Daniel and Jorge Explain the UniverseDaniel and Kelly talk about how cosmic rays and carbon decay let us put dates on ancient deaths!
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Hey Daniel, it's been a long time. Did you have a nice holiday with your family?
We did, Thank you very much. We did some of our favorite holiday traditions.
Ooh, like you cooked specific meals that you only cook once a year or something like that.
Well, you know, one of our favorite holiday traditions is a little bit unusual. It's trash Day.
Like when you roll your bins to the street because I call that Tuesday. Does that count as a holiday? Peal?
No?
No, No. Trash Day is a much bigger deal. It's when you take all the stuff out of your closets, decide what you actually need, and donate or throw away the rest of it.
Oh, I bet you find some real treasures.
You know, every year we try to go one layer deeper into the archaeology of our closets.
Do you think you'll ever reach the back.
I'm not even convinced a back exists anymore. It might just be like an infinite stack of useless junk.
Well, maybe you'll get lucky and it will have compressed for long enough that it will become diamonds or something.
Or there'll be a black hole in there that'll just eat you.
That's not lucky, that's less lucky. That's still interesting.
Hi. I'm Daniel. I'm a professor at UC Irvine and a particle physicist, and I love throwing stuff away.
I'm Kelly Waiter Smith. I'm a parasitologist with Rice University, and I love throwing stuff away. But my family doesn't know.
Do you guys have arguments about how many nostalgic elements to keep.
Yes, that might be the thing we argue about the most often.
Actually, and are you ever right where they really need something? And you're like, Hi, you shouldn't have thrown away that melon baller. I knew we would come in handy very rarely.
And also, you know, with things like Amazon, it could show up at your house in less than a day. So even if I'm wrong, you can just get a new melon baller.
That's true. Amazon has undermined your argument.
That's right, that's right. You can always just choose this spoom right.
Well, I feel like you might appreciate the melon baller, but you'll appreciate the lack of the melon baller even more. You know, nothing is better than negative space if you've succeeded in like cleaning out a closet or cleaning out some corner of your room, and you can just appreciate the emptiness.
I agree. I like not tripping over things in the middle of the night because I get up a lot in the middle of the night. I value that well.
Welcome to the podcast Daniel and Jorge explain the Universe, in which we explore the meaning of nothingness and the very fabric of space itself. We take the whole universe apart from the junk in your closet it's to the junk between your toes, to the junk that fills the whole cosmos. We want to understand it. We want to take it apart. We want to understand what it means when it's there and it's not there, and how it all got to be the way that it is.
So what are we working on today? Then?
Today we are thinking about the universe as a sort of mystery. I like to think of physicists as like detectives trying to crack the murder mystery of the universe, like what happened here? We are showing up at the scene and wondering, like, how did it all get to be the way that it is?
And today you get to be the forensic scientist that dates the dead body.
That's always a good character, that's right, because when you show up at the scene, you want to understand what happened. You want to put things in order. You want to say this happened, then that happened, then the other happened, like the Solar System formed and then the Earth cooled, and then life evolved on the planet, or the Big Bang happened, and then heally informed and then much much later stars happened. The whole point of doing science is to put the universe in some sort of order. That's what a store is. And I'm a firm believer in the philosophy that human thinking is mostly about storytelling, and so in order to do that, you have to figure out like how old is stuff, what came before what other stuff? You need to have dates and clocks and times if you're going to unravel the murder mystery of the universe.
And there's a lot of different ways that we've done that throughout the years, and some of them work better for particular kinds of mysteries than others. So what sort of mysteries are we solving today?
Well, of course we are interested in the mysteries of deep time. What happened in the first few moments of the universe. How long has the Earth been around, and what's been going on? How did everything come together? One of my favorite things about dating stuff is discovering those surprises, those realizations like wow, oh my gosh, the Earth is so much older than anybody ever imagined, or that the universe is shockingly old, or you know, for some people the universe might be shockingly young. Maybe some people thought the universe had been around forever or millions of years. So anytime you get one of these dates, it's this glorious moment where you get to like pin down the universe and say, aha, now I know something about you, and that like eliminates a bunch of possibilities. When you're a reader digesting a mystery novel, you have like lots of different options in your head. Maybe it was this person, maybe it was that person, maybe it was this third person. And as you get clues, you get to eliminate possibilities, which finally reveals to you the truth, right, what actually happened in the universe. The story is revealed by elimination, and so those clues are absolutely essential. But you're right tell you that there's lots of different ways to date things in the universe, and today we're going to be focusing on something a little bit more recent how we can use physics to understand fairly recent history here on Earth.
Ooh, that sounds a lot less disgusting than using fly maggots.
Fly maggots, what are you talking about?
That's for dating things like you know, dead bodies that you find, like what stage are the fly eggs that have been laid on the body, So that that's you know, we're working with like days to weeks there, So are we working on that scale or are we working like past the rotting dipter and phase?
