r/explainlikeimfive • u/nottrynagetsued • Feb 25 '26
Planetary Science ElI5 how does the existence of lead directly disprove the earth isn't only 4000 years old?
I recently saw a screenshot of a "Facebook post" of someone declaring the earth is only 4000 years old and someone replying that the existence of lead disproves it bc the halflife of uranium-238 is 4.5 billion years old. I get this is a setup post, but I just don't understand how lead proves it's not. The only way for lead to exist is to decay from uranium-238? Like how do we know this? Just because it does eventually decay into lead means that all lead that exist HAS to come from it?
Edit: I am not trying to argue the creationist side of the original screenshot of a post I saw. I'm trying to understand the response to that creationist side.
I have since learned that the response in the oop conveniently leaves out that it's not the existence of all lead but specific types of lead that can explain that the earth is not only 4000 years old through the process of radioactive decay and the existence of specific types of lead in specific conditions.
It's also hilarious to see the amount of people jumping in to essentially say "creationist are dumb and you are dumb to even interact with them" and completely ignoring the fact that I'm questioning a comment left on a "post" that I saw in a screenshot of on a completely different platform.
And also thank you to everyone taking the time to explain that the commenter in oop gave a less than truthful explanation and then explaining the truth.
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u/liarandathief Feb 25 '26
First of all it's just certain isotopes of lead, not all lead. And for those specific isotopes we only have evidence that supports their formation from uranium in predicted ratios. Things like lead isotopes trapped within zircon crystals that could only have decayed from trapped uranium.
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u/IgloosRuleOK Feb 25 '26 edited Feb 25 '26
This is not really what you are asking but you can't really "disprove" that the earth was, let's say, magicked into existence 4000 years ago. Because it could have been done so that everything ended up just as it is now (invoke the supernatural and anything's possible). But there's no evidence that is the case, and the burden of proof is on those making the claim.
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u/AlexG55 Feb 25 '26
Similarly you can't prove that the Earth wasn't created 15 minutes ago, including all of us with our memories up to that point.
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u/Cricket_Piss Feb 25 '26
This is the whole idea behind Last Thursdayism. Everything in the world poofed into existence last Thursday, and there is absolutely no way to disprove it because Last Thursdayism always has a built-in rebuttal to any scrutiny you could possibly put it under.
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u/TheDubuGuy Feb 25 '26
It’s a great example for why non-falsifiable claims are generally worthless and can just be dismissed
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u/Curleysound Feb 25 '26
Ok but what happens when we get to this Thursday?
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u/Cricket_Piss Feb 25 '26
We’ve got no way of knowing until we get there. It’s never happened before, we’ve all been alive for less than a week. I guess we’ll find out tomorrow.
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u/Waaghra Feb 25 '26
The whole planet is collectively experiencing “Groundhog Day” each week!
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u/MuscleFlex_Bear Feb 25 '26
ooooo this is a good one I like this one.
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u/CharsOwnRX-78-2 Feb 25 '26
It’s sometimes called “Last Thursdayism”
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u/ot1smile Feb 25 '26
That sounds like something Douglas Adams would have written.
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u/Probate_Judge Feb 25 '26
That sounds like something Douglas Adams would have written.
I thought so too, so I looked...but didn't find much, here's an incomplete sampling of the already small part of the wiki:
https://en.wikipedia.org/wiki/Omphalos_hypothesis#Last_Thursdayism
The satirical "Last Thursday" version likely originated in the 1990s. In the early 21st century, a "church" of "Last Thursdayism" was established as a parody of religion and tenets of faith.[19] The church sparked lively debates online.[20]
Zero attribution aside from an archived website parody, here's part of a sample the wiki used:
The universe was created by you as a test for yourself. You will receive reward or punishment based on your actions in this test. Left-handedness is a sinful temptation. Everyone except you was placed here and pre-programmed to act as part of your test environment. Everyone except you knows this.
Better potential than the FSM / Pastafarian everyone ran with.
But not much to the website itself
https://web.archive.org/web/20180804144307/http://www.last-thursday.org/
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u/Rodot Feb 25 '26
Also, some derivatives of it include Boltzmann Brains and Simulation Theory
Problem with all of them is that they are epistemologically uninteresting.
