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Here's an idea that I've been toying with. The aliens are real, they are here and they dominate at least a part of our galaxy.

How did they overcome the vast distances and why haven't we noticed? Same reason. This alien race is s-l-o-w. Their life span is orders of magnitude longer than ours. (Their metabolism is slower in a similar manner) So they can explore, conquer and colonize one solar system after another without FTL. What's a hundred year journey to a fifty thousand old person?

Now my question is about scale. As I said, these aliens now dominate a part of our galaxy, so they must be "slow" enough to manage an empire at least five thousand light years across without FTL tech. They also must be so slow that we haven't noticed them yet (we might have detected them, but we mistook their actions for geological/astronomical events) At the same time I don't want to make them too slow. Otherwise they couldn't realistically have developed in our galaxy's lifespan, and we would also never notice them making the story less interesting.

So:

  • How much slower than us do they have to be to become a local galactic superpower given that communication is limited by c and travel by fractions of c? (Hand waved propulsion for now)
  • Where is the sweet spot where the are just fast enough for us to realize they exist and are coming?

Some clarification after excellent comments:

The effect I'm after is that the alien precence becomes obvious once you take a (long) step back. (Wait a minute, the continental drift isn't random?) If this concept turns into a story it will be around a somehow balanced conflict between us (who can do small-scale things quickly) and them (who can do large-scale things slowly)

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  • $\begingroup$ Iain M Banks' novel The Algebraist is largely about a species that meets your specifications rather well. $\endgroup$ – John Dallman Nov 2 '16 at 12:50
  • $\begingroup$ Thanks. I haven't read that. (Stephen Baxter has based a few novels around similar concepts as well) $\endgroup$ – Guran Nov 2 '16 at 13:06
  • $\begingroup$ There could be a human-like civilization populating 99% of the galaxy and we would have no idea. How are you envisioning we humans discover your long-lived aliens? $\endgroup$ – Kys Nov 2 '16 at 16:57
  • $\begingroup$ @Kys True, we wouldn't detect civilization (order, signals, structures and patterns that would give us a clue) from remote stars. But in my scenario, they are here. They are meddling with our solar system, even earth itself. But (due to their different time scale) they are doing so very slowly. We might first notice that the trajectories of some planet or asteroid are a bit off, but we dismiss that as poor measurements by earlier astronomers. And that crater-like feature on the moon? It wasn't there three hundred years ago, but who trusts eighteenth century drawings over modern photos? $\endgroup$ – Guran Nov 3 '16 at 7:15
  • $\begingroup$ And of course Lockstep. $\endgroup$ – JDługosz Nov 3 '16 at 10:35
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The suggestion made by YoustayIgo that a biologically immortal doesn't need to slow down makes considerable sense. The proposition that a galactic superpower with its "communication is limited by c and travel by fractions of c" also makes considerable sense.

It is worth looking at some of the queries forming part of your whole post to clarify the issues for a galactic superpower.

"What's a hundred year journey to a fifty thousand old person?" Well compared to human beings with a life expectancy of eighty years (it is true that many countries have populations with longer life expectancies, but this figure will serve as a benchmark). This is the of a 58.4 day journey one way for a human being. In this day and age, journeys with this duration are rare. Although some sea journeys will be this long. prior to the twentieth century such journeys and much longer were commonplace. So definitely your galactic aliens will have to deal with many such long journeys. These may not be insuperable, but they could be discomforting.

If the region they dominate covers several thousand light years either the aliens must travel extremely close to lightspeed to reduce their travel time with relativistic time dilation or use some form of biosuspension like cryogenic suspended animation. Other options include disassembling their bodies and then reconstituting themselves at their destination or even living through the entire journeys, presumably undertaking whatever work is required to maintain their galactic domain.

The main drawback to relativistic interstellar travel, of travelling close to lightspeed, is that it is extremely energy hungry. A sensible galactic superpower might be more inclined to travel more slowly, though this can still be at a reasonable fraction of lighspeed.

Currently human technology is not able to detect alien life on the exoplanets we know about to any degree of certainty. New missions in the near-future will start to change this. But we will still be far from being able to detect evidence of alien civilizations on any exoplanets.

If this galactic superpower maintains a large fleet of interstellar vehicles that are engaged in traversing across their domain of, at least, five thousand light years could this be detected?

