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## Main Question or Discussion Point

Every particle in the universe has been, at a given point of its history, interacting with other particles. So, how is it possible to decide whether a given particle can be treated as an isolated quantum system the states of which are vectors of a single Hilbert space? Altenatively, what conditions will force us to consider electron pairs emitted by radioactive decay or photon pairs resulting of radiative cascade, as a single sytem describe by a tensor product of two Hilbert spaces?

What is the criteria to decide whether a system is bipartite?

What is the criteria to decide whether a system is bipartite?