# How long would it take to conquer a galaxy? [closed]

I will explain this as briefly as I can. I'm thinking about beginning a novel that has an MC that is born into a race of super-efficient alien invaders.

They're able to take over entire planets without the natives realizing they're there until it is much too late. The way they do this is by taking it one town/city/village at a time. The alien homeworld initially sends out a single conqueror alien to each galaxy.

The conquering alien begins by taking over a world within the galaxy that has a needed commodity to take over the other worlds of the galaxy. They take over that world by using the same selection process with the cities/towns/villages in that world. They conquer it city by city, world by world, using a combination of ingenious manipulation, their race's unique powers, and reinforcements.

Once the conquering alien has gotten the desired city to relinquish its authority to them, they call in warriors to act as law enforcement, then they establish a new set of rules for the natives to adhere to, and then finally they bring purebloods from the homeworld to be rulers. Then, the conqueror alien moves on to the next city, town, village, or world. The conqueror only has to worry about getting the natives to give up and establishing the laws, maintenance is handled by reinforcements.

I have already thought through the why, the how, and established that this method of conquest is feasible. Assuming that not every celestial body in the galaxy is inhabited and that the majority of those that are habited are homogenous societies or societies with smaller populations than that of earth, approximately how long (in years) would it take for an entire galaxy to be conquered?

• Since we don't have even a remote estimate of how many civilizations there are (bottom limit: 1. Top limit: many millions) there is no way to answer your question. Also, you have 't said how fast they travel, whether they use self-replicating machines to move through the galaxy exponentially or take on one star system at a time, whether they can detect all the civilizations remotely or if they have to find them through exploration, etc. – Dan Hanson Dec 31 '19 at 20:56
• Seems like a not very complex math problem. It takes X amount of time to conquer a planet. There are about Y planets that are conquerable. The aliens can work on Z amount of planets at once. So, it's about X * (Y / Z). Give or take some time for logistics but this can easily overlap with other planet conquering activities. – VLAZ Dec 31 '19 at 20:56
• If you envision these aliens as moving through the galaxy enslaving one star system at a time, you are talking about distances many multiples of the width of the galaxy. Assuming they can travel near the speed of light, travel times alone would be many millions of years. Probably more than enough time for other civilizations to prepare for them and attack them. Finally, compared to all the travel time, the time it takes to actually subdue a planet is probably a rounding error and not important. – Dan Hanson Dec 31 '19 at 21:02
• @VLAZ I bet travel time dominates that entire computation. Galaxies are big. Traveling between them is bigger. I bet the time needed to conquer a world is generally a tail on the travel time. I haven’t run the numbers, but that’s how most “how long in space” questions work out. – SRM Dec 31 '19 at 21:04
• In terms of reopening, there's a critical variable value missing in the question which reflects directly on the answer. That being the speed of travel. The answer is literally, whatever the speed of travel is in the universe times the distance to the furthermost planet from the point of origin. – Separatrix Jan 9 at 13:41

Galaxies are big. Let’s say it takes 100 Earth years to conquer one planet. Let’s say that in the entire Milky Way, there are only two sentient races: humans and The Others, on the opposite side of the galaxy. Your aliens arrive at one edge and spend 100 years conquering Earth. Then they fly to the other side and spend another 100 years on The Others. In between, your conquerors spend:

“At 17.3 km/s, it would take Voyager over1,700,000,000 years to traverse the entire length of the Milky Way. Even traveling at the speed of light, it would take nearly a hundred thousand years!” — NASA

So who cares how long it takes to conquer. Just traversing is probably, even at extreme speeds, you’re looking at MILLIONS of years of travel time AT BEST.

If you have to move back and forth across the galaxy hitting lots of stops along the way, that travel time goes up by A LOT because of acceleration and deceleration time.

I think you should rethink your story in light of “Space Is Biiiiiiig!”

