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 is always an element of doubt within scientific research, you will rarely find someone saying ‘this is 100% accurate’.
The accuracy of carbon dating for example has been questioned recently.
Really you look for numerous sources to corroborate/verify a conclusion, and it is worth remaining mindful that the methods used to calculate a given date are not perfect and may need to be refined as new information/methods become available, as you suggest.
Accepted conclusions for a given phenomena are often ‘the best we have got at the moment’, until something better comes along.
Sometimes when something better comes along, prior conclusions may be discredited or placed under further scrutiny.
Some conclusions to studies though are so replicable, the margin for error for so low, and the real world application stable, that further research is not prioritised. There is a ‘if it is not broken no need to fix it’ concept here, as there is a finite amount of resource (in all meanings of the term), that further study is simply not urgent, this is not to say further research would not be valid.
They do have a method to test bone age. Radio Carbon dating.
Radioactive things decay at a steady and fixed rate. This is a known rate from physics and chemistry, called the half life. It’s a physical and universal constant.
Things take in a slightly radioactive isotope from the atmosphere when alive, Carbon 14. When they die, they stop taking in carbon because they are dead. They can use the decay of the radioactive carbon to see how long something has been dead.
This will get us to about 50,000 years since the half life is only about 5,000 years.
To go farther, you can use other longer half life isotopes but getting into the geologic time scale at that point with billions of years. Accuracy goes down a bit though, and that’s accepted. At some point the accepted range gets far higher since that is the highest degree of sensitivity tests can run. To test for decay, instruments must be able to measure how much has decayed.
There are other methods like layers(stratigraphy), if something is known change over time it can be seen in buried layers. An example is the KT boundary. A layer of iridium from a meteor strike that has been very well researched, so it’s possible to have a relative “Was this before or after 66 million years ago?”
Most dating systems use radioactive decay. I’m going to explain the general principle behind it, and two specific cases.
In general, how radioactive dating works is that you pick a useful radioactive isotope, and figure out how much of it was present when something got made, and how much of it is left now. That ratio tells you about what percentage is left – which tells you about how many “half-lives” of the isotope have passed. Multiply that by the half life, and you get the approximate age of the thing. A “half life” is the amount of time it takes half of an isotope to undergo radioactive decay – you can think about it like flipping coins: after one flip, half the coins have flipped heads, and you don’t flip them again. If you started with enough coins, you can guess about how many flips have been made by looking at how many heads are on the table, and how many are tails (meaning, they’ve never flipped heads). In radioactive dating, as long as a thing is between 1/10 and 10 half-lives of the isotope you are using, you can get an accurate-ish guess of how old the thing is.
For life forms, scientists use “carbon-dating”, which uses Carbon-14 as the isotope. Carbon-14 has a half-life of about 5700 years, and is constantly being made in the atmosphere when sunlight hits Nitrogen-14. There’s not a lot of it; but we think there’s been a pretty constant amount (there is some controversy over this), so anything that is alive has the same amount. However, when something dies, it stops getting new Carbon-14, so the amount of Carbon-14 in a dead thing starts decreasing.
For rocks, scientists use a few different isotopes, but Uranium-lead dating is one of the more common ones. This works slightly differently: some minerals allow uranium in their crystals, but not lead. This means that when crystals form, there’s only uranium, and no lead. However, over time, that uranium decays into lead – and because you know there was no lead to start, ALL of the lead in the crystal must have come from uranium decay. This method is slightly more complicated, because there’s two isotopes: Uranium-238 decays into Lead-206, and Uranium-235 decays into Lead-207; and at different speeds. However, this also gives a wider range of usable dates: anything older than 1 million years can be dated in this way (in theory, it should work through 45 billion years, giver or take – but that’s older than the universe…).
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As for how we know this is accurate:
With modern technology, it is relatively easy to isolate a pure sample of many isotopes, and to watch the sample for radioactive decay. If you’re starting with a pure sample of even a gram or two of a material, you have over 10^20 atoms. With that many atoms, it’s pretty easy to keep track of how often a decay occurs – and from there, figure out the half-life of the isotope. Many isotopes we have the half life measured to three to six significant digits (Carbon-14 is 5730 years, plus or minus 40 years; Uranium 235 has a half life of 703.8 million years, accurate to the nearest .1 million).
The harder part is knowing how much of the isotope was there at first. As I noted earlier, Carbon-14 dating has some issues because there is ongoing debate about how much Carbon-14 was present at various times in the past (there’s more today than there was before the 1940s, because nuclear tests produced a lot of it); which makes carbon dating less accurate. In contrast, Uranium-Lead dating is more accurate because you have two different isotopes that generally behave the same, and so it’s a lot easier to correct for things like lead leeching out of a rock.
