Speaker 1: Hey, Daniel, do you keep a family photo album? You know,
Speaker 1: we used to, but these days it's all digital. I
Speaker 1: think Mark Zuckerberg owns all of our family photos. It
Speaker 1: owns your family period. Probably that too. But and how
Speaker 1: do you feel when you look back and see pictures
Speaker 1: of yourself when you were younger? Um? I try to
Speaker 1: ignore all those signs of aging, you know, pretend that
Speaker 1: I looked the same ten or twenty years ago as
Speaker 1: I do today. I see myself as a fine wine
Speaker 1: because you have turned, because I just get more expensive
Speaker 1: the older I get. For some reason. Well, you know,
Speaker 1: I've known you for over ten years or so, and
Speaker 1: I can I can say with confidence for those people
Speaker 1: listening that you are so or no Paul Rudd. Unfortunately
Speaker 1: that's true in many ways, unfortunately. But it is kind
Speaker 1: of interesting how little clues can show you how old
Speaker 1: someone is or how old something is. Right, It's true,
Speaker 1: you know, a few wrinkles here and a few gray
Speaker 1: hairs there, and all of a sudden, you look like
Speaker 1: you're in your midlife crisis. What's that look like a
Speaker 1: look of panic? It's a look of gravitas. I always
Speaker 1: remember how dan Quail died his temples gray and which
Speaker 1: not look so much like a little boy Dan quail Man.
Speaker 1: That's a that's a reference that totally tells everyone how
Speaker 1: old you are. Nobody's fascinating how almost everything around us
Speaker 1: ages right. Everything from people around you, to the rocks
Speaker 1: around you, um to the buildings around you always show
Speaker 1: their signs of age. Nothing in this world is permanent.
Speaker 1: Hi am poor handmade cartoonists and the creator of PhD comics. Hi.
Speaker 1: I'm Daniel. I'm a particle physicist, and I'm young and spry,
Speaker 1: and I have not written any web comics, but I'm
Speaker 1: the co author of the book We Have No Idea
Speaker 1: All about the unknown Questions in the Universe. And welcome
Speaker 1: to our podcast, Daniel and Jorge Explain the Universe, a
Speaker 1: production of I Heart Radio, in which we take weird
Speaker 1: and amazing and young and old stuff in the universe
Speaker 1: and try to explain it to you, and not just
Speaker 1: telling you the answers, but sometimes digging in deeply and
Speaker 1: explaining how we know so that you can explain it
Speaker 1: to skeptical folks around you. Yeah, we try to get
Speaker 1: you to think about the world around you and even
Speaker 1: the world you're standing on right now or sitting on
Speaker 1: or lying down on and one of the depending depending
Speaker 1: on how old you are. But one of the goals
Speaker 1: of this podcast is for you to have sort of
Speaker 1: a new perspective on the world around you. We want
Speaker 1: you to see everything around you as sort of a clue,
Speaker 1: something that can tell you how the world works, that
Speaker 1: can reveal secrets. I can answer questions because almost every
Speaker 1: question we have about the universe, the answers are all
Speaker 1: around us, and it just takes sort of a trained
Speaker 1: mind and a careful eye to see those clues everywhere.
Speaker 1: And so there are clues all around you about where
Speaker 1: we come from and how long we've been here, and
Speaker 1: how long everything has been here? Right, Yeah, exactly. And
Speaker 1: sometimes answers are easy and obvious, and sometimes the answers
Speaker 1: are mind mogglingly surprising. And so today on the podcast,
Speaker 1: we'll be tackling, quite possibly the oldest question on earth.
Speaker 1: Would you say that's true, Daniel, is that who stole
Speaker 1: my dessert? Um? I think the answer is obvious, obviously,
Speaker 1: it's your your children. Well you know, Um, that's actually
Speaker 1: a question my kids asked me today. Um, when was
Speaker 1: dessert invented? It sounds like a topic for a pope,
Speaker 1: you know, if that falls into a physics question. But
Speaker 1: it's sort of a fascinating question, and they were actually
Speaker 1: hypothesizing that dessert might be older than people that maybe,
Speaker 1: like our prehuman ancestors, enjoyed a sweet, sweet treat after dinner.
Speaker 1: What do you think the little bacteria the good that
Speaker 1: well the ocean? They're like, you know, I could use
Speaker 1: a little two remy suit after consuming the other other bacteria.
Speaker 1: I don't know if we have enough in common with
Speaker 1: bacteria to say what they might like, but I can
Speaker 1: imagine it's some pre human homaid, you know, eating some
Speaker 1: berries and going and that goes nicely with my antelope
Speaker 1: or whatever they had for dinner. Well anyway, so so weely. Uh.
Speaker 1: What I mean by it's the oldest question on Earth
Speaker 1: is that it's it's probably really the oldest question that
Speaker 1: could be on Earth, right, Yeah, I think that's probably true,
Speaker 1: And it's a it's an important question, right. It's the
Speaker 1: kind of question that we'd like to dig into because
Speaker 1: it's the kind of question where the answer could change
Speaker 1: the way we think about life and the universe and
Speaker 1: the humanity's rolling it right, it has all sorts of
Speaker 1: things to do with with creation and the context of
Speaker 1: our lives and therefore how we should live our lives
Speaker 1: and whether they have meanings and so to be. On
Speaker 1: the podcast, we'll be tackling the question how old is
Speaker 1: the Earth? And is it even polite to ask? That's right?
Speaker 1: And what does the earth do when he gets upset?
Speaker 1: All right, when he gets offended? Um? And not just
Speaker 1: how old had any worked? One? Has the Earth had
Speaker 1: any work? One? Is it trying to? I think the
Speaker 1: Earth looks great. Doesn't look a day past four billion? Um?
Speaker 1: It is full of buetos. They're probably right, um. And
Speaker 1: just as important as the question how old is the
Speaker 1: Earth and the answer, which is you know, just a number,
Speaker 1: is how do we know? How do we have confidence
Speaker 1: in this answer? Because you know, there are people out
Speaker 1: there who hold beliefs about the various age of the Earth.