Definitely past the rotting corpse phase. But I think you put your finger on a really interesting point, which is that there are all these clocks build into nature, right, like maggots take a certain amount of time to digest a corpse, or rocks take a certain amount of time to cool, and these clocks are things we can take advantage of in order to figure out how old things are, how long they've been digesting, or how long they've been cooling, or how long they've been radioactively decaying. We don't get to like design these things go back in the past and like leave clocks in place to tell us how long things took. Instead, we have to just take advantage of the clocks that we find. And some of these clocks, as you say, are really short lived, like they tell us about things that happened just over weeks. Some of them can tell us about things that happened for billions of years, and other ones can tell us about things that happened in the last ten fifty thousand years, which is a super interesting time for the story of human civilization, how we got to be who we are, And of course, it turns out physics plays a big role.
I love learning about this topic because it's so exciting to me how we figure out what tools are the best ones to use to explore the past. And I specifically like hearing about sort of physics and chemistry sorts of tools because my sense is that they're a little bit less susceptible to uncertainty. So, for example, you know, temperature can impact how fast those flies are developing. But maybe today you're going to explain to me that chemistry and physics aren't that straightforward either.
It turns out to be a twisty and complicated topic, and of course sewage is involved.
Yay.
My wife is very happy to hear about that. And so today on the podcast, we'll be answering the question how does radiocarbon dating work?
Well, should we start by seeing what the listeners think?
Yes, absolutely, I was curious to hear what people people had to say about this. Radiocarbon dating is something I think a lot of people have heard about. It's on television all the time, and we date this and we date that. But I was wondering if people like really understood the mechanism of it, like what the physics is of these clocks? Why is there a clock? Why does it start when you die? And how do we read it out? So thanks very much to everyone who participates in this segment of the podcast, sharing your thoughts without a chance to prepare. If you would like to participate in the future, please don't be shy. I'll be gentle. It's fun. Just write to me two questions at Danielandhorge dot com. So before you hear these answers, think to yourself, do you know how radiocarbon dating works? Here's what people had to say.
If we find some animal spoon, let's say some dinosaurs poon, but it inside the earth, and then we calculate the amount of carbon s forteen specifically present in it. Through those calculations, we can easily calculate the age of that skeleton.
I've heard it get called carbon fourteen dating before, so that makes me think that it has something to do with the isotope and the decay of that carbon atom.
You can detect the age of organic objects by looking at the ratio of carbon isotopes.
I feel like carbon dating is when like two particles of carbon are like connected. I don't know, I don't really know anything about that.
I felt like the answers to these fell into two camps, people who sounded like they really knew what they were talking about and people who had maybe not even heard of carbon dating. And to me, that's sort of interesting that it's like an either or scenario, Like maybe either you're into the kinds of TV shows that this is shown in, or maybe you're not watching NCIS Or what was that shit bones where the archaeologist was doing carbon fourteen dating? Yeah, what do you think?
I was pretty tickled by the answer that suggested that, like carbon and fourteen adams are like hooking up, you know, they're like on Tinder, finding each other and making magic.
That was a clever off the cuff response.
There.
Yeah, I liked it.
Our listeners are funny, funny people, absolutely, and I had a lot of fun digging into the details of this. This is not something I use in my research and so not something i'd really ever wrapped my mind around. And one of my favorite things about this podcast is that I get an excuse to go off and learn about something I'd always wanted to understand but never really given myself the time to dig into. So thanks for the listener who suggested this topic.
How can listeners suggest topics to you? Just shoot you an email.
Absolutely, if there's something you'd like to understand, please just write to me too. Questions at Danielandhorhe dot com. We put it on our list and we get to every single one eventually, And.
I'm with you. There are a few things I enjoy more than an excuse to study a topic that's interesting that isn't part of my research program.
I know, And it's weird that sometimes you feel like you need an excuse. You know, you don't just like get slice off a few hours of your day and go read about how something works. For some reason, I feel like I have to give myself the opportunity and having to do a podcast on it to explain it of the folks. Is a good excuse to go and actually learn how something works.
That is one of the weird things about academia. You don't feel like you have a lot of time to just sit and think. But I too, have created a bunch of tricks in my life, you know, like, oh, I'm gonna write a book about ten emerging technologies, so I have to read about all of them. So yeah, but at least we have these tricks, all right. So what is carbon fourteen?
Right? So radio carbon dating or carbon fourteen dating uses this weird form of carbon called carbon fourteen. And when we say carbon fourteen, that number fourteen tells us how many nucleons there are in the atom. So remember that an atom has electrons around it, and in the nucleus there are protons and neutrons. Now, usually carbon has six protons and six neutrons, and so that's carbon twelve. That's like the vanilla kind of carbon, the kind that makes up most of you, and if it's in carbon dioxide and everywhere in the atmosphere, So that's carbon twelve, which is stable. Carbon fourteen is a weird version of carbon it's still carbon, so there are six protons inside, but there's two extra neutrons, so it's like a heavier nucleus. So that's what carbon fourteen is.
And do those neutrons want to stay there or do they want to escape?
