World Building 101: Today's Topic: That's Heavy, Man!

Gravity is what holds the universe together on the grand scale of things. All mass has gravity, which means, everything has gravity, because nothing, so far, has been found to be without mass. What is mass? Mass is energy, in one form or another. So all the universe is but an ephemeral net of energy, bent and twisted by the four basic forces: Gravity, Electromagnetism, Weak and Strong. I'm going to concentrate on the heavy subject of Gravity in today's lecture.

Gravity holds the Earth together. It keeps the atmosphere on the surface of the planet, and the oceans, and all the stuff living on it. The steady pull of gravity is what makes us weighable. If you step on a scale, you are measuring the pull of the Earth. If you were on Mars, the pull would be less, since Mars is less massive than the Earth.

What is the gravity on your fantasy world? Can it be less? Can it be more? Yes, and yes, within a certain range.

If your world is as massive as Mars, which is 38% the size of the Earth, then the pull would be 0.38 that of Earth's. Meaning a 100 pound rock would weigh only 38 pounds on your planet. It would still have the same mass, it would just be more buoyant... as if you had help hefting it.

This is where things can get confusing. Mass and weight are two different things. Only on Earth does a 100 lbs rock have 100 lbs of mass. Out in space it may be weightless, but it's not massless. It is always a 100 lbs mass.

Life on a world with less gravity would provide for larger growth. Large creatures are curtailed in their growth by the square-cube law. Larger objects get bigger by volume, as in cubic feet, while they must support themselves as a cross-section of their skeletons. Cross-sections are measure in square inches. If an animal doubles its size, then it has become eight times more massive (2x2x2), while its bone support has only increased by a factor of four (2x2). So the bones have to become even more massive to support the extra mass. This becomes progressively more difficult as the animal gets bigger and bigger.

(Learn more about The Square-cube law)

The whole subject of large creatures is still debated, with gravity providing but one of many limitations on being big, including landmass and biodiversity.

On a planet more massive than Earth, the animals would have a lesser upper limit, since the square-cube law applies stronger. On a planet twice as massive as Earth, you probably wouldn't have elephants, or if you did, they would have much thicker legs.

Gravity also greatly determines the atmospheric pressure of the planet. The lighter the planet, the lighter the atmosphere; the heavier the planet, the thicker the air. (Smaller planets can have thick atmospheres, from volcanic eruptions and other chemical events, but it will be short-lived, geologically speaking). I'll get into atmospheric composition in another lecture.

So what is a fantasy author to do? Make your planet smaller, larger, or about the same as Earth? Make it too small and there's not enough gravity to hold an atmosphere. Make it too large and the surface becomes slushy from the strong internal convections.

A small world makes feats of strength more believable -- but remember that mass stays the same. Lifting a car would be easier, but stopping a speeding train would be just as hard. And a less massive planet would make it easier to fly, except that smaller planets usually have less air (If you want to have large flying creatures, you'd have to have a world that is still producing its atmosphere, making it thick enough for flying, and coupled with the lesser gravity).

A large world would likely have a thicker atmosphere, smaller animal and plant life, and more chaotic plate tectonics. Cloud formation is easier under more pressure, and wind dynamics are more persistent. A storm may persist for several years on a heavy atmospheric world, circling the globe many times as it rages.