August 6, 2019 • 41 mins
How do we know the age of the earth?
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Speaker 1 (00:08):
Hey, Daniel, do you keep a family photo album? You know,
we used to, but these days it's all digital. I
think Mark Zuckerberg owns all of our family photos. It
owns your family period. Probably that too. But and how
do you feel when you look back and see pictures
of yourself when you were younger? Um? I try to
ignore all those signs of aging, you know, pretend that
I looked the same ten or twenty years ago as
(00:29):
I do today. I see myself as a fine wine
because you have turned, because I just get more expensive
the older I get. For some reason. Well, you know,
I've known you for over ten years or so, and
I can I can say with confidence for those people
listening that you are so or no Paul Rudd. Unfortunately
(00:50):
that's true in many ways, unfortunately. But it is kind
of interesting how little clues can show you how old
someone is or how old something is. Right, It's true,
you know, a few wrinkles here and a few gray
hairs there, and all of a sudden, you look like
you're in your midlife crisis. What's that look like a
look of panic? It's a look of gravitas. I always
(01:13):
remember how dan Quail died his temples gray and which
not look so much like a little boy Dan quail Man.
That's a that's a reference that totally tells everyone how
old you are. Nobody's fascinating how almost everything around us
ages right. Everything from people around you, to the rocks
around you, um to the buildings around you always show
(01:37):
their signs of age. Nothing in this world is permanent.
Hi am poor handmade cartoonists and the creator of PhD comics. Hi.
(02:00):
I'm Daniel. I'm a particle physicist, and I'm young and spry,
and I have not written any web comics, but I'm
the co author of the book We Have No Idea
All about the unknown Questions in the Universe. And welcome
to our podcast, Daniel and Jorge Explain the Universe, a
production of I Heart Radio, in which we take weird
(02:20):
and amazing and young and old stuff in the universe
and try to explain it to you, and not just
telling you the answers, but sometimes digging in deeply and
explaining how we know so that you can explain it
to skeptical folks around you. Yeah, we try to get
you to think about the world around you and even
the world you're standing on right now or sitting on
(02:42):
or lying down on and one of the depending depending
on how old you are. But one of the goals
of this podcast is for you to have sort of
a new perspective on the world around you. We want
you to see everything around you as sort of a clue,
something that can tell you how the world works, that
can reveal secrets. I can answer questions because almost every
(03:02):
question we have about the universe, the answers are all
around us, and it just takes sort of a trained
mind and a careful eye to see those clues everywhere.
And so there are clues all around you about where
we come from and how long we've been here, and
how long everything has been here? Right, Yeah, exactly. And
sometimes answers are easy and obvious, and sometimes the answers
(03:23):
are mind mogglingly surprising. And so today on the podcast,
we'll be tackling, quite possibly the oldest question on earth.
Would you say that's true, Daniel, is that who stole
my dessert? Um? I think the answer is obvious, obviously,
it's your your children. Well you know, Um, that's actually
a question my kids asked me today. Um, when was
dessert invented? It sounds like a topic for a pope,
(03:46):
you know, if that falls into a physics question. But
it's sort of a fascinating question, and they were actually
hypothesizing that dessert might be older than people that maybe,
like our prehuman ancestors, enjoyed a sweet, sweet treat after dinner.
What do you think the little bacteria the good that
well the ocean? They're like, you know, I could use
a little two remy suit after consuming the other other bacteria.
(04:11):
I don't know if we have enough in common with
bacteria to say what they might like, but I can
imagine it's some pre human homaid, you know, eating some
berries and going and that goes nicely with my antelope
or whatever they had for dinner. Well anyway, so so weely. Uh.
What I mean by it's the oldest question on Earth
is that it's it's probably really the oldest question that
(04:31):
could be on Earth, right, Yeah, I think that's probably true,
And it's a it's an important question, right. It's the
kind of question that we'd like to dig into because
it's the kind of question where the answer could change
the way we think about life and the universe and
the humanity's rolling it right, it has all sorts of
things to do with with creation and the context of
our lives and therefore how we should live our lives
(04:53):
and whether they have meanings and so to be. On
the podcast, we'll be tackling the question how old is
the Earth? And is it even polite to ask? That's right?
And what does the earth do when he gets upset?
All right, when he gets offended? Um? And not just
(05:14):
how old had any worked? One? Has the Earth had
any work? One? Is it trying to? I think the
Earth looks great. Doesn't look a day past four billion? Um?
It is full of buetos. They're probably right, um. And
just as important as the question how old is the
Earth and the answer, which is you know, just a number,
is how do we know? How do we have confidence
(05:36):
in this answer? Because you know, there are people out
there who hold beliefs about the various age of the Earth.
And one of our listeners actually wrote in and said, hey,
could you tell me how we know the age of
the Earth, so that when I discussed with my friends
who have strong opinions about the age of the Earth,
I can tell them not just what scientists think, but
how they know it. Yeah, And that's the thing I
think we always try to do in this podcast, right,
(05:58):
is try to get at how people know these things,
not just tell people what scientists know, but how they
know it. That's right, And that also helps you understand
with what confidence we know it, Like is this just
sort of an educated guess the back of the envelope
calculation or is this something that people have been working
on for hundreds of years and we have great confidence
and have it nailed down to like point oh one percent.
(06:19):
And it's important because different things in science are are
known well and some things since science are are known
not very well. So it's important to know how we know. Yeah,
And so how old is the Earth is the question
for today, and it's kind of um, I think it's
a question most people know a little bit about, right.
I mean, I think everyone knows or has a sense
(06:40):
that the Earth is very old. Yeah, but you know,
that sense that the Earth is like extremely old, like
many orders of magnitude older than you or me or
your parents, you know, is a fairly modern idea. I
think thousands of years ago people thought the Earth was
only hundreds or thousands of years old. They thought it
(07:01):
might have been created fairly recently, and it was only
sort of in the eighteen hundreds that people started to
understand that there were processes around us that we could see,
specifically rocks and evolutions or geology and biology started to
give hints that the answer was much much bigger than
anything anybody had imagined. And I think that's wonderful. I
(07:22):
I love to live in a scientific era like that,
where you're where something you thought you knew is upended
and replaced with a completely different answer. That's like oors
a magnitude bigger. Wow, is that really true? Like it
only in the last couple of hundred years do we
have a sense of how old the Earth really is?
Before it was totally up in the air. Yeah, it
(07:44):
was up in the air until about the eighteen hundreds,
and then people started trying to do calculations and they're like, well,
how long would it take to form a planet, you know,
so you gathered it together with gravity, or you know,
how how long would it take to lay down layers
of rock? You know, how long would it take glaciers
to this kind of stuff? And you know, geology is
a fairly young field and that you know, it's a
(08:04):
couple of hundred years old and so people started to
look around, and as soon as they started to explore
these geological processes, they realized, wow, this stuff takes a
long time. And you know, it's sort of like um
walking into a room and seeing somebody has written a
book that's like five million pages long, and you're like, wow,
you've been in this room a long time, you know,
(08:26):
so it shocks you into realizing the things things are
a much different scale than you imagined. And that's the
we've seen the story the earth all around us. We
realize it's staggeringly long, much longer than we imagined, and that, yeah,
that's a fairly new idea. Yeah, that's a lot of
party you've missed exactly we should. Things have been around
(08:46):
for a long time. Do you have a fear of
missing out? There? Is that fomo about what happened? All
the good desserts? Fear of being in parties? Actually, to
be honest, what about all the desserts? The usually have
good dessertsive party old desserts or I just like I
just like a fine dessert. Wine? So you should you
show U at the beginning of the party, how the
(09:07):
desserts drink the wine and then go home? That's how
you know you're old or that was like a good
party to me. That is how your age old track exactly.
That's very age appropriate. Um. The beginning of this field
like becoming scientific before that was just speculation of the
beginning of the field. Becoming scientific was in the eighteen hundreds. Um.
But still that's very inaccurate. Like looking at rocks gives
(09:29):
you clues about the sort of order of magnitude, But
then people wanted to know very precisely, and that took
that took some time to to like basically defined clocks
inside the earth, clocks that have been ticking since it
was born. And that's in the end signs of aging, Yeah,
signs of aging, very precise signs of aging that could
tell us how old the earth was. So you're saying, like,
you know, the people who started the United States, like
(09:52):
the founding father fathers, like the Founding Fathers, they had
no idea how old we've been around, Like did they
think we've been around for a few thousand years? For
a few but didn't they know about the you know,
like ancient Egypt and things like that. Yeah, but you
know that's thousands of years, and so I think they
had the idea that the Earth was, you know, thousands,
(10:12):
many thousands of years old, right, because that's as long
as written history is, and so they imagine it's on
that time scale because that's the oldest thing, you know.
And that's basically the game is like find the oldest
thing you know and then assume that that's basically the
age of the world, right, And that's frankly what we're
still doing, um, except that we think that we found
stuff that comes from the very beginning of the world.
(10:34):
But you know, they knew about stuff that was thousands
of years old, so they had no reason to believe
that the Earth might be millions or even God forbid,
billions of years old, right. Plus I think those guys
are pretty religious, all right. So it is an age
old question and one that a lot of people have
had answers to and seems seemed to have answers to.
