Testing causality violation on spacetimes with closed timelike curves

Generalized quantum mechanics is used to examine a simple two-particle scattering experiment in which there is a bounded region of closed timelike curves (CTCs) in the experiment’s future. The transitional probability is shown to depend on the existence and distribution of the CTCs. The effect is therefore acausal, since the CTCs are in the experiment’s causal future. The effect is due to the nonunitary evolution of the pre- and postscattering particles as they pass through the region of CTCs. We use the time-machine spacetime developed by Politzer, in which CTCs are formed due to the identification of a single spatial region at one time with the same region at another time. For certain initial data, the total cross section of a scattering experiment is shown to deviate from the standard value (the value predicted if no CTCs existed). It is shown that if the time machines are small, sparsely distributed, or far away, then the deviation in the total cross section may be negligible as compared to the experimental error of even the most accurate measurements of cross sections. For a spacetime with CTCs at all points, or one where microscopic time machines pervade the spacetime in the final moments before the big crunch, the total cross section is shown to agree with the standard result (no CTCs) due to a cancellation effect.