RADIOCARBON dating relies on carbon-14 to decode an object’s age, but the isotope has steadfastly refused to divulge the key to its own unusual longevity. The answer, it seems, lies in the bizarre rules of quantum physics.

Carbon-14 decays with a half-life of 5730 years, so it is often used to date objects up to about 50,000 years old (anything older would have negligible amounts of the stuff).

But most other atoms that decay in the same way – by converting one of their neutrons into a proton – disappear in less than a day. So what’s different about carbon-14?

The nucleus of the carbon-14 isotope has six protons and eight neutrons. When it decays, one of the neutrons turns into a proton, and also releases an electron and a neutrino. The result is a nitrogen-14 nucleus with seven protons and seven neutrons.


But in the weird world of quantum mechanics, an atomic nucleus is not a single object. Instead, multiple ghost-like states of the nucleus with different amounts of angular momentum coexist at the same time. It has long been known that in carbon-14, two of these doppelgänger states are constantly trying to decay into nitrogen-14.

As in other quantum processes, the results of these two potential decays combine like waves, which can either reinforce or cancel each other out. The bigger the resulting wave, the more likely the decay.

Now Pieter Maris of Iowa State University in Ames and colleagues have performed the most detailed calculations of the process yet, and found that the resulting waves happen to cancel out to almost nothing (Physical Review Letters, DOI: 10.1103/PhysRevLett.106.202502).

The near-perfect cancellation means carbon-14 has a low probability of decay, giving it its unusual lifetime. “One can now say confidently that the problem is solved,” says Jeremy Holt of the Technical University Munich, Germany.

A similar effect might explain the unexpectedly long life of beryllium-10, says team member James Vary, also of Iowa State University.