Physicists know how to describe single atoms, and they know how to describe a large collection of particles, like air. But they don’t always know when to switch from one description to the other.
As few as four atoms could signal the switch, scientists now report October 24 in Science.
Andre Wenz of the University of Heidelberg and colleagues added extremely cold lithium atoms one by one to an atom trap. They then changed the internal state of one of the atoms and measured how it interacted with the others.
When only two atoms were in the trap, modeling them like individual particles best described their interaction. But when the number of atoms increased to five, the physicists found that describing the four unchanged atoms as a bulk material worked best.
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