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… The possible existence of closed timelike curves raises many questions. Consider one posed in French writer René Barjavel’s 1944 novel Future Times Three: What if a person travels into the past and causes the death of their grandfather before he could father any children? In that case, the time traveler cannot exist, creating a paradox.

Russian physicist Igor Dmitriyevich Novikov grappled with such questions and formulated his self-conformity principle: a time traveler can influence the past but cannot change it. Thus, the time traveler’s intervention cannot transform the future into a different one from which they started. This principle has been explored in many TV shows and films, including Doctor Who, The Umbrella Academy, 12 Monkeys and the German TV series Dark. The protagonists attempted to alter the past, but their actions ultimately caused the very events they originally sought to prevent.

To support the self-conformity principle, Thorne and several of his students developed an exemplary model of the grandfather paradox described by Barjavel. Imagine a ball rolling through a wormhole and into the past that emerges in such a way that it collides with its future self, thus preventing it from rolling into the wormhole in the first place. The question that occupied the physicists was: For every initial condition—position and initial velocity of the ball—is there a solution in which the described scenario does not occur? In other words, could the ball always roll out of the wormhole in such a way that it would collide with its future self but the future self would still find its way into the hole? Indeed, the calculations by Thorne’s group from 1991 seemed to confirm precisely that.

We can imagine that his wife, Penelope, would have asked him about that particular time. Odysseus might have replied, “It was nothing. In fact, it was less than nothing. Negative five years I dwelt with Calypso. How else could I have arrived home after only ten years? If you don’t believe me, ask her.”

Quantum particles, it turns out, are just as wily as Odysseus, as we have shown in an experiment published in Physical Review Letters. Not only can their arrival time suggest that they dwelt with other particles for a negative amount of time, but if one asks those other particles, they will corroborate the story.