arXiv subjects

Taeseung Choi

Publications and source records attributed to Taeseung Choi.

At least 19 recordsLinked to original sources

arXiv2026

Energy-momentum tensor from diffeomorphism invariance in classical electrodynamics

We reexamine the energy-momentum tensor in classical electrodynamics from the perspective of spacetime-dependent translations, i.e., diffeomorphism invariance in flat spacetime. When energy-momentum is identified through local translations rather than constant ones, a unique, symmetric, and gauge-invariant energy-momentum tensor emerges that satisfies a genuine off shell Noether identity without invoking the equations of motion. For the free electromagnetic field, this tensor coincides with the familiar Belinfante-Rosenfeld and Bessel-Hagen expressions, but arises here directly from spacetime-dependent translation symmetry rather than from improvement procedures or compensating gauge transformations. In interacting classical electrodynamics, comprising a point charge coupled to the electromagnetic field, diffeomorphism invariance yields well-defined energy-momentum tensors for the field and the particle, while the interaction term itself generates no independent local energy-momentum tensor. Its role is instead entirely encoded in the coupled equations of motion governing energy-momentum exchange, thereby resolving ambiguities in energy-momentum localization present in canonical and improvement-based approaches.

hep-th↗

arXiv2025

Probability Density in Relativistic Quantum Mechanics

In the realm of relativistic quantum mechanics, we address a fundamental question: Which one, between the Dirac or the Foldy-Wouthuysen density, accurately provide a probability density for finding a massive particle with spin $1/2$ at a certain position and time. Recently, concerns about the Dirac density's validity have arisen due to the Zitterbewegung phenomenon, characterized by a peculiar fast-oscillating solution of the coordinate operator that disrupts the classical relation among velocity, momentum, and energy. To explore this, we applied Newton and Wigner's method to define proper position operators and their eigenstates in both representations, identifying 'localized states' orthogonal to their spatially displaced counterparts. Our analysis shows that both densities could represent the probability of locating a particle within a few Compton wavelengths. However, a critical analysis of Lorentz transformation properties reveals that only the Dirac density meets all essential physical criteria for a relativistic probability density. These criteria include covariance of the position eigenstate, adherence to a continuity equation, and Lorentz invariance of the probability of finding a particle. Our results provide a clear and consistent interpretation of the probability density for a massive spin-$1/2$ particle in relativistic quantum mechanics.

quant-ph↗

arXiv2024

Lorentz-covariance of Position Operator and its Eigenstates for a massive spin $1/2$ field

We present a derivation of a position operator for a massive field with spin $1/2$, expressed in a representation-independent form of the Poincaré group. Using the recently derived Lorentz-covariant field spin operator, we obtain a corresponding field position operator through the total angular momentum formula. Acting on the Dirac spinor representation, the eigenvalues of the field position operator correspond to the spatial components of the Lorentz-covariant space-time coordinate $4$-vector. We show that the field position operator preserves the particle and the antiparticle character of the states. Thus, the field position operator can serve as a one-particle position operator for both particles and antiparticles, thereby avoiding an unusual fast-oscillating term, known as the Zitterbewegung, associated with the Dirac position operator. We show that the field position operator yields the same velocity as a classical free particle. The eigenstates of the field position operator satisfy the Newton-Wigner locality criteria and transform in a Lorentz-covariant manner. The field position operator becomes particle position and antiparticle position operators when acting on the particle and the antiparticle subspaces, both of which are Hermitian. Additionally, we demonstrate that within the particle subspace of the Dirac spinor space, the field position operator is equivalent to the Newton-Wigner position operator.

quant-ph↗

arXiv2023

Lorentz-Covariant Spin Operator for Spin 1/2 Massive Fields As a Physical Observable

We derive a relativistic-covariant spin operator for massive case directly from space-time symmetry in Minkowski space-time and investigate the physical properties of a derived spin operator. In the derivation we require only two conditions: First, a spin operator should be the generator of the SU(2) little group of the Poincare group. Second, a spin operator should covariantly transform under the Lorentz transformation. A space inversion transformation is shown to play a role to derive a unique relativistic-covariant spin operator, we call the field spin operator, whose eigenvalue labels the spin of a massive (classical) field that provides the irreducible representation space of the Poincare group. The field spin becomes the covariant spin in the covariant Dirac representation, which is shown to be the only spin that describes the Wigner rotation properly in the covariant Dirac representation. Surprisingly, the field spin also gives the non-covariant spin, which is the FW spin for the positive energy state. We also show that the field spin operator is the unique spin operator that generate the (internal) SU(2) little group transformation of the Poincare group properly.