Firstly, this depends on their means of propulsion. For example, fusion plasma propulsion systems could leave an exhaust trail. Again whether this is detectable depends on the scale and duration of the acceleration and deceleration phases of the spacecraft. For example, if the spacecraft accelerates to 0.1 c at 1 g the duration will be approximately five weeks. Ditto for its deceleration phase. So if we kept observing plasma jets either beginning or terminating at specific stars this would indicate the presence of fusion propulsion spaceships.

Of course, this could involve long observation periods. Basically we would need to be lucky to see, at least, one fusion starship during its active phase. Then we would need to look for more, spotting those again depends on how many vehicles leave or arrive at that star. If the rate was one per century, then it will be a long wait. While one per month might stand out as completely obvious.

Fusion propulsion has been used as an example. It is also that the galactic superpower's vessels use propulsion systems that don't have observable effects. For example, if they used an Alcubierre sublight drive, while this could have major local effects there might be nothing to be seen at a distance. In that case, billions of starships could be cruising through the domain of the galactic superpower and be unobserved.

What about their communications? Could these be observed? Probably, a galactic superpower will have learned to operate in an energy efficient and conservative manner. This will ensure their long-term survival. Communications will be by laser or maser channels aimed directly at the systems of their recipients. This means little in the way of side-channel signals that can be eavesdropped upon or observed.

Any navigation beacons won't blasting out high-powered signals, highly advanced technology will be capable of detecting even very weak or low-powered signals. They won't be broadcasting their presence to the rest of the galaxy.

In essence, even a galactic superpower spread across five thousand light years won't necessarily be sending out signs or signals that will clearly advertise its presence or its existence. Such a civilization will have learned how use its energy effectively and efficiently without leaking out any signals that show it is there.

Even against the vast scale of the galaxy and the niche in it that the galactic superpower occupies it will be extremely hard to detect them. It is little wonder the human species hasn't done so.

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  • $\begingroup$ Good answer. About communication and energy efficiency: Remember, these aliens are not in a hurry. They can afford to sacrifice bandwidth for reliability. Perhaps they are tinkering with the solar activity, turning stars into Yotta-watt, nano-bit/second beakons. (We would see the pattern eventually, but not before we looked at century-spanning data) $\endgroup$ – Guran Nov 3 '16 at 9:40
  • $\begingroup$ @Guran This is an interesting question I enjoyed answering it. The aliens don't need to hurry. So efficiency and reliably will be a major feature of their operations. Tinkering with stars might be excessive. Refer to Fred Hoyle's OCTOBER THE FIRST IS TOO LATE (1966) where an alien intelligence modulates the Sun's infrared output. Signalling that could be seen right across the galaxy. $\endgroup$ – a4android Nov 3 '16 at 11:17
  • $\begingroup$ Thanks. I have lots of follow up questions if I ever make something out of this, but my first priority is to get the scale right. It started as a thought experiment on how to have realistic interstellar empires without any FTL. $\endgroup$ – Guran Nov 3 '16 at 11:33
  • $\begingroup$ @Guran Quite understand. I've tried similar thought experiments with realistic, non-FTL galactic civilizations. The scales are mind-boggling. Trying to grasp the social, economic and political structures needed for galactic civilizations is non-trivial. I will look forward to your further questions. Have fun! $\endgroup$ – a4android Nov 3 '16 at 11:40
  • $\begingroup$ I'm accepting this as the answer, though it can still be improved with more thoughts about the relations between physical scale, timeframe, speed etc. $\endgroup$ – Guran Nov 7 '16 at 9:12
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I don't think you can/should make space faring species orders of magnitude slower than us. For one, space travel requires fast decisions, sometimes in the span of minutes, even seconds perhaps. You don't have to make your aliens slow in order to give them exaggeratedly long lifespans.

For example, bowhead whales have lifespans easily exceeding 200 years, yet they swim much faster than us, humans, and are quite dexterous. Your aliens could be the same way. You could even make them biologically immortal without slowing them down at all.

As far as the question of why we haven't discovered them yet is concerned, it is easily answered. Our technology for discovering exoplanets is very crude and we can only detect planets quite close to their parent stars. Even large planets, farther away from their parent stars, are not discovered through our current technology. With this in mind, it would be utterly impossible to see or otherwise detect any aliens living on some distant planets, tens of light years away.

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  • $\begingroup$ Perhaps "slow" is not the best description. (After all we're talking about a species that achieves inter-stellar travel.) The driving idea behind this concept is that these aliens work on a completely different time scale than us. Not sure yet where this will lead, but I'm picturing a conflict between momentum and agility. $\endgroup$ – Guran Nov 2 '16 at 13:04

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