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The Probability Density Function from Schroedinger's Equation for a Particle Moving in a Central Field |
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This material is for the further development of the proposition that the probability density function from Schroedinger's equation is related to the time-spent probability density function of classical physics .

Niels Bohr noted that the validity of classical physics has been well established at the macroscopic level . Therefore he asserted that for quantum physics to be valid its results must match or approach in the limit those of classical physics as the energy level increases without bound. This is Bohr's Correspondence Principle. It has been modified to include the limit of other parameters besides energy. It also needs to be modified to apply to the spatial average of the quantum results rather than the quantum results themselves . The observed results of classical physics are necessarily time or space averages. Phyical quantities cannot be measured at instants or points.

Consider what the correspondence principle implies . At each energy level the quantum physics determines a probability density distribution (PDD). In the limit it also determines a PDD. What classical physics result can this correspond to? Classical physics is deterministic; the only PDD that the limit can be is the time-spent PDD.

This has been verified for one dimensional physical systems because there are analytical
solutions for them . The material here is for extending the results to two dimensional
systems.
Surprisingly enough there are no valid analytical solutions for particles
in two and three dimensional
systems. Physicists have resorted to what is called the
*Separation of Variables* assumption to get solutions,
but
the Separation of Variables assumption is not compatible with particality (particleness). Therefore
such solutions
are mathematically valid but irrelevant physically .

The general form of this equation is

where i is the imaginary unit, ~~h~~ is Planck's constant divided by 2π and t is time. H^ is the Hamiltonian operator derived
from Hamiltonian function for the physical system. The wave function ψ is such that |ψ|² is the probability density function
for the system.

Schroedinger's Time-Dependent Equation provides a numerical method for approximating solutions. The only problem is what should be the initial conditions for starting the iterative process of numerical solution. Note that Schroedinger's Time-Dependent Equation works backwards in time as well as forward.

Here the initial condition will be the analytical solution that would prevail if Planck's constant were zero. If Planck's constant is zero the solution is the classical one.

Perturbation analysis also may be used to obtain the physical solution for the increase in Planck's constant from zero to its real world level. For this approach the relevant equation is Schroedinger's time independent equation

where E is the energy of the system.

The system being considered is that of a particle of mass m moving in a potential field −K/r. The energy of this system is then

The Hamiltonian operator corresponding to this Hamiltonian function is

Consider a circular orbit about the origin for the particle. The orbit radius is given by

with

½mv² = E

Thus

and hence

R = K/(2E)

The linear probability density for the system 1/(2πR) if r equals R and zero other wise. If the polar plane is
divided into rectangles of dimensions Δr×(rΔθ) then the areal probability density
is 1/(rΔθΔr) for R−Δr<r<R+Δr and zero otherwise. This means
that the wave function ψ is 1/(rΔθΔr)^{½} in the band about r=R
and zero otherwise.

Thus at t=0

When the wave function is always circularly symmetric the derivative with respect to θ is zero and therefore Schroedinger's time dependent equation for the system is

which reduces to

(∂ψ/∂t) = (i

The derivatives with respect to r may be approximated with centered differences; i.e.,

and

(∂²ψ/∂r²) ≅ [ψ(r+Δr)−2ψ(r)+ψ(r−Δr)]/(Δr)²

The wave function at the next time point is then given by

The initial condition and first iteration for the wave function are

(To be continued.)

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