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arXiv:quant-ph/0003075 · Physical Review A

Causality, delocalization, and positivity of energy

Evgueni Karpov, Gonzalo E. Ordonez, Tomio Yamakoshi Petrosky, Ilya Prigogine, G. P. Pronko

Published Jun 8, 2000• 37 citations
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Abstract

In a series of interesting papers Hegerfeldt has shown that quantum systems with positive energy initially localized in a finite region immediately develop infinite tails. In our paper Hegerfeldt's theorem is analyzed using quantum and classical wave packets. We show that Hegerfeldt's conclusion remains valid in classical physics. No violation of Einstein's causality is ever involved. Using only positive frequencies, complex wave packets are constructed which at $t=0$ are real and finitely localized and which, furthermore, are superpositions of two nonlocal wave packets. The nonlocality is initially cancelled by destructive interference. However, this cancellation becomes incomplete at arbitrary times immediately afterwards. In agreement with relativity the two nonlocal wave packets move with the velocity of light, in opposite directions.

DOI
10.1103/physreva.62.012103

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As shown by Hegerfeldt, positivity of energy in quantum mechanics leads to nonlocal effects, where a state that is initially localized immediately develops infinite tails. In a previous paper we have described the mechanism of such effects for relativistic free fields. In this paper we discuss the nonlocal effects for a more complex system that consists of a two-level atom interacting with a radiation field. We study the dressing process of the bare ground state in terms of the photon density. In this process there appears a nonlocal cloud that remains around the atom and ``Zeno'' photons that move away from the atom with the speed of light. Nonlocality is seen to be to due to a ``curtain'' effect where the cloud is initially covered by the Zeno photons, and immediately afterwards is uncovered as the photons move away from the atom. No superluminal speeds are involved. We also consider nonlocal effects associated with a partially dressed state where the atom is excited starting from the true ground state. The excitation of the atom produces a nonlocal rearrangement of the photon cloud, which leads to precursors to the usual retarded field.

Physical Review A•Sep 15, 2000•26 citations

Quantum time and spatial localization: An analysis of the Hegerfeldt paradox

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Two related problems in relativistic quantum mechanics, the apparent superluminal propagation of initially localized particles and dependence of spatial localization on the motion of the observer, are analyzed in the context of Dirac’s theory of constraints. A parametrization invariant formulation is obtained by introducing time and energy operators for the relativistic particle and then treating the Klein–Gordon equation as a constraint. The standard, physical Hilbert space is recovered, via integration over proper time, from an augmented Hilbert space wherein time and energy are dynamical variables. It is shown that the Newton–Wigner position operator, being in this description a constant of motion, acts on states in the augmented space. States with strictly positive energy are nonlocal in time; consequently, position measurements receive contributions from states representing the particle’s position at many times. Apparent superluminal propagation is explained by noting that, as the particle is potentially in the past (or future) of the assumed initial place and time of localization, it has time to propagate to distant regions without exceeding the speed of light. An inequality is proven showing the Hegerfeldt paradox to be completely accounted for by the hypotheses of subluminal propagation from a set of initial space–time points determined by the quantum time distribution arising from the positivity of the system’s energy. Spatial localization can nevertheless occur through quantum interference between states representing the particle at different times. The nonlocality of the same system to a moving observer is due to Lorentz rotation of spatial axes out of the interference minimum.

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Causal localizations describe the position of quantum systems moving not faster than light. They are constructed for the systems with finite spinor dimension. At the center of interest are the massive relativistic systems. For every positive mass, there is the sequence of Dirac tensor-localizations, which provides a complete set of inequivalent irreducible causal localizations. They obey the principle of special relativity and are fully Poincaré covariant. The boosters are determined by the causal position operator and the other Poincaré generators. The localization with minimal spinor dimension is the Dirac localization. Thus, the Dirac equation is derived here as a mere consequence of the principle of causality. Moreover, the higher tensor-localizations, not known so far, follow from Dirac’s localization by a simple construction. The probability of localization for positive energy states results to be described by causal positive operator valued (PO-) localizations, which are the traces of the causal localizations on the subspaces of positive energy. These causal Poincaré covariant PO-localizations for every irreducible massive relativistic system were, all the more, not known before. They are shown to be separated. Hence, the positive energy systems can be localized within every open region by a suitable preparation as accurately as desired. Finally, the attempt is made to provide an interpretation of the PO-localization operators within the frame of conventional quantum mechanics attributing an important role to the negative energy states.

