Journal of Modern Physics
Vol.08 No.05(2017), Article ID:75601,17 pages
10.4236/jmp.2017.85050
Statistical Description of Nonrelativistic Classical Systems
George H. Goedecke
Physics Department, New Mexico State University, Las Cruces, NM, USA

Copyright © 2017 by author and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/



Received: March 5, 2017; Accepted: April 22, 2017; Published: April 25, 2017
ABSTRACT
We prove that any nonrelativistic classical system must obey a statistical wave equation that is exactly the same as the Schrödinger equation for the system, including the usual “canonical quantization” and Hamiltonian operator, provided an unknown constant is set equal to
. We show why the two equations must have exactly the same sets of solutions, whereby this classical statistical theory (CST) and nonrelativistic quantum mechanics may differ only in their interpretations of the same quantitative results. We identify some of the different interpretations. We show that the results also imply nonrelativistic Lagrangian classical mechanics and the associated Newtonian laws of motion. We prove that the CST applied to a nonrelativistic rigid rotator yields spin angular momentum operators that obey the quantum commutation rules and allow both integer and half-odd-integer spin. We also note that the CST applied to systems of identical massive particles is mathematically equivalent to nonrelativistic quantum field theory for those particles.
Keywords:
Stochastic Classical Mechanics, Stochastic Quantum Mechanics

1. Introduction
During the latter part of the nineteenth century, Ludwig Boltzmann initiated the application of statistics to many-particle systems. He pursued this approach despite the strongly prevailing belief in a continuum structure of matter during that period. With the introduction of the ensemble concept by Willard Gibbs in 1902, the resulting statistical mechanics achieved several great successes, including the statistical definition of entropy and derivations of thermodynamics and fluid mechanics.
During the past century, the belief that quantum mechanics (QM) is the truly fundamental theory of nature has become overwhelmingly prevailing. On the basis of an enormous number of correct predictions, and apparently no incorrect ones, this belief continues to strengthen despite the puzzling fact that QM makes only statistical predictions. How can a statistical theory be fundamental? Any such theory must involve a statistical treatment of some number of underlying quantities.
Many attempts have been made to establish a classical statistical foundation for the single-particle Schrödinger or Dirac equation, e.g., Bohm’s hidden variable theory [1] [2] ; the stochastic mechanics approach of Nelson [3] and Baublitz [4] ; Okamoto’s approach using a complex Langevin equation [5] ; Srinivasan and Sudarshan’s use of quaternion measures and the Langevin equation (to obtain the Dirac equation) [6] ; use of the Fokker-Planck equation [7] ; and extensive work on the global statistical hidden variable theory known as stochastic electrodynamics (SED) [7] [8] [9] [10] [11] . (The first two of these SED references are to review articles that contain references to the very many seminal and comprehensive papers on SED published during the years 1960-1990. The last three are to more recent papers on SED). Also, Gilson [12] and Collins [13] [14] [15] [16] used the Madelung transform [17] in reverse to derive a wave equation that must be obeyed by any system that satisfies a continuity equation for a non-negative density and associated flux in three-dimensional Eulidean space. Their wave equation has exactly the same form as the Schrödinger equation for a single pointlike massive spinless particle, but contains unknown functions instead of the potential energy and electromagnetic vector potential, and an unknown constant instead of Planck’s constant. The unknown functions and constant would be different for, say, a classical fluid than for the statistical description of a one-particle system.
None of the abovementioned approaches has been shown to apply to all classical systems. The principal goals of the work reported in this paper are to develop a statistical description of the nonrelativistic classical motions of the coordinates of any system, based on the probability continuity relation in the coordinate configuration space, and to investigate how close is that description to the nonrelativistic quantum mechanics of the system.
We pursue those goals as follows: In section 2, we consider a general non- relativistic classical system involving
generalized (curvilinear or rectilinear) coordinates that define the
-dimensional configuration space (not the phase space) of the system. We note that if for any reason we wish to treat the system statistically, we may apply the continuity equation for the probability density and flux in that configuration space. Then we prove that this equation alone implies the existence of a statistical wave equation (SWE) that is exactly the same as the Schrödinger equation (SEQ) for the system, provided an unknown constant is set equal to
and two unknown functions are set equal to the physical potentials in the
-space. The new results include an actual derivation of general “canonical quantization” and the operator Hamiltonian, as well as a derivation of nonrelativistic Lagrangian classical mechanics and the associated Newton’s laws of motion. Furthermore, we show that the quantitative solutions of the SWE must be exactly the same as those of the axiomatic SEQ for any given nonrelativistic system, but that some interpretations of these solutions must be different. In section 3, we provide analyses for three important illustrative examples, including particles with spin and systems of arbitrarily many identical particles. In sections 4 and 5, we offer a brief summary and discussion of our results, as well as a few conclusions.
2. General Statistical Wave Equation
In this section we treat general nonrelativistic classical systems, which are invariably described in terms of
of generalized coordinates, where
may be any integer
. For example, for a classical nonrelativistic system of
identical particles, each having only translational and rigid rotational degrees of freedom, the coordinates for each particle could be three Cartesian coordinates for the center-of-mass (CM) motion, and three Euler angles for the rigid rotational motion about the CM, or altogether
generalized coordinates that comprise the configuration space of the system.
2.1. Generalized Coordinates for Nonrelativistic Classical Systems
We include this subsection to establish our notation, which is the notation used by Lichnerowicz [18] and Hartle [19] , simply extended to
dimensions. We represent the generalized-coordinate manifold of a system under consideration by the set
, where each coordinate
is an independent real continuous variable that may have any physical dimension and any range. A classical nonrelativistic system moves on a trajectory in this
-space given by
, where
is the continuous time variable as read on a reference clock.
For such systems, the kinetic energy
is always a positive definite quadratic form in the coordinate velocities
:

where we use the extended Einstein summation convention that all repeated indices are summed over from 1 to







where 









2.2. Statistical Wave Equation
In treating classical motions statistically, one may always begin with the coor- dinate probability density. The fine-grained coordinate probability density in the 
where 










where
These fine-grained probability densities are almost never useful in application, because it would be virtually impossible to solve for the detailed coordinate trajectories 








Note that Equation (4) must be satisfied irrespective of what stochastic process is considered, e.g., Markovian or not, and independently of what kind of stochastic dynamics is considered, e.g., the Langevin equation, the Fokker- Planck equation, etc., and independently of what kind of position-velocity or position-momentum phase space treatment may be valid. Therefore, the statistical description of a system’s classical coordinates that evolves from just this conti- nuity equation will be incomplete, but still must be obeyed.
Now we proceed by following Collins’ method [13] (which mathematically is essentially Bohm’s approach in reverse order), but with our new generalization to the 



Then 









where 







This relation is known as the Madelung transform [17] ; it is usually applied to a given SEQ in a Euclidean 3-space, to obtain the Madelung/Bohm “hydrody- namical” equations, which did not include the non-gradient vector field







Then, requiring that the continuity Equation (4) be satisfied yields easily

where 

and 



where 

This 

Note that we are not getting something for nothing: The quantities












Of course, one can identify








If






These identifications and the above derivation of the SWE from the probability continuity relation actually provide a derivation of classical mechanics (CM) for any nonrelativistic Lagrangian system as well, since the Hamilton-Jacobi equation implies the existence of a Hamiltonian, a Lagrangian containing the kinetic energy and metric (Equation (1)), the Euler-Lagrange equations, and thereby Newton’s laws of motion, for arbitrarily chosen potentials 


Canonical quantization. We define the vector conjugate momentum operator

Then the covariant components of 

and the Hamiltonian operator of Equation (13) is

Therefore, the general rule for obtaining the SWE for any nonrelativistic classical Lagrangian system is simply to write down the classical Hamiltonian and then replace the conjugate momentum 









3. Examples
In this section, we consider three examples that should help clarify the gene- ralized-coordinate approach. One is a system of two spinless pointlike particles that may have different masses. Another is a system of arbitrarily many identical spinless pointlike particles with two-body central force instantaneous internal interactions. A third is a system of one nonrelativistic rigid rotator. In these examples, we put
3.1. Two Pointlike Particles
The designation “pointlike” does not mean that the particles are actual points; instead, it means that the model particles considered are allowed no coordinates other than their CM coordinates. In this example, let the masses be





where the index 

with other components zero. Since 



where 

For this example, we consider the unperturbed central force case, by choosing 

is the distance between the particle CM’s. The classical Hamiltonian is




At this point, one may go to the conventional notation
One reason for choosing this particular example is that it is probably the simplest two-particle example of the general method derived in section 2. Another reason is to emphasize that what you get in the Hamiltonian operator in the derived SWE is exactly what you have included in the classical Hamiltonian. For example, it is clearly physically incorrect to choose 
3.2. Many Identical Pointlike Particles
Consider the extension of the two-particle system above to 












where terms with 




where 







For the identical particles in this example, the total Hamiltonian is invariant under all pair interchanges of particle indices. This invariance leads immediately to the result that the total wavefunction solution of the general many-particle SWE must either change sign under each pair interchange, yielding Fermions, or not change sign, yielding Bosons. As discussed in detail by Schweber [26] , the set of all Schrödinger equations for 
3.3. Nonrelativistic Rigid Rotator
Many authors have considered classical spinning top models and their possible connections to quantum spin and magnetic moment, e.g. [27] - [37] . Their treatments either postulate the usual commutation rules for the Cartesian components of the spin operator in the non-rotating coordinate system, by analogy with the rules for orbital angular momentum, or postulate that the momenta conjugate to the Euler angles become operators equal to 
3.3.1. Euler Angles and Angular Velocity
We include this subsection to establish our notation and method. For a rigid rotator with a fixed CM, the coordinates are a set of Euler angles,





where the orthogonal matrices 


Thus, the complete rotation is specified by the orthogonal matrix

The angular velocity 3-vector can be found from the relations defining rigidly rotating Cartesian coordinates,

where × is the cross-product, and 



where we have specified the time-dependent trajectories of the Euler angles by



3.3.2. Model Rotator, Metric, and Conjugate Momenta
In this introductory work, we treat a very simple model rotator, a non-translating but freely rotating rigid extended symmetric object having only the attributes of mass




