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 by Setterfield and Trevor Norman, hosted at Lambert Dolphin's web site.

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 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.

The bulk of the paper is huge tables, footnotes, and descriptions of many constants and many historical measurements of same. That part is extremely difficult to read. The readable part is "APPENDIX I: NON-TECHNICAL SUMMARY", which is the last 7% of the paper. The important points made are that:

• "Atomic time" is determined by atomic process. And, very importantly, the speed of light, as measured in atomic time, is unchanged. This is said to be because "All light comes from atomic processes, and the speed of light is measured in kilometers per second." The possibility that distance measurements have changed is not considered.
• "Dynamical time" is measured by the Earth's daily rotation or yearly revolution. It is said to be related to gravity.
• Historical measurement of time (and hence of the speed of light) has, in the past, been tied to dynamical time, not atomic time. This makes sense, because the Cesium clock was invented fairly recently, and, through most of the historical observations, the second was defined as one 86,400^th of the time of the Earth's rotation.
• Most importantly, the observed decay of the speed of light (discussed below) is due to either the "atomic clock" being faster in the past than the "dynamical clock", or the dynamical clock being slower. Either way, there were more orbits of an electron per Earth rotation in the past.
• Setterfield argues that it is the atomic clock that has been speeding up, rather than the dynamical clock slowing down. The dynamical time scale can't be the one that is changing. He gives a few reasons for this conclusion:

• "It can be demonstrated that gravitational phenomena are completely independent of any changes in the atom or c. In other words, if the atom is varying, the dynamical clock is not affected."
• "Furthermore, the physical behavior of the gravitational constant, G, would be in contradiction to conservation laws and theory if the atom were not changing."
• "Things tend to slow down, wear out, and get older, rather than speed up and go faster with time as a dynamical variation would require. Dynamical variation would seem to break this physical principle which is called the 2nd law of Thermodynamics."

Setterfield then gives the evidence that the speed of light (as measured in dynamical time) has been decaying during the period of a few hundred years that scientists have made measurements. The data are summarized in two graphs.

Setterfield's Figure 3. The main graph, covering the period from 1740 to 1983.

Setterfield's Figure 4. Zooming in on the period from 1860 to 1983.