duffin-Kemmer-Petiau algebra and the generalized phase space, Foundations of Physics, vol. 29, no. 2, 1999 (abstract)</ref>

Relation to other quantities and theories

Relation to the Fisher information

It can be shown<ref>M. Reginatto, Phys. Rev. A 58, 1775 (1998), cited after: Roumen Tsekov: Towards nonlinear quantum Fokker‐Planck equations, Int. J. Theor. Phys. 48 (2009) 1431–1435 (arXiv 0808.0326, p. 4).</ref> that the mean value of the quantum potential <math>Q = - \hbar^2 \nabla^2 \sqrt{\rho} / (2m \sqrt{\rho})</math> is proportional to the probability density's Fisher information about the observable <math>\hat{x}</math>

<math> \mathcal{I} = \int \rho \cdot (\nabla \ln \rho)^2 \, d^3x = - \int \rho \nabla^2 (\ln \rho) \, d^3x.</math>

Using this definition for the Fisher information, we can write:<ref>Robert Carroll: On the Emergence Theme of Physics, World Scientific, 2010, ISBN 981-4291-79-X, Chapter 1 Some quantum background, p. 1.</ref>

<math> \langle Q \rangle = \int \psi^* Q \psi \, d^3x = \int \rho Q \, d^3x = \frac{\hbar^2}{8m} \mathcal{I}.</math>

Relation to the Madelung pressure tensor

In the Madelung equations presented by Erwin Madelung in 1927, the non-local quantum pressure tensor has the same mathematical form as the quantum potential. The underlying theory is different in that the Bohm approach describes particle trajectories whereas the equations of Madelung quantum hydrodynamics are the Euler equations of a fluid that describe its averaged statistical characteristics.<ref name="tsekov-2009-bohmian">Tsekov, R. (2012) Bohmian Mechanics versus Madelung Quantum Hydrodynamics doi:10.13140/RG.2.1.3663.8245</ref>

Relation to the von Weizsäcker correction

In 1935,<ref>C. F. von Weizsäcker: Zur Theorie der Kernmassen, Zeitschrift für Physik, Volume 96, pp. 431–458 (1935).</ref> Carl Friedrich von Weizsäcker proposed the addition of an inhomogeneity term (sometimes referred to as a von Weizsäcker correction) to the kinetic energy of the Thomas–Fermi (TF) theory of atoms.<ref>See also section "Introduction" of: Rafael Benguria, Haim Brezis, Elliott H. Lieb: The Thomas–Fermi–von Weizsäcker theory of atoms and molecules, Commun. Math. Phys., Volume 79, pp. 167–180 (1981), doi:10.1007/BF01942059.</ref>

The von Weizsäcker correction term is<ref name="tsekov-2009-dissipative">See also Roumen Tsekov: Dissipative time dependent density functional theory, Int. J. Theor. Phys., Vol. 48, pp. 2660–2664 (2009), arXiv:0903.3644.</ref>

<math>

E_W[\rho] = \int dr\, \frac{\rho \hbar^2 (\nabla \ln \rho)^2}{8m} = \frac{\hbar^2}{8m} \int dr\, \frac{(\nabla \rho)^2}{\rho} = \int dr\, \rho\,Q. </math>

The correction term has also been derived as the first-order correction to the TF kinetic energy in a semi-classical correction to the Hartree–Fock theory.<ref>Kompaneets, Alexander Solomonovich; Pavlovskii, E. S.; Sov. Phys. JETP, volume 4, pp. 328–336 (1957). Cited in section "Introduction" of: Rafael Benguria, Haim Brezis, Elliott H. Lieb: The Thomas–Fermi–von Weizsäcker theory of atoms and molecules, Commun. Math. Phys., volume 79, pp. 167–180 (1981), doi:10.1007/BF01942059.</ref>

It has been pointed out<ref name="tsekov-2009-dissipative"/> that the von Weizsäcker correction term at low density takes on the same form as the quantum potential.

