Intriguingly, this type of decoherence occurs wherever there is a horizon that allows information to flow in only one direction, creating the potential for causality paradoxes. The edge of the known universe called the cosmological horizon is another example. Or think of the “Rindler Horizon” that forms behind an observer accelerating and approaching the speed of light, so light rays can no longer catch up with it. All of these “killing horizons” (named after the late 19th-early 20th-century German mathematician Wilhelm Killing) result in the decoherence of quantum superpositions. “These horizons are really watching you the same way,” said Satishchandran.
Exactly what it means that the edge of the known universe is observing everything within the universe is not entirely clear. “We don’t understand the cosmological horizon,” said Lupsasca. “It’s super fascinating, but much harder than black holes.”
In any case, physicists hope to learn something about the behavior of a unified theory from thought experiments like this one, in which gravity and quantum theory collide. “This likely gives us further clues about quantum gravity,” Wald said. For example, the new effect could help theorists understand how entanglement is related to spacetime.
“These effects must be part of the ultimate story of quantum gravity,” Lupsasca said. “Well, will they be a crucial clue on the way to gaining insight into this 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 appear to be in a subtle superposition until observed. Certainty arises from interactions. “I think that’s what Wheeler had in mind,” Danielson said.
And the realization that black holes and other killing horizons are constantly watching everything “like it or not” is “more evocative” for the participatory universe than the other types of decoherence, the authors said.
Not everyone is ready to buy into Wheeler’s philosophy in a big way. “The idea that the universe is observing itself? That sounds a little Jedi to me,” said Lupsasca, who agrees, however, that “everything is constantly observing itself through interactions.”
“Poetically, you could think of it that way,” Carney said. “Personally, I would just say that the presence of the horizon means that the fields living around it are going to stick to the horizon in a really interesting way.”
When Wheeler first drew the “big U” as a student in the 1970s, Wald didn’t think much of it. “Wheeler’s idea didn’t seem that solid to me,” he said.
And now? “A lot of what he did was enthusiasm and some vague ideas that later turned out to be really apt,” Wald said, noting that Wheeler foresaw the Hawking radiation long before the effect was calculated.
“He saw himself as a light of the lamp to illuminate possible paths for other people to follow.”
Original story Reprinted with permission from quanta magazine, an editorially independent publication Simons Foundation whose mission is to improve public understanding of science by covering research developments and trends in mathematics and the natural and life sciences.