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Physics and Archaeology
Guinnessy writes: "In 1960 Willard Frank Libby won a Nobel Prize for his work on radiocarbon dating, a technique that truly revolutionize archaeology. Now Physics Today magazine has an article describing how new methods are yielding more accurate dates for our prehistoric ancestors, profoundly affecting our understanding of the past. Neat stuff."
I didn't really see a lot of new information in the article, but it did mention some radio-dating techniques I had heard of.
What's left to consider are the reprocussions from this kind of discovery. It's important to remember that all of human social sciences... language, philosiphy, psychology... all of them will benifit dramatically from knowing not only the exact time of origin of the human species, but early human's movement patterns.
One of the problems about human history that this kind of dating will help solve is the origin of human language. When did humans learn to speak? What languages descended from which? Why do many 'fairy tales' appear in more than one culture? Was there a single human 'parent' language that was responsible for this?
This kind of 'early' human history dating will help us probe out these kind of conundrums.
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From hypertextbook.com
potassium-argon dating
Potassium-argon dating is used to determine the age of igneous rocks based on the ratio of an unstable isotope of potassium to that of argon. Potassium is a comon element found in many minerals. The isotopic distribution of potassium on the earth is approximately 93% 39K and 7% 41K. Since these values are only approximate, the total percent abundance of these two isotopes is not 100%, but 99.9883%. The remaining 0.0117% is 40K -- an unstable isotope with a half life of 1.26 x 10^9 years. 40K has three decay modes: beta decay, positron emission, and electron capture.
1.26*10^9 = 1.26 BILLION. On a logarithmic basis, the article is much closer than you.
My field work has suggested to me that "what it means to be an archaeologist" is basically sitting around and spouting lots of complex scenerios from rather trivial amounts of data. Whoever pushes the grandest vision gets the biggest funding and that's what determines our view of reality. I thought about going into the field at one time but was quickly discouraged. While it's an interesting (and useful) mental exercise to sit around and think up these scenerios, I think the amount of real hard knowledge that we derive from them is essentially meaningless. Fact is, pretty much all of what we know comes from written records. The rest, we owe mostly to the harder sciences (carbon dating, genetic analysis, etc). And the speculation, well, that's not so easy to test...
Devon
I'm a geologist and I use some of these techniques fairly regularly (40Ar/39Ar, U-Pb, 14C, ams cosmogenic dating). Most often, anomalies in the ages you get are most readily explained by geologic uncertainty rather than gross flaws in the techniques themselves. Sloppy field work and sample collection/documentation can get you in trouble when you try and interpret the geochronology. Also, non-idealities of the materials we use to date and other factors come into play. The dating techniques are sound, the "critical assumptions" you seem to question regarding the mechanics of calculating a radiometric age and the theory behind it really come down to radioactive decay and our technical ability to measure isotopic ratios very precisely, both of which are far from dispute. The diffuculty is in interpeting the resulting ages in a meaningful way. For instance, fluid infiltration and other processes often impart "extraneous argon" to a sample that results in an anomalously old 40Ar/39Ar age. We can analyze the
isotopic data to see if the extraneous Ar is there and we can look at the minerals and the geologic context of the sample and assess the likelihood of it. But unless we do those things throughly, we can misinterpret the isotopic data and thus the age of the sample. The isotopes don't lie, but we can be fooled.
> What if it's on an exponential rate itself?
> What if it decayed at a slower/faster rate
> 1 million years ago?
I suspect that couldn't happen unless the balance between the strong and weak forces changed. And if that balance did change, the results would be far more catastrophic than a mere asteroid collision.
Imagine entire galaxies unravelling into bundles of plasma - or switching off like a light switch.
No, I don't see that assumption as a major problem.
Actually, it isn't the decay rate that is changing over time. It's the exact opposite. The production rate of 14C in our atmosphere (due to interactions between N and cosmic rays) is changing over time. As with all cosmogenic nuclide techniques (e.g. 26Al, 10Be, 36Cl), our constraints on the production rate of the nuclide of interest is key. Dendrochronology (tree ring dating) has helped to put firm constraints on the 14C production rate going bakc 9kyr or so.
is that you can tell easily item one is just a s old as item two. But unless you have a way to date one of the items some other way, all you can tell is that the items are the same age, but not how old they are.
When you are talking about artifacts millions of years old, there is no "proof" of the age of any item.
Some scientists put together a theory of how carbon acts over millions of years (and obviously because of the timeframe involved have no empiracle evidence about the behavior of carbon over millions of years) Then they date things relative to the theory.
Note I am not saying carbon dating is wrong, but it certainly hasn't been proven.
In the case of 14C, our understanding of production rates over time is actually remarkably good given the dendrochronologic record (tree-rings). Also, for other radiometric systems, such as 40Ar/39Ar and U-Pb, there is little discrepancy in ages from the same sample (taking into account technical details such as closure temperatures, etc.). At any rate, my point is that our geochronologic techniques are (a) ground in robust and simple theoretical physics and (b) much more often than not yield self-consistent results. But like you say, unless we get a time machine, we'll never know for sure. But that's kind of a moot point.