Physics 2000 Science Trek Isotopes & Radioactivity

Meaning of Halflife

I'd like to illustrate what this really means. If living creatures had halflives the way radioactive atoms do, the world would be a very different place.

What do you mean?

Suppose there's an alien species with a halflife of, say, 70 years. You randomly pick out 16 baby aliens and track them to see how long they live. After 70 years, of course, 8 of them will still be alive.


That doesn't sound so weird. If you tracked a group of human babies, you might get the same results.

True...but remember that the halflife is always the same, regardless of how old the aliens are. After another 70 years, 4 of those 8 will still be living, now 140 years old. And half of those--2 aliens--will survive to the age of 210. Another 70 years go by, and there's one alien left, age 280.


That is kind of strange.

It gets stranger. Let's say that your great-great-great-great-grandchildren, who have been faithfully continuing your study, now decide they want to examine the lives of some aliens of the younger generation. In addition to our 280-year-old friend--let's call him Methuselah--they also begin tracking 15 new babies. Now there are 16 aliens in the study again, and, as before, 8 of them will be alive after 70 years.


Methuselah isn't going to make it, though.

Methuselah has just as much chance of surviving the next 70 years as any one of the 15 babies. In fact, he has just as good a chance as any one of them of living another 280 years. The probability of decay has nothing to do with the history of any individual atom, or alien; otherwise the halflife wouldn't be constant. Radioactive atoms just don't grow old the way we do.



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