首先让我们换一角度看这一问题，地球上的所有人一共多重？当我写这篇文章的时候地球上大约有70亿人，我希望这篇文章写完地球仍有70亿人。
     Let’s look at this in different parts. First, what is the mass of all the people on the Earth? As I am writing this, there are about seven billion humans living on the Earth. Hopefully when this book is finished, there will still be around seven billion people, but let’s wait and see.
     如果我知道人类的平均质量，那么我就能知道地球上人的总质量。美国的普通男性大概70公斤，女性大约50公斤。我觉得这些都是成年人的。但美国人较其他地区的人更为强壮，这将使平均质量约在60公斤左右。如果我考虑到孩子，那么平均体重将是40公斤左右。
     If I know the average mass of a person, I can get the mass of all the people. Here’s where we guess. Well, not a completely random guess. No, it will be a well-aimed guess. An average human male is maybe 160 pounds (70 kg) and a female is about 110 pounds (50 kg). I suspect those are a little high for all of the adults. I would guess that adults in the United States are a bit more massive than other parts of the world. Now, what about chigamesen? I could say there are about as many men as women. That would put the average mass some-where around 60 kg. If I take into account chigamesen, maybe the average mass is around 40 kg.
     虽然40kg只是个估计，但这远远比100kg更容易接受
     Yes, 40 kg is an estimate. However, it isn’t a crazy estimate. If I were to actually go around and put every human on a scale, what would I get? I’d get a broken scale, that’s what. But I could also get the exact average human mass. I don’t think this mass could be lower than maybe 20 kg. Also, it couldn’t be higher than 60 kg (unless we discovered a new race of giant humans somewhere). So, again, 40 kg isn’t a crazy average. A crazy average would be something like 100 kg, that’s crazy.
     现在，我们得到了人类的平均质量。根据总人口的质量=人类总数 X 平均质量，我们将求出地球上所有人的质量280000000000公斤（2.8×10^9公斤）。这个质量看似十分巨大，但它与地球的质量6×10^24公斤相比，却只是个零头罢了。
     Now that I have an estimate for the average mass of a human, the total human population mass is just going to be the total number of humans multiplied by the average human mass. This would give a mass of 280 billion kg (2.8 x 109 kg). That’s a large mass, but how does it compare to the mass of the Earth? At about 6 x 1024 kg, all the humans are just a tiny fraction of the total mass (4.7 x 10-14 %).
     很难想象人类对于地球是多么的微不足道。让我再举一个例子。
It’s hard to realize just how small a percentage of mass humans are. Let me give another example. What if we look at the mass of a 60 kg human? The mass of all the humans on the Earth is proportionally like the mass of one single yeast cell on a human. So as you can see, the mass of the human population just doesn’t matter in regards to the total mass of the Earth.
     However, there is another part to this question: does the mass of the stuff on the Earth change? By “stuff” I mean all the living things and consumable things like air and water. In short, the answer is no. Where do humans come from? Or perhaps I should ask where the mass that makes up humans comes from? As a person grows or as new people are created, the mass for this new mate-rial comes from three sources: air, water, and food. Actually, I’m not too sure how much of our material comes from the air but it could be a small contributing factor.
     Where does the food come from? If everyone was a vegetarian, the food would come from plants. But where does the mass of plants come from? Most of the plant mass comes from the air. Yes, the air. Plants take in carbon dioxide and produce oxygen. They save the carbon (and water and other stuff) to use as building blocks for growing. It might seem strange, but it’s true.
     Which means that, indirectly at least, the mass of the human population comes from the air. When people die, they decompose and produce more carbon dioxide. It’s an endless cycle. Just about all of the mass comes from something that is already on the Earth. Yes, I said “just about.”
Is there mass that leaves the Earth? Is there mass that is added to the Earth? The answer for both questions is yes.
     When does the Earth lose mass? First, there is loss of gas in the atmosphere. Think of our air as a bunch of gas particles bouncing around. That’s essentially what is happening in reality. Some of these particles of gas (oxygen or nitrogen molecules) are going quite a bit faster than other particles. If the particle is going fast enough and is near the top of the atmosphere, it can escape the Earth’s gravitational influence. This does happen, but the effect is quite small. The other way for the Earth to lose mass is when humans send objects into space. Again, the total mass of all the man-made space objects is quite small.
     The Earth also gains mass. According to NASA, there are about a hundred tons of meteoroids hitting the Earth each day. Over a year, this would be 3 x 107 kg. It would take about a hundred years’ worth of meteors to equal the mass of the human population. However, doubling something super small is still some-thing super small.
     In the end, the mass of the Earth does change, but not from the human population. This mass is still relatively small compared to the mass of the Earth and doesn’t make a significant contribution to the Earth’s gravitational field. Even if the mass of the humans was much greater than it is now, the overall mass of the Earth is still mostly constant.