physics, mathematical physics, philosophy of physics
theory (physics), model (physics)
experiment, measurement, computable physics
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Types of quantum field thories
In quantum theory the state of a physical system is given by a vector in a complex vector space (usually a Hilbert space). This means that given any two states $\psi_1$ and $\psi_2$, also their sum as vectors $\psi_1 + \psi_2$ is represents a state. This is called the superposition of the two states.