Decoherence and Quantum Darwinism:
From quantum foundations to classical reality

by Wojciech Hubert Zurek
Cambridge, 2025, 357 pages
reviewed by T. Nelson

In this book, Wojciech Zurek says that previous work on quantum mechanics, notably the Copenhagen interpretation, presumed that the universe was classical and asked how quantum states emerged from a classical system. We should, he says, turn it around: the universe is essentially quantum and we should be asking how a classical system can emerge from a quantum universe. Why do we perceive the quantum universe as a classical one?

His answer leads to quantum Darwinism. To find out what that is, you must read through 192 pages of formulas and discussion of the so-called measurement problem. And you must digest new concepts like einselection or environment-induced superselection, where the environment picks the result; envariance, or entanglement-assisted invariance; and environment-induced decoherence. The inclusion of Zurek’s favorite letter ‘e’ in these terms tells us that the environment will be playing a key role.

This has obvious implications for quantum computing, and we get a discussion of concepts from information theory such as entropy and mutual informa­tion, as well as quantum discord (i.e., the amount of quantum­ness in a system), and pointer states.

Pointers are a key concept in Zurek’s theory. The environment interaction with the system depends on an observable (Λ), which is Hermitian so it has various eigenstates |s_k⟩, which are stable states the observable thing can be in. The measuring apparatus can be thought of as a pointer that stores the informa­tion where it is saved from quantum entangle­ment.

After defining these concepts, Zurek then recasts statistical mechanics as a quantum phenomenon, doing away with the traditional probabilities, which he says are bad because they represent arbitrary groupings of particles.

Many experimental physicists, such as Jordan and Siddiqi, agree with the importance of the environment, saying that wavefunctions can be un-collapsed and that only measurement outcomes are really meaning­ful. These two physicists say the wave­function is only a model.

That’s good to keep in mind. All we have are models. On page 182 Zurek presents a color plot of the Wigner function (an important formula in decoher­ence) as it evolves in a chaotic system. The model predicts complicated interference patterns at the sub-Planck scale, which is impossible. It reminds us that these are only theories, and a theory, no matter how well founded, is always one step away from a fantasy.

What is quantum Darwinism

In spite of all the talk about qubits, Zurek’s theory is not really about qubits or quantum computation. It is an attempt to explain why the universe appears classical when in fact it is fundamentally quantum. That’s the reverse of the usual approach. Objective reality, he says, should reflect consensus between observers, not any underlying reality.

Zurek says the environ­ment is split into many sub-environments, much as classical statistical mechanics split the system into arbitrary groupings of particles, which he criticized earlier. Scattering of light in the air imprints multiple copies of the information onto the environ­ment. Photons, he says, provide massive redundancy to everyday perception, tricking us into thinking the world is classical. So in some sense this gives us a population that could undergo ‘natural selection’ by the observer.

The observer selects which subset represents the reality by picking the most stable one. Only the eigenstates of the pointer observable singled out by their predictability can be recorded in the environ­ment. So, he says, the information flow from the system S to the environment E (pretend that S and E are in mathcal font here) acts as an amplifier for the observable, while shredding the quantum part.

Exponential instability of the chaotic evolution delocalizes the wave packet, so wave packets representing quantum states become delocalized and you get breakdown of the quantum-classical corre­spond­ence. Decoherence suppresses the discrep­ancy and re-establishes the correspondence. Once this happens, Zurek says, the change becomes irreversible.

You can see why this theory is popular among those who think reality is subjective. As Alain Aspect says on the back cover, be glad he’s not invoking quantum consciousness. But is there anything more to this theory than a catchy name? To be Darwinism, it would need some mechan­ism for selecting by fitness, some way of producing diversity, and some kind of evolution that changes the nature of the entities. So far, we only have redund­ant copies of the environment that get selected on the basis of their stability. Though Zurek talks about evolution, it’s not really evolution in the Darwinian sense. The particles all have the same fate: they get eaten by the detector. Why are some copies more stable than others, and if they’re all otherwise identical, does it matter?

So quantum Darwinism is not really Darwinism at all. And to me it all seems a bit contrived. But perhaps the theory still has value anyway by giving us a fresh perspective on the quantum-to-classical transition. Maybe I’m biased: so far most of the public discussion seems to be praise from uncritical “gee-whiz” websites—“mind-blowing,” as one calls it, is definitely an exaggeration—but we’re also starting to see real physic­ists consider the ideas. In all probability the theory will evolve to find its true ecological niche.

may 8, 2026

Einstein and the Quantum Revolutions

by Alain Aspect, trans. by T.L. Fagan
U of Chicago Press, 2024, 81 pages
reviewed by T. Nelson

Einstein's relativity was the first revolution. Quantum mechanics was the second one. But Einstein has always been accused of doubting quantum mechanics when he doubted “spooky action at a distance.” He believed that some unknown variable had to be involved.

In this tiny popular style book, Alain Aspect, perhaps the most famous experimentalist in quantum optics today, describes how these doubts led to a third revolution in physics: a recognition of the importance of quantum entanglement.

Ironically, it was John Bell's attempt to refute Einstein's view in a paper that almost nobody read that gave physicists the key. But it would be hard to prove using the two-slit experiment, the experiment that would be needed. The problem was that photons leaving the slit could be sending information back to the source.

Aspect built a remarkable experimental apparatus to change the polarity of the polarizer on his detector so rapidly this would be impossible. This was the proof that was needed and it made waves well outside of physics.

Can you learn quantum mechanics in an 81-page book that can be read in twenty minutes? No. But a reader knowing little about science can get a good nontech­nical explanation of why Einstein deserves more credit for it than he gets.

Aspect deserves praise for setting the record straight. It’s also a valuable lesson: if somebody has reserva­tions about your theory, it might mean there’s something important to be discovered.

may 14, 2026