Isaac Newton’s classical view of light (consisting of very small material bodies) appeared to lose validity following Thomas Young’s double-slit experiments. These convincing demonstrations in the first decade of the nineteenth century showed that light must propagate like ocean waves, but transverse rather than normal to the direction of travel.
Then, once again, uncertainty reigned when in 1877 Albert Michelson and Edward Morley failed to detect the existence of an ether “wind” created by the earth’s motion through a luminiferous medium.
Following Albert Einstein’s assertion that light has a dual wave-particle nature, the idea of a photon regained validity. It was further refined by a number of twentieth-century quantum theoreticians, whose work came together first in the Copenhagen Interpretation and then in the Standard Model.
The Copenhagen Interpretation, primarily the work of Niels Bohr and Werner Heisenberg in the late 1920s, is more of a doctrine than the word “interpretation” would seem to suggest. There are three principle assertions that apply to space, time and material bodies:
All changes that take place in nature consist of indeterminate physically discontinuous and abrupt changes from one state to another.
It is, in principle, impossible to define disparate attributes of an elementary particle for a unique location in time and space.
The wave-particle dualism is absolute and objective.
The Standard Model is more or less an international consensus among subatomic particle theoreticians and experimenters. It surfaced in the final decades of the twentieth century and is currently widely accepted subject to as-yet unverified refinements such as contained in string theory.
At present we have to say that the Standard Model remains a work in progress as it does not incorporate a complete theory of gravitation. Nor does it account for the observed accelerating expansion of the universe.
The Standard Model describes the electromagnetic, weak and strong nuclear reactions. Additionally, it lists with principle attributes all known subatomic particles. The Standard Model as a theoretical construct has been remarkably successful in that its predictions have been repeatedly verified by experiment. A major triumph was the discovery of the Higgs Boson in 2013, the existence of which had been predicted, required even, by the Standard Model on theoretical grounds.
A pre-eminent elementary particle is the photon. We all experience it every day. (Even the visually impaired feel the warming rays of the sun.) But what do we know for certain about the photon?
Except for virtual photons (that’s another story), photons are never at rest. They travel at the nominal speed of light, whatever that may be for various media.
A photon has zero mass or rest energy. It can be created or destroyed whenever radiation is absorbed or emitted. Emission can be spontaneous or stimulated by an outside energy source, as in a laser.
Because it is a boson, not a fermion, the photon is a spin-1 particle. Polarization of light is made possible by the fact that the spin axis is parallel to the direction of travel.
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