Curiously, this type of decoherence will occur wherever there is a horizon that only allows information to travel in one direction, creating the potential for causal paradoxes. The edge of the known universe, called the cosmological horizon, is another example. Or consider “Rindler’s horizon,” which forms behind an observer who is continually accelerating and approaching 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 Wilhelm Killing) cause quantum superpositions to decohere. “These horizons really look exactly the same to you,” Satishchandran said.
What exactly this means for the edge of the known universe to look at 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.”
Either way, by posing thought experiments like this, where gravity and quantum theory collide, physicists hope to learn more about how a unified theory behaves. “That probably gives us more clues about quantum gravity,” Wald said. For example, the new effect can help theorists understand how entanglement relates to spacetime.
“These effects must be part of the final quantum gravity story,” Lupsasca said. “Now are they going to be a crucial clue on the way to gleaning some insight into this theory? It is worth investigating.
The participative universe
As scientists continue to learn more about decoherence in all its forms, Wheeler’s concept of a participatory universe is becoming clearer, Danielson said. All particles in the universe, it seems, are in subtle superposition until they are observed. Definition emerges through interactions. “That’s kind of what I think Wheeler had in mind,” Danielson said.
And the observation that black holes and other horizons of Killing observe everything, all the time, “whether we like it or not”, is “more evocative” of the participative universe than are the other types of decoherence, according to the authors.
Not everyone is ready to accept Wheeler’s philosophy on a large scale. “The idea that the universe observes itself? It seems a bit Jedi to me,” said Lupsasca, who nevertheless agrees that “everything is observed all the time 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 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 didn’t seem so solid to me,” he said.
And now? “A lot of what he did was enthusiasm and a few vague ideas that later turned out to be really relevant,” Wald said, noting that Wheeler had anticipated Hawking’s radiation long before the effect. not be calculated.
“He saw himself as holding a lamp to illuminate possible paths that other people might follow.”
Original story reproduced with permission from Quanta Magazine, an editorially independent publication Simons Foundation whose mission is to enhance the public understanding of science by covering developments and trends in research in mathematics and the physical and life sciences.
