frequency or amplitude) properties of a photon can be measured, but not both at the same time. Which characteristic is measured depends on whether experimenters use a device intended to observe particles or to observe waves. When this statement is applied very strictly, one could argue that by determining the detector type one could force the photon to become manifest only as a particle or only as a wave. Detection of a photon is generally a destructive process (see quantum nondemolition measurement for non-destructive measurements). For example, a photon can be detected as the consequences of being absorbed by an electron in a photomultiplier that accepts its energy, which is then used to trigger the cascade of events that produces a "click" from that device. In the case of the double-slit experiment, a photon appears as a highly localized point in space and time on a screen. The build up of the photons on the screen gives an indication on whether the photon must have traveled through the slits as a wave or could have traveled as a particle. The photon is said to have traveled as a wave if the build up results in the typical interference pattern of waves (see double-slit experiment#Interference of individual particles for an animation showing the build up). However, if one of the slits is closed or two orthogonal polarizers are placed in front of the slits (making the photons passing through different slits distinguishable) then no interference pattern will appear, and the build up can be explained as the result of the photon traveling as a particle. Quantum mechanics predicts that the photon always travels as a wave, however one can only see this prediction by detecting the photon as a particle. Thus, the question arises: Could the photon decide to travel as a wave or a particle depending on the experimental setup? And if yes, when does the photon decide whether it is going to travel as a wave or as a particle? Suppose that a traditional double-slit experiment is prepared so that either of the slits can be blocked. If both slits are open and a series of photons are emitted by the laser then an interference pattern will quickly emerge on the detection screen. The interference pattern can only be explained as a consequence of wave phenomena, so experimenters can conclude that each photon "decides" to travel as a wave