The gold standard is radiometric dating. This process looks at the ratio of radioactive isotopes in a sample of a material to determine when it was formed. Everything has tiny amounts of radioactive isotopes in them. These isotopes decay into isotopes at known, fixed rates. Since we know the decay rates and we know that the decay rate doesn’t change, we can look at the ratio of the isotopes to determine when the object was formed, or in the case of organic remains, when the organism died. This does not require historical records and it is not a guess or an assumption – it is a precise (with a small margin of error) and scientific way to date objects.
They use multiple independent methods to corroborate.
First is relative dating. There idea that objects buried deeper, are older. It’s also documented at any site you find the object or one similar to it. This allows you to compare, and try to account for any missing layers or irregularities.
For example you find layers of rock types, other recognizable objects, fossils etc. Say in the order below
134a89
At another site the layers are
23a698
“A” our mystery object appears it same sequence in both sites. There is some variation, as some markers didn’t survive in both sites (never got deposited, eroded, extinct in the area at that time etc)
And every site we check we compare and see how consistent it is, how it is placed based on others etc.
This doesn’t tell us exactly how old it is. But we can pretty confidently say from the two samples down that it’s older than 123 and younger than 689.
But this is only one method.
Another is looking for any market that has a fairly consistent change over time. A big one is the way ice looks when deposited in the summer or the winter. It’s more clear when deposited in dinner months and more cloudy with trapped gas in the winter. This allows researchers to essentially count cycles or years to get a good estimate of the age. There is some uncertainty of course, as layers can get destroyed.
But again, it’s checked against multiple sites, and researchers examine samples for abnormal signs and document possible lost years. Say signs of a summer layer that bleeds into a winter layer may indicate an event that melted a lot of ice.
These are compared to relative dating, if we find an object embedded in such a sample.
Then there is the radioactive dating as described by other posters. Again once this value is find, it’s checked against the others to see if it lines up.
So you find an object, let’s say a bronze knife, and plane it in the local record based on relative dating. It’s found “above” obsidian and flint tools, and under an interesting layer of ash.
Across the country someone else finds a bronze pot. No obsidian tools, but they did find the ash layer and their bronze pot is under that.
We’re getting an indication that bronze work occurred before that eruption, and not that this knife was buried or accidentally put lower.
Radioactive dating out the two objects within a couple bonded years of each other, another supporting piece.
And then geologists start placing the ash layer the same way we placed the bronze knife, fitting it into the geologic record using both relative and radioactive dating. This places it younger than bronze knife at every location.
Sunshine finds an ash layer in artic ice cores… And use the later method to date it. They find it’s date and composition of trace minerals matches the one in our region. And the later method backs up the radioactive and relative dating.
One single method of dating isn’t ironclad, but having several all give similar results puts it beyond reasonable doubt. Having all of those be wrong, across multiple sites is incredibly unlikely.
And we also find that the radioactive dating method is incredibly reliable, so much so that it becomes the primary line of dating evidence. Good enough that even if it’s all we get, it’s viewed as correct because it works fine and again when checked against other methods.
Ideally, we have multiple test methods that reach the same conclusion, or similar.
For example, we might use Carbon dating to find a fossil is 9,000 years old.
We also look at the rocks layer that the fossil was found in. If we based on our models of how sedimentary rock builds up, and it predicts that the rock layer the fossil was found in are 11,000 years old.
At the same time, we try to identify the species of the animal the bone belonged to. We might know that this species went extinct 5,000 years ago.
We might also use Flourine dating to determine the bone was underground for 10,000 years.
Based on all of these dating methods, and the fact they line up roughly, we can estimate the bone’s age at 10,000 years.
Followup research might show that due to heavy volcanic activity 10,000 years ago, caused an abnormal concentrations of Carbon 12, so our Carbon dating would underestimate the age by 600-1200 years. Yay! more accuracy.
Our sedimentary rock models might update, and show we over estimate by 500-2000 years. Better!
So ideally we use lots of different tests that say the same thing.
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.
You have a phone right? With a battery in it? Let’s assume it lasts a full 24h before dying.
Every day you charge that phone, and as long as you’re alive, the phone gets charged, you login etc.
Now you die with the phone in your pocket, no more charging.
When you’re found, someone may look in your pocket and find your phone and from the state of charge they can measure how long ago you were alive to charge it for the last time, so that gives them at least a day when you were still alive.
Of the phone is dead and the battery’s completely depleted, you’ve been dead for over a day right?
Well you also drink water. How dry is your body? From the level of dehydration you get a scale of weeks where you were still alive.
Well you also eat right? From the level of decay you can get months, perhaps even years.
Well that food also has radioactive isotopes that stopped being replenished once you stopped eating. From the level of radioactive isotopes in your body you can get accuracy within thousands, even millions of years.
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