Is radioactive decay reversible in time?

[Izzhov]

I ask this because I've heard repeatedly that all the equations in current physics models are reversible in time, save the second law of thermodynamics. But does this mean the process of radioactive decay is also reversible in time?

 

[Avodyne]

Yes, radioactive decay is reversible in time. If you imagine filming the decay process, and then running the movie backwards, what you would see is a perfectly allowed process: the decay products come together and form the original nucleus.

In practice, however, it's very difficult to set up the converging decay products as an initial state, which is why the decay process is much more common than the inverse decay process.

But in, for example, computations of nucleosynthesis in the early universe, the inverse-decay processes must be included to get the right answer.

 

[Nugatory]

I ask this because I've heard repeatedly that all the equations in current physics models are reversible in time, save the second law of thermodynamics. But does this mean the process of radioactive decay is also reversible in time?

Any single radioactive decay is in principle reversible, but the reverse reaction is far less probable (easy to imagine an alpha particle flying out from a nucleus, much harder to imagine one approaching a nucleus at exactly the right speed and direction to "stick"). Thus, within a sample of radioactive material the rate of decay will enormously exceed the rate of reversed decays, and the overall decay of the sample is as irreversible as the transfer of heat from a hotter body to a cooler one.