Where does Quantum meet Classical?

On what scale do the quantum world and the classical world begin to cross into each other? How big does an "observer" have to be?

Researchers at the Lawrence Berkeley National Laboratory and their collaborators at the University of Frankfurt, Germany; Kansas State University; and Auburn University have established that quantum particles start behaving in a classical way on a scale as small as a single hydrogen molecule. They reached this conclusion after performing what they call the world's simplest — and smallest — double slit experiment.

See Science Daily.