Gell-Mann, Hartle spin a quantum narrative about reality | Science News



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Tom Siegfried

Gell-Mann, Hartle spin a quantum narrative about reality

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Second of two parts (read part 1)

Science is a map of nature. It helps you navigate your way through the world. And like all good maps, it provides just the essential information, not too much detail. (In technical terms, maps are “coarse-grained.”)

But science is more than a map. Science is also story about nature. And as a story, science also leaves details out, the way some movies omit a lot from their original books.

In the case of the natural world, the original book seems to have been written in the language of quantum physics. And for several decades now, scientists reading the book of nature have been trying to figure out what kind of story quantum physics is telling about reality. It turns out that this text resists unambiguous interpretation. Scientists can’t even agree on whether it’s written in the first person or by an omniscient narrator. In other words, whether scientists are the authors or are just characters within the story.

In some interpretations of quantum physics, people tell the story. Reality depends on the observations they make. Schrödinger’s cat becomes fully dead or fully alive only when somebody looks in the box to see. Some physicists have even gone so far as to suggest that reality resides in the consciousness of the observer.

Another viewpoint, known as Quantum Bayesianism, attempts to balance the role of the subjects telling the story of reality and the objects they study. In this view the math describing a quantum system — a quantum state — is information possessed by an individual for use in placing bets about the results of measurements. Quantum physics from this perspective is not a description of some ultimate reality at the foundations of existence, but rather a way for science to bring order to the multiple manifestations of human experience.

Even so, observers are part of the narrative, not merely narrators. Quantum physics applies to people just as it applies to photons. From some perspectives, the quantum story of reality should regard people as characters participating in the plot, not as spectators evaluating the performance of subatomic particles.

Reasoning along these lines led the late Hugh Everett III, in the 1950s, to formulate the “many worlds” interpretation of quantum mechanics. All the possible realities contained in the quantum math do in fact come into existence, Everett argued. As experimenters record different possible results, various new universes pop into existence to satisfy each of the possible outcomes. Nobody notices because each person splits into quantum clones that carry on in the new universes.

Everett’s interpretation still has its advocates. But it has also been embellished into more elaborate interpretations to explain why people never notice the multiple quantum realities. Various possibilities for the location of say, a chair, may in fact all be real. But most of those possibilities disappear too rapidly for anybody to see — if the story of the universe’s history is going to be self-consistent.

A chair, for instance, does not exist in isolation. It is constantly bombarded by air molecules and photons and other stuff in its surroundings. Before very long — fractions of a second, much too short for Olympic timing clocks to register — the subsequent positions of all those air molecules and particles of light will be consistent with only one location of the chair. So the story of the universe turns into a history of interactions producing a consistent narrative.

Various physicists have pursued this idea to cope with quantum conundrums, leading to a set of viewpoints known generally as the “consistent histories” approach to quantum physics. Note the plural. A quantum universe can contain many consistent histories.

Among the many experts who have labored to provide mathematical rigor for this approach are Murray Gell-Mann, the originator of quarks, and James Hartle, well known for work with Stephen Hawking on the quantum state of the universe. In a series of papers over the last quarter century, Gell-Mann and Hartle have articulated the “decoherent histories” version of the consistent histories idea. (Because a quantum state possessing multiple possibilities is said to be coherent, the interactions with the environment that eliminate possible outcomes is called “decoherence.”)

For some quantum physicists, decoherence is enough — the environment plays the role of an observer that selects concrete results from the fog of quantum possibilities. But Gell-Mann and Hartle point out that a complete system (say, the universe) does not possess an external environment. Quantum physics should describe the whole enchilada. In this case, the enchilada, or a closed quantum system, can be represented by a big box (something like 65 billion light-years wide), possibly expanding, containing the particles and fields described by the laws of physics.

“Everything is contained within the box, galaxies, planets, observers and observed, measured subsystems, any apparatus that measures them, and, in particular, any human observers including us,” Gell-Mann and Hartle write in their most recent paper.

That paper refines some of their previous work into a new manifesto addressing the question of how the universe tells its story. The answer, in a form suitable for tweeting, is “narrative coarse grainings.”