But we were wondering what people out there right now
(10:56):
in the world think about this question. Yeah, and this
is one this is a set of interviews. I was
very curious to see what people thought. I was wondering
if I would run into some sort of young Earth
folks who thought the Earth was from biblical ages and uh,
and even those people who were scientific. I was wondering, like,
how precisely do people know the age of the Earth.
So this I was very fascinating to hear the answers
(11:17):
like is it a million years, a bazillion years, twenty
seven point two bazillion? And so, as usual, Daniel went
out and asked people on the street how old they
thought the Earth was. So think about it for a second.
If you ran into a physicist in the street, uh today,
how would you answer this question? And no googling. Well,
(11:38):
here's what people had to say. Um, I have no
idea probably billion years. I don't exactly, but my understanding
is it's a couple of hundred million years old. Okay,
you know how we know? So I don't actually like
my my physics knowledge is pretty limited. Um. I think
like we've used carbon dating to date things within various
geological epox but I don't actually know how we arrived
(12:02):
at the calculation of the Earth stage and maybe three
billion years, tens of million years, four billion years, and
how do we know rock formation? It's probably like millions
of years old. I feel like, all right, Um, a
lot of millions and billions, right, yeah, exactly, not not
(12:24):
a whole lot of really specific answers. I have to
say I was a little bit disappointed. I guess I
thought people were more interested in this, Like I remember
wanting to know this number as precisely as possible when
I was a kid. I was really curious about this, um,
and so I was a little surprised that people were
just sort of knew it was super old and were
satisfied and didn't really care about the details. Although I'll
say every single person here after I asked them the
(12:46):
question then they did turn around and ask me. They're like, well,
how old is the Earth? So they wanted to know
abous just hadn't really spend any time looking it up.
All right, Well, let's get started on this topic, Daniel
and so, um. First of all, I think I feel
like we should define what we mean by the Earth
by the age of the Earth, right, Like, um, do
you mean like the the planet, like the ball that
(13:07):
it's Earth, or the rocks in the Earth, or the
atoms that are that make up the Earth? What do
you mean, like what's the age of the Earth. Yeah,
it turns out that that's a bit of a fuzzy question, right,
because the Earth has a sort of continuous history. It's
not like somebody snapped their fingers and boom, there was
the Earth. And you can start from the clock from then, right, Um,
(13:28):
that you know of, right, that we know of, that's right, unless,
of course it was a simulation, which case things is easy.
But in the case that it wasn't the simulation, we
have an idea about how the Earth was formed, and
there's just a lot of stages there, and so you
have to sort of make an arbitrary definition and into
like a lot of things in science, you know, Um,
there's not necessarily a clear and sower. So you just
(13:49):
define it to be something arbitrary and at least we
can agree on it. But maybe we should recap sort
of the brief history of the of the Earth and
how it was formed, yeah, like how it how it
came to be. Yeah, And then at some point we're
gonna say, this is when the earth started, this is
when the Earth was born. That's right. This is when
the party began, right, um, Because you know, you invite
people to party at nine o'clock and then they don't
show up till ten, and by then you've eating all
(14:10):
the brownies, um, and parties only really get started at
one a m. That's right, unless you're in Spain, in
which cases four am um. But you know, the very
early history is that we basically started with a huge
nebula of gas and dust. And this is stuff that's
left over from the explosions of other stars and just
gas from the Big Bang. And then of course gravity
(14:31):
did its thing and it pulls out all that stuff
together and you get a sun, right, So most of
the stuff went to the Sun, and then you get
a disc of stuff that's rotating around the Sun. It's
like rings around the Sun. And then the gravity pulls
the stuff in that disc together to turning to turn
that disc into planets, right, And uh so we think
that the Solar system started about four and a half
(14:53):
billion years ago, and it took like a few million
or a hundred million years to pull all that stuff
and the disk together into sort of protoplanets. Right. So
you have this sort of loose collection of stuff that
gravity is pulled together, and then gravity really gets to
work and it starts squeezing it and it collects all
this stuff and the planet heats up and sort of
(15:15):
melts the metal and you get the iron dropping to
the core and the crust forming, and so you can
sort of ask, like, what moment do you count the
earth forming? Is it like when you have the first
big cluster of rocks or maybe when it got gravitationally
heated enough to sort of melt the metals and form
the core um It's not really very clear. Well, I
(15:38):
always find this really fascinating because we we talked about
this the other day, that something like the rings around
Saturn are only temporary, right, Like at some point those
rings might turn into little blobs. That's right. Gravity is
very weak but very very patient, and so it just
keeps working and you give it enough time, it will
gather things together. And so this stuff that's orbiting Saturn
(16:01):
eventually grab gravity will pull it together in two moons.
Even the asteroid belt, is that at some point gonna
turn into more planets for our Solar system? Yeah, eventually.
Gravity it means very slow and those things are mostly
stable orbits, but eventually that stuff will get pulled together.
I mean also there's disruption. Right if you just left
the Solar system all by itself, then yes, eventually you'll
(16:22):
just gather together into a smaller and smaller number of objects.
But this disruption from stuff that comes outside, you know
and comets things from the word cloud. Um even even
things like supernovas from other solar system can disrupt things
happening in our solar system. Um. So, But yes, eventually
gravity will pull all this stuff together. It's really incredible
what over over these time scales, what gravity can do.
(16:45):
And that's one of my favorite things. It's just started
realizing that these processes are really slow, so things must
have been around for a super long time to make
it happen, right right, Well, I guess I would maybe
count that the birth of the Earth is when it
was form, you know, when all those asteroids and rocks
out there clumped together into a ball about the size
(17:06):
of the ball that we have now, like, you know,
when when it kind of snapped into shape. I would
maybe count that as the age of when the Earth started. Yeah, yeah,
and that's totally reasonable. That's totally reasonable. Um. I think
one common definition is that you measured the age of
the Earth is sort of the age of the oldest
rock in the Earth. And then you can ask the question,
(17:27):
what does it mean for a rock to be old.
Like you know, a rock is just a gravitational pulled
together clump of dust. But rocks are are formed right
there through melt and then they're cooled. And so the
age of a rock is usually like the last time
it was melted, right, So it's the time since it
was last melted, and so a lot of time people
just like the time that it cooled off, the last
(17:48):
time it really chilled out. And so you can imagine
like all these rocks came together, as you said, and
formed the Earth and got squeezed together and probably melted
and then cooled right, and the crust at least cooled,
and so you can asked, like maybe that last moment
the last moment of cooling of the crust. So the
age of the oldest cooled rock on the Earth could
be a definition at the age of the Earth, because otherwise,
(18:09):
how do you know when these rocks came together? Right?
Otherwise I don't know unless you have, like you know,
CCTV of the formation of the Solar system. It's pretty
hard to pin that down. Or you could say, is
it well, but you just gave me a number you
said that it was maybe it took around a hundred
million years to to form that the ball of the Earth. Yeah,
that's you know, I said, I said between a few
(18:31):
and a hundred million years, right, So that's a pretty
big uncertainty. Um, we don't really know how long it
took and and what moment do you define, like do
you needed one more rock? Because you know, caustic stuff
kept getting added to the Earth, and so some people say, well,
the age of the Earth is sort of just just
just call it the age of the Solar System because
that's pretty close to the age of the Earth. Or
you could say it's the age of the the oldest
(18:54):
rock on the Earth. So I think both of those
are sort of acceptable. And anyway, that's really all we
can probe and end. What we do is we measured
the age of the oldest rock we can find on
the Earth, and we say that's pretty close to the
age of the Earth. So the time that it formed
into a ball is kind of in between those two numbers, right, Yeah, exactly,
it's between when the Solar System formed and when the
(19:16):
ball that is the Earth started getting crusty. Exactly exactly
when it started getting crusty and grumpy and eating desserted
parties and going home early like a teenager. The suwhere
between conception and teenage years that's when you're born. That's
that's right. And you know, we have the same question
about people, right, I wonder about that sometimes. You know, Um,
(19:38):
how do you count somebody's age if there if somebody
is born premature, they're older. You know, even then somebody
who was born it was conceived at the same moment
but then born at full term. So even the age
of people is sort of hard to define sometimes. All right, well,
we've determined, we've determined what we mean by the age
of the Earth, and so now let's get into how
we know how old the Earth is and what what
(20:00):
that means. But first let's take a quick break. Okay,
so we're defining the age of the Earth is how
old the oldest rock was formed that we know about
(20:23):
right on Earth. That's right, exactly. Let's say, so when
the Earth was like a ball of lava, that doesn't count.
But once it started solidifying, then we're saying that's when
the Earth was born. Yeah, exactly, because we're imagining that, Yeah,
we're imagining that with that the formation, the Earth probably
melted a lot of the rocks because there was this
early lava period as you as you mentioned, and so
(20:44):
as it's cooled down and formed the crust, then it
made rocks and and it turns out we can figure
out how old a rocky is, and so that's really
our best handle in figuring out how old the Earth is.
