Examples
“(Presumably, the need for a license is a matter of degree, given that our own bodies contain radioactive potassium-40, and given that the walls of most buildings contain the radioactive isotopes from the natural uranium-238 and thorium-232 decay chains).”
“That 4 million tonnes of coal contains 5.2 tonnes of uranium, 12.8 tonnes of radioactive thorium, as well as 0.22 tonnes of radioactive potassium-40!”
“Presumably the individual particles of silt or sand that go to make sedimentary rocks contain potassium-40 and other radioactive isotopes, and therefore could be said to contain radioactive clocks; but unfortunately these clocks are no use to us because they are not properly zeroed, or are zeroed at different times from each other.”
“If you start with some quantity of potassium-40, after 1.26 billion years half of the potassium-40 will have decayed to argon-40.”
“Igneous rocks typically contain many different radioactive isotopes, not just potassium-40.”
“So geologists, by measuring the ratio between potassium-40 and argon-40 in a piece of igneous rock that they pick up today, can tell how long ago the rock first crystallized out of its molten state.”
“After another 1.26 billion years, half of the remaining potassium-40 will have been converted to argon-40, and so on.”
“When the clock was zeroed, the ratio was 100 per cent in favour of potassium-40.”
“But, for the reason I have just explained, this quantity is meaningful only if expressed as the ratio of potassium-40 to argon-40.”
“If, before that, the half-life of potassium-40 was radically different, only a few centuries, say, rather than 1.26 billion years?”
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