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Humans have achieved FTL and have colonized several planets in our greater stellar neighborhood. While we have discovered several planets that harbor life as we know it, we have not yet discovered other intelligent life. However, this isn't because we haven't yet encountered it; within the same stellar neighborhoods we've come to inhabit, there exist one or more intelligent and technologically advanced civilizations, none of which have yet become aware of each other or us. It may even be the case that a single star system plays host to all of them simultaneously, and they still haven't found each other.

How do you keep FTL-capable, intelligent, advanced civilizations from noticing or even bothering each other?

EDIT: Constraints: The creatures themselves must be similar enough to humans that they could be easily recognized as intelligent if encountered in-person. Individuals should be roughly similar in size to humans (they could be as large as whales or as small as bugs, but not, for example, microscopic or telescopic in scale.)

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    $\begingroup$ That's kinda hard, still ant sized might work. But that rules out energy beings and the like. perhaps they only life in gas clouds or deep underground and use teleportation over starship FTL travel. What kind of FTL system do you use? $\endgroup$
    – Mormacil
    Commented Mar 30, 2017 at 16:46
  • $\begingroup$ Perhaps that's one element of it: the mode of FTL that each uses is unknown to the other, so in that respect they remain invisible to each other. Combined with some of the answers, you would have each in different environments (perhaps gas giants would shield radio transmissions from getting out enough to be recognized as being caused by intelligent beings), each using different modes of transportation, and each probably appearing so different from each other it might be difficult to even recognize the other as a life form at all without significant context. $\endgroup$
    – Adam
    Commented Mar 30, 2017 at 17:21
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    $\begingroup$ In Ender's Game the buggers didn't realise that humans were sentient because humans aren't telepathic. Maybe humans wouldn't think buggers are sentient because they don't speak. In Starship Troopers the specialist Brain Bugs are unknown at the start. $\endgroup$
    – Grimm The Opiner
    Commented Mar 31, 2017 at 7:26
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    $\begingroup$ Pardon me, but how hard-sci-fi you want your answer? Because the first thing I had in mind when passing by this question was, that they are phase shifted. Sadly that is usually used to handwave the most astounding effects, but still... they could stand next to each other without noticing, except for the one scientist who happens to invent... something. $\endgroup$
    – Confused Merlin
    Commented Mar 31, 2017 at 8:00
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    $\begingroup$ @GrimmTheOpiner Wasn't it more to do with the fact that they are one large controlling hive mind per queen (which just happen to be telepathic), and didn't understand that individual humans were more significant/sentient/life-worthy than their expendable drones until they had one of their queens killed by humans (which was a huge no-no in their culture) and had a whole big "oh god what have we been doing" awakening because getting one of their own murdered made them realize that's what they were doing to us the whole time? $\endgroup$
    – mtraceur
    Commented Mar 31, 2017 at 14:35

20 Answers 20

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Easy

Space is big.

Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.

Douglas Adams "The Hitchhikers Guide to the Galaxy"

We like to think that radio waves we send out can be picked up by another civilisation, but this is just a huge comforting lie. Distance to Alpha Centauri system is about 4.37 light years. Radio signals lose power as a square of distance. For comparison, as of last year, Voyager 1 is 137 AU away from Earth (1 ly is about 63000 AU).

According to this answer power of about 20 kW is needed to communicate with Voyager probes. Knowing that, we can estimate that since Alpha Centauri is about 2000 times further away, communicating with it would require 4000000 more power, for a total requirement of about 81 GW (Giga-Watts). And that's using large focused antennae. Radio and TV transmissions dissipate into noise much sooner, I doubt they are detectable beyond Pluto's orbit.

Basically, detecting random radio transmissions is a pipe dream. You can have interstellar radio communications only if you use massive antennae, pumping terawatts directly at other systems. Unless you actually know there is someone to talk to, it's rather pointless, even without considering light-speed communication lag.

Because of that, you can have two civilisations inhabit neighbouring stars without ever finding out about each other.

