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arXiv:2511.15935

Relativistic Covariance and Nonlinear Quantum Mechanics: Tomonaga-Schwinger Analysis

Stephen D. H. Hsu

hep-thquant-phSubmitted Nov 19, 2025• Announced Nov 21, 2025• 0 citations
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Abstract

We use the Tomonaga-Schwinger (TS) formulation of quantum field theory to determine when state-dependent additions to the local Hamiltonian density (i.e., modifications to linear Schrodinger evolution) violate relativistic covariance. We derive new operator integrability conditions required for foliation independence, including the Frechet derivative terms that arise from state-dependence. Nonlinear modifications of quantum mechanics affect operator relations at spacelike separation, leading to violation of the integrability conditions.

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5 pages

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We use the Tomonaga-Schwinger (TS) formulation of quantum field theory to determine when state-dependent additions to the local Hamiltonian density (i.e., modifications to linear Schrodinger evolution) violate relativistic covariance. We derive new operator integrability conditions required for foliation independence, including the Frechet derivative terms that arise from state-dependence. Nonlinear modifications of quantum mechanics affect operator relations at spacelike separation, leading to violation of the integrability conditions.
Nonlinear modifications of quantum mechanics generically lead to nonlocal effects which violate relativistic causality. We study these effects using the functional Schrödinger equation for quantum fields and identify a type of nonlocality which causes nearly instantaneous entanglement of spacelike separated systems. We describe a simple example involving widely separated wave packet (coherent) states, showing that nonlinearity in the Schrödinger evolution causes spacelike entanglement, even in free field theory.
Covariant classical particle dynamics is described, and the associated covariant relativistic particle quantum mechanics is derived. The invariant symmetric bracket is defined on the space of quantum amplitudes, and its relation to a generalized Hamiltonian dynamics and to a covariant Schrödinger type equation is shown. Examples for relativistic potential problems are solved. Mathematically and physically acceptable probability densities for the Klein-Gordon equation and for the Dirac equation are derived, and a new continuity equation for each case is given. The quantum distribution for mass is discussed, and unambiguous representations of four-velocity and four-acceleration operators are given.
Abstract This chapter develops the theory of measurements in relativistic quantum mechanics. The presentation is based on the idea to consider the state of a relativistic quantum system as a functional on the set of space-like hypersurfaces in Minkowski space. This idea leads to a manifest covariant time-evolution equation, known as Schwinger–Tomonaga equation, and allows a relativistic formulation of the projection postulate of quantum measurement theory. This postulate is used to study a number of applications to local and non-local measurements, to the erasing of local information, and to the restrictions on the measurability of non-local quantities imposed by the causality principle. The relativistic projection postulate further enables the discussion of many important problems from a unified perspective, such as EPR-type experiments, measurements of Bell state operators, exchange measurements, and quantum teleportation.
The conventional phase space of classical physics treats space and time differently, and this difference carries over to field theories and quantum mechanics (QM). In this paper, the phase space is enhanced through two main extensions. First, we promote the time choice of the Legendre transform to a dynamical variable. Second, we extend the Poisson brackets of matter fields to a spacetime symmetric form. The ensuing ``spacetime phase space'' is employed to obtain an explicitly covariant version of Hamilton equations for relativistic field theories. A canonical-like quantization of the formalism is then presented in which the fields satisfy spacetime commutation relations and the foliation is quantum. In this approach, the classical action is also promoted to an operator and retains explicit covariance through its nonseparability in the matter-foliation partition. The problem of establishing a correspondence between the new noncausal framework (where fields at different times are independent) and conventional QM is solved through a generalization of spacelike correlators to spacetime. In this generalization, the Hamiltonian is replaced by the action, and conventional particles by off-shell particles. When the foliation is quantized, the previous map is recovered by conditioning on foliation eigenstates, in analogy with the Page and Wootters mechanism. We also provide an interpretation of the correspondence in which the causal structure of a given theory emerges from the quantum correlations between the system and an environment. This idea holds for general quantum systems and allows one to generalize the density matrix to an operator containing the information of correlators both in space and time.
We show that the Tomonaga-Schwinger formalism in quantum field theory provides a manifestly Lorentz covariant description of an Einstein-Podolsky-Rosen (EPR) correlated state defined on a curved-space-like surface. This avoids the notion of a universal time and clearly demarcates between the completion of the measuring process on a member of an EPR pair and its nonlocal effect on the state of its partner. Being reciprocal and restricted to a spacelike surface, the latter is deterministic but not causal and is consistent with Lorentz invariance.