Mutual information challenges entropy bounds

TL;DR

This paper investigates the limitations of entropy bounds in quantum field theory using mutual information, revealing violations of certain bounds and implications for the nature of degrees of freedom and non-locality.

Contribution

It introduces mutual information as a finite, regularization-independent measure to analyze entropy bounds and demonstrates violations of the FMW and Bousso bounds in specific scenarios.

Findings

Mutual information remains finite and cutoff independent in QFT.

Vacuum entropy violates the FMW bound in Minkowski space.

Violations of the Bousso bound may occur in conformal theories at large distances.

Abstract

We consider some formulations of the entropy bounds at the semiclassical level. The entropy S(V) localized in a region V is divergent in quantum field theory (QFT). Instead of it we focus on the mutual information I(V,W)=S(V)+S(W)-S(V\cup W) between two different non-intersecting sets V and W. This is a low energy quantity, independent of the regularization scheme. In addition, the mutual information is bounded above by twice the entropy corresponding to the sets involved. Calculations of I(V,W) in QFT show that the entropy in empty space cannot be renormalized to zero, and must be actually very large. We find that this entropy due to the vacuum fluctuations violates the FMW bound in Minkowski space. The mutual information also gives a precise, cutoff independent meaning to the statement that the number of degrees of freedom increases with the volume in QFT. If the holographic bound holds, this points to the essential non locality of the physical cutoff. Violations of the Bousso bound would require conformal theories and large distances. We speculate that the presence of a small cosmological constant might prevent such a violation.