In 2021, Wald, Satishchandran and Danielson were exploring a paradox brought about when hypothetical observers gather information in this way. They imagined an experimenter called Alice who creates a particle in a superposition. At a later time, she looks for an interference pattern. The particle will only exhibit interference if it hasn’t become too entangled with any outside system while Alice observes it.

Then along comes Bob, who is attempting to measure the particle’s position from far away by measuring the particle’s long-range fields. According to the rules of causality, Bob shouldn’t be able to influence the outcome of Alice’s experiment, since the experiment should be over by the time the signals from Bob get to Alice. However, by the rules of quantum mechanics, if Bob does successfully measure the particle, it will become entangled with him, and Alice won’t see an interference pattern.

The trio rigorously calculated that the amount of decoherence due to Bob’s actions is always less than the decoherence that Alice would naturally cause by the radiation she emits (which also becomes entangled with the particle). So Bob could never decohere Alice’s experiment because she would already have decohered it herself. Although an earlier version of this paradox was resolved in 2018 with a back-of-the-envelope calculation by Wald and a different team of researchers, Danielson took it one step further.

He posed a thought experiment to his collaborators: “Why can’t I put [Bob’s] detector behind a black hole?” In such a setup, a particle in a superposition outside the event horizon will emanate fields that cross over the horizon and get detected by Bob on the other side, within the black hole. The detector gains information about the particle, but as the event horizon is a “one-way ticket,” no information can cross back over, Danielson said. “Bob cannot influence Alice from inside of the black hole, so the same decoherence must occur without Bob,” the team wrote in an email to Quanta. The black hole itself must decohere the superposition.

“In the more poetic language of the participatory universe, it is as if the horizon watches superpositions,” Danielson said.

Using this insight, they set about working on an exact calculation of how quantum superpositions are affected by the black hole’s space-time. In a paper published on the preprint server arxiv.org in January, they landed on a simple formula that describes the rate at which radiation crosses over the event horizon and so causes decoherence to occur. “That there was an effect at all was, to me, very surprising,” Wald said.

Hair on the Horizon

The idea that event horizons gather information and cause decoherence isn’t new. In 2016, Stephen Hawking, Malcolm Perry and Andrew Strominger described how particles crossing over the event horizon could be accompanied by very low-energy radiation that records information about these particles. This insight was suggested as a solution to the black hole information paradox, a profound consequence of Hawking’s earlier discovery that black holes emit radiation.

The problem was that Hawking radiation drains energy from black holes, causing them to completely evaporate over time. This process would appear to destroy any information that has fallen into the black hole. But in doing so, it would contradict a fundamental feature of quantum mechanics: that information in the universe can’t be created or destroyed.

The low-energy radiation proposed by the trio would get around this by allowing some information to be distributed in a halo around the black hole and escape. The researchers called the information-rich halo “soft hair.”

Wald, Satishchandran and Danielson were not investigating the black hole information paradox. But their work makes use of soft hair. Specifically, they showed that soft hair is created not only when particles fall across a horizon, but when particles outside a black hole merely move to a different location. Any quantum superposition outside will become entangled with soft hair on the horizon, giving rise to the decoherence effect they identified. In this way the superposition is recorded as a kind of “memory” on the horizon.

The calculation is a “concrete realization of soft hair,” said Daniel Carney, a theoretical physicist at Lawrence Berkeley National Laboratory. “It’s a cool paper. It could be a very useful construction for trying to make that idea work in detail.”

But to Carney and several other theorists working at the forefront of quantum gravity research, this decoherence effect isn’t all that surprising. The long-range nature of the electromagnetic force and gravity means that “it’s hard to keep anything isolated from the rest of the universe,” said Daniel Harlow, a theoretical physicist at the Massachusetts Institute of Technology.

Total Decoherence

The authors argue that there is something uniquely “insidious” about this kind of decoherence. Usually, physicists can control decoherence by shielding their experiment from the outside environment. A vacuum, for example, removes the influence of nearby gas molecules. But nothing can shield gravity, so there’s no way to insulate an experiment from gravity’s long-range influence. “Eventually, every superposition will be completely decohered,” Satishchandran said. “There’s no way of getting around it.”

The authors therefore regard black hole horizons as taking a more active role in decoherence than was previously known. “The geometry of the universe itself, as opposed to the matter within it, is responsible for the decoherence,” they wrote in an email to Quanta.

Carney disputes this interpretation, saying that the new decoherence effect can also be understood as a consequence of electromagnetic or gravitational fields, in combination with rules set by causality. And unlike Hawking radiation, where the black hole horizon changes over time, in this case the horizon “has no dynamics whatsoever,” Carney said. “The horizon doesn’t do anything, per se; I would not use that language.”

To not violate causality, superpositions outside the black hole must be decohered at the maximum possible rate that a hypothetical observer inside the black hole could be collecting information about them. “It seems to be pointing toward some new principle about gravity, measurement and quantum mechanics,” Gralla said. “You don’t expect that to happen more than 100 years after gravity and quantum mechanics were formulated.”

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- early 20th-century German mathematician Wilhelm Killing) 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.”