In lectures or on-the-spot hallway conversations at Princeton University in the early 1970s, renowned theoretical physicist John Wheeler may be seen sketching a large “U.” The leftmost point of the letter stood for the beginning of the cosmos, where everything was unknowable and all quantum possibilities occurred simultaneously. The right tip of the letter, which occasionally had an eye, represented a time traveler looking back, giving rise to the left side of the U.
The cosmos cooled and expanded around the U in this “participatory universe,” as Wheeler referred to it, generating structures and eventually observers, such as people and measuring equipment. These observers somehow made the early universe real by looking back at it.
“He would say things like ‘No phenomenon is a true phenomenon until it’s an observed phenomenon,'” recalled Robert M. Wald, a theoretical physicist at the University of Chicago who was Wheeler’s Ph.D. student at the time.
Wald and his colleagues have now computed a new impact that is suggestive of Wheeler’s participatory universe by examining how quantum theory behaves on the horizon of a black hole. They have discovered that the sheer existence of a black hole is sufficient to transform a particle’s fuzzy “superposition”—the state of being in numerous conceivable states—into a clear reality. According to co-author and Princeton theoretical physicist Gautam Satishchandran, “It suggests that these black hole horizons are watching.”
The third author, Daine Danielson, who is also from Chicago, speculated that what they had discovered might be a quantum mechanical manifestation of [the participatory cosmos], but one in which space-time itself acts as the spectator.
Currently, theorists are arguing what to infer from these vigilant black holes. Sam Gralla, a theoretical astrophysicist at the University of Arizona, said: “This seems to be telling us something deep about the way gravity influences measurement in quantum mechanics.” It’s still unclear whether this will help scientists who are edging closer to developing a comprehensive theory of quantum gravity.
The effect is one of many that scientists researching what occurs when quantum theory and gravity interact at low energies have discovered in the last ten years. Theorists, for instance, have had a lot of success considering Hawking radiation, which causes black holes to gradually evaporate. According to Alex Lupsasca, a theoretical physicist at Vanderbilt University who was not involved in the new discovery, “subtle effects that we hadn’t really noticed before give us constraints from which we can glean clues about how to go up toward quantum gravity.”
According to Lupsasca, these observant black holes seem to provide an impact that is “very arresting” since it “feels like it’s deep,”
Black Holes and Superpositions
Start small to comprehend how a black hole could watch the universe. Think about the traditional double-slit experiment, when quantum particles are shot at two slits in a barrier. A screen on the opposite side then detects those who have already passed through.
Each flying particle initially seems to arrive on the screen at random. The pattern of light and dark stripes, however, starts to form as more particles flow through the slits. This pattern shows that each particle behaves as though it is a wave that simultaneously passes through both apertures. The waves’ peaks and troughs add up or cancel each other out to form the bands, which is a process known as interference.
To determine which of the two slits the particle passes through, add a detector now. The bright and black striped pattern will vanish. When a particle is observed, its wavelike properties completely vanish, changing the particle’s state. According to physicists, the data collected by the sensor device “decoheres” the quantum possibilities into a concrete reality.
It’s significant that you can determine the particle’s path with your detector regardless of how close to the slits it is. For instance, if a charged particle passes through the right-hand or left-hand slit, it may emit a long-range electric field with somewhat differing strengths. If you measure this field from a distance, you can still determine the direction the particle followed, and will cause decoherence.
Wald, Satishchandran, and Danielson were investigating a paradox that results from this kind of information gathering by hypothetical observers in 2021. They made up an Alice-like scientist who produces particles in superposition. She searches for an interference pattern later. Only if the particle hasn’t become excessively entangled with any external systems while Alice is observing it will it show interference.
Then along comes Bob, who is trying to measure the particle’s long-range fields in order to determine the particle’s position from a great distance. Given that Alice’s experiment should have concluded by the time Bob’s signals reach Alice, Bob shouldn’t be able to affect how it turns out, according to the laws of causality. However, if Bob is successful in measuring the particle, it will get entangled with him and Alice won’t notice an interference pattern because of the laws of quantum physics.
The group carefully calculated that the decoherence brought on by Bob’s actions is never more than the decoherence that Alice would inevitably bring about by the radiation she releases (which also entangles the particle). As a result, Bob could never decode Alice’s experiment because she would have done it independently. Although Wald and a different group of academics were able to answer a previous iteration of this contradiction in 2018 with a back-of-the-envelope calculation, Danielson went a step further.
He put his team members through a thought experiment by asking, “Why can’t I put [Bob’s] detector behind a black hole?” In this scenario, a particle in superposition outside the event horizon will emit fields that cross it and are picked up by Bob inside the black hole on the other side. The detector gathers data about the particle, but Danielson noted that since the event horizon is a “one-way ticket,” no data can cross back over. The researchers emailed Quanta, “Bob cannot influence Alice from inside the black hole, so the same decoherence must occur without Bob.” The superposition must be decohered by the black hole itself.
“In the more poetic language of the participatory universe, it is as if the horizon watches superpositions,” Danielson observed.
They used this realization to begin calculating precisely how the space-time of the black hole affects quantum superpositions. They came up with a straightforward formula that defines the rate at which radiation breaches the event horizon and results in decoherence in an article that was posted on the preprint domain arxiv.org in January. I was extremely surprised that there was any effect at all, Wald remarked.