[Update] "Let's do it in parallel!" VLAZ suggested in comments that the conquering might happen with multiple agents working in parallel. That still doesn't help you much... the agent who is going to conquer that last civilization still has to travel for millions of years to reach the civilization on the far side. All the worlds in between are very far apart. Just getting to Alpha Centuari, 4.39 lightyears away, would take Voyager 40,000 years by some estimates. A better craft running under continuous acceleration could peel time off of that, but still you're looking at serious travel time. The lightsail project is talking 20 years to reach Alpha Centauri by getting up to 20% of lightspeed for a ship the size of a computer chip. That's not a conquering vessel! That's a surveillance drone!

And that's just the nearest star to Earth. I haven't even discussed the time to get BETWEEN galaxies. That's REALLY a long time. The nearest galaxy to the Milky Way is the Andromeda Galaxy... it is 2,540,000 lightyears away from us. If we get a ship that can get to Alpha Centauri in 20 years away, that still means Andromeda is 12 MILLION YEARS AWAY.

All in all, you're still talking vast spans of time to reach most stars. Vast. You're not thinking big enough. Seriously... our whole species timeline is insignificant compared to Earth, much less to the solar system, and way much less to space!

# Space is BIG!

• Okay, understood, so in order add any sort of realism, I need to explain how they travel, either FTL, some kind of teleportation, or time warping. Thank you! – RE Lavender Dec 31 '19 at 22:51
• Even worse, how do the planets stay subjugated? – NomadMaker Dec 31 '19 at 22:55
• “Realism” goes in quotes. Anyone who has ANY kind of FTL also has time travel. They can freely violate causality. So your story has to either account for that or toss hard science aside. We give Star Trek a pass because this wasn’t a hard wall back in the 1960s... people thought there might be a way around it. But nowadays, the math is pretty irrefutable. You can write great galactic stories, but not with modern science. – SRM Dec 31 '19 at 23:03
• @RELavender You may want to read this: Why FTL Implies Time Travel: physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel – SRM Dec 31 '19 at 23:40
• @RELavender By the way, there's a sci-fi book called "Timelike Diplomacy" by Charles Stross that says, "Ok. Let's assume we have FTL and causality CAN be violated by anyone with a ship. What happens to human history at that point?" amazon.com/Timelike-Diplomacy-Singularity-Iron-Sunrise/dp/… You may enjoy it. – SRM Jan 9 at 6:00

Let me put here a rough estimate which should give you some idea of how much work it might take to conquer a galaxy, planet by planet. The Milky Way is about 100,000 light years in diameter. Suppose you have about N many planets with intelligent life in a typical 1 x 1 x 1 light-year region somewhere in the Milky Way, and these are evenly distributed across the galaxy. Then you'd be looking at something in the order of 125000000000000N (125 trillion times N) planets to conquer in the Milky Way.

Now, N is of course unknown in real life and probably much smaller than one. If we are the only intelligent life in the universe then it is basically 0. But presumably if you are writing a story about aliens conquering things then N can't be too small. Maybe, say, 1/1000000; out of a million light-year cubes of space, you can find at least one planet worth conquering. Then you still have to deal with roughly 125,000,000 planets. That's already a lot of conquering to do, even assuming you can travel anywhere instantaneously.

Not to mention that if N = 1/1000000, most of your story will be about your aliens wandering around space bored, searching for their next target.

If they were to conquer and spread in a similar way to humans, it should take somewhere between a century and a Millenium.

• 100 to 1000 years to spread to multiple planets? We've been on ours for about 200 000 years and we've barely set foot on a couple of other celestial bodies. – VLAZ Dec 31 '19 at 21:10
• Assuming that planetary conquest as such is instantaneous, and given that a galaxy is about 100 thousand light-years across, how do they even travel across the galaxy in a mere millennium? (And it has a double n, because it comes from annus "year", not from anus.) – AlexP Dec 31 '19 at 21:16
• @AlexP It does when your answers are spoken out of your anus. ;) – Muuski Dec 31 '19 at 21:37
• my answer was meant to be somewhat satirical but okay – 2024oyefold Jan 1 at 12:42