Quantum Phenomena at Large

Abstract

The advent of quantum theory was closely related to investigations of the microscopic world, and motivated by the necessity of understanding the stability of individual atoms, the nature of their spectra, the photoelectric effect, etc. As a result, quantum phenomena are commonly thought to be essentially microscopic ones, in the realms of atomic and molecular, or nuclear and subnuclear physics. And, inded, we are currently witnessing, as in this Colloquium, wonderful advances in our mastering ever more subtle quantum effects on the microscopic scale — one should even speak nowadays of the 'nanoscopic' and 'picoscopic' scales. It is my purpose here to stress that on our macroscopic scale as well (and even on the 'megascopic' one), essential phenomena require a genuine quantum understanding; after all, let us not forget that quantum theory found one of its roots in the explanation of the blackbody radiation, a phenomenon which certainly belongs to the macroscopic world. In a sense, of course, the statement that most natural phenomena, as we commonly experience them, are genuine evidence for quantum theory is rather trivial, since no chemical reactions and no biological transformations can be explained without recourse to quantum notions. But I have in mind here phenomena much less subtle and much more basic, that is, the very existence of matter in bulk, with its gross properties of cohesion and density.