The possibility of time travel, both into the past and into the future of a given observer, has left the realm of pure imagination to come into physical plausibility, starting with the seminal works by Chandrasekhar and Carter, followed by those of de Felice, Clarke, Thorne, Novikov, and several others.
Time may run forward but also backwards with respect to a selected observer and under extreme space-time conditions. One of these conditions is the occurrence of a curvature singularity which is naked with respect to asymptotic regions. Curvature singularities hide themselves inside black holes so their existence is implicitly accepted once black holes are accepted as astrophysical objects. Black holes however may not necessarily form as a result of gravitational collapse so naked singularities may stand as a physical reality responsible of unusual effects.
Once a naked singularity forms, a general theorem by Clark and de Felice (C.J.S. Clarke and F. de Felice, “Globally non causal space-times. II-naked singularities and curvature conditions”, Gen. Rel. Gravitation 16 (1984) 139-148 and references therein) assures that a region around the singularity exists where time runs backwards with respect to null infinity. A practical example of such a situation is offered by the Kerr solution where an effective cosmic time machine can be realized. Chronology can be violated and consequently causality as well.
However we shall not consider this as a real possibility, being the conservation of causality a fundamental principle of physics, hence it is essential to find the way how Nature operates in order to preserve causality according to the “causality preservation principle” .
A strong impetus to the deepening of such studies came also from the findings of K. S. Thorne, Novikov and collaborators, who revealed the possible presence of wormholes (Phys. Rev. D 44 1077, 1991, Phys. Rev. Lett. 61 1446, 1988, Int. J. Mod Phys. D4 557, 1995).
Despite of all this, recent developments have opened new perspectives for the “realization” of a time machine (Phys. Rev. Lett. 106, 040403, 2011) related to the issue of quantum non-locality resulting from entanglement and, in particular, to the phenomenon of teleportation (Nature 390, 575-579, 1997, Phys. Rev. Lett. 80, 1121-1125, 1998) and CTCs/OTCs (Pienaar, J. L., Myers, C. R. & Ralph, T. C. “Quantum fields on closed timelike curves.” Phys. Rev. A 84, 062316 (2011), Ralph, T. C., Milburn, G. J. & Downes, T.; “Quantum connectivity of space-time and gravitationally induced decorrelation of entanglement” Phys. Rev. A 79, 022121 (2009); Oreshkov, O., Costa, F. & Brukner, C. “Quantum correlations with no causal order. Nat. Commun. 3, 1092 (2012); C. Marletto, et al., “Theoretical description and experimental simulation of quantum entanglement near open time-like curves via pseudo-density operators”, Nat. Commun. 10, 182 (2019)).
The topological nature of spacetime in the framework of Quantum Mechanics suggests that both causality and locality need to be analyzed at a fundamental level, also by allowing the existence of entities that are more elementary than the spatial dimensions of everyday life.
Focusing on these issues would help unveiling the ultimate nature of causality, its role in fundamental physics and in the evolution of the Universe. This would allow for a better comprehension of the “direction” of large scale structures, including the formation of complex entities such as black holes or hierarchical structures like the Galaxy, with obvious implications on the process of mapping and measuring the Universe on local and global scales, which must remain consistent with the theories of General Relativity and Quantum Mechanics.