Wrong Signs are Alright

TL;DR

This paper investigates Lorentz-invariant quantum field theories with negative coefficients for higher-dimensional operators, showing that quantum effects prevent classical superluminal and closed time-like curve formation, thus protecting causality.

Contribution

It demonstrates that quantum backreaction invalidates classical superluminal and Cauchy horizon predictions, supporting the viability of theories with 'wrong signs' at low energies.

Findings

Quantum effects excite unknown short-distance degrees of freedom.

Classical superluminality and Cauchy horizons are prevented by quantum backreaction.

No low-energy obstruction exists for theories with negative coefficients.

Abstract

It has been shown that some Lorentz-invariant quantum field theories, such as those with higher-dimensional operators with negative coefficients, lead to superluminality on some classical backgrounds. While superluminality by itself is not logically inconsistent, these theories also predict the formation of closed time-like curves at the classical level, starting from initial conditions without such curves. This leads to the formation of a Cauchy Horizon which prevents a complete description of the time evolution of such systems. Inspired by the chronology protection arguments of General Relativity, we show that quantum mechanical effects from low energy quanta strongly backreact on such configurations, exciting unknown short-distance degrees of freedom and invalidating the classical predictions. Thus, there is no obvious low-energy obstruction to the existence of these operators.