Roman ring

A Roman ring, in theoretical physics, is a type of wormhole where the time difference across its mouths is such that it may not allow a closed timelike curve (CTC), or 'closed-time loop'. If these wormholes and their mouths are arranged in a suitable configuration, a closed time loop will be again possible.

A Roman ring is a very stable configuration. The chronology protection conjecture postulated by physicist Stephen Hawking fails to prevent the formation of such rings.