Speaker 1: And one of our listeners actually wrote in and said, hey,
Speaker 1: could you tell me how we know the age of
Speaker 1: the Earth, so that when I discussed with my friends
Speaker 1: who have strong opinions about the age of the Earth,
Speaker 1: I can tell them not just what scientists think, but
Speaker 1: how they know it. Yeah, And that's the thing I
Speaker 1: think we always try to do in this podcast, right,
Speaker 1: is try to get at how people know these things,
Speaker 1: not just tell people what scientists know, but how they
Speaker 1: know it. That's right, And that also helps you understand
Speaker 1: with what confidence we know it, Like is this just
Speaker 1: sort of an educated guess the back of the envelope
Speaker 1: calculation or is this something that people have been working
Speaker 1: on for hundreds of years and we have great confidence
Speaker 1: and have it nailed down to like point oh one percent.
Speaker 1: And it's important because different things in science are are
Speaker 1: known well and some things since science are are known
Speaker 1: not very well. So it's important to know how we know. Yeah,
Speaker 1: And so how old is the Earth is the question
Speaker 1: for today, and it's kind of um, I think it's
Speaker 1: a question most people know a little bit about, right.
Speaker 1: I mean, I think everyone knows or has a sense
Speaker 1: that the Earth is very old. Yeah, but you know,
Speaker 1: that sense that the Earth is like extremely old, like
Speaker 1: many orders of magnitude older than you or me or
Speaker 1: your parents, you know, is a fairly modern idea. I
Speaker 1: think thousands of years ago people thought the Earth was
Speaker 1: only hundreds or thousands of years old. They thought it
Speaker 1: might have been created fairly recently, and it was only
Speaker 1: sort of in the eighteen hundreds that people started to
Speaker 1: understand that there were processes around us that we could see,
Speaker 1: specifically rocks and evolutions or geology and biology started to
Speaker 1: give hints that the answer was much much bigger than
Speaker 1: anything anybody had imagined. And I think that's wonderful. I
Speaker 1: I love to live in a scientific era like that,
Speaker 1: where you're where something you thought you knew is upended
Speaker 1: and replaced with a completely different answer. That's like oors
Speaker 1: a magnitude bigger. Wow, is that really true? Like it
Speaker 1: only in the last couple of hundred years do we
Speaker 1: have a sense of how old the Earth really is?
Speaker 1: Before it was totally up in the air. Yeah, it
Speaker 1: was up in the air until about the eighteen hundreds,
Speaker 1: and then people started trying to do calculations and they're like, well,
Speaker 1: how long would it take to form a planet, you know,
Speaker 1: so you gathered it together with gravity, or you know,
Speaker 1: how how long would it take to lay down layers
Speaker 1: of rock? You know, how long would it take glaciers
Speaker 1: to this kind of stuff? And you know, geology is
Speaker 1: a fairly young field and that you know, it's a
Speaker 1: couple of hundred years old and so people started to
Speaker 1: look around, and as soon as they started to explore
Speaker 1: these geological processes, they realized, wow, this stuff takes a
Speaker 1: long time. And you know, it's sort of like um
Speaker 1: walking into a room and seeing somebody has written a
Speaker 1: book that's like five million pages long, and you're like, wow,
Speaker 1: you've been in this room a long time, you know,
Speaker 1: so it shocks you into realizing the things things are
Speaker 1: a much different scale than you imagined. And that's the
Speaker 1: we've seen the story the earth all around us. We
Speaker 1: realize it's staggeringly long, much longer than we imagined, and that, yeah,
Speaker 1: that's a fairly new idea. Yeah, that's a lot of
Speaker 1: party you've missed exactly we should. Things have been around
Speaker 1: for a long time. Do you have a fear of
Speaker 1: missing out? There? Is that fomo about what happened? All
Speaker 1: the good desserts? Fear of being in parties? Actually, to
Speaker 1: be honest, what about all the desserts? The usually have
Speaker 1: good dessertsive party old desserts or I just like I
Speaker 1: just like a fine dessert. Wine? So you should you
Speaker 1: show U at the beginning of the party, how the
Speaker 1: desserts drink the wine and then go home? That's how
Speaker 1: you know you're old or that was like a good
Speaker 1: party to me. That is how your age old track exactly.
Speaker 1: That's very age appropriate. Um. The beginning of this field
Speaker 1: like becoming scientific before that was just speculation of the
Speaker 1: beginning of the field. Becoming scientific was in the eighteen hundreds. Um.
Speaker 1: But still that's very inaccurate. Like looking at rocks gives
Speaker 1: you clues about the sort of order of magnitude, But
Speaker 1: then people wanted to know very precisely, and that took
Speaker 1: that took some time to to like basically defined clocks
Speaker 1: inside the earth, clocks that have been ticking since it
Speaker 1: was born. And that's in the end signs of aging, Yeah,
Speaker 1: signs of aging, very precise signs of aging that could
Speaker 1: tell us how old the earth was. So you're saying, like,
Speaker 1: you know, the people who started the United States, like
Speaker 1: the founding father fathers, like the Founding Fathers, they had
Speaker 1: no idea how old we've been around, Like did they
Speaker 1: think we've been around for a few thousand years? For
Speaker 1: a few but didn't they know about the you know,
Speaker 1: like ancient Egypt and things like that. Yeah, but you
Speaker 1: know that's thousands of years, and so I think they
Speaker 1: had the idea that the Earth was, you know, thousands,
Speaker 1: many thousands of years old, right, because that's as long
Speaker 1: as written history is, and so they imagine it's on
Speaker 1: that time scale because that's the oldest thing, you know.
Speaker 1: And that's basically the game is like find the oldest
Speaker 1: thing you know and then assume that that's basically the
Speaker 1: age of the world, right, And that's frankly what we're
Speaker 1: still doing, um, except that we think that we found
Speaker 1: stuff that comes from the very beginning of the world.
