﻿ The Asymptotic Equality of a Spatial Average of the Probability Density from Schroedinger's Equation for a Particle in a Potential Field and the Classical Time-Spent Probability Density for that System
San José State University

applet-magic.com
Thayer Watkins
Silicon Valley
U.S.A.

The Asymptotic Equality of a Spatial Average
of the Probability Density from Schroedinger's
Equation for a Particle in a Potential Field
and the Classical Time-Spent Probability
Density for that System

## The Classical Analysis

Let V(x) be the potential energy function for the particle with V(0)=0. If m is the mass of the particle and v is its velocity then the total energy E is

Therefore

#### v = (2/m)½(E−V)½which can be expressed as v = (2/m)½(K(x))½

where K(x) is the kinetic energy of the system expressed as a function of the location of the particle.

The time spent in an interval dx at x is then

#### dt = dx/|v(x)|

Thus the probability density PC(x) of finding the particle in an interval about x is

#### PC(x) = 1/(T|v(x)|)

where T the time required by the particle to complete its orbit; i.e.,

#### T = ∫dx/|v(x)| = ∫[(m/2)½/(K(x))½]dx

But the factor (m/2)½ cancels out in the normalization process and is thus irrelevant and can be discarded. Thus

#### PC = 1/(T1K(x))½)

where T1=∫dz/K(z))½)

The crucial result is that the classical time-spent probability density distribution is inversely proportional to (K(z))½.

## Lemma

If a probability density function P(z) is proportional to a function g(z), say

#### P(z) = αg(z)

then the constant of proportionality is irrelevant because

But

#### P(z) = αg(z)/G = αg(z)/(α∫g(ζ)dζ = g(z)/∫g(ζ)dζ

Thus the coefficient α is irrelevant in determining P(z).

## The Solution to Schroedinger's Equation for the System

The Hamiltonian function for the system is

#### H = p²/(2m) + V(x)

where p is the momentum of the particle.

The Hamiltonian operator H^ for the system is created by substituting −ih(∂/∂x) for p in the Hamiltonian function, where h is Planck's constant divided by 2π and i is the imaginary unit. The time-independent Schroedinger's equation for the system is then

#### [(−h²/(2m))(∂²/dx²) + V(x)]φ = Eφ

where E is the total energy and φ is the wave function.

This equation can be expressed as

#### (∂²φ/dx² = −(2m/h²)(E − V(x))φ or, equivalently (∂²φ/dx² = −(2m/h²)K(x)φ

with K(x)=E−V(x).

This may be considered the very simple case of a generalized Helmholtz equation

#### d²φ/dx² = −k²(x)φ

with k²(x) being equal to (2m/h²)K(x).

## An Example

The graph below shows the probability density distributions for a harmonic oscillator. The thin line is the quantum mechanical solution for a principal quantum number of 60 and the thick line is the time-spent probability density distribution based upon classical analysis.

## A Sketch of a Proof

If k(x) is a constant over some interval then the solution over that interval is

#### φ(x) = A·cos(kx) + B·sin(kx)

Let φ(0)=0 so the solution is

Then

#### (dφ/dx)0 = Bk

The maxima of φ are equal to

#### φ(π/2) = B

This occurs at kx=π/2 and hence x=π/(2k).

The average value of φ²(x) over the interval from 0 to π/(2k) is ½B² since the average value of sin²(x) over that interval is ½. Thus the probability of the particle being in the interval [0, π/(2k)] is πB²/(4k). If there are N equal intervals over the range of x then

#### NπB²/(4k) = 1 and hence B² = 4k/(Nπ)

N and k are related to the energy of the system by the relations

Hence

#### N = 2πE/h = E/h k = (2m)½(K(x))½ and thus ½B² = 2(2m)½h(K(x))½/(π(K(x)+V(x))) which is equivalent to ½B² = [2(2m)½h/π](K(x))½) /((K(x))½+V(x))

Thus the spatial average of the probability density from Schroedinger's equation is

#### PSch = γ(K(x))½/((K(x))½+V(x)

where γ is the irrelevant constant factor of [2(2m)½h/π].

Now consider the limit as E→∞ at a fixed x. As E increases without bound so does K(x) and hence V(x)/(K(x))½ goes to zero. Thus asymptotically PSch goes to PC. Thus the probability density distribution from Schroedinger's equation is tied to the classical time-spent probability distribution rather than, as in the Copenhagen Interpretation, to some intrinsic uncertainty of the particle.

## The Correspondence Principle

As articulated by Niels Bohr the Correspondence Principle is that for Quantum theory to be valid its solution must asymptotically approach that of the classical analysis. It is often thought that there are two regimes: the microscopic quantum regime and the macroscopic classical regime with a boundary scale where the two match. This is most likely erroneous. A better characterization of the situation is that the quantum theory applies at all scales but there is a sale at which the smoothed version of the quantum theoretic behavior is indistinguishable from the results of classical analysis. This is analogous to relativistic effects applying at all velocities but the difference between them and classical analysis being infinitesimal at low velocities.

In order for the quantum theoretic analysis of a particle in a potential field to be valid according to the Correspondence Principle it must involve a particle trajectory. Whether the setting is for such a system is one dimensional, two dimensional or three dimensional the existence of an orbit trajectory reduces the analysis to the one dimesnsional. The analysis presented here would then apply with path length being the coordinate. Thus in a 1D, 2D or 3D setting the spatial average of the probability density distribution from Schroedinger's equation for a particle in a potential field is asymptotically equal to the classical time-spent probability distribution.

## A Proposed Principle of Universality

The Copenhagen Interpretation holds that particles do not have a physical existence until they are subjected to measurement, which causes their probability density functions to collapse to spiked one involving a definite location.

It cannot be that the nature of reality is one thing for solvable models and yet another thing for more complex models that cannot be analytically solved. Thus the Copenhagen Interpretation applies for both the simple and complex models or for neither. Since it does not apply for a particle in a potential field it does not apply anywhere.