There is a famous experiment in which photons or electrons are fired individually towards a detector screen through an intermediate screen containing two small holes. On arriving at the detector screen each particle makes a dark spot. As the impacting particles accumulate, however, what emerges is neither a random arrangement of spots nor the repeated arrival of particles behind one or other of the holes.
Surprisingly, the detector screen instead displays the characteristic pattern made by a wave of light diffracting and interfering on passing through two holes simultaneously. The shocking conclusion drawn by observers of the "double-slit experiment" (as it is known) is generally cushioned by the word "seem": singly emitted particles seem not only able to pass through two holes simultaneously but to have an awareness of past and future enabling them to "select" a point of arrival on the detector screen that contributes to the classical interference pattern for waves.
Moreover, when the set-up includes a detector indicating which of the two holes the particle passed through on its way to the detector screen, the result is equally astounding: the particle passes through one hole only and there is no interference pattern. Instead a single spot appears behind that hole. The particle seems to know when it is being observed.
