Roman ring
In general relativity, a Roman ring (proposed by Matt Visser in 1997 and named after the Roman arch, a concept proposed by Mike Morris and Kip Thorne in 1988 and named after physicist Tom Roman) is a configuration of wormholes where for each individual wormhole the time difference across its mouths (caused by the mouths moving relative to each other) is such that it may not allow a closed timelike curve (CTC), or 'closed time loop', but if these wormholes are arranged in a suitable configuration, a closed time loop is formed. == Examples == For example, an Earth–Moon wormhole whose far end is 0.5 seconds in the "past" will not violate causality, since information sent to the far end via the wormhole and back through normal space will still arrive back on Earth (–0.5 + 1) = 0.5 seconds after it was transmitted; but an additional wormhole in the other direction will allow information to arrive back on Earth 1 second before it was transmitted (time travel).