Main article: Thomas–Fermi model § Inaccuracies and improvements

Quantum potential as energy of internal motion associated with spin

Giovanni Salesi, Erasmo Recami and co-workers showed in 1998 that, in agreement with the König's theorem, the quantum potential can be identified with the kinetic energy of the internal motion ("zitterbewegung") associated with the spin of a spin-½ particle observed in a center-of-mass frame. More specifically, they showed that the internal zitterbewegung velocity for a spinning, non-relativistic particle of constant spin with no precession, and in absence of an external field, has the squared value:<ref>G. Salesi, E. Recami, H. E. Hernández F., Luis C. Kretly: Hydrodynamics of spinning particles, submitted 15 February 1998, arXiv.org, arXiv:hep-th/9802106v1</ref>

<math>\mathbf V^2 = \frac{(\nabla \rho \land \mathbf s)^2} {(m \rho)^2} = \frac{(\nabla \rho)^2 \mathbf s^2 - (\nabla \rho \cdot \mathbf s)^2}{(m \rho)^2}</math>

from which the second term is shown to be of negligible size; then with <math>| \mathbf s | = \hbar/2</math> it follows that

<math>| \mathbf V | = \frac{\hbar}{2} \frac{\nabla \rho}{m \rho}</math>

Salesi gave further details on this work in 2009.<ref>G. Salesi: Spin and Madelung fluid, submitted 23 June 2009, arXiv:quant-ph/0906.4147v1</ref>

In 1999, Salvatore Esposito generalized their result from spin-½ particles to particles of arbitrary spin, confirming the interpretation of the quantum potential as a kinetic energy for an internal motion. Esposito showed that (using the notation <math>\hbar</math>=1) the quantum potential can be written as:<ref name="esposito-1999">Salvatore Esposito: On the role of spin in quantum mechanics, submitted 5 February 1999, arXiv:quant-ph/9902019v1</ref>

<math>Q = - \frac{1}{2} m \mathbf v_S^2 - \frac{1}{2} \nabla \cdot \mathbf v_S</math>

and that the causal interpretation of quantum mechanics can be reformulated in terms of a particle velocity

<math>\mathbf v = \mathbf v_B + \mathbf v_S \times \mathbf s</math>

where the "drift velocity" is

<math>\mathbf v_B = \frac {\nabla S}{m}</math>

and the "relative velocity" is <math>\mathbf v_S \times \mathbf s</math>, with

<math>\mathbf v_S = \frac {\nabla R^2}{2m R^2}</math>

and <math>\mathbf s</math> representing the spin direction of the particle. In this formulation, according to Esposito, quantum mechanics must necessarily be interpreted in probabilistic terms, for the reason that a system's initial motion condition cannot be exactly determined.<ref name="esposito-1999"/> Esposito explained that "the quantum effects present in the Schrödinger equation are due to the presence of a peculiar spatial direction associated with the particle that, assuming the isotropy of space, can be identified with the spin of the particle itself".<ref>p. 7</ref> Esposito generalized it from matter particles to gauge particles, in particular photons, for which he showed that, if modelled as <math>\psi = (\mathbf E - i \mathbf B) / \sqrt 2</math>, with probability function <math>\psi^* \cdot \psi = (\mathbf E^2 + \mathbf B^2)/2</math>, they can be understood in a quantum potential approach.<ref>S. Esposito: Photon wave mechanics: A de Broglie–Bohm approach, p. 8 ff.</ref>

James R. Bogan, in 2002, published the derivation of a reciprocal transformation from the Hamilton-Jacobi equation of classical mechanics to the time-dependent Schrödinger equation of quantum mechanics which arises from a gauge transformation representing spin, under the simple requirement of conservation of probability. This spin-dependent transformation is a function of the quantum potential.<ref>James R. Bogan: Spin: The classical to quantum connection, arXiv.org, submitted 19 December 2002, arXiv:quant-ph/0212110</ref>

