Radiometric Clock
How do you know how old you are? Of course, you would say your age is calculated based on the number of days in a year (365), months in a year (12) and days within a month (28 – 31). You could go a step further and talk about hours (24), minutes (1440) and seconds (86,400) in a day. Time, as we understand it, is measured in relationship to the number of times the Earth rotates on its axis as it makes one complete trip in its orbit around the Sun. In general, we can measure time in this way back a few thousand years, but then absolute time becomes rather murky. For example, when you visit a T-Rex in the museum, do you have any idea when it might have lived on the planet? Very challenging since there were no humans around at the time to report on the dinosaur’s life and death. FYI, T-Rex lived ~ 62 million years ago.
How old is the Earth and the fossils, strata, and minerals you find within the Earth? Prior to 1905, dating of geologic features was relative. In general, strata were viewed as a layered cake with the youngest fossils at the top and oldest at the bottom. But when asked how old a fossil might be, in years, the response would be crickets and shrugged shoulders – there was no standard with which to measure absolute age. But in 1905 British physicist, Ernest Rutherford, known as “the father of nuclear physics,” discovered a way to measure absolute time – radiometric dating, based on known rates of radioactive decay.
Figure 1 Radioactive Decay (credit BioNinja)
I remember the exercise. As a child in grade school, we would go through mock drills, such as lying beneath our desks; somehow this maneuver was supposed to protect us in case Russia dropped an atom bomb on Pittsburgh. From the horribly distorted images of victims from Hiroshima, we were quite aware of the effects of radiation from a nuclear weapon. I continued to learn about radiation when the dentist put a lead vest on me before taking an x-ray, or when I played for too long in the Sun. Thanks to Rutherford, a whole new understanding of invisible radiation was revealed to the world. Uranium – bomb material - is radioactive.
Uranium is a very heavy element; it is composed of 92 protons and 92 electrons, but its number of neutrons varies. For example, U-238 has 146 neutrons while U-235 has 143 neutrons. These unstable forms of uranium are called radioactive isotopes. Figure 1 (to the left) starts with a radioactive “parent isotope”. When it tries to morph into a more stable “daughter isotope” form through the decay process, it sheds some of its energy in three ways. Alpha decay involves the forcible release of a Helium nucleus from the parent; beta decay is the shedding of an electron, and gamma decay is the release of high energy in the form of gamma ray photons. Radiation from a nuclear blast can harm us in two ways: 1) breathing or eating dust or food contaminated by alpha radiation; 2) exposure to intense gamma radiation released directly from the explosion.
So, what might radioactive decay have to do with the absolute age of anything? When parent isotope A has a decay episode, the new daughter isotope B transforms into a completely different chemical element, e.g., Carbon-14 decays into Nitrogen-14. Because radioactive decay is a function of quantum mechanics, the time between an episode of decay for a single nucleus can never be predicted. But when a large population of parent isotope A samples is analyzed, an average rate for decay can be predicted. By analogy, we know the average human life span is 74 years, but we have no way of predicting how long a single individual will live. If an average human life span is 74, then an average human half-life would be 37 years. A half-life of radioactive decay is the time it takes to convert 50 percent of a parent isotope into its daughter isotope. Knowledge of the half-life of an isotope can assist scientists in calculating the age of a rock containing it.
Figure 2 Carbon Dating (credit WordPress.com)
Consider radio-carbon age dating (Figure 2). Our atmosphere is composed mostly of nitrogen (78%) and oxygen (21%). Cosmic rays from the Sun create energetic neutrons that collide with Nitrogen-14 in the air to produce a Carbon-14 (C-14) radioactive isotope. Carbon-14 from the atmosphere enters plants and animals, which already contain much greater amounts of very stable Carbon-12 (C-12). The ratio of C-14 to C-12 in living plants and animals is a fixed constant. Once the plant or animal dies, it no longer takes in any new C-14. Over time, the radioactive C-14 decreases as it decays back to Nitrogen-14 (N-14); all the while the amount of C-12 remains stable. Scientists have calculated that it takes 5,370 years to convert half of the C-14 into N-14. Thus, when studying a dead plant or animal, if only half of the original C-14 remains, then the date of death occurred 5,370 years ago. If 1/4th C-14 remains, the age would be 10,740 years, and so on. Carbon dating is accurate for dates up to about 75,000 years.
Figure 3 Element Pairs Used for Radiometric Dating (credit AirKhraung.com)
Carbon dating is only accurate for very young samples. Figure 3 shows examples of other parent-daughter chemical pairs used to date much older rocks and the fossils contained within. Suppose I wanted to know the age of an extinct Cambrian Period (~500 mya) trilobite. I would select Potassium-40 (P-40) to Argon-40 (A-40) to date my trilobite. If 3/4 of the P-40 had decayed to A-40, I would know the strata (and fossil) was ~ 540 million years old.
Figure 4 Half-life Graph (credit Luminlearning.com)
Figure 4 is a standard chart to measure the half-life of any radioactive parent isotope. Suppose we insert Carbon-14 to Nitrogen-14 into this chart. At the point of death and burial, the amount of C-14 parent isotope in the specimen would be 100% while the amount of N-14 daughter isotope is 0%. At exactly one half-life both lines intersect at 50%, which would represent 5370 years of elapsed time. Notice by the 4th half-life the remaining C-14 is around 6%. By eight half-lives (75,000 years) it is nearly impossible to accurately date a sample using C-14.
God has given man an amazing tool to measure deep time. From biblical names ascribed to the Creator, like El Olam – the Eternal or Ancient of Days - we always suspected God was ancient, but until 100 years ago, we had no idea just how archaic. From radiometric dating, scientists have determined the age of the Earth to be over 4.5 billion years. From the doppler movement of stars and galaxies, astronomers have extended the beginning of our universe to over 13 billion years. And yet our God, the Eternal One, has no beginning or no end. His age cannot be calculated or even imagined! Many Believers have envisioned a very young Earth, perhaps less than 10,000 years old, because they think it agrees with biblical genealogies. But Scripture does not provide us with a beginning date. When I contemplate Jesus’ promise of eternal life for those who follow him, I am comforted to know that he speaks from a perspective of unimaginably deep time, giving us hope for an inconceivably distant future. Geologic time with its billions of years is but a foretaste of what is to come.
