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Decay rates are well understood, and have been observed in distant galaxies.
=== Rate of decay ===
=== Rate of decay ===
Another assumption is that the rate of decay is constant over long periods of time, which is particularly implausible as energy levels changed enormously over time.
Another assumption is that the rate of decay is constant over long periods of time. Radiometric dating requires that the decay rates of the isotopes involved be accurately known, and that there is confidence that these decay rates are constant. Fortunately, this is the case. The physical constants (nucleon masses, fine structure constant) involved in radioactive decay are well characterized, and the processes are well understood. Careful astronomical observations show that the constants have not changed significantly in billions of years—spectral lines from distant galaxies would have shifted perceptibly if these constants had changed. In some cases radioactive decay itself can be observed and measured in distant galaxies when a supernova explodes and ejects unstable nuclei.
There are a few effects that can alter radioactive half-lives, but they are mostly well understood, and in any case would not materially affect the radiometric dating results. That is, the analysis of the isotopic and chemical composition of the sample has far greater uncertainty than any uncertainty in the decay rate itself.
The major reason that decay rates can change is that the electric field, from the atom's electron cloud, can change due to chemical changes. That is, electrons can move closer to or farther away from the nucleus depending on the chemical bonds. This affects the coulomb barrier involved in [[Alpha decay]], and therefore changes the height and width of the barrier through which the alpha particle must [[Quantum tunnelling|tunnel]]. The effect of this on alpha decay, which is the most common decay mode in radiometric dating, is utterly insignificant.
There is another effect that takes place in the "electron capture" type of [[Beta decay]]. This is an example of the [[Weak force]], and is fairly rare. Electron capture requires that there be an electron in the vicinity of the nucleus, so its activity depends strongly on the configuration of the electron cloud, which depends on the chemical state. In fact, it is possible to shut down electron capture completely—simply ionize the substance so that there are no electrons nearby.
There is a fairly well-known example of chemical state affecting electron capture activity. The <sup>7</sup>Be nucleus (Beryllium-7) is an electron capturer with a half-life of about 53 days, turning into Lithium-7. The variation is about 1.5%. While this half-life is way too short to be useful for radiometric dating, the effect of the chemical state is noticeable. The reason is that, because the atomic number is only four, the 2s valence electrons are very close to the 1s electrons involved in capture.<ref>Huh, C.-A., Dependence of the decay rate of <sup>7</sup>Be on chemical forms, Earth and Planetary Science Letters 171:325–328, 1999.</ref> See [[Systematics of beta decay]] for more details.
There is no reason to expect that the rate of decay of a radioactive material is largely constant,<ref>At least one example of a change in the rate has been observed in laboratory experiments. See Walker, 2000.</ref> and it was almost certainly not constant near the creation or beginning of the universe.<ref>Multiple references:
There is no reason to expect that the rate of decay of a radioactive material is largely constant,<ref>At least one example of a change in the rate has been observed in laboratory experiments. See Walker, 2000.</ref> and it was almost certainly not constant near the creation or beginning of the universe.<ref>Multiple references:
*Sibley, Andrew (August 2013). [https://creation.com/radioactive-decay-rates-and-solar-activity Variable radioactive decay rates and the changes in solar activity]. ''Creation Ministries International'' (from the ''Journal of Creation 27(2):3–4''). Retrieved January 4, 2018.
*Sibley, Andrew (August 2013). [https://creation.com/radioactive-decay-rates-and-solar-activity Variable radioactive decay rates and the changes in solar activity]. ''Creation Ministries International'' (from the ''Journal of Creation 27(2):3–4''). Retrieved January 4, 2018.