It is argued that only open systems can be observed and pointed out that the renormalization group is designed to deal with them in a systematic manner. A more careful view of open dynamics leads to the Schwinger-Keldysh formalism. A distinguished feature of this scheme, the formal redoubling of the degrees of freedom, is motivated in classical and quantum mechanics in the first part of the talk. The second part is devoted to the universal nature of statistical physics, characterized by few thermodynamical variables. It is shown in the framework of an open harmonic oscillator that the description of the quantum dynamics, usually covered by the Kubo-Martin-Schwinger approach, requires further parameters. Finally, in the third part, the renormalization group method is applied to quantum field theory. It is argued that quantum field theories are always open owing to their UV divergences and shown that the 3+1 dimensional open scalar field theory displays a pre-classical phase with strong IR-UV entanglement. Furthermore, it is conjectured that the renormalization conditions for the open parameters can replace the maximal entropy principle of statistical physics.

(Szilárd Leó Colloquium)