Researchers from Austria have devised a looking-glass world in which the normal roles of light and matter are reversed. In the experiments of Anton Zeilinger's group at the University of Innsbruck, crystals are made of pure light, and the rays that bounce off them are made of atoms.
This strange reversal is made possible by quantum physics, which ascribes some very counterintuitive properties to atoms. Albert Einstein's studies of the so-called photoelectric effect, in which light knocks electrons out of metals, showed at the beginning of the century that light can sometimes act like a stream of particles, not a continuous wave. And in 1923 the French aristocrat and scientist Louis de Broglie argued that if light could be both wave and particle, then so should matter be capable of this duality. De Broglie's hypothesis built on the new framework of quantum mechanics, the science of very small particles, and it paved the way for Erwin Schrödinger's 'wave mechanics', which described the fundamental constitution of matter in terms of electron waves.
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