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arXiv:2404.09205

Apparent violation of causality in relativistic quantum mechanics

Iwo Białynicki‐Birula, Zofia Białynicka-Birula

Published Apr 14, 2024• 0 citations
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Abstract

In relativistic theories the principle of microscopic causality states that ``information cannot travel faster than the speed of light'' \cite{kaku}. In the present work we show that the time evolution of relativistic wave functions violates this principle. We consider here the wave functions of massless and massive particles. In the case of massless particles the wave functions which violate the microscopic causality have an analytic form while in the case of massive particles we have to rely on numerical calculations. In both cases the wave functions which are strictly localized at $t=0$, at later times do not vanish {\it outside} the future light cone. \end{abstract}

DOI
10.48550/arxiv.2404.09205

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In relativistic theories the principle of microscopic causality states that ``information cannot travel faster than the speed of light'' \cite{kaku}. In the present work we show that the time evolution of relativistic wave functions violates this principle. We consider here the wave functions of massless and massive particles. In the case of massless particles the wave functions which violate the microscopic causality have an analytic form while in the case of massive particles we have to rely on numerical calculations. In both cases the wave functions which are strictly localized at t=0t=0t=0, at later times do not vanish {\it outside} the future light cone. \end{abstract}
We compare the relativistic time evolution of an initially localized quantum particle obtained from the relativistic Schr\"odinger, the Klein-Gordon and the Dirac equations. By computing the amount of the spatial probability density that evolves outside the light cone we quantify the amount of causality violation for the relativistic Schr\"odinger Hamiltonian. We comment on the relationship between quantum field theoretical transition amplitudes, commutators of the fields and their bilinear combinations outside the light cone as indicators of a possible causality violation. We point out the relevance of the relativistic localization problem to this discussion and comment on ideas about the supposed role of quantum field theory as a vehicle of making a theory causal by introducing antiparticles.
Two statements in physics are frequently conflated: that nothing can travel faster than light, and that two entangled particles behave as if "instantaneously" connected regardless of their separation. This paper addresses both questions together: whether a faster-than-light particle could exist and, if so, whether it would count as matter; and whether quantum entanglement actually violates the light-speed limit. Starting from the energy–velocity relation of special relativity, it is shown that massive particles cannot reach the speed of light, and the properties of hypothetical tachyons (imaginary mass, absence of a rest frame and violation of causality) are examined. Using the singlet state, the non-separability, reduced state and correlation properties of entanglement are derived, and the quantum violation of the CHSH inequality up to 2√2 is computed. Finally, via the no-signaling theorem, it is shown that entanglement produces non-local correlations without permitting superluminal communication. In conclusion, nature accommodates non-local correlations without allowing any object to travel faster than light.
We show that states of systems which, in a very general sense, are approximately localized at time t=0t=0t=0 in a finite region, with exponentially bounded tails outside, violate Einstein causality at later times. Implications are discussed.
We prove that the classical Dirac equation in the presence of an external (nondynamical) electromagnetic field is a relativistically causal theory. As a corollary, we show that it is impossible to use quantum tunneling to transmit particles or information faster than light. When an electron tunnels through a barrier, it is bound to remain within its future light cone. In conclusion, the relativistic quantum tunneling (if modeled using the Dirac equation) is an entirely subluminal process, and it is not instantaneous.
In the current formalism of quantum mechanics, collapse of the wave function is envisioned as occurring “instantaneously”. Since this is in conflict with the fact that “simultaneity” is not a relativistic invariant, quantum mechanics and relativity are in conflict with each another. This conflict is benign as long as faster-than-light (FTL) phenomena are disallowed. This paper explores this conflict and considers the implications if collapse can be specified to occur in a way that is consistent with relativity. In particular, relativistically consistent wave function collapse forces us to reconsider our preconceptions of causality. We are led to consider two cases; timelike and spacelike. For two events that are causally connected by a timelike interval, the earlier-in-time event is the cause in all reference frames, but the spatial ordering is not preserved. For two events that are causally connected by a spacelike interval, the temporal ordering is not preserved but the spatial ordering is, just the reverse. This is philosophically disturbing since for spacelike causes half the reference frames see the cause as proceeding backwards in time. However, in and of itself this is not logically inconsistent. Backwards-in-Time Causality (BTC) seems at first glance to permit temporal paradox, since two spacelike causal connections can be used to create a closed causal loop in spacetime. While temporal paradox is logically inconsistent, BTC can still be allowed if there is also a censor mechanism that prevents temporal paradox. In relativity, FTL and BTC are two sides of the same coin, so the major implication of relativistically consistent wave function collapse is that quantum systems should be able to support both FTL and BTC phenomena despite Eberhard’s “proof.”