And in the end it's a lower bound, right we say, well,
here's an old, super old rock on the Earth, so
that Earth must be at least that old, because like
we've talked about, like you mentioned, like um, trying to
(21:05):
figure out how old Earth is, the only thing we
can do is sort of look around and see how
old the things in it are, right, and like, what's
the oldest thing in it? We know it has to
be at least that old. Yeah, yeah, exactly. That's about
all we can do. And you know what you can
do otherwise is sort of form of theory. You say,
I have a concept for how this might have worked,
and then you can fit data to and say, well,
(21:27):
I expect you know that to see these sort of
rocks in that case, and you can use that a
theory to sort of extrapolate a little bit past the
day to you you observe. But that's very but that's speculative, right,
it's based on your concept of how things happen. So
the hardest number, the one that you know best, is
the oldest thing that you can find. And the oldest
thing we find our rocks, and and so what we
(21:49):
do is we try to turn one question into another question.
The question we can answer is what's the age of
the oldest rock on the Earth. Wouldn't that tell us
like if we found a really old raw, what if
that rock came from before the Earth was formed? Do
you know what I mean? Yeah, yeah, there are some
of those actually, like meteorites, Right, meteorites tell us the
age of the Solar system because we think meteorites were
(22:12):
formed in the beginning of the Solar system. Right, the
first stage is gathered together gas and dust into rocks, right,
And so those rocks from space tell us the age
of the Solar System. But we can tell which rocks
came from space and which rocks are from Earth, And
so we can distinguish those two. How can we tell? Oh,
they're clearly labeled. You know, they have their turn address, right,
(22:35):
it says somewhere near Neptune Um. It's it's actually quite
interesting that the so the distribution of metals in these
rocks is very different from meteorites than it is for
rocks on the Earth. It's like a different group of rocks. Yeah,
it's like they have more nickel in them where they
have this other rare stuff. And that's a whole other
fascinating field. Um. And also, so these rocks are different
(22:58):
from Earth, and they tell us like what else is
out there in the Solar System and what was out
there when the Solar system was formed? There like little
time capsules from the very beginning of the Solar system.
And they actually tell us another interesting story. Right, So
those rocks, we can tell how old they are, and
they tell us how old the Solar system is. So
we can answer two questions. We can find a bunch
of rocks. We can say, oh, these came from somewhere
in the Solar System. There x y z old. These
(23:20):
rocks are here on Earth, they're only you know, ABC
old all right. So then so it all comes down
to the question of how do you tell how old
the rock is, yeah, right, exactly exactly, or a meteorite
which is the same, which is the same because the
meteorite is just basically a rock from space that landed
on Earth. And the way we do it is something
called radiometric dating, which is not the same as carbon dating, right,
(23:41):
but it works in which is not the same as
online dating either. It turns out to be actually kind
of similar. There's a lot of swiping, there's a lot
of lying about your age as well. Um. No, carbon
dating and radio metric dating have similar principles, and we
should have a whole podcast episode of how carbon dating works.
Carbondating relates to organic stuff like living beings. Radiometric dating
(24:05):
relates to how how old rocks are. Okay, so you
find a rock and you're you're staring at it, and
you're like, I wonder when this rock became a rock? Yeah,
exactly when this turn from lava to this heavy thing
that I'm holding, And so you can we can actually tell, right,
we can look at the stuff in it and tell
how old window was formed, how many millions of years
(24:26):
ago exactly? And I have so much respect for scientists
who develop these techniques. You know, they look at the
rock and they say, I want to know how old
this rock is. How could I tell? Can I find
something inside the rock that I can use as a clock,
something which changes reliably and steadily every year, so that
I can count it and and project back and figure
(24:47):
out when that clock started or something. And this is
hard to do, you know. You have to find something
which is steady and reliable and can be calibrated. Um,
it's not trivial, right. You can't just say, oh, I'm
just gonna your X Y Z property. You have to
invent these techniques. And it takes time and sweat and tears,
and so I have so much respect for folks who
(25:08):
did this. And and in the case of rocks, it
turns out to be very dependent on the special kind
of mineral called zircon. Zircon like the fake diamond like, yeah,
it's related exactly. Zirconium and zircon is a special kind
of mineral. And it's special because it has two properties. One,
it likes it's a crystal, and it likes to absorb
(25:29):
uranium into it right, gobbles up uranium. So when the
rock forms, if it has some zircon, then it will
grab any uranium that's around it. And the second important property,
but wait, what is zircon made out of? Is it?
What is it like a made out of iron or
what is it carbon? What is this mineral made out
of it? I think it's made out of leftover desserts. No,
(25:53):
it's it's a mineral um. It's made out of zirconium
and silicon and oxygen. Oh Sirgonium is like an element
like carbon or irn. Yes, Zirconium is an element, and
it's sort of like a silver metal. It's atomic number forty.
It's you know, it doesn't really play a lot of
roles and stuff other than like cubics urconium. It's not
(26:15):
like a famous metal like some of its friends and
neighbors um, but it plays an important role in aging rocks.
So this element is all around us and to all
the rocks, and it forms a special crystal. You're saying
that when it forms, it likes to capture or uranium.
That's right. It captures uranium and it really rejects lead.
So the moment that the rock is created, it creates
(26:37):
a special thing, which is zircon with just uranium in it.
And you might wonder like, well, how is that helpful? Right? Well, uranium,
as you might know, is radioactive. It doesn't just sit
around and stay uranium forever. It very reliably decays and
it decays into lead. But wait, why does the durk
circon like uranium and white, doesn't it? You mean, like
(26:58):
when it forms the crystal les, it forms a little
structure and it just it just likes uranium like uranium
likes being inside of zircon crystal. I mean, I don't
want to get in the mind of uranium on the
mind of zircon. I mean, they did some carbon dating
online and they decided they were a good match. Um,
but no, there's some chemical some business with the chemical bonds.
(27:19):
You know, the way the crystal structure of zircon forms
fits very nicely with uranium and doesn't fit very nicely
with lead. Right, Yeah, So like a rock, you have
a have a bunch of lava and it cools and
it slowly turns into rock. And in that process is
when you you're saying this crystal form, yeah, and zirkon
is it doesn't have to be all over the rock.
(27:39):
It's not like a whole rock full of zirkon. It
is just these little chips of zircon. And what they
do is they repel all the lead inside them and
they grab some uranium and your uranium is like a clock.
It decays very reliably. So every million years or so,
some uranium turns into lead. And so after one million years,
some fraction the uranium and then zircon has turned into lead.
(28:02):
After two million years, another fraction has turned into lead.
After a billion years, a very predictable fraction is turned
into lead. So what you do is you take a rock,
you look at that, you find some zircon crystals, and
you ask how much of the uranium has turned into lead,
and that tells you how much time it's had to
turn into lead. It's kind of like a like an
(28:23):
aging process, right like wine. If you leave wine there,
it's gonna change, it's characteristic, it's gonna change into something
else exactly, and you need something. That's why it works
so well as a clock, because uranium is something that
we know reliably. We understand it, We know how it decays,
we know how long it takes, we know, you know,
after a hundred thousand years, what fraction of uranium atoms
(28:44):
will turn into lead. And the other key thing, of
course is that it starts with a blank slate. That
when you form these zircon crystals, it rejects lead. Right,
So you start out like with an empty bucket, and
then slowly the uranium bucket fills the lead bucket. And
so you can just measure the uranium lead fraction and
that tells you how long the uranium has been turning
into lead. It's really amazing. It's it's really just chance
(29:07):
that this happens, that somebody figured this out. So like
if you find a rock and you find a little
little zircon chip inside of it, and it's like, let's
say it's full of uranium, then you know it's a
pretty new rock exactly. But if it's full of lead,
then you know it's really old rock exactly. It's exactly right.
So rock start start out pure uranium, no lead, and
(29:28):
then as they age, the uranium turns into lead. And
so a rock with almost no uranium is going to
be like super duper old, right, And new lead can't
just like come in from the outside because now it's crystallized, right,
It's hard for lead to get in any other way.
All the only way you can get lead inside the
zircon is for uranium to turn into lead. So the
(29:49):
rock sort of you might say like it's spoils or
ages by looking at that. Yeah, like you, it turns
into lead right exactly, And whether it's spoils or ages
be to fully depends on you know, your preference for
how rocks taste. I suppose whether you swipe right or
left for rocks. But you know, it took a long
time to figure this out. People tried all sorts of
(30:10):
other strategies first before they discovered this one. And there's
lots of scientific careers and pH d PCs that were
frustrated in people trying to figure out a way to
do radiometric dating before people stumbled onto this one. Wow,
and this is not one of these things that you
um have to calibrate, right because I imagine you have
probably very scientific information about the decay of uranium, right like,
(30:35):
it's very physics space like. You don't have to calibrate
it to anything else, do you. You're right, And we
can basically use simple physics arguments to argue how long
uranium takes to decay and to lead and uh, and
so it gives us pretty clear descriptions, but you know,
we also want to get confidence in it, and so
what people do to have various other sort of radiometric
(30:58):
methods not just uranium to lead, but other stuff, things
that decay faster, things that decay slower, and so you
can use those to sort of cross calibrate, but uranium
to let this sort of the cleanest one. But you know,
like everywhere in science, you want to check everything three
different ways, um and then make sure you get consistent answers.
