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arXiv:2408.03941

The Born rule for quantum probabilities from Newton's third law

Sergei Sergeevich Afonin

Published Jun 24, 2024• 0 citations
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Abstract

According to the Born rule, the probability density in quantum theory is determined by the square of the wave function. A generally accepted derivation of this rule has not yet been proposed. In the given work, a simple physical picture is constructed within which the Born rule arises in a natural way. In the proposed scheme, the interaction of a particle with a measuring apparatus is equivalent to creation of a "mirror image" of particle wave function in the space region of interaction. The observable quantity is the product of the particle wave function and its "image". The phase of the latter is reversed due to Newton's third law, thus leading to the Born rule.

DOI
10.48550/arxiv.2408.03941

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According to the Born rule, the probability density in quantum theory is determined by the square of the wave function. A generally accepted derivation of this rule has not yet been proposed. In the given work, a simple physical picture is constructed within which the Born rule arises in a natural way. In the proposed scheme, the interaction of a particle with a measuring apparatus is equivalent to creation of a "mirror image" of particle wave function in the space region of interaction. The observable quantity is the product of the particle wave function and its "image". The phase of the latter is reversed due to Newton's third law, thus leading to the Born rule.
The Born’s rule to interpret the square of wave function as the probability to get a specific value in measurement has been accepted as a postulate in foundations of quantum mechanics. Although there have been so many attempts at deriving this rule theoretically using different approaches such as frequency operator approach, many-world theory, Bayesian probability and envariance, literature shows that arguments in each of these methods are circular. In view of absence of a convincing theoretical proof, recently some researchers have carried out experiments to validate the rule up-to maximum possible accuracy using multi-order interference (Sinha et al, Science, 329, 418 [2010]). But, a convincing analytical proof of Born’s rule will make us understand the basic process responsible for exact square dependency of probability on wave function. In this paper, by generalizing the method of calculating probability in common experience into quantum mechanics, we prove the Born’s rule for statistical interpretation of wave function.
The Born rule is a rule that a probability we observe a small particle like an electron is proportional to the square of the absolute value of the wave function. In this paper, we try to derive the Born rule from the many-worlds interpretation. Many researchers have tried to derive the Born rule (also called Born's rule, Born's law, or probability interpretation) from Many-Worlds Interpretation (MWI). However, no one succeeds. Thus, the derivation of Born’s rule had become an important issue for MWI. We try to derive Born’s rule by introducing an elementary event of probability theory to the quantum theory as a new method. We interpret the wave function as a manifold like a three-dimensional sphere, and interpret the absolute value of the wave function as the surface area of the manifold. We suppose that the manifold exists in the discrete space that has lattice points. We interpret a point on the surface of the manifold as a state that we cannot divide any more, an elementary state. We draw an arrow from any point to any point. We interpret an arrow as an event that we cannot divide any more, an elementary event. Probability is proportional to the number of elementary events, and the number of elementary events is the square of the number of elementary states. The number of elementary states is proportional to the surface area of the manifold, and the surface area of the manifold is the absolute value of the wave function. Therefore, the probability is proportional to the square of the absolute value of the wave function.
The Born rule—that measurement probabilities equal the squared amplitude of the wavefunction—has been treated as a fundamental postulate of quantum mechanics since 1926. Every major interpretation either assumes it or struggles to derive it from other postulates. This paper derives the Born rule as the uniform-coupling limit of a physical measurement interaction within the Field Intrinsic Gravity Induced Density (FIGID) framework. A companion paper [1] established that quantum measurement is gradient-driven localization: the field density gradient of a macroscopic apparatus forces a diffuse field condensation to concentrate at the point of strongest interaction. Here we formalize this mechanism as an interaction Hamiltonian coupling the apparatus gradient profile G(x) to the system’s condensation density ρ₂(x) = |ψ(x)|². The apparatus gradient is a macroscopic, static field configuration—the system responds to it as a background potential, not as a propagating quantum exchange. This static-coupling structure, which parallels FIGID’s treatment of gravitational interactions, yields a measurement law with the transition rate linear in G. In the uniform-coupling limit—when the apparatus gradient varies negligibly across the system’s coherence length—the measurement probabilities reduce exactly to the Born rule. Deviations from Born statistics are predicted when the dimensionless parameter L₂|∇G|/G₀ approaches unity, where L₂ is the coherence length determined by the inverse-roughness law L₂ = Kₗₐₘ/Rₙ [2]. The derivation requires one structural assumption—local scalar coupling between apparatus and system—and produces the Born rule as its leading-order consequence, with quantitative correction terms that are experimentally testable.
The Born rule postulates that the probability of measurement in quantum mechanics is related to the squared modulus of the wave function ψψψ. We rearrange the equation for energy eigenfunctions to define the energy as the real part of E^ψ/ψ\hat{E}ψ/ψE^ψ/ψ. For an eigenstate, this definition gives a constant energy eigenvalue. For a general wave function, the energy fluctuates in space and time. We consider a particle in a one-dimensional square well potential in a superposition of two states and average the energy over space and time. We show that, for most cases, such an energy expectation value differs by only a few percent from that calculated using the Born rule. This difference is consistent with experimental tests of the expectation value and suggests that the Born rule may be an approximation of spacetime averaging.
In order to make the quantum mechanics a closed theory one has to derive the Born rule from the first principles, like the Schroedinger equation (SEq), rather than postulate it. The Born rule was in certain sense derived in several articles, e.g. in [1] and [2]. In this work some arguments of previous authors are simplified and made more “physical”. It is shown how to derive the Born rule using the conservation of quantum state norm (Ψ|Ψ) that is the unitary evolution property determined by the SEq. It is this property that makes the probability equal to the square of the amplitude modulus. Possible modification of the Born rule is briefly discussed in the case when the evolution of the system is described by a nonlinear Schroedinger equation. We also present arguments in the spirit of the Many-World Interpretation to explain the origin of probabilistic behavior. Simply speaking, the randomness appears as a result of representing the wave function by using a detector of discrete nature that is found only in one state at a time, out of two or more possible states.
Physical theories assign values of quantities to physical systems, and they assign probabilities to the possible outcomes of measurements of those quantities. In quantum theory, probabilities are assigned using the Born rule. Here, I show that the Born rule is uniquely determined by the conditions under which quantum systems instantiate values of quantities as they are specified by the eigenstate-eigenvalue link, together with minimal theory-independent assumptions about how probabilities are sensitive to the structure of physical quantities.