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arXiv:2611.03099

On physical origin of Berry connection of free relativistic electrons

Chun-Fang Li

quant-phSubmitted Nov 2, 2026• Announced Nov 3, 2026
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Abstract

No consensus has been reached on the appropriate operators for the spin and orbital angular momentum of free relativistic electrons. But there is a tendency to believe the existence of a spin-orbit coupling through the Berry connection. Here we show that this is not the case. Because the positive-energy solution of the Dirac equation is associated with the state of the electron, it is reasonable to postulate that only when projected onto the positive-energy subspace can observable operators on the Hilbert space of the Dirac equation represent physical quantities of the electron. On this basis, we find, after mapping the projected operators onto an irreducible representation, that the Berry connection describes the position of a quantum reference point with respect to the laboratory reference frame, relative to which the position of the electron is canonical. The orbital angular momentum of the quantum reference point about the origin of the laboratory reference frame is expressed in terms of the Berry connection. It combines with the canonical orbital angular momentum of the electron about the quantum reference point to give the orbital angular momentum of the electron about the origin of the laboratory reference frame.

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12 pages

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No consensus has been reached on the appropriate operators for the spin and orbital angular momentum of free relativistic electrons. But there is a tendency to believe the existence of a spin-orbit coupling through the Berry connection. Here we show that this is not the case. Because the positive-energy solution of the Dirac equation is associated with the state of the electron, it is reasonable to postulate that only when projected onto the positive-energy subspace can observable operators on the Hilbert space of the Dirac equation represent physical quantities of the electron. On this basis, we find, after mapping the projected operators onto an irreducible representation, that the Berry connection describes the position of a quantum reference point with respect to the laboratory reference frame, relative to which the position of the electron is canonical. The orbital angular momentum of the quantum reference point about the origin of the laboratory reference frame is expressed in terms of the Berry connection. It combines with the canonical orbital angular momentum of the electron about the quantum reference point to give the orbital angular momentum of the electron about the origin of the laboratory reference frame.
Motivated by recent interest in relativistic electron vortex states, we revisit the spin and orbital angular momentum properties of Dirac electrons. These are uniquely determined by the choice of the position operator for a relativistic electron. We consider two main approaches discussed in the literature: (i) the projection of operators onto the positive-energy subspace, which removes the Zitterbewegung effects and correctly describes spin-orbit interaction effects, and (ii) the use of Newton-Wigner-Foldy-Wouthuysen operators based on the inverse Foldy-Wouthuysen transformation. We argue that the first approach [previously described in application to Dirac vortex beams in K. Y. Bliokh et al., Phys. Rev. Lett. 107, 174802 (2011)] has a more natural physical interpretation, including spin-orbit interactions and a nonsingular zero-mass limit, than the second one [S. M. Barnett, Phys. Rev. Lett. 118, 114802 (2017)].
The problem of the position and spin in relativistic quantum mechanics is analyzed in detail. It is definitively shown that the position and spin operators in the Foldy-Wouthuysen representation (but not in the Dirac one) are quantum-mechanical counterparts of the classical position and spin variables. The probabilistic interpretation is valid only for Foldy-Wouthuysen wave functions. The relativistic spin operators are discussed. The spin-orbit interaction does not exist for a free particle if the conventional operators of the orbital angular momentum and the rest-frame spin are used. Alternative definitions of the orbital angular momentum and the spin are based on noncommutative geometry, do not satisfy standard commutation relations, and can allow the spin-orbit interaction.
Quantum-mechanical spin is often thought of in terms of classical angular momentum. In fact spin is defined by its commutation relations and the spin and orbital angular momentum operators are very different. Here we solve the Dirac equation in a rotating frame of reference and create a localized wave packet from the resulting wave functions to examine the consequences of the rotational motion. We highlight some unexpected effects in the properties of the wave packet and show that the spin operator and orbital angular momentum operator describe different aspects of the rotational properties of the wave packet. It is also observed that neither quantum-mechanical spin nor orbital angular momentum can be fully understood within an inertial frame.
A connection is established between the classical and wave mechanics of a spinning relativistic particle. In the case of a plane wave comprising both positive and negative energies, the 4-dimensional stream-lines of the current density 4-vector coincide with the world-lines of a classical particle. Moreover, the wave function represents the spinor (or quaternion) which determines the generalized intrinsic frame of reference attached to the world-line of the classical particle. This provides a geometrical interpretation for the unquantized Dirac field as well as for the total angular momentum.
A neo-classical relativistic mechanics theory is presented where the spin of an electron is a natural part of its space-time path as a point particle. The fourth-order equation of motion corresponds to the same Lagrangian function in proper time as in special relativity except for an additional spin energy term. The total motion can be decomposed into a sum of a local spin motion about a point and a global motion of this point, called the spin center. The global motion is sub-luminal and obeys Newton's second law in proper time, the time for a clock fixed at the spin center, while the total motion occurs at the speed of light c, consistent with the eigenvalues of Dirac's velocity operators having magnitude c. The local spin motion corresponds to Schrödinger's zitterbewegung and is a perpetual motion, which for a free electron has a circular path in the spin-center frame. In an electro-magnetic field, this spin motion generates magnetic and electric dipole energies through the Lorentz force on the electron's point charge. The corresponding electric dipole energy is consistent with the spin-orbit coupling term in the corrected Pauli non-relativistic Hamiltonian but the magnetic dipole energy is one half of that in Dirac's theory. By defining a spin tensor as the angular momentum of the electron's total motion about its spin center, the fundamental equations of motion can be re-written in an identical form to those of the Barut-Zanghi electron theory. These equations of motion can then be expressed using operators applied to a state function of proper time satisfying a Dirac-Schrödinger spinor equation. The operators produce dynamic variables without any probability implications. For the free electron, the state function satisfies Dirac's relativistic wave equation when the Lorentz transformation is applied to express proper time in terms of an observer's space-time coordinates.
Abstract Non-inertial physics is seldom considered in quantum mechanics and this contrasts with the ubiquity of non-inertial reference frames. Here, we show an application to the Dirac oscillator which provides a fundamental model of relativistic quantum mechanics. The model emerges from a term linearly dependent on spatial coordinates added to the momentum of the free-particle Dirac Hamiltonian. The definition generates peculiar features (mutating vacuum energy, non-Hermitian momentum, accidental degeneracies of the spectrum, etc). We interpret these anomalies in terms of inertial effects. The demonstration is based on the decoupling of the Dirac equation from the stereographic projection that maps the 3D geometry of the dynamical problem to the complex plane. The decoupling shows that the fundamental mechanical model underpinning the Dirac oscillator reduces to the representation of the oscillator in the rotating reference frame attached to the orbital angular momentum. The resulting Coriolis-like contribution to the Hamiltonian accounts for the peculiarities of the model (mutating vacuum energy, form of the non-minimal correction to the momentum, classical intrinsic spin and gain of its quantum value, accidental degeneracies of the energy spectrum, supersymmetric potential). The suggested interpretation has an interdisciplinary character where stereographic geometry, classical physics of the Coriolis effect and quantum physics of Dirac particles contribute to the definition of one of the few exactly soluble models of relativistic quantum mechanics.