Journal of Mathematical Physics•Jul 1, 2015•10 citations
In a series of interesting papers Hegerfeldt has shown that quantum systems with positive energy initially localized in a finite region immediately develop infinite tails. In our paper Hegerfeldt's theorem is analyzed using quantum and classical wave packets. We show that Hegerfeldt's conclusion remains valid in classical physics. No violation of Einstein's causality is ever involved. Using only positive frequencies, complex wave packets are constructed which at t=0t=0t=0 are real and finitely localized and which, furthermore, are superpositions of two nonlocal wave packets. The nonlocality is initially cancelled by destructive interference. However, this cancellation becomes incomplete at arbitrary times immediately afterwards. In agreement with relativity the two nonlocal wave packets move with the velocity of light, in opposite directions.
As shown by Hegerfeldt, positivity of energy in quantum mechanics leads to nonlocal effects, where a state that is initially localized immediately develops infinite tails. In a previous paper we have described the mechanism of such effects for relativistic free fields. In this paper we discuss the nonlocal effects for a more complex system that consists of a two-level atom interacting with a radiation field. We study the dressing process of the bare ground state in terms of the photon density. In this process there appears a nonlocal cloud that remains around the atom and ``Zeno'' photons that move away from the atom with the speed of light. Nonlocality is seen to be to due to a ``curtain'' effect where the cloud is initially covered by the Zeno photons, and immediately afterwards is uncovered as the photons move away from the atom. No superluminal speeds are involved. We also consider nonlocal effects associated with a partially dressed state where the atom is excited starting from the true ground state. The excitation of the atom produces a nonlocal rearrangement of the photon cloud, which leads to precursors to the usual retarded field.
Two related problems in relativistic quantum mechanics, the apparent superluminal propagation of initially localized particles and dependence of spatial localization on the motion of the observer, are analyzed in the context of Dirac’s theory of constraints. A parametrization invariant formulation is obtained by introducing time and energy operators for the relativistic particle and then treating the Klein–Gordon equation as a constraint. The standard, physical Hilbert space is recovered, via integration over proper time, from an augmented Hilbert space wherein time and energy are dynamical variables. It is shown that the Newton–Wigner position operator, being in this description a constant of motion, acts on states in the augmented space. States with strictly positive energy are nonlocal in time; consequently, position measurements receive contributions from states representing the particle’s position at many times. Apparent superluminal propagation is explained by noting that, as the particle is potentially in the past (or future) of the assumed initial place and time of localization, it has time to propagate to distant regions without exceeding the speed of light. An inequality is proven showing the Hegerfeldt paradox to be completely accounted for by the hypotheses of subluminal propagation from a set of initial space–time points determined by the quantum time distribution arising from the positivity of the system’s energy. Spatial localization can nevertheless occur through quantum interference between states representing the particle at different times. The nonlocality of the same system to a moving observer is due to Lorentz rotation of spatial axes out of the interference minimum.
Whether the quantum mechanics (QM) is non-local is an issue disputed for a long time. The violation of the Bell-type inequalities was considered as proving this non-locality. However, these inequalities are constructed on a class of local hidden variables, which obey the calculus with positive probabilities. Such a calculus is rather suitable for billiard balls while the QM deals with wave-packets of complex amplitudes. There is no wonder that a calculus with positive numbers does not match a calculus with complex numbers. The present text describes a different model of hidden variables for entanglements, model that reproduces the quantum predictions in different experiments, and also explains why the QM is nonlocal. The model deals with waves, some of them full and the others empty, and the hidden variables mark which waves are full. The basic physical concept with which the model operates is joint amplitudes of probability, and not probabilities. The latter are a secondary concept, the probability of a combination of results being equal to the absolute square of sum of all the contributing joint amplitudes. Thus the non-locality appears: a) a joint amplitude ignores distance, it handles distant particles as if they were one single particle at one single place, b) joint amplitudes are complex numbers and the sum of several contributions may vanish, blocking the respective combination of wave-packets and therefore of results. Although showing the success of the model, this text does not advocate for full/empty waves. It is shown that this hypothesis works only as long as one does not consider moving observers, and does not compare their conclusions. The real purpose here is to point to a severe impasse: assuming the existence of a preferred frame contradicts the theory of relativity, while refuting the full/empty waves idea one runs into other insurmountable difficulties.
