nt - p h / 98 05 05 7 v 1 1 9 M ay 1 99 8 Arrival Times in Quantum Theory from an Irreversible Detector Model

We investigate a detector scheme designed to measure the arrival of a particle at x = 0 during a finite time interval. The detector consists of a two state system which undergoes a transition from one state to the other when the particle crosses x = 0, and possesses the realistic feature that it is effectively irreversible as a result of being coupled to a large environment. The probabilities for crossing or not crossing x = 0 thereby derived coincide with earlier phenomenologically proposed expressions involving a complex potential. The probabilities are compared with similar previously proposed expressions involving sums over paths and the decoherent histories approach.