quant-ph↗

arXiv2020

Proper relativistic position operators in 1+1 and 2+1 dimensions

We have revisited the Dirac theory in 1+1 and 2+1 dimensions by using the covariant representation of the parity-extended Poincaré group in their native dimensions. The parity operator plays a crucial role in deriving wave equations in both theories. We studied two position operators, a canonical one and a covariant one that becomes the particle position operator projected onto the particle subspace. In 1+1 dimensions the particle position operator, not the canonical position operator, provides the conserved Lorentz generator. The mass moment defined by the canonical position operator needs an additional unphysical spin-like operator to become the conserved Lorentz generator in 1+1 dimensions. In 2+1 dimensions, the sum of the orbital angular momentum given by the canonical position operator and the spin angular momentum becomes a constant of motion. However, orbital and spin angular momentum do not conserve separately. On the other hand the orbital angular momentum given by the particle position operator and its corresponding spin angular momentum become a constant of motion separately.

quant-ph↗

arXiv2020

Spin operators and representations of the Poincaré group

We present the rigorous derivation of covariant spin operators from a general linear combination of the components of the Pauli-Lubanski vector. It is shown that only two spin operators satisfy the spin algebra and transform properly under the Lorentz transformation, which admit the two inequivalent finite-dimensional representations for the Lorentz generators through the complexification of the $SU(2)$ group. In case that the Poincaré group is extended by parity operation, the spin operator in the direct sum representation of the two inequivalent representations, called the new spin distinguished from the Dirac spin, is shown to be equivalent to axial and Hermitian spin operators for particle and antiparticle. We have shown that for spin $1/2$, the Noether conserved current for a rotation can be divided into separately conserved orbital and spin part for the new spin, unlike for the Dirac spin. This implies that the new spin not the Dirac spin provides good quantum observables.

physics.gen-ph↗

arXiv2019

Singularity of relativistic vortex beam and proper relativistic observables

We have studied the phase singularity of the relativistic vortex beams for the two sets of relativistic operators. One includes the new spin and orbital angular momentum (OAM) operators, which is derived from the parity-extended Poincaré group, and the other is composed of the (usual) Dirac spin and OAM operators. The first set predicts the same singular circulation as the nonrelativistic vortex beams. On the other hand, the second set anticipates that the singularity of the circulation is spin orientation-dependent and can be disappeared especially for relativistic paraxial electron beam with spin parallel to the propagating direction. These contradistinctive predictions suggest the relativistic electron beam experiment with spin-polarized electrons for the first time to answer the long-standing fundamental question, i.e., what are the proper relativistic observables, raised from the beginning of relativistic quantum mechanics since the discovery of the Dirac equation.

quant-ph↗

arXiv2016

Spin Operators for Massive Particles

Since the discovery a century ago, spin describing the intrinsic angular momentum of massive elementary particles has exposed its nature and significant roles in wide ranges of (relativistic) quantum phenomena and practical applications for future quantum technology. Emerging inconsistencies have also disclosed its telltale incomplete description. Finding relativistic spins (operators) of massive particles is a long-standing fundamental problem from the beginning of relativistic quantum mechanics. Here we present the rigorous derivation and the representation of spin operators from the spacetime symmetry. The covariant parity operation, defined by the spin operators, naturally leads to a fundamental equation equivalent to the covariant Dirac equation, which manifests existent relativistic spins. Proper understanding position operator in the Dirac theory on account of the spin operator through total angular momentum predicts no Zitterbewegung as well as conserving orbital and spin currents. The spin operators can be applicable for unraveling the inconsistencies and for exploring unveiled physics of massive particles.

quant-ph↗

arXiv2016

Classical understanding of the electron vortex beams in a uniform magnetic field

Recently interesting observations on electron vortex beams, which have angular momentum about the center of the vortex beams, have been made. We have shown that the basic features of the electron vortex beams in a uniform magnetic field are understandable by using the classical motions of electrons. We have constructed a classical vortex-like motion by the collective motion of individual electrons in their cyclotron motions with a constant canonical angular momentum in the symmetric gauge, which models electron vortex beams, in a uniform magnetic field. With this model the various properties of circulating currents and the relation between energy and kinetic angular momentum in the electron vortex beams are well explained. We have also shown that the mismatch between the centers of the electron vortex beam and the classical cyclotron orbits naturally induces the parallel axis theorem and also the time-varying kinetic angular momentum of the electron vortex beam for certain distributions of classical electrons.

quant-ph↗

arXiv2015

Quantum Probability assignment limited by relativistic causality

The quantum nonlocality is limited by relativistic causality, however, the reason is not fully understood yet. The relativistic causality condition on nonlocal correlations has been usually accepted as a prohibition of faster-than-light signaling, called no-signaling condition. We propose another causality condition from the observation that space-like separate events should have no causal relationship. It is proved that the new condition is stronger than no-signaling condition for a pair of binary devices. We derive the standard probability assignment rule, so-called Born rule, on quantum measurement, which determines the degree of quantum nonlocality, by using relativistic causality constraint. This shows how the causality limits the upper bound of quantum nonlocality through quantum probability assignment.

quant-ph↗

arXiv2014

Newton-Wigner position operator and its corresponding spin operator in relativistic quantum mechanics

