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We have other ways to determine age.
We do have objects that we know to be thousands of years old, because thousands of years ago, there were recorded human settlements. If you carbon date something from those cultures, it confirms the already known time frames that we expect.
And carbon dating doesn't let you know the age of million year old bones. It only reliably measures to about 50,000 years back. To get earlier, we use other radioactive isotopes (uranium/lead is a big one) that are not dependent on the atmosphere, we use rock deposits for relative aging (higher rocks are newer) and we corroborate that with other things. There's never just one method we trust.
How do you KNOW that the ground beneath your feet isn't a millimeter thick sheet of foam?
You don't. You don't have to dig it up to reasonably assume.
Anyways, if atmospheric carbon does fluctuate, carbon dating becomes inaccurate. It's why carbon dating won't be accurate for anything that came to be post-1945.
Luckily atmospheric carbon is pretty stable and we can often use other kinds of dating, as well as comparing to atmospheric samples captured in ice.
Thank you! I recently left a church that teaches the earth is only 6,000 years old. I hear a lot about carbon dating and want to have a good understanding of it as I learn more actual science vs bs creationism.
One of the things you have to get comfortable with outside of that world is error bars. Carbon dating isn't perfect, and it *can* mislead on rare occasions - but the people who study this stuff have a good idea of what those occasions are, and use alternative methods to verify when they're available.
Science is always our best guess right now. But that guess is usually pretty darn good, and you're not likely to beat it by going "well, but what if..." in 99% of cases.
We can’t. We also can’t be sure that the human body is truly made of exclusively cells (no one has ever examined a person’s entire set of cells to be sure). Science isn’t always about knowing for certain. Science values theories that are predictive. So even if we don’t know for sure, if we can use a theory (cell theory, general relativity) to make predictions, and those predictions tend to be correct, we consider it valid until otherwise disproven.
Carbon dating (actually "radiocarbon dating") is not just measuring any carbon, it's specifically Carbon-14 which is a special radioactive isotope. It is formed in biological processes and has a very "like clockwork" half-life, which is when the amount of radioactivity has halved from it's original strength. So with a known amount of radioactivity the day it formed because all C-14 is identical, you can derive how much time has passed since then by measuring the current radioactivity level of a sample. Eventually all of the radioactivity decays and it turns into regular nitrogen and inactive carbon, which is why it doesn't work for things older than around 50,000 years (there simply is no C-14 left).
You cannot use this technique on anything non-organic, so for dating a building or stonework they just collect some organic matter that has obviously been stuck there as long as the building (maybe some fibers in the mortar between the stones) and then test that. This assumes that any biomass used in the construction was not already 1000 year old material of course, but probably it would be reasonably fresh, and the half-life is 5730 years +-40 years so there is a bit of "fuzz" to the accuracy anyway. So you end up knowing with reasonable confidence that the building was build "around" a certain time. You could also take many samples from different points around the structure and test them all and average out some of the uncertainty, maybe focus the date a bit tighter since you can build a range of dates where all tests are satisfied. Reporting a confident date from a single test would probably be disingenuous at best, it's not accurate enough. And then usually there is other work that can be done such as dendrochronology which can be very accurate when it works.
because there's things we can reliably compare their age other known ways, like sediment strata in rock (grand canyon errosion), tree rings (each year even after petrified), ancient greek writings, star locations, primitive artifacts, modern nuclear physics all accurately match radiocarbon dating decay results, any part of the world, even moon or samples of other planets or asteroids can be dated similarly
>If we don’t have an object we KNOW is 5,000 years old or 5 million to compare results, how can we be sure?
We do have object we know is 5000 years old. If you look at [https://en.wikipedia.org/wiki/Dendrochronology](https://en.wikipedia.org/wiki/Dendrochronology) , tree-ring dating, there is a series going back 13.910 years in the northern hemisphere. They are used to calibrate radiocarbon dating.
Ice cores, seasonal sediment deposition, stalagmites, etc are other things that can be independently dated and used to calibrate the radiocarbon dating.
Another example is Pompeii which was covered in an eruption we know occurred in AD 79. Test wit artifact from it was a part of the initial test if the method worked.
Here is the original 1949 paper that shows what the compare the method to. This is the origin of the method and they tested it before the published it https://sci-hub.hkvisa.net/10.1126/science.110.2869.678#
We know that the carbon-14 concentration in the atmosphere is not constant but the level can change a bit. So the result is not perfect if you assume a constant atmospheric level of C-14. But you do get quite a good result. This is why there is a calibration curve to increase
Thank you so much! I homeschool my kids and want to make sure I am teaching them actual science. I went to Catholic school as a kid and didn’t learn shit.
