Intriguingly, this kind of decoherence will occur anywhere there is a horizon that only allows information to travel in one direction, creating the potential for causality paradoxes. The edge of the known universe, called the cosmological horizon, is another example. Or consider the “Rindler horizon,” which forms behind an observer who continuously accelerates and approaches the speed of light, so that light rays can no longer catch up with them. All of these “Killing horizons” (named after the late-19th- and early-20th-century German mathematician) cause quantum superpositions to decohere. “These horizons are really watching you in exactly the same way,” Satishchandran said.
Exactly what it means for the edge of the known universe to watch everything inside the universe isn’t entirely clear. “We don’t understand the cosmological horizon,” Lupsasca said. “It’s super fascinating, but way harder than black holes.”
In any case, by posing thought experiments like this, where gravity and quantum theory collide, physicists hope to learn about the behavior of a unified theory. “This is likely giving us some more clues about quantum gravity,” Wald said. For example, the new effect may help theorists understand how entanglement is related to space-time.
“These effects have to be part of the final story of quantum gravity,” Lupsasca said. “Now, are they going to be a crucial clue along the way to gleaning insight into that theory? It’s worth investigating.”
The Participatory Universe
As scientists continue to learn about decoherence in all its forms, Wheeler’s concept of the participatory universe is becoming clearer, Danielson said. All particles in the universe, it seems, are in a subtle superposition until they are observed. Definiteness emerges through interactions. “That’s kind of what, I think, Wheeler had in mind,” Danielson said.
And the finding that black holes and other Killing horizons observe everything, all the time, “whether you like it or not,” is “more evocative” of the participatory universe than the other types of decoherence are, the authors said.
Not everyone is ready to buy Wheeler’s philosophy on a grand scale. “The idea that the universe observes itself? That sounds a little Jedi for me,” said Lupsasca, who nevertheless agrees that “everything is observing itself all the time through interactions.”
“Poetically, you could think of it that way,” Carney said. “Personally, I’d just say that the presence of the horizon means that the fields living around it are going to get stuck on the horizon in a really interesting way.”
When Wheeler first drew the “big U” when Wald was a student in the 1970s, Wald didn’t think much of it. “Wheeler’s idea struck me as not that solidly grounded,” he said.
And now? “A lot of the stuff he did was enthusiasm and some vague ideas which later turned out to be really on the mark,” Wald said, noting that Wheeler anticipated Hawking radiation long before the effect was calculated.
“He saw himself as holding out a lamp light to illuminate possible paths for other people to follow.”
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