Physical Interpretation of Antigravity

TL;DR

This paper explores the concept of antigravity regions in geodesically complete theories of gravity, addressing apparent issues of ghosts and instabilities by proposing an observer-based interpretation that preserves unitarity.

Contribution

It introduces a framework where antigravity regions are physically interpretable without violating unitarity, resolving ghost-related problems through an observer perspective.

Findings

Antigravity regions are connected to gravity regions via singularities.

Unitarity remains intact despite negative kinetic energies in antigravity.

Observers in the gravity region cannot directly detect antigravity negativity.

Abstract

Geodesic incompleteness is a problem in both general relativity and string theory. The Weyl invariant Standard Model coupled to General Relativity (SM+GR), and a similar treatment of string theory, are improved theories that are geodesically complete. A notable prediction of this approach is that there must be antigravity regions of spacetime connected to gravity regions through gravitational singularities such as those that occur in black holes and cosmological bang/crunch. Antigravity regions introduce apparent problems of ghosts that raise several questions of physical interpretation. It was shown that unitarity is not violated but there may be an instability associated with negative kinetic energies in the antigravity regions. In this paper we show that the apparent problems can be resolved with the interpretation of the theory from the perspective of observers strictly in the gravity region. Such observers cannot experience the negative kinetic energy in antigravity directly, but can only detect in and out signals that interact with the antigravity region. This is no different than a spacetime black box for which the information about its interior is encoded in scattering amplitudes for in/out states at its exterior. Through examples we show that negative kinetic energy in antigravity presents no problems of principles but is an interesting topic for physical investigations of fundamental significance.