Assumptions vs. inductive logic: is radiometric dating based on assumptions?

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.

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20 Responses to Assumptions vs. inductive logic: is radiometric dating based on assumptions?

  1. Ally says:

    Good post. I’m really enjoying the blog so far – keep up the good work!

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  2. You understate the case for radiometric dating. Isochron methods, using a non-radiogenic isotopes to tell us the amount of daughter present to start with, avoids assumptions about initial amounts. And the constancy of decay rates is not merely an observation made over the past century or so but confirmed by observations on distant supernovas that we are observing them at times ling past. Moreover, these rates are consequences of such fundamental physical laws that we know they cannot have changed. For if those laws had been different, the whole of physical science would have been different, and we would not have had rocks of recognisable chemistry being laid down in the first place.

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    • Fallacy Man says:

      Thank you for your comment. You are certainly correct that there is a lot more to it than just measuring decay rates, but I’m trying to keep things on a very basic level, at least for now, so that even people with no scientific training can follow the argument easily. In the future I may write a more detailed post about the physics and math behind it, but in my many discussions with creationists, I have generally found that it is best to just stick to some basic points that are easy to grasp and avoid overloading them with too many facts.

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      • Indeed. And yet I was responding the arguments that creationists actually make, and that you will find spelt out in any creationist text.

        It is always a difficult judgement call; to what extent should we simply ignore the details of creationist arguments, and to what extent should we explicitly rebut them. The former risks giving a free pass to fallacies, while the latter risks spreading the creationist meme.

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        • Could these rates be affected by forces such as temperature, magnetic fields, or quantum vacuum fluctuations?

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          • Fallacy Man says:

            There is a considerable amount of literature on the topic of external factors affecting decay rates, and occasionally someone reports an anomalous result, but the overwhelming consensus is that they are not affected by things like temperature (many of the anomalies are likely the result of user error).

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            • Regarding changing radioactive decay rates; some rates do depend, in known and well understood ways, on the charge of the decaying atom, but the effects are minor except under conditions such as those inside stars. Nor could it be otherwise; these rates depend on the basic laws of physics; if they had been different when the rocks were laid down, we wouldn’t have had those rocks.

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  3. arekexcelsior says:

    What’s so idiotic about this argument is that it’s saying, “You have some imperfect assumptions that may have some margin of error to them, so my position which is based on no data at all is superior”. Geology, radiology, astronomy and biology all point to pretty consistent date ranges, and none of them can support anything remotely close to a literalist interpretation of the Bible. They’re wrong by orders of magnitude.

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  4. tom says:

    It has been known since at least 1748 that there is no such thing as “inductive logic”. It is very strange to encounter someone still proselytizing it. There is no sense in which you can go from a series of observations to a general law.

    Your example of gravity is quite revealing. You don’t claim that the law is arrived at by a process of induction (which it cannot be) but merely that the value of a constant is. This is nonsense. G is a measured quantity. And despite the powers of induction, we now know there is no force of gravity.

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    • Fallacy Man says:

      It is well known that inductive logic does not provide a proof, but that does not mean that there is no such thing as inductive logic, or that inductive logic isn’t useful. It is merely a form of probabilistic argument.

      All scientific theories and laws are arrived at by inductive logic. That is inherent in their nature. For example, cell theory states that all living things are made of cells. To actually prove that, we would need to test all living things, which is impossible, but every living thing that we have tested has been made of cells. Therefore, we went form all of those observations to the general conclusion that all living things are made of cells. That is by definition inductive logic.

      Let me ask you this. If we take two objects, for which we know the masses and the distance between them, if we plug G into the equation, will it work? Everyone on the entire planet agrees that it will, but we agree because of inductive logic. We have not measured G between those two objects, but we know that G will work because G has always worked. That’s simple inductive logic.

      “we now know there is no force of gravity” ummm what? can you elaborate?

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      • “we now know there is no force of gravity” ummm what? can you elaborate?”

        I assume he is referring to the Higgs-Boson particle. I was curious, what is the Higgs-Boson particle made out of?

        The neutrons you mention above, when referring to Uranium-lead dating – what are they made out of?

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        • Fallacy Man says:

          neutrons are made of quarks (I’m not sure about the Higgs-Boson). Now, what are quarks made of? Quarks (along with leptons) are the smallest units of matter that we have confirmed to date. So in short, we don’t know if there is anything smaller.