Hair on the Horizon
It is not a novel idea that event horizons collect data and lead to decoherence. 2016 saw the publication of Stephen Hawking, Malcolm Perry, and Andrew Strominger’s explanation of how particles crossing the event horizon may be accompanied by very low-energy radiation that captures data about these particles. The black hole information paradox, a significant result of Hawking’s earlier discovery that black holes leak radiation, was said to be resolved by this realization.
The issue was that black holes lose energy as a result of Hawking radiation, eventually dissipating altogether. It would seem that any information that has fallen into the black hole will be destroyed throughout this procedure. But doing so would go against a key tenet of quantum mechanics, which holds that there is no such thing as the creation or destruction of information in the universe.
This would be overcome by the low-energy radiation the trio has proposed by allowing some data to be dispersed in a halo around the black hole and escape. The information-rich halo was referred to by the researchers as “soft hair.”
Danielson, Wald, and Satishchandran weren’t looking into the black hole information paradox. However, soft hair is used in their craft. They specifically demonstrated that soft hair is produced when particles outside a black hole just migrate to a different point rather than falling across a horizon. They found that the decoherence effect arises from the fact that any quantum superposition outside will become entangled with soft hair on the horizon. This leaves a kind of “memory” of the superposition on the horizon.
According to Daniel Carney, a theoretical physicist at Lawrence Berkeley National Laboratory, the calculation offers a “concrete realization of soft hair.” “I like this paper. It might be a very helpful construction for attempting to implement that concept in detail.
But this decoherence effect isn’t that startling to Carney and a number of other theorists at the forefront of quantum gravity research. According to Daniel Harlow, a theoretical physicist at the Massachusetts Institute of Technology, “it’s hard to keep anything isolated from the rest of the universe” due to the long-range nature of the electromagnetic force and gravity.
Total Decoherence
The authors contend that this form of decoherence is particularly “insidious” in nature. Decoherence is typically managed by insulating an experiment from its surroundings. For instance, a vacuum eliminates the impact of neighboring gas molecules. However since gravity is unavoidable, there is no way to protect an experiment from its long-range effects. Every superposition will eventually become entirely undecoherent, according to Satishchandran. There is no avoiding it, in other words.
Because of this, the authors believe black hole horizons have a more significant role in decoherence than was previously thought. They stated in an email to Quanta that “the geometry of the universe itself, as opposed to the matter within it, is responsible for the decoherence.”
Carney challenges this interpretation, arguing that the new decoherence effect can alternatively be explained as a result of electromagnetic or gravitational forces in conjunction with causality’s principles. Additionally, contrary to Hawking radiation, in which the black hole’s horizon varies over time, in this instance the horizon “has no dynamics whatsoever,” according to Carney. “The horizon doesn’t really do anything; I wouldn’t use that terminology.”
Superpositions outside the black hole must be decohered as fast as a hypothetical observer inside the black hole might gather information about them in order to maintain causality. Gralla stated that the evidence “seems to be pointing toward some new principle about gravity, measurement, and quantum mechanics.” You wouldn’t anticipate that to occur more than a century after the formulation of gravity and quantum physics.
It’s interesting to note that causality paradoxes could result from this kind of decoherence anywhere there is a horizon that only permits information to move in one direction. Another illustration is the cosmic horizon, which is the boundary of the known cosmos. Consider the “Rindler horizon,” which develops behind an observer moving at a constant rate that approaches the speed of light and prevents light rays from catching up to them. All of these “Killing horizons” (so named after the German mathematician Wilhelm Killing, active in the late 19th and early 20th centuries) result in the collapse of quantum superpositions. These vistas are actually keeping a close eye on you, according to Satishchandran.
It’s unclear exactly what it means for the edge of the known universe to observe everything within the cosmos. Lupsasca declared, “We don’t comprehend the cosmological horizon. It’s extremely fascinating, but far more difficult than black holes.
In any event, physicists seek to get insight into the behavior of a unified theory by conducting thought experiments like this one where gravity and quantum theory collide. Wald remarked, “This is probably giving us some more hints about quantum gravity.” The novel effect, for instance, might aid theorists in comprehending how entanglement relates to space-time.
According to Lupsasca, “These effects have to be a part of the quantum gravity story in the end.” “Are they going to be a key piece of information that will help us understand that theory? It merits more investigation.
The Participatory Universe
Wheeler’s idea of the participatory universe is coming into focus as scientists learn more about decoherence in all of its manifestations, according to Danielson. Until they are noticed, it appears that all particles in the cosmos are in a delicate superposition. Interactions lead to definiteness. That’s kind of what Wheeler had in mind, according to Danielson.
The authors concluded that the observation of everything by black holes and other Killing horizons, “whether you like it or not,” is “more evocative” of the participatory world than the other sorts of decoherence.
Some people are not yet prepared to accept Wheeler’s philosophy on a broad basis. The notion that the universe keeps track of itself? Lupsasca agreed that “everything is observing itself every moment through interactions, but that sounds a little Jedi to me.”
You may consider it that way poetically, Carney added. Personally, I’d merely remark that the existence of the horizon indicates that the fields residing in its vicinity are going to get incredibly interestingly stuck on the horizon.
Wald didn’t think much of the “big U” when Wheeler originally drew it when he was a student in the 1970s. He said, “Wheeler’s theory struck me as not that well anchored.
And this? Wald noted that Wheeler anticipated Hawking radiation long before the impact was measured, saying that “a lot of the stuff he did was enthusiasm and some vague ideas which later turned out to be really on the mark.”
He envisioned himself as a lamp, shining a light on potential directions for others to take.