Only real chance is if their probes, scout or colony ships happen to run into each other in some system. If you want to eliminate this risk, you simply need to make them completely different in terms of biology.

Through observation of other stars, including planetary transition effects, we have a pretty good idea where to find rocky planets like Earth, located in local goldilocks zone. There is little reason to visit systems lacking such planets, because those are the ones which could possibly be seeded with Earth-like biosphere and as such, are the most interesting for us. Simply give aliens vastly different requirements, and they will not bother visiting systems targeted by humans. This can go on for a very long time, with both civilisations being entirely spatially intertwined without ever learning of the others.

Until they happen upon system having planets preferred by both...

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    $\begingroup$ I think that the question specifies that they must be similar enough that they would recognize the other as intelligent, so I think the part about making them different is wrong. But the rest seems really good. +1 from me. $\endgroup$
    – 3C273
    Commented Mar 31, 2017 at 2:59
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    $\begingroup$ Even with the edit requiring aliens being similar to humans I think this is the right answer. Space is big, there are millions of stars and most of them have planets. Too many to search them all. Here in our solar system there are for bodies in the Goldilocks zone: Venus, Earth, Luna and Mars and you have every possibility from frozen hell to burning one. Once humans start visiting other solar system and realize the goldilocks zone is no guarantee of anything a solar system is not interesting enough to merit a visit just because it has some ordinary planets in it. $\endgroup$
    – Rekesoft
    Commented Mar 31, 2017 at 7:13
  • $\begingroup$ Arguably, this all depends on speed and cost of FTL, something I glossed over. If it's cheap and instantaneous, there's no reason not to pop over to any nearby system. If it takes a month to travel a light year and requires entire planetary energy output to produce fuel, then we can expect years of astronomical observation before choosing next exploration destination. $\endgroup$
    – M i ech
    Commented Mar 31, 2017 at 8:43
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    $\begingroup$ @Miech they don't have to be extremely different. Humanoids that live on cold environements will look for very cold planets, and cold-blooded humanoids will aim for very hot ones. we're temperate, so it's hard unless there is another "gaia" planet like earth that has all environements that we even meet. $\endgroup$
    – CptEric
    Commented Mar 31, 2017 at 11:36
  • $\begingroup$ @Miech Sure, but even so, you’d have to hit exactly the right solar system by chance. If you have FTL, there’s no reason to visit only the neighbouring systems rather than some much larger away, so how close the other civilisations are becomes mostly irrelevant, and whether you point your ships at the right system is then a question of luck: you sample more or less uniformly. And since “space is big”, the chance of finding even very close intelligent species becomes incredibly unlikely. $\endgroup$
    – Konrad Rudolph
    Commented Mar 31, 2017 at 13:13
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Haste makes waste!

You have your brand new shiny FTL drive, and the bureaucrats are demanding progress. You have to catalogue 50 new planets this week. Never mind that you can barely check the boxes on your planetary assays with that schedule. The bureaucrats demand progress, and this is the metric they chose!

That cliff with 30 foot tall giant golden squiggles certainly looks interesting. Maybe it's the product of intelligence. No time for that, though. The paperwork required to properly name every one of the 70 oceans on this particular planet is going to be a killer! You gaze at the pile of forms to be filled out if you find intelligent life. You'll never get a promotion if you fall behind on the catalogue. Time to jet off to the next planet! Maybe future adventurers will come back and figure out what the squiggles are

