Most radioactive isotopes of the lighter elements decay in minutes or less. But one particular isotope of carbon takes 6000 years to decay, and that fact has revolutionized archaeology. But why it does that has long been a complete mystery.
This isn't really a small problem - the isotpe in question, carbon-14, takes roughly three billion times longer than its comparable isotopes to decay. That fact has baffled physicists for decades, but that ignorance hasn't stopped researchers from using carbon-14 to estimate the ages of various artifacts with tremendous precision, transforming forever our understanding of history and archaeology.
Now researchers Pieter Maris and James Vary, both from Iowa State University, have figured out what makes carbon-14 special, and why it's taken so long to figure this out. It all comes down to the three-way interactions between particles in the nucleus. While it's fairly easy to calculate how two particles - since this is the atomic nucleus, we could be talking about either neutrons or protons - might interact, but when you add a third into the mix the whole thing becomes fiendishly complex to simulate.
Basically, under normal circumstances, the forces that govern the normal decay of radioactive isotopes is always pairwise, meaning it involves interactions between only two particles. But the structure of the carbon-14 nucleus means that those two-particle interactions constantly turn into three-particle interactions, and that cancels out the effects that, in any other similar isotope, would cause it to decay in about a minute or so.
All it took to figure this out was a billion-by-billion matrix, a computer capable of handling 30 trillion different elements, and about three months of processing time. But, as Vary explains, now that they've figured out this answer, an even more complicated question arises:
"The whole story doesn't come together until you include the three-particle forces. The elusive three-nucleon forces contribute in a major way to this fact of life that carbon-14 lives so long...Everybody now knows about these three-nucleon forces. But what about four-nucleon forces? This does open the door for more study."