A relativistic spin operator is to be the difference between the total and orbital angular momentum. As the unique position operator for a localized state, the remarkable Newton-Wigner position operator, which has all desirable commutation relations as a position operator, can give a proper spin operator. Historically important three spin operators respectively proposed by Bogolubov et al., Pryce, and Foldy-Woutheysen are investigated to manifest a corresponding spin operator to the Newton-Wigner position operator. We clarify a unique spin operator in relativistic quantum mechanics described by the Dirac Hamiltonian.

quant-ph↗

arXiv2013

Comment on "Aharonov-Casher and Scalar Aharonov-Bohm Topological Effects"

In this Comment we point out (i) that the Hamiltonian, Eq. (17) in the Letter(Phys. Rev. Lett. 108, 070405 (2012)), is not a relativistic Hamiltonian, (ii) then that the conditions in the Letter are irrelevant for a topological AC and SAB effects, and (iii) conclusively that the non-relativistic Hamiltonian employed by Peshkin and Lipkin (Phys. Rev. Lett. 74, 2847 (1995)) has the same $U(1)_{mm}$ gauge structure for a fixed spin and then is not wrong, but their incorrect interpretation of the spin autocorrelations led to the incorrect conclusion.

cond-mat.mes-hall↗

arXiv2013

Relativistic Spin and Dirac Spin in relativistically covariant Stern-Gerlach Experiment

We have studied a relativistically covariant Stern-Gerlach (SG) experiment for a relativistic spin and a Dirac spin. We have obtained the relativistic spin in an arbitrary frame by using the classical spin dipole tensor, which gives the covariant spin dipole interactions, and the relation between a spin and a spin magnetic dipole moment. The relativistic spin is shown to have problems to become a proper spin operator for a massive relativistic particle because of two reasons. First, the relativistic spin three-vector operators cannot satisfy the spin algebra. Second, the SG experiment for the relativistic particle provides a paradox between two observers in the particle rest frame and the laboratory frame, in which the particle is moving. We have shown that the paradox in the SG experiment is resolved by the Dirac spin, which is covariantly defined by a Lorentz transformation in the Dirac spinor representation. The Dirac spin three-vector operators satisfy the spin algebra. It is shown that the SG experiment for the Dirac spin in the inertial frame, where there is only magnetic field, can determine the spin without the information of the momentum of the particle. This shows that the reduced spin density matrix for the Dirac particle can be well-defined by integrating out the momentum degrees of freedom.

quant-ph↗

arXiv2013

Relativistic spin operator and Lorentz transformation of spin state of a massive Dirac particle

We have shown the covariant relativistic spin operator is equivalent to the spin operator commuting with the free Dirac Hamiltonian. This implies that the covariant relativistic spin operator is a good quantum observable. The covariant relativistic spin operator has the pure quantum contribution which does not exist in the classical covariant spin operator. Based on this equivalence reduced spin states can be claerly defined. We have shown the change in the entropy of a reduced spin density matrix sweeps through the whole range according to the relative motion of an observer.

quant-ph↗

arXiv2012

Limits on Quantum Probability Rule by no-Signaling Principle

We have studied the possibility of post-quantum theories more nonlocal than the (standard) quantum theory using the modification of the quantum probability rule under the no-signaling condition. For this purpose we have considered the situation that two spacelike separate parties Alice and Bob share an entangled two qubit system. We have modified the quantum probability rule as small as possible such that the first local measurements are governed by the usual Born rule and the second measurement by the modified quantum probability rule. We have shown that only the maximally entangled states can have higher nonlocality than the quantum upper bound while satisfying the no-signaling condition. This fact could be a partial explanation for why the nonlocality of the quantum theory is limited. As a by-product we have found the systematic way to obtain a variety of nonlocal boxes.

quant-ph↗

arXiv2010

Mean spin entanglement of two massive Dirac particles under Lorentz transformations

We have studied the relativistic effects on the mean spin entanglement of two massive Dirac particles using the simultaneous eigen-spinors of the Foldy-Woutheysen mean spin operator and the Dirac Hamiltonian. We have obtained the transformation matrix from the spinor with specific momentum to the spinor with a transformed momentum under an arbitrary Lorentz transformation. Using the transformation matrix we have shown the consistent monotonic behavior between the concurrence and the maximum value of Bell parameter in Bell inequality of transformed spin states.

quant-ph↗

arXiv2009

Generalized Faraday law derived from classical forces in a rotating frame

We show the additional spin dependent classical force due to the rotation of an electron spin's rest frame is essential to derive a spin-Faraday law by using an analogy with the usual Faraday law. The contribution of the additional spin dependent force to the spin-Faraday law is the same as that of the spin geometric phase. With this observations, Faraday law is generalized to include both the usual Faraday and the spin-Faraday laws in a unified manner.

quant-ph↗

arXiv2008

Entanglement and Berry Phase in Two Interacting Qubits

Entanglement and Berry phase are investigated in two interacting qubit systems. The XXZ spin interaction model with a slowly rotating magnetic field is employed for the interaction between the two qubits. We show how the anisotropy of interaction reveals unique relations between the Berry phases and the entanglements for the eigenstates of the system.

quant-ph↗