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    $\begingroup$ This is definitely not something I would have come up with myself. Bureaucracies will most certainly play a role in interstellar exploration, and hastily picked metrics for said exploration would be a great way to make fun of corporate culture. $\endgroup$
    – Adam
    Commented Mar 30, 2017 at 16:58
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    $\begingroup$ While this is creative, it seems highly unbelievable that not just one, but multiple, FTL-capable spacefaring civilizations care so little about examining new worlds that they could manage to miss another spacefaring civilization while cataloguing the planets they’re present on. $\endgroup$
    – Avernium
    Commented Mar 30, 2017 at 18:01
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    $\begingroup$ @Avernium I think it depends on how colonized the average planet is. I think this could happen early on, when there's a few heavily colonized planets and then mostly outposts from there. Planets are big. I do not doubt however, that a FTL capable civilization would realize something was going on if they came across Courescant. $\endgroup$
    – Cort Ammon
    Commented Mar 30, 2017 at 18:03
  • $\begingroup$ @Avernium Looking at the photos of Earth I don't see much of human activity. If someone took photos of Earth from sufficiently large distance they might just miss humans. $\endgroup$
    – Maja Piechotka
    Commented Apr 3, 2017 at 10:39
  • $\begingroup$ @Avernium if you look at the day side, sure... but when you are the opposite side of the planet... $\endgroup$
    – Baldrickk
    Commented Apr 3, 2017 at 14:13
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Make them inhabit vastly different environments inhospitable for the other species.

Example: Jovian gas giant dwellers with a different perspective of time (living longer with slower metabolism etc.) might not bother talking to those pesky fast humans on their little rock worlds, or the humans might have been moved in for a few decades before even being noticed.

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    $\begingroup$ Deep ocean is another great way to avoid contact - humans have searched a mere fraction of the ocean bed. Problem with this is that it might make them too alien to communicate, and that's part of the question's goal. $\endgroup$
    – Jeutnarg
    Commented Mar 31, 2017 at 21:16
  • $\begingroup$ Or comet clouds. Or magma. $\endgroup$
    – Beta
    Commented Apr 2, 2017 at 16:25
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Plot twist: the star system IS the intelligent civilization.

The starship captain finally made it! After years of training, her first FTL flight had been a success. The approach was perfect, landing textbook and her disembarkment catchphrase worthy of Neil Armstrong himself. The last thing to do was to use the hydraulic ram to ensure the Comms Pole / Earth Flag wouldn't get dislodged after they left. She primed the Pole into the guide tube, powered on the ram and flicked the switch-

Kranthos-5, youngest of the planet people jolted as a sudden itch broke out on its surface. Young and rash at 200 million years old, Kranthos volcanoed out a jet of lava to remove the offending sensation. "Strange, it didn't feel like an asteroid."

...

Differences in scale make communication difficult. When was the last time you had a meaningful conversation with a cold virus?

Edit: Oh you changed the scope of the question while I was writing this :(

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    $\begingroup$ Edit sadly invalids this answer. $\endgroup$
    – Mormacil
    Commented Mar 30, 2017 at 16:45
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    $\begingroup$ @mormacil but how is Kranthos-5 FTL-capable? This answer fails the first premises anyway, even before the edit $\endgroup$
    – Mindwin Remember Monica
    Commented Mar 30, 2017 at 17:09
  • $\begingroup$ @Mindwin - there was no prerequisite that the second species was FTL capable, only humans were stated to be FTL. Your point does stand that I hadn't demonstrated technology-capability of Kranthos-5. $\endgroup$
    – Hugh Nolan
    Commented Mar 30, 2017 at 17:11
  • $\begingroup$ "How do you keep FTL-capable, intelligent, advanced civilizations from noticing or even bothering each other?" - This implies both are FTL capable. $\endgroup$
    – Mormacil
    Commented Mar 30, 2017 at 17:18
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    $\begingroup$ @Mormacil Hugh just did not bother to write about Kranthos-5's black-hole-organ which it can use to generate a black hole and control the black-hole's size (horizon-size) and can scale it up to twice its own size. Whenever Kranthos-5 wants to try out a new star, it creates such a black hole at full size out at arm's length, then quickly scales it down again, repeating as necessary as Kranthos-5 rides the gravity waves around. Sometimes the planet people do this for fun, as the effects of this massive gravitational distortion affect them as alcohol does us; hangovers last many years. $\endgroup$
    – Loduwijk
    Commented Mar 30, 2017 at 22:10
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Bigotry.

Make one race like tigers and the other like deer, and they won't recognize each other as "human".

