I am designing a hard sci-fi universe where humanity is confined to the Solar system and travels using fusion.

One of the obstacles I am facing is how to make humans meet one alien race which is roughly on the same technology level as humans are.

Namely :

  • Reliable interplanetary travel
  • No reliable interstellar travel
  • No space elevators
  • No antigravity
  • No antimatter production
  • No strong AI

It seems unbelievable that aliens will encounter us while being on the same technological level. They will be vastly technologically superior to us if they can travel between the stars.

The same applies to humans if we meet aliens in their solar system.

The only viable way is to come up with some way to connect different star systems to bypass the interstellar journey: the gate system of some kind.

gates from "The Expanse"

Naturally, such a system requires gate-builders, which is a trope beaten so hard, that it barely breathes.

The other way is to make these gates form naturally, but it requires very advanced technology to harness their power. The closest thing that is similar to a pair of gates is a wormhole. Essentially, the ability to handle those will bring humanity to the same level as gate-builders themselves.

Is there any other way to meet aliens different from just accidentally meeting them, while maintaining a reasonable technological level?

  • $\begingroup$ How much contact do you need? Is catching a slightly more advanced version of the Voyager probes enough? If interplanetary travel is straight forward it should be possible to build a bunch of these and aim that at all nearby stars, