C decay
C decay is a theory that postulates that the speed of light (c) gets slower over time. The concept was developed Dr. Barry Setterfield and V. S. Troitskii, based on past measurements of the speed of light. It is described in detail in the paper The Atomic Constants, Light, and Time[1] by Setterfield and Trevor Norman, hosted at Lambert Dolphin's web site[2].
The paper claims an apparent decay trend that levels off as time goes on, and which has the added benefit of accelerating nuclear decay.
This theory is overwhelmingly rejected by the scientific community.
The data are summarized in two graphs.
The paper seems to make two definitions of a time scale. One, called "atomic time", is calibrated from the orbital speed of the electrons in the Hydrogen atom. It, for all practical purposes, matches the time scale that the scientific community uses. (The scientific community currently uses a hyperfine transition in the Cesium spectrum, but they depend on the same fundamental constants and are, for all practical purposes, the same.) If "atomic time" had changed significantly during the time that scientific measurements were being made, it would have been noticed. In fact, if it differed at any point in human history, such things as colors of light would have changed noticeably.
The other scale he refers to as "dynamical time". It is apparently calculated from the Earth's orbital period around the sun. Prior to the adoption of the Cesium standard in 1967, the Earth's daily rotational period was used as the scientifically accepted time scale. Whether the Earth's daily rotation or yearly revolution is used doesn't matter; any disconnect between them at any time in human history would have been clearly noticed.
The paper seems to associate changes in the speed of light with divergence between the two time scales.