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      • tom says:

        Restating a fallacy often doesn’t make it true. Hume identified the problem of induction and various philosophers have grappled with it. Induction is not just a fallacy, it is a myth. See in particular Popper.

        Did you not notice in your example of “call theory” you began with the theory, then came up with a test? That is not induction by any stretch of the imagination!

        The answer to your question about gravity is NO! Your equations, despite all the powers of induction and the belief of “everyone on the planet” don’t work. In particular Newton’s law of universal gravitation is inadequate planetary orbits and GPS systems.

        A theory called General Relativity was conjectured 100 years ago. It works for planets and GPS and in it there is no force of gravity.

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        • Fallacy Man says:

          Regarding cell theory, I began with the theory because the theory already exists, but lets back the clock up to before cell theory was proposed. Why did we propose it? Well, every time we had ever examined a living thing, it had been made of cells. So, we went from those observations to the general conclusion that all living things are made of cells. If that isn’t inductive logic, then what is it? Similarly, every time we have examined a piece of matter, it has been made of atoms. Therefore, we proposed atomic theory which states that all matter is made of atoms. Again, if that isn’t inductive logic, then what is it? We have also gone from countless observations in the fossil record, zoology, biogeography, etc. the the general conclusion known as the theory of evolution by natural selection. Again, if that isn’t inductive logic, then what is it? You should re-read your philosophy of science books. All theories are based on inductive logic.

          The problem of induction, which is well recognized, is not that inductive logic doesn’t exist or is fallacious, but rather that it doesn’t provide proofs (as opposed to deductive logic which does). Thus, it is always possible that somewhere in the universe there is a living thing that is not made of cells, but until we find such a thing, there is no reason not to accept cell theory. Thus, inductive logic tells us what is probably true, not what is definitely true.

          Regarding gravity, you are misunderstanding relativity. It in no way shape or form shows that gravity isn’t a thing. Rather, it simply shows the Newton’s laws were incomplete. Gravity absolutely is a real thing, but under certain conditions (such as when dealing with objects moving near the speed of light or objects with an extraordinarily large mass) we have to modify Newtonian physics to include relativity. You are simply incorrect that gravity is not included in relativity. Many of the calculations involved use the gravitational constant.

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          • tom says:

            So you claim that “cell theory” is an induction from the observation that all living things are made up of cells. Your “Cell Theory” is just the statement that “all living things that are made of cells are made of cells”. It doesn’t even preclude the possibility that once we have a better theory of life, we might be able to discover or construct non-cellular life.

            But what about the related “Germ Theory”? Germ theory eclipsed Miasma theory after significant effort to discover germs was ultimately successful. Note that the theory existed before any germs were discovered.

            Famously Higgs theory existed for 50 years before any evidence was found. Also quantum entanglement was discovered 50 years before any experiments became possible. I don’t think there was any evidence for E=mc2 in 1905 either!

            Atomic theory was proposed by the ancient Greeks. Boltzmann committed suicide in 1906 in part because the scientific establishment refused to accept the existence of atoms. There was no direct evidence so the Inductivists refused to accept the explanatory power of atomic theory. If only he had read Einstein’s paper of 1905 that settled the matter! So atomic theory existed as a theory first.

            You choose to ignore the theories of evolution prior to Darwin’s. Do you think he was unaware of Anaximander, Empedocles and Lucretius? How about Maupertius, Leclerc and Lamarck? Surely he was aware of the evolutionary theory of his own grandfather! Anyway, I find it amusing to note that Lamarckism is essentially an inductive theory. Darwinism is not inductive.

            There have been many proofs that induction is impossible. One of my favourites is this one:

            http://www.nature.com/nature/journal/v302/n5910/abs/302687a0.html

            In it, Popper and Miller prove that induction cannot provide even probabilistic support.

            So your characterization of science is logically impossible, historically inaccurate and rather naive. Yet you tell me to reread some books. Well, at least I HAVE read some books.

            I’m going to ignore the gibberish you wrote about gravity for now.