  • For example, as every tiger knows, the defining characteristic of intelligence is "undaunted ferocity" and these timid herds of all-look-the-same grass-eaters obviously have none of that.

  • Similarly deer-people know that the symptoms of intelligence are social harmony ("consensus") and ecological harmony ("wisdom"). Tigers (whose landers leave scorch-marks on the alpine meadows) clearly have less than none of that: and are less inteligent than rabbits, even.

According to our own history books, not very long ago, there were humans who didn't even recognize each other as intelligent.

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  • $\begingroup$ This is so delightfully pessimistic, it might just work. $\endgroup$
    – geometrian
    Commented Apr 2, 2017 at 5:24
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You need to make their anatomies, societies, means of communication, nature of technology, and other seemingly small details so radically different from each other, that even when they see each other, they fail to recognize it.

I can see a small parallel with the Real World (TM) humans, chimpanzees, ants, and dolphins. All four species live in cooperative social groups, often with hierarchies. All four use language in some form. All four have what could be argued as tool use. All four exhibit signs of intelligence (although ant intelligence is on a collective level.)

Humans arose last of all, but we have co-existed on this Earth for a million-ish years. Yet only recently has one of us begun to realize the potential of the other three. To this day, ants see humans only as large, dangerous things that might step on them or dig up their homes. Chimps see ants as food. Dolphins probably don't even know what an ant is. Humans debate whether a bubble net is actually a tool.

As another precedent, just a few hundred years ago, some humans looked at other humans and labeled them savage animals, just because their definitions of society differed.

Expand this to an interplanetary level, and one particularly anthropocentric (or whatever it is called for their species) creature could easily overlook another, dismissing it as just another puny insignificant life form.

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    $\begingroup$ If you can prove that chimpanzees, ants or dolphins use a signalling system with an expressive power even remotely similar to human language you will be considered the greatest linguist of times. $\endgroup$
    – AlexP
    Commented Mar 30, 2017 at 16:43
  • $\begingroup$ @AlexP I see you're referring to Dr Dolittle. Great man $\endgroup$
    – Jeutnarg
    Commented Mar 31, 2017 at 21:17
  • $\begingroup$ @AlexP Already kinda demonstrated in published peer reviewed studies ("remotely similar to human language") with prairie dogs, not even any of the more sophisticated beings. They describe numerous details about creatures around their colony, including things like the color of clothes and things in the hands of approaching humans (somewhat crudely, but they can distinguish between an unarmed human and one carrying a long stick-like tool, so there's that). I'm not sure if you already knew this and were just making the reference for humor, or not. $\endgroup$
    – mtraceur
    Commented Apr 2, 2017 at 6:28
  • $\begingroup$ @mtraceur: The point is that it was not demonstrated. What was demonstrated is that they can communicate data about the world; this is nothing exceptional, very many species can, for example, bees. Human language can communicate data about the communication itself (for example, "I heard from Tom that a fox is comming, but I don't think it's true"), and, crucially, it's unbounded (what is called "recursion", for example, "John said that Mary said that Anne thought that Mark thought he had seen a fox"). Demonstrating any of those two in a non-human species would be a fantasic achievement. $\endgroup$
    – AlexP
    Commented Apr 2, 2017 at 6:43
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    $\begingroup$ @AlexP Fair enough - the problem with phrases like "even remotely similar" is that they are rather broadly open to interpretation - looks like you and I had very different ideas for the scope of "remotely similar" in your original statement. $\endgroup$
    – mtraceur
    Commented Apr 2, 2017 at 8:44
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Well, I am afraid you really will have to rely on space is large.

There are two requirements for that to work:

Neither is civilization is noisy and will not be detected unless you actively look.

No radio or other broadcast communications. No routine use of nuclear explosives. FTL does not cause Nova-like lightshows on arrival. No routine large scale terraforming.

Neither civilization is actively looking.

Basically both civilizations assume that they are alone in the neighbourhood, so they are not actively looking. Presumably because they have never found any sign of anyone. Which is because they have not really looked.