Speaker 1: But you know, they knew about stuff that was thousands
Speaker 1: of years old, so they had no reason to believe
Speaker 1: that the Earth might be millions or even God forbid,
Speaker 1: billions of years old, right. Plus I think those guys
Speaker 1: are pretty religious, all right. So it is an age
Speaker 1: old question and one that a lot of people have
Speaker 1: had answers to and seems seemed to have answers to.
Speaker 1: But we were wondering what people out there right now
Speaker 1: in the world think about this question. Yeah, and this
Speaker 1: is one this is a set of interviews. I was
Speaker 1: very curious to see what people thought. I was wondering
Speaker 1: if I would run into some sort of young Earth
Speaker 1: folks who thought the Earth was from biblical ages and uh,
Speaker 1: and even those people who were scientific. I was wondering, like,
Speaker 1: how precisely do people know the age of the Earth.
Speaker 1: So this I was very fascinating to hear the answers
Speaker 1: like is it a million years, a bazillion years, twenty
Speaker 1: seven point two bazillion? And so, as usual, Daniel went
Speaker 1: out and asked people on the street how old they
Speaker 1: thought the Earth was. So think about it for a second.
Speaker 1: If you ran into a physicist in the street, uh today,
Speaker 1: how would you answer this question? And no googling. Well,
Speaker 1: here's what people had to say. Um, I have no
Speaker 1: idea probably billion years. I don't exactly, but my understanding
Speaker 1: is it's a couple of hundred million years old. Okay,
Speaker 1: you know how we know? So I don't actually like
Speaker 1: my my physics knowledge is pretty limited. Um. I think
Speaker 1: like we've used carbon dating to date things within various
Speaker 1: geological epox but I don't actually know how we arrived
Speaker 1: at the calculation of the Earth stage and maybe three
Speaker 1: billion years, tens of million years, four billion years, and
Speaker 1: how do we know rock formation? It's probably like millions
Speaker 1: of years old. I feel like, all right, Um, a
Speaker 1: lot of millions and billions, right, yeah, exactly, not not
Speaker 1: a whole lot of really specific answers. I have to
Speaker 1: say I was a little bit disappointed. I guess I
Speaker 1: thought people were more interested in this, Like I remember
Speaker 1: wanting to know this number as precisely as possible when
Speaker 1: I was a kid. I was really curious about this, um,
Speaker 1: and so I was a little surprised that people were
Speaker 1: just sort of knew it was super old and were
Speaker 1: satisfied and didn't really care about the details. Although I'll
Speaker 1: say every single person here after I asked them the
Speaker 1: question then they did turn around and ask me. They're like, well,
Speaker 1: how old is the Earth? So they wanted to know
Speaker 1: abous just hadn't really spend any time looking it up.
Speaker 1: All right, Well, let's get started on this topic, Daniel
Speaker 1: and so, um. First of all, I think I feel
Speaker 1: like we should define what we mean by the Earth
Speaker 1: by the age of the Earth, right, Like, um, do
Speaker 1: you mean like the the planet, like the ball that
Speaker 1: it's Earth, or the rocks in the Earth, or the
Speaker 1: atoms that are that make up the Earth? What do
Speaker 1: you mean, like what's the age of the Earth. Yeah,
Speaker 1: it turns out that that's a bit of a fuzzy question, right,
Speaker 1: because the Earth has a sort of continuous history. It's
Speaker 1: not like somebody snapped their fingers and boom, there was
Speaker 1: the Earth. And you can start from the clock from then, right, Um,
Speaker 1: that you know of, right, that we know of, that's right, unless,
Speaker 1: of course it was a simulation, which case things is easy.
Speaker 1: But in the case that it wasn't the simulation, we
Speaker 1: have an idea about how the Earth was formed, and
Speaker 1: there's just a lot of stages there, and so you
Speaker 1: have to sort of make an arbitrary definition and into
Speaker 1: like a lot of things in science, you know, Um,
Speaker 1: there's not necessarily a clear and sower. So you just
Speaker 1: define it to be something arbitrary and at least we
Speaker 1: can agree on it. But maybe we should recap sort
Speaker 1: of the brief history of the of the Earth and
Speaker 1: how it was formed, yeah, like how it how it
Speaker 1: came to be. Yeah, And then at some point we're
Speaker 1: gonna say, this is when the earth started, this is
Speaker 1: when the Earth was born. That's right. This is when
Speaker 1: the party began, right, um, Because you know, you invite
Speaker 1: people to party at nine o'clock and then they don't
Speaker 1: show up till ten, and by then you've eating all
Speaker 1: the brownies, um, and parties only really get started at
Speaker 1: one a m. That's right, unless you're in Spain, in
Speaker 1: which cases four am um. But you know, the very
Speaker 1: early history is that we basically started with a huge
Speaker 1: nebula of gas and dust. And this is stuff that's
Speaker 1: left over from the explosions of other stars and just
Speaker 1: gas from the Big Bang. And then of course gravity
Speaker 1: did its thing and it pulls out all that stuff
Speaker 1: together and you get a sun, right, So most of
Speaker 1: the stuff went to the Sun, and then you get
Speaker 1: a disc of stuff that's rotating around the Sun. It's
Speaker 1: like rings around the Sun. And then the gravity pulls
Speaker 1: the stuff in that disc together to turning to turn
Speaker 1: that disc into planets, right, And uh so we think
Speaker 1: that the Solar system started about four and a half
Speaker 1: billion years ago, and it took like a few million
Speaker 1: or a hundred million years to pull all that stuff
Speaker 1: and the disk together into sort of protoplanets. Right. So
Speaker 1: you have this sort of loose collection of stuff that
Speaker 1: gravity is pulled together, and then gravity really gets to
Speaker 1: work and it starts squeezing it and it collects all
Speaker 1: this stuff and the planet heats up and sort of
Speaker 1: melts the metal and you get the iron dropping to
Speaker 1: the core and the crust forming, and so you can
Speaker 1: sort of ask, like, what moment do you count the
Speaker 1: earth forming? Is it like when you have the first
Speaker 1: big cluster of rocks or maybe when it got gravitationally
Speaker 1: heated enough to sort of melt the metals and form
Speaker 1: the core um It's not really very clear. Well, I
Speaker 1: always find this really fascinating because we we talked about
Speaker 1: this the other day, that something like the rings around
Speaker 1: Saturn are only temporary, right, Like at some point those
Speaker 1: rings might turn into little blobs. That's right. Gravity is
Speaker 1: very weak but very very patient, and so it just
Speaker 1: keeps working and you give it enough time, it will
Speaker 1: gather things together. And so this stuff that's orbiting Saturn
Speaker 1: eventually grab gravity will pull it together in two moons.
