A phenomenon in which constant observation of an unstable particle prevents it from decaying. In general, such a particle would be expected to decay within a time τ‎. If numerous measurements are made at intervals within time τ‎, the wave function is constantly collapsed, and the lifetime of the particle is prolonged. It can also be shown that if measurements are made at time intervals longer than τ‎ there is an anti-Zeno effect in which the decay rate increases. These effects were predicted theoretically in 1977 and have subsequently been observed to occur in a number of experiments. The name is an allusion to a paradox put forward by the Greek philosopher Zeno of Elea (b. c.490 bc). In the ‘arrow paradox’ he argued that at every moment of time, a moving arrow is occupying a definitive position. Therefore at every moment, the arrow is immobile, and the arrow never moves.