They'd basically see astronomical objects when on a system and then sends scouts to physically look at interesting planets.

They would discover each other when scouts stumbled on something. Presumably this would be a permanent settlement. So you'd manage the odds of detection by limiting the extent of permanent settlement. I think something like densely populated cores distant from each other and then overlapping sparsely populated fringes where settlements are small and rare enough to avoid random detection.

I see no reasons why the empires could not claim the same system or even have mining or science stations in the same system. Or even the same planet. If you do not actively look even few hundred kilometers hides things pretty well.

The needed distance and thus density possible will probably hinge on whatever engines used for lifting off or coming down on the planet. Something like a rocket lift off or shuttle re-entry would be visible for far way. Coming down or going down on anti-grav might be hard to notice even for people right below the vessel.

So you'll want anti-grav and some sort of reactionless space drives to match your FTL.

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If both civilizations are mutually recognizable then the only way the them to be unaware of each other is for them to not be in the same space.

Perhaps they have vastly different definitions of what habitable and interesting are when selecting where to travel to.

The odds are pretty high that they would become aware of each other as soon as their radio signals are picked up by the other. This is unless one race or the other has never communicated by radio, which is unlikely.

To see how far our own signals have propigated check out this.

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  • $\begingroup$ Plus one for the closest answer to, nope. $\endgroup$
    – Mazura
    Commented Mar 30, 2017 at 23:44
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    $\begingroup$ Primitive radio communications would be noticed. However, advanced radio communications are all but indistinguishable from noise if you don't know the encoding. If you're looking for aliens you can still detect them by noting that planet is awfully radio-noisy and the spectrum of the noise looks very strange indeed! However, the average person is unlikely to even have the equipment, let alone recognize the significance of what it's telling him. $\endgroup$
    – Loren Pechtel
    Commented Mar 31, 2017 at 3:17
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    $\begingroup$ We aren't talking about people we are talking about civilizations advanced enough to break the fundamental laws of physics. $\endgroup$
    – sphennings
    Commented Mar 31, 2017 at 4:57
  • $\begingroup$ As Loren Pechtel said: advanced coding always aims to optimise Shannon entropy, and the signal with the highest entropy is noise. But more relevantly: isotropic broadcasting is only effective at short range, and at sufficiently high frequency (which you need anyway for bandwidth) it's an obvious step to use directed transmission. $\endgroup$
    – leftaroundabout
    Commented Mar 31, 2017 at 20:13
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Advanced civilization #1 is human.

Advanced civilization #2 evolved on a gas giant and can only live under extreme pressure--they can't build spaceships because the only way to get the pressure they need to survive is gravity, there is nothing strong enough to contain the pressure.

This does not mean they can't build probes that can leave their planets, though. They are extremely difficult to construct due to the changes that will take place as it rises from the depths, not to mention the rare materials that must go into making them. Hence their space program is tiny and the craft suborbital (so the materials can be recovered)--until they finally discovered how to open a wormhole. The energies must balance, a wormhole can only be opened to another location at the same pressure. Hence the hop from world to world without visiting the space in between.

Humans get around by the same wormholes but the same pressure rules apply--we can no more jump into their realms than they can into ours.

There could even be more than one such race with different temperature requirements. The Jovians and the Neptunians never want the same world so they don't find each other.

Advanced race #4 lives in space and at an extremely slow rate. (Handwave--while I believe such a race is possible none can exist as their slow life process means slow evolution--there hasn't been enough time for them to evolve to civilization.) They aren't common, none of our scientists have noticed that some rocks in space aren't rocks. Our life processes are too fast to have come to their attention yet.

