Anyone who has tried to debate a creationist has invariably encountered their liberal use of the word, “assumption.” This is one of their trump-card, catch-all arguments that they use to handily “defeat” any evidence that opposes their position. For example, if you present them with the fact that coral reefs grow much too slowly to have formed in the past 4,500 years (their calculated time since the supposed world-wide flood), they will say, “well, you’re assuming that corals couldn’t grow faster in the past.” Similarly, if you point out that ice cores clearly show that the earth cannot possibly be less than 10,000 years old, they will retort, “well you’re assuming that layers only form annually.” Most infamously, when faced with the realization that radiometric dating completely obliterates the notion of a young earth, they choose to ignore that evidence because scientists are, “assuming a constant rate of decay/the amount of material in the original rock.” These blind dismissals of evidence are often accompanied by a rhetorical, “were you there?” The problem is that creationists are misusing the term “assumption,” and, as usual, are completely misconstruing how science actually works. As I will demonstrate, coral growth rates, radioactive decay rates, etc. are not assumptions. Rather, they are the conclusions of simple inductive logic.

The definition of “assumption”

First, we have to define “assumption.” At the broadest level, you could define an assumption as something that cannot be proved with 100% certainty, but that is an extremely problematic definition because it makes virtually everything an assumption. This is the problem that Descartes was describing when he famously proclaimed, “cogito ergo sum” (I think therefore I am). You see, I cannot prove with 100% certainty that I am not currently dreaming, or that we are not currently in the Matrix. The only thing that I can be 100% certain of is my own existence. Thus, at this broad definition, I am “assuming” that I am actually in a real, physical universe. Fortunately, I think that even creationists would agree with me that this definition is not really useful, and I don’t think that it is the definition that they are operating under.

A more restrictive definition is that an assumption is something that was not directly observed. Indeed, this seems to be the definition that creationists use, but this definition is also fraught with problems and inconsistencies. The most fundamental problem is one which I have previously elaborated on. Namely, direct observation is actually very unreliable, and you can use simple inductive logic to reach conclusions about something without directly observing it (i.e., there is no difference between “historical” and “observational” science). Remember that inductive logic is the type of logic that goes from a series of observations to a general conclusion.

I have used the theory of gravity to illustrate this before, but it is such a good, clean example that I am going to use it again. The theory of universal gravity states that all objects with mass produce gravity and are acted on by the gravity from other bodies. It also goes on to detail the math of how these bodies interact with each other, ultimately producing what is known as the gravitational constant (G) where G = 6.672×10^-11N m^2 kg^-2. This value is exceedingly useful and lets us do something really neat. For any two bodies, if we know the mass of each object and the distance between them, then we can use G to calculate the force of gravity between those two objects. These calculations will, however, only work if G is actually constant.

The question is, of course, how do we know that the gravitational constant is in fact constant? Well, quite simply, we have tested it over and over again and it has always been correct. In other words, we accept it as true because of inductive logic (i.e., we went from a series of observations to a general conclusion). Importantly, we can never prove that G actually is a constant, because doing that would require us to test G against every single piece of matter in the universe. That’s clearly impossible, so instead we rely on inductive logic (also there are very strong mathematical reasons to think that G is constant).

This is where things get interesting (and problematic for creationists). We use G all the time, even in situations where we can’t actually observe it to confirm that G works for the object in question. Any high school level physics course will go over calculations that use G, and it is extremely important for astrophysics. This is important because no one would claim that G is just an “assumption,” but that is exactly what creationists’ definition of assumption does. Imagine for a moment that an astrophysicist derived an explanation for some phenomena, and the math for that explanation involved G. It would be utterly absurd for you to say, “I don’t have to accept that explanation because you are assuming that G is constant.” We know that G is constant because we have measured it over and over again and it has always been constant. Therefore, via inductive logic, we must accept that it is constant until we have been shown a compelling reason to think that it is not constant. Even so, we have measured the rates of radiometric decay over and over again and they have always been constant. Therefore, via inductive logic, we must accept that they are constant until we have been shown a compelling reason to think that they are not constant. Similarly, we have repeatedly measured coral growth rates, and we know that even their fastest growth rate is nowhere near fast enough for them to have formed in only a few thousand years. When we make a statement like that, we aren’t “assuming” that growth rates weren’t faster in the past; rather we are applying inductive logic.

