Causal cycle experiments are shown to be valid indictments of any tachyon theory which allows all parts of the cycle to occur. This is to be contrasted with claims to the contrary made in a recent paper by Parmentola and Yee. A means of uniquely determining the Lorentz-invariant causal direction of a beam is described.

• Received 6 March 1972

DOI:https://doi.org/10.1103/PhysRevD.6.2300