TRAVELING BETWEEN ALTERNATE WORLDS

How do you get far-flung empires without space travel? Lots of Earth-like planets? Humanoid aliens that can eat the same food as humans? By visiting lots of alternate Earths. But certain limitations need to be enforced to tell the kinds of stories I wanted to tell.

The Goal
I didn't want to tell alternate history stories. I didn't want to tell stories about how magical societies clash with modern technological cultures. I didn't want to tell Amber-style stories.

But in a wide-open alternate world setting, those types of stories are almost unavoidable. where anything you can imagine must exist somewhere among the alternate universes. There's nothing wrong with those kinds of stories. Other people are telling them quite well, however.So I came up with some rules governing travel between alternate worlds. I want these to be logical, but their real purpose is to reinforce the kinds of stories I am interested in.

The Rules of Travel
There are probably an infinite number of alternate worlds, and with great effort and under certain limitations, you can travel between them. You can’t access all the alternate possibilities, however, for three reasons:

• Law of Similarity: If the timelines of a pair of alternate worlds are too similar, you can’t pass between them. Think of it as the principle of like repels like in magnetism. You can’t go to an alternate world and meet a doppleganger version of yourself, for example. You can’t even find two versions of history in which Alexander the Great did different things. But you can find many cases of parallel evolution and development.
• Law of Opposition: If a pair of worlds are too divergent in their timelines, you can’t create the necessary magical formulas for the gateway, and travel between those worlds is impossible. The underlying natures of the worlds are just too alien to be reconciled.
• Law of Magic: Related to the magical Laws of Similarity and Opposition, this simply boils down to the maxim that you can’t forge a gateway between a world in which magic works and one in which it doesn’t. The fundamental laws governing each universe are too distinct for the gap to be bridged.

These conditions leave a large but still limited “window” of worlds that can freely interact with each other. In my setting there are dozens of worlds connected to each other by social, political, and economic ties. Worlds that belong to this interplanetary network are known as Named Worlds. Most of the Named Worlds lie within the sphere of influence of one or more of the major interplanetary Powers.

Cousins, Not Twins
The Earth is very similar in its general aspects from one alternate world to the next: the continents are the same basic shape, the world is the same age, the Moon is still there, the same principles of geology and meteorology apply. There is a representation of the fundamental geography shared by all Earths called The Map of All Worlds.

However, there are many differences that, while small on the global scale, are significant on the local scale. Coastlines are different, rivers run slightly different courses, and ocean currents may shift. Extreme events such as severe weather, volcanic eruptions, major earthquakes, and even meteor strikes can vary from world to world, leading to differences in climate.

And different species may have evolved, survived, or gone extinct. Mutations like blue eyes and red hair or violet eyes and white hair may have spread on one world and not another.

So each world has its own flavor. But all the variation is within certain parameters.