The metric of the Euler angle 3-space is easily identified as

We may define the conjugate momentum 3-vector by

where 



The (angular) momenta conjugate to the angles are

Contraction with 


where the 

3.3.3. Statistical Wave Equation and Spin Operators
The relevant SWE for any nonrelativistic system having three coordinates is the three-dimensional version of the general SWE (12). In this case, the fields 


where

just as postulated by some of the authors mentioned above. Writing Equation (31) in summation notation and making these substitutions yields

where, from Equations (33) and (35),

It is not difficult to show that the operators 



where 

Equations (28) and (39) then yield easily

Note the minus sign, compared to Equation (38). These “left-handed” commu- tation rules must be obeyed by the rotating system Cartesian components of
Note that


















In a paper to follow, we will derive several relevant detailed results for a charged nonrelativistically spinning top of arbitrary shape and structure immersed in a magnetic field, including the following: 1) The commutation rules of Equations (38) and (40) are unchanged, whereby the commuting operators 















4. Summary, Discussion, and Conclusions
In this work, using a derivation from first principles with no approximations, we proved that any nonrelativistic classical physical system must obey a statistical wave equation (SWE) that has the same form and the same quantitative solu- tions as the Schrödinger equation (SEQ) for the system. In the non-statistical (“classical”) limit, the SWE yields a system Hamiltonian and Lagrangian and thus the Euler-Lagrange equations and Newton’s laws of motion for the system coordinates. The SWE also yields quantum spin and many-body quantum field theory for nonrelativistic systems. On the basis of these results, should we not conclude that the classical statistical theory (CST) developed in this work actually provides a derivation of nonrelativistic classical and quantum dynamics for all Lagrangian systems composed of massive particles?
Our answer to that question is “not yet, and maybe never”. In earlier work [21] [22] [23] [24] and in this work (see Section 2.2) we found that a classical statistical theory based on continuity equations does not allow the conventional QM interpretations of the same quantitative results. In particular, the SWE by itself does not provide a complete statistical description of any system. For example, restrictions on initial conditions may exist such that the system cannot be in a single excited state initially, but only in a mixture of excited states [23] [24] . Also, the SWE is an ensemble-averaged equation, so its quantized Ha- miltonian eigenvalues, the same as those of the SEQ, are ensemble-averaged energies; the actual energies are not quantized. Nevertheless, standard per- turbation theory applied to a perturbed SWE still yields the Einstein rule in- volving the differences of the unperturbed eigenvalues for the frequencies of the dominant transitions, as well as the conventional transition rules and pro- babilities. (Prediction of transition probabilities for spontaneous emission using either the SEQ or the SWE requires either quantized electromagnetic fields or the inclusion of a radiation reaction vector portential in the vector potential
Detailed analysis of such profound interpretational differences is well beyond the scope of this introductory paper. Our results would require an ensemble interpretation of QM, as well as other re-interpretations; see e.g. work by Ballentine [41] [42] . Furthermore, all our derivations follow conventional logical paths, but in reverse. For example, in this work we started our derivation with classical systems that do have coordinate trajectories, and used the Madelung transform in reverse to obtain the SWE/SEQ. On the other hand, Erwin Schrödinger inferred his SEQ nearly a century ago; the Madelung transform was applied to the SEQ to obtain its full hydrodynamic form, Equation (14). How- ever, scientists are still discussing whether classical trajectories are compatible with conventional QM [43] .
The CST derived herein makes sense only if there are reasons why even a one-particle classical system might require a statistical treatment. One possible reason is the known fact that any classical system is continually bombarded by thermal and other highly fluctuating background radiation fields. One model background field used in Stochastic Electrodynamics [7] [8] is the electro- magnetic stochastic zero-point field (SZPF), which is a stochastic analog of the quantized electromagnetic field; see e.g. [21] . Up to a multiplicative adjustable constant, chosen to be
5. Conclusion
In conclusion: Before we are willing to conclude that the CST actually provides a derivation of a good part of modern physics, we feel that at least two things must be accomplished. One is a statistical treatment of relativistic classical systems, with a closer correspondence to our nonrelativistic treatment herein than our work reported in 2010 [25] . The other is a resolution of the interpretational differences between the CST and conventional QM. This latter accomplishment may require clever experiments that will take some time.
Acknowledgements
The author would like to thank Stephen Pate, Michael Engelhardt, and Stefan Zollner for helpful discussions and assistance in preparing the manuscript.
Cite this paper
Goedecke, G.H. (2017) Statistical Description of Nonrelativistic Classical Systems. Journal of Modern Physics, 8, 786-802. https://doi.org/10.4236/jmp.2017.85050
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Appendix A: Relativistic Rotation
As mentioned above, one objection to a nonrelativistic extended spinning electron model having semidefinite charge density is that it seems to require tangential linear speeds 

















Evidently we should have used relativistic expressions for the momenta. For this model rotator, any infinitesimal ring segment of restmass 











It is interesting to to evaluate the magnitude 



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