COMPLEMENTARITY PRINCIPLE DEMONSTRATED FOR ELECTRONS. When light waves pass through a pair of slits in a screen, an interference pattern will form at a detector further along. If one of the slits is closed, or if one tries to take a peek at which way the light went then the interference pattern starts to go away. Quantum reality is shy; if you look at it, it disappears. Now a group at the Weizmann Institute in Israel have done a sort of double slit experiment with electrons and observed (for the first time with fermions, spin-one-half particles) how the resultant interference pattern dissipates the more you watch the electrons as they go through the slits, thus demonstrating Niels Bohr's complementarity principle which states that objects can have wave and particle properties, but not both at the same time. In the Weizmann experiment, led by Mordehai Heiblum, the electrons (or electron waves, depending on whether you look or not) slalom through a two-dimensional obstacle course, where they must negotiate a pair of channels, one of which (via a separate circuit called a "quantum point contact," or QPC) gives a hint as to whether an electron passed that way. Essentially, as a wave the electron passes through both channels; but if it senses that it is being watched, the electron (as a particle) goes through only the one path, diminishing the interference thereby. (E. Buks et al., Nature, 26 Feb. 1998.)