Pilot-wave

In theoretical physics, the pilot wave theory, also known as Bohmian mechanics, was the first known example of a hidden-variable theory, presented by Louis de Broglie in 1927. Its more modern version, the de Broglie–Bohm theory, interprets quantum mechanics as a deterministic theory, avoiding troublesome notions such as wave–particle duality, instantaneous wave function collapse, and the paradox of Schrödinger's cat. To solve these problems, the theory is inherently nonlocal.

The de Broglie–Bohm pilot wave theory is one of several interpretations of (non-relativistic) quantum mechanics.
An extension to the relativistic case has been developed since the 1990s.

History

Louis de Broglie's early results on the pilot wave theory were presented in his thesis (1924) in the context of atomic orbitals where the waves are stationary. Early attempts to develop a general formulation for the dynamics of these guiding waves in terms of a relativistic wave equation were unsuccessful until in 1926 Schrödinger developed his non-relativistic wave equation. He further suggested that since the equation described waves in configuration space, the particle model should be abandoned. Shortly thereafter, Max Born suggested that the wave function of Schrödinger's wave equation represents the probability density of finding a particle. Following these results, de Broglie developed the dynamical equations for his pilot wave theory. Initially, de Broglie proposed a ''double solution'' approach, in which the quantum object consists of a physical wave (''u''-wave) in real space which has a spherical singular region that gives rise to particle-like behaviour; in this initial form of his theory he did not have to postulate the existence of a quantum particle. He later formulated it as a theory in which a particle is accompanied by a pilot wave.

De Broglie presented the pilot wave theory at the 1927 Solvay Conference. However, Wolfgang Pauli raised an objection to it at the conference, saying that it did not deal properly with the case of inelastic scattering. De Broglie was not able to find a response to this objection, and he abandoned the pilot-wave approach. Unlike David Bohm years later, de Broglie did not complete his theory to encompass the many-particle case. The many-particle case shows mathematically that the energy dissipation in inelastic scattering could be distributed to the surrounding field structure by a yet-unknown mechanism of the theory of hidden variables.

In 1932, John von Neumann published a book, part of which claimed to prove that all hidden variable theories were impossible. This result was found to be flawed by Grete Hermann three years later, though this went unnoticed by the physics community for over fifty years.

In 1952, David Bohm, dissatisfied with the prevailing orthodoxy, rediscovered de Broglie's pilot wave theory. Bohm developed pilot wave theory into what is now called the de Broglie–Bohm theory. The de Broglie–Bohm theory itself might have gone unnoticed by most physicists, if it had not been championed by John Bell, who also countered the objections to it. In 1987, John Bell rediscovered Grete Hermann's work, and thus showed the physics community that Pauli's and von Neumann's objections "only" showed that the pilot wave theory did not have locality.

Yves Couder and co-workers in 2010 reported a macroscopic pilot wave system in the form of '' walking droplets''. This system was said to exhibit behavior of a pilot wave, heretofore considered to be reserved to microscopic phenomena. However, more careful fluid dynamics experiments have been carried out since 2015 by two American groups and one Danish team led by Tomas Bohr (grandson of Niels Bohr). These new experiments have not replicated the results of the 2010 experiment as of 2018.

The pilot wave theory

Principles

The pilot wave theory is a hidden-variable theory. Consequently:
* the theory has realism (meaning that its concepts exist independently of the observer);
* the theory has determinism.

The positions of the particles are considered to be the hidden variables. The observer doesn't know the precise values of these variables; they cannot know them precisely because any measurement disturbs them. On the other hand, the observer is defined not by the wave function of their own atoms but by the atoms' positions. So what one sees around oneself are also the positions of nearby things, not their wave functions.

A collection of particles has an associated matter wave which evolves according to the Schrödinger equation. Each particle follows a deterministic trajectory, which is guided by the wave function; collectively, the density of the particles conforms to the magnitude of the wave function. The wave function is not influenced by the particle and can exist also as an empty wave function.

The theory brings to light nonlocality that is implicit in the non-relativistic formulation of quantum mechanics and uses it to satisfy Bell's theorem. These nonlocal effects can be shown to be compatible with the no-communication theorem, which prevents use of them for faster-than-light communication, and so is empirically compatible with relativity.

Mathematical foundations

To derive the de Broglie–Bohm pilot-wave for an electron, the quantum Lagrangian

:$L(t)=mv^2-(V+Q),$

where $V$ is the potential energy, $v$ is the velocity and $Q$ is the potential associated with the quantum force (the particle being pushed by the wave function), is integrated along precisely one path (the one the electron actually follows). This leads to the following formula for the Bohm propagator:

:K^Q(X_1, t_1; X_0, t_0) = \frac \exp\left frac\int_^L(t)\,dt\right

This propagator allows to track the electron precisely over time under the influence of the quantum potential $Q$.

Derivation of the Schrödinger equation

Pilot wave theory is based on Hamilton–Jacobi dynamics, rather than Lagrangian