A while ago, I decided I wanted to read some science fiction about a galaxy-spanning empire. I briefly browsed Amazon’s kindle offerings under “galactic empire”, but all I found was military sci-fi where the galaxy-spanning nature of the empire wasn’t a major element in the story. Thinking back to classic sci-fi, I’m only coming up with Asimov’s foundation series. It’s possible novels set in the Star Wars or Star Trek universes might have some of what I’m looking for, but off-hand, I don’t even know how big their empires are. I’d appreciate reading recommendations in the comments below.
Since I didn’t find many galactic-empire stories, I decided to put down some thoughts to help sci-fi authors. I’m thinking of a mostly-human galaxy as I write this, but the concepts could apply in an empire composed of thousands of alien species.
First, let’s make sure we understand how huge a galaxy is. If you don’t know, read this wikipedia article. I’ve read a number of sci-fi stories where it’s obvious the author didn’t know the difference between a galaxy and a star. You can see what I think of such authors by reading the second short-story review on this page.
Since some or all of the characters in our story will be humans, let’s set our galactic empire in the Milky Way. Assuming people still live primarily on planets, how big will the empire be?
Most astronomers think that the Milky Way has 100-400 billion stars, although there are some much higher estimates. Astronomy isn’t my field, but I skimmed a scientific paper, found at http://arxiv.org/pdf/1107.1286, whose authors think that 1.2% of stars might have habitable planets. However, they assume that 75% of those planets would be tidally locked. I’m thinking that a tidally-locked planet wouldn’t be a nice habitat, due to high winds and the planet’s moisture being deposited on the dark side. Still, even using the conservative 100 billion figure, 0.3% gives our empire three hundred million planets to work with!
The above paper uses the word “habitable” differently from science fiction. The authors are thinking in SETI terms, looking at planets with the potential for life to evolve independently over billions of years, without being wiped out by radiation from a supernova. If our science-fiction galaxy was populated by other means such as panspermia, an elder alien race, or creation by God (or gods), then many more planets could be “habitable”. The same would be true if our empire puts time and effort into terraforming. For now, I’m going to stick with the most-conservative 300 million figure.
Does our empire actually rule the entire galaxy? Let’s look at some empires on Earth. The well-known empires of Alexander the Great, Rome, and the Ottoman Turks didn’t even reach 4% of the Earth’s surface area. Still, 4% of our galaxy would give an empire over ten million planets. Alexander’s empire didn’t last long enough to say much about, but Rome and the Ottoman Empire were overrun militarily, so maybe 4% doesn’t cut it.
The Russians and Mongols weighed in closer to 16%, which might give our empire a respectable forty-eight million planets. Genghis’ Mongol Empire didn’t last, and Russia faced some pretty serious invasions that were defeated with outside help.
Let’s go to the largest one, the British Empire. Its 24% would give us a whopping seventy-two million planets. Still, Earth’s British Empire fell after two victorious wars against Germany, and needed outside help for both victories.
Would 50% be enough to keep our empire secure? Maybe, but what if the other 50% were in the hands of a second empire? Cold War scenarios could play out, or there could be a never-ending war of attrition between the two empires. Jack Campbell’s Lost Fleet Series heads in this direction, but doesn’t go full galaxy.
If our empire has a technological edge, maybe it rules most or all of the galaxy. The full 300 million planets. You can easily double, triple, or quadruple that number if you want to use higher star system estimates, increase the likelihood of planets being habitable, or give your empire extreme terraforming capabilities.
My next few posts will look at economic, military, social, and political aspects of a galaxy-spanning empire. I’ll try not to think too small, but I will fail. The galaxy is just too dang big.
Update: Part 2 has been posted