So, you know, the stability of the nucleus is a really interesting and complicated question. Some collections of protons and neutrons are stable. You can like build carbon twelve and have it sit in space and come back a billion years later and it'll still be carbon twelve. Other constructions are not stable, Like you add two more neutrons and all of a sudden you have carbon fourteen. It's not stable. It will fall apart after a few thousand years. And this all comes down to how those protons and neutrons like to put themselves together inside the nucleus. Remember that the nucleus is only protons and neutrons, and the protons are positively charged. The neutrons are neutral, of course, and so initially you might wonder, well, how does the nucleus stay together anyway? It's all positive charges, why don't they just blow each other apart? And there is that desire, right, They definitely are pushing against each other. But on the other hand, they're held together by a much stronger force. The strong nuclear force sticks the protons and the neutrons all together. So it's a delicate balance in some cases between the strong force sticking it together and the electromagnetic force trying to push it apart.
You have one prouton and one neutron held together by the strong nuclear force. That's stable. And then when you've got extra neutrons floating around so that they don't pair evenly, is that what makes it unstable?
Yeah, that's right. You can actually think about the construction of the nucleus in a similar way to how we think about electron orbitals. You remember learning in like high school chemistry that electrons aren't all just buzzing around the nucleus the same way. It's like one in the lowest energy level and another one in the next energy level, and they sort of fill up and they get to like more and more elaborate orbitals as they get to higher and higher energy. Well, the nucleus is constructed sort of in the same way. The picture you often have of the nucleus is just like a scoop full of protons and neutrons, but that's not really very accurate. It's more like there are shells. You're like an inner core where protons and neutrons have clicked together, and then you can surround that with another layer of like protons and neutrons, and the most stable atoms are the ones where those layers are filled. You've like clicked in all the protons and neutrons. It's sort of like making a roman arch. When you have all the bricks together, they click together to make a very stable structure. If you're just missing one, then it can be very unstable.
So I didn't do great in high school chemistry. Is the reason that that sounds new to me because we've learned this in the twenty plus years since I've been in high school. Or is that something we usually just sort of, you know, skip over in high school or what's the story there? How long have we known that?
Well? I think we're gonna have to interview your high school chemistry teacher to really find the answer to that. Let's dig deep into your past. In fact, we have them here on the podcast Surprise Surprise Your Life.
I don't think they like to be very much.
Let's move on. No, my high school chemistry teacher would not be happy to hear from me either. I think it's a combination of both things. We have understood what's going on in the nucleus much much better in the last few decades because we've been shooting stuff at it and breaking it up and seeing what's inside of it, and it's a hard project. We've also been building heavier and heavier nuclei to understand, like what are the limits of stability? How many protons and neutrons can you stick together and have something which will stick around for billions of years. We have a whole podcast episode about what are the heaviest stable atoms and we think, for example, there might be an island of stability where you get like hundreds of protons and neutrons stuck together to make new super heavy stable elements nobody's ever seen before. So it's a complex field of study, and it's probably not taught in high school chemistry because it's evolving, and also because it's messy, and I don't think that high school sophomores aren't necessarily ready for it.
I'm not sure that I was ready for the electron stuff either, but all right, so these things aren't stable, and every once in a while the neutrons get kicked out. How long are we talking before the neutrons get get kicked out? Does it take like weeks, years, millennia? What are we looking at?
So every atom is different. Uranium, for example, has a half life which is like the age of the Earth, but carbon fourteen only lasts about fifty seven hundred years. Now, remember when we say that, we don't mean that there's like a little clock inside every carbon and when the time expires, it dies. What we mean is that that's how long it takes, like a population of carbon atoms for half of them to decay. Each individual one might take longer or less time, because fundamentally it's a quantum mechanical effect. There's a randomness to when these things decay.
So like thousand, seven hundred years, that's much longer than any of us live. And so certainly there was no scientist who was sitting around, you know, watching these things for five thousand, seven hundred years. How do we figure something like that out?
That's a great question. And you know, if it did take fifty seven hundred years for every carbon atom to decay, then you couldn't measure that half life without waiting for the first one to decay would literally take thousands of years. After one thousand years or three thousand years, you would have seen none decay, and you still wouldn't know is the half life five thousand years or five billion years right?
And you wouldn't get tenure.
You need a very understanding department chair area that's right. And so it's the statistical nature of that, the randomness that really helps you, because even though the half life is five thousand, seven hundred years, after one hundred years, there is a chance that a few of them will have decayed. So all you need is like a lot of carbon atoms, millions and billions and zillions. Unfortunately, there are a lot of them around, and you just watch for a few years or even a few months, and a few of them will decay, and from that you can extrapolate, right as soon as you start to see some of them decay, you know how likely it is for any of them to decay, and from that you can calculate how long it would take for half of them to decay.
As it decay, does it go from carbon fourteen, the carbon thirteen, the carbon twelve, or does it go right from fourteen to twelve does it lose two neutrons all in one step.
