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.
