---
abstract: 'We compute, for massive particles, the explicit Wigner rotations of one-particle states for arbitrary Lorentz transformations; and the explicit Hermitian generators of the infinite-dimensional unitary representation. For a pair of spin 1/2 particles, Einstein-Podolsky-Rosen-Bell entangled states and their behaviour under the Lorentz group are analysed in the context of quantum field theory. Group theoretical considerations suggest a convenient definition of the Bell states which is slightly different from the conventional assignment. The behaviour of Bell states under arbitrary Lorentz transformations can then be described succinctly. Reduced density matrices applicable to systems of identical particles are defined through Yang’s prescription. The von Neumann entropy of each of the reduced density matrix is Lorentz invariant; and its relevance as a measure of entanglement is discussed, and illustrated with an explicit example. A regularization of the entropy in terms of generalized zeta functions is also suggested.'
author:
- Chopin Soo
- 'Cyrus C. Y. Lin'
title: |
    Wigner Rotations, Bell States, and Lorentz Invariance\
    of Entanglement and von Neumann Entropy
---

Introduction and Overview
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It can be argued that, aside from theories with infinite number of particle types such as string theory, quantum [*field*]{} theory is the only way to 