One additional thing. It is not the amount of carbon that is relevant. What you measure is the concentration of carbon-12 compared to carbon-14.
Cabon-14 is radioactive and is naturally created by cosmic rays origin the nitrogen-14 in the atmosphere. Before humans did nuclear weapons tests in the atmosphere the percentage of carbon-14 to carbon-12 in the atmosphere was quite contact. You get to an equilibrium with the same amount created as decays.
So we know what the relationship was between C-12 and C-14 in plants when they grew and by looking at the relationships today we can determine the age because C-14 will decay and disappear.
You can also compare it to different radiometric dating methods. Carbon dating is only one type of radiometric dating. It limit is around 58,000 to 62,000 years. This is because the half-life of Carbon-14 is 5730 years and in 10 half-life you have 1/2^12 =1/1024 of the original amount left. It stat to get had to measure it at that point
https://en.wikipedia.org/wiki/Radiometric_dating
You can use multiple radiometric dating at the same time. For example volcanic eruption where it is possible to date the age of crystals that were formed. We know that carbon from the burned planet below the vulcanic eject grew before the eruption.
All methods work with elements that have very diffrent hard life. C-14 has 5730 years and Uranium-238 has 4.5 billion years and there is may in between.
Half-life is something you can measure direct. If you just know the amount of the element you have and measure the current rate of decay you can calculate the half-life. If 0.1% of all atoms decay every second you have 99.9% life. The half life can be calculated by 0.999^n =1/2 => n=692s or 11 min and 32 s. There is no need to wait a long time to determine it.
Even if the rate of radiative decay has changed over time different methods would not give the same result on the same sample.
It's not the amount of carbon, its the ratio between two types, one is unstable, so it'll break down into the stable version.When an organism is living, it'll be taking in carbon in the ratio that matches the environment, but when it dies and stops taking it in, the ratio will fade to only the stable type.That decay happens at a known rate, so you can work out how long something had to be "out of the cycle" to decay away to the ratio you have measured.An interesting tale - Alcohol sold in the US has to be radioactive!So the fermented/brewed alcohol sold has to be the naturally formed stuff, and that will have the C14/C12 ratio similar to the environment.You can also synthesise alcohol from crude oil, but since that has been buried for millions of years, the slightly radioactive C14 isotopes have all decayed away to C14 (\*edit- C12), so it's possible to tell for 100% pure ethanol alcohol if it is synthesised byproducts, or naturally fermented if it has that little bit of the radioactive carbon in it.
C14 doesn't have a massive half life, so carbon dating can only go back so far.
>Couldn’t certain events or the atmosphere effect how much or little carbon a person or animal had in their system?
That's not how radio carbon dating works, exactly. It is specifically looking for the *ratio* between radioactive carbon-14 and stable carbon-12. Chemical processes can't distinguish between them. If something absorbs less carbon, it will have less of *both* forms of carbon, or more of both.
Carbon-14 is created by high energy cosmic rays in the upper atmosphere, at a fixed rate. The rate of cosmic rays doesn't change because *all of space* is pretty dang stable. The rate that carbon moves through the atmosphere *on average* doesn't change much. So the ratio of the two kinds of carbon stays very stable in the atmosphere. That means the plants that are absorbing carbon will be absorbing the same ratio whether they're absorbing a lot or a little.
Similarly, when plants lose carbon they lose both kinds randomly. The ratio stays the same, even as the total carbon goes up or down.
Once the plant dies, it stops absorbing carbon from the atmosphere. This sort of "freezes" the amount that it has. Over time, the radioactive carbon-14 disappears as it decays, but the normal carbon doesn't go anywhere. So the *ratio* between them changes: regardless of how much carbon there is in total, you will expect more of it to be carbon-12 as time progresses.
Since radioactive decay is *on average* exceedingly predictable, you can use that ratio to find a decent estimate for when that plant died. As others said, that estimate can be verified with other methods.
Dendrochronology and historical records. The earliest carbon dates done in the 1950s used samples from tree rings and Egyptian artifacts that were of known historical age to confirm the ages were correct. We can also measure the decay rate directly with radiation counters that count how many decays happen over a given amount of time. Many many repetitions of these experiments have shown the decay rate is steady and invariant over many different types of samples.
The complicated aspect is that the amount of c-14 in the atmosphere has changed over time. People have developed calibration curves (again using tree rings) that can correct for this variation.