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By definition, neither they nor us could be a Kardashev Level >1 civilization since utilization of most or all of stellar output would definitely be notable. Given our utilization of energy, it seems that what you'd need is these other civilizations exist at either much faster or much slower rates, so that any heat or emr signals from them would be indistinguishable from noise. An alternative would be that they exist somewhere which we have not adequately explored. Obvious candidates include the interior of planets, or in the Oort Cloud. I suppose there is no requirement for an "advanced" civilization to be curious, so if they developed a million years before we did (our species is estimated to be roughly 200,000 years old, although it seems that significant changes occurred about 50,000 to 100,000 years ago). And if they "kept their heads down" during our electromagnetic "noisy" period (still on-going), then if they were few in number - a "civilization" existing in a Matrix type machine might only occupy a very very small space maybe only a few hundred or thousand kilometers².

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  • $\begingroup$ Partially offtopic, but nice idea: there are a lot of periodally pulsating variable stars. Maybe some ot them is controlled by kardashev 2 civilizations to adapt some periodicity in their society. :-) $\endgroup$
    – Gray Sheep
    Commented Mar 30, 2017 at 17:24
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FTL means time travel. So let's assume there's some magic that allows for much faster than light travel, but nothing between that and conventional travel. So to get from here to a neighboring star, you either have to go way back in time, or wait thousands of years (for a direct trip.) Thus these civilizations could co-exist on 'nearby' stars, but not notice each other because the FTL travel path takes them too far back in time in the process. Traditional radio-emissions might still take decades to reach the other star system in present time, and would only do so at a very weak and possibly undetectable level.

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    $\begingroup$ FTL within our universe means the possibility of time travel but not the certainty of it. However, it's quite possible to have other means of going fast that don't--any drive that imposes a fixed reference frame upon it's use avoids the time travel problem. Einstein said our universe doesn't use fixed frame, that doesn't say there can't be one that simply doesn't matter to our physics. $\endgroup$
    – Loren Pechtel
    Commented Mar 31, 2017 at 3:24
  • $\begingroup$ Time's Arrow imposes a fixed reference frame on three-dimensional beings, you can't time travel, it would be fun but no. $\endgroup$
    – Ash
    Commented Sep 25, 2017 at 11:28
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As has already been posted: space is VERY big and radio signals tend to get lost even within a single solar system. It is absolutely possible for multiple advanced FTL civilizations to be in the same system without finding one another for an extended period of time.

Here's a point that wasn't brought up previously: Why are we in the system? What are we looking for? We are assuming that humans are there to look for Earth like planets, right? We further assume that these aliens would be similar enough to humans to be instantly recognizable, and that does imply enough similarity that they might be interested in the same kind of planet, right?

What if one party's FTL technology was so limiting that it demanded the primary objective in going to a new system be servicing the FTL drive and NOT looking for new, colonizable planets?

For example: let's say that the FTL drive (which also serves as a communication unit, since FTL travel without FTL comms is problematic) uses mass to energy conversion that demands a very specific and rare type of material to be stable. The very first thing a brand new colony in a brand new system has to do is camp out next to a deposit of this stuff and mine enough of it to have a ride home (or even a phone call home to let others know they are ok). Unfortunately for them, this stuff is rare and usually only found in Oort belts way out on the outskirts of a solar system (or maybe in the molten cores of hot planets very close to the sun). So, for the first few years of colonization, this species is living in very uncomfortable pressurized domes in a location that is NOT optimal for them.

Species #2 on the other hand has a very different type of FTL technology. Their drives work in a different way. They aren't as worried about making a phone call back, so they head directly to the planet that BOTH species find most optimal for colonization in the "goldilocks zone". Now, unless they were specifically looking for one another, it could take a long time for either side to realize that they aren't alone. Random radio chatter wouldn't do it either, since there is SO much noise out there and signals get so weak so fast. This would be particularly true if one of the two was camped out very close to the star.

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It would be easy to fail to notice an intelligent species if they did not make significant alterations to their environment, if those significant alterations were structured in such a way that they did not appear "artificial".

I did initially also consider an underground civilisation, but we're already in a position where our probes routinely scan for gravitational anomalies, so this would not go unnoticed. At the very least this would be a trigger for investigation due to never-before-seen geological structures, and first contact would then be inevitable.

