Keele Research Repository

This short paper summarises the proof that a causality violation formed from a bounded region requires matter that violates the null energy condition. Therefore the answer to the question in the title is yes. This is one of the results on which Stephen Hawking based his Chronology protection conjecture. It is conjectured that the required negative mass density fields cannot cover sufficiently large regions of space-time to make causality violations a reality. This lends support to the idea that the evolution of the universe is an entirely unitary process. This work is intended to be more accessible than Hawking’s original, which was cast in terms of the Newman-Penrose formalism.