EP quantum mechanics with quantum potential as Schwarzian derivative

In a different approach, the EP quantum mechanics formulated on the basis of an Equivalence Principle (EP), a quantum potential is written as:<ref name="faraggi-matone">Alon E. Faraggi, M. Matone: The Equivalence Postulate of Quantum Mechanics, International Journal of Modern Physics A, vol. 15, no. 13, pp. 1869–2017. arXiv hep-th/9809127 of 6 August 1999</ref><ref>Robert Carroll: Aspects of quantum groups and integrable systems, Proceedings of Institute of Mathematics of NAS of Ukraine, vo. 50, part 1, 2004, pp. 356–367, p. 357</ref>

<math>Q (q) = \frac{\hbar^2}{4m} \{ S ; q \}</math>

where <math>\{ \cdot \, ; \cdot \}</math> is the Schwarzian derivative, that is, <math> \{ S ; q \} = (S' / S') - (3/2) (S/S')^2</math>. However, even in cases where this may equal

<math>Q (q) = - \frac {\hbar^2}{2m} \frac {\Delta R}{R}</math>

it is stressed by E. Faraggi and M. Matone that this does not correspond with the usual quantum potential, as in their approach <math>R \exp (i S /\hbar)</math> is a solution to the Schrödinger equation but does not correspond to the wave function.<ref name="faraggi-matone"/> This has been investigated further by E.R. Floyd for the classical limit <math>\hbar \to 0</math>,<ref>Edward R. Floyd: Classical limit of the trajectory representation of quantum mechanics, loss of information and residual indeterminacy, arXiv:quant-ph/9907092v3</ref> as well as by Robert Carroll.<ref>R. Carroll: Some remarks on time, uncertainty, and spin, arXiv:quant-ph/9903081v1</ref>

Re-interpretation in terms of Clifford algebras

B. Hiley and R. E. Callaghan re-interpret the role of the Bohm model and its notion of quantum potential in the framework of Clifford algebra, taking account of recent advances that include the work of David Hestenes on spacetime algebra. They show how, within a nested hierarchy of Clifford algebras <math>C\ell_{i,j}</math>, for each Clifford algebra an element of a minimal left ideal <math>\Phi_L(\mathbf r, t)</math> and an element of a right ideal representing its Clifford conjugation <math>\Phi_R(\mathbf r, t) = \tilde{\Phi}_L(\mathbf r, t)</math> can be constructed, and from it the Clifford density element (CDE) <math>\rho_c(\mathbf r, t) = \Phi_L(\mathbf r, t) \tilde{\Phi}_L(\mathbf r, t)</math>, an element of the Clifford algebra which is isomorphic to the standard density matrix but independent of any specific representation.<ref>B. Hiley, R. E. Callaghan: The Clifford algebra approach to quantum mechanics A: The Schrödinger and Pauli particles, 14 March 2010, p. 6</ref> On this basis, bilinear invariants can be formed which represent properties of the system. Hiley and Callaghan distinguish bilinear invariants of a first kind, of which each stands for the expectation value of an element <math>B</math> of the algebra which can be formed as <math>{\rm Tr} B \rho_c</math>, and bilinear invariants of a second kind which are constructed with derivatives and represent momentum and energy. Using these terms, they reconstruct the results of quantum mechanics without depending on a particular representation in terms of a wave function nor requiring reference to an external Hilbert space. Consistent with earlier results, the quantum potential of a non-relativistic particle with spin (Pauli particle) is shown to have an additional spin-dependent term, and the momentum of a relativistic particle with spin (Dirac particle) is shown to consist in a linear motion and a rotational part.<ref>B. Hiley, R. E. Callaghan: The Clifford algebra approach to quantum mechanics A: The Schrödinger and Pauli particles, 14 March 2010, p. 1-29</ref> The two dynamical equations governing the time evolution are re-interpreted as conservation equations. One of them stands for the conservation of energy; the other stands for the conservation of probability and of spin.<ref name="clifford-direc-bohm-hj">B. Hiley: Clifford algebras and the Dirac–Bohm Hamilton–Jacobi equation, 2 March 2010, p. 22</ref> The quantum potential plays the role of an internal energy<ref>B. J. Hiley: Non-commutative geometry, the Bohm interpretation and the mind–matter relationship, p. 14</ref> which ensures the conservation of total energy.<ref name="clifford-direc-bohm-hj"/>

