4 ms·

In the case of carbon dating, plants take in carbon-14 from the atmosphere. Carbon-14 is radioactive and decays at a known rate. Once a plant dies, the carbon-1

by jff 14y ago

In the case of carbon dating, plants take in carbon-14 from the atmosphere. Carbon-14 is radioactive and decays at a known rate. Once a plant dies, the carbon-14 begins to decay. By finding the ratio of carbon-14 to carbon-12 and carbon-13 in a sample, we can estimate how long it has been since the sample died. Since animals consume plants directly or indirectly, they also take in carbon-14 and can thus have their ages estimated in a similar fashion.

ars 14y ago
Just so you know, in practice it's considerably more complicated than that. The main limitation is that carbon dating only works to about 60,000 years (so older figures are never from carbon dating). This is well known. Less well known is that it only works if the sample has never been soaked in water. This is because the CO2 in water will dissolve into the sample completely overwhelming the original carbon ratios. This probably causes a huge number of errors in the field, unfortunately. Because how many samples have been completely shielded from water for that long? There are other datings though, but they are less accurate. Usually they are only able to date the rocks near the sample, but not the sample itself. http://en.wikipedia.org/wiki/Radiometric_dating http://en.wikipedia.org/wiki/Radiometric_dating Most commonly things are actually dated based on fossil and geological evidence, not any type of direct dating. But there is a bit of circular logic in those types of datings. First the rock is dated, and the sample is assumed to be of the same age. Next when other samples of the same type are found they are assumed to be of the same age as the first. The rock formations are then recorded and the appropriate layer is given an estimated age. Which is then used to date other things. Basically, things older than 60,000 years are dated based on what fits the theory best.
- ericd 14y ago
  Thanks for one of those rare concentrated knowledge nuggets that are one of the best parts of HN.
- Alex3917 14y ago
  Kind of like how we know the universe is 13 billion years old because of the speed of light. And we know the speed of light is constant because it hasn't changed in 100 years. Hmmm.
  - ars 14y ago
    Let's not overreact here. We know the speed of light is constant because it's impossible for it not to be - too much is tied into it. If you change the speed of light the energy content of matter changes, and where would that energy come from (or go to)? If you tried to balance out the change by changing other constants you would end up changing even more things in an impossible cascade. As for the age of the universe, it's not calculated from the speed of light, but from other things which I do not understand well enough to explain to others.
    - Alex3917 14y ago
      For what it's worth I was kind of joking, I agree that it would be basically impossible for the speed of light to change over time.

by jff 14y ago

ars 14y ago

Just so you know, in practice it's considerably more complicated than that. The main limitation is that carbon dating only works to about 60,000 years (so older figures are never from carbon dating). This is well known. Less well known is that it only works if the sample has never been soaked in water. This is because the CO2 in water will dissolve into the sample completely overwhelming the original carbon ratios. This probably causes a huge number of errors in the field, unfortunately. Because how many samples have been completely shielded from water for that long? There are other datings though, but they are less accurate. Usually they are only able to date the rocks near the sample, but not the sample itself. http://en.wikipedia.org/wiki/Radiometric_dating http://en.wikipedia.org/wiki/Radiometric_dating Most commonly things are actually dated based on fossil and geological evidence, not any type of direct dating. But there is a bit of circular logic in those types of datings. First the rock is dated, and the sample is assumed to be of the same age. Next when other samples of the same type are found they are assumed to be of the same age as the first. The rock formations are then recorded and the appropriate layer is given an estimated age. Which is then used to date other things. Basically, things older than 60,000 years are dated based on what fits the theory best.

ericd 14y ago
Thanks for one of those rare concentrated knowledge nuggets that are one of the best parts of HN.
Alex3917 14y ago
Kind of like how we know the universe is 13 billion years old because of the speed of light. And we know the speed of light is constant because it hasn't changed in 100 years. Hmmm.
- ars 14y ago
  Let's not overreact here. We know the speed of light is constant because it's impossible for it not to be - too much is tied into it. If you change the speed of light the energy content of matter changes, and where would that energy come from (or go to)? If you tried to balance out the change by changing other constants you would end up changing even more things in an impossible cascade. As for the age of the universe, it's not calculated from the speed of light, but from other things which I do not understand well enough to explain to others.
  - Alex3917 14y ago
    For what it's worth I was kind of joking, I agree that it would be basically impossible for the speed of light to change over time.