Eli5: how can scientists determine the age of anything past x amount of years if there’s no records to prove it?

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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…).

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.