All right, that's pretty cool. That's how we can tell
how old the rock is. Man, rock can't lie. I
(31:21):
can't lie about its age. That's right, that's right. You
can lie, but we'll know you are. All right, Let's
get into the answer now, Daniel, how old Earth is
and how old the Solar system is. But first let's
take another quick break. All right, Daniel, Let's let's break
(31:48):
it down for people. How old is the Earth, or
at least the oldest rocks on Earth. The oldest rocks
on Earth come from Australia. And I don't know why,
but that's not surprising to me. It seems like Australia
is the land of extremes, and so you expect the craziest, weirdest,
oldest rock to be in Australia. And they have the
biggest spiders, the biggest crocodiles and the oldest rocks, the
(32:12):
deadliest everything, the biggest, best stars and the best podcast
listening fans. Right. Um, you know, we've got a lot
of good email questions from Australia. But the oldest rock
on Earth come from the jack Hills of Australia and
it's four point zero four billion years old. So these
are rocks that we do we have to dig up
for them, dig for them or are they were they
(32:34):
pretty close to the surface or yeah, it's it's fascinating.
Actually these rocks just sort of sit on outcrops and
people recently discovered that they have that the oldest rocks
are there, and you know it's um. The Earth itself,
of course, has had lots of geologic activity and so
some of these really old rocks get buried and then
there's geological activity that brings them up to the surface,
(32:54):
and so it just happens to be at this spot
on the Earth, these really old rocks got pushed up
and so they're not that far underground there on these
outcropping of rocks in Western Australia, but you almost kind
of expect them to be close to the surface, right,
because that's you know, I would imagine that if we
were once a ball of lava, then the stuff sort
of cools from the outside inwards. That's true. But you know,
(33:16):
then there's all these sedimentary processes where old things get buried. Um.
But then there's tectonic activity and all sorts of other
crazy stuff that brings stuff up and mixes everything around. Um.
And so there's a lot of stuff going on there.
I think it's not that not that simple. Um. But
in the end, the oldest thing we found on Earth
is uh, is almost four and a half billion years old.
Is four billion, four hundred and four million years old.
(33:39):
And you know, that's a staggering answer. It's hard to
really comprehend what that means. For billion years old, that's
a lot. It's a it's a lot of dessert parties. Yeah,
I mean, if theysed, a lot of party missed. Yeah.
I think I think what you said earlier about how
we came to the party pretty late. It's sort of shocking.
You know, the first people to like learn these number,
(34:00):
first to know that the Earth was billions of years old,
it must have made humans feel sort of small and
recent and uh. And you know, maybe transient tried like
the Earth has had a long history and we are
only the very last little bit of it. Yeah. I
mean it's a difference between I mean human histories maybe
what like ten thousand two years old? Yeah, a hundred
(34:20):
thousand if you're really generous. Yeah, but the Earth has
been around for four point four billion years old exactly.
It's not too impressed. It's been there, It's seen a
lot of stuff. You know, it'll be here, remember done. Um.
You know, people always talk about how humans are going
to destroy the Earth, and uh, I don't think that's likely.
I think the humans might destroy ourselves or life on Earth,
(34:41):
but the Earth will be here when we're done. Unless
I hear some physics are trying to make black holes
down at the Large Mattern Collider. Yeah, but we just
want to understand the universe. Man, We're not threatening the world.
There's no danger there. Trust me, And can I have
ten more billion dollars to build a bigger one one
year for every one daughter, for every year that the earth?
(35:03):
That's right, That sounds good. Um, But you know, that's
the oldest rock on Earth that we think is part
of the Earth. It was part of the formation of
the Earth. But of course we have found rocks on
Earth that are meteorites, right, they came from space that
are older than the Earth itself. So late comers to
the party, that's right. They heard there was something cool
happening down here in the third rock from the Sun,
(35:24):
and they they've fell on in to figure it out.
And those rocks are four point five six billion years old,
So that's like a hundred and fifty million years older
than the oldest rocks on Earth. That's the oldest meteorite
that we found. And I think that you were saying
that that kind of tells us what the oldest media
right at all is in our solar system, right, Yeah,
(35:46):
using the same logic, this is the oldest thing we found. Yeah,
so it's probably the sort of the age of the
Solar system. Right, that's when we think the gas and
the dust got accumulated together to form the Sun and
the planets and all that stuff. But the first age
of that was to form smaller rocks, and so we
think these things are remnants of those times. Right, But wait,
didn't the Sun. Hasn't the Sun our son already been
(36:08):
through a couple of cycles, Like, hasn't it exploded a
few times already. Well, our son hasn't exploded, but it's materials, right,
have been through several solar cycles. Everything around us, all
that stuff is left over from one or two or
three solar cycles of a sun lasting for you know,
one to see three billion years and then blowing up
and spreading its materials around. So everything around us is
(36:31):
all all the components of the solar system are probably
been recycled. But it's not like our son formed and
then we started in formed and we started it was
a different from a different star. Oh I see, So
our current son is is maybe around four point five
billion years old. Yeah, that's what we think. That's the
history of our solar system. And remember, for context, the
(36:51):
Milky Way is something like thirteen billion years old, and
the universe is just a little bit older than that,
almost fourteen billion years old. So there's a lot of
history before even our son, our solar system was formed,
and then a lot of history before we came around.
And so like that's the cosmic context that we're standing in.
(37:12):
And so that's the all this thing we found in
the solar system, but that's older than all the things
we found. That is part of the Earth. So somewhere
between those two numbers is when maybe the ball of
the Earth form. That's right. That's what we call the
Joorge date, the Joge definition of the age of the Earth,
the Bald date or his Bald date. That's right, that's right. Um.
(37:33):
And so you know there's a window there about a
hundred and fifty million years, which seems like a huge number,
you know, that's like, but it's pretty small, I think
compared to exactly. Geologically speaking, it's a blink of an eye, yeah,
or universe speaking, it's it's pretty accurate, yeah, exactly. And uh,
you know, we we know these numbers two plus or
(37:53):
minus twenty five or fifty million years based on the
uncertainty of radiometric dating and so um, because of the
hard work of a lot of scientists to develop these
things and across check them and understand them across various
other processes. Now we've looked around us and we've gathered
these clues from everyday objects around us, you know, the
rocks at our feet, the rocks in Australia give us
(38:15):
clues about these fantastic numbers that would just blow the
minds of our ancestors and those clues were around for
them to discover also, and so of course that makes
you wonder, like, what clues are there laying around at
our feet now that would tell us secrets of the
universe that our descendants will know and wonder why we
couldn't figure it out. It's not just like an opinion, right,
(38:35):
It's pretty based on evidence, and it's pretty based on
science and physics, the numbers. Yeah, exactly, these things we
know pretty well. We have a lot of confidence in them.
People have been working on these processes for decades and decades,
a lot of you know, grumpy competition between rivals to
make more and more accurate measurements. You know, this is
not a conspiracy of scientists all working together in harmony.
(38:58):
You know, this is a bunch of people racing to
develop better and better techniques and trying to find older
and older rocks. Remember, sciences a competitive field, and everybody
wants to one up the other to get the more accurate,
the more reliable, the more verifiable answer. So, yeah, we
have a lot of confidence in this knowledge. Yeah, it's
like it's all around the evidence, it's all around us,
and you're you're standing on it. Yeah, exactly, and even
(39:19):
if you can't do radiometric dating in your head of course,
because you can't see the xericon crystals. You know, you
just look around you and you see lots of things
around you which take a long time to form. You know,
how long does it take a mountain to form? It's
a very slow process. And so all around us on
the Earth we see evidence of very very slow processes
which have accomplished a great deal, which tells you that
(39:42):
they've been doing it for a long long time. Yeah,
like you said, who knows what else is lying under
our feet? That's right exactly, So I think we answered
that question about the age of the earth. Yeah, yeah,
so grab a glass of wine and sit back and enjoy.
That's right. And uh, no matter how old you are,
(40:03):
remember you can always enjoy one more brownie. See you
next time. Thanks for tuning in, and thanks for asking
great questions. This question was inspired by an email from
a listener, So if you have a question you'd like
to know the answer to, please send it to us
at questions at Daniel and Jorge dot com. Before you
(40:28):
still have a question after listening to all these explanations,
please drop us a line. We'd love to hear from you.
You can find us on Facebook, Twitter, and Instagram at
Daniel and Jorge That's one Word, or email us at
Feedback at Daniel and Jorge dot com. Thanks for listening
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio from More podcast from
(40:50):
my Heart Radio. Visit the I heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. Nine
Hey, Daniel, do you keep a family photo album? You know,
we used to, but these days it's all digital. I
think Mark Zuckerberg owns all of our family photos. It
owns your family period. Probably that too. But and how
do you feel when you look back and see pictures
of yourself when you were younger? Um? I try to
ignore all those signs of aging, you know, pretend that
I looked the same ten or twenty years ago as
(00:29):
I do today. I see myself as a fine wine
because you have turned, because I just get more expensive
the older I get. For some reason. Well, you know,
I've known you for over ten years or so, and
I can I can say with confidence for those people
listening that you are so or no Paul Rudd. Unfortunately
(00:50):
that's true in many ways, unfortunately. But it is kind
of interesting how little clues can show you how old
someone is or how old something is. Right, It's true,
you know, a few wrinkles here and a few gray
hairs there, and all of a sudden, you look like
you're in your midlife crisis. What's that look like a
look of panic? It's a look of gravitas. I always
(01:13):
remember how dan Quail died his temples gray and which
not look so much like a little boy Dan quail Man.
