The Real Physics Meaning of “probability density”
In QM, the square of (absolute value of) amplitude of wave function is “probability density”, which is calculated with wave function multiplies its conjugate wave function:
Formula quoted from QM textbook: ω =︱ Ψ ︱² = ΨΨ*
Ψ(r, t)= N exp[i(p•r – Et)] note: Plank constant and vector symbol ignored for easy observation.
Ψ*(r, t)= N exp[i (p•r + Et)]
Then, ω =︱ Ψ ︱² = ΨΨ* = N exp[i(p•r)] • N exp[i(p•r)] = N²exp[i(p•r)] exp[i(p•r)]
Next, we go to explore the real physics meaning of this mathematical operation in QM.
In the mathematical model of unit charge, the “electric wave” can be represented by the equation:
Ψ(r ) =(1 / r)sin r p. The amplitude of wave function naturally means the intensity of electric field E.
Now we add the Et item to make a full wave function and associates it with its conjugate wave function. Denote them in the form of complex number.
Ψ(r, t ) =(1 / r)exp [i (r p – Et)] ① note: Plank constant and vector symbol ignored for easy observation.
Ψ*(r, t ) =(1 / r)exp [i (r p + Et)] ②
We notice the item “r p” actually is the doc product of vector “r” and vector “p” in case of angle θ equals to zero.
Design a simplest system of proton – electron: Assume the proton on the left side while the electron on the right side. They move away from each other relatively in a straight line.
If we consider ① is the “electric wave” activated by the “layer” in the proton (namely “p” is the momentum of the “layer” in the proton) and propagates rightward, then, ② can be considered as the conjugate “electric wave” responded by the “layer” in the electron and propagates leftward. (Note: thinking from the concept of “interaction”, it’s understandable why the “electric wave” propagates that way.)
If we consider ② is the “electric wave” activated by the “layer” in the electron (namely “p” is the momentum of the “layer” in the electron) and propagates leftward, then, ① can be considered as the conjugate “electric wave” responded by the “layer” in the proton and propagates rightward.
This is the real physics meaning of conjugate wave functions.
The conjugate wave functions reflect the spirit of interaction.
Below is the focus of this chapter.
Now we multiplies ① and ②.
Then, Ω = Ψ(r, t ) Ψ*(r, t ) = (1 / r²) exp[i (r p)] exp[i (r p)]
We see that the factor (1/ r²) actually has something to do with the electric force of unit charge interaction. (For details, please see the chapter “Unilateral conception of field vs bilateral conception of field”.)
Compare ω and Ω.
We see that the real physics meaning of “probability density” ︱ Ψ ︱² in QM should be to describe the electric force.
In QM, the formula of ω ahead describes a “quantum mechanics stationary state” in case of conservation of energy, which has nothing to do with time.
Then by comparison, we can see the “electric wave” function of Ψ(r ) =(1 / r)sin r p (in the mathematical model of unit charge) which has nothing to do with time actually describes a “stationary state” too. Just it’s a bilateral conception of field. The difference is the condition of “stationary state” of “electric wave” is conservation and quantization of charge.
(We should notice the dynamic electric force actually is wave characteristic. It’s a bit different from classical electromagnetism. We will analyze this point in details in the Theory of “electric wave” in the future.)