So actually carbon fourteen doesn't decay to carbon twelve. What it does when it decays is that it goes to nitrogen. Like normal nitrogen is nitrogen fourteen, but it has seven protons. So you go from something which has eight neutrons and six protons that's carbon fourteen, into something that has seven neutrons and seven protons. So you take one of the neutrons and you do a beta decay into a proton, and now carbon fourteen has flipped into nitrogen fourteen. And to me, this is really cool because this is the quantum mechanics of it. Like carbon fourteen is not totally stable, but it's also not totally unstable. It will stick around for a long long time. It's like a particle trapped in a little potential well, and nitrogen fourteen is other state it can flip intwo is also a little potential well. And like classically, if you had a particle trapped in a little well, it could never get out. So what happens is that this atom switches from one state to another state even though there's like a potential barrier in between it. It does this by quantum tunneling. It's like an electron stuck in one little hole that ends up in another little hole, even though it doesn't have the energy to go over the barrier. So carbon fourteen turns into nitrogen fourteen through this random quantum mechanical tunneling process that lets us switch from one state to a lower energy state without having enough energy to actually go over the barrier in between them.
That's another thing I'm amazed we ever managed to figure out.
I know, so like quantum mechanics is happening all over the place in the atmosphere right in front of us.
That's crazy. So where do we get carbon fourteen in the first place?
Yeah, carbon fourteen only sticks around for a few thousand years. So you might wonder, like the Earth is billions of years old, why do we have any carbon fourteen? And the only reason we do is that we have a source of it, right this new carbon fourteen being made all the time, and of course it comes from space. The Earth is not just sitting out in empty space. Space is very far from empty. We're being inundated with high energy particles all the time from the Sun, from the Black hole, accretion, disks from other galaxies. All sorts of stuff are smashing into the Earth all the time. And we call these things cosmic rays, and when they hit the upper atmosphere, they cause all sorts of reactions. They smash into stuff and they change it. What happens is a cosmic ray will smash into like a proton that's in nitrogen, which is seven P seven N, and convert it into a neutron. So you have nitrogen fourteen, smash bang with a proton, and you end up with carbon fourteen because one of those protons has gotten converted into a neutron.
WHOA Okay, And that's all happening in the atmosphere and not happening much on Earth. Is that right?
That's right, it's happening mostly in the upper atmosphere. Cosmic grays can't just fly through the atmosphere. They interact with particles. It's sort of like running into a crack, right, You're going to bang into all the other particles because cosmic areas, they're just particles. They sound spooky and weird, but they're just particles. So these things are created in the upper atmosphere and then they sort of drift down to the rest of the planet.
Okay, all right, so now we know how we get carbon fourteen, and we should take a break, and when we come back we'll talk about how it goes from the atmosphere to being incorporated into living things.
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And we're back. Okay. So, carbon fourteen, made by cosmic rays in the atmosphere, falls down towards the living stuff that lives below, and then what happens, Daniel.
So carbon fourteen interacts with oxygen in the atmosphere to make carbon dioxide, and so now you have these special molecules of carbon dioxide. Most of the carbon dioxide in the atmosphere has carbon twelve in it the normal stuff, but about one in a trillion has a carbon fourteen atom, So it's carbon fourteen plus two normal oxygens. And this is just floating out there around in the atmosphere. And so when plants, for example, do photosynthesis and they breathe in carbon dioxide, most of the time they're breathing in normal, the nilla carbon dioxide, but sometimes one in a trillion they get the extra spicy version of carbon dioxide that has carbon fourteen in it, so they take it in. And then when you eat your impossible burger, which is out of plants, you're eating that carbon fourteine that got incorporated into the plant.
So do plants have more carbon fourteen than animals because they're sucking so much of it out of the atmosphere or is that not how it works?
Carbon fourteen definitely flows through the sort of biosphere, and it's richest at the source, Like if you go into the upper atmosphere, that's the highest fraction of carbon fourteen. As you get further and further away from the source, you get less and less carbon fourteen because it starts to decay. Now it decays all really really slowly. So like most plants and most animals on the surface have about as much carbon fourteen in them as exists in the upper atmosphere. But like the bottom of the ocean, the deep ocean doesn't interact with the atmosphere as often, and so the rate of carbon fourteen down there is much less because it takes longer to get down there, and by the time it does, some of it has decayed. And so like deep ocean animals tend to have less carbon fourteen than like birds that live in the high atmosphere or you know, cows that live on the surface. Actually map the carbon fourteen fraction over the biosphere and see how that flows.
Okay, And so in general, we're also not really interested in like you know, there are one hundred units of carbon fourteen in a tree. We're interested in the relative amounts of carbon twelve compared to carbon fourteen. Is that right? So it's about you know, relative amounts as opposed to absolute quantities.
Yeah, exactly. And this is the really fascinating bit about carbon fourteen. Right. First of all, it's a natural clock. It's this thing that happens in the universe, you create carbon fourteen, you have about fifty seven hundred years until it decays. The fascinating thing about carbon fourteen is that we use it to date when something dies, right, we can tell when something has died. And when I first heard about this, I thought, what, how does like the carbon know that you've died. Is it like as soon as you die your carbon fourteen atoms start ticking or something. I thought that was really weird, because you know, life and death is this holistic property of an object, and like the carbon fourteen doesn't care if you're alive or dead. Right, So the reason that carbon fourteen is sensitive to your moment of death is because of what you stop doing when you die, which is you stop breathing and you stop eating, so you stop getting new carbon fourteen. So all the carbon fourteen in your body is always decaying. Like that clock started when the cosmic array hit in the upper atmosphere, and maybe it took a few hundred years before you ate that carbon fourteen, and so that clock has already started. What doesn't happen anymore when you die is that you don't get any fresh carbon fourteen. And so now the carbon fourteen in your body is decaying and it's not being replaced. So something that died a long time ago will have almost no carbon fourteen in it, whereas if you died ten minutes ago, you still have a lot of carbon fourteen in you. So it's not really like the moment of death. It's more like the moment you stopped participating in the biosphere, which I guess is really the same thing.