That's a that's a reference that totally tells everyone how
old you are. Nobody's fascinating how almost everything around us
ages right. Everything from people around you, to the rocks
around you, um to the buildings around you always show
(01:37):
their signs of age. Nothing in this world is permanent.
Hi am poor handmade cartoonists and the creator of PhD comics. Hi.
(02:00):
I'm Daniel. I'm a particle physicist, and I'm young and spry,
and I have not written any web comics, but I'm
the co author of the book We Have No Idea
All about the unknown Questions in the Universe. And welcome
to our podcast, Daniel and Jorge Explain the Universe, a
production of I Heart Radio, in which we take weird
(02:20):
and amazing and young and old stuff in the universe
and try to explain it to you, and not just
telling you the answers, but sometimes digging in deeply and
explaining how we know so that you can explain it
to skeptical folks around you. Yeah, we try to get
you to think about the world around you and even
the world you're standing on right now or sitting on
(02:42):
or lying down on and one of the depending depending
on how old you are. But one of the goals
of this podcast is for you to have sort of
a new perspective on the world around you. We want
you to see everything around you as sort of a clue,
something that can tell you how the world works, that
can reveal secrets. I can answer questions because almost every
(03:02):
question we have about the universe, the answers are all
around us, and it just takes sort of a trained
mind and a careful eye to see those clues everywhere.
And so there are clues all around you about where
we come from and how long we've been here, and
how long everything has been here? Right, Yeah, exactly. And
sometimes answers are easy and obvious, and sometimes the answers
(03:23):
are mind mogglingly surprising. And so today on the podcast,
we'll be tackling, quite possibly the oldest question on earth.
Would you say that's true, Daniel, is that who stole
my dessert? Um? I think the answer is obvious, obviously,
it's your your children. Well you know, Um, that's actually
a question my kids asked me today. Um, when was
dessert invented? It sounds like a topic for a pope,
(03:46):
you know, if that falls into a physics question. But
it's sort of a fascinating question, and they were actually
hypothesizing that dessert might be older than people that maybe,
like our prehuman ancestors, enjoyed a sweet, sweet treat after dinner.
What do you think the little bacteria the good that
well the ocean? They're like, you know, I could use
a little two remy suit after consuming the other other bacteria.
(04:11):
I don't know if we have enough in common with
bacteria to say what they might like, but I can
imagine it's some pre human homaid, you know, eating some
berries and going and that goes nicely with my antelope
or whatever they had for dinner. Well anyway, so so weely. Uh.
What I mean by it's the oldest question on Earth
is that it's it's probably really the oldest question that
(04:31):
could be on Earth, right, Yeah, I think that's probably true,
And it's a it's an important question, right. It's the
kind of question that we'd like to dig into because
it's the kind of question where the answer could change
the way we think about life and the universe and
the humanity's rolling it right, it has all sorts of
things to do with with creation and the context of
our lives and therefore how we should live our lives
(04:53):
and whether they have meanings and so to be. On
the podcast, we'll be tackling the question how old is
the Earth? And is it even polite to ask? That's right?
And what does the earth do when he gets upset?
All right, when he gets offended? Um? And not just
(05:14):
how old had any worked? One? Has the Earth had
any work? One? Is it trying to? I think the
Earth looks great. Doesn't look a day past four billion? Um?
It is full of buetos. They're probably right, um. And
just as important as the question how old is the
Earth and the answer, which is you know, just a number,
is how do we know? How do we have confidence
(05:36):
in this answer? Because you know, there are people out
there who hold beliefs about the various age of the Earth.
And one of our listeners actually wrote in and said, hey,
could you tell me how we know the age of
the Earth, so that when I discussed with my friends
who have strong opinions about the age of the Earth,
I can tell them not just what scientists think, but
how they know it. Yeah, And that's the thing I
think we always try to do in this podcast, right,
(05:58):
is try to get at how people know these things,
not just tell people what scientists know, but how they
know it. That's right, And that also helps you understand
with what confidence we know it, Like is this just
sort of an educated guess the back of the envelope
calculation or is this something that people have been working
on for hundreds of years and we have great confidence
and have it nailed down to like point oh one percent.
(06:19):
And it's important because different things in science are are
known well and some things since science are are known
not very well. So it's important to know how we know. Yeah,
And so how old is the Earth is the question
for today, and it's kind of um, I think it's
a question most people know a little bit about, right.
I mean, I think everyone knows or has a sense
(06:40):
that the Earth is very old. Yeah, but you know,
that sense that the Earth is like extremely old, like
many orders of magnitude older than you or me or
your parents, you know, is a fairly modern idea. I
think thousands of years ago people thought the Earth was
only hundreds or thousands of years old. They thought it
(07:01):
might have been created fairly recently, and it was only
sort of in the eighteen hundreds that people started to
understand that there were processes around us that we could see,
specifically rocks and evolutions or geology and biology started to
give hints that the answer was much much bigger than
anything anybody had imagined. And I think that's wonderful. I
(07:22):
I love to live in a scientific era like that,
where you're where something you thought you knew is upended
and replaced with a completely different answer. That's like oors
a magnitude bigger. Wow, is that really true? Like it
only in the last couple of hundred years do we
have a sense of how old the Earth really is?
Before it was totally up in the air. Yeah, it
(07:44):
was up in the air until about the eighteen hundreds,
and then people started trying to do calculations and they're like, well,
how long would it take to form a planet, you know,
so you gathered it together with gravity, or you know,
how how long would it take to lay down layers
of rock? You know, how long would it take glaciers
to this kind of stuff? And you know, geology is
a fairly young field and that you know, it's a
(08:04):
couple of hundred years old and so people started to
look around, and as soon as they started to explore
these geological processes, they realized, wow, this stuff takes a
long time. And you know, it's sort of like um
walking into a room and seeing somebody has written a
book that's like five million pages long, and you're like, wow,
you've been in this room a long time, you know,
(08:26):
so it shocks you into realizing the things things are
a much different scale than you imagined. And that's the
we've seen the story the earth all around us. We
realize it's staggeringly long, much longer than we imagined, and that, yeah,
that's a fairly new idea. Yeah, that's a lot of
party you've missed exactly we should. Things have been around
(08:46):
for a long time. Do you have a fear of
missing out? There? Is that fomo about what happened? All
the good desserts? Fear of being in parties? Actually, to
be honest, what about all the desserts? The usually have
good dessertsive party old desserts or I just like I
just like a fine dessert. Wine? So you should you
show U at the beginning of the party, how the
(09:07):
desserts drink the wine and then go home? That's how
you know you're old or that was like a good
party to me. That is how your age old track exactly.
That's very age appropriate. Um. The beginning of this field
like becoming scientific before that was just speculation of the
beginning of the field. Becoming scientific was in the eighteen hundreds. Um.
But still that's very inaccurate. Like looking at rocks gives
(09:29):
you clues about the sort of order of magnitude, But
then people wanted to know very precisely, and that took
that took some time to to like basically defined clocks
inside the earth, clocks that have been ticking since it
was born. And that's in the end signs of aging, Yeah,
signs of aging, very precise signs of aging that could
tell us how old the earth was. So you're saying, like,
you know, the people who started the United States, like
(09:52):
the founding father fathers, like the Founding Fathers, they had
no idea how old we've been around, Like did they
think we've been around for a few thousand years? For
a few but didn't they know about the you know,
like ancient Egypt and things like that. Yeah, but you
know that's thousands of years, and so I think they
had the idea that the Earth was, you know, thousands,
(10:12):
many thousands of years old, right, because that's as long
as written history is, and so they imagine it's on
that time scale because that's the oldest thing, you know.
And that's basically the game is like find the oldest
thing you know and then assume that that's basically the
age of the world, right, And that's frankly what we're
still doing, um, except that we think that we found
stuff that comes from the very beginning of the world.
(10:34):
But you know, they knew about stuff that was thousands
of years old, so they had no reason to believe
that the Earth might be millions or even God forbid,
billions of years old, right. Plus I think those guys
are pretty religious, all right. So it is an age
old question and one that a lot of people have
had answers to and seems seemed to have answers to.
But we were wondering what people out there right now
(10:56):
in the world think about this question. Yeah, and this
is one this is a set of interviews. I was
very curious to see what people thought. I was wondering
if I would run into some sort of young Earth
folks who thought the Earth was from biblical ages and uh,
and even those people who were scientific. I was wondering, like,
how precisely do people know the age of the Earth.
So this I was very fascinating to hear the answers
(11:17):
like is it a million years, a bazillion years, twenty
seven point two bazillion? And so, as usual, Daniel went
out and asked people on the street how old they
thought the Earth was. So think about it for a second.
