The quantum inequalities do not forbid spacetime shortcuts

TL;DR

This paper challenges the idea that quantum inequalities prohibit spacetime shortcuts like wormholes and warp drives, showing they can exist without requiring unphysical energy densities or total negative energy.

Contribution

It demonstrates through explicit examples that quantum inequalities do not necessarily forbid superluminal spacetime shortcuts, countering previous assumptions.

Findings

Quantum inequalities do not always imply large energy densities.

Large energy densities do not necessarily mean large total negative energy.

Spacetime shortcuts can exist without unphysical energy conditions.

Abstract

A class of spacetimes (comprising the Alcubierre bubble, Krasnikov tube, and a certain type of wormholes) is considered that admits `superluminal travel' in a strictly defined sense. Such spacetimes (they are called `shortcuts' in this paper) were suspected to be impossible because calculations based on `quantum inequalities' suggest that their existence would involve Planck-scale energy densities and hence unphysically large values of the `total amount of negative energy' E_tot. I argue that the spacetimes of this type may not be unphysical at all. By explicit examples I prove that: 1) the relevant quantum inequality does not (always) imply large energy densities; 2) large densities may not lead to large values of E_tot; 3) large E_tot, being physically meaningless in some relevant situations, does not necessarily exclude shortcuts.