Do like the microbes that break you down put some more carbon fourteen in there to mess the clock up a little. I guess it doesn't matter, because if the half life is five thy seven hundred years, we're talking about a week or two of microbes messing up the values.
Yeah, good question. I guess it depends a little bit if they're aerobic or anaerobic. Right, are they consuming CO two? I'll have to ask mycrobiologist about that, check in with my wife about it later.
Thank you, no one. Yeah, that's right. That's very convenience.
And so this has a really interesting history. It was like in the forties that people figured out, oh maybe this is possible. People were studying carbon fourteen just from like a chemistry point of view, like what is this stuff? How long does it last? And at the time, you know, we didn't really understand the nucleus very well. People were making estimates for like how long it might last, and the measurements they made were really surprising. They discovered, oh my gosh, this stuff lasts a lot longer than we thought. They expected to have a much shorter half life, like tens or hundreds of years. So when they discovered that it has a half life of almost six thousand years, they were surprised, But it also opened up this really cool possibility.
Why were they surprised, Like, did we expect that half lives would be like so similar between objects on the periodic table and they ended up not being similar?
Yah?
Is there a reason they were surprised or just sometimes sits surprising?
They were surprised because we just didn't understand the nucleus of the atom very well. I mean, this is the forties, right, Quantum mechanics was very very new, and so all of our calculations of how things work in the nucleus were very very rough and a lot of handwaving. I mean even today, it's not easy to do these calculations to say, like what would be the mass of this particle or how stable would that be. You know, we have theories about whether the super heavy elements would be stable, but we can't say these things for certain because we don't know how to do a lot of these calculations. Because the strong nuclear force is very complicated. It's very very strong, which means the calculations are very very sensitive to getting things wrong. It's a kind of calculation where if you start to get things a little bit wrong, the wrongness gets amplified by the strength of the force instead of like decaying away. Like with gravity, if you get the location of an asteroid a little bit wrong, it's not going to propagate to becoming really really wrong later on because gravity is so week, so you can make approximations and mostly get the right answer, at least for the foreseeable future. With the strong force, as soon as you get something a little bit wrong, everything goes totally wrong. So the short version is that nuclear physics is hard.
Well, that's not surprising, and so maybe they shouldn't have been surprised because nuclear physics is hard. Okay, So you had mentioned at the top of the show that sewage was gonna come into the story at some point, and so I'm hoping that now that we're talking about, you know, when we figure out the whole carbon fourteen thing, is this one we get to talk about the sewage.
This is when we get to talk about the sewage. Absolutely. So it was in the mid nineteen forties and Willard Libby, who was at Berkeley, learned about these results that carbon fourteen was surprisingly long lasting, and he thought, hey, that would be a really cool way to figure out how long something has been dead. And so what he did is he moved to a new job at University of Chicago and he proposed this idea. He said, maybe this will work. And the first thing they did was to study methane from the Boston sewage system. It's a methane is a gas, right that's produced when sewage basically ferments, migrooves are consuming your sewage. And so they gathered this and they measure the carbon fourteen fraction in methane basically from Boston's dark matter, and then they compared that to how much carbon fourteen there was in methane from fossil fuels. Right, like the methane that maybe you burn in your house comes from plants that died millions and millions and millions of years ago, and what they found is no surprise to us now, is that the methane from the Boston sewage system has a lot of carbon fourteen and the methane from fossil fuels has none.
All right, So that's beginning to tell us the limits for how we can use carbon fourteen for dating. But I'll note you said that Libby had moved to Chicago, but he looked at the Boston sewage is there just like is Boston sewage The best sewage he traveled all the way to Boston was the Chicago sewage. Wasn't disgusting with us.
I'm sure those people who live in Boston are very proud of their sewage. I have a little glimpse into this because of my wife's research. You know, she studies the microbes and sewage, and it can be surprisingly tricky to get access to it. So I suspect it's just a question of like politics and access. Not every waste management system is interested in having scientists like dig around in their facility, while others are, you know, free to share the science gold that is flowing through their pipes.
Politics and sewage.
Yeah, so I suspect they had a good Boston connect, you know, for some real primo Boston sewage.
Got it, Okay, So by the time you ask petroleum, there's no carbon fourteen, and you start losing carbon fourteen when you die. So can you use this for? Is this helpful for like certain kinds of fossils? What kind of things have we used this for so far?
