Physics believes that macroscopic matter consists of microscopic matter; the macroscopic physical world described by classical mechanics is causal, and the microscopic world described by quantum mechanics is non-causal [1][2]; the wave function that describes a quantum is interpreted as the probability of the quantum's appearance at a certain location. The wave function, then, is the bridge between randomness and causality, the dividing line between the micro and macro worlds. Either according to the Schrödinger equation (1), or the Dirac equation (2),
iħ ∂/∂t {Ψ} = H{Ψ} (1)
(iħγ'∂μ - mc){Ψ} = 0 (2)
The changes of the wave function are all deterministic. What governs this determinism? For example, how does the derivative of the probability ∂/∂t {Ψ}, i.e., the rate of change of the probability in time, occur and by whom?
The essence of an equation is its invariance, and no matter how many solutions there are, the common feature of the solutions is that they maintain the invariance of the equation. In equation (1), (2), it is required that the total probability of the amplitude of the wave function is conserved and the energy-momentum is conserved. If there are more than one conserved quantity in a process at the same time, there must be a definite relationship between them, or one conserved quantity must dominate and the others are additives. If we must assume that the probabilistic interpretation is correct, what is the relationship between energy-momentum and probability?
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[1] Born, M. 1955. Statistical Interpretation of Quantum Mechanics. Science 122 (3172):675-679.
[2] Bassi, A., K. Lochan, S. Satin, T. P. Singh, and H. Ulbricht. 2013. Models of wave-function collapse, underlying theories, and experimental tests. Reviews of Modern Physics 85 (2):471.
"Moreover, the theory does not explain why during a quantum measurement, deterministic evolution is replaced by probabilistic evolution, whose random outcomes obey the Born probability rule."