Radiometric Dating For Sonic.

Question about half-lives? How do they know that the half-lives are corerct as to, the years. For example: how do we know that the half-life for uranium 238 is actually 4.46 billion years and not just 4.46 thousand years?------------------
Thank You
Sonic

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A half-life is the amount of time it takes for a nuclei of isotope to decay 1/2 way. So each half-life has a timeframe in yrs?

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Some, especially up in the really heavy elements, have half-lives that are tiny fractions of a second. On the other hand, uranium-238 has a half-life of 4.47 billion years. One of the things that makes radiometric dating so versatile is the presence of elements with such diverse half-lives. For example, carbon-14 dating works well for ages of a few thousand years but is useless beyond 50k or so; other elements don't give useful results at those ages but are good for dating hundreds of millions of years, with an error of a few million each way.This page may help you some more:
National Safety Council

Because we have equipment that can measure the decay of a radioactive mass. This has been done over and over and over again, and the measurements have been verified beyond a reasonable doubt. Additionally, we can calculate what the radiation on the surface of the earth would be if the known radioactive materials in the crust were to decay at rates much higher than what we know them to be today. I have not done the math myself but am told that we would be cooked alive in short order. Pardon the pun

Interesting.------------------
Thank You
Sonic

I think that some of the (rather large amount) of reading you've been pointed to explains this. But to repeat, it is actually measured. A sample is monitored, in some cases for years.I don't know enough to be able to comment on calculations also being able to predict anything.From good ol' talk origins
TalkOrigins Archive - Feedback for January 2003Decay rates are measured in the lab, not merely "assumed." For example, in 1955, Kovarik and Adams reported an experimental determination of the U238 half-life, of 4.507 billion years. Their paper describes one experimental setup:
Alpha particles from a thin layer of natural uranium in the form of oxide U3O8 were allowed to pass through a grid of known geometry into an ionization chamber. The electrical impulses there produced were amplified and applied to an electromechanical recording system. By this means the specific alpha activity of natural uranium was observed to be 1486 disintegrations per minute per milligram.
[Kovarik, A.F., and N.I. Adams, Jr., 1955. "Redetermination of the Disintegration Constant of U238" in Physical Review 98, No. 1, p. 46]The half-life can be computed directly from the measured rate of disintegration. It doesn't actually require waiting around for half of the atoms to decay, as some might think. The uncertainty in the measurement is related to the number of decays counted, so scientists compensate for slow-decaying isotopes by measuring the decays among a large number of atoms. (Even one milligram of U238 contains over 2,000,000,000,000,000,000 atoms.)

Interesting. I am still reading, so I will find it eventually.------------------
Thank You
Sonic

Interesting that you picked uranium as an example. The general process has already been explained, but it's interesting that one of the reasons for the popularity of U-Th-Pb dating methods is that the half-life of uranium is knwown to significantly greater precision, IIRC far far less than 1%, than the half-life of any other radioactive substance.Why, you ask?Bombs.Several governments have invested a lot of money in bigger and better bombs, and the more precision in the known half-life of uranium the better they can build the bomb.