You can use this for basically anything that's died in the last you know, maybe fifty thousand years, because those things were accumulating carbon fourteen as they were alive and participating in the biosphere, and as soon as they died, then they stopped and the carbon fourteen started to decay away. Now, things that are much much older than that, they just have zero carbon fourteen. So you can't tell is this thing one hundred thousand years old or one hundred million years old or four billion years old, because you just get zero. So what you got to do is measure the carbon fourteen the carbon twelve ratio in your thing, and then you're going to extrapolate back to when did this thing last have the sort of normal rate of carbon fourteen?
Got it all right? So dinosaurs are out, but helpful for things like human archaeology questions exactly.
Human archaeology was really like revolutionized by this subject. The first time that they used it was actually to date some Egyptian tombs. These are some things that archaeologically, we already knew what the dates were based on writing and other analysis, Like archaeologists already knew when somebody had been buried, and now they were able to take a sample and measure the carbon fourteen fraction in like a piece of linen or in a piece of wood from the two boom and independently establish the date of the death of that object, like here's when the tree that formed this plank must have been killed, or here's when this plant was harvested to make this linen. And that's really powerful. It's like a completely separate clock that lines up archaeologically with your records.
Okay, So one, that's awesome, and two, I bet this has been used at some point in ways that have made people angry, like people who thought they had a thing that was old but it ended up not being that old, or the other way around. But before I ask you about that, let's take another quick break.
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All right, So, has this method been used in a way that ended up making people ling?
Why do I feel like you're digging for the controversy here, Klly?
Well, you know it makes good radio.
It does. Absolutely. Yeah. This is a really cool technique and it's powerful because they can date things to like within a few decades. You know, the thing is that there's a lot of carbon in living stuff, and the more carbon you have, the more precise measurement can be, so you can like pin down within a few decades when something died. And for example, when people discovered like the Dead Sea Scrolls, these ancient scrolls outside of Jerusalem that had been in the desert for who knows how long and may have been one of the earliest written records of you know, the books of the Bible, people wanted to know like are these real? Are they actually thousands of years old or is this like a forgery from last week that's been stained with tea? And so they were able to use radiocarbon dating to confirm that these things were one nine hundred and seventeen years old, which really told people like, these are an ancient relic. You know, these were not created recently, or at least the materials on which they were made are fairly old. So either it's a really old book or it's new writing on a really old sheet of paper.
Well, I'm going to have to give you that. That's awesome. That's not what I asked. I asked you for controversy, but you gave me confirmation, but still all right, so that's pretty awesome.
Well, this controversy also, you may have heard about the Shroud of Turin. This is this piece of cloth that I think is held in Milan that's supposed to have like the face of Jesus on it, and the mythology around it is that it wrapped to Jesus's corpse and it was sort of imprinted by the power of his unknown spiritual personality with his face. And so this is a relic that's been celebrated in people like pilgrimage to go and see it, and so they radiocarbondated it and discovered Oops, it's actually from the fourteenth century, which means it's pretty old. It's like six hundred years old. But the plants that made this shroud were grown and planted like fourteen hundred years after Jesus died.
That's disappointing. Is it safe to assume that there are people who don't buy the science on this to this day?
Oh? Absolutely, the way people deny evolution or that the Earth is round or the dinosaurs existed well before humans. There are people who say radiocarbon dating is not reliable and that you can't use it, especially, you know, when it contradicts their belief. The thing I love about the Shroud of Turn story is not just that it reveals that the whole thing is a hoax, but it's an ancient hoax. The hoax is now hundreds of years old, right, The hoax itself is of historical interest. That's how old it is, right, A six hundred year old hoax is pretty cool. Okay, it's not the face of Jesus, but wow, that's really cool insight and like what people were doing and why they wanted to do it, and all sorts of stuff. I love how like today's mundanity, right, and even lies can turn into something fascinating for future anthropologists.
Ye know, humans have enjoyed messing with each other for a really long time. It's good to know that.
But it is tricky to date things with carbon fourteen because it turns out that the assumption we made at the beginning, that like carbon fourteen is produced in the upper atmosphere and spreads around mostly evenly to everything not exactly true. And so you got to like make some corrections and calibrations to get things as precise as you'd like.
So, like how not exact. Are we talking here like you know the flies that you find in bodies? I think things like temperature impact development time. How many things impact our ability to date things based on carbon fourteen And are they things that we can learn about and control for or.
No, Yes, absolutely we can learn about them. And it's a whole field of people calibrating carbon fourteen dating. We first discovered this actually when carbon fourteen dating got some Egyptian tombs wrong, like some of them were bang on the archaeological records, and other ones that archaeologists were pretty sure about. Radiocarbon dating got a little wrong, and people thought, hmm, that's weird. I wonder what it means. And so they discovered a few interesting effects. For example, carbon fourteen production is not constant over time. If you want to assume that you can extrapolate backwards from the carbon fourteen ratio today to the carbon fourteen ratio when this object died, you have to assume that carbon fourteen is being replenished at the same rate over time. But it turns out that it's not that there's a variation in the carbon fourteen rates in the atmosphere.
WHOA and is that because cosmic rays hit us at different rates. Is this like a solar Now that's not a solar wind thing? Do we know? Yeah? Why I am confused.