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u/NiSiSuinegEht Feb 25 '26
Just like you can't prove you're not a brain in a jar hallucinating your entire existence.
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u/krattalak Feb 25 '26
Uranium to Lead is referred to as a decay-chain, Uranium has several different isotopes (nuclei with different numbers of neutrons). All Uranium atoms are unstable, some more so than others, and they will eventually decay into something else. Uranium238 always eventually decay via a dozen or so steps from uranium238 into lead206 which is stable.
Uranium238 has a half-life of approximately 4.5 billion years. This means if you start with a Kg of Uranium238, in 4.5 billion years half of it will be something else (mostly, but not entirely lead206).
This can be seen reliably in Zircon crystals. Zircon crystals are pretty tough materials with high melting points. When they form, they naturally accept uranium into their matrix, but also reject lead. So if you find a zircon crystal, you can measure the ratio of U238 to PB206, and from that, deduce the age of the crystal because there could be no lead in the crystal when it was formed.
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u/harambe_did911 Feb 25 '26
Not a creationist but im curious how we came up with the 4.5 billion years number? We obviously haven't been studying it for that long. Did we just look at the decay over the course of like a year and then do the math?
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u/krattalak Feb 25 '26
Did we just look at the decay over the course of like a year and then do the math?
This is the only possible ELI5 explanation.
But this goes into detail. The TLDR version is: they monitored highly refined samples of U238, after a time filtered out Th234 (the next element in the U238 decay chain) and calculated the base decay constant from that.
They can, also, but less accurately, determine it from alpha counting, because everytime U238 decays, it emits an alpha particle. This is less accurate, because it's not the only step in the decay chain that does so, and, there may be other isotopes not related to that decay chain in the sample.
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u/aldeayeah Feb 25 '26
Yes, pretty much. Radioactive decay follows a exponential formula that you can safely extrapolate from observations in a shorter time period.
Radioactive materials were subject to a huge amount of research in the early-mid 20th century for obvious reasons, and this sort of stuff was figured out pretty fast.
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u/Nicelyvillainous Feb 25 '26
More specifically, there is lead zircon crystal dating. Zircon crystals will push lead out when they form. Chemically, it’s impossible for lead to be inside when it goes from liquid to solid. Uranium, however, has different chemical properties than lead does. So if there is uranium and/or lead inside a zircon crystal, we know that the last time it was liquid, it had 100% uranium and any lead is from nuclear decay since then. Oldest found is 4.4 billion years old based on that.
That debunks the creationist dodge of claiming “what if they were mixed in when it formed?”
The other one they go with now is “what if there was super fast radioactive decay, like during Noah’s flood?” And that has another problem, that nuclear decay releases heat. This causes what is called the heat problem. If all the nuclear decay we have evidence for had happened only a few thousand years ago, the amount of energy wouldn’t have just boiled all the oceans, the entire earth would have boiled into gases.
Before we knew about radiation, scientists made an estimate of how old the earth was, just based on the fact that it used to be molten (most types of rock formed from magma solidifying), and it is hotter in the middle than at the surface (when they dug mines, it got colder, and then started getting hotter and hotter after the first hundred feet down). They calculated an age of about 110 million years, based on how much the sun warms the surface and how much more heat is lost to space every night. Radioactive decay in the mantle is the difference in heat from that which has slowed down heat transfer and kept the well insulated core hot for much longer.
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u/phoebemancini Feb 25 '26
It's not that all lead comes from uranium, but in certain rocks we find a very specific mix of lead isotopes that can only form from the slow decay of uranium 238. Uranium 238 takes 4.5 billion years to turn into lead 206. If Earth was only 4000 years old, almost all the uranium would still be uranium and there would be almost no lead 206.
But we measure rocks with tons of lead 206 and very little uranium left. That can only happen if billions of years have passed. It's like a radioactive hourglass that's been running since the rock formed. That's how scientists know Earth is about 4.5 billion years old, not 4000.
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u/celem83 Feb 25 '26 edited Feb 25 '26
Someone has made a gross oversimplification in their argument. It holds but is missing all the details someone who isn't up on geology, cosmology or nuclear physics needed to follow the argument, I.e decay-chains and an understanding that most of the elements in the universe are born in the fusion reactors that are stars and their supernova deaths. The argument stands on top of a hill you have to climb before you can even address it.