Speaker 1: Even the asteroid belt, is that at some point gonna
Speaker 1: turn into more planets for our Solar system? Yeah, eventually.
Speaker 1: Gravity it means very slow and those things are mostly
Speaker 1: stable orbits, but eventually that stuff will get pulled together.
Speaker 1: I mean also there's disruption. Right if you just left
Speaker 1: the Solar system all by itself, then yes, eventually you'll
Speaker 1: just gather together into a smaller and smaller number of objects.
Speaker 1: But this disruption from stuff that comes outside, you know
Speaker 1: and comets things from the word cloud. Um even even
Speaker 1: things like supernovas from other solar system can disrupt things
Speaker 1: happening in our solar system. Um. So, But yes, eventually
Speaker 1: gravity will pull all this stuff together. It's really incredible
Speaker 1: what over over these time scales, what gravity can do.
Speaker 1: And that's one of my favorite things. It's just started
Speaker 1: realizing that these processes are really slow, so things must
Speaker 1: have been around for a super long time to make
Speaker 1: it happen, right right, Well, I guess I would maybe
Speaker 1: count that the birth of the Earth is when it
Speaker 1: was form, you know, when all those asteroids and rocks
Speaker 1: out there clumped together into a ball about the size
Speaker 1: of the ball that we have now, like, you know,
Speaker 1: when when it kind of snapped into shape. I would
Speaker 1: maybe count that as the age of when the Earth started. Yeah, yeah,
Speaker 1: and that's totally reasonable. That's totally reasonable. Um. I think
Speaker 1: one common definition is that you measured the age of
Speaker 1: the Earth is sort of the age of the oldest
Speaker 1: rock in the Earth. And then you can ask the question,
Speaker 1: what does it mean for a rock to be old.
Speaker 1: Like you know, a rock is just a gravitational pulled
Speaker 1: together clump of dust. But rocks are are formed right
Speaker 1: there through melt and then they're cooled. And so the
Speaker 1: age of a rock is usually like the last time
Speaker 1: it was melted, right, So it's the time since it
Speaker 1: was last melted, and so a lot of time people
Speaker 1: just like the time that it cooled off, the last
Speaker 1: time it really chilled out. And so you can imagine
Speaker 1: like all these rocks came together, as you said, and
Speaker 1: formed the Earth and got squeezed together and probably melted
Speaker 1: and then cooled right, and the crust at least cooled,
Speaker 1: and so you can asked, like maybe that last moment
Speaker 1: the last moment of cooling of the crust. So the
Speaker 1: age of the oldest cooled rock on the Earth could
Speaker 1: be a definition at the age of the Earth, because otherwise,
Speaker 1: how do you know when these rocks came together? Right?
Speaker 1: Otherwise I don't know unless you have, like you know,
Speaker 1: CCTV of the formation of the Solar system. It's pretty
Speaker 1: hard to pin that down. Or you could say, is
Speaker 1: it well, but you just gave me a number you
Speaker 1: said that it was maybe it took around a hundred
Speaker 1: million years to to form that the ball of the Earth. Yeah,
Speaker 1: that's you know, I said, I said between a few
Speaker 1: and a hundred million years, right, So that's a pretty
Speaker 1: big uncertainty. Um, we don't really know how long it
Speaker 1: took and and what moment do you define, like do
Speaker 1: you needed one more rock? Because you know, caustic stuff
Speaker 1: kept getting added to the Earth, and so some people say, well,
Speaker 1: the age of the Earth is sort of just just
Speaker 1: just call it the age of the Solar System because
Speaker 1: that's pretty close to the age of the Earth. Or
Speaker 1: you could say it's the age of the the oldest
Speaker 1: rock on the Earth. So I think both of those
Speaker 1: are sort of acceptable. And anyway, that's really all we
Speaker 1: can probe and end. What we do is we measured
Speaker 1: the age of the oldest rock we can find on
Speaker 1: the Earth, and we say that's pretty close to the
Speaker 1: age of the Earth. So the time that it formed
Speaker 1: into a ball is kind of in between those two numbers, right, Yeah, exactly,
Speaker 1: it's between when the Solar System formed and when the
Speaker 1: ball that is the Earth started getting crusty. Exactly exactly
Speaker 1: when it started getting crusty and grumpy and eating desserted
Speaker 1: parties and going home early like a teenager. The suwhere
Speaker 1: between conception and teenage years that's when you're born. That's
Speaker 1: that's right. And you know, we have the same question
Speaker 1: about people, right, I wonder about that sometimes. You know, Um,
Speaker 1: how do you count somebody's age if there if somebody
Speaker 1: is born premature, they're older. You know, even then somebody
Speaker 1: who was born it was conceived at the same moment
Speaker 1: but then born at full term. So even the age
Speaker 1: of people is sort of hard to define sometimes. All right, well,
Speaker 1: we've determined, we've determined what we mean by the age
Speaker 1: of the Earth, and so now let's get into how
Speaker 1: we know how old the Earth is and what what
Speaker 1: that means. But first let's take a quick break. Okay,
Speaker 1: so we're defining the age of the Earth is how
Speaker 1: old the oldest rock was formed that we know about
Speaker 1: right on Earth. That's right, exactly. Let's say, so when
Speaker 1: the Earth was like a ball of lava, that doesn't count.
Speaker 1: But once it started solidifying, then we're saying that's when
Speaker 1: the Earth was born. Yeah, exactly, because we're imagining that, Yeah,
Speaker 1: we're imagining that with that the formation, the Earth probably
Speaker 1: melted a lot of the rocks because there was this
Speaker 1: early lava period as you as you mentioned, and so
Speaker 1: as it's cooled down and formed the crust, then it
Speaker 1: made rocks and and it turns out we can figure
Speaker 1: out how old a rocky is, and so that's really
Speaker 1: our best handle in figuring out how old the Earth is.
