Science1 publisher2 min readPublished
Jonathan Halliwell puts environmental decoherence where the textbooks put the observer
Jonathan Halliwell of Imperial College London told Quanta's The Joy of Why that interactions with the surrounding environment carry the transition from quantum to classical behaviour, and that he tracks that transition with the histories approach.
The Scientist · Science desk
What happened
- Quanta's podcast The Joy of Why put Steven Strogatz in conversation with Jonathan Halliwell, a theoretical physicist at Imperial College London who has worked on the foundations of quantum theory.
- Halliwell tells Strogatz that decoherence, the dispersal of fragile quantum behaviour through interactions with the surrounding environment, is key to the transition from quantum to classical behaviour.
- He explains why that transition does not require a conscious observer to be present and looking at the system.
- The tool he uses to understand the transition is the histories approach, which he describes over the course of the episode.
- He also takes up Einstein's question of whether the moon is really there when no one is looking at it.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint More decimal places will not decide what quantum mechanics means: precision constrains predictions, and the interpretive dispute has survived all of it.
- capability Locating the transition in environmental interaction makes it a modelling problem: specify what a system is coupled to, and the loss of quantum behaviour becomes something you can calculate.
- decision A reader who wants to check the machinery has to go past the write-up to the episode and the published work; the probability rule is the part the write-up leaves out.
- precedent A senior practitioner closing an interview on how he lives with competing perspectives is a signal about the working norm in foundations, where interpretations coexist for decades without being eliminated.
Halliwell came to these questions from quantum cosmology, where quantum mechanics gets applied to the universe as a whole [11]. Strogatz named the picture that makes that awkward. In his college course and in the popular accounts he read, an observer sat outside the experiment and collapsed wave functions by making measurements [12]. If the system under study is everything, there is no outside left for that observer to occupy [18].
Janna Levin, co-hosting, stated the standing problem in one breath: "Quantum mechanics, the most precisely tested paradigm in all of science to the largest number of decimal points, and still there's no coherent interpretation" [8]. She added, "I don't know anyone who's gonna claim to understand quantum mechanics fully" [14]. She also noted where the precision shows up in daily life: "we can send things to space. It works in your phones, and it works to that level of precision in an accelerator experiment" [15]. Strogatz put the figure at "one part in 10 to the 12th or some astonishing precision", hedge included [9]. One part in 10 to the 12th is one part in a trillion [10].
Quanta's own framing holds both halves at once: quantum mechanics is among the most precisely tested paradigms in science, and there is still no universally accepted interpretation of what it tells us about reality [7]. On that record, more decimal places have not produced interpretive agreement [16].
A practitioner who has spent his career on quantum foundations [2] puts interaction with the surrounding environment where the textbook put the measurement, and says the transition needs no conscious observer [3][5]. Whether that fully dissolves the measurement problem is not something an interview establishes. Quanta's written summary names the histories approach without setting out how it assigns probabilities [17].
Levin, on why physicists keep at it, said "nobody's ever made a great discovery by throwing their hands in the air and giving up, right?" [19]. The episode ends with Halliwell explaining how yoga and meditation help him live with competing perspectives in science [13].
What to watch
- Whether the histories program yields a prediction that differs measurably from other interpretations, which would move it from interpretation to experiment.
- Whether proponents of rival interpretations answer Halliwell's claim that the transition needs no conscious observer.
- Whether Quanta publishes the section of the conversation in which Halliwell lays out how histories are assigned probabilities.