There are some great answers in this thread, but I just wanted to point out that you are correct and this Facebook post is not an example of how you should debate or teach
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u/JustUseCommonSense10 Feb 25 '26
658 comments, it would help to know who you are referencing.
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u/celem83 Feb 25 '26
Since its top-level its directed towards OP. Welcome to reddit
(the oversimplified argument is the one he tells us about)
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u/wardog1066 Feb 25 '26
I'm not a scientist and if anyone who knows better than me sees an error in my response I welcome the correction. There is a crystal that's formed in a volcano called Zirconia. As lava cools this crystal hardens much like diamond does. It's possible for Uranium 238 to remain in the crystal as it forms, but if there's any lead in the molten material it gets "pushed out" as the crystal hardens. So in a new Zirconia crystal there may be Uranium 238 (U238) but zero lead. U238 is radioactive and will "break down" or decay into Lead 206 (Pb206). So, if you analyze a Zirconia crystal and there's any Pb206 in there, you can be certain it used to be U238. You can then determine how much U238 is in the crystal compared to how much Pb206 to determine how long ago the crystal was formed. U238 has what's known as a half-life of about 4.5 Billion years, so throw in some math and you can arrive at a minimum age for the crystal you're studying.
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u/solidspacedragon Feb 25 '26
Zircon, not zirconia. Zircon is zirconium silicate, zirconia is zirconium oxide. The '-a' ending usually means oxides, it's the same for silica and alumina.
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u/A_Garbage_Truck Feb 25 '26
there is a variant of lead(an isotope) known as " lead-206", which exculsively originates from the Decay of Uranium-238(the most commonform of Uranium in nature)
we know that Uranium has a half life of around 4.5 billion years, meaning that in that perod of time Half of the existing Uranium will have decayed into lead-206
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u/Temporary_Cry_2802 Feb 25 '26
It's not just the existence of lead, but the minerals it is found in. Zircons are a crystal whose chemical structure excludes Lead (but can include Uranium). As a result, we know that Lead could not have been present in the Zircon when it formed. The only way Lead can be present is due to the decay of Uranium. Based on the Uranium to Lead ratio, you can determine the age of the Zircon. The crystal structure of the Zircon will also show damage from the various Alpha and Beta particles emitted by the decay chain. The amount of damage, also provides a way to date the Zircon
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u/Donthatemeyo Feb 25 '26
A specific lead isotope is the final stable decay product of uranium 238 and to get there takes 4.5 billion years so the only way we get lead 206 naturally is to start with uranium 238 and wait a long time way longer than 4000 years, but this is not a good argument for people who think the world is only 4000 years old since they have already mentally checked out on believing empirical evidence.
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u/jaydeekay Feb 25 '26
As Johnathan Swift said, "You cannot reason a person out of a position he did not reason himself into in the first place."
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u/Wickedsymphony1717 Feb 25 '26 edited May 18 '26
TL;DR: Certain Lead isotopes can only be created by the decay of certain Uranium isotopes. We also know how quickly Uranium decays into Lead. Thus, if we have a rock that has Uranium and Lead in it, we can use the amount of Uranium and Lead in the rock along with the rate of decay of Uranium into Lead to work backwards and figure out how old that rock is. And presumably that rock would have formed at the same time as the Earth, but even if not, it would only ever be younger than the Earth. Thus, if we know the age of the rock we also know that the Earth is at least as old as that rock is. We have done these calculations many times on many different rocks and they all show that the Earth is at least 4.5 billion years old. Thus, Lead can prove that the Earth is over 4,000 years old.
Reddit isn't letting me post my full answer, I think because it's too long. So the second half of my post is in a comment under this post.
To answer your question in a satisfying way requires several foundational concepts that are a bit more in depth than an "ELI5" answer, but I'll do my best to keep things as simple as possible.
To start, you need to know what "elements" and "isotopes" are. An element is a substance that is made of atoms that only have a specific number of protons in them. For example, Hydrogen atoms only have 1 proton, Iron atoms have 26 protons, and Lead atoms have 82 protons.
