We understand quantum mechanics well enough to make stunningly accurate predictions, ranging from atomic spectra to the structure of neutron stars, and to successfully exploit these predictions in devices such as lasers, MRI machines, and tunneling microscopes. Yet there is no generally accepted explanation of how the solid reality of such devices—or of objects such as cats, moons, and people—arise from a nebulous quantum wave in an abstract mathematical space. Some physicists prefer to ignore the problem, suggesting that we should just “shut up and calculate!” Others seek answers by modifying quantum theory in various ways or by searching for ways to explain how stable structures can emerge from quantum theory itself. Taking the latter approach, Philipp Strasberg and colleagues at the Autonomous University of Barcelona in Spain use simulations to show that, on large scales, a robust reality with classical features can emerge for a broad class of quantum systems, independently of their detailed microstructure [1]. Their conclusion suggests how the emergence of our classical world can be explained in the context of the “many-worlds interpretation” of quantum mechanics, in which countless parallel worlds branch off from each other each time a measurement is performed. Loosely speaking, the idea is that large-scale classical features emerge from underlying quantum dynamics much like the stable macroscopic jets of a garden sprinkler emerge from the countless microscopic tumbling trajectories of individual water molecules (Fig. 1). The results have broad potential implications, ranging from cosmology to statistical mechanics.

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