If you ran into a physicist in the street, uh today,
how would you answer this question? And no googling. Well,
(11:38):
here's what people had to say. Um, I have no
idea probably billion years. I don't exactly, but my understanding
is it's a couple of hundred million years old. Okay,
you know how we know? So I don't actually like
my my physics knowledge is pretty limited. Um. I think
like we've used carbon dating to date things within various
geological epox but I don't actually know how we arrived
(12:02):
at the calculation of the Earth stage and maybe three
billion years, tens of million years, four billion years, and
how do we know rock formation? It's probably like millions
of years old. I feel like, all right, Um, a
lot of millions and billions, right, yeah, exactly, not not
(12:24):
a whole lot of really specific answers. I have to
say I was a little bit disappointed. I guess I
thought people were more interested in this, Like I remember
wanting to know this number as precisely as possible when
I was a kid. I was really curious about this, um,
and so I was a little surprised that people were
just sort of knew it was super old and were
satisfied and didn't really care about the details. Although I'll
say every single person here after I asked them the
(12:46):
question then they did turn around and ask me. They're like, well,
how old is the Earth? So they wanted to know
abous just hadn't really spend any time looking it up.
All right, Well, let's get started on this topic, Daniel
and so, um. First of all, I think I feel
like we should define what we mean by the Earth
by the age of the Earth, right, Like, um, do
you mean like the the planet, like the ball that
(13:07):
it's Earth, or the rocks in the Earth, or the
atoms that are that make up the Earth? What do
you mean, like what's the age of the Earth. Yeah,
it turns out that that's a bit of a fuzzy question, right,
because the Earth has a sort of continuous history. It's
not like somebody snapped their fingers and boom, there was
the Earth. And you can start from the clock from then, right, Um,
(13:28):
that you know of, right, that we know of, that's right, unless,
of course it was a simulation, which case things is easy.
But in the case that it wasn't the simulation, we
have an idea about how the Earth was formed, and
there's just a lot of stages there, and so you
have to sort of make an arbitrary definition and into
like a lot of things in science, you know, Um,
there's not necessarily a clear and sower. So you just
(13:49):
define it to be something arbitrary and at least we
can agree on it. But maybe we should recap sort
of the brief history of the of the Earth and
how it was formed, yeah, like how it how it
came to be. Yeah, And then at some point we're
gonna say, this is when the earth started, this is
when the Earth was born. That's right. This is when
the party began, right, um, Because you know, you invite
people to party at nine o'clock and then they don't
show up till ten, and by then you've eating all
(14:10):
the brownies, um, and parties only really get started at
one a m. That's right, unless you're in Spain, in
which cases four am um. But you know, the very
early history is that we basically started with a huge
nebula of gas and dust. And this is stuff that's
left over from the explosions of other stars and just
gas from the Big Bang. And then of course gravity
(14:31):
did its thing and it pulls out all that stuff
together and you get a sun, right, So most of
the stuff went to the Sun, and then you get
a disc of stuff that's rotating around the Sun. It's
like rings around the Sun. And then the gravity pulls
the stuff in that disc together to turning to turn
that disc into planets, right, And uh so we think
that the Solar system started about four and a half
(14:53):
billion years ago, and it took like a few million
or a hundred million years to pull all that stuff
and the disk together into sort of protoplanets. Right. So
you have this sort of loose collection of stuff that
gravity is pulled together, and then gravity really gets to
work and it starts squeezing it and it collects all
this stuff and the planet heats up and sort of
(15:15):
melts the metal and you get the iron dropping to
the core and the crust forming, and so you can
sort of ask, like, what moment do you count the
earth forming? Is it like when you have the first
big cluster of rocks or maybe when it got gravitationally
heated enough to sort of melt the metals and form
the core um It's not really very clear. Well, I
(15:38):
always find this really fascinating because we we talked about
this the other day, that something like the rings around
Saturn are only temporary, right, Like at some point those
rings might turn into little blobs. That's right. Gravity is
very weak but very very patient, and so it just
keeps working and you give it enough time, it will
gather things together. And so this stuff that's orbiting Saturn
(16:01):
eventually grab gravity will pull it together in two moons.
Even the asteroid belt, is that at some point gonna
turn into more planets for our Solar system? Yeah, eventually.
Gravity it means very slow and those things are mostly
stable orbits, but eventually that stuff will get pulled together.
I mean also there's disruption. Right if you just left
the Solar system all by itself, then yes, eventually you'll
(16:22):
just gather together into a smaller and smaller number of objects.
But this disruption from stuff that comes outside, you know
and comets things from the word cloud. Um even even
things like supernovas from other solar system can disrupt things
happening in our solar system. Um. So, But yes, eventually
gravity will pull all this stuff together. It's really incredible
what over over these time scales, what gravity can do.
(16:45):
And that's one of my favorite things. It's just started
realizing that these processes are really slow, so things must
have been around for a super long time to make
it happen, right right, Well, I guess I would maybe
count that the birth of the Earth is when it
was form, you know, when all those asteroids and rocks
out there clumped together into a ball about the size
(17:06):
of the ball that we have now, like, you know,
when when it kind of snapped into shape. I would
maybe count that as the age of when the Earth started. Yeah, yeah,
and that's totally reasonable. That's totally reasonable. Um. I think
one common definition is that you measured the age of
the Earth is sort of the age of the oldest
rock in the Earth. And then you can ask the question,
(17:27):
what does it mean for a rock to be old.
Like you know, a rock is just a gravitational pulled
together clump of dust. But rocks are are formed right
there through melt and then they're cooled. And so the
age of a rock is usually like the last time
it was melted, right, So it's the time since it
was last melted, and so a lot of time people
just like the time that it cooled off, the last
(17:48):
time it really chilled out. And so you can imagine
like all these rocks came together, as you said, and
formed the Earth and got squeezed together and probably melted
and then cooled right, and the crust at least cooled,
and so you can asked, like maybe that last moment
the last moment of cooling of the crust. So the
age of the oldest cooled rock on the Earth could
be a definition at the age of the Earth, because otherwise,
(18:09):
how do you know when these rocks came together? Right?
Otherwise I don't know unless you have, like you know,
CCTV of the formation of the Solar system. It's pretty
hard to pin that down. Or you could say, is
it well, but you just gave me a number you
said that it was maybe it took around a hundred
million years to to form that the ball of the Earth. Yeah,
that's you know, I said, I said between a few
(18:31):
and a hundred million years, right, So that's a pretty
big uncertainty. Um, we don't really know how long it
took and and what moment do you define, like do
you needed one more rock? Because you know, caustic stuff
kept getting added to the Earth, and so some people say, well,
the age of the Earth is sort of just just
just call it the age of the Solar System because
that's pretty close to the age of the Earth. Or
you could say it's the age of the the oldest
(18:54):
rock on the Earth. So I think both of those
are sort of acceptable. And anyway, that's really all we
can probe and end. What we do is we measured
the age of the oldest rock we can find on
the Earth, and we say that's pretty close to the
age of the Earth. So the time that it formed
into a ball is kind of in between those two numbers, right, Yeah, exactly,
it's between when the Solar System formed and when the
(19:16):
ball that is the Earth started getting crusty. Exactly exactly
when it started getting crusty and grumpy and eating desserted
parties and going home early like a teenager. The suwhere
between conception and teenage years that's when you're born. That's
that's right. And you know, we have the same question
about people, right, I wonder about that sometimes. You know, Um,
(19:38):
how do you count somebody's age if there if somebody
is born premature, they're older. You know, even then somebody
who was born it was conceived at the same moment
but then born at full term. So even the age
of people is sort of hard to define sometimes. All right, well,
we've determined, we've determined what we mean by the age
of the Earth, and so now let's get into how
we know how old the Earth is and what what
(20:00):
that means. But first let's take a quick break. Okay,
so we're defining the age of the Earth is how
old the oldest rock was formed that we know about
(20:23):
right on Earth. That's right, exactly. Let's say, so when
the Earth was like a ball of lava, that doesn't count.
But once it started solidifying, then we're saying that's when
the Earth was born. Yeah, exactly, because we're imagining that, Yeah,
we're imagining that with that the formation, the Earth probably
melted a lot of the rocks because there was this
early lava period as you as you mentioned, and so
(20:44):
as it's cooled down and formed the crust, then it
made rocks and and it turns out we can figure
out how old a rocky is, and so that's really
our best handle in figuring out how old the Earth is.
And in the end it's a lower bound, right we say, well,
here's an old, super old rock on the Earth, so
that Earth must be at least that old, because like
we've talked about, like you mentioned, like um, trying to
(21:05):
figure out how old Earth is, the only thing we
can do is sort of look around and see how
old the things in it are, right, and like, what's
the oldest thing in it? We know it has to
be at least that old. Yeah, yeah, exactly. That's about
all we can do. And you know what you can
do otherwise is sort of form of theory. You say,
I have a concept for how this might have worked,
and then you can fit data to and say, well,
(21:27):
I expect you know that to see these sort of
rocks in that case, and you can use that a
theory to sort of extrapolate a little bit past the
day to you you observe. But that's very but that's speculative, right,
it's based on your concept of how things happen. So
the hardest number, the one that you know best, is
the oldest thing that you can find. And the oldest
thing we find our rocks, and and so what we
(21:49):
do is we try to turn one question into another question.