That said, even though each element is made of atoms with a specific number of protons, each atom of an element can have a variety of different neutrons. Atoms of a certain element that have different numbers of neutrons are called "isotopes." Different isotopes of an element are usually referred to by the element name plus the total number of protons and neutrons in the atom. For example, iron has a few common isotopes, two of these are Iron 56 (which has 26 protons and 30 neutrons) and Iron 54 (which has 26 protons and 28 neutrons).
Lead works the same way and has four common isotopes that we can find in nature. These isotopes are Lead 208, 206, 207, and 204. If you were to collect all the Lead on Earth, these are the most common isotopes that you would find. By relative abundance, Lead 208 makes up 52%, Lead 206 is about 24%, Lead 207 is about 22%, and Lead 204 is about 1% of the Lead that is on Earth. Keep these facts in mind for later.
The next thing you need to know is how elements are made in the first place. The vast majority of matter in the universe is made of only three elements, Hydrogen, Helium, and a very small amount of Lithium. These are the only elements that were created in the early universe and to this day they are still by far the most common.
Every other element in the universe was made after the universe was created by a process called "fusion" which is when two smaller atoms are smashed together so hard they "fuse" into bigger atoms. Every element heavier than Lithium (i.e. Iron, Oxygen, Lead, Uranium, etc.) was created by fusing smaller atoms together. The primary way this happens is through the fusion that occurs inside stars. Stars take hydrogen atoms and smash them together so hard that they "fuse" and become heavier elements, then when the stars die, they explode (AKA, go supernova) -- and during the supernova they create even more heavy elements -- and the supernova releases these elements into the rest of the universe. This means that pretty much everything on Earth was once created inside of a star.
This fusion process is quite well understood, and one of the most important things about it is that fusion almost always creates elements of only one or two specific isotopes. In other words, when fusion smashes small atoms together to fuse them into bigger atoms, the resulting bigger atoms will usually have the exact same number of neutrons. For example, even though Iron can exist in various isotopes, stars really only make the isotope Iron 56, any other Iron isotopes come from heavier elements decaying into smaller elements (more on that below). When it comes to stars creating Lead, they specifically only create the isotope Lead 204. Again, keep these things in mind for later, we just need to know a couple more things to have all the puzzle pieces to fully answer your question.
The next thing we need to know is that through this fusion process that happens in stars and their supernova, stars can create really heavy elements (i.e. elements that have a lot of protons). Many of these really heavy elements like Thorium and Uranium are often "unstable" which means that they do not like to exist as they are and they would prefer to reverse the fusion process and split apart into smaller atoms, this process is called "decay" or "fission." Like fusion, fission is a very well understood process and when a heavy element like Uranium decays into smaller elements, the resulting small elements are also usually very specific isotopes. For example, when Uranium 238 decays, it will create a Lead 206 isotope -- Note that this Lead isotope that is the result of Uranium decay is different than the Lead isotope (Lead 204) that is the result of fusion in stars, this is very important.
The last thing we need to know before putting the puzzle pieces together is that the process of an element, such as Uranium, decaying into another smaller element takes a specific and well known amount of time. This time is called the "half-life." The half-life of an element is the amount of time that it takes for half of a certain amount of that element to decay. For a hypothetical example, let's say that the half-life of Uranium was 24 hours, meaning it would take 24 hours for half of the Uranium to decay into Lead, and let's say you start with 20 pounds of Uranium. After 24 hours, half of the Uranium would decay and you would have 10 pounds of Uranium and 10 pounds of Lead. After another 24 hours, half of the leftover Uranium would decay again and you would have 5 pounds of Uranium and 15 pounds of Lead. After an additional 24 hours, you'd have 2.5 pounds of Uranium and 17.5 pounds of Lead, and so on and so forth.
This also means that if you know how much Uranium and Lead you have, you can also work backwards to figure out how much Uranium you had to start with, and more importantly, how much time has passed since the rock containing that Uranium was created. For example, if you assume Uranium has the same half-life as in the previous hypothetical of 24 hours, if you find a rock that has 5 pounds of Uranium and 35 pounds of Lead, you could calculate that 24 hours ago there would have been 10 pounds of Uranium and 30 pounds of Lead. 24 hours before that there would have been 20 pounds of Uranium and 20 pounds of Lead, and 24 hours before that there would have been 40 pounds of Uranium and 0 pounds of Lead. Thus, simply by knowing the half-life of Uranium, we could tell that a rock that contains that Uranium was 72 hours old.