It's really interesting and there's some cool physics there. One is that, yes, cosmic rays have sort of like seasons. They are not totally constant. It's sort of like the solar weather. You know, where these cosmic rays come from depend on like magnetic fields and the galaxy. You know what's going on with the objects that are created them. So there's those kinds of effects. But we can actually measure that in independent ways because they can look at super ancient trees. Right. Trees are really cool because they grow and they add rings, and those new rings interact with the biosphere, but the old rings don't. So every year a tree is basically like taking a sample of the carbon fourteen fraction in the atmosphere and storing it forever. And so if you slice open a really old tree and look back at the carbon fourteen fraction in each of the rings, you can get like a history of the carbon fourteen fraction at the time that the tree was growing.
Do you happen to know what the oldest tree? We've done that on is are we talking about, like hundreds of years worth of data are thousands We.
Have lots that are hundreds of years and a few that are thousands of years, and so that really helps us calibrate like the more recent fluctuations in carbon fourteen. But also humans have really altered the carbon fourteen fraction in the atmosphere. For example, we've been burning a lot of fossil fuels over the last couple of hundred years. Fossil fuels have carbon but no carbon fourteen, So we've been releasing a lot of carbon twelve into the atmosphere, really bringing down the carbon fourteen fraction in the atmosphere. So, like human effects have really changed this and made it more complicated to interpret the past.
Wow. So like if you were an alien trying to age stuff that was happening down here on Earth and you wanted to something you know, thousands of years from now that happened during our period, it would be a mess.
It would be a mess if you didn't have ways to calibrate it. And so fortunately, like trees can help us understand these things. Because trees have been around for hundreds of years since, we can understand the effect the humans have had on the carbon fourteen fraction in the atmosphere, but more confusingly, we've had effects in both directions. So burning fossil fuels lowers the carbon fourteen fraction because you're pumping out super old carbon where everything is already decayed. But nuclear testing in the atmosphere increases the carbon fourteen fraction because it makes new carbon fourteen. Is all this radiation and all the products of the nuclear testing makes lots and lots of carbon fourteen, much more than it's made from cosmic rays. So like in the middle of this century, we had like twice as much carbon fourteen in the atmosphere as we usually do.
Where are the worst That'll be another interesting problem for the aliens to solve. Then where did all this carbon fourteen come from? Oh, they were setting off new weapons in the sky, of course there were.
It's fascinating because there are two sides to that. Archaeologists are frustrated by that, because we're like poisoning the historical record and making it harder to figure out when things came from. But geologists actually really like it because it's like a really bright signal. They're like, okay, cool, we can use this to calibrate and we can tell when something happened because there's so much carbon fourteen in that layer. So geologists, I think would like us to be like regularly nuking the atmosphere in predictable and periodic ways because it leaves like this ruler back in the record.
You know. I know a geologist I think is not quite that self interested, but I can imagine him thinking that that's a silver lining of an author thing. Humans are complicated.
So yeah, so you have to take all this into account, and you have to know, like what was the rate of carbon fourteen in the atmosphere over the last few thousand years, And you also have to take into account where you have found something. If you found it in the deep ocean, then it was going to get less carbon fourteen to begin with, and then if you found it in the upper atmosphere. Also, the different hemispheres of Earth have different depositions of carbon fourteen because there's different like patterns of winds, and the North and the South hemisphere actually have totally separate, independent wind systems that don't really mix very well. So there's less carbon fourteen in the southern atmosphere because it's like more surface area of ocean, which sucks up more carbon fourteen, and there's more carbon fourteen in the northern hemisphere where there's less ocean surface.
Do you have any sense for like for the hemisphere thing? Are we talking like if you didn't correct for that, you'd be off by about ten years or one thousand years or it just kind of depends on a lot of other stuff too.
These all are really small effects, and everybody wants really precise dating of things, you know, down to the decades, and so this is the kind of thing that happens in science. First you have a very approximate effect. You're like, okay, let's just assume carbon fourteen is constant everywhere and over time. What do we get? Oh my gosh, it can teach us something already. Then you start to hone in on the details you want, like the second digit to be accurate, and then the third, and then the fourth, and by now we're like, you know, seventy years into this research project, we're getting down to the nitty gritty detail. So a lot of these things will affect our estimate for the dates of things by decades or maybe up to hundreds of years, not thousands of years. It's not going to like upend everything we thought we knew. We also need a really precise estimate of the half life of carbon fourteen, right, we have to be able to calibrate this clock to know how long does it take carbon fourteen to decay? And as you said, you can't wait around for five thousand years, which would be the best way to get an accurate measurement. But people have been developing more and more precise experiments with like larger samples of carbon fourteen, so they actually had to update the official half life of carbon fourteen from fifty five sixty eight later to fifty seven thirty, so it changes like one hundred and fifty years. And this is super fascinating because it then required a change of all the archaeological dates. Like all the archaeologists who thought that their thing was data a certain date. Oops, I got updated because the nuclear physicists or the chemists got the number wrong.
Does that mean that you get to redo all of your publications with the updated date and double the number of lines you have on your CV because in that case, thank you physicists.
