That's a pretty cool discovery in its own right.

Wow! That's in https://en.wikipedia.org/wiki/Xenon now.

Xe-124, half-life 1.1 * 10^22 years. That's crazy.

Makes me wonder if all atoms with 2+ nucleus elements (protons and neutrons) are radioactive but the halflife is so far out as to make something we'll never detect.

Probably not, they have a pretty good handle on why atoms decay, to the point they can predict some to be radioactive before it's actually observed to be (like Bismuth-209).

Also even if something is REALLY REALLY long lasting, you can still check for the halflife by observing enough of it, they've been able to rule out proton halflives under 10^34 years (the universe is on the order of 10^10 years old) but by observing enough protons (like say 50,000 tons of water) you would expect at least some to decay.

> they have a pretty good handle on why atoms decay

Oh, they actually don't. Radioactive decay, AFAIK, is still an open physics mystery. We know it happens, we don't know why, what causes it, or if there even is a cause. We can predict factors that make it more likely.

> to the point they can predict some to be radioactive before it's actually observed to be (like Bismuth-209).

Right, but Xenon 124 wasn't predicted to be radioactive which is what makes it fascinating. It shows holes in what we can predict as being radioactive which is what makes me wonder about everything being radioactive but the timetable is too far out.

Where is the mystery? Any system can spontaneously transform into a new state with a probability greater than zero unless some conservation law prevents it. In a sense it's just quantum tunneling.

The mystery is the details, not in that it happens at all. Feel free to submit a paper if you have all the answers.

Can you be more specific? I believe quantum mechanics explains all of radioactive decay. Unless the GP meant that QM is mysterious, I don't understand the problem.

I mean questions like could you predict the decay of Xe-124 correctly? AFAIK theory overestimates the rate

Quantum Chromodynamics, path integrals, and what other mechanisms you need for deriving the half life are extremely complicated, especially for that many particles, so of course it's hopeless without approximations, which can be wrong, or are even wrong by definition. But the fact that it's prohibitively hard to solve the equations for such complicated systems doesn't indicate a gap in our understanding. You don't even have to go to xenon, this is already the case for tritium.

I don't actually think Xenon-124 being radioactive was a surprise. All the publicity related to observing the decay for the first time is phrased around 'hey we observed something that's very rare' not 'hey this thing happened we didn't expect to happen'.

Which seems to point even more towards, scientists have a pretty good handle on which ones are radioactive.

There's just no way deuterium is radioactive, unless hydrogen is radioactive too.

Some theories predict that protons decay, but the half life is like 1E31 or 1E35 years (compare to the Xe124 that has a half life of only 1E24 years). All experiments so far to measure the proton decay have failed, anyway. https://en.wikipedia.org/wiki/Proton_decay

I don't remember anything specific about deuterium, and the method that Xe124 uses is not available, and I can't imagine a razonable alternative method, so my guess is that deuterium is as stable as protons.

Too late to edit: Some conclusion, wrong number. The correct half life of Xe124 is 1E22, instead of 1E24.

Those double beta decays are also interesting because they can probe whether the neutrino is a Majorana particle.