VIEWS: 4 PAGES: 8 POSTED ON: 6/13/2011
Hello, and Welcome to Mr Crowder’s World! This is the Second in a series of Podcasts aimed at detailing many of the most tantalizing Scientific questions an intelligent student would ask when learning something interesting in a middle School Science class. This podcast asks and answers the question, “Why do we think we know how old really old things are?” This is a very good question, and a tough one to answer since humans only live at most a century and usually only half that. Our Gregorian calendar only goes back about 2000 years, yet we say the earth is 5 billion years old.. It doesn’t seem like we’ve been around long enough to make such a claim! Why do we think we know that mankind only reaches back into history about 50,000 years? Or perhaps more than tripling this figure, the recent human fossil find in Ethiopia by Geologists Richard Leakey and his team of paleontologists who traveled in 1967 to the Kibish Formation along the Omo River, and later in 2001 by Frank Brown, dean of mines and Earth sciences at the University of Utah. Brown and his colleagues determined that these fossilized bones of a Homo sapiens were 195,000 years old -- the oldest fossils of our human species ever found. On what methods could these rather exacting figures, rattled off by scientists - like the time for the extinction of the dinosaurs - 65 million years ago, or the age of the earth, be based? And why do we believe their accuracy? The dating of things older than recent history probably begins with tree rings. Since we know exactly how old a living tree is by it’s rings, then we can start there. The differing growth rate of a tree caused by the change of seasons creates its rings. When the tree’s growth slows in the cooler winter weather, the ring being formed is darker due to the nutrients concentrating in the ring and not spreading out as much as they do in the warmer summer months. Thus, alternating bands of dark and light rings allows the observer to count the years of growth. The outer layer just under the bark - usually a green color, is the one growing and it represents this year. The others below this outer layer are dead and no longer growing. Being a tree for you would be like your skin is all that is alive and everything underneath is just a framework for your skin to grow on. The assumption is that in the big rain years, all of the trees in the same forest will have thick rings. Though, thick, on one tree might be thinner on another, so it’s relative to each tree’s own particular growth rate. And, in drought years, all of them will be thinner.. We can accurately date wood back to about 20,000-50,000 years in some temperate forests with tree rings thanks to Andrew Ellicott Douglass who first formulated the tree ring method for dating known as dendrochronology in the early 1900s. Actually, Douglass was an astronomer, not an archeologist. But, he figured the change of solar energy would affect tree growth and that he might be able to extend the sunspot cycle, his focus of study, further back into history. Dendrochronology is the analysis, cross comparison, and tracking - in extensive data collections usually with short and long lines tabulated into vertical columns. The scientists can then overlap data creating an unbroken chain starting with the current year’s outer ring cross-referencing through more and more ancient trees felled, fossilized, or cored with an instrument, called an increment borer, all in the same forest. Obviously forests in other parts of the world might have different weather systems for these same sampled years. So, it’s important to note that your cross-referencing to form a chain of known dates is only good for each particular forest, so tree ring data is not global. Since a sort of morse code develops with patterns of drought and rain over decades and centuries, “thick”, “thin”, “thick, thick thick thin thick thin thin thin thin....” you can take data that overlaps from tree to log to petrified log to perhaps the dawn of man in some places. This is how the Anasazi ruins on the Western Colorado Plateau were dated. Looking at the tree ring patterns in the wooden supports of buildings found under an outcropping of rock. Okay, Fabulous! So, we can date back to 50,000 years by cross- referencing tree ring patterns in a sample of wooden spoon, table, building or artifact,, but how do we date ancient bones, rocks, or really old things like the age of the earth itself? Around the same time as Douglass’s dendrochronology, discoveries were being made in radioactivity by Henri Becquerel, Marie Curie, Ernest Rutherford, & Wilhelm Roentgen. Radioactivity is the steady loss of small particles flying out of the nucleus of atoms. Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that make up the nucleus are balanced. An atom is unstable, the term more commonly used (radioactive), first coined by Marie Curie,, if these nuclear forces are unbalanced apparently because it has an excess of internal energy. An unstable nucleus will continually vibrate and contort and, sooner or later, attempt to reach stability by some combination of: • ejecting neutrons, and protons • converting one to the other with the ejection of a beta particle or positron • the release of additional energy by photon or (, gamma ray) emission. As the unstable nucleus emits radiation or (disintegrates), the radionuclide transforms itself into different nuclides. This process is called radioactive decay. It will continue until the forces in the nucleus are balanced. For example, as a radionuclide decays, it will become a different