Placeholder Image

字幕列表 影片播放

已審核 字幕已審核
  • Hey there, and welcome to Brain Stuff.

    嘿各位,歡迎來到 Brain Stuff。

  • I'm Josh Clark and this is the Brain Stuff where I explain to you how carbon-14 dating works.

    我是 Josh Clark,而我將在 Brain Stuff 和你解釋碳十四年份測定法如何運作。

  • Carbon-14 dating, which we also just call carbon dating, is a form of radiometric dating.


  • And all it is, is measuring the decay of a certain type of atom found in a once living organism to determine when it was last alive.


  • And all of this starts in the up, up, upper atmosphere of Earth, where it's constantly bombarded by cosmic rays.


  • And one of the things these cosmic rays do is knock neutrons off of some atoms and protons off of others and before you know it, the nice, stable, family-man nitrogen-14 atom is all wrapped up and gone crazy, and becomes what's known as a carbon-14 atom, which is radioactive.


  • Now, carbon-14 atoms aren't the only ones in the upper atmosphere.


  • There's also carbon-12 atoms.


  • Carbon-12 atoms are in much more abundance and they're pretty stable.


  • Carbon-14 atoms are, again, radioactive and unstable.


  • But, they're formed at a reliable, steady rate, so at any point in time, we have a pretty good idea of the ratio of carbon-12 atoms to carbon-14 atoms.


  • Got that?


  • It's important.


  • Now, carbon dioxide is essential to life here on Earth.


  • Plants breathe it in, animals eat the plants, we eat the animals and the plants, and these carbon molecules, carbon-14 and 12 that make up the carbon dioxide, get in everything.


  • What's neat is that the ratio of carbon-12 to carbon-14 found in all these living things here on Earth is pretty much the same as what's in the atmosphere.


  • Which means it's predictable.


  • And some very, very smart scientists have figured out that carbon-14 actually decays at a predictable rate.


  • Carbon-14, like all radioactive particles, has what's called a half-life.


  • Now the half-life is the amount of time it takes for the number of radioactive particles in a sample to decrease by half.


  • Carbon-14 has a half-life of 5,730 years.

    碳十四的半衰期是 5,730 年。

  • That means that after 5,730 years, the amount of carbon-14 atoms in a plant that you found fossilized will be half of what it was the last time that plant took a breath of life.

    這代表經過 5,730 年後,你在一種變成化石的植物中測得的碳十四原子的量,比起上次它還活著時會減半。

  • After 11,460 years, which is two half-lives, there'll be a quarter of the amount of carbon-14 that was originally present.

    在 11,460 年後,也就是兩個半衰期,碳十四的量會剩下原來的四分之一。

  • And then after 17,190 years, you'll have just an eighth of the number of carbon-14.

    而在 17,190 年後,你只會有原本碳十四量的八分之一。

  • And so on, and so on, until there's none left.


  • And this is actually one of the limitations of carbon-14 dating, that eventually you're going to go far enough back in time that all of the carbon-14 atoms have decayed.


  • And you don't know whether this took place a day before or 100,000 years before.


  • Which means the time limit that you can date things using carbon-14 is roughly 50,000 years.


  • But as long as you are trying to date something that lived on Earth within the last 50,000 years, you can figure out roughly when it was last alive.


  • The way that you do that is by measuring the rate of decay of carbon-14 atoms compared to the slow and steady carbon-12 atoms that are also present in there.


  • Presto, chango, you've got some carbon-14 dating, and all of a sudden you say, "Oh my God, this wooden axe handle is 12,000 years old."


  • Pretty sweet stuff.


  • And this whole thing gets even more interesting when you realize that future archaeologists are going to have a lot of trouble using carbon-14 dating thanks to us, humans of the present time.


  • Our industrial activities have been pumping CO2 into the atmosphere and really messing with the ratio of carbon-12 to carbon-14 and, even more astounding, all of the nuclear bombs that we set off in the mid-20th century, well those messed with the atmospheric ratios, too.


  • So, good luck with all of that, future anthropologists and archaeologists.


  • Sorry.


Hey there, and welcome to Brain Stuff.

嘿各位,歡迎來到 Brain Stuff。

已審核 字幕已審核

單字即點即查 點擊單字可以查詢單字解釋