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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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.
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