Speaker 1: And in the end it's a lower bound, right we say, well,
Speaker 1: here's an old, super old rock on the Earth, so
Speaker 1: that Earth must be at least that old, because like
Speaker 1: we've talked about, like you mentioned, like um, trying to
Speaker 1: figure out how old Earth is, the only thing we
Speaker 1: can do is sort of look around and see how
Speaker 1: old the things in it are, right, and like, what's
Speaker 1: the oldest thing in it? We know it has to
Speaker 1: be at least that old. Yeah, yeah, exactly. That's about
Speaker 1: all we can do. And you know what you can
Speaker 1: do otherwise is sort of form of theory. You say,
Speaker 1: I have a concept for how this might have worked,
Speaker 1: and then you can fit data to and say, well,
Speaker 1: I expect you know that to see these sort of
Speaker 1: rocks in that case, and you can use that a
Speaker 1: theory to sort of extrapolate a little bit past the
Speaker 1: day to you you observe. But that's very but that's speculative, right,
Speaker 1: it's based on your concept of how things happen. So
Speaker 1: the hardest number, the one that you know best, is
Speaker 1: the oldest thing that you can find. And the oldest
Speaker 1: thing we find our rocks, and and so what we
Speaker 1: do is we try to turn one question into another question.
Speaker 1: The question we can answer is what's the age of
Speaker 1: the oldest rock on the Earth. Wouldn't that tell us
Speaker 1: like if we found a really old raw, what if
Speaker 1: that rock came from before the Earth was formed? Do
Speaker 1: you know what I mean? Yeah, yeah, there are some
Speaker 1: of those actually, like meteorites, Right, meteorites tell us the
Speaker 1: age of the Solar system because we think meteorites were
Speaker 1: formed in the beginning of the Solar system. Right, the
Speaker 1: first stage is gathered together gas and dust into rocks, right,
Speaker 1: And so those rocks from space tell us the age
Speaker 1: of the Solar System. But we can tell which rocks
Speaker 1: came from space and which rocks are from Earth, And
Speaker 1: so we can distinguish those two. How can we tell? Oh,
Speaker 1: they're clearly labeled. You know, they have their turn address, right,
Speaker 1: it says somewhere near Neptune Um. It's it's actually quite
Speaker 1: interesting that the so the distribution of metals in these
Speaker 1: rocks is very different from meteorites than it is for
Speaker 1: rocks on the Earth. It's like a different group of rocks. Yeah,
Speaker 1: it's like they have more nickel in them where they
Speaker 1: have this other rare stuff. And that's a whole other
Speaker 1: fascinating field. Um. And also, so these rocks are different
Speaker 1: from Earth, and they tell us like what else is
Speaker 1: out there in the Solar System and what was out
Speaker 1: there when the Solar system was formed? There like little
Speaker 1: time capsules from the very beginning of the Solar system.
Speaker 1: And they actually tell us another interesting story. Right, So
Speaker 1: those rocks, we can tell how old they are, and
Speaker 1: they tell us how old the Solar system is. So
Speaker 1: we can answer two questions. We can find a bunch
Speaker 1: of rocks. We can say, oh, these came from somewhere
Speaker 1: in the Solar System. There x y z old. These
Speaker 1: rocks are here on Earth, they're only you know, ABC
Speaker 1: old all right. So then so it all comes down
Speaker 1: to the question of how do you tell how old
Speaker 1: the rock is, yeah, right, exactly exactly, or a meteorite
Speaker 1: which is the same, which is the same because the
Speaker 1: meteorite is just basically a rock from space that landed
Speaker 1: on Earth. And the way we do it is something
Speaker 1: called radiometric dating, which is not the same as carbon dating, right,
Speaker 1: but it works in which is not the same as
Speaker 1: online dating either. It turns out to be actually kind
Speaker 1: of similar. There's a lot of swiping, there's a lot
Speaker 1: of lying about your age as well. Um. No, carbon
Speaker 1: dating and radio metric dating have similar principles, and we
Speaker 1: should have a whole podcast episode of how carbon dating works.
Speaker 1: Carbondating relates to organic stuff like living beings. Radiometric dating
Speaker 1: relates to how how old rocks are. Okay, so you
Speaker 1: find a rock and you're you're staring at it, and
Speaker 1: you're like, I wonder when this rock became a rock? Yeah,
Speaker 1: exactly when this turn from lava to this heavy thing
Speaker 1: that I'm holding, And so you can we can actually tell, right,
Speaker 1: we can look at the stuff in it and tell
Speaker 1: how old window was formed, how many millions of years
Speaker 1: ago exactly? And I have so much respect for scientists
Speaker 1: who develop these techniques. You know, they look at the
Speaker 1: rock and they say, I want to know how old
Speaker 1: this rock is. How could I tell? Can I find
Speaker 1: something inside the rock that I can use as a clock,
Speaker 1: something which changes reliably and steadily every year, so that
Speaker 1: I can count it and and project back and figure
Speaker 1: out when that clock started or something. And this is
Speaker 1: hard to do, you know. You have to find something
Speaker 1: which is steady and reliable and can be calibrated. Um,
Speaker 1: it's not trivial, right. You can't just say, oh, I'm
Speaker 1: just gonna your X Y Z property. You have to
Speaker 1: invent these techniques. And it takes time and sweat and tears,
Speaker 1: and so I have so much respect for folks who
Speaker 1: did this. And and in the case of rocks, it
Speaker 1: turns out to be very dependent on the special kind
Speaker 1: of mineral called zircon. Zircon like the fake diamond like, yeah,
Speaker 1: it's related exactly. Zirconium and zircon is a special kind
Speaker 1: of mineral. And it's special because it has two properties. One,
Speaker 1: it likes it's a crystal, and it likes to absorb
Speaker 1: uranium into it right, gobbles up uranium. So when the
Speaker 1: rock forms, if it has some zircon, then it will
Speaker 1: grab any uranium that's around it. And the second important property,
Speaker 1: but wait, what is zircon made out of? Is it?