The wave-particle duality introduced by de Broglie and developed mathematically by Schrödinger remains one of the deepest conceptual achievements of modern physics. Yet Schrödinger's original physical intuition contained a problem that has never entirely disappeared: if a particle is represented by a wave, how can an extended and generally dispersive wave structure correspond to the localized events observed in experiments? Schrödinger initially hoped that a particle could be understood as a localized wave packet formed by the coherent superposition of many elementary waves. The program was attractive because localization could then arise from interference rather than being imposed as an independent particle property. However, generic wave packets disperse under free evolution, while stable bound states of quantum systems are not generally localized packets moving along classical trajectories. The subsequent Born interpretation provided an extraordinarily successful connection between the wave function and experimental statistics, identifying ∣Ψ∣2|\Psi|^2 with a probability density. Nevertheless, this interpretation leaves open a distinct physical question: how does a spatially extended complex amplitude structure give rise to localized, discrete detection events? This paper revisits that question from the historical development of wave mechanics to the operational structure of early scattering experiments. Particular attention is given to the experiments of Davisson and Kunsman, Ramsauer's measurements of slow-electron scattering, and Dymond's measurements of the velocity and angular distribution of electrons scattered in helium. Although these experiments differ in physical arrangement and measured quantities, they share a common operational structure: an extended incident quantum state undergoes propagation and interaction, after which the experiment registers discrete events within a finite spatial, angular, or energy acceptance region. The central proposal of this paper is therefore not a new derivation of Born's rule, but a reformulation of the problem that precedes such a derivation. Instead of asking only why a quantum state has a probability distribution, one may ask how a propagating wave-amplitude structure is dynamically converted into a localized event rate. In symbolic form, Ψ⟶D⟶Γ,\Psi \quad \longrightarrow \quad \mathcal{D} \quad \longrightarrow \quad \Gamma, where Ψ\Psi denotes the propagating amplitude structure, D\mathcal{D} represents the interaction and detection dynamics, and Γ\Gamma denotes the resulting localized event rate. Born's rule then appears as the empirical relation Γ(r)∝∣Ψ(r)∣2,\Gamma(\mathbf r)\propto |\Psi(\mathbf r)|^2, whose dynamical origin remains to be established. The paper concludes by reconsidering Schrödinger's original wave intuition rather than simply declaring it defeated. The failure of the persistent localized wave packet may indicate not that wave ontology was fundamentally misguided, but that propagation and localization are physically distinct stages of quantum dynamics. This distinction provides a conceptual starting point for a subsequent series of investigations into the dynamical origin of quantum probability.
Causal localizations describe the position of quantum systems moving not faster than light. They are constructed for the systems with finite spinor dimension. At the center of interest are the massive relativistic systems. For every positive mass, there is the sequence of Dirac tensor-localizations, which provides a complete set of inequivalent irreducible causal localizations. They obey the principle of special relativity and are fully Poincaré covariant. The boosters are determined by the causal position operator and the other Poincaré generators. The localization with minimal spinor dimension is the Dirac localization. Thus, the Dirac equation is derived here as a mere consequence of the principle of causality. Moreover, the higher tensor-localizations, not known so far, follow from Dirac’s localization by a simple construction. The probability of localization for positive energy states results to be described by causal positive operator valued (PO-) localizations, which are the traces of the causal localizations on the subspaces of positive energy. These causal Poincaré covariant PO-localizations for every irreducible massive relativistic system were, all the more, not known before. They are shown to be separated. Hence, the positive energy systems can be localized within every open region by a suitable preparation as accurately as desired. Finally, the attempt is made to provide an interpretation of the PO-localization operators within the frame of conventional quantum mechanics attributing an important role to the negative energy states.