This is the second in a series of posts. The first one can be found at http://sigma5.blogspot.com/2012/07/50-years-of-science-part-1.html. Taking the Isaac Asimov book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science as it was when he wrote the book (1959 - 1960) I am examining what has changed since. For this post I am starting with the chapter Asimov titled "The Birth of the Universe".
In this chapter Asimov reviews in more detail than in previous chapters what science has determined about the age of things. He reviews various "origin stories" for the Earth, including the one in the Bible. He then moves quickly on to scientific attempts to determine how old Earth is. Based on the salt content of the ocean the earth is at least a billion years old. Based on various radioactive decay-based measurements the Earth is at least 3.3 billion years old. Both of these estimates contradict "young earth" creationists. Asimov doesn't mention them anywhere in the book because at the time the book was written no one took them seriously. They did not have a political home in the Republican party and a well established network of religion channels on cable and mega churches to support and maintain their belief system. In the decades since this book was written science has developed and enhanced the lines of reasoning Asimov lists, along with dozens of others, all indicating that the Earth is billions of years old.
No one has come up with any credible evidence that even one of these multiple lines of reasoning is wrong. But we live in a world where people's knowledge of science has diminished to the point where most people are unfamiliar with the reasoning or the evidence that supports the reasoning. Instead they are drowned in a sea of "facts" that are factually wrong, and people whose idea of a scientifically valid argument is " I believe it because my faith demands I believe it" or "I believe it because I wish it were so and 'wishing it were so' is enough to make something true".
In any case, the basic methods Asimov discusses have been refined and extended so that we now know that the Earth is 4.7 billion years old. The primary line of evidence for this is radioactive decay. Why is the modern number different from the number in 1960? The big reason is that a concerted effort has been made to date lots and lots of rock formations. When rocks melt then many radioactivity "clocks" reset resulting in a misleadingly young estimate of how old the rocks are. Scientists have now located rock formations that are substantially older than the oldest ones they were familiar with in the '60s. The scientific methods of radioactive dating have also gotten better. More isotopes can now be used as the basis for these radioactive studies. The amount of material necessary to make an accurate measurement is now much smaller. And the overall accuracy and sensitivity of the measurements have improved. Scientists are now also able to measure different "isotope systems" in the same rock and compare the results. This makes it easier to identify situations where a sample appears to be pristine but has actually been processed (e.g. heated up by a geologic process).
Now is probably a good time to spend some time explaining how radioactive clocks work. The thing that makes an atomic element what it is is the number of protons in the nucleus. Hydrogen is Hydrogen because its nucleus has one Proton. Helium is Helium because it has two Protons in its nucleus. But there are actually multiple kinds of Hydrogen, Helium, and other elements. Each kind is called an isotope, The three isotopes of Hydrogen are called "Hydrogen", "Deuterium", and "Tritium". Regular Hydrogen has a nucleus consisting of one Proton. That's it. Deuterium has a Proton but also a Neutron in its nucleus. The name references the two (deu) nucleons. Tritium has a Proton and two Neutrons, hence the "tri" in the name. The isotopes of other elements don't have such cute names. Chemists and Physicists also have various superscripts and subscripts they use to indicate isotopes but it is essentially impossible to get these to print correctly in the blog. So I am instead going to use H-1 to indicate Hydrogen with just the one nucleon in its nucleus, H-2 to indicate Deuterium, the isotope of Hydrogen with two nucleons, and H-3 to indicate the three nucleons in Tritium.
Now from a chemical point of view H-1, H-2, and H-3 are indistinguishable. They all behave like Hydrogen in every way when it comes to chemical reactions. The same is true for the isotopes of Helium: He-2, He-3, and He-4. In each case there are two Protons in the nucleus along with 0, 1, or 2 Neutrons. As a result each isotope acts just like the others from a chemical reaction point of view. But in other ways each isotope differs. For one thing the weight of each differs. An atom of H-2 weighs about twice as much as an atom of H-1. Both have one Proton and, in normal circumstances one electron. But the electron weighs about one 2000th as much as a Proton, whereas a Neutron weighs roughly the same as a Proton. So H-1 has one Proton and 1 electron and weighs about as much as a Proton. But H-2 has a Proton, an Electron, and a Neutron. So it weighs about the same as two Protons. When you get to heavy atoms like U-235 versus U-238 the difference is much smaller. U-235 weighs roughly as much as 235 Protons and U-238 weighs roughly as much as 238 Protons. But here the difference in weight is roughly 1%. In some cases the weight difference can be important but in most cases the difference in not enough to make a big difference. And in any case that is not what we are interested in.