All previous answers seem to assume an "FTL drive". If instead you assume "FTL portals" allowing rapid transit to another place (via wormholes or whatever other handwavey pseudo-science) then you could very easily avoid significant changes to the environment. Most of our infrastructure comes from transport requirements; if you remove this then you immediately remove all roads, large buildings, need for lighting (if you want light to work by, you can simply portal to a place with daylight), and so on. If you want food, you can simply portal to a field and pick stuff; and telling a field (without walls or fences, naturally) from a naturally-occuring area of plant-covered land would be very hard.

Examples in fiction would be the Foxen from Grass (and other advanced races from the Tepperverse), or the way farcasters allow travel in the Hyperion Cantos.

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  • $\begingroup$ Only works if portals are very cheap and easy, what about houses, factories...? $\endgroup$
    – Donald Hobson
    Commented Apr 1, 2017 at 19:39
  • $\begingroup$ @DonaldHobson That's true enough, but if they are then it's as cheap to walk through a portal as it is to walk through a regular doorway. The Hyperion example is particularly interesting, with "houses" having rooms distributed around worlds. As for factories, unless you're building something physically large then you simply don't need most of that building. And with this level of tech, I'd assume massive automation anyway. $\endgroup$
    – Graham
    Commented Apr 3, 2017 at 10:20
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They live in a multiverse, and FTL actually involves going to other sheafs of the multiverse.

Targetting is a problem; most of the multiverse is chaotic noise. Nothing interesting there.

To find an appropriate target, we have figured out how to use massive body location patterns (stars and planets) to produce a fingerprint, then target our FTL based on those coordinates.

The multiverse isn't infinite, so if we input random patterns we'll end up nowhere and lost. So we very carefully survey nearby solar systems and target a local pattern of the planets and stars that actually exist.

We do not, however, actually arrive in our universe. We arrive in a nearby one.

If we want to get back to Earth, our location in the multiverse is also anchored into a hyperplane defined by Earth itself.

In effect, there is one alpha centauri we can reach (or a small limited set). It isn't in our universe; if you FTL there, and fly back to Earth, there are no humans there.

To keep colonies from being lost, FTL beacons are set up, and we FTL from one beacon to another.

Possibly the FTL is actually planet-to-planet, and we don't go into space for our FTL travel. Conventional space travel remains expensive and mostly pointless.

The other civilizations exist within our space, but possibly in other sheafs. Our beacons are incompatible, and we haven't noticed each other yet.

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  • $\begingroup$ Related idea: the geometry of FTL is largely independent of that of normal space, so what's near for our eyes may not be near for a ship. $\endgroup$
    – Anton Sherwood
    Commented Apr 2, 2017 at 3:34
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So, if I understand this right, humans have found several species who are able to go FTL and travel but they have not chosen to yet. They come in contact with humans, who have the ability already to and you want to know how humans can basically police them?

Before I get started, it would probably be worth noting that, as soon as a technology is shown, others will try to achieve it. Humans showing up on planets with species with technology capable of FTL are going to inspire them to build it too so they can explore. Preventing them from exploring will be pretty impossible unless you find some way to control them through culture or religious means. If they have FTL technology... I have a feeling though that religion may not play a factor.. but who knows they could be like the covenant in Halo XD

Okay, for one... Humans need to establish credibility. If we don't establish any reputation and credibility, no one will listen to us. If species A meets species B and you try to walk in between them and say stop fighting, if they have no respect for you, they will just push you away and keep fighting. They may even team up against the humans to get us away so that they can do as they please.

As I said above... it won't be about how to prevent them from seeing each other, but rather, how to keep them to being friendly neighbors. Again that falls into how the humans interact. If you can set a good example and show them how to explore and be friendly, they in turn will be inspired to do the same and you will have a quadrant of species exploring and being good neighbors. Of course not everyone will be friendly. Some people are aggressive by nature no matter what you do.

