Waves particles and interfering observers

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So far, so good. However, things quickly start to get complicated. An experiment a hundred years earlier, the double-slit experiment, proposed by Thomas Young, sought to determine whether light consisted of waves or particles. Young arrived at the celebrated and long-uncontroversial result that light consisted of waves: this experiment shows interference patterns, crests and troughs that support the notion that light waves exist.

However, when this experiment was repeated in 1927, with a quantum interpretation, the results were different: if you start with a coherent source of light (such as a laser), you can mostly observe wave-like behavior—except when you spain email list try to detect which of the slits the light particle (the photon) in question passes through. In other words, particles don’t like being spied on; if you try to find out the position of a particle, the very act of doing so destroys the information about its state (this is known as the observer effect). Going even one step further, Werner Heisenberg proposed the uncertainty principle in 1927: the more certainty you seek in determining the position of a particle, the less you can know its linear momentum and, therefore, its speed.