Relativistic and field-theoretic extensions

Quantum potential and relativity

Bohm and Hiley demonstrated that the non-locality of quantum theory can be understood as limit case of a purely local theory, provided the transmission of active information is allowed to be greater than the speed of light, and that this limit case yields approximations to both quantum theory and relativity.<ref>D. Bohm, B. J. Hiley: Non-locality and locality in the stochastic interpretation of quantum mechanics, Physics Reports, Volume 172, Issue 3, January 1989, Pages 93-122, doi:10.1016/0370-1573(89)90160-9 (abstract)</ref>

The quantum potential approach was extended by Hiley and co-workers to quantum field theory in Minkowski spacetime<ref>P.N. Kaloyerou, Investigation of the Quantum Potential in the Relativistic Domain, PhD. Thesis, Birkbeck College, London (1985)</ref><ref>P.N. Kaloyerou, Phys. Rep. 244, 288 (1994).</ref><ref>P.N. Kaloyerou, in "Bohmian Mechanics and Quantum Theory: An Appraisal", eds. J.T. Cushing, A. Fine and S. Goldstein, Kluwer, Dordrecht, 155 (1996).</ref><ref>D. Bohm, B. J. Hiley, P. N. Kaloyerou: An ontological basis for the quantum theory, Physics Reports (Review section of Physics Letters), volume 144, number 6, pp. 323–348, 1987 (PDF)</ref> and to curved spacetime.<ref>B. J. Hiley, A. H. Aziz Muft: The ontological interpretation of quantum field theory applied in a cosmological context. In: Miguel Ferrero, Alwyn Van der Merwe (eds.): Fundamental problems in quantum physics, Fundamental theories of physics, Kluwer Academic Publishers, 1995, ISBN 0-7923-3670-4, pages 141-156</ref>

Carlo Castro and Jorge Mahecha derived the Schrödinger equation from the Hamilton-Jacobi equation in conjunction with the continuity equation, and showed that the properties of the relativistic Bohm quantum potential in terms of the ensemble density can be described by the Weyl properties of space. In Riemann flat space, the Bohm potential is shown to equal the Weyl curvature. According to Castro and Mahecha, in the relativistic case, the quantum potential (using the d'Alembert operator <math>\scriptstyle\Box</math> and in the notation <math>\hbar=1</math>) takes the form

<math>Q = - \frac {1}{2m} \frac {\quad \Box \sqrt \rho}{\sqrt \rho}</math>

and the quantum force exerted by the relativistic quantum potential is shown to depend on the Weyl gauge potential and its derivatives. Furthermore, the relationship among Bohm's potential and the Weyl curvature in flat spacetime corresponds to a similar relationship among Fisher Information and Weyl geometry after introduction of a complex momentum.<ref>Carlo Castro, Jorge Mahecha: On nonlinear quantum mechanics, Brownian motion, Weyl geometry and Fisher information, submitted February 2005, In: F. Smarandache and V. Christianto (Eds.): Quantization in Astrophysics, Brownian Motion, and Supersymmetry, pp.73–87, MathTiger, 2007, Chennai, Tamil Nadu, ISBN 81-902190-9-X, page 82, eq.(37) ff.</ref>

Diego L. Rapoport, on the other hand, associates the relativistic quantum potential with the metric scalar curvature (Riemann curvature).<ref>Rapoport, Diego L. (2007). "Torsion fields, Cartan-Weyl space-time, and state-space quantum geometries, Brownian motion, and their topological dimension". In Smarandache, F.; Christianto, V. (eds.). Quantization in Astrophysics, Brownian Motion, and Supersymmetry. Chennai, Tamil Nadu: MathTiger. pp. 276–328. CiteSeerX 10.1.1.75.6580. ISBN 978-81-902190-9-9.</ref>

