Something usually seems real only when we can detect it with our own senses. The famous 1972 photograph of Earth as a blue marble rising above the Moon is a dramatic case in point. It makes our planet appear round in a vivid way unseen before.

What, then, can we make of atoms, which will always be too small to see with ordinary visible light? The speculation that matter is made of atoms goes back millennia to Lucretius’s De Rerum Natura (On the Nature of Things). Compelling scientific evidence is not that old, but it does go back centuries to an unexpected corner of physics—the fracture of solids—where the influence of atoms, if not the atoms themselves, is visible.

Fracture is the process by which external forces break a solid object into pieces. It can happen in any solid but is most suggestive as it occurs in...

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