c-decay theory 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."
 Are physical constants really constant?
Real scientists do care about investigating whether the physical constants are in fact constant. Indeed, the study of possibly varying constants has been motivated by several emerging theories such as the string theory. The first experimental tests of this question examined the spectral lines of distant astronomical objects, and the products of radioactive decay in the Oklo natural nuclear fission reactor. The findings were consistent with no change.
More recently, improved technology has made it possible to probe the value of the fine-structure constant α at much larger distances and to a much greater accuracy. In 1999, a team led by John K. Webb of the University of New South Wales claimed the first detection of a variation in α. Using the Keck telescopes and a data set of 128 quasars at redshifts 0.5 < z < 3, Webb et al.. found that their spectra were consistent with a slight increase in α over the last 10–12 billion years. Specifically, they found that
However, in 2007 simple flaws were identified in the analysis method of Chand et al., discrediting those results. Nevertheless, systematic uncertainties are difficult to quantify and so the Webb et al. results still need to be checked by independent analysis, using quasar spectra from different telescopes.
King et al. have used Markov Chain Monte Carlo methods to investigate the algorithm used by the UNSW group to determine from the quasar spectra, and have found that the algorithm appears to produce correct uncertainties and maximum likelihood estimates for for particular models. This suggests that the statistical uncertainties and best estimate for stated by Webb et al. and Murphy et al. are robust.
Lamoreaux and Torgerson analyzed data from the Oklo natural nuclear fission reactor in 2004, and concluded that α has changed in the past 2 billion years by 4.5 parts in 108. They claimed that this finding was "probably accurate to within 20%." Accuracy is dependent on estimates of impurities and temperature in the natural reactor. These conclusions have to be verified.
In 2007, Khatri and Wandelt of the University of Illinois at Urbana-Champaign realized that the 21 cm hyperfine transition in neutral hydrogen of the early Universe leaves a unique absorption line imprint in the cosmic microwave background radiation. They proposed using this effect to measure the value of α during the epoch before the formation of the first stars. In principle, this technique provides enough information to measure a variation of 1 part in 109 (4 orders of magnitude better than the current quasar constraints). However, the constraint which can be placed on α is strongly dependent upon effective integration time, going as t−1/2. The European LOFAR radio telescope would only be able to constrain Δα/α to about 0.3%. The collecting area required to constrain Δα/α to the current level of quasar constraints is on the order of 100 square kilometers, which is economicallly impracticable at the present time.
In 2008, Rosenband et al. used the frequency ratio of Al+ and Hg+ in single-ion optical atomic clocks to place a very stringent constraint on the present time variation of α, namely Δα̇/α = (−1.6 ± 2.3) × 10−17 per year. Note that any present day null constraint on the time variation of alpha does not necessarily rule out time variation in the past.
In September 2010 researchers from Australia said they had identified a dipole-like structure in the fine structure constant across the observable universe. According to studies on quasars using the Very Large Telescope, the fine structure constant appears to have been larger by one part in 100,000 in the southern direction, 10 billion years ago. Similarly, the constant appeared to have been smaller by a similar fraction in the northern direction, billions of years ago.
 Definition of c
Currently, the second is defined in terms of the oscillation of 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. Thus, according to current definitions, it is actually logically impossible for c to change. This is not merely some hyper-technical "gotcha" defense; if one measures that the time that light takes to travel a distance is decreasing, there are three possible interpretations: everything is getting smaller, and we can't tell because all of our measuring devices are getting smaller as well; time is speeding up, and we can't tell because all of our clocks are speeding up as well; or light is indeed traveling faster. It is completely arbitrary which of these interpretations we pick, and there can be no objective meaning to saying that c is "slowing down."
 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
- ↑ J.-P. Uzan (2003); The Fundamental Constants and Their Variation: Observational Status and Theoretical Motivations;Reviews of Modern Physics; 75; pg 403-455}}
- ↑ J.-P. Uzan (2004); Variation of the Constants in the Late and Early Universe; astro. ph.
- ↑ K. Olive, Y.-Z. Qian (2003); Were Fundamental Constants Different in the Past?; Physics Today; 57; 10; pg 40-45
- ↑ J.D. Barrow (2002);The Constants of Nature: From Alpha to Omega—the Numbers That Encode the Deepest Secrets of the Universe;Vintage
- ↑ J.-P. Uzan, B. Leclercq (2008);The Natural Laws of the Universe: Understanding Fundamental Constants;Springer Praxis
- ↑ F. Yasunori (2004);Oklo Constraint on the Time-Variability of the Fine-Structure Constant, Astrophysics, Clocks and Fundamental Constants; pg 167–185; Lecture Notes in Physics;Springer Berlin;
- ↑ J.K. Webb et al. (1999); Search for Time Variation of the Fine Structure Constant; Physical Review Letters;82;5;pg. 884–887
- ↑ M.T. Murphy et al. (2001); Monthly Notices of the Royal Astronomical Society;327; pg 1208
- ↑ J.K. Webb et al. (2001); Further Evidence for Cosmological Evolution of the Fine Structure Constant;Physical Review Letters;87;9
- ↑ M.T. Murphy, J.K. Webb, V.V. Flambaum (2003); Further Evidence for a Variable Fine-Structure Constant from Keck/HIRES QSO Absorption Spectra;Monthly Notices of the Royal Astronomical Society;345; pg. 609
- ↑ H. Chand et al. (200) ; Probing the Cosmological Variation of the Fine-Structure Constant: Results Based on VLT-UVES Sample; Astronomy & Astrophysics; 417; pg. 853
- ↑ R. Srianand et al. (2004);Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars; Physical Review Letters;92;pg. 121302
- ↑ M.T. Murphy, J.K. Webb, V.V. Flambaum (2007); Comment on "Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars;Physical Review Letters;99;pg. 239001
- ↑ M.T. Murphy, J.K. Webb, V.V. Flambaum (2008); Revision of VLT/UVES Constraints on a Varying Fine-Structure Constant;Monthly Notices of the Royal Astronomical Society; 384; pg. 1053
- ↑ J. King, D. Mortlock, J. Webb, M. Murphy (2009);Markov Chain Monte Carlo methods applied to measuring the fine structure constant from quasar spectroscopy
- ↑ R. Kurzweil (2005); The Singularity Is Near;Viking Penguin;pg.139–140
- ↑ S.K. Lamoreaux, J.R. Torgerson (2004);Neutron Moderation in the Oklo Natural Reactor and the Time Variation of Alpha;Physical Review D;69
- ↑ E.S. Reich (30 June 2004); Speed of Light May Have Changed Recently;New Scientist
- ↑ Scientists Discover One Of The Constants Of The Universe Might Not Be Constant (12 May 2005);ScienceDaily
- ↑ 24.0 24.1 R. Khatri, B.D. Wandelt (2007); 21-cm Radiation: A New Probe of Variation in the Fine-Structure Constant; Physical Review Letters;98; pgs 111301
- ↑ T. Rosenband et al(2008);Frequency Ratio of Al+ and Hg+ Single-Ion Optical Clocks; Metrology at the 17th Decimal Place;Science;319;5871;pg. 1808
- ↑ H. Johnston (2 September 2010);Changes spotted in fundamental constant;Physics World