In relation to the Klein–Gordon equation for a particle with mass and charge, Peter R. Holland spoke in his book of 1993 of a ‘quantum potential-like term’ that is proportional <math>\Box R/R</math>. He emphasized however that to give the Klein–Gordon theory a single-particle interpretation in terms of trajectories, as can be done for nonrelativistic Schrödinger quantum mechanics, would lead to unacceptable inconsistencies. For instance, wave functions <math>\psi(\mathbf{x},t)</math> that are solutions to the Klein–Gordon or the Dirac equation cannot be interpreted as the probability amplitude for a particle to be found in a given volume <math>d^3 x</math> at time <math>t</math> in accordance with the usual axioms of quantum mechanics, and similarly in the causal interpretation it cannot be interpreted as the probability for the particle to be in that volume at that time. Holland pointed out that, while efforts have been made to determine a Hermitian position operator that would allow an interpretation of configuration space quantum field theory, in particular using the Newton–Wigner localization approach, but that no connection with possibilities for an empirical determination of position in terms of a relativistic measurement theory or for a trajectory interpretation has so far been established. Yet according to Holland this does not mean that the trajectory concept is to be discarded from considerations of relativistic quantum mechanics.<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. 498 ff.</ref>

Hrvoje Nikolić derived <math>Q = - (1/2m) \, \Box R/R</math> as expression for the quantum potential, and he proposed a Lorentz-covariant formulation of the Bohmian interpretation of many-particle wave functions.<ref>Hrvoje Nikolić: 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 also developed a generalized relativistic-invariant probabilistic interpretation of quantum theory,<ref>Hrvoje Nikolić: Time in relativistic and nonrelativistic quantum mechanics, arXiv:0811/0811.1905 (submitted 12 November 2008 (v1), revised 12 Jan 2009)</ref><ref name="nikolicqft">Nikolic, H. 2010 "QFT as pilot-wave theory of particle creation and destruction", Int. J. Mod. Phys. A 25, 1477 (2010)</ref><ref>Hrvoje Nikolić: 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.<ref>Hrvoje Nikolić: Bohmian mechanics in relativistic quantum mechanics, quantum field theory and string theory, 2007 J. Phys.: Conf. Ser. 67 012035</ref><ref>See also: De Broglie–Bohm theory#Relativity</ref>

Quantum potential in quantum field theory

Starting from the space representation of the field coordinate, a causal interpretation of the Schrödinger picture of relativistic quantum theory has been constructed. The Schrödinger picture for a neutral, spin 0, massless field <math>\Psi \left[ \psi(\mathbf{x},t) \right] = R \left[ \psi(\mathbf{x},t) \right] e^{S \left[ \psi(\mathbf{x},t) \right]}</math>, with <math>R \left[ \psi(\mathbf{x},t) \right], S \left[ \psi(\mathbf{x},t) \right]</math> real-valued functionals, can be shown<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. 520 ff.</ref> to lead to

<math>Q \left[ \psi(\mathbf{x},t) \right] = - (1/2R) \int d^3 x \, \delta^2 R / \delta \psi^2 </math>

This has been called the superquantum potential by Bohm and his co-workers.<ref>Basil Hiley: The conceptual structure of the Bohm interpretation of quantum mechanics, Kalervo Vihtori Laurikainen et al (ed.): Symposium on the Foundations of Modern Physics 1994: 70 years of matter waves, Editions Frontières, ISBN 2-86332-169-2, p. 99–117, p. 144</ref>

