NO, an assumed rate, calculated over a long time. We use radio active decay to try simulate randomness, it's not constant, it is extremely difficult to predict. Just like the growth of human population, you don't know who is going to get pregnant next, but observed over time it seems constant.
Actually you can derive the decay rate from quantum mechanics and the probability of an alpha particle (or beta particle) to leave a nucleus.
Yes, you cannot predict which one or which atom, just like you can't predict coin tosses, but you can predict overall trends.
We can also look at evidence of the decay rate remaining constant, both in the stars (decay curves observed in nova explosions match curves on earth for the elements that are emitting light with the same wavelengths as they do here on earth) AND in uranium halos (alpha decay results in alpha particles having specific energies unique to the isotope that decays and that is related to the decay rate, and the alpha particles form rings around inclusions in crystal that show long age stability - hundreds of millions of years - in decay rates).
Enjoy.
Edited by RAZD, : .
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