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Physical Review A

Test of local causality with very short light pulses

Karl Svozil

Published Feb 1, 1989• 4 citations
Publisher

Abstract

It is explicitly shown that the nonvanishing spacelike or timelike contributions to the Feynman propagator of quantum-field theory do not reflect any violation of local causality, such as propagation faster than light. An experiment is proposed to test the causality of quantum-field theory by measuring the broadening of a very short light pulse. For these pulses theory predicts a nondissipative behavior, with the pulse broadening only stemming from preparation and measurement uncertainties.

DOI
10.1103/physreva.39.2222

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Comment on ‘‘Test of local causality with very short light pulses’’

Morton H. Rubin

It is shown in this Comment that the claim of Svozil that his paper [Phys. Rev. A 39, 2222 (1989)] demonstrates that nonvanishing spacelike and timelike contributions to the Feynman propagator do not lead to a violation of microscopic causality as claimed in the work of Hegerfeldt [Phys. Rev. D 10, 3320 (1974); Phys Rev. Lett. 54, 2395 (1985)] and Rubin [Phys. Rev. D 35, 3836 (1987)] is incorrect, and that his calculation is irrelevant to the issues discussed by these authors.

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Reply to ‘‘Comment on ‘Test of local causality with very short light pulses’ ’’

Karl Svozil

A clarification concerning the solution of the initial-value problem for the propagation of light pulses in dispersive media is given. In a proper interpretation, this solution corresponds to two single pulses traveling in opposite directions. Furthermore, a critical review of important research related to Rubin's arguments but not mentioned by him in the preceding Comment [Phys. Rev. A 41, 1727 (1990)] is presented.

Physical Review A•Feb 1, 1990•1 citation
It is explicitly shown that the nonvanishing spacelike or timelike contributions to the Feynman propagator of quantum-field theory do not reflect any violation of local causality, such as propagation faster than light. An experiment is proposed to test the causality of quantum-field theory by measuring the broadening of a very short light pulse. For these pulses theory predicts a nondissipative behavior, with the pulse broadening only stemming from preparation and measurement uncertainties.
We analyze potential violations of causality in Unruh DeWitt-type detector models in relativistic quantum information. We proceed by first studying the relation between faster-than-light signaling and the causal factorization of the dynamics for multiple detector-field interactions. We show in what way spatially extended nonrelativistic detector models predict superluminal propagation of the field's initial conditions. We draw parallels between this characteristic of detector models, stemming from their nonrelativistic dynamics, and Sorkin's ``impossible measurements on quantum fields'' [R. D. Sorkin, in Directions in General Relativity: Proceedings of the 1993 International Symposium, Maryland (Cambridge University Press, 1993), Vol. 2, pp. 293--305.]. Based on these features, we discuss the validity of measurements in quantum field theory when performed with nonrelativistic particle detectors.
Quantum mechanics predicts correlations between measurements performed in distant regions of a spatially spread entangled state to be higher than allowed by intuitive concepts of Locality and Realism. These high correlations forbid the use of nonlinear operators of evolution (which would be desirable for several reasons), for they may allow faster-than-light signaling. As a way out of this situation, it has been hypothesized that the high quantum correlations develop only after a time longer than L/c has elapsed (where L is the spread of the entangled state and c is the velocity of light). In shorter times, correlations compatible with Locality and Realism would be observed instead. A simple hidden variables model following this hypothesis is described. It is based on a modified Wheeler–Feynman theory of radiation. This hypothesis has not been disproved by any of the experiments performed to date. A test achievable with accessible means is proposed and described. It involves a pulsed source of entangled states and stroboscopic record of particle detection during the pulses. Data recorded in similar but incomplete optical experiments are analyzed, and found consistent with the proposed model. However, it is not claimed, in any sense, that the hypothesis has been validated. On the contrary, it is stressed that a complete, specific test is absolutely needed.
Explaining observations in terms of cause-and-effect (causal) relations is central to empirical science. However, in his seminal 1964 work [1], John Bell showed that the correlations between entangled quantum systems cannot be explained in terms of, possibly hidden, cause and effect relations. After four decades of experimental effort, Bell's prediction has recently been confirmed in an unambiguous way by a series of loophole-free Bell-test experiments [2-4]. These experiments, however, cannot reveal where exactly quantum mechanics breaks with our classical notion of cause and effect. Specifically, Bell's theorem is based on the central assumptions of realism and local causality. Realism posits that physical systems have real, objective properties, independent of whether we observe them or not, and local causality assumes that causal influences cannot propagate faster than the speed of light.
Causal quantum theory is an umbrella term for ordinary quantum theory modified by two hypotheses: state vector reduction is a well-defined process, and strict local causality applies. The first of these holds in some versions of Copenhagen quantum theory and need not necessarily imply practically testable deviations from ordinary quantum theory. The second implies that measurement events which are spacelike separated have no nonlocal correlations. To test this prediction, which sharply differs from standard quantum theory, requires a precise definition of state vector reduction. Formally speaking, any precise version of causal quantum theory defines a local hidden variable theory. However, causal quantum theory is most naturally seen as a variant of standard quantum theory. For that reason it seems a more serious rival to standard quantum theory than local hidden variable models relying on the locality or detector efficiency loopholes. Some plausible versions of causal quantum theory are not refuted by any Bell experiments to date, nor is it evident that they are inconsistent with other experiments. They evade refutation via a neglected loophole in Bell experiments---the collapse locality loophole---which exists because of the possible time lag between a particle entering a measurement device and a collapse taking place. Fairly definitive tests of causal versus standard quantum theory could be made by observing entangled particles separated by $\ensuremath{\approx}0.1$ light seconds.