AQFT and operator algebra
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Fell’s theorem is about a property of vector states of a C-star algebra, it says that if the kernels of two representations of the algebra coincide, then the vector states are mutually weak-* dense. This has a profound consequence for the AQFT interpretation: A state represents the physical state of a physical system. Since one can always only perform a finite number of measurements, with a finite precision, it is only possible to determine a weak-* neigborhood of a given state. This means that it is not possible - not even in principle - to distinguish representations with coinciding kernels by measurements.
For this reason representations with coinciding kernels are sometimes called physically equivalent in the AQFT literature.
Let A be a unital $C^*-$algebra and $\pi_1, \pi_2$ be two representations of A on a Hilbert space H.
equivalence theorem Every vector state of $\pi_1$ is the weak-* limit of vector states of $\pi_2$ iff the kernel of $\pi_1$ contains the kernel of $\pi_2$.