Small things are quantum and big things are classical. But where is the boundary between the two regimes and what happens there? Once the preserve of gedanken experiments, such questions are increasingly tackled in the lab. Markus Arndt, Anton Zeilinger, and their colleagues at the University of Vienna explore the quantum-classical boundary by subjecting ever larger molecules to matter interferometry. Their latest experiment, on hot C70 buckyballs, supports the emerging consensus that an object’s interaction with the environment, rather than its mere size or even its complexity, dictates its classicality. 1  

The Vienna group’s experiment is conceptually simple: Heat buckyballs, send them through an interferometer, then watch how the fringes depend on the buckyballs’ temperature. The higher the temperature, runs the theory, the more a molecule radiates as it cools and the more it should lose its fringe-forming coherence to the environment.

Freshly baked buckyballs emerge from the lab’s...

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