The question we can answer is what's the age of
the oldest rock on the Earth. Wouldn't that tell us
like if we found a really old raw, what if
that rock came from before the Earth was formed? Do
you know what I mean? Yeah, yeah, there are some
of those actually, like meteorites, Right, meteorites tell us the
age of the Solar system because we think meteorites were
(22:12):
formed in the beginning of the Solar system. Right, the
first stage is gathered together gas and dust into rocks, right,
And so those rocks from space tell us the age
of the Solar System. But we can tell which rocks
came from space and which rocks are from Earth, And
so we can distinguish those two. How can we tell? Oh,
they're clearly labeled. You know, they have their turn address, right,
(22:35):
it says somewhere near Neptune Um. It's it's actually quite
interesting that the so the distribution of metals in these
rocks is very different from meteorites than it is for
rocks on the Earth. It's like a different group of rocks. Yeah,
it's like they have more nickel in them where they
have this other rare stuff. And that's a whole other
fascinating field. Um. And also, so these rocks are different
(22:58):
from Earth, and they tell us like what else is
out there in the Solar System and what was out
there when the Solar system was formed? There like little
time capsules from the very beginning of the Solar system.
And they actually tell us another interesting story. Right, So
those rocks, we can tell how old they are, and
they tell us how old the Solar system is. So
we can answer two questions. We can find a bunch
of rocks. We can say, oh, these came from somewhere
in the Solar System. There x y z old. These
(23:20):
rocks are here on Earth, they're only you know, ABC
old all right. So then so it all comes down
to the question of how do you tell how old
the rock is, yeah, right, exactly exactly, or a meteorite
which is the same, which is the same because the
meteorite is just basically a rock from space that landed
on Earth. And the way we do it is something
called radiometric dating, which is not the same as carbon dating, right,
(23:41):
but it works in which is not the same as
online dating either. It turns out to be actually kind
of similar. There's a lot of swiping, there's a lot
of lying about your age as well. Um. No, carbon
dating and radio metric dating have similar principles, and we
should have a whole podcast episode of how carbon dating works.
Carbondating relates to organic stuff like living beings. Radiometric dating
(24:05):
relates to how how old rocks are. Okay, so you
find a rock and you're you're staring at it, and
you're like, I wonder when this rock became a rock? Yeah,
exactly when this turn from lava to this heavy thing
that I'm holding, And so you can we can actually tell, right,
we can look at the stuff in it and tell
how old window was formed, how many millions of years
(24:26):
ago exactly? And I have so much respect for scientists
who develop these techniques. You know, they look at the
rock and they say, I want to know how old
this rock is. How could I tell? Can I find
something inside the rock that I can use as a clock,
something which changes reliably and steadily every year, so that
I can count it and and project back and figure
(24:47):
out when that clock started or something. And this is
hard to do, you know. You have to find something
which is steady and reliable and can be calibrated. Um,
it's not trivial, right. You can't just say, oh, I'm
just gonna your X Y Z property. You have to
invent these techniques. And it takes time and sweat and tears,
and so I have so much respect for folks who
(25:08):
did this. And and in the case of rocks, it
turns out to be very dependent on the special kind
of mineral called zircon. Zircon like the fake diamond like, yeah,
it's related exactly. Zirconium and zircon is a special kind
of mineral. And it's special because it has two properties. One,
it likes it's a crystal, and it likes to absorb
(25:29):
uranium into it right, gobbles up uranium. So when the
rock forms, if it has some zircon, then it will
grab any uranium that's around it. And the second important property,
but wait, what is zircon made out of? Is it?
What is it like a made out of iron or
what is it carbon? What is this mineral made out
of it? I think it's made out of leftover desserts. No,
(25:53):
it's it's a mineral um. It's made out of zirconium
and silicon and oxygen. Oh Sirgonium is like an element
like carbon or irn. Yes, Zirconium is an element, and
it's sort of like a silver metal. It's atomic number forty.
It's you know, it doesn't really play a lot of
roles and stuff other than like cubics urconium. It's not
(26:15):
like a famous metal like some of its friends and
neighbors um, but it plays an important role in aging rocks.
So this element is all around us and to all
the rocks, and it forms a special crystal. You're saying
that when it forms, it likes to capture or uranium.
That's right. It captures uranium and it really rejects lead.
So the moment that the rock is created, it creates
(26:37):
a special thing, which is zircon with just uranium in it.
And you might wonder like, well, how is that helpful? Right? Well, uranium,
as you might know, is radioactive. It doesn't just sit
around and stay uranium forever. It very reliably decays and
it decays into lead. But wait, why does the durk
circon like uranium and white, doesn't it? You mean, like
(26:58):
when it forms the crystal les, it forms a little
structure and it just it just likes uranium like uranium
likes being inside of zircon crystal. I mean, I don't
want to get in the mind of uranium on the
mind of zircon. I mean, they did some carbon dating
online and they decided they were a good match. Um,
but no, there's some chemical some business with the chemical bonds.
(27:19):
You know, the way the crystal structure of zircon forms
fits very nicely with uranium and doesn't fit very nicely
with lead. Right, Yeah, So like a rock, you have
a have a bunch of lava and it cools and
it slowly turns into rock. And in that process is
when you you're saying this crystal form, yeah, and zirkon
is it doesn't have to be all over the rock.
(27:39):
It's not like a whole rock full of zirkon. It
is just these little chips of zircon. And what they
do is they repel all the lead inside them and
they grab some uranium and your uranium is like a clock.
It decays very reliably. So every million years or so,
some uranium turns into lead. And so after one million years,
some fraction the uranium and then zircon has turned into lead.
(28:02):
After two million years, another fraction has turned into lead.
After a billion years, a very predictable fraction is turned
into lead. So what you do is you take a rock,
you look at that, you find some zircon crystals, and
you ask how much of the uranium has turned into lead,
and that tells you how much time it's had to
turn into lead. It's kind of like a like an
(28:23):
aging process, right like wine. If you leave wine there,
it's gonna change, it's characteristic, it's gonna change into something
else exactly, and you need something. That's why it works
so well as a clock, because uranium is something that
we know reliably. We understand it, We know how it decays,
we know how long it takes, we know, you know,
after a hundred thousand years, what fraction of uranium atoms
(28:44):
will turn into lead. And the other key thing, of
course is that it starts with a blank slate. That
when you form these zircon crystals, it rejects lead. Right,
So you start out like with an empty bucket, and
then slowly the uranium bucket fills the lead bucket. And
so you can just measure the uranium lead fraction and
that tells you how long the uranium has been turning
into lead. It's really amazing. It's it's really just chance
(29:07):
that this happens, that somebody figured this out. So like
if you find a rock and you find a little
little zircon chip inside of it, and it's like, let's
say it's full of uranium, then you know it's a
pretty new rock exactly. But if it's full of lead,
then you know it's really old rock exactly. It's exactly right.
So rock start start out pure uranium, no lead, and
(29:28):
then as they age, the uranium turns into lead. And
so a rock with almost no uranium is going to
be like super duper old, right, And new lead can't
just like come in from the outside because now it's crystallized, right,
It's hard for lead to get in any other way.
All the only way you can get lead inside the
zircon is for uranium to turn into lead. So the
(29:49):
rock sort of you might say like it's spoils or
ages by looking at that. Yeah, like you, it turns
into lead right exactly, And whether it's spoils or ages
be to fully depends on you know, your preference for
how rocks taste. I suppose whether you swipe right or
left for rocks. But you know, it took a long
time to figure this out. People tried all sorts of
(30:10):
other strategies first before they discovered this one. And there's
lots of scientific careers and pH d PCs that were
frustrated in people trying to figure out a way to
do radiometric dating before people stumbled onto this one. Wow,
and this is not one of these things that you
um have to calibrate, right because I imagine you have
probably very scientific information about the decay of uranium, right like,
(30:35):
it's very physics space like. You don't have to calibrate
it to anything else, do you. You're right, And we
can basically use simple physics arguments to argue how long
uranium takes to decay and to lead and uh, and
so it gives us pretty clear descriptions, but you know,
we also want to get confidence in it, and so
what people do to have various other sort of radiometric
(30:58):
methods not just uranium to lead, but other stuff, things
that decay faster, things that decay slower, and so you
can use those to sort of cross calibrate, but uranium
to let this sort of the cleanest one. But you know,
like everywhere in science, you want to check everything three
different ways, um and then make sure you get consistent answers.
All right, that's pretty cool. That's how we can tell
how old the rock is. Man, rock can't lie. I
(31:21):
can't lie about its age. That's right, that's right. You
can lie, but we'll know you are. All right, Let's
get into the answer now, Daniel, how old Earth is
and how old the Solar system is. But first let's
take another quick break. All right, Daniel, Let's let's break
(31:48):
it down for people. How old is the Earth, or
at least the oldest rocks on Earth. The oldest rocks
on Earth come from Australia. And I don't know why,
but that's not surprising to me. It seems like Australia
is the land of extremes, and so you expect the craziest, weirdest,
oldest rock to be in Australia. And they have the
biggest spiders, the biggest crocodiles and the oldest rocks, the
(32:12):
deadliest everything, the biggest, best stars and the best podcast
listening fans. Right. Um, you know, we've got a lot
of good email questions from Australia. But the oldest rock
on Earth come from the jack Hills of Australia and
it's four point zero four billion years old. So these
are rocks that we do we have to dig up
for them, dig for them or are they were they
(32:34):
pretty close to the surface or yeah, it's it's fascinating.