The difference that matters to us is that the stability of various isotopes varies considerably. H-1 is stable. If you sit around and watch a H-1 atom for a very long time it won't do anything. H-2 is also stable. But if you watch H-3 for about 12 years there is a 50-50 chance that it will "decay" into something else. It will stop being H-3 and become a different isotope of a different element. It will spontaneously become He-3. One of the Neutrons will turn into a Proton. If you have a bunch of H-3 atoms and wait a little over 12 years 50% of it will spontaneously decay into He-3.
There are 92 naturally occurring elements. They range from e.g. H-1 to e.g. U-238. Hydrogen comes in three isotopes as does Helium. Other elements like Uranium come in a dozen or so isotopes. All together there are hundreds of isotopes. Many like H-1 are stable. They never decay into something else. But most isotopes are like H-3 and U-235 and U-238. They decay spontaneously into other isotopes. This is a complicated process. U-235, for instance, can decay into one of several isotopes. And sometimes the isotope it decays into is radioactive (e.g. unstable) so it decays into something else. But scientists have carefully studied many isotopes and for the radioactive ones they have studied what they decay into. H-3 always decays into He-3. And for a combination like H-3 to He-3 there is a single magic number called the "half life". In the case of the H-3 to He-3 decay the half life is exactly 12.32 years. This means that if you put 10 lbs of H-3 into a container and wait exactly 12.32 years, when you look into the container you will find 5 lbs of H-3 and 5 lbs of He-3.
U-235 is more complicated. It can decay into a number of different isotopes. But most of the time it decays into Th-231. The half life of this decay is 700 million years. U-238 has three different decay paths. The most common one is to Th-234 and its half life is 4.5 billion years. What's important is for each decay path (e.g. H-3 to He-3 or U-235 to Th-231) you have three things: the starting isotope, the ending isotope, and a very specific half life. As we have seen half lives can be relatively short (e.g. 12.32 years) or very long (e.g. 4.5 billion years). They can even be much shorter. The half life of some isotopes is less than a second. And they can be even longer than 4.5 billion years. But, since 4.5 billion years is about as long as the Earth has been around, decay paths that have a half life longer than 4.5 billion years are not very useful as radioactive clocks.
And this whole half life thing is a little more complicated than it looks. If we look in on our container of H-3 after 12.32 years we have half as much H-3, namely 5 lbs. But what if we seal it back up and wait another 12.32 years? Is it all gone? No! "Half life" means the amount of time it takes for half the remaining material to decay. So after a total of 24.64 years we will have 2 1/2 lbs of H-3 (half the 5 lbs we had at the 12.32 year mark). Radioactive decay is what mathematicians call an exponential process. After one half life we have half the material. After two half lives we have a quarter of the material. After three half lives we have an eighth of the material. And so it goes to a sixteenth (4 half lives) a thirty-second (5 half lives) a sixty-fourth (6 half lives). If a large number of half lives are involved there is a shortcut that can be used. After ten half lives we will have about a thousandth of the material left. After twenty half lives we will have about a millionth, etc. Every additional ten half lives will reduce the amount of original material by a factor of a about a thousand.
This whole "isotopes and half lives" thing gives us a clock for measuring times. If we know how much of a specific isotope we started with and we know how much we have now then we can measure time. For periods of hundreds to tens of thousands of years C-14 (carbon fourteen) works really well. Lots of things like wood have carbon in them. Most Carbon is stable C-12. There is also some C-13, which we will ignore. But there is usually a small amount of C-14 mixed in with the other isotopes of Carbon. The half life of C-14 is 5,730 years. If by careful analysis we find that exactly half the C-14 we started with is gone we can conclude that the artifact containing the Carbon is 5,730 years old. If a quarter remains then the artifact is a little over 11,000 years old. If a little less than a thousandth of the C-14 is left then the artifact must be about 57,000 years old. In theory the process is that simple. In actual practise it is more complicated than that.
The most obvious problem is with an artifact that we suspect is a little over a hundred thousand years old. In this case we expect to measure about a millionth of the C-14 we started with. That's not very much. So C-14 dating is not very good for artifacts that are more than about 50,000 years old as the remaining amount of C-14 is so small. But there is an even bigger problem for an artifact that we suspect is say 20,000 years old, what should be in the butter zone where we should have enough C-14 left over to get an accurate enough measurement to produce a pretty sharp age estimate. Now the issue hangs on the question of how much C-14 we started with. And that turns out to be a much harder question than it would seem.
