This means something for the measurement process: The quantum objects under observation become entangled with the atoms of the measuring instrument. “Measurement” here doesn’t have to imply probing the object with some fancy bit of scientific kit; it applies to any quantum object interacting with its environment. The molecules in an apple are described by quantum mechanics, and photons of light bouncing off the surface molecules get entangled with them. Those photons carry information about the molecules to your eyes — say, about the redness of the apple’s skin, which stems from the quantum energy states of the molecules that constitute it.

In other words, Zurek and Zeh realized, entanglement is ubiquitous, and it is the information conduit between quantum and classical. As a quantum object interacts with its environment, it becomes entangled with it. Using nothing but regular quantum math, Zeh and Zurek showed that this entanglement “dilutes” the quantumness of the object because it becomes a shared property with the entangled environment, so that quantum effects quickly become unobservable in the object itself. They call this process decoherence. For example, a superposition of the quantum object becomes spread out among all its environmental entanglements, so that to deduce the superposition we’d need to examine all the (rapidly multiplying) entangled entities. There’s no more hope of doing that than there is of reconstructing a blob of ink once it has dispersed in the ocean.

Zurek shows that pointer states can be efficiently and robustly imprinted again and again in the environment. Such states are the “fittest,” he told me. “They can survive the process of copying, and so the information about them can multiply.” They are, by analogy with Darwinian evolution, “selected” for translation to the classical world because they are good at becoming amplified — replicated, you could say — in this way. This is the “quantum Darwinism” of Zurek’s book title.

This leads us to another revelation of decoherence theory, the one that persuades me that Zurek’s theory now tells a complete story. It predicts that all the imprints must be identical. Thus, quantum Darwinism insists that a unique classical world can and must emerge from quantum probabilities. This imposition of consensus obviates the rather mysterious and ad hoc process of collapse, in favor of something more rigorous. The object being observed, surrounded by a cloud of identical, observable imprints of it in its macroscopic environment, forms an element of “relatively objective existence,” as Zurek puts it. It becomes a part of our concrete classical reality, which he calls an extanton.