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Name: Robert
Status: other
Grade: other
Location: NV
Country: USA
Date: Spring 2012

Question:
All research I see regarding Carbon Dating accuracy state that the oldest reliable dates of this testing method to around 50,000+ years. Why do we still see and hear comments from even sources such as NASA using billions of years? What testing methods are these scientists using to come up with those kinds of time lines?


Replies:
The useful time scale of a radioactive dating method depends on the length of the half-life of the nuclide that decays. Carbon-14 has a fairly short half-life, so carbon dating is useful for recent times. Other useful isotopes, such as uranium-235, uranium-238, and potassium-40, have much longer half-lives, so dating techniques based on their decays are useful for much longer times.

Richard E. Barrans Jr., Ph.D., M.Ed.


Robert,

Carbon dating only works when the material being studied used to be alive and we actually have similar examples of that living thing currently. For example, we can study the carbon from a bowl or pottery that was made from a tree because we have samples of that tree right now. Here is how it works: while a thing is alive it is continuously taking in nutrients and excreting - this means that the balance between the two isotopes of carbon (12 and 13) is maintained at a constant value. When the thing dies, it no longer takes in nutrients and the carbon-13 isotope steadily degrades to carbon-12. By measuring how far the C-13 in a sample from a used-to-be living thing has degraded, we can estimate the age in which the living thing died and started the process of degradation of it's C-13. ...but this only works if we have samples of that living thing currently alive because by analyzing those current samples, we can have a baseline of what the C-13 to C-12 ratio ought to be if the balance is being maintained by the living thing taking nutrients and excreting. We cannot do carbon dating for objects, such as rocks, that do not have this balance of C-13/C-12, and we cannot have reliable data if we do not have live samples of that object. This is why the reliability of carbon dating only goes back to the time where we still have live samples of the studied objects (humans, cloth spun from plants, wood implements from trees, etc.)

There are other dating techniques similar to carbon dating that can be used for rocks (objects that existed prior to there being anything alive that we can measure) - and again this depends on the rock samples being relatively undisturbed (not having gone through erosion or been melted by some cataclysm), so this dating techniques only works for the life-span of the Earth. These dating techniques are good for objects that formed when the Earth formed and became relatively stable.

To have a reliable measure of the age of the universe (which is much older than the Earth) we do primarily three things: (1) microwave background radiation, (2) measurements on the rate of expansion of the universe, and (3) setting lower limits by the oldest observable objects in the universe.

The microwave background radiation measurement postulates that the current temperature of deep space (which is not zero) results from the remnants of the heat resulting from the time when the universe was incredibly hot (commonly referred to as the "Big Bang"). If we know how quickly the universe can cool (remembering that the heat has nowhere to go since the entire universe was hot, and so we can model what objects in the universe can take in that heat) then we can estimate how old the universe must be - at least since the Big Bang - in order for the universe to be its current temperature.

Measurements of the rate of expansion of the universe assumes that the universe started from a singularity and has been expanding ever since. By measuring how fast galaxies are receding from our own, connecting that to the distance of those galaxies from our own, we can estimate how long the universe has been expanding, and therefore how old.

We can also observe some truly old objects in the universe (white dwarfs, etc.) and conclude that the universe must be at least as old as the oldest objects in it. Since we know the process in the development of objects like white dwarfs, we know the lower limit of the age of the universe in order to have contained such objects.

The nice thing about these three independent measurements is that there is a strong agreement between the values they come up with. So we can say that we have a pretty good idea of how old the universe is.

Greg (Roberto Gregorius) Canisius College


Robert,

Using unstable isotopes to date events is one of the more reliable methods for determining absolute age. The relatively short half-life of 14C (5730 years), precludes its use for dating really old materials. Roughly speaking, we can reliably date something for about five half-lives (for 14C that equates to 5*5730=28,650 years).

The object that is being dated also needs to have C as part of its chemical make-up. Not all minerals have C as part of its chemistry.

So, what do we need to date older events? Something with a longer half life and something that is found in the minerals we are dating. There are numerous geologically useful isotopes that we use to date old events.

In addition to 14C, some of the isotopic ratios commonly used to date geological events are listed below.

U-Th/Pb

40Ar/39Ar

Rb/Sr

Nd/Sm

238U/235U

210Pb

K/Ar

Note that Rb/Sr half life of 4.8x1010 years (48,000,000,000 years), so we can date really old events.

Yet the oldest samples that we've dated are the moon rocks, some meteorites, and some old rocks on Earth. None of the ages comes close to the age of the universe. So, how do you think we put a number on the age of the universe?

(Please note: the numbers that preceed the abbreviation for the element should be superscripts, but if they did not translate well in your email, then they will look like a standard font).

--- Leslie Kanat, Ph.D. Professor of Geology


The method of carbon dating has evolved over the decades since it was first “discovered”. Various “corrections” have been incorporated into the method. You are correct that its range of utility is about 50,000 +/- . What various sources have done is to get “sloppy” about what method has been used to determine the date of various older specimens. Other radioactive isotopes decay by different schemes that can be analyzed using different decay schemes.

Vince Calder


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