### Speaker

Wojciech Zurek
(Los Alamos)

### Description

A system in equilibrium does not evolve: time independence is its telltale characteristic. However, in Newtonian physics microstate of an individual system (a point in its phase space) evolves incessantly in accord with its equations of motion. Ensembles were introduced in XIX century to bridge that chasm between continuous motion of phase space points in Newtonian dynamics and stasis of thermodynamics: While states of individual classical system inevitably evolve, a phase space distribution of such states -- an ensemble -- can be time-independent. I show that entanglement (e.g., with the environment) can yield time-independent equilibrium in an individual quantum system. This allows one to eliminate ensembles -- an awkward stratagem introduced to reconcile thermodynamics with Newtonian mechanics -- and use individual system interacting with its heat bath to represent equilibrium and to elucidate the role of information and measurements in physics. Thus, in our quantum Universe one can practice statistical physics without ensembles. I illustrate classical difficulties and the need for ensembles with Szilard's engine, and show that in a similar quantum engine a single system entangled with the environment is enough. The role of Maxwell's demon (which in this quantum context resembles Wigner's friend) is also discussed.

### Primary author

Wojciech Zurek
(Los Alamos)