Basil Hiley showed that the energy–momentum-relations in the Bohm model can be obtained directly from the energy–momentum tensor of quantum field theory and that the quantum potential is an energy term that is required for local energy–momentum conservation.<ref>B. J. Hiley: The Bohm approach re-assessed (2010 preprint), p. 6</ref> He has also hinted that for particle with energies equal to or higher than the pair creation threshold, Bohm's model constitutes a many-particle theory that describes also pair creation and annihilation processes.<ref>B. J. Hiley (2013-03-25). "Bohmian Non-commutative Dynamics: History and New Developments". Pre-print arXiv:1303.6057 (submitted 25 March 2013)</ref>

Interpretation and naming of the quantum potential

In his article of 1952, providing an alternative interpretation of quantum mechanics, Bohm already spoke of a "quantum-mechanical" potential.<ref>Bohm, David (1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables" I". Physical Review. 85 (2): 166–179. Bibcode:1952PhRv...85..166B. doi:10.1103/PhysRev.85.166. p. 170 Archived 2012-10-18 at the Wayback Machine</ref>

Bohm and Basil Hiley also called the quantum potential an information potential, given that it influences the form of processes and is itself shaped by the environment.<ref name="information-quantum-theory-and-the-brain-P207">B. J. Hiley: Information, quantum theory and the brain. In: Gordon G. Globus (ed.), Karl H. Pribram (ed.), Giuseppe Vitiello (ed.): Brain and being: at the boundary between science, philosophy, language and arts, Advances in Consciousness Research, John Benjamins B.V., 2004, ISBN 90-272-5194-0, pp. 197-214, p. 207</ref> Bohm indicated "The ship or aeroplane (with its automatic Pilot) is a self-active system, i.e. it has its own energy. But the form of its activity is determined by the information content concerning its environment that is carried by the radar waves. This is independent of the intensity of the waves. We can similarly regard the quantum potential as containing active information. It is potentially active everywhere, but actually active only where and when there is a particle." (italics in original).<ref>David Bohm: Meaning And Information Archived 2011-10-09 at archive.today, In: P. Pylkkänen (ed.): The Search for Meaning: The New Spirit in Science and Philosophy, Crucible, The Aquarian Press, 1989, ISBN 978-1-85274-061-0</ref>

Hiley refers to the quantum potential as internal energy<ref name="hiley-reappraisal-bohm-2005"/> and as "a new quality of energy only playing a role in quantum processes".<ref>B.J. Hiley: Non-commutative quantum geometry: A reappraisal of the Bohm approach to quantum theory. In: Avshalom C. Elitzur, Shahar Dolev, Nancy Kolenda (es.): Quo vadis quantum mechanics? Springer, 2005, ISBN 3-540-22188-3, pp. 299 ff., therein p. 310</ref> He explains that the quantum potential is a further energy term aside the well-known kinetic energy and the (classical) potential energy and that it is a nonlocal energy term that arises necessarily in view of the requirement of energy conservation; he added that much of the physics community's resistance against the notion of the quantum potential may have been due to scientists' expectations that energy should be local.<ref>Basil Hiley & Taher Gozel, episode 5, YouTube (downloaded 8 September 2013)</ref>

Hiley has emphasized that the quantum potential, for Bohm, was "a key element in gaining insights into what could underlie the quantum formalism. Bohm was convinced by his deeper analysis of this aspect of the approach that the theory could not be mechanical. Rather, it is organic in the sense of Whitehead. Namely, that it was the whole that determined the properties of the individual particles and their relationship, not the other way round."<ref>B. J. Hiley: Some remarks on the evolution of Bohm's proposals for an alternative to quantum mechanics, 30 January 2010</ref><ref>See also: Basil Hiley#Quantum potential and active information</ref>

Peter R. Holland, in his comprehensive textbook, also refers to it as quantum potential energy.<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. 72</ref> The quantum potential is also referred to in association with Bohm's name as Bohm potential, quantum Bohm potential or Bohm quantum potential.

