This chapter considers three concepts that were central to popular articulations of quantum physics: wave–particle duality, the function of observation in Erwin Schrödinger’s wave-mechanics, and the idea that physics describes an insubstantial world, opposed to the world of common sense. Taking these concepts in turn, it shows that in Woolf’s novels, as well as in scientific radio broadcasts, in the popular science writing of G. P. Thomson, Arthur Eddington, and James Jeans, and in modernist culture more broadly, they become associated with the idea that identity is multiple. The first section, ‘Rays around a Point: Wave–Particle Duality’, examines a passage from The Wavesin relation to accounts of G. P. Thomson’s electron-diffraction experiment, which confirmed the wave–particle duality of matter. The second section, ‘“Its Beam Strikes Me”: Schrödinger’s Wave Equation’, considers the role of observation in limiting identity in The Waves and The Years, identifying resonances with popular accounts of Erwin Schrödinger’s wave function. Tthe last section, ‘“We Ripple in Light”: Insubstantial Selves’, examines the interplay between solidity and insubstantiality in Woolf’s novels and in contemporary popular science.