Originally scientists just assumed that everything started out with pretty much the same percentage of C-14. So they would measure the total carbon, apply the magic percentage to estimate how much C-14 there originally was, and go from there. But it turns out the magic percentage trick doesn't work very well. C-14 comes from high altitude cosmic rays hitting the upper atmosphere. If the rate of cosmic rays stays constant then after a while the carbon in the atmosphere will contain a specific percentage of C-14. This C-14 will end up in carbon dioxide in the air. And plants will absorb the carbon dioxide and end up with a specific percentage of C-14 in their tissues. If the plant lives for a very short time compared to the 5,730 year half life of C-14 then we will end up with plant material with a predictable initial C-14 percentage and we are good to go. But this process is complicated and it turns out that there are variations in the efficiencies of some of the steps. So the percentage of carbon in plant material that is C-14 varies somewhat. And this introduces errors. We can still measure what is now called the C-14-age of material containing carbon. Scientists have developed elaborate adjustment procedures that work pretty well most of the time for turning C-14 age into real age. But they are complicated and don't work all the time.
So some times there are problems with C-14 based radioactive dating. Scientists have reacted to this in two ways. First, they have developed and continued to refine their C-14 adjustment procedures. The second way is to come up with other isotope systems. That way they can compare the results for the C-14 isotope system with the results of the other isotope system. Other isotope systems also allow artifacts to be dated that are much older than 50,000 years. For instance, if you can find some Uranium in a rock and you can estimate how much of that Uranium was originally U-238, you can use radioactive dating on a very old rock. If you measure the remaining U-238 and it turns out to be half of the amount you calculated was originally there you can estimate that the rock was 4.5 billion years old. Other isotope systems can be used in situations where your age estimate is different. If you can use an isotope system that has the right half life you can get an accurate and reliable date for a range of from hundreds of years to billions of years and anything in between.
This digression has turned out to be much longer than I originally planned. So let me stick with it just a little longer and explain how scientists figured out that the C-14 isotope system had problems. They didn't match it against a different isotope system. Instead they matched it against a completely different dating system called dendrochronology. This is just a fancy name for counting tree rings. People have known for a long time that if you cut tree down you will see rings. And each ring represents a year in the life of the tree. The rings represent wood of different colors. And the explanation is simple. In the Spring when the weather is nice the tree grows quickly and typically creates light material. In the winter the tree grows more slowly and typically creates darker material. This idea of annual tree rings has been around a long time and was certainly not invented by scientists. But scientists took this basic idea and built on it.
Scientists observed that a wide ring represented a year with good growth weather and a narrow ring represented a year with poor growth weather. Originally this idea was used to determine weather patterns for times and places where there weren't good weather records. But scientists found a way to do even further. All the trees in a specific stand experience the same weather so they will have the same pattern of narrow rings for poor growth years and wide rings for good growth years. This allows the pattern of rings to be synchronized between different trees. Specifically, if you can find the stump of an old tree in a stand with younger trees you can match rings from late in the life of the stump with rings early in the life of the younger trees. This allows you to establish the time period when the old tree was alive. You now have access to weather information going farther back than the age of the oldest tree still alive.
This idea can be extended to trees in different stands as long as the stands are subject to similar weather. And this method can be used to develop a weather record that spans not just two trees but several trees. So a record can be developed that spans hundreds, in some cases thousands of years. And the method does not require a whole tree. A beam from a house or any piece of wood big enough to contain a number of rings can be used. So a beam from a building or a piece of furniture can be dated. You know the object containing the piece of wood was constructed some time after the tree that originally contained the piece of wood died (e.g. was cut down). This allows you to date the piece of wood as being after the newest date represented by the newest ring in the piece of wood. This can be very useful.
Specifically, wood contains carbon. You can take a small sample from piece of wood and C-14 date it. You can then compare this C-14 date to the tree ring date for the larger piece of wood. You may even know the exact year the rings were laid down that ended up in the small piece that was C-14 dated. Scientists did that. They had complete confidence in the tree ring dates. They found, however, that the C-14 date did not match. That caused them to go back and look harder at the C-14 system and decide it had problems. They now know what these problems are. But there is not always a method of correcting the C-14 date that works.
Scientists do this kind of thing all the time. They test one method against another method to see if they agree. It's nice when the do but they don't always. When there is disagreement they go back and look at both methods to see if they can figure out what went wrong. Most of the time when it turns out that something is wrong it is scientists and not the critics that figure out that there is a problem. When it comes to legitimate criticism, criticism that turns out to be justified when all the facts are in, Scientists are much harder on Science than critics are.
