Every now and then, it is worth pausing for a second and giving thanks to the many, ever so slightly different versions of you that exist in parallel realities. It is these alternative selves that help to keep these universes in balance.

At least, that is what’s going on if you happen to subscribe to the many-worlds interpretation of quantum theory. First proposed more than 65 years ago, the idea is that reality is constantly splitting off into parallel paths, due to subtle interactions at the level of quantum particles. Though it may boggle the mind, it also smooths over some devilishly tricky problems in physics and, for that reason, plenty of clear-eyed physicists believe it to be true.

But now this strange idea might be facing a huge challenge, thanks to physicists Sandu Popescu and Daniel Collins at the University of Bristol, UK. They initially set out to solve a 100-year-old puzzle in quantum theory, but ended up undermining the idea of parallel universes. “We’ve essentially demolished one of the arguments for it,” says Collins.

It might sound like a destabilising development, but it may actually prove to be a shot in the arm for quantum theory. Already, Popescu and Collins’s work is helping to resolve other long-standing quantum paradoxes and, in the eyes of some theorists, it points to a fresh way of thinking about the cosmos as a singular quantum reality built from the inside out. “This is something deep and new. I think it could become really important,” says Nicolas Gisin, who researches…