In the early 1970s, John Wheeler, a notable theoretical physicist at Princeton University, could often be seen in informal hallway debates or seminars sketching a large “U.” The left end of the symbol symbolized the origin of the cosmos, where everything was indefinite and all imaginable quantum events were happening concurrently. The right end of the symbol, sometimes embellished with an eye, illustrated an observer gazing back in time, thus bringing the initial side of the U into existence.
Within this concept of the “participatory universe,” as Wheeler labeled it, the universe burgeoned and cooled around the U, forming configurations and ultimately engendering observers, such as humans and measurement devices. By looking back at the early universe, these observers in some way actualized it.
“He would express ideas like ‘No phenomenon is a true phenomenon until it’s an observed phenomenon,’” recounted Robert M. Wald, a theoretical physicist at the University of Chicago who served as Wheeler’s doctoral candidate during that period.
Today, by examining how quantum theory behaves at the fringe of a dark hole, Wald and his colleagues have computed a novel phenomenon that resonates with Wheeler’s participatory model of the universe. The mere existence of a dark hole, they have ascertained, is adequate to transform a particle’s ambiguous “superposition”—its state of being in several potential conditions—into a distinct reality. “It provokes the notion that these dark hole borders are observant,” stated coauthor Gautam Satishchandran, a theoretical physicist at Princeton.
“The discovery we have made could potentially be a quantum mechanical illustration of [the participatory universe], but in this case, the spacetime itself assumes the position of the observer,” mentionedDaine Danielson, the third contributor affiliated with Chicago.
Theorists are currently engaged in discussions about the implications of these vigilant black holes. “This appears to convey a profound insight into how gravity affects observations in quantum mechanics,” stated Sam Gralla, a theoretical astrophysicist at the University of Arizona. However, whether this will be beneficial to researchers striving for a comprehensive quantum gravity theory remains uncertain.
This phenomenon is one of several discoveries made in the last decade by physicists exploring the interplay of quantum theory and gravity under low-energy conditions. For instance, researchers have made significant progress in analyzing Hawking radiation, which leads to the gradual evaporation of black holes. “Previously unnoticed subtle effects offer us limitations from which we can extract clues on progressing towards quantum gravity,” remarked Alex Lupsasca, a theoretical physicist at Vanderbilt University who was not involved in the recent research.
These attentive black holes seem to generate an effect that is “extremely striking,” as Lupsasca described it, “because it gives the sensation that it is profound in some way.”
Black Holes and Superpositions
To comprehend how a black hole could witness the universe, let’s delve into a basic scenario. Consider the traditional double-slit experiment where quantum particles are directed towards two openings in a barrier. Subsequently, those particles that pass through are detected by a screen positioned on the opposite side.
Initially, each particle in transit appears to land randomly on the screen. Nevertheless, as more particles travel through the slits, a pattern of bright and dark stripes emerges. This pattern implies that each particle behaves akin to waves passing through both openings simultaneously. The stripes result from the waves’ peaks and valleys either merging or nullifying each other—a phenomenon known as interference.
Now, introduce a sensing device to ascertain which slit the particle traverses. The pattern of bright and dark stripes will vanish. The act of observation alters the particle’s state—its wave-like attributes completely vanish. Physicists explain that the information obtained by the detection apparatus “decoheres” the quantum probabilities into a definite reality.
Importantly, the sensor need not be in proximity to the slits to decipher the particle’s path. For instance, a charged particle releases an extensive electric field that might vary slightly based on whether it passed through the right or left opening. Assessing this field from a distance still allows you to deduce the particle’s path and thus initiates decoherence.
In 2021, Wald, Satishchandran, and Danielson delved into a paradox arising when hypothetical observers collect information in this manner. They envisioned an experimenter named Alice who generates a particle in a superposition. Later on, she examines it for an interference pattern. The particle will only manifest interference if it hasn’t become too entangled with any external system while under Alice’s scrutiny.
Then, Bob comes into play, endeavoring to measure the particle’s position remotely by gauging its long-range fields. According to causality principles, Bob should not be able to influence the outcome of Alice’s experiment, considering that the experiment should have concluded by the time Bob’s signals reach Alice. Nevertheless, in the realm of quantum mechanics, if Bob effectively measures the particle, it becomes entangled with him, leading to the absence of an interference pattern as observed by Alice.
The trio meticulously calculated that the level of decoherence due to Bob’s actions is perpetually lower than the decoherence naturally caused by the radiation emitted by Alice (which also becomes entangled with the particle). As a result, Bob could never induce decoherence in Alice’s experiment because she would have already induced it herself. Although a previous iteration of this paradox was resolved in 2018 through a rudimentary calculation by Wald and a distinct group of researchers, Danielson took it a step further.
He proposed a hypothetical scenario to his colleagues: “Why couldn’t I position [Bob’s] detector behind a black hole?” In this setup, a particle in a superposition outside the event horizon will emit fields that traverse the horizon and get perceived by Bob on the opposite side, within the black hole. The detector acquires information about the particle, but since the event horizon acts as a ‘one-way passage,’ information cannot return, according to Danielson. “Bob is unable to influence Alice from within the black hole; thus, the same decoherence”must transpire devoid of Bob,” conveyed the team in an electronic message to Quanta. The black hole is required to disrupt the superposition.
“In the more expressive phrasing of the participatory cosmos, it is as though the fringe observes superpositions,” Danielson expressed.
Employing this perspective, they commenced laboring on a precise computation of how quantum superpositions are impacted by the space-time of the black hole. In a manuscript published on the preprint site Arxiv.org in January, they unveiled a simple formula that delineates the pace at which radiation traverses the event horizon, consequently inducing decoherence. “I found it markedly unexpected that there was any impact at all,” Wald remarked.
