What the hell is wave-particle duality and why do we care?

     At the close of the 19th century, the case for atomic theory, that matter was made of particulate objects or atoms, was well established. Electricity, first thought to be a fluid, was now understood to consist of particles called electrons, as demonstrated by J. J. Thomson who, led by his research into the work of Ernest Rutherford, had discovered using cathode rays that an electrical charge would actually travel across a vacuum from cathode to anode. In brief, it was understood that much of nature was made of particles. At the same time, waves were well understood, together with wave phenomena such as diffraction and interference. Light was believed to be a wave, as Thomas Young's double-slit experiment and effects such as Fraunhofer diffraction had clearly demonstrated the wave-like nature of light.

    But as the 20th century turned problems had emerged. Albert Einstein's analysis of the photoelectric effect in 1905 demonstrated that light also possessed particle-like properties, and this was further confirmed with the discovery of the Compton scattering in 1923. Later on, the diffraction of electrons would be predicted and experimentally confirmed, thus showing that electrons must have wave-like properties in addition to particle properties.

     This confusion over particle versus wave properties was eventually resolved with the advent and establishment of quantum mechanics in the first half of the 20th century, which ultimately explained wave–particle duality. It provided a single unified theoretical framework for understanding that all matter may have characteristics associated with particles and waves, as explained below. By the very end of the 20th century extremely precise results were obtained quantifying this duality, in the form of the Englert-Greenberger duality relation.

     This is my rambling on light for now.  I love the fact that light painters use such a magnificently complex "substance" for their "paint"....