Radiometric Popcorn II: Isochrons

After Touchstone's magnificent explanation of radiometric dating, we needed a follow-up.  

A number of people interested in Young Earth Creationism began to point out what they take to be flaws in the technique.  One of these is that there is no "control" on the process.  That is, they assumed that if something were interfering with the stable decay of an isotope, we'd never be able to tell.

So I added this epilogue to his lesson, about how science can and does control for such uncertainties.

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Imagine a second bag of popcorn, Isochron Brand. It pops at a half-life of 7 days (meaning half the kernels turn into popcorn every seven days).  The kernels are obviously a slightly different "type" than the first bag TS described. By measuring BOTH bags of kernels, we get a heightened accuracy.  For example, if (let's say) we KNOW that TS's Radiometric Popcorn is a little apt to change its popping-patterns if somebody's been using CosmicRadiation Brand Cooking Spray in the kitchen, and we have a pretty good idea that it changes Radiometric's half-life by .3%, then we can't be certain how long Radiometric has been popping... we aren't sure if our "readings" are accurate or whether somebody used CosmicRadiation in the kitchen.  BUT if we ALSO have Isochron popping, we have something to check it against. Thus, we find that half of Isochron has popped, and (I've forgotten TS's math) ALMOST enough of Radiometric has popped to tell us it's been one week, + or - .3%, we can do the math and get an extremely accurate reading for the time the two bags have been popping. Isochronic dating therefore measures the decay / transformation of some isotope that is different from the "main" isotope we measure, but which produces the same "daughter" element. Not popcorn in most cases.