Life exists in the universe only because the carbon atom possesses certain exceptional properties. -James Jeans
Here on Earth, every living thing is based around four fundamental, elemental building blocks of life: hydrogen, oxygen, nitrogen and, perhaps most importantly, carbon.
From diamonds to nanotubes to DNA, carbon is indispensable for constructing practically all of the most intricate structures we know of. Most of the carbon in our world comes from long-dead stars, in the form of Carbon-12: carbon atoms containing six neutrons in their nucleus. About 1.1% of all carbon is Carbon-13, with one extra neutron. But there is another form of carbon that, while not at all abundant, is definitely worth talking about.
Carbon-14, or carbon atoms with eight neutrons in their nuclei, is unstable, and is so rare that only one-in-a-trillion carbon atoms are carbon-14. With a half-life of just over 5,000 years, any Carbon-14 atoms that were created in stars, billions of years ago, have long since decayed away into nitrogen atoms.
But there are small, but not quite negligible amounts of carbon-14 present in all the organic life that we know, including in our own bodies. The way it gets here is, literally, cosmic.
From across the galaxy and across the Universe, from stars (including our Sun), pulsars, black holes and more, space is flooded with high-energy particles known as cosmic rays. Most frequently, cosmic rays are protons, but a handful are heavier ions and a few are even humble electrons. But once they interact with the atmosphere, look out!
They produce showers of subatomic particles of many different types, including for our purposes the all important neutron. The reason neutrons are so important is because our atmosphere is 78% nitrogen, which you may remember as the thing that carbon-14 decays into.
Well, if carbon-14 can decay into nitrogen-14 and other stuff, then we can create carbon-14 by combining nitrogen-14 with the proper stuff. In this case, that happens to be a neutron, which allows us to do this:
Once you create carbon-14, it behaves just like any other atom of carbon, readily forming CO2 (a.k.a., carbon dioxide) and mixing throughout the atmosphere and oceans, easily making its way into living organisms into a well-understood equilibrium. As far as we can tell, the levels of carbon-14 throughout the world have remained roughly constant throughout the past few millenia, so that when an organism dies and the carbon-14 decays, we can measure how long ago it became deceased by measuring the ratio of carbon-14 to its normal carbon-12.
The only major fluctuation we know of occurred when we began detonating nuclear weapons in the open air, back in the mid-20th Century. If you ever wondered why nuclear tests are now performed underground, this is why.
So you can imagine it came as a shock when, just yesterday, nature released a paper showing a big, short-lived spike in carbon-14 levels way back in the 8th Century! By looking at the tree rings of ancient Japanese Cedars, you can see a rise in the concentration of carbon-14 that starts in the 770s, peaks in the 780s and then falls off.
What does this correspond to, in terms of creating this carbon-14? Well, there were no nearby supernovae that happened at that time, so thats out. Theres no evidence of an unusually large solar flare or any other bizarre solar activity, so that cant be the culprit, either. What this appears to correspond to, at least at this preliminary stage, is an increase in cosmic rays during the year 774-775.
Now, since weve been watching the skies, weve never seen an increase in levels like this, but its only recently that our sophistication in measuring the carbon-14 levels in old tree rings like this has allowed us to test this.
The follow-up? Looks like were going to have to unearth more old trees that can be radiocarbon-dated back to these years, and see whether they have elevated levels of carbon-14 in them. If not, then its conceivable that these trees are just flukes, or that there was a mistake done in the analysis. But that doesnt seem likely; there is data from North American and European trees that this is consistent with! If this is confirmed, then there was very likely an extremely large increase in cosmic radiation over a very short period of time, the likes of which weve never seen or recorded, until now.
What couldve caused an influx of cosmic rays like this? While there are many possibilities, I wouldnt count out a relatively nearby, flaring black hole!
The Universe may never cease to surprise us, but we may never cease, as long as we exist, to figure out exactly why it does the things that it does. How remarkable is this!
Thanks to Sarah Kavassilis for suggesting this story; its a great one!