On Negative Energies, Strings, Branes, and Braneworlds: A Review of Novel Approaches

TL;DR

This review explores unconventional ideas in quantum gravity, such as negative energies, higher derivative theories, and braneworlds, highlighting their potential viability contrary to previous dismissals.

Contribution

It compiles and discusses recent developments suggesting the physical plausibility of previously rejected theories in quantum gravity and string theory.

Findings

Negative energies may be physically meaningful in certain theories

Higher derivative theories and ultrahyperbolic spaces can be consistent

Braneworld quantization offers new insights into extra dimensions

Abstract

On the way towards quantum gravity and the unification of interaction, several ideas have been rejected and avenues avoided because they were perceived as physically unviable. But in the literature there are works in which it was found the contrary, namely that those rejected topics make sense after all. Such topics, reviewed in this article, are negative energies occurring in higher derivative theories and ultrahyperbolic spaces, ordering ambiguity of operators in curved spaces, the vast landscape of possible compactifications of extra dimensions in string theory, and quantization of a 3-brane in braneworld scenarios.