Fakeons and microcausality: light cones, gravitational waves and the Hubble constant

Abstract

The concept of a fake particle, or ‘fakeon’, allows us to make sense of quantum gravity as an ultraviolet complete theory, by renouncing causality at very small distances. We investigate whether the violation of microcausality can be amplified or detected in the most common settings. We show that it is actually short range for all practical purposes. Due to our experimental limitations, the violation does not propagate along the light cones or by means of gravitational waves. In some cases, the Universe even conspires to make the effect disappear. For example, the positivity of the Hubble constant appears to be responsible for the direction of time in the early Universe.

Finite Quantum Gravity Amplitudes: No Strings Attached

“…These constraints constitute challenges for many quantum gravity programs. For instance, Stelle gravity [12,13] resolves the growth of the amplitudes at high energy at the expense of unitarity or causality [14,15], while Lee-Wick-type gravity models [16][17][18][19][20][21] break causality at sub-Planckian scales (also see Refs. [22,23] for a related discussion of massive ghost modes in nonlocal gravity models).…”

Quantum field theories of arbitrary-spin massive multiplets and Palatini quantum gravity

“…At present we cannot answer in the affirmative, since fake particles should stay massive, and possibly heavy, in realistic models. The reason is that they trigger violations of microcausality [32,33] at energies larger than their masses, so a massless fakeon would be responsible for the violation of causality at all energies. Nevertheless, we do discuss some properties of the massless limits when we talk about RG fixed points.…”

“…In realistic models the fakeons are required to be massive. Indeed, they trigger violations of microcausality [32,33] at energies larger than their masses, so a massless fakeon would be responsible for the violation of causality at all energies. The masses make a higher-spin multiplet disappear at low energies.…”

The quest for purely virtual quanta: fakeons versus Feynman-Wheeler particles

“…The simplest attempts we can think of to amplify the effect do not lead too far. Actually, in some cases the universe conspires to recover causality for free [12]. In the classical limit, we may have to downgrade the violation of causality to an unusual form of the equations of motion and a fuzziness of their solutions [10,12], which still lead to predictions that can be tested experimentally.…”

“…Actually, in some cases the universe conspires to recover causality for free [12]. In the classical limit, we may have to downgrade the violation of causality to an unusual form of the equations of motion and a fuzziness of their solutions [10,12], which still lead to predictions that can be tested experimentally. Finally, we stress that even if time loses sense at small scales, scattering processes in momentum space make sense to arbitrarily high energies.…”