Also, note that the argument that creationists are making here is nothing more than an ad hoc fallacy. There is absolutely no reason to think that coral reefs grew faster in the past, or ice cores and varves formed multiple layers annually, or radioactive particles decayed faster, etc., but creationists are assuming that those things occurred even though there is absolutely no evidence to support those notions. That is the proper use of “assumption.” An assumption is something which you choose to accept as true despite a lack of supporting evidence. So, despite what creationists would like you to believe, scientist’s methods for dating the earth are based on inductive logic, not assumptions; whereas creationists’ arguments are based entirely on assumptions and ad hoc fallacies (note: I am using “assumption” synonymously with “unfounded assumption” because that is the way in which creationists seem to use it).

How radiometric dating actually works

Hopefully at this point you realize that scientists aren’t just making haphazard assumptions, but just to be sure, I want to quickly walk through how radiometric dating actually works because there is a lot of confusion and misinformation about it. First, realize that there are many different types of radiometric dating. Each method is specific to the type of rock that it can date, and which one you use depends on what type of material you are working with (on a side note, you may see creationists claim that they have dated something that we know is recent, such as a rock from Mt St. Helen, and the radiometric dating said it was old. These reports are generally a result of creationists using the wrong method for the rock in question).

To illustrate how radiometric dating works, I am going to focus on one method (uranium-lead dating), but all other types of radiometric dating follow the same general steps (note: technically there are two types of uranium-lead dating and both are generally used simultaneously, but I am going to focus on the cycle of 235U to keep things simple). Uranium-lead dating is used on a type of rock known as a zircon. Zircons are useful because when they form, the formation process incorporates uranium, but it strongly repels lead, which means that a newly formed zircon will never have any lead in it. This resolves creationists’ claim that scientists “assume the amount that was in the rock to begin with.” We aren’t “assuming,” rather we have tested the formation processes of zircons, and we understand the chemistry, and we know that lead simply isn’t incorporated. That’s simple inductive logic (note: the amount of uranium in the parent rock is irrelevant).

Uranium exists in several isotopes (same element, different numbers of neutrons), and the one we are interested in is 235U. 235U decays into 207Pb (an isotope of lead) at a rate known as a half-life. A half-life is the amount of time that it takes for half the atoms to decay. For 235U, a half-life is roughly 704 million years. How do we know what the half-life is? Simple: we have measured the rate of decay over and over again and it has always been the same (i.e., inductive logic). Also, as with the theory of gravity, there are strong mathematical reasons for thinking that the rate is constant (in fact, it’s a scientific law known as the law of radioactive decay). So, once again, saying that scientists “assume” that decay rates are constant is no different from saying that scientists “assume” that gravity is constant. Saying that we shouldn’t trust decay rates is just as absurd as saying that we shouldn’t trust gravity.

To illustrate how a half-life works, let’s say we have a rock that starts off with 80 atoms of 235U. After 704 million years, it will have a 1:1 ratio (40 atoms of 235U and 40 atoms of 207Pb) because half of the particles will have decayed. After another 704 million years (1,408 million total), the ratio will be 1:3 (20 atoms of 235U and 60 atoms of 207Pb). After 704 million more years (2,112 million total), the ratio will be 1:7 (10 atoms of 235U and 70 atoms of 207Pb), etc. The ratios are the important things here, and they are why the amount of uranium in the original rock is irrelevant. We can take a zircon, measure the amount of 235U and the amount of 207Pb, and the ratio of those two chemicals will tell us how old the rock is. For example, if the ratio is 1:7, then we know that it is 2.1 billion years old. It doesn’t matter if that ratio is from 1 atom of 235U and 7 atoms of 207Pb or from 1,000 atoms of 235U and 7,000 atoms of 207Pb, the ratio is still 1:7.

In summary, radiometric dating is based on well tested, scientific results, not assumptions. We know that there was no lead in zircons to begin with, because zircons strongly repel lead when they are forming. That is not an “assumption,” that is an inductive conclusion based on multiple experiments. We don’t know how much uranium was present in the original rock, but we don’t need to because the ratios are all that we care about. Finally, we know the rate at which uranium decays into lead because we have repeatedly measured it, and it has always been the same. So you see, when creationists claim that radiometric dating relies on “assumptions” they are grossly mischaracterizing how the process works, and they have demonstrated that they are either dishonest or ignorant about the science. Either way, they aren’t a trustworthy source of information.