It means that there's a bunch of papers out there with old dates that we now like know need corrections. So you can't just read the old papers in archaeology and take the dates at face value. You have to know when that date was calculated, like what effects did they take into account? And what effects do we now take into account. So the whole thing has gotten really really complicated. That is frustrating, but it also has been an enormous boon to archaeology. I mean, what a powerful tool, Like anything that was alive now you can date the moment of its death. And that's not a perfect tool, right, There's still thinks like when was this metal forged? Well, we can't tell because it was never alive. It doesn't participate in the carbon biosphere, so you can't date like jewelry or swords or stuff like that. But you can look at what else is any and you can tell maybe when that person died. And before this, archaeologists had much more rudimentary methods. You know, they had this like layer method where they would like count down from the ground and try to like find things that they knew happened that they could use to like sandwich when their relics might have been buried. So it was very, very rough, and now we have this way to like measure for these objects when they died. It's super powerful.
It's super powerful. But I wonder if there was also a generation of scientists who were, like, you know, the whole reason I got into archaeologies because I don't want to be in the lab, and now I have to be in the lab to get these dates. But on the other hand, I think we all benefit when we get accurate, precise information.
Yeah, I think that's probably true. After this was discovered, very rapidly, a bunch of labs were set up around the world to start doing radiocarbon dating. So it's not the kind of thing that like a typical archaeologist so does in the field, or every archaeologist has their own radiocarbon setup. It's a little bit of an involved process. What you have to do is measure the carbon fourteen ratio. In the old days, what they did was just sort of like count the radiation emitted because it's emitting betaies when it's decaying. So that was the old strategy. More recently they have a more precise strategy which uses mass spectrometer, so it like takes a sample of it, accelerates it bends it through a magnetic field, and then it'll bend more if it has low mass and less if it has high mass. So it gives you like a spectrum of the mass of the object that you're sampling, and you can tell how much carbon fourteen, how much carbon twelve is in there. You don't have to wait for the carbon fourteen to decay, so you're getting to take advantage of like all the carbon fourteen in there, not just the ones that happen to be decaying as you're watching. So these are pretty specialized techniques now, and I think most archaeologists like will send a sample to the lab rather than like doing it themselves. So I think you still have like old school Indiana Jones types out there in the field, gathering stuff, plundering sites exactly. Oh man, the ethics of ourology, and that's something I want to get into.
Yeah, nodalistic with science.
Yeah, but then sending those samples to a lab to do the dating. So you probably have like a division. You know, we have the radiocarbon archaeologists in the lab and the folks who are not in the lab.
Okay, well, let's step away from the ethical quandaries presented to us by archaeology and talk about dinosaurs. So sorry, So we've we've established unfortunately early on, that carbon fourteen is not helped for dinosaurs. Is there something we can use if we're interested in when a dinosaur bone, you know, when the dinosaur died.
Dinosaurs are tricky, absolutely, because this clock has all run out. All carbon fourteen that was in dinosaurs has now decayed. So what you need are longer clocks. We did an episode about using uranium to date stuff because this is this cool relationship between uranium and lead. Uranium two thirty eight and uranium two thirty five have half lives of more than a million years, and so they're useful for dating stuff that's super due old. And when they form, this cool thing happens. They get these zircon crystals zr con. And when those zircon crystals form, they reject any lead and they like expel lead from inside the crystal. So when they're formed, they're like lead free, but they do take uranium in them, and uranium decays into lead. So each one is like a little clock. If you pick up a zircon crystal and measure how much lead is inside of it, you can tell when it was formed. So now we're dating like when a rock cooled into these crystals, which is pretty cool.
That's awesome. And so then you're measure in the rock around the fossils or something.
Yeah, exactly, No, dinosaur bones don't have this stuff in them. But sometimes around the dinosaur bones there's like cooled magma. So this igneous rock. Because fossils only form in sedimentary rock. But if you have like layers of igneous rock above and below your fossil, then you can use the zircon crystals in those rocks to figure out when those crystals were formed and therefore racket your dinosaur bones, so you can tell roughly when it must have existed. Yay science, Yay science. I was always puzzled as a kid when people would talk about like the age of rocks. I'm like, what does that mean, Like when it's a rock born? Right, rock was never alive. And it's only later that I understood that they're talking about when the rock cooled. Because you have like that blob of magma. It was still basically rock. It was just like liquid rock. But it's when it's cooled into a rock and formed crystal. That's what they're interested and that's what they're measuring. It's like saying, how old is my ice cube? Well, you know, the water in it has been water forever, but it's only been an ice cube since you put it in the freezer last Tuesday. So that's like the age of your ice cube in the same way.
That's a good way to explain it.
So yeah, that's the story of radiocarbon dating. There's an incredible cool process where clocks are created in the upper atmosphere and then drift down into the biosphere inhaled by plants eaten by you, and then stop ticking as soon as you stop eating and breathing. Thanks very much Kelly for joining us on this trip into debunking the Shroud of Turn and confirming the Dead Sea scrolls.
Thanks for bringing me along on the trip. I had a great time, all.
Right, And thanks everyone for listening. And if you have questions about how something works, please don't be shy. Write to us two questions at Daniel and Jorge dot com. Tune in next time.
Bye.
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