放射性测定法还有一个基本假设，某种放射性元素的衰变速率是恒定不变的。如果这个假设不成立，测定的结果当然就靠不住。因此“年轻地球论者”最喜欢攻击这一假设。但是这一假设有充分的量子力学理论依据，攻击它也就是在攻击量子力学。这个假设也被实验所证实：物理学家做了很多试图改变衰变率的实验，对放射性同位素施加不同的温度（高达2000摄氏度，低达零下186摄氏度），不同的气压（从真空到几千个大气压），不同的引力场和磁场，都未能显著改变衰变率。
On the time scale of interest to the article, the fact that the assumption of constant decay rate is supported by theoretical quantum mechanics does not carry any weight. To hold that the decay rate is not constant on the time scale of a billion year is not a serious attack on quantum mechanics. Quantum mechanics is developed to account for experimental observations on microscopic time scales, consequently it is much less concrete on the billion year time scale.