Radiometric Dating is a reliable tool that scientists use to figure out how old things are. Here's how it works: Uranium, specifically 235U, decays over time into 207Pb (Lead). In the laboratory, scientists can measure how long it would take for half of any amount of 235U to decay into a 207Pb. The time it takes for half the amount of 235U to decay into 207Pb is called the Half Life.
When scientists attempt to date something by this method, they look at how much of each of these isotopes are present, and calculate how long it would take for that amont of decay to occur.
But there are complications: Creationists often ask, "How do you know how much 207Pb was there to begin with?" The answer is that when a rock is formed, it takes in an equal amount of 204Pb, which is an isotope that is never, ever produced by radiometric decay. So we can look at the amount of 204Pb and figure out how much 207Pb was there to begin with. We subtract that amount from the present amount of 207Pb, and we end up with a very precise estimate of how much radiometric decay occured. Please see "Finding Darwin's God" By Ken Miller, page 67.
Another good question concerning radiometric decay is this: How is it we can be certain that the rate of decay has not changed over time? Let's take a look at how scientists have tested radiometric dating: In the essay "Radiometric Dating: A Christian Perspective" Dr. Roger Wiens states on p.24 that, "Radioactive atoms used for dating have been subjected to extremes of heat, cold, pressure, vacuum, acceleration, and strong chemical reactions far beyond anything experienced by rocks, without any significant change." In addition, Geophysicist Joe Meert reports that if 4 billion years of radiometric decay had happened in only ten thousand years, it would have destroyed the earth!
One last question to be asked is this: Are Rocks "closed systems"? Can 207Pb atoms be added? I will again quote Dr. Roger Wiens:
"Some doubters have tried to dismiss geologic dating with a sleight of hand by saying that no rocks are completely closed systems (that is, that no rocks are so isolated from their surroundings that they have not lost or gained some of the isotopes used for dating). Speaking from an extreme technical viewpoint this might be true--perhaps 1 atom out of 1,000,000,000,000 of a certain isotope has leaked out of nearly all rocks, but such a change would make an immeasurably small change in the result."
Finally, we should consider the consistency of radiometric dating methods. Over 70 Meteorites have been dated using three different methods of radiometric dating, all giving the same age of about 4.5 billion years old. Geologists have long known that the deeper the rcok is, the older it is (assuming no geologic processes have disturbed the rock). Radiometric dating is very consistent with this "deeper is older" concept.
Other Creationist quibbles about Radiometric Dating:
Helium Zircons and the work of "RATE"
