Two approaches toward the arrow of time for scattering processes have beenproposed in rigged Hilbert space quantum mechanics. One, due to Arno Bohm,involves preparations and registrations in laboratory operations and results intwo semigroups oriented in the forward direction of time. The other, employedby the Brussels-Austin group, is more general, involving excitations andde-excitations of systems, and apparently results in two semigroups oriented inopposite directions of time. It turns out that these two time arrows can berelated to each other via Wigner's extensions of the spacetime symmetry group.Furthermore, their are subtle differences in causality as well as thepossibilities for the existence and creation of time-reversed states dependingon which time arrow is chosen.
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