係一篇1992年嘅論文中，霍金使用比喻嘅講法「時序保護機構/機制」（Chronology Protection Agency）嚟解釋物理定律唔畀宏觀尺度嘅時間旅行發生，噉就唔會出現時間悖論。佢認為： 好似有個時間順序保護機制，唔畀封閉類時曲線出現，噉宇宙對歷史學家嚟講就安全咗。It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.
In a 1992 paper, Hawking used the metaphor of a "chronology protection agency" to explain that the laws of physics do not allow time travel on a macroscopic scale to occur, so that there would be no time paradox. He argues that there seems to be a chronological protection mechanism that prevents the occurrence of closed time-like curves, so that the universe is safe for historians for anything. It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.