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 wavefunction 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.