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.

碳十四年份測定法，我們又把它稱為碳測定法，是一種用放射性性質測量的形式。

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.

而它所指的就是測量一個曾經活著的有機體中的特定元素，測量這特定元素的衰退，以推測它先前活到甚麼時候。

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

而這些全始自地球上、上、上層的大氣層，在那裡一直都充滿了宇宙射線。

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.

而這些宇宙射線做的其中一件事是擊走一些原子的中子和其他原子的質子，而在你發現前，那美好、穩定、愛家的氮十四變得無法控制，變成了我們所知的，具有放射性的碳十四原子。

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

碳十四並非唯一存在於上層大氣的原子。

There's also carbon-12 atoms.

那裡也存在著碳十二原子。

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

碳十二原子的量多很多，而且它們也很穩定。

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

碳十四原子，一樣，具放射性且不安定。

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.

但是，它們以確切穩定的速度形成，所以不管在哪個時間點，我們都知道碳十二和碳十四的比例。

Got that?

有理解嗎？

It's important.

這很重要。

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

現今，二氧化碳對地球的生命體是不可或缺的。

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.

植物吸入二氧化碳，動物吃這些植物，我們吃這些動物和植物，而這些組成二氧化碳的碳分子，碳十四和碳十二都參與其中。

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.

很美妙的是在這些地球上生命體中發現的碳十二和碳十四的比例，和大氣中的比例幾乎一樣。

Which means it's predictable.

這意味著它是可以被預測的。

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

而一些非常、非常聰明的科學家已經發現碳十四事實上以一個可以預測的速率衰退。

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

碳十四，和所有具放射性的粒子一樣，都有半衰期。

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

半衰期指的是在樣品中，具放射性的粒子量減半所需的時間。

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.

而這會不斷發生，直到甚麼都不剩。

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.

這事實上是碳十四年分測定法的其中一個限制，最後你將追溯到連碳十四都衰退殆盡的久遠過去。

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

而你也不知道這究竟發生在一天前還是十萬年前。

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

這意味著你可以用碳十四測定年分的時間限制大約是五萬年。

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.

但只要你試著測的東西是生存在地球過去五萬年內的，你大概可以得知牠上次還活著的時間。

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.

你所使用的方法是測量碳十四原子對上也存在大氣中，緩慢且穩定的碳十二原子的衰退的速率。

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."

很快地如魔法般，你將得到一些碳十四定年的資訊，然後你就會突然說，「喔我的天，這個木製的斧頭把手已經有一萬兩千年了。」

Pretty sweet stuff.

非常厲害的東西。

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.

而這一切在當你發現未來的考古學家將會因為我們，這些現今的人類，在使用碳十四測定法時遭遇很多麻煩，而變得更有趣。

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.

我們的工業活動不停地排放出二氧化碳到大氣層中，這真的打亂了碳十二和碳十四的比例，且更驚人的是，所有我們在二十世紀中投下的炸彈也打亂了大氣的比例。

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

所以，祝福一切都好運，未來的人類學家和考古學家。

Sorry.

抱歉了。