The Davey Dialogues - An Exploration of the Scientific Foundations of Human Culture. John C. Madden
personal story is an interesting one and is ably told in a book by Simon Winchester, The Map that Changed the World. Over a period of twenty years, this observant and self-taught son of an English blacksmith compiled a large map of England and Wales depicting the relative age of the strata to be found near the surface. This map was published in 1815 and caused a small sensation. The credit he received for all his work was initially disappointing to say the least. Smith paid a huge penalty for not being a recognized member of the scientific establishment. His work was plagiarized, and he spent time in debtor’s prison before finally, in his later years, enjoying the recognition he deserved.
Smith’s discoveries did much to promote the scientific field of palaeontology – the study of life in the prehistoric past. There is now a large body of knowledge relating to long-extinct life forms. These studies demonstrate conclusively that humankind is very much a newcomer to the parade of life forms over the past few billion years. In common with most other life forms, many of which have disappeared, its future is by no means assured. This was a message that the Western world of Smith’s time was ill-prepared to receive, since it was still fixated on the idea of spontaneous creation as outlined in the Old Testament, and buttressed by Archbishop Ussher’s precise calculation of the time when that creation occurred.
Palaeontology has allowed scientists to determine the relative age of various rock strata, but it was a long time before non-specialists accepted its basic message about the existence of life long before 4004 BC. In part this was because palaeontology did little to help us determine the actual age of a given rock or relic. Our ability to make such identifications came much later (for the most part after the Second World War) with the discovery that radioactive isotopes of some commonly found elements could be used to determine the age of a variety of substances.
– Well, those were some quaint stories you told me. You humans seem to have had a lot of difficulty sorting out the age of things. It is not a problem I had thought about much before listening in on you and others. However, I now suspect it had much greater importance to my vanished friends than I had realized.
But I missed something in what you said. Can you tell me what radioactive isotopes are?
– I’m surprised you don’t know! Some elements have variants (called isotopes) that are not stable and that decay into other elements. [Isotopes are explained in greater detail in Appendix 1.]
– How strange. You mean that the ninety-two natural elements, which I thought were the basic building blocks of your planet, are not always stable?
– That’s right. Furthermore, the rates of decay vary widely from one unstable variant of an element to another. This turns out to be very useful for calculating time lapses. Don’t you have radioactivity in your universe?
– Not that I am aware of. This is quite a surprise for me. I need to learn more.
– And so you shall.
I tried to mask my astonishment that Davey’s universe had no radioactive elements. In fact, while I was prepared to talk about the application of radioactivity to the dating of old artifacts, I had not guessed that radioactivity itself would be a mystery to him. So, I began at the beginning with a quick summary of the discovery of radioactivity in 1896.
– Henri Becquerel was awarded the first-ever Nobel Prize in Physics in 1901 for his discovery of radioactivity. Marie and Pierre Curie and Ernest Rutherford were subsequently awarded Nobel prizes, too, for their follow-on research that yielded a basic understanding of the phenomenon. The world of the early 1900s was just as surprised and mystified as you apparently are today to encounter the phenomenon.
By their very nature, radioactive elements are unstable. They give up energy in the form of radiation and decay into the isotope of another element. That new isotope may also be unstable, in which case it, too, will decay. Ultimately, the last radioactive isotope in the chain will decay into a stable (i.e. non-radioactive) isotope.
The first watch I ever owned had the numbers on the watch face as well as the hour and minute hands painted with a special luminescent paint containing traces of radium. The radioactivity in the radium excited luminescence in the zinc sulphide with which it was mixed. As a young boy, I loved looking at the watch face in the dark, and wondered at this miraculous appearance of light without an obvious power source.
For health reasons, the use of radium-powered luminescent paints was banned in most countries in the two decades starting in 1960. The deleterious health effects were first noted in the women who painted the watch face dials, many of whom licked their paintbrushes to give them a sharp point and subsequently developed cancerous growths on their tongues. Today’s luminescent paints are either powered by tritium, an isotope of hydrogen, which decays much faster and more safely than radium, or they are pumped by external radiation sources, such as ultra-violet light. The luminescent face on my watch today has only a very feeble glow as compared to that of my first watch!
Although clocks and watches with radium paint dials have now virtually disappeared, there is another kind of radioactive clock that has revolutionized our measurement of historical time.
As already mentioned, fossils provide a marvellous record of the comparative ages of different sedimentary rock strata. Using the simple assumption that a layer of sedimentary rock that is on top of another layer is therefore more recent in origin, it is possible to establish the relative ages of different fossils by correlating layers of similar fossils that appear around the world. As Gould so nicely pointed out, the very complexity of the evolutionary process means that the same animal has, for practical purposes, no chance of evolving twice in the same form. This makes comparison of fossils to other fossils found elsewhere in the world an excellent way of determining the comparative age of a rock or a fossil. The process is aided by the finding that in the early days of Earth’s history there was only one, large, contiguous landmass, making it much easier for one animal to appear in many divergent places.
However Gould went too far when he wrote: “The best signs of history are objects so complex and so bound in webs of unpredictable contingency that no state, once lost, can ever arise again in precisely the same way.”[23] For while the fossil record – the pre-eminent exemplar of this phenomenon – is very good at giving us relative time, it is of very little use in telling us exactly how old a given fossil might be. For that, we need a clock with a regular beat that leaves a record of how long it has been ticking away since the clock was “started”.
Radioactive dating (or radiometry) provides just such a facility. Used in combination with the fossil records, it has enabled us to gain a spectacular insight into events that took place long before the arrival of humankind on Earth, providing some humbling news about our place in the world’s history.
There were two main streams of radiometry development. Geologists were most interested in dating the age of rocks (whose ages are typically millions [and occasionally billions] of years old) and started work soon after the discovery of radioactivity in 1895. They tended to work with isotopes with long periods of radioactive decay, measured in millions or billions of years. Their estimates were rather inaccurate until after the Second World War, when much better instruments for measuring radioactivity were developed.
The second main stream of research focussed on isotopes whose radioactivity decayed much faster. These substances decay too quickly to be useful for measuring time scales as long as billions of years but very useful, for measuring much shorter time frames. Radioactive carbon-14 is the best known of the elements used for shorter-term dating, but it is not the only useful isotope applied to this task.
The undoubted pioneer in developing radioactive carbon–dating techniques was Willard Frank Libby (1908–80), an American chemist. According to Libby, carbon-14 and tritium dating had their origins in studies of the effects of cosmic rays on Earth’s atmosphere.[24] In 1939, just before the Second World War, Libby developed an instrument that permitted him to detect very low concentrations of carbon-14 and tritium – respectively radioactive isotopes of carbon and hydrogen. This instrument allowed him to discover that interactions of cosmic rays with the upper atmosphere led to the generation of about two neutrons every second over each square centimetre of Earth’s atmosphere. The