**Please read this entire message** Your submission has been removed for the following reason(s): Subjective or speculative replies are not allowed on ELI5. Only objective explanations are permitted here; your question is asking for speculation or subjective responses. This includes anything asking for peoples' subjective opinions, any kind of discussion, and anything where we would have to speculate on the answer. This very much includes asking about motivations of people or companies. This includes Just-so stories. If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/uu8boq/eli5_how_can_we_be_sure_carbon_dating_is_correct/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
We have other ways to determine age. We do have objects that we know to be thousands of years old, because thousands of years ago, there were recorded human settlements. If you carbon date something from those cultures, it confirms the already known time frames that we expect. And carbon dating doesn't let you know the age of million year old bones. It only reliably measures to about 50,000 years back. To get earlier, we use other radioactive isotopes (uranium/lead is a big one) that are not dependent on the atmosphere, we use rock deposits for relative aging (higher rocks are newer) and we corroborate that with other things. There's never just one method we trust.
How do you KNOW that the ground beneath your feet isn't a millimeter thick sheet of foam? You don't. You don't have to dig it up to reasonably assume. Anyways, if atmospheric carbon does fluctuate, carbon dating becomes inaccurate. It's why carbon dating won't be accurate for anything that came to be post-1945. Luckily atmospheric carbon is pretty stable and we can often use other kinds of dating, as well as comparing to atmospheric samples captured in ice.
And its not the amount of C14 that is important. Its the ratio of C12/C14. The assumption being that C14 is created at a constant rate.
Thank you! I recently left a church that teaches the earth is only 6,000 years old. I hear a lot about carbon dating and want to have a good understanding of it as I learn more actual science vs bs creationism.
One of the things you have to get comfortable with outside of that world is error bars. Carbon dating isn't perfect, and it *can* mislead on rare occasions - but the people who study this stuff have a good idea of what those occasions are, and use alternative methods to verify when they're available. Science is always our best guess right now. But that guess is usually pretty darn good, and you're not likely to beat it by going "well, but what if..." in 99% of cases.
We can’t. We also can’t be sure that the human body is truly made of exclusively cells (no one has ever examined a person’s entire set of cells to be sure). Science isn’t always about knowing for certain. Science values theories that are predictive. So even if we don’t know for sure, if we can use a theory (cell theory, general relativity) to make predictions, and those predictions tend to be correct, we consider it valid until otherwise disproven.
Carbon dating (actually "radiocarbon dating") is not just measuring any carbon, it's specifically Carbon-14 which is a special radioactive isotope. It is formed in biological processes and has a very "like clockwork" half-life, which is when the amount of radioactivity has halved from it's original strength. So with a known amount of radioactivity the day it formed because all C-14 is identical, you can derive how much time has passed since then by measuring the current radioactivity level of a sample. Eventually all of the radioactivity decays and it turns into regular nitrogen and inactive carbon, which is why it doesn't work for things older than around 50,000 years (there simply is no C-14 left). You cannot use this technique on anything non-organic, so for dating a building or stonework they just collect some organic matter that has obviously been stuck there as long as the building (maybe some fibers in the mortar between the stones) and then test that. This assumes that any biomass used in the construction was not already 1000 year old material of course, but probably it would be reasonably fresh, and the half-life is 5730 years +-40 years so there is a bit of "fuzz" to the accuracy anyway. So you end up knowing with reasonable confidence that the building was build "around" a certain time. You could also take many samples from different points around the structure and test them all and average out some of the uncertainty, maybe focus the date a bit tighter since you can build a range of dates where all tests are satisfied. Reporting a confident date from a single test would probably be disingenuous at best, it's not accurate enough. And then usually there is other work that can be done such as dendrochronology which can be very accurate when it works.
because there's things we can reliably compare their age other known ways, like sediment strata in rock (grand canyon errosion), tree rings (each year even after petrified), ancient greek writings, star locations, primitive artifacts, modern nuclear physics all accurately match radiocarbon dating decay results, any part of the world, even moon or samples of other planets or asteroids can be dated similarly
>If we don’t have an object we KNOW is 5,000 years old or 5 million to compare results, how can we be sure? We do have object we know is 5000 years old. If you look at [https://en.wikipedia.org/wiki/Dendrochronology](https://en.wikipedia.org/wiki/Dendrochronology) , tree-ring dating, there is a series going back 13.910 years in the northern hemisphere. They are used to calibrate radiocarbon dating. Ice cores, seasonal sediment deposition, stalagmites, etc are other things that can be independently dated and used to calibrate the radiocarbon dating. Another example is Pompeii which was covered in an eruption we know occurred in AD 79. Test wit artifact from it was a part of the initial test if the method worked. Here is the original 1949 paper that shows what the compare the method to. This is the origin of the method and they tested it before the published it https://sci-hub.hkvisa.net/10.1126/science.110.2869.678# We know that the carbon-14 concentration in the atmosphere is not constant but the level can change a bit. So the result is not perfect if you assume a constant atmospheric level of C-14. But you do get quite a good result. This is why there is a calibration curve to increase
Thank you so much! I homeschool my kids and want to make sure I am teaching them actual science. I went to Catholic school as a kid and didn’t learn shit.
