Half-life is the time it takes for half of the original sample of a radioactive isotope to decay. Different isotopes have different half-lives, some being very short and difficult for scientists to study. If the exponential decay of a radioisotope is expressed as , then the half-life is equal to ln 2. Knowing these decay rates is important for dating, such as with carbon-14.
It should be noted that the measurement of half-life is based on recent observations, and the numbers that follow from radioactive dating based on nuclear decay extrapolate from this assumption. For example, if a sample of biological material has 1/4 of the Carbon-14 relative to the rate a similar biological material has, constant half-life extrapolation dates this sample as two half-lifes old (in the order of 11,000 years). The creationist blog Answers in Genesis suggests that half-life varies over time. An analogy for this extrapolation would be taking the population growth of Germany today (a negative rate), extrapolating backwards, and concluding that there were 100 billion Germans during the time of the fall of the Roman Empire.
- Wile, Dr. Jay L. Exploring Creation With Physical Science. Apologia Educational Ministries, Inc. 1999, 2000
- Avison J. The World of Physics; Thomas Nelson and Sons; Cheltenham. p. 395, (1984) 
- Raising the Bar on Creation Research - by Don DeYoung, Ph.D.