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  • $\begingroup$ The idea is that humans and one or more other civilizations come to colonize the same parts of space at roughly the same time (within several thousand years of each other) without actually discovering each other, as a setup for a future plot. $\endgroup$
    – Adam
    Commented Mar 30, 2017 at 16:47
  • $\begingroup$ @Adam ok thanks for the clarification. May want to put that in the question because as I read your question now, it doesn't read the way your comment suggests. For me, I read it as there are just planets sitting there with capable species but who haven't tried yet. $\endgroup$
    – ggiaquin16
    Commented Mar 30, 2017 at 16:50
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The FTL is not detectable, or not easily detectable. Maybe they all use different types of FTL so they aren’t looking for each other.

FTL is possible from the surface of one planet to the surface of another (Star Gate). So everyone travels directly from their home planet to another planet of ideal habitability. Species could share the same planet if one preferred to live under the ocean, and another preferred to live in the magma core.

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We don't even know what 'intelligent life' means. Whales have far more complex, and far larger, brains than ours - do they count as 'intelligent'? (I'm not suggesting that they are FTL capable, though Star Trek suggests they have FTL relatives).

Putting that to one side, the answer is simple: What makes us 'noticeable' is the use of radio, which sucks as a communications medium since we have FTL. Let's assume that we have ansible comms, and that ansible communications is a directional closed signal, so you can only notice it if you are the receiving end of the device (some sort of fictional 'quantum entanglement' magic - which doesn't allow ansible communications in the real world), then it's very easy for many very similar species to co-habit our region of space.

Space is huge, and so are planets. If you aren't looking out for another entry vehicle, you won't notice someone else's. You could even have a relatively stable population on a planet before other alien colonies on that planet were discovered.

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The Universe is expanding at an increasing rate.

Even now there are galaxies in the visible universe which are moving away from us faster than the speed of light. What this would signify is that any light from the galaxy would never reach us ever, let alone reaching the galaxy we can never see it again.

So even a civilisation capable of doing FLT would not be able to see galaxies moving away at a speed greater than light.

Long time from now ( billions of years ), we would only be able to nearby galaxies. In such a situation even galaxies which we consider neighbourhood would just freeze and fade away from us.

Even if FTL is cheap to do, it is extremely unlikely that a civilisation would decide to venture into nowhere without any reason. ( Because they can travel FTL hence they can definitely reach galaxies which are not visible to them )

Finally place the other civilisations in galaxies moving away at faster than speed of light. Impossible to see them / communicate with them despite them being exactly human.

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    $\begingroup$ The question was asked pertaining to nearby civilizations, not placing them so far away from each other that physics dictates that they can't even see each other. $\endgroup$
    – Jarred Allen
    Commented Apr 1, 2017 at 15:47
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if the different technological civilizations inhabit totally different biospheres on totally different types of planets they might never notice their colonies in the same solar system. If type A aliens only visit the surfaces of Earth like planets and type B aliens only visit the subsurface oceans of ice moons like Europa, they might never meet.

Unless they have space radar for interplanetary traffic control in their solar systems and they detect a lot of space ships that aren't theirs going to places they would never go to.

Detecting the spaceships or other advanced technology of the other race would reveal their existence.

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Several thoughts:

  1. If the two/three/ten races use point-to-point communications, laser would be the obvious option, then they won't see each others' communications except by accident.

  2. If they live in different environments like David Brin's Oxygen/Hydrogen divide in the Uplift books then entire civilisations could co-habitat a section of space without bumping into each other.

  3. Time and movement are big factors, most estimates make the odds of multiple intelligent species actually overlapping in time exceedingly small but assuming they did then a migrating species and a sedentary group would have little opportunity to encounter each other. Imagine if Sol had been visited by a migratory harvester race that skimmed some atmosphere from Jupiter and a few Oort and Kuiper bodies on the way through in the 1200s on a 10000 year migration route we'd never know about it. A number of migratory species could knock around a given volume of space nearly forever without much noticing each other unless one, or more, of them was sloppy and left equipment lying around.

  4. Early life stages, if a creature with a long life cycle left eggs or pupae to mature unsupervised then other species probably wouldn't realise they were sharing living space until mum and dad came to check on the kids.

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