isotope of the same element if the number of neutrons changes, or into a different element altogether, if the number of protons changes. So, This distinguished group of Nobel Laureates were finding out things like radioisotopes, or emitters were charged particles and that they would veer off their paths when coming in contact with a magnetic field, meaning they were particles and not waves like x-rays, discovered by Roentgen, and that they would make air become an electrical conductor by ionizing the air - this discovery by Becquerel. Since radiation ionized - or gave a charge to air particles, then two electrodes held really close and almost touching would carry a charge across the gap of air, completing a circuit like a wire made of air! This concept is what eventually lead to the geiger counter, named after its inventor. Geiger realized he could carry around a battery powered device that uses two electrodes mounted extremely close together, but not touching, to complete a circuit only when the air became ionized by radiation. The device then would make a beeping sound, flash some lights, or produce a digital read-out. So the ground work was being laid on how radioactive isotopes and elements behaved. By 1911 Charles Wilson had invented the cloud chamber. This would allow a person to actually see the trails of the radioactive particles very similar to those trails left by a jet airplane flying so high in the atmosphere that the plane itself is invisible, but the contrail, or vaporization trail made by its exhaust interacting with the particles in the air, is easily recognizable. The same thing happens with radioactive particles. Yes, they are super tiny atomic sized particles completely invisible to even a microscope, but their paths can be revealed by a cloud of condensation left behind by the ionization, or charging of the atoms in cooled humid air. This caused by the quickly moving radioactive particles emitted from a radioisotope. There are many forms of these radioactive particles, some are the size of a Helium nuclei, and some are massless. The Greek letters alpha beta and gamma are a few of their names. During this early miasma of discoveries, Frederick Soddy and Ernest Rutherford 1903 came up with the Law of Radioactivity. It seems that all radioactive isotopes, called parents, decay at the same rate into their particular daughter elements. The daughters are whatever the parents have become once they’ve decayed. Now that doesn’t mean that all the elements have the same decay rate or speed of radioactivating,, but that each radioactive element decays at its own particular rate forever, or until all of the atoms are done radioactivating, once that particular element’s rate of decay has been established on a graph. Now, how do they know that? How can they be so sure this is the case for EVERY element, and that there is no end to the decay? And what is so important about this graph anyway? Soddy and Rutherford would establish a rate for whatever element they found by using a counting mechanism like Wilson’s cloud chamber, or the geiger counter, and the element in every sample would always continue decaying at this same rate for as long as they measured it. Also, each element was found to have its own unique rate of decay. For some, the decay rate slowed to half in less than a millionth of a second and some apparently would decay into the 10s of billions of years before their rate slowed to half speed! Rubidium-87 for example is said to take, 40 billion years before it finally slows to half! They don’t “know” that - how could they? No one has ever lived that long! This figure is extrapolated from a graph which is the EXACT SAME GRAPH for EVERY radioactive element. The only difference being the increments of time you use on the graph’s x-axis. That’s the other part of the Law. Every radioactive element uses the same decay graph just with different numbers of time on the x-axis. Interestingly enough, it’s the same graph used for the odds of flipping coins. And like coins, we feel that the more items there are to flip (in this case the atoms) the closer to a predicted amount that turn over, with each successive flip. In other words, if you have only one coin to flip, you have no idea whether it is going to be heads or tails - In fact the odds are 50-50. Not good odds. But, if you flip two coins at the same time, the odds are now slightly better that at least one coin will turn over. If you had a million coins, then then you can be almost certain half will turn over if you were to flip them all at the same time. The classic experiment proving the law of probability was done by Mathematician John Kerrich, who tossed a coin 10,000 times while interned in a prison camp in Denmark during World War I. At various stages of the experiment, the relative frequency would climb or fall below the theoretical probability of 0.5, but as the number of tosses increased, the relative frequency tended to vary less and stay near 0.5, or 50 percent. Kerrich recorded the number of heads within each set of ten flips. In his first ten flips, the coin landed heads-up four times for a relative frequency of 0.4. In the next ten flips, he observed six heads, bringing his overall relative frequency back closer to 0.5. After 30 tosses, the proportion of heads was .567. After 200 tosses, it was .502. With this small number of tosses, the proportion of heads fluctuates. But after 10,000 tosses, Kerrich counted a total of 5,067 heads for a relative frequency of .5067, which is really close to the theoretical probability of 50-50 or batting 500 Like Kerrich’s