I must admit my interest is whetted when an article on physics starts out,
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.
This is from “Life’s subatomic secret: How we’re cracking the Hoyle state,” by (NewScientist, 22 October 2016, paywall), and I’d never heard of this particular mystery before. It’s fascinating stuff – but, being high energy physics, is way beyond me. I can sort of follow this:
Source: openclipart.org
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 – to fuse to make beryllium-8. This would then grab another helium to make carbon-12. Except there is a snag. Beryllium-8 is highly unstable, meaning it decays in the blink of an eye – too quickly to produce the amount of carbon that exists in the universe.
The other possibility is that three helium-4 nuclei come together simultaneously inside bloated, dying stars known as red giants, where all the hydrogen has burned off to leave an extremely dense and hot core of helium. But this process is so rare that even over the aeons since the big bang, it couldn’t have produced enough carbon.
But after that I get lost (why is beryllium-8 unstable?1 for example) , except to understand they’re using supercomputers to computationally model the problem.
I think there’s two reasons an article like this fascinates me. First, physicists are some of the smartest folks in the world, so it’s good to see them bemused.
Second, the activity of attacking a puzzling mystery often leads to all sorts of interesting knowledge. I can’t wait to see if that’s true of this mystery.
1The Isotopes of beryllium page in Wikipedia is actually fascinating, not for the why, but the what. For example,
The rate at which the short-lived 7Be is transferred from the air to the ground is controlled in part by the weather. 7Be decay in the sun is one of the sources of solar neutrinos, and the first type ever detected using the Homestake experiment. Presence of 7Be in sediments is often used to establish that they are fresh, i.e. less than about 3–4 months in age, or about two half-lives of 7Be.
