To explain.
Let’s say they have a method that can test bone age. Up to let’s say 1-2k years we can know for sure it’s accurate, since we might have believable records on the bones proving that the age test is accurate.
Past a certain age though there’s no more records. How can we know the testing is accurate and not just the method only going up to that limit and being inaccurate on anything older? Or are we just assuming?
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There are many different methods of “dating” something (meaning to determine the age of things). All of these methods fall into two categories, relative dating and absolute dating. I’ll give a brief overview of each as well as some examples, however, the science of dating could almost be a field of science in and of itself, so I’d encourage you to Google these as well. Just search “relative/absolute dating techniques.”
Absolute dating is probably what most people think of when scientists say they’ve found the age of something. Absolute dating uses some sort of methodology to determine the actual age of something, i.e. the scientists can say “this thing is x years old.”
Relative dating is also very common when scientists date things. Relative dating typically places the age of something as either older, younger, or between the ages of other things, i.e. the scientists can say “object x is older than object y but younger than object z.” In many instances relative dating is the only way to date something, however, often times the thing that you are comparing the age to may be dated using an absolute dating method which can allow relative dating techniques to be used as absolute techniques.
I’ll give two specific examples of each from different fields of science.
For absolute dating, arguably the most famous and by far the most used method is radiometric dating. Scientists have worked out to remarkable precision the rates at which radioactive elements decay. We know that carbon 14 takes 5,700 +/-30 years for half of it to decay into nitrogen 14. This means if you find a bone that you want to know the age of, what you can do is measure its chemical composition to determine how much nitrogen 14 is in the bone and how much carbon 14 is in the bone. Then, using the known rate of carbon 14 to nitrogen 14 decay, you can use those values to determine how old the bone is. There are many other elements that can be used in radiometric dating, like uranium-lead or potassium-argon that allow us to determine the age of things anywhere from 100 to 4.6 billion years ago. Another absolute dating method is one you’ve probably heard of, Dendrochronology, commonly called tree ring dating (though it’s not exclusive to trees). There are many physical and biological processes that occur at different rates depending on the time of year. For trees, they tend to grow faster in summer months when the air is warmer and there is more water and light around. This means in these months the rings inside them will be larger than the rings in the colder months. You can then count these different rings to determine the age of the tree. As I mentioned though, this isn’t exclusive to trees. Some processes like erosion create more runoff in the rainier months, creating thicker deposit layers in those months. This means you can count the thick and thin layers to determine the age of that deposit layer.
For relative dating, as I mentioned you don’t determine an actual age of something, you just determine if something is older or younger than other things. My favourite example are craters on planets/moons. You can tell how old an area of a planet is relative to other areas of the planet by counting the craters. Since asteroids/comets have no preference where they hit a planet, craters *should* be spread out evenly over a planet’s surface. However, if the planet is geology active (volcanoes, tectonic plates, etc) some surfaces can get renewed when new lava flows over them, erasing the craters. This means you can count the craters in two different areas on the planet and whichever area has more craters is almost certainly older than the other one. Again, this doesn’t tell us their actual ages, but can still be very useful, because it could tell us that something geologically interesting happened recently on a planet. Another common relative dating technique in geology is the “principle of superposition” which basically states that new rock layers are only ever placed on top of older rock layers. This means that the deeper you go into a system of rock layers the older those layers get. So hypothetically, say that the scientists found two fossils at a dig site and one of the fossils was much deeper than the other fossil. The scientists can then conclude that the deeper fossil was a creature that lived before the shallower fossil since the layers of rock it was in were older.
One of the neat things about relative dating is that you can often use it to get an absolute dating method. For example let’s go back to the principle of superposition, and let’s say we found a fossil between two layers of rock. We don’t know what the age of the fossil is, but we do know it’s younger than the deep rock layer and older than the shallow rock layer. Now let’s imagine that the rock layers have uranium in them which allows for radiometric dating. If we then date the rock layers we could find that the young rock layer is 60 million years old and the old rock layer is 80 million years old. Since we found the fossil between these two, we then know that the fossil has to be between 60 and 80 million years old.
One caveat that should be mentioned about ANY measurement (doesn’t matter what that measurement is, time, distance, electric charge, voltage, mass, etc) is that there is ALWAYS an uncertainty to it. This basically means that, any measurement will be an expected value plus or minus a range of values where the actual value could potentially be. For example, say we were dating something using carbon 14 dating, and we found it was 500 years old. The scientists would actually report an age of something like 500 +/- 10 years. Basically saying that it’s anywhere between 490 and 510 years.
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