In a previous question, I asked about the measurement problem in quantum mechanics. The sense I got is that the best interpretation we have is the many worlds interpretation, and one of the answers mentioned measurement as "anything that entangles with the system". What can cause systems to entangle with each other?
What can cause systems to entangle with each other?
It is the basic framework of quantum mechanics. If the systems are small enough,( comensurate to h, the Planck constant ) so that any interaction has to be described by quantum mechanics, the system is entangled. I.e. a system of particles is entangled if a single wavefunction can describe it in space and time .