What are physical systems?

Empirical sciences thrive on empirical predictions. Empirical predictions refer to experiences. Experiences are experienced by us. We are observers. Observers are physical systems. Physical systems are … what?

System, as usually understood, is a collection of interacting parts that form a unity. For instance, atomic nucleus and electrons interact to form atoms that behave as a unity.

Identifying systems by Hilbert space factors misses this crucial aspect of systemness (Jia2025). Instead, I propose to characterize classical and quantum systems taking this aspect into account (Jia2025). In this approach, Schrödinger’s cat is not a physical system, because the dead and alive cats do not form a unity.

This opens the door to an interpretation of quantum theory that treats observers as experiencing and living physical systems exhibiting unity.

In the future, I hope to understand if transitioning from the same to different unities determines the preferred-basis for quantum events. Grouping elements according to unities might also determine an objective coarse-graining that is not subject to the choice of observers. The latter could be useful for understanding the foundations of statistical physics and arrows of time).