test, each year my students get boxes with 100 m&ms per group all ms facing down. They shut the lid and shake the box once. This is called a half life because you would expect half to turn over when you shake it. They reach in to pull out all the ms that turned over to face upwards. We call these ms we take out the daughters (the analogy being that the ms facing down are like the radioactive atoms, and the ms that turn over are the daughters.) They go through four half lives. In this case each half life is a shake of the box. The first is with 100 mms. When they plot the amounts of parents and daughters at each half life, they generate the same graph as the law of probabilities and the law of radioactivity. Then, they do the same probability experiment with 50 mms, then 25, and finally only 10. We find that when there are more mms, the closer to the actual curve, and the less out of whack are their experimental graphs. What we proved is that when there or more items, each with an equal chance of being turned over, or turned into a daughter, then the more likely you are able to predict how many turned over on the next half life shake, and the next, and the next. The ultimate goal for all of this probability testing is to realize that there are billions and billions of radioactive atoms in any given sample that a scientist tests.. Therefore they can be reasonably sure of any predictions they make on their decay graphs, even when going well beyond human or geologic time scales. They are so sure, that it’s one of science’s laws!! THE LAW OF RADIOACTIVITY Since one can analyze the percentages of parent and daughter elements in a rock or any sample, using a spectral analyzer, or mass spectrometer, and the rate of decay can be measured by any of the counting methods like the cloud chamber or geiger counter, the age of that object can be determined. The way that is done is finding the percentages of the parent and daughter element in the clump of matter in question. If it’s 100% parent element, it is 0 time, because as soon as the time clock starts, the parent begins to decay. So that would mean less than 100% parent if only slightly. Then, If it decays down to 50% parent and 50% daughter, since it has decayed half way, it’s age is appropriately named - one half life. With the Law of Radioactivity, you can say, since there is this amount of parent and this amount of daughter in this sample, it must have decayed for this long. But, what people have difficulty believing, is that we seem to have some elements with half lives much older than our current understanding of the earth’s age or even the current estimate for the age of the universe! How could we know that? Well, remember the tree rings? We can be relatively sure of the age of some pieces of wood that date back to between 20,000 and 50,000 years. In 1949 Willard Libby invented the radiocarbon dating method that would use the percentage of parent element C-14 and its non-radioactive daughter N-14 from a sample and be able to determine the age of the sample. It was assumed that the cycle of C-14 and N-14 creation and decay caused by cosmic rays colliding with Nitrogen in the Atmosphere remained constant throughout time. At least as long as the atmosphere had been able to sustain life. Then, one could calibrate the law of radioactivity by looking for a tree ring sample that had exactly half C-14 and half N-14. They promptly found it, and a tree ring exactly 5,730 years old was one half life because it contained exactly 50% C-14 & 50% N-14. Then, a tree ring exactly 11,460 years was two half lives old, and a 17,190 year had gone through 3 half lives. So, the Law of Radioactivity was proven yet again, and calibrated pretty solidly, since each successive 1/2 life was the perfect sum of the previous. And now to something really old like dinosaurs, or the age of the earth. All you have to do is find the percentage of parent and daughter elements that have long half lives in a fossil or rock sample - like Uranium, or often for Dinosaur bones - Potassium-40. Recently, geologists found rocks from Canada in an area on the eastern shore of Hudson Bay in northern Quebec that formed 4.28 billion years ago. That’s 250 million years older than any other known rocks. They used a new instrument called a thermal ionization mass spectrometer to measure the percentages of parent and daughter elements. But, of course with plate tectonics and an active mantle, the fact that the crust continues to recycle as it plummets back into the earth about a centimeter per year in most places, perhaps it is older than the oldest rocks. It could be infinitely old since we can’t know how many times the surface of the earth might have recycled itself, so,, Somebody had the brilliant idea to double-check with moon samples. If we could find the oldest moon rocks, then the moon not having a liquid mantle and not recycling its surface like the earth, and assuming the moon’s age is probably close to Earth’s, the oldest rocks found there could lead us to our lovely planet’s age. So, During the Apollo missions to the Moon from 1969-1972, astronauts collected 2,414 samples of Moon rocks, weighing 383 kilograms, or 842 pounds. One of these rocks, collected in 1971 during Apollo 15, was named the Genesis Rock. It is the oldest known piece of lunar crust, having formed as the Moon cooled and solidified over 4 billion years ago. Now, most scientists agree on a 4-5 billion year range. And that concludes this podcast on how do we know how old things are? From Mr Crowders World, to you...
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