How carbon dating reveals the age of ancient objects

HostIt's pretty wild to think that we can pick up a piece of charcoal from an old campfire or a bit of bone and know almost exactly when that animal was walking around. We just sort of take it for granted that scientists have these tools, but the actual way it works feels a bit like magic. How do we turn a piece of old wood into a date on a calendar?

GuestIt starts way up in the sky, well above where planes fly. These tiny bits of energy from deep space are hitting our air all the time. When they bash into the gas up there, they change it and create a special, heavy kind of carbon. We call it carbon fourteen. It's a bit different from the normal carbon that makes up most of our world, but to a plant, it looks exactly the same as the regular stuff. So plants breathe it in to grow, then we eat the plants, and suddenly, every living thing on earth is a little bit sparkly with this space carbon. We're all walking around with a tiny amount of this unstable stuff inside us as long as we're alive.

HostBut if this special carbon is unstable and always breaking down, why don't we just run out of it while we're still walking around?

GuestBecause you never stop eating and breathing. As long as you're alive, you're constantly bringing in fresh carbon from the world around you. You're part of a big loop. The sky makes it, the plants take it in, and you eat the plants. So the amount inside you stays the same as the amount in the air. You're basically a mirror of the sky. But the second a plant or an animal dies, that loop snaps shut. No more eating, no more breathing. The clock starts ticking because the space carbon you already have starts to disappear, and nothing is coming in to replace it.

HostSo it's like a tiny battery that stops charging the moment something dies?

GuestThat's a great way to put it. It's like an hourglass that gets flipped the second a living thing stops living. This special carbon breaks down at a very steady speed. It's so steady that we can actually use it as a timer. We know that it takes about five thousand seven hundred years for half of that space carbon to go away. We call that a half life. So, if a scientist finds an old wooden bowl and sees that half of the space carbon is gone, they know that tree was cut down about five or six thousand years ago. If three quarters of it are gone, it's even older.

HostWait, if it works for wood and bone, can I use this to find out how old a stone tool is, or maybe a gold coin?

GuestNo, and that's a mistake people make all the time. Stones and metal were never alive. They never breathed in the air or ate plants, so they never had that space carbon in them to begin with. You can only date things that were part of that big life loop, like wood, leather, hair, or seeds. If you find a stone spearhead, you can't date the stone itself. You have to find a bit of bone or charcoal buried right next to it and hope they're from the same time.

HostOkay, that makes sense. So if I find a bone from a huge dinosaur, I can use this to see if it lived sixty million years ago?

GuestActually, you can't. That's another big wall we hit. Because that space carbon disappears so fast, after about fifty thousand years, there's almost none of it left to measure. It would be like trying to use a stopwatch that only runs for an hour to time a race that lasts a week. For dinosaurs, the clock has already run out and hit zero. We have to use different kinds of tests with different minerals for things that old. Carbon dating is really just for the relatively recent past, like the history of humans and the last ice age.

HostSo we're trusting that the amount of space carbon in the air has always been the same, but how do we know the sky wasn't different thousands of years ago?

GuestWe actually found out that the sky does change. The sun goes through phases and the earth's shield against those space rays can get stronger or weaker. This means our clock was a little bit off at first. To fix it, we had to find a way to check our work. We used tree rings. We can find very old trees, some that have been dead for a long time but are still preserved, and count their rings one by one. Since each ring is one year, we have a perfect record of time. We take a tiny bit of wood from a ring we know is exactly four thousand years old and check its carbon. If the carbon says it's only three thousand eight hundred years old, we know the clock was running a bit slow back then, and we adjust our math.

HostIt sounds like we're constantly tuning this giant planetary clock.

GuestWe're, and it's getting harder. The trickiest part now is that by burning fuels like coal and oil, we're putting a lot of very old carbon into the air that has no space carbon left in it at all, which is starting to make new things today look like they're already thousands of years old.

HostThat piece of charcoal from an old fire isn't just burnt wood, it's a clock that started ticking the moment the flame went out.