Speaker 1: What is it like a made out of iron or
Speaker 1: what is it carbon? What is this mineral made out
Speaker 1: of it? I think it's made out of leftover desserts. No,
Speaker 1: it's it's a mineral um. It's made out of zirconium
Speaker 1: and silicon and oxygen. Oh Sirgonium is like an element
Speaker 1: like carbon or irn. Yes, Zirconium is an element, and
Speaker 1: it's sort of like a silver metal. It's atomic number forty.
Speaker 1: It's you know, it doesn't really play a lot of
Speaker 1: roles and stuff other than like cubics urconium. It's not
Speaker 1: like a famous metal like some of its friends and
Speaker 1: neighbors um, but it plays an important role in aging rocks.
Speaker 1: So this element is all around us and to all
Speaker 1: the rocks, and it forms a special crystal. You're saying
Speaker 1: that when it forms, it likes to capture or uranium.
Speaker 1: That's right. It captures uranium and it really rejects lead.
Speaker 1: So the moment that the rock is created, it creates
Speaker 1: a special thing, which is zircon with just uranium in it.
Speaker 1: And you might wonder like, well, how is that helpful? Right? Well, uranium,
Speaker 1: as you might know, is radioactive. It doesn't just sit
Speaker 1: around and stay uranium forever. It very reliably decays and
Speaker 1: it decays into lead. But wait, why does the durk
Speaker 1: circon like uranium and white, doesn't it? You mean, like
Speaker 1: when it forms the crystal les, it forms a little
Speaker 1: structure and it just it just likes uranium like uranium
Speaker 1: likes being inside of zircon crystal. I mean, I don't
Speaker 1: want to get in the mind of uranium on the
Speaker 1: mind of zircon. I mean, they did some carbon dating
Speaker 1: online and they decided they were a good match. Um,
Speaker 1: but no, there's some chemical some business with the chemical bonds.
Speaker 1: You know, the way the crystal structure of zircon forms
Speaker 1: fits very nicely with uranium and doesn't fit very nicely
Speaker 1: with lead. Right, Yeah, So like a rock, you have
Speaker 1: a have a bunch of lava and it cools and
Speaker 1: it slowly turns into rock. And in that process is
Speaker 1: when you you're saying this crystal form, yeah, and zirkon
Speaker 1: is it doesn't have to be all over the rock.
Speaker 1: It's not like a whole rock full of zirkon. It
Speaker 1: is just these little chips of zircon. And what they
Speaker 1: do is they repel all the lead inside them and
Speaker 1: they grab some uranium and your uranium is like a clock.
Speaker 1: It decays very reliably. So every million years or so,
Speaker 1: some uranium turns into lead. And so after one million years,
Speaker 1: some fraction the uranium and then zircon has turned into lead.
Speaker 1: After two million years, another fraction has turned into lead.
Speaker 1: After a billion years, a very predictable fraction is turned
Speaker 1: into lead. So what you do is you take a rock,
Speaker 1: you look at that, you find some zircon crystals, and
Speaker 1: you ask how much of the uranium has turned into lead,
Speaker 1: and that tells you how much time it's had to
Speaker 1: turn into lead. It's kind of like a like an
Speaker 1: aging process, right like wine. If you leave wine there,
Speaker 1: it's gonna change, it's characteristic, it's gonna change into something
Speaker 1: else exactly, and you need something. That's why it works
Speaker 1: so well as a clock, because uranium is something that
Speaker 1: we know reliably. We understand it, We know how it decays,
Speaker 1: we know how long it takes, we know, you know,
Speaker 1: after a hundred thousand years, what fraction of uranium atoms
Speaker 1: will turn into lead. And the other key thing, of
Speaker 1: course is that it starts with a blank slate. That
Speaker 1: when you form these zircon crystals, it rejects lead. Right,
Speaker 1: So you start out like with an empty bucket, and
Speaker 1: then slowly the uranium bucket fills the lead bucket. And
Speaker 1: so you can just measure the uranium lead fraction and
Speaker 1: that tells you how long the uranium has been turning
Speaker 1: into lead. It's really amazing. It's it's really just chance
Speaker 1: that this happens, that somebody figured this out. So like
Speaker 1: if you find a rock and you find a little
Speaker 1: little zircon chip inside of it, and it's like, let's
Speaker 1: say it's full of uranium, then you know it's a
Speaker 1: pretty new rock exactly. But if it's full of lead,
Speaker 1: then you know it's really old rock exactly. It's exactly right.
Speaker 1: So rock start start out pure uranium, no lead, and
Speaker 1: then as they age, the uranium turns into lead. And
Speaker 1: so a rock with almost no uranium is going to
Speaker 1: be like super duper old, right, And new lead can't
Speaker 1: just like come in from the outside because now it's crystallized, right,
Speaker 1: It's hard for lead to get in any other way.
Speaker 1: All the only way you can get lead inside the
Speaker 1: zircon is for uranium to turn into lead. So the
Speaker 1: rock sort of you might say like it's spoils or
Speaker 1: ages by looking at that. Yeah, like you, it turns
Speaker 1: into lead right exactly, And whether it's spoils or ages
Speaker 1: be to fully depends on you know, your preference for
Speaker 1: how rocks taste. I suppose whether you swipe right or
Speaker 1: left for rocks. But you know, it took a long
Speaker 1: time to figure this out. People tried all sorts of
Speaker 1: other strategies first before they discovered this one. And there's
Speaker 1: lots of scientific careers and pH d PCs that were
Speaker 1: frustrated in people trying to figure out a way to
Speaker 1: do radiometric dating before people stumbled onto this one. Wow,
Speaker 1: and this is not one of these things that you
Speaker 1: um have to calibrate, right because I imagine you have
Speaker 1: probably very scientific information about the decay of uranium, right like,
Speaker 1: it's very physics space like. You don't have to calibrate
Speaker 1: it to anything else, do you. You're right, And we
Speaker 1: can basically use simple physics arguments to argue how long
Speaker 1: uranium takes to decay and to lead and uh, and
Speaker 1: so it gives us pretty clear descriptions, but you know,
Speaker 1: we also want to get confidence in it, and so
Speaker 1: what people do to have various other sort of radiometric
Speaker 1: methods not just uranium to lead, but other stuff, things
Speaker 1: that decay faster, things that decay slower, and so you
Speaker 1: can use those to sort of cross calibrate, but uranium
Speaker 1: to let this sort of the cleanest one. But you know,
Speaker 1: like everywhere in science, you want to check everything three
Speaker 1: different ways, um and then make sure you get consistent answers.