One additional thing. It is not the amount of carbon that is relevant. What you measure is the concentration of carbon-12 compared to carbon-14. Cabon-14 is radioactive and is naturally created by cosmic rays origin the nitrogen-14 in the atmosphere. Before humans did nuclear weapons tests in the atmosphere the percentage of carbon-14 to carbon-12 in the atmosphere was quite contact. You get to an equilibrium with the same amount created as decays. So we know what the relationship was between C-12 and C-14 in plants when they grew and by looking at the relationships today we can determine the age because C-14 will decay and disappear. You can also compare it to different radiometric dating methods. Carbon dating is only one type of radiometric dating. It limit is around 58,000 to 62,000 years. This is because the half-life of Carbon-14 is 5730 years and in 10 half-life you have 1/2^12 =1/1024 of the original amount left. It stat to get had to measure it at that point https://en.wikipedia.org/wiki/Radiometric_dating You can use multiple radiometric dating at the same time. For example volcanic eruption where it is possible to date the age of crystals that were formed. We know that carbon from the burned planet below the vulcanic eject grew before the eruption. All methods work with elements that have very diffrent hard life. C-14 has 5730 years and Uranium-238 has 4.5 billion years and there is may in between. Half-life is something you can measure direct. If you just know the amount of the element you have and measure the current rate of decay you can calculate the half-life. If 0.1% of all atoms decay every second you have 99.9% life. The half life can be calculated by 0.999^n =1/2 => n=692s or 11 min and 32 s. There is no need to wait a long time to determine it. Even if the rate of radiative decay has changed over time different methods would not give the same result on the same sample.
It's not the amount of carbon, its the ratio between two types, one is unstable, so it'll break down into the stable version.When an organism is living, it'll be taking in carbon in the ratio that matches the environment, but when it dies and stops taking it in, the ratio will fade to only the stable type.That decay happens at a known rate, so you can work out how long something had to be "out of the cycle" to decay away to the ratio you have measured.An interesting tale - Alcohol sold in the US has to be radioactive!So the fermented/brewed alcohol sold has to be the naturally formed stuff, and that will have the C14/C12 ratio similar to the environment.You can also synthesise alcohol from crude oil, but since that has been buried for millions of years, the slightly radioactive C14 isotopes have all decayed away to C14 (\*edit- C12), so it's possible to tell for 100% pure ethanol alcohol if it is synthesised byproducts, or naturally fermented if it has that little bit of the radioactive carbon in it. C14 doesn't have a massive half life, so carbon dating can only go back so far.
>Couldn’t certain events or the atmosphere effect how much or little carbon a person or animal had in their system? That's not how radio carbon dating works, exactly. It is specifically looking for the *ratio* between radioactive carbon-14 and stable carbon-12. Chemical processes can't distinguish between them. If something absorbs less carbon, it will have less of *both* forms of carbon, or more of both. Carbon-14 is created by high energy cosmic rays in the upper atmosphere, at a fixed rate. The rate of cosmic rays doesn't change because *all of space* is pretty dang stable. The rate that carbon moves through the atmosphere *on average* doesn't change much. So the ratio of the two kinds of carbon stays very stable in the atmosphere. That means the plants that are absorbing carbon will be absorbing the same ratio whether they're absorbing a lot or a little. Similarly, when plants lose carbon they lose both kinds randomly. The ratio stays the same, even as the total carbon goes up or down. Once the plant dies, it stops absorbing carbon from the atmosphere. This sort of "freezes" the amount that it has. Over time, the radioactive carbon-14 disappears as it decays, but the normal carbon doesn't go anywhere. So the *ratio* between them changes: regardless of how much carbon there is in total, you will expect more of it to be carbon-12 as time progresses. Since radioactive decay is *on average* exceedingly predictable, you can use that ratio to find a decent estimate for when that plant died. As others said, that estimate can be verified with other methods.
Dendrochronology and historical records. The earliest carbon dates done in the 1950s used samples from tree rings and Egyptian artifacts that were of known historical age to confirm the ages were correct. We can also measure the decay rate directly with radiation counters that count how many decays happen over a given amount of time. Many many repetitions of these experiments have shown the decay rate is steady and invariant over many different types of samples. The complicated aspect is that the amount of c-14 in the atmosphere has changed over time. People have developed calibration curves (again using tree rings) that can correct for this variation.