Quantum physics once shocked scientists by revealing that particles can behave like waves—and now, that strange behavior has been pushed even further. For the first time, researchers have observed wave-like interference in positronium, an exotic “atom” made of an electron and its antimatter partner, a positron. This breakthrough not only strengthens the weird reality of quantum mechanics but also opens the door to new experiments involving antimatter, including the possibility of testing how gravity affects it—something never directly measured before.

As Homer tells us, Odysseus made an epic journey, against the odds, from Troy to his home in Ithaca. He visited many lands, but mostly dwelt with the nymph Calypso on her island. 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.