Speaker 1: All right, that's pretty cool. That's how we can tell
Speaker 1: how old the rock is. Man, rock can't lie. I
Speaker 1: can't lie about its age. That's right, that's right. You
Speaker 1: can lie, but we'll know you are. All right, Let's
Speaker 1: get into the answer now, Daniel, how old Earth is
Speaker 1: and how old the Solar system is. But first let's
Speaker 1: take another quick break. All right, Daniel, Let's let's break
Speaker 1: it down for people. How old is the Earth, or
Speaker 1: at least the oldest rocks on Earth. The oldest rocks
Speaker 1: on Earth come from Australia. And I don't know why,
Speaker 1: but that's not surprising to me. It seems like Australia
Speaker 1: is the land of extremes, and so you expect the craziest, weirdest,
Speaker 1: oldest rock to be in Australia. And they have the
Speaker 1: biggest spiders, the biggest crocodiles and the oldest rocks, the
Speaker 1: deadliest everything, the biggest, best stars and the best podcast
Speaker 1: listening fans. Right. Um, you know, we've got a lot
Speaker 1: of good email questions from Australia. But the oldest rock
Speaker 1: on Earth come from the jack Hills of Australia and
Speaker 1: it's four point zero four billion years old. So these
Speaker 1: are rocks that we do we have to dig up
Speaker 1: for them, dig for them or are they were they
Speaker 1: pretty close to the surface or yeah, it's it's fascinating.
Speaker 1: Actually these rocks just sort of sit on outcrops and
Speaker 1: people recently discovered that they have that the oldest rocks
Speaker 1: are there, and you know it's um. The Earth itself,
Speaker 1: of course, has had lots of geologic activity and so
Speaker 1: some of these really old rocks get buried and then
Speaker 1: there's geological activity that brings them up to the surface,
Speaker 1: and so it just happens to be at this spot
Speaker 1: on the Earth, these really old rocks got pushed up
Speaker 1: and so they're not that far underground there on these
Speaker 1: outcropping of rocks in Western Australia, but you almost kind
Speaker 1: of expect them to be close to the surface, right,
Speaker 1: because that's you know, I would imagine that if we
Speaker 1: were once a ball of lava, then the stuff sort
Speaker 1: of cools from the outside inwards. That's true. But you know,
Speaker 1: then there's all these sedimentary processes where old things get buried. Um.
Speaker 1: But then there's tectonic activity and all sorts of other
Speaker 1: crazy stuff that brings stuff up and mixes everything around. Um.
Speaker 1: And so there's a lot of stuff going on there.
Speaker 1: I think it's not that not that simple. Um. But
Speaker 1: in the end, the oldest thing we found on Earth
Speaker 1: is uh, is almost four and a half billion years old.
Speaker 1: Is four billion, four hundred and four million years old.
Speaker 1: And you know, that's a staggering answer. It's hard to
Speaker 1: really comprehend what that means. For billion years old, that's
Speaker 1: a lot. It's a it's a lot of dessert parties. Yeah,
Speaker 1: I mean, if theysed, a lot of party missed. Yeah.
Speaker 1: I think I think what you said earlier about how
Speaker 1: we came to the party pretty late. It's sort of shocking.
Speaker 1: You know, the first people to like learn these number,
Speaker 1: first to know that the Earth was billions of years old,
Speaker 1: it must have made humans feel sort of small and
Speaker 1: recent and uh. And you know, maybe transient tried like
Speaker 1: the Earth has had a long history and we are
Speaker 1: only the very last little bit of it. Yeah. I
Speaker 1: mean it's a difference between I mean human histories maybe
Speaker 1: what like ten thousand two years old? Yeah, a hundred
Speaker 1: thousand if you're really generous. Yeah, but the Earth has
Speaker 1: been around for four point four billion years old exactly.
Speaker 1: It's not too impressed. It's been there, It's seen a
Speaker 1: lot of stuff. You know, it'll be here, remember done. Um.
Speaker 1: You know, people always talk about how humans are going
Speaker 1: to destroy the Earth, and uh, I don't think that's likely.
Speaker 1: I think the humans might destroy ourselves or life on Earth,
Speaker 1: but the Earth will be here when we're done. Unless
Speaker 1: I hear some physics are trying to make black holes
Speaker 1: down at the Large Mattern Collider. Yeah, but we just
Speaker 1: want to understand the universe. Man, We're not threatening the world.
Speaker 1: There's no danger there. Trust me, And can I have
Speaker 1: ten more billion dollars to build a bigger one one
Speaker 1: year for every one daughter, for every year that the earth?
Speaker 1: That's right, That sounds good. Um, But you know, that's
Speaker 1: the oldest rock on Earth that we think is part
Speaker 1: of the Earth. It was part of the formation of
Speaker 1: the Earth. But of course we have found rocks on
Speaker 1: Earth that are meteorites, right, they came from space that
Speaker 1: are older than the Earth itself. So late comers to
Speaker 1: the party, that's right. They heard there was something cool
Speaker 1: happening down here in the third rock from the Sun,
Speaker 1: and they they've fell on in to figure it out.
