Quadratic gravity in 4D preserves unitarity because the extra spin-2 sector is a dual inverted harmonic oscillator with vanishing spectral density, yielding a principal-value propagator that satisfies the optical theorem with only virtual contributions.
Quantum gravity, fakeons and microcausality
2 Pith papers cite this work. Polarity classification is still indexing.
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abstract
We investigate the properties of fakeons in quantum gravity at one loop. The theory is described by a graviton multiplet, which contains the fluctuation $h_{\mu \nu }$ of the metric, a massive scalar $\phi $ and the spin-2 fakeon $\chi _{\mu \nu }$. The fields $\phi $ and $\chi _{\mu \nu }$ are introduced explicitly at the level of the Lagrangian by means of standard procedures. We consider two options, where $\phi $ is quantized as a physical particle or a fakeon, and compute the absorptive part of the self-energy of the graviton multiplet. The width of $\chi _{\mu \nu }$, which is negative, shows that the theory predicts the violation of causality at energies larger than the fakeon mass. We address this issue and compare the results with those of the Stelle theory, where $\chi _{\mu \nu }$ is a ghost instead of a fakeon.
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2026 2verdicts
UNVERDICTED 2representative citing papers
Causality is a misleading metaphysical concept that should be abandoned in fundamental physics, making renouncing microcausality acceptable for consistent quantum gravity theories such as those using fakeons.
citing papers explorer
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Unitary Quadratic Quantum Gravity in 4D
Quadratic gravity in 4D preserves unitarity because the extra spin-2 sector is a dual inverted harmonic oscillator with vanishing spectral density, yielding a principal-value propagator that satisfies the optical theorem with only virtual contributions.
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On Causality and Predictivity
Causality is a misleading metaphysical concept that should be abandoned in fundamental physics, making renouncing microcausality acceptable for consistent quantum gravity theories such as those using fakeons.