Coarse graining is a familiar idea in physics. Just as road maps don’t show every pothole, physicists seldom study a system at the level of every individual molecule or atom or subatomic particle. In a roughly analogous way, the history of the universe does not lend itself to a single consistent story. Instead, a quantum universe evolves sets of consistent histories. Within a set, the histories all produce the same large-scale observable objects and processes. Differences among the histories within a set are at too small a scale for anybody to detect. So they don’t affect the reality observed at the macroscopic, or “classical” level, the predictable world of ordinary experience.

Quantum effects do show up occasionally, though, so Gell-Mann and Hartle call the set of consistent histories of the ordinary world a “quasiclassical realm.”

Other sets of consistent histories may exist. Histories can “branch” into alternative quasiclassical realms, like the multiple universes of Everett’s many worlds.

Measurement results, and the large-scale reality we observe in general, depend on this branching. In some branches, for instance, the sun might never have formed. But in our branch, it did, as one part of a protostellar gas cloud became denser than the rest. In a denser region, more particles collide, so decoherence happens more rapidly, leading to the construction of objects and their environments. Records of what happens to those objects persist in the environment, preserving a narrative history of our quasiclassical realm.

In their new paper, Gell-Mann and Hartle tackle these issues with elaborate mathematical depth. Words can convey only some of the flavor of it. In a blog, however, it is permissible to summarize the key points without the math.

First is the notion of “adaptive coarse graining.” The level of coarse graining found in a quasiclassical realm is the most refined graining possible consistent with the features of classical physics. Such fortunate coarse graining occurs only in some branches of history, so whether a quasiclassical realm contains things like planets and stars depends on the specific features on the immediately preceding branch. “That way the quasiclassical realms can be a property of our universe, and not just our choice,” Gell-Mann and Hartle note.

Second, adaptive coarse graining in turn permits the evolution of narrative histories (or sets of histories) that tell a consistent story of how the universe evolves and its features change over time. “Narrative sets of histories … allow the construction of an environment,” Gell-Mann and Hartle write. “They therefore put the notion of environment in its proper place as a consequence of a narrative coarse graining, and not as a separate postulate of quantum mechanics.”

Third, decoherence needs to be strong — that is, not only the chair gets a precise position, but so do other things that you aren’t particularly paying attention to, creating “records” that are preserved over time.

“Strong decoherence ensures that the past remains permanent as a set of histories is fine-grained by extending it into the future,” Gell-Mann and Hartle explain. In other words, consistent histories permit the classical behavior that is well described by Newton’s pre-quantum laws. And give us a consistent narrative for relating the history of the universe. Reality is as it is recorded. It may not be the only reality, or the best reality, but it’s our reality.

As you may have noticed, the coarse-grained level of blogging does not fully capture the technical nuances of the Gell-Mann/Hartle interpretation. Their view is simply not as easy to grasp as some of the other interpretations, even Quantum Bayesianism.

But even if both these (and all the other) quantum interpretations could be quickly comprehended by a bright 12-year-old, there are still personal preferences at work that render consensus unlikely.

Quantum Bayesianism’s advocates, do after all, make the valid point that people are involved in using the quantum math to compute probabilities and make measurements. And Gell-Mann and Hartle are undeniably correct that people exist within a closed-system universe and should be governed by the same physical laws as everything else.

So maybe there’s some sense in which these competing quantum interpretations are not mutually exclusive. After all, quasiclassical realms do contain observers. Gell-Mann and Hartle call them information gathering and utilizing systems, or IGUSes. And Gell-Mann has said that the role of observers in a quantum universe is to “make bets” based on the quantum probabilities.

Or maybe these approaches are mutually exclusive, but complementary, in the way that some quantum processes can be described using either waves or particles, just not both at the same time.

In any case, similar issues about the nature of reality occupied deep thinkers long before quantum physics came along. In a rough way, the array of competing quantum interpretations today parallels the old question of whether reality is an objective feature of the world or something in the mind, a question that mathematician-philosopher-theologian Alfred North Whitehead addressed in Science and the Modern World.

Interactions of objects in space and time cannot be separated from perception of them, Whitehead pointed out. “You cannot tear any one of them out of its context,” he wrote. “Yet each of them within its context has all the reality that attaches to the whole complex.” The universe evolves during the continuous process of perceptions being unified with what’s being perceived.

“Thus nature is a structure of evolving processes,” Whitehead asserted. “The reality is the process.”

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