Speaker 1: And those rocks are four point five six billion years old,
Speaker 1: So that's like a hundred and fifty million years older
Speaker 1: than the oldest rocks on Earth. That's the oldest meteorite
Speaker 1: that we found. And I think that you were saying
Speaker 1: that that kind of tells us what the oldest media
Speaker 1: right at all is in our solar system, right, Yeah,
Speaker 1: using the same logic, this is the oldest thing we found. Yeah,
Speaker 1: so it's probably the sort of the age of the
Speaker 1: Solar system. Right, that's when we think the gas and
Speaker 1: the dust got accumulated together to form the Sun and
Speaker 1: the planets and all that stuff. But the first age
Speaker 1: of that was to form smaller rocks, and so we
Speaker 1: think these things are remnants of those times. Right, But wait,
Speaker 1: didn't the Sun. Hasn't the Sun our son already been
Speaker 1: through a couple of cycles, Like, hasn't it exploded a
Speaker 1: few times already. Well, our son hasn't exploded, but it's materials, right,
Speaker 1: have been through several solar cycles. Everything around us, all
Speaker 1: that stuff is left over from one or two or
Speaker 1: three solar cycles of a sun lasting for you know,
Speaker 1: one to see three billion years and then blowing up
Speaker 1: and spreading its materials around. So everything around us is
Speaker 1: all all the components of the solar system are probably
Speaker 1: been recycled. But it's not like our son formed and
Speaker 1: then we started in formed and we started it was
Speaker 1: a different from a different star. Oh I see, So
Speaker 1: our current son is is maybe around four point five
Speaker 1: billion years old. Yeah, that's what we think. That's the
Speaker 1: history of our solar system. And remember, for context, the
Speaker 1: Milky Way is something like thirteen billion years old, and
Speaker 1: the universe is just a little bit older than that,
Speaker 1: almost fourteen billion years old. So there's a lot of
Speaker 1: history before even our son, our solar system was formed,
Speaker 1: and then a lot of history before we came around.
Speaker 1: And so like that's the cosmic context that we're standing in.
Speaker 1: And so that's the all this thing we found in
Speaker 1: the solar system, but that's older than all the things
Speaker 1: we found. That is part of the Earth. So somewhere
Speaker 1: between those two numbers is when maybe the ball of
Speaker 1: the Earth form. That's right. That's what we call the
Speaker 1: Joorge date, the Joge definition of the age of the Earth,
Speaker 1: the Bald date or his Bald date. That's right, that's right. Um.
Speaker 1: And so you know there's a window there about a
Speaker 1: hundred and fifty million years, which seems like a huge number,
Speaker 1: you know, that's like, but it's pretty small, I think
Speaker 1: compared to exactly. Geologically speaking, it's a blink of an eye, yeah,
Speaker 1: or universe speaking, it's it's pretty accurate, yeah, exactly. And uh,
Speaker 1: you know, we we know these numbers two plus or
Speaker 1: minus twenty five or fifty million years based on the
Speaker 1: uncertainty of radiometric dating and so um, because of the
Speaker 1: hard work of a lot of scientists to develop these
Speaker 1: things and across check them and understand them across various
Speaker 1: other processes. Now we've looked around us and we've gathered
Speaker 1: these clues from everyday objects around us, you know, the
Speaker 1: rocks at our feet, the rocks in Australia give us
Speaker 1: clues about these fantastic numbers that would just blow the
Speaker 1: minds of our ancestors and those clues were around for
Speaker 1: them to discover also, and so of course that makes
Speaker 1: you wonder, like, what clues are there laying around at
Speaker 1: our feet now that would tell us secrets of the
Speaker 1: universe that our descendants will know and wonder why we
Speaker 1: couldn't figure it out. It's not just like an opinion, right,
Speaker 1: It's pretty based on evidence, and it's pretty based on
Speaker 1: science and physics, the numbers. Yeah, exactly, these things we
Speaker 1: know pretty well. We have a lot of confidence in them.
Speaker 1: People have been working on these processes for decades and decades,
Speaker 1: a lot of you know, grumpy competition between rivals to
Speaker 1: make more and more accurate measurements. You know, this is
Speaker 1: not a conspiracy of scientists all working together in harmony.
Speaker 1: You know, this is a bunch of people racing to
Speaker 1: develop better and better techniques and trying to find older
Speaker 1: and older rocks. Remember, sciences a competitive field, and everybody
Speaker 1: wants to one up the other to get the more accurate,
Speaker 1: the more reliable, the more verifiable answer. So, yeah, we
Speaker 1: have a lot of confidence in this knowledge. Yeah, it's
Speaker 1: like it's all around the evidence, it's all around us,
Speaker 1: and you're you're standing on it. Yeah, exactly, and even
Speaker 1: if you can't do radiometric dating in your head of course,
Speaker 1: because you can't see the xericon crystals. You know, you
Speaker 1: just look around you and you see lots of things
Speaker 1: around you which take a long time to form. You know,
Speaker 1: how long does it take a mountain to form? It's
Speaker 1: a very slow process. And so all around us on
Speaker 1: the Earth we see evidence of very very slow processes
Speaker 1: which have accomplished a great deal, which tells you that
Speaker 1: they've been doing it for a long long time. Yeah,
Speaker 1: like you said, who knows what else is lying under
Speaker 1: our feet? That's right exactly, So I think we answered
Speaker 1: that question about the age of the earth. Yeah, yeah,
Speaker 1: so grab a glass of wine and sit back and enjoy.
Speaker 1: That's right. And uh, no matter how old you are,
Speaker 1: remember you can always enjoy one more brownie. See you
Speaker 1: next time. Thanks for tuning in, and thanks for asking
Speaker 1: great questions. This question was inspired by an email from
Speaker 1: a listener, So if you have a question you'd like
Speaker 1: to know the answer to, please send it to us
Speaker 1: at questions at Daniel and Jorge dot com. Before you
Speaker 1: still have a question after listening to all these explanations,
Speaker 1: please drop us a line. We'd love to hear from you.
Speaker 1: You can find us on Facebook, Twitter, and Instagram at
Speaker 1: Daniel and Jorge That's one Word, or email us at
Speaker 1: Feedback at Daniel and Jorge dot com. Thanks for listening
Speaker 1: and remember that Daniel and Jorge Explain the Universe is
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