We are told that scientists use a technique called radiometric dating to measure the age of rocks. We are also told that this method very reliably and consistently yields ages of millions to billions of years, thereby establishing beyond question that the earth is immensely old — a concept known as deep time.
This apparently contradicts the biblical record in which we read that God created in six days, with Adam being made on the sixth day. From the listed genealogies, the creation of the universe happened about years ago. Has science therefore disproved the Bible? Is radiometric dating a reliable method for estimating the age of something? How does the method attempt to estimate age?
Can Science Measure Age? People often have grave misconceptions about radiometric dating. First, they tend to think that scientists can measure age.
However, age is not a substance that can be measured by scientific equipment. The former quantities are physical properties that can be directly measured using the right equipment.
But age is not a physical property. Age is the concept of the amount of time an object has existed. It is the present time minus the time at which the object came into existence.
The only way that this can be known scientifically is if a person observed the time of creation. This may seem like a trivial or obvious point. But it is a very important one. Instead, it would be far more accurate to say that scientists attempt to estimate the age of something. This is an important distinction because a measurement is direct, objective, repeatable, and relatively independent of starting assumptions.
An estimate, on the other hand, is indirect and highly dependent on starting assumptions. Sometimes deep time advocates ignore this important distinction.
Of course, there is nothing wrong at all with attempting to estimate the age of something. We simply need to remember that such estimates are not nearly as direct or objective as a measurement of something like mass or length — measurements that are directly repeatable in the present.
And, as we will find below, age estimates are highly dependent upon starting assumptions. Estimating Age Since age cannot be measured, how is it estimated? This is done by measuring a proxy and performing a calculation. In science, a proxy is something that substitutes for something else and correlates with it.
As one example, age is not a substance that accumulates over time, but dust is. The amount of dust can serve as a proxy for the amount of time since a room was last cleaned.
Though age cannot be measured, the depth of dust can be measured. The estimated age is then computed based on the measured dust. In order for this kind of estimate to work, certain assumptions must be used. One set of assumptions concerns the initial conditions. These are assumptions about the state of the system when it first started. In the case of estimating the time since a room was last cleaned by measuring dust, we might reasonably assume that the room had zero dust at the time of its cleaning.
Another assumption concerns the rate of change of our proxy. In this case, we must know something about the rate at which dust accumulates. Often the rate can be measured in the present. We might measure the amount of dust at one time, and then measure it again a week later. We might find that dust accumulates at one millimeter per week.
But we must still make an assumption about the rate at which dust accumulated in the past. Perhaps dust always accumulates at the same rate it does today. But it is difficult to know for certain; hence, this remains an assumption.
In the case of our hypothetical example, we might assume that no one has gone into the room and added dust, or blown dust away using a fan. The assumptions of initial conditions, rates, and closed-ness of the system are involved in all scientific attempts to estimate age of just about anything whose origin was not observed. Suppose a room has 5 millimeters of dust on its surfaces. If dust accumulates at one millimeter per week and always has, if no one has disturbed the room, and if the room started with zero dust at the time of its cleaning, we can reasonably estimate the time since the last cleaning as five weeks.
Our estimate will be as good as our assumptions. If any of the assumptions is wrong, so will our age estimate be wrong. The problem with scientific attempts to estimate age is that it is rarely possible to know with any certainty that our starting assumptions are right. Radiometric Dating In radiometric dating, the measured ratio of certain radioactive elements is used as a proxy for age.
Radioactive elements are atoms that are unstable; they spontaneously change into other types of atoms. For example, potassium is radioactive.
The number 40 refers to the sum of protons 19 and neutrons 21 in the potassium nucleus. Most potassium atoms on earth are potassium because they have 20 neutrons. Potassium and potassium are isotopes — elements with the same number of protons in the nucleus, but different numbers of neutrons. Potassium is stable, meaning it is not radioactive and will remain potassium indefinitely.
No external force is necessary. The conversion happens naturally over time. The time at which a given potassium atom converts to argon atom cannot be predicted in advance. It is apparently random. However, when a sufficiently large number of potassium atoms is counted, the rate at which they convert to argon is very consistent.
Think of it like popcorn in the microwave. You cannot predict when a given kernel will pop, or which kernels will pop before other kernels. But the rate of a large group of them is such at after 1. This number has been extrapolated from the much smaller fraction that converts in observed time frames.
Different radioactive elements have different half-lives. The potassium half-life is 1. But the half-life for uranium is about 4. The carbon half-life is only years. Cesium has a half-life of 30 years, and oxygen has a half-life of only The answer has to do with the exponential nature of radioactive decay.
The rate at which a radioactive substance decays in terms of the number of atoms per second that decay is proportional to the amount of substance. So after one half-life, half of the substance will remain. After another half-life, one fourth of the original substance will remain. Another half-life reduces the amount to one-eighth, then one-sixteenth and so on. The substance never quite vanishes completely, until we get down to one atom, which decays after a random time.
Since the rate at which various radioactive substances decay has been measured and is well known for many substances, it is tempting to use the amounts of these substances as a proxy for the age of a volcanic rock. So, if you happened to find a rock with 1 microgram of potassium and a small amount of argon, would you conclude that the rock is 1. If so, what assumptions have you made?
The Assumptions of Radiometric Dating In the previous hypothetical example, one assumption is that all the argon was produced from the radioactive decay of potassium But is this really known?
How do you know for certain that the rock was not made last Thursday, already containing significant amounts of argon and with only 1 microgram of potassium? In a laboratory, it is possible to make a rock with virtually any composition. Ultimately, we cannot know. But there is a seemingly good reason to think that virtually all the argon contained within a rock is indeed the product of radioactive decay.
Volcanic rocks are formed when the lava or magma cools and hardens. But argon is a gas. Since lava is a liquid, any argon gas should easily flow upward through it and escape. Thus, when the rock first forms, it should have virtually no argon gas within it. But as potassium decays, the argon content will increase, and presumably remain trapped inside the now-solid rock.
So, by comparing the argon to potassium ratio in a volcanic rock, we should be able to estimate the time since the rock formed. This is called a model-age method.
In this type of method, we have good theoretical reasons to assume at least one of the initial conditions of the rock. The initial amount of argon when the rock has first hardened should be close to zero. Yet we know that this assumption is not always true. We know this because we have tested the potassium-argon method on recent rocks whose age is historically known. That is, brand new rocks that formed from recent volcanic eruptions such as Mt.