Published: 10 February, 2005

Earth doesn't gain mass, it just redistributes it

Does the mass of the Earth increase with the increasing population? If not, why not?

I know several friends who have recently had a baby. You probably know someone who just had a baby too, because across the world about three babies are born every second! About 90 million new people come into the world every year. If an average baby weighs about 6 pounds at birth, then we're looking at roughly 500,000,000 pounds of baby every year, about the mass of 50,000 full grown elephants. Still, this two hundred million kilograms of new babies is only 0.000000000000004 % of the total mass of the Earth (6*10^24 Kg). In fact, the total biomass of the planet (all plant and animal matter), is roughly only 0.00000003% of the total mass of the Earth (most of that biomass is in forests).

So, even if all the babies born did add to the mass of the earth, it would hardly be noticeable. But the population changing doesn't change the mass of the Earth. Before a baby is born, it grows inside its mother. When someone grows, baby or otherwise, they're not adding to the mass of the planet; rather,they're converting other mass into them. In some sense, you really are what you eat. If you eat a hamburger for instance, your body breaks that food down into molecules and atoms, incorporates what it can use into your body, and passes what it can't use out as waste. You're just taking matter from something else and incorporating it into yourself. The famous chemist Antoine Laurent Lavoisier did a series of careful experiments back in the 1780's that showed mass was not created or destroyed in chemical reactions. By performing chemical reactions in sealed containers, he showed that the total mass is conserved. If you burn something like a piece of wood, the ash left doesn't have the same mass as the log, some of the mass of the log has escaped in the smoke. However, if you could keep track of where all the atoms from the log went up in smoke, you'd be able to tell that the total mass of the log and the air involved in burning it hadn't changed when it was smoke and ash afterwards.

Incorporating food into your body is a much more complicated chemical reaction, but also doesn't change the amount of mass on Earth. In the case of an unborn baby, most of the digesting is done by the mother, but the baby still converts the matter she passes to it into baby.

There are ways that the mass of the Earth can change. For instance, the lunar rover was left on the Moon by the Apollo astronauts, so that mass is no longer part of the Earth. Meteorites that crash into our planet, or even burn up in our atmosphere, can contribute some small mass to the planet. One of the most famous equations in physics, E=mc^2, actually tells us another way for total mass to change. Albert Einstein put forth the idea that matter and energy are just different forms of the same thing. Matter can be changed to energy, and vice versa. It doesn't happen much on Earth, but the massive amount of energy coming out of our Sun actually represents a gradual decrease in the Sun's mass. But that's a subject for another "Ask a Scientist."