Locks on the Horizon
The concept that event horizons compile data and induce decoherence is not recent. In 2016, Stephen Hawking, Malcolm Perry, and Andrew Strominger outlined how particles moving across the event horizon could be accompanied by exceedingly low-energy radiation that logs information about said particles. This revelation was put forth as a resolution to the black hole information enigma, a profound ramification of Hawking’s preceding revelation that black holes emit radiation.
The quandary was that Hawking radiation saps energy from black holes, leading to their complete dissipation over time. This process would seem to obliterate any information that has descended into the black hole. However, in doing so, it would flout a fundamental tenet of quantum mechanics: that information in the cosmos cannot be brought into existence or annulled.
The low-energy radiation proposed by the trio would circumvent this by enabling some information to be spread in a halo surrounding the black hole and escape. The scientists termed the data-laden halo “soft hair.”
Wald, Satishchandran, and Danielson were not investigating the black hole information issue. Nevertheless, their research makes use of soft hair. Specifically, they demonstrated that soft hair materializes not only when particles cascade over a horizon but also when particles external to a black hole merely relocate to a distinct position. Any quantum superposition external to the hole will intertwine with soft hair on the horizon, instigating the decoherence effect they pinpointed. In this manner, the superposition is documented as a sort of “memory” on the horizon.
The computation is a “tangible manifestation of soft hair,” articulated Daniel Carney, a hypothetical physicist at Lawrence Berkeley National Laboratory. “It’s a compelling article. It could be a highly advantageous framework for attempting to execute that notion meticulously.”
However, for Carney and numerous other theorists at the vanguard of quantum gravity exploration, this decoherence impact isn’t notably surprising. The far-reaching nature of the electromagnetic force and gravity implies that “it’s challenging to maintain anything isolated from the remainder of the cosmos,” stated Daniel Harlow, a hypothetical physicist at the Massachusetts Institute of Technology.
Total Disruption
The authors assert that there is something distinctively “sinister” about this form of decoherence. Usually, physicists can regulate decoherence by shielding their experiment from the external environment. For instance, a vacuum eliminates the impact of nearby gas molecules. However, nothing can buffer gravity, so there is no means to segregate an experiment from the protracted influence of gravity. “Eventually, every superposition will be entirely disrupted,” affirmed Satishchandran. “There is no evasion.”
The authors consequently view black hole horizons as undertaking a more proactive role in decoherence than formerly recognized. “The structure of the cosmos itself, as opposed to the constituents within it, is accountable for the decoherence,” they conveyed in an electronic message to Quanta.
Carney refutes this interpretation, stating that the new decoherence effect can also be comprehended as a consequence of electromagnetic or gravitational fields, along with rules established by causality. Unlike Hawking radiation where the black hole horizon evolves over time, in this scenario, the horizon “possesses no dynamics whatsoever,” Carney clarified. “The horizon doesn’t undertake any action, per se; I would not articulate it in that manner.”
To uphold causality, superpositions exterior to the black hole must undergo decoherence at the maximum feasible rate at which a hypothetical observer positioned inside the black hole could assimilate information about them. “It appears to be gesturing toward a novel principle about gravity, measurement, and quantum mechanics,” Gralla remarked. “One does not anticipate that to unfold more than a century after the formulation of gravity and quantum mechanics.”
The occurrence of this kind of decoherence is fascinatingly observed in regions with a horizon that permits one-way information flow, potentially creating causality paradoxes. An instance is the cosmological horizon, which marks the boundary of the known universe. Another example is the “Rindler horizon,” emerging behind an object accelerating continuously and nearing the speed of light, making it impossible for light beams to reach them. All these horizons, known as “Killing horizons” (in honor of mathematician Wilhelm Killing from the late 19th and early 20th centuries), cause quantum superpositions to decohere. Satishchandran stated, “These horizons are essentially monitoring you in a similar manner.”
The implications of the universe’s edge observing everything within it remain somewhat ambiguous. Lupsasca commented, “We have limited understanding of the cosmological horizon. It’s incredibly intriguing but more challenging than black holes.”
By proposing such speculative scenarios where gravity and quantum theory intersect, physicists aim to gain insight into a unified theory’s behavior. Wald highlighted, “This likely offers more clues regarding quantum gravity.” This new phenomenon could aid theorists in comprehending the relationship between entanglement and spacetime.
Lupsasca added, “These ramifications must be integrated into the overarching narrative of quantum gravity. Will they be pivotal in unraveling profound insights into the theory? It certainly merits exploration.”
The Participatory Universe
With ongoing discoveries about decoherence, Wheeler’s notion of the participatory universe is becoming clearer, according to Danielson. It appears that all particles across the universe are subtly superposed until observation occurs. Definiteness arises through interactions, aligning with Wheeler’s vision, as Danielson indicated.
Discovering that black holes and other Killing horizons constantly observe everything echoes the participatory universe concept more vividly than other decoherence types, as noted by the authors.
Not everyone readily embraces Wheeler’s grand philosophical ideas. Lupsasca jokingly remarked, “The concept of the universe observing itself? Sounds a bit Jedi to me,” but acknowledged that “everything continuously observes itself through interactions.”
Carney expressed, “One could metaphorically interpret it that way. Personally, I’d suggest that the horizon’s presence results in intriguing entanglement of surrounding fields with the horizon.”
During Wald’s student days in the 1970s, Wheeler’s original “big U” illustration didn’t capture much of his attention. Reflecting on those times, Wald said, “Wheeler’s concept seemed somewhat speculative.” However, he recognized Wheeler’s foresight when the Hawking radiation effect was eventually established.
“He viewed himself as a beacon illuminating possible paths for others to explore.”
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