|The divine comedy|
c-decay theory[note 1] is a pseudoscientific creationist cosmology put forward by cdesign proponentsists. It attempts to solve the starlight problem by claiming that the speed of light in a vacuum was faster in the past and has since decayed to the value we observe it to be today.
Since the development of electronic digital counters and pulsed lasers, it has been possible to measure the speed of light in the laboratory with extraordinary precision. Even if the speed of light reached the proximity of its final value decades ago, there would be enough residual decay as the value reached its limit asymptotically for our modern apparatus to detect. There is none, forcing the proponents of the theory to toss out the exponential decay which governs nearly every phenomenon in the universe in favor of wild trigonometric functions they found by brute force curve-fitting, without an underlying explanation of "why". Ultimately, even many creationists have abandoned c-decay.
Keep in mind that, in order to be useful for validating an age of the universe less than 10,000 years rather than more than 10 billion years, the speed of light needs to be more than a million times faster, a difference which would be difficult to miss. We're not talking about a difference of a fraction of a percent.
 Notes about c
Creationists would have us believe that the speed of light is arbitrary and somehow separated from the rest of reality; however, c is not just "the speed of light." It is a universal constant which is observed as unchanging no matter who is observing it. It can be thought of as the speed which all objects fly through the four dimensions of spacetime — if you move along "space" you have less of the speed left to go through "time" and you experience the effect of time dilation. "c" is also used in many equations related to electromagnetic phenomenon such as Maxwell's equations; and it is the fact that these equations mention c without asking what the speed is relative to as should happen in a relativistic universe that lead to the notion that c is constant for all observers. It is also a key component in Einstein's famous E = mc2 equation; in this case, if c was larger in the past, then matter would have had more unit of energy per unit of mass in the past. Spontaneously losing this energy would at least violate the law of conservation of energy in some way.
The speed of light may be intimately related to two other physical constants: the vacuum permittivity ε0 and the vacuum permeability μ0:
Therefore, a variation on the value of c implies variation either on ε0 or μ0 or both. These properties of space are measured in experiments that don't even involve light, such as experiments with capacitors and magnets.
c-decay, therefore, does not simply mean that "light travels a bit faster;" it means that the very fabric of reality would be subject to change in the temporal dimension. So believe us, anyone who postulates that "c may decay" needs to understand that it has very serious and very far-reaching consequences. One of the most far-reaching being that the speed of light changing with time implies that energy is not conserved.
 c-decay and the fine-tuned universe
c-decay may be in conflict with the argument that the fundamental constants of the universe are, and must be, fine-tuned for life as we know it. Indeed, the speed of light is closely related to the fine-structure constant α, which may be defined as
- e is the elementary charge;
- ħ = h/2π is the reduced Planck constant;
- c is the speed of light in vacuum;
- ε0 is the electric constant or permittivity of free space;
- or µ0 is the magnetic constant or permeability of free space;
- or ke is the Coulomb constant.
J.D Barrow published in 2001 that stable matter, and therefore life and intelligent beings, could not exist if the value of α were much different. Barrow wrote that if α to increase by 4%, "there would be a disaster... Stars would rapidly... collapse to degenerate states or black holes"; while if α were 10% smaller, the "pathways to the buildup of biological elements would be blocked."
 Definition of the meter
Currently, the second is defined in terms of the frequency of the hyperfine transition in the cesium atom, and the meter is defined in terms of how far light travels in a vacuum in
one 1/299,792,458 of a second. In other words, the unit of distance depends on the speed of light and a measure of time, not on the length of some specific physical object (although it used to between 1889 and 1960). If an experiment were to indicate that it takes more cesium transitions for light to travel to a previously measured distance, then based on the definition of our measurement units, we would actually be forced to conclude that the measured distance has increased, i.e., that everything is getting larger, not that the speed of light has decreased.
 Not to be confused with
- Carbon dating, a scientific method for dating organic matter, based on the radioactive decay of 14C.
- ↑ This is notably a lowercase c, as the constant representing the speed of light in science is always given in the lowercase, even when this causes a grammatical nightmare. Such are the problems when combining scientific notation with the language the rest of us speak.
- ↑ The Decay of c-decay
- ↑ http://www.icr.org/index.php?module=articles&action=view&ID=283
- ↑ page 147 in: J.D. Barrow (2001), "Cosmology, Life, and the Anthropic Principle" Annals of the New York Academy of Sciences volume 950 number 1 (December 2001) pages 139-153. doi: 10.1111/j.1749-6632.2001.tb02133.x