Applications

The quantum potential approach can be used to model quantum effects without requiring the Schrödinger equation to be explicitly solved, and it can be integrated in simulations, such as Monte Carlo simulations using the hydrodynamic and drift diffusion equations.<ref>G. Iannaccone, G. Curatola, G. Fiori: Effective Bohm Quantum Potential for device simulators based on drift-diffusion and energy transport, Simulation of Semiconductor Processes and Devices, 2004, vol. 2004, pp. 275–278</ref> This is done in form of a "hydrodynamic" calculation of trajectories: starting from the density at each "fluid element", the acceleration of each "fluid element" is computed from the gradient of <math>V</math> and <math>Q</math>, and the resulting divergence of the velocity field determines the change to the density.<ref>Eric R. Bittner: Quantum tunneling dynamics using hydrodynamic trajectories, arXiv:quant-ph/0001119v2, 18 February 2000, p. 3.</ref>

The approach using Bohmian trajectories and the quantum potential is used for calculating properties of quantum systems which cannot be solved exactly, which are often approximated using semi-classical approaches. Whereas in mean field approaches the potential for the classical motion results from an average over wave functions, this approach does not require the computation of an integral over wave functions.<ref>E. Gindensberger, C. Meier, J.A. Beswick: Mixing quantum and classical dynamics using Bohmian trajectories Archived 2012-03-28 at the Wayback Machine, Journal of Chemical Physics, vol. 113, no. 21, 1 December 2000, pp. 9369–9372</ref>

The expression for the quantum force has been used, together with Bayesian statistical analysis and Expectation-maximisation methods, for computing ensembles of trajectories that arise under the influence of classical and quantum forces.<ref name="maddox-bittner-2003"/>

Further reading

Fundamental articles

• Bohm, David (1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables" I". Physical Review. 85 (2): 166–179. Bibcode:1952PhRv...85..166B. doi:10.1103/PhysRev.85.166. (full text)
• Bohm, David (1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden Variables", II". Physical Review. 85 (2): 180–193. Bibcode:1952PhRv...85..180B. doi:10.1103/PhysRev.85.180. (full text)
• D. Bohm, B. J. Hiley, P. N. Kaloyerou: An ontological basis for the quantum theory, Physics Reports (Review section of Physics Letters), volume 144, number 6, pp. 321–375, 1987 (full text), therein: D. Bohm, B. J. Hiley: I. Non-relativistic particle systems, pp. 321–348, and D. Bohm, B. J. Hiley, P. N. Kaloyerou: II. A causal interpretation of quantum fields, pp. 349–375

Recent articles

• Spontaneous creation of the universe from nothing, arXiv:1404.1207v1, 4 April 2014
• Maurice de Gosson, Basil Hiley: Short Time Quantum Propagator and Bohmian Trajectories, arXiv:1304.4771v1 (submitted 17 April 2013)
• Robert Carroll: Fluctuations, gravity, and the quantum potential, 13 January 2005, asXiv:gr-qc/0501045v1

Overview

• Davide Fiscaletti: About the Different Approaches to Bohm's Quantum Potential in Non-Relativistic Quantum Mechanics, Quantum Matter, Volume 3, Number 3, June 2014, pp. 177–199(23), doi:10.1166/qm.2014.1113.
• Ignazio Licata, Davide Fiscaletti (with a foreword by B.J. Hiley): Quantum potential: Physics, Geometry and Algebra, AMC, Springer, 2013, ISBN 978-3-319-00332-0 (print) / ISBN 978-3-319-00333-7 (online)
• 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 June 25, 1993), ISBN 0-521-35404-8 hardback, ISBN 0-521-48543-6 paperback, transferred to digital printing 2004
• David Bohm, Basil Hiley: The Undivided Universe: An Ontological Interpretation of Quantum Theory, Routledge, 1993, ISBN 0-415-06588-7
• David Bohm, F. David Peat: Science, Order and Creativity, 1987, Routledge, 2nd ed. 2000 (transferred to digital printing 2008, Routledge), ISBN 0-415-17182-2