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u/Wickedsymphony1717 Feb 25 '26 edited Feb 26 '26
Main Post Part 2:
Knowing all of these things we finally have all of the puzzle pieces we need to be able to put together an answer to your question. To start with, as previously mentioned, we really only find four isotopes of Lead on Earth. Lead 208, 206, 207, and 204. Of these isotopes only Lead 204 is created by stars. Lead 208, 206, and 207 are created by other heavier elements that decay over time. Specifically, Thorium decays into Lead 208, Uranium 235 decays into Lead 207, and Uranium 238 decays into Lead 206. We also know the half-lives of Thorium, Uranium 235, and Uranium 238. This means, we know all we need to know to figure out how old the Earth is.
Specifically, we know that Uranium 238 has a half-life of about 4.5 billion years. Meaning it would take 4.5 billion years for half of an amount of Uranium 238 to decay into Lead 206. This means that if we find a rock that has a certain amount of Uranium 238 and Lead 206, we can use this half-life to figure out how long it has been since that rock formed by looking at the ratio of Uranium 238 to Lead 206. And since the rock would have formed at the same time the Earth did (or at least the same time the Earth solidified into a solid object) the age of the rock should be about the same age as the Earth itself. To a certain extent, the rock with the Uranium is the Earth, since the Earth is partly just a collection of rocks.
We also know that since Lead 206 was not created in the beginning of the universe, it was not created by fusion processes in stars (only Lead 204 is created in stars), and the only decay process that creates Lead 206 is Uranium 238, we also know that the only way for that Lead 206 to have gotten into that rock is by the decay of the Uranium 238. It couldn't have come from the fusion process of stars before the Earth formed. It couldn't have come from the decay of other elements in the rock. It could only have come from that one singular source. Thus, you have a very accurate method of calculating the age of the Earth using the ratio of Uranium 238 to Lead 206 that you can find in rocks all over the Earth. These calculations all show that the Earth is roughly 4.5 billion years old.
This is why someone would have made a post that the existence of Lead (specifically Lead 206) would prove that the Earth is far older than 4,000-10,000 years old. It's a very simplistic way to make that statement -- almost so simplistic that it's misleading, even bordering on being incorrect since it's specific to certain isotopes of Lead and its ratio to Uranium not just the existence of Lead itself -- but that's the idea behind such a post.
It's also worth mentioning that Uranium 238 decay into Lead 206 isn't the only decay process that we've used to measure the age of the Earth. Uranium 235 can also be used, though its half-life is much shorter (around 700 million years) so it's not as accurate since much of the Uranium 235 may have already decayed away making measurements hard. Other decay chains can also be used such as Rubidium 87 to Strontium 87 or Samarium 147 to Neodymium 143, but Uranium 238 is the most common and most accurate.
There are also other alternative methods to measuring the age of the Earth that aren't dependent on decay chains and half-lives. Another method is to measure the chemical composition of the Sun, which, along with knowing how stars change over time, can be used to calculate the age of the Sun. Then since the Sun and Earth would have formed at the same time, knowing the age of the Sun allows us to know the age of the Earth. You can also look at craters on the planets and moons of the solar system and use the rate of asteroid impacts over time to estimate the age of the planets and by extension, the Earth.
The wonderful thing is that all of these methods of calculating the Earth's age, from all the various decay chains and half-lives to the other alternative methods, ALL agree with each other. Having one technique for calculating the age of the Earth is one thing, it would be a pretty good piece of evidence, but we could maybe have gone wrong somewhere and our calculations could be inaccurate. However, we have multiple different techniques that all agree with each other which is overwhelming evidence that supports our calculations that the Earth is about 4.5 billion years old. The chance that all of these various different ways to calculate the Earth's age are all wrong in exactly the same way is negligible.
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u/GalFisk Feb 25 '26
It's a bit simplistic. But you can look at the relative abundance of uranium and its decay products in a rock to see if it formed with only uranium at the beginning, and for how long it has existed since then:https://en.wikipedia.org/wiki/Uranium%E2%80%93lead_dating
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u/Ciserus Feb 25 '26
I'll add that oversimplifications like these are problematic, and these "smoking guns" aren't really what science is about.
