YOU are made of carbon. So are your pets and all your houseplants. Every living thing on Earth owes its existence to carbon atoms’ ability to join up with other elements in a bewildering number of ways and form complex molecules. But the abundance of this element in our universe depends on a seemingly miraculous coincidence – an excited state of the carbon nucleus that our best models say shouldn’t exist, but clearly does.
The nature of this weird form of carbon has baffled us for more than 60 years, much to the distress of nuclear physicists. Its existence is so essential in the sequence of reactions making life possible that our failure to explain it is deeply embarrassing. “We need this state to exist for us to be here and yet it is extremely unusual in nuclear physics terms,” says David Jenkins at the University of York, UK. “Cracking this problem has become a matter of pride.” And yet the more we learn, the more confusing things seem to become.
The story starts 13.8 billion years ago, when everything erupted out of nothing – or at least the ingredients for everything did. Actually, the only elements forged in the big bang were the very lightest: hydrogen, helium and a smattering of others. All the heavy stuff, starting with carbon, was forged later inside stars.
The first step in carbon manufacture is to fuse nuclei of the lightest element, hydrogen, to make the second-lightest, helium. The next step ought to be for two helium-4 nuclei – each containing two protons and two neutrons –