## Emergent Quantum Mechanics

### An Approach via Sub-Quantum Thermodynamics

Considering a theory as emergent if it “contains or reduces to another theory in a significant manner or if its laws are tied to those of another theory via mathematical connections” [Robert Carroll], we propose that quantum mechanics is such a theory. More precisely, we propose that quantum theory emerges from a deeper, more exact theory on a sub-quantum level. In our approach, one assumes that the latter can be described with the aid of nonequilibrium thermodynamics. We ask ourselves how quantum theory would have evolved, had the “tool” of modern nonequilibrium thermodynamics existed, say, a century ago. As has recently been shown, one can derive the exact Schrödinger equation with said tool, where the relation between energy and frequency, respectively, is used as the only empirical input [Grössing], with the additional option that even the appearance of Planck’s constant may have its origin in classical physics. For an extensive review of our respective papers, and for connections to similar work, and, in particular, to Fisher information techniques, see [Carroll 2010].

In a review for "Entropy" (2010), we have summarized the early results of our works relating to the derivation from purely classical physics of the following quantum mechanical features: Planck’s relation E=hbar.omega for the energy of a particle, the Schrödinger equation for conservative and non-conservative systems, the Heisenberg uncertainty relations, the quantum mechanical superposition principle, Born’s rule, and the quantum mechanical “decay of a Gaussian wave packet”.

We have, a.o., also proven that free quantum motion exactly equals sub-quantum anomalous (i.e., “ballistic”) diffusion, and, via computer simulations with coupled map lattices, we have shown how to calculate averaged (Bohmian) trajectories purely from a real-valued classical model. This was illustrated with the cases of the dispersion of a Gaussian wave packet, both for free quantum motion and for motion in a linear (e.g., gravitational) potential. The results are shown to be in excellent agreement with analytical expressions as they are obtained both via our approach, and also via the Bohmian theory. However, in the context of the explanation of Gaussian wave packet dispersion, quantitative statements on the trajectories’ characteristic behavior are presented, which cannot be formulated in any other existing model for quantum systems.

As is well known, the main features of quantum mechanics, like the Schrödinger equation, for example, have only been postulated, but never derived from some basic principles. (Cf. Murray Gell-Mann: “Quantum mechanics is not a theory, but rather a framework within which we believe any correct theory must fit.”) Even in causal interpretations of the quantum mechanical formalism, such as the de Broglie-Bohm theory, the quantum mechanical wave function, or the solution of the Schrödinger equation, respectively, is taken as input to the theory (sometimes even as a “real” ontological field), without further explanation of why this should have to be so. Still, the Bohmian approach has brought some essential insight into the nature of quantum systems, particularly by exploiting the physics of the “guiding equation” (in what is called “Bohmian mechanics”) or, respectively, by providing a detailed analysis of the “quantum potential”. The latter was shown, in the context of the Hamilton-Jacobi theory, to represent the only difference to the dynamics of classical systems.

However, in 1965, Edward Nelson suggested a derivation of the Schrödinger equation from classical, Newtonian mechanics via the introduction of a new differential calculus. Thus it was possible to show, e.g., that the quantum potential can be understood as resulting from an underlying stochastic mechanics, thereby referring to a hypothesized sub-quantum level. However, ambiguities within said calculus, e.g., as to the formula for the mean acceleration, as well as an apparent impossibility to cope with quantum mechanical nonlocality (which had become rather firmly established in the meantime) has led to a temporary decline of interest in stochastic mechanics. Still, it is legitimate to enquire also today whether the stochastic mechanics envisioned is not just one part of a necessarily larger picture, with the other part(s) of it yet to be established.

Considering the history of quantum mechanics, for example, with its many differences in emphasizing particle and wave aspects of quantum systems, one must concede that in general the particle framework was the dominant one throughout the twentieth century. (Cf., as a representative example, Richard Feynman: “It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told about light behaving like waves. I’m telling you the way it does behave – like particles.”) However, a purely particle-centered approach may not be enough, as the quantum phenomena to be explained may just be more complex than to be reducible to a one-level point-particle mechanics only. In other words, it is possible that by the attempts to reduce quantum dynamics to simple point-by-point interactions, the phenomenon to be discussed would remain without reach, because it is too complex to be described on just one (i.e., an assumed “basic”) level. In still other words, a quantum system may be an emergent phenomenon, where a stochastic point-mechanics on just one level of description would still be a necessary ingredient for its description, but not the only relevant one. So, there may exist two or more relevant levels (e.g., on different time and/or spatial scales), where only the combination, or interactions, of them would result in the possibility to completely describe quantum systems. The latter may thus be more complex than it is assumed in any one-level stochastic mechanics model. In fact, recent results from classical physics suggest that this more complex scenario is even highly probable, since the said new results exhibit phenomena which previously were considered to be possible exclusively as quantum phenomena.

One is here reminded of Feynman’s famous discussion of the double slit, and his introductory remark: "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way and has in it the heart of quantum mechanics. In reality, it contains the only mystery." However, the above-mentioned recent classical physics experiments not only disprove Feynman’s statement w.r.t. the double slit, but prove that a whole set of “quantum” features can be shown to occur in completely classical ones, among them being the Heisenberg uncertainty principle, indeterministic behaviour of a particle despite a deterministic evolution of its statistical ensemble over many runs, nonlocal interaction, tunnelling, and, of course, a combination of all these. We are referring to the beautiful series of experiments performed by the group of Yves Couder using small liquid drops that can be kept bouncing on the surface of a bath of the same fluid for an unlimited time when the substrate oscillates vertically. These “bouncers” can become coupled to the surface waves they generate and thus become “walkers” moving at constant velocity on the liquid surface. A “walker” is defined by a lock-in phenomenon so that the drop falls systematically on the forward front of the wave generated by its previous bouncings. It is thus a “symbiotic” dynamical phenomenon consisting of the moving droplet dressed with the Faraday wave packet it emits. Couder and Fort report on single-particle diffraction and interference of walkers. They show “how this wavelike behaviour of particle trajectories can result from the feedback of a remote sensing of the surrounding world by the waves they emit”.

Of course, the “walkers” of Couder’s group, despite showing so many features they have in common with quantum systems, cannot be employed one-to-one as a model for the latter, with the most obvious difference being that quantum systems are not restricted to two-dimensional surfaces. However, along with the understanding of how the Schrödinger equation can be derived via nonequilibrium thermodynamics, also the mutual relationship of particle and wave behaviour has become clearer. Just as in the experiments with walkers, there exists an average orthogonality also for particle trajectories and wave fronts in the quantum case. This is going to be of central importance for our modelling of quantum mechanics with the aid of an assumed sub-quantum thermodynamics.

Note that in 2011 we have organized a first international conference at the University of Vienna (2011) on

### Emergent Quantum Mechanics (EmerQuM11). See our 2011 conference webpage

Moreover, recently we have also co-organized the follow-up conference,

### Emergent Quantum Mechanics 2013 (EmQM13). For this event, see our 2013 conference webpage, https://emqm13.org .