At Princeton University in the early 1970s, the celebrated theoretical physicist John Wheeler could be spotted in seminars or impromptu hallway discussions drawing a big “U.” The letter’s left tip represented the beginning of the universe, where everything was uncertain and all quantum possibilities were happening at the same time. The letter’s right tip, sometimes adorned with an eye, depicted an observer looking back in time, thus bringing the left side of the U into existence.
In this “participatory universe,” as Wheeler called it, the cosmos expanded and cooled around the U, forming structures and eventually creating observers, like humans and measuring apparatus. By looking back to the early universe, these observers somehow made it real.
“He would say things like ‘No phenomenon is a true phenomenon until it’s an observed phenomenon,’” said Robert M. Wald, a theoretical physicist at the University of Chicago who was Wheeler’s doctoral student at the time.
Now, by studying how quantum theory behaves on the horizon of a black hole, Wald and his collaborators have calculated a new effect that is suggestive of Wheeler’s participatory universe. The mere presence of a black hole, they’ve found, is enough to turn a particle’s hazy “superposition” — the state of being in multiple potential states — into a well-defined reality. “It evokes the idea that these black hole horizons are watching,” said co-author Gautam Satishchandran, a theoretical physicist at Princeton.
“What we have found might be a quantum mechanical realization of [the participatory universe], but where space-time itself plays the role of the observer,” said Daine Danielson, the third author, also at Chicago.
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