Expert: Daniel Mazur - 10/22/2010

Question
Hi Daniel,

I have read about radioactivity and do know that we cannot
predict when a nucleus will decay.

Where does this uncertainty come from? Does it come from
Quantum Mechanics (I doubt it does) or where?

Or is it just that nobody has been able to work this out
from even more fundamental laws(if they are there) of
nuclei, due to the complexity involved?

Or is it simply the case that nobody has any clue where this
uncertainty comes from?

A similar question would be related to the emission of
radiation of atoms or the change in state of polarization of
unpolarized light with time. We have similar kinds of  
"randomness" associated with these phenomenon too. Why?

Thanks,
Shikhin

Answer
Hi Shikhin,

radioactive decay is described by quantum theory. Processes like this one have no better description for two reasons: 1) the uncertainty relations don't allow us, from principle, to know both position and momentum to an arbitrary precision 2) we have no way to measure the momentum of a subatomic particle inside an atom nucleus anyway.

The first point is fairly straightforward, when you realize that you need those quantities to describe motion of any body classically. The second point stems from the fact, that we can measure anything about nuclei only by detecting some kind of radiation that has been emitted from the nucleus or else radiated from an external source and scattered by the nucleus. Both approaches lead to a purely statistical analysis, because neither our detectors nor our radiation sources work with anything near a size-of-nucleus precision. Remember that there are only a handful of experimental methods, which allow us to distinguish atoms, i.e. objects 10000 times larger than nuclei.

Take care,
Daniel