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            • Fallacy Man says:

              First, we have to clear up exactly what is meant by the term “theory.” You seem to be using it very generally to refer to what scientists today would refer to as a hypothesis. You can propose a hypothesis without solid evidence to support it, but it doesn’t become a theory (in the modern use of the word which I have been using) until it has evidence to support it (the term is admittedly used slightly differently in fields such as physics where theories are largely mathematically derived, so one of my previous comments is somewhat erroneous as currently written, I apologize for not being more careful with my wording earlier on).

              So, for example, germ theory was proposed as a hypothesis before bacteria, viruses, etc. were discovered, but could not fairly be considered a theory until it had evidence to support it. Similarly, there were many hypotheses about evolution prior to Darwin, but only Darwin and Wallace were able to produce a theory, because only they had the supporting evidence. Your comment in those regards was also a straw man because I specified the “theory of evolution by natural selection” which did not exist prior to Darwin/Wallace.

              Regarding the link you posted, that proof relies on a very bizarre and specific definition of inductive logic, and its not the definition that most people (myself included) use. So all it proves is that inductive logic, as they uniquely define it, is impossible. You can find a brief explanation of the situation here, as well as sources to other articles about it
              http://epiresearch.org/wp-content/uploads/2014/07/Greenland_EPI_2008.pdf

              Now, I want to return to the topic of cell theory, because we seem to be talking past each other, so I want to take your comment point by point.

              “Your ‘Cell Theory’ is just the statement that ‘all living things that are made of cells are made of cells’.” No, cell theory states that “ALL living things are made of cells.” Theories are judged by their predictive powers, and this theory predicts that anytime we take a living thing and examine it, we will find that it is made of cells. Simply saying that “every living thing that we have examined has been made of cells” is not a theory, it is a statement of fact. The theory predicts the results of future tests.

              “It doesn’t even preclude the possibility that once we have a better theory of life, we might be able to discover or construct non-cellular life.” Of course it doesn’t because science doesn’t deal with proofs (excluding a few fields which are capable of mathematical proofs). It deals with probabilities. So, based on all available evidence, all living things are made of cells, but it is always possible that we will find evidence in the future that discredits that. This is true for all science. For example, based on all available evidence, life on this planet has slowly evolved for millions of years and natural selection has been the dominant force of that evolution (i.e., the theory of evolution by natural selection), but it is always possible that at some point in the future we will find something that discredits that theory (even though that possibility is exceedingly remote).

              I asked you several questions earlier which you did not answer, and I would really appreciate it if you actually answered them.

              First, do you think that all living things are probably made of cells? If no, defend your answer, if yes, explain how you reached that conclusion without using inductive logic.

              Second, explain how the theory of evolution by natural selection is not inductive. We cannot PROVE that it is true, but every piece of evidence that we have says that it is. Therefore, from all of that evidence, we draw the general inductive conclusion that it is most likely true. If that isn’t inductive logic, then what is it?

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            • Fallacy Man says:

              let me add one more question to that list. Suppose we went out to some tribe of people that had never been studied before, and we took a blood sample from one of them and put it under a microscope. Do you expect to see that his blood is made of cells? Please explain how you reached your conclusion via a means other than inductive logic.

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        • Fallacy Man says:

          Let me ask you this simple question to avoid you and I going back and forth forever. Do you think that all living things are probably made of cells? If “no,” please justify your answer. If “yes,” please explain your answer without using inductive logic.

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  5. Richard Good says:

    BTW, not trying to undermine your useful analogy, but measuring G has been so challenging, and there are so many inconsistent measurements (http://scienceblogs.com/principles/2010/08/26/measuring-gravity-aint-nothin/) that people have seriously proposed it might be time-varying (http://phys.org/news/2015-04-gravitational-constant-vary.html), or maybe orientation dependent (http://arxiv.org/abs/physics/0202058).

    Of course, more likely is that there’s something people are failing to account for in their measurements, but there’s always that exciting chance of new physics.

    (Of course, a few hundred ppm of discrepancy, while gargantuan by fundamental physics standards, is nothing like the multiple-orders-of-magnitude discrepancies that YECs require.)

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    • Fallacy Man says:

      That’s a good point which I didn’t go into for sake of time and clarity, but, as you suggest, the slight variations still render it essentially constant, and hardly qualifies it as an assumption. In all fairness, I will fully admit that radiometric dating may have a similar situation with extremely tiny variations in decay rates, but nothing anywhere near the magnitude proposed by creationists.

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