Actually these rocks just sort of sit on outcrops and
people recently discovered that they have that the oldest rocks
are there, and you know it's um. The Earth itself,
of course, has had lots of geologic activity and so
some of these really old rocks get buried and then
there's geological activity that brings them up to the surface,
(32:54):
and so it just happens to be at this spot
on the Earth, these really old rocks got pushed up
and so they're not that far underground there on these
outcropping of rocks in Western Australia, but you almost kind
of expect them to be close to the surface, right,
because that's you know, I would imagine that if we
were once a ball of lava, then the stuff sort
of cools from the outside inwards. That's true. But you know,
(33:16):
then there's all these sedimentary processes where old things get buried. Um.
But then there's tectonic activity and all sorts of other
crazy stuff that brings stuff up and mixes everything around. Um.
And so there's a lot of stuff going on there.
I think it's not that not that simple. Um. But
in the end, the oldest thing we found on Earth
is uh, is almost four and a half billion years old.
Is four billion, four hundred and four million years old.
(33:39):
And you know, that's a staggering answer. It's hard to
really comprehend what that means. For billion years old, that's
a lot. It's a it's a lot of dessert parties. Yeah,
I mean, if theysed, a lot of party missed. Yeah.
I think I think what you said earlier about how
we came to the party pretty late. It's sort of shocking.
You know, the first people to like learn these number,
(34:00):
first to know that the Earth was billions of years old,
it must have made humans feel sort of small and
recent and uh. And you know, maybe transient tried like
the Earth has had a long history and we are
only the very last little bit of it. Yeah. I
mean it's a difference between I mean human histories maybe
what like ten thousand two years old? Yeah, a hundred
(34:20):
thousand if you're really generous. Yeah, but the Earth has
been around for four point four billion years old exactly.
It's not too impressed. It's been there, It's seen a
lot of stuff. You know, it'll be here, remember done. Um.
You know, people always talk about how humans are going
to destroy the Earth, and uh, I don't think that's likely.
I think the humans might destroy ourselves or life on Earth,
(34:41):
but the Earth will be here when we're done. Unless
I hear some physics are trying to make black holes
down at the Large Mattern Collider. Yeah, but we just
want to understand the universe. Man, We're not threatening the world.
There's no danger there. Trust me, And can I have
ten more billion dollars to build a bigger one one
year for every one daughter, for every year that the earth?
(35:03):
That's right, That sounds good. Um, But you know, that's
the oldest rock on Earth that we think is part
of the Earth. It was part of the formation of
the Earth. But of course we have found rocks on
Earth that are meteorites, right, they came from space that
are older than the Earth itself. So late comers to
the party, that's right. They heard there was something cool
happening down here in the third rock from the Sun,
(35:24):
and they they've fell on in to figure it out.
And those rocks are four point five six billion years old,
So that's like a hundred and fifty million years older
than the oldest rocks on Earth. That's the oldest meteorite
that we found. And I think that you were saying
that that kind of tells us what the oldest media
right at all is in our solar system, right, Yeah,
(35:46):
using the same logic, this is the oldest thing we found. Yeah,
so it's probably the sort of the age of the
Solar system. Right, that's when we think the gas and
the dust got accumulated together to form the Sun and
the planets and all that stuff. But the first age
of that was to form smaller rocks, and so we
think these things are remnants of those times. Right, But wait,
didn't the Sun. Hasn't the Sun our son already been
(36:08):
through a couple of cycles, Like, hasn't it exploded a
few times already. Well, our son hasn't exploded, but it's materials, right,
have been through several solar cycles. Everything around us, all
that stuff is left over from one or two or
three solar cycles of a sun lasting for you know,
one to see three billion years and then blowing up
and spreading its materials around. So everything around us is
(36:31):
all all the components of the solar system are probably
been recycled. But it's not like our son formed and
then we started in formed and we started it was
a different from a different star. Oh I see, So
our current son is is maybe around four point five
billion years old. Yeah, that's what we think. That's the
history of our solar system. And remember, for context, the
(36:51):
Milky Way is something like thirteen billion years old, and
the universe is just a little bit older than that,
almost fourteen billion years old. So there's a lot of
history before even our son, our solar system was formed,
and then a lot of history before we came around.
And so like that's the cosmic context that we're standing in.
(37:12):
And so that's the all this thing we found in
the solar system, but that's older than all the things
we found. That is part of the Earth. So somewhere
between those two numbers is when maybe the ball of
the Earth form. That's right. That's what we call the
Joorge date, the Joge definition of the age of the Earth,
the Bald date or his Bald date. That's right, that's right. Um.
(37:33):
And so you know there's a window there about a
hundred and fifty million years, which seems like a huge number,
you know, that's like, but it's pretty small, I think
compared to exactly. Geologically speaking, it's a blink of an eye, yeah,
or universe speaking, it's it's pretty accurate, yeah, exactly. And uh,
you know, we we know these numbers two plus or
(37:53):
minus twenty five or fifty million years based on the
uncertainty of radiometric dating and so um, because of the
hard work of a lot of scientists to develop these
things and across check them and understand them across various
other processes. Now we've looked around us and we've gathered
these clues from everyday objects around us, you know, the
rocks at our feet, the rocks in Australia give us
(38:15):
clues about these fantastic numbers that would just blow the
minds of our ancestors and those clues were around for
them to discover also, and so of course that makes
you wonder, like, what clues are there laying around at
our feet now that would tell us secrets of the
universe that our descendants will know and wonder why we
couldn't figure it out. It's not just like an opinion, right,
(38:35):
It's pretty based on evidence, and it's pretty based on
science and physics, the numbers. Yeah, exactly, these things we
know pretty well. We have a lot of confidence in them.
People have been working on these processes for decades and decades,
a lot of you know, grumpy competition between rivals to
make more and more accurate measurements. You know, this is
not a conspiracy of scientists all working together in harmony.
(38:58):
You know, this is a bunch of people racing to
develop better and better techniques and trying to find older
and older rocks. Remember, sciences a competitive field, and everybody
wants to one up the other to get the more accurate,
the more reliable, the more verifiable answer. So, yeah, we
have a lot of confidence in this knowledge. Yeah, it's
like it's all around the evidence, it's all around us,
and you're you're standing on it. Yeah, exactly, and even
(39:19):
if you can't do radiometric dating in your head of course,
because you can't see the xericon crystals. You know, you
just look around you and you see lots of things
around you which take a long time to form. You know,
how long does it take a mountain to form? It's
a very slow process. And so all around us on
the Earth we see evidence of very very slow processes
which have accomplished a great deal, which tells you that
(39:42):
they've been doing it for a long long time. Yeah,
like you said, who knows what else is lying under
our feet? That's right exactly, So I think we answered
that question about the age of the earth. Yeah, yeah,
so grab a glass of wine and sit back and enjoy.
That's right. And uh, no matter how old you are,
(40:03):
remember you can always enjoy one more brownie. See you
next time. Thanks for tuning in, and thanks for asking
great questions. This question was inspired by an email from
a listener, So if you have a question you'd like
to know the answer to, please send it to us
at questions at Daniel and Jorge dot com. Before you
(40:28):
still have a question after listening to all these explanations,
please drop us a line. We'd love to hear from you.
You can find us on Facebook, Twitter, and Instagram at
Daniel and Jorge That's one Word, or email us at
Feedback at Daniel and Jorge dot com. Thanks for listening
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio from More podcast from
(40:50):
my Heart Radio. Visit the I heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. Nine
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
Popular Podcasts
Las Culturistas with Matt Rogers and Bowen Yang
Ding dong! Join your culture consultants, Matt Rogers and Bowen Yang, on an unforgettable journey into the beating heart of CULTURE. Alongside sizzling special guests, they GET INTO the hottest pop-culture moments of the day and the formative cultural experiences that turned them into Culturistas. Produced by the Big Money Players Network and iHeartRadio.
40s and Free Agents: NFL Draft Season
Daniel Jeremiah of Move the Sticks and Gregg Rosenthal of NFL Daily join forces to break down every team's needs this offseason.
Crime Junkie
Does hearing about a true crime case always leave you scouring the internet for the truth behind the story? Dive into your next mystery with Crime Junkie. Every Monday, join your host Ashley Flowers as she unravels all the details of infamous and underreported true crime cases with her best friend Brit Prawat. From cold cases to missing persons and heroes in our community who seek justice, Crime Junkie is your destination for theories and stories you won’t hear anywhere else. Whether you're a seasoned true crime enthusiast or new to the genre, you'll find yourself on the edge of your seat awaiting a new episode every Monday. If you can never get enough true crime... Congratulations, you’ve found your people. Follow to join a community of Crime Junkies! Crime Junkie is presented by audiochuck Media Company.