Photonic Mode in a Space-Time with a Causal Loop: Comparison of D-CTC and S-CTC Models

The evolution of a quantized electromagnetic mode in a space-time toy model with nontrivial topology, allowing closed timelike and null world lines, is considered. The physical consequences of adopting an ontological or epistemological view on a quantum state are compared. It is done within a framework of two alternative interpretations of mode evolution—Deutsch’s D-CTC model (ontological) and S-CTC model (epistemological). The future states of the mode (with respect to the causal loop) are calculated for two types of interaction with the mode’s previous version coming from the future. The found differences of the predictions may be helpful for building a future fundamental theory unifying quantum physics and gravity.