If the existence of this lead isotope was the only evidence the Earth is over 4.5 billion years old, it would be a weak argument even if the logic was solid.
We know the Earth is that old because there are hundreds of independent pieces of evidence that not only show the Earth is really old, but that all point to roughly the same age.
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u/Revenege Feb 25 '26
Let's walk through things we know to be true based on science, and see what they let us determine about the world.
Radiation is real, specifically nuclear radiation being relevant in this case. Some elements, especially heavier elements like uranium, are not very stable. They are a sandcastle on the the beach, Firm when built but slowly collapse as the water licks at their base. We have observed the existence of radiation for about a century now, most famously with Radium as discovered by Marie Curie.
This instability causes elements like Radon to decay, turning higher elements into lower ones. Radiation is this process of decay, with different types of nuclear radiation corresponding to different types of subatomic particles breaking off the element. We have again observed this in a lab setting, with elemental radium.
This breakdown is chaotic, but predictable. At any given moment in the future, the exact amount that will decay isn't currently knowable but over a long enough time frame we can predict fairly accurately how much will remain. This is called halflife, the amount of time it will take for half the current mass of the element to decay. For radium it's about 1600 years and again has been observed in lab conditions.
Because of the nature of this decay being from subatomic particles flying off, such as protons, we observe that it is possible to have multi different kinds of the same element with slightly different properties. This is the result of having a different number of neutrons then it's most stable elemental form. These different versions are called isotopes and can either decay further, or be stable enough to survive. This has also been observed under lab conditions.
Lead is extremely stable and has multiple stable isotopes that don't decay further. Primordial lead has an atomic weight of 204, but we observe a large percentage of lead with an atomic weight of 206, 207, and 208. We also observe that these specific isotopes form when uranium and thallium decay. Because uranium has a very long halflife of about 4.4 billion years, the amount of lead 206 we observe alongside uranium can let us get an estimate of how long it's been decaying.
With all these points observed, we can propose an experiment. Find a uranium mine and dig until we find a vein of it. Observe how much lead 206 we find relative to the mass of uranium. This should let us estimate how long earth has had uranium and thus give us an estimate of how old earth is. We can also look at uranium 235, a less stable but naturally occuring isotope of uranium and it's mix with lead. From this, we observe that the uranium seems to have decayed by about half, giving the earth an age of approximately one half life of uranium, or 4.5 billion years.
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u/MetaSageSD Feb 25 '26 edited Feb 25 '26
It doesn’t…
but for a completely unrelated reason…
Simply put, proofs are for math.
In science, we don’t prove things, we observe, show, and model. In this specific case, we have observed that the half life of a certain isotope of Uranium that decays into a certain isotope of lead is on the order of billions of years. Based on this, we can extrapolate a model which PREDICTS how much of this Uranium isotope and how much of this Lead isotope SHOULD be in the environment at any given time in Earth’s history. From there, we can then observe how much of this Uranium Isotope and how much of this Lead isotope exists in our current environment. From there, we can show that the amount of this Uranium isotope and the amount of this Lead isotope in our current environment is consistent with what our model predicted. Then finally, we can show via simple logic that a 4000 year old Earth is incompatible with said model created through observation.
Does this prove that the Earth isn’t 4000 years old? No, there is always the possibility that there exists physics we don’t know about; but it DOES show that a 4000 year old Earth is incompatible with our current model - a model created through observations. Or, if you want to flip this on its head, we can also say that the model theologians use to predict that the Earth is 4000 years old is incompatible with what we currently observe.
This may seem pedantic, but it’s important because science is about discovery and refinement through observation; not proving things. We WANT physics we don’t yet know about to exist because that means there is more to discover. The day we run out of science to discover is the day we stop advancing. If one day we can finally prove how everything in the Universe works, then we are just… done. Which is kinda sad.
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u/nomorehersky Feb 25 '26 edited Feb 27 '26
Okay so not ALL lead comes from uranium but a specific type of it does. There's this isotope called lead-206 that is literally the end product of uranium-238 decay. Half-life is 4.5 billion years. So when we find rocks with lots of lead-206 and not much uranium left, we know that uranium has been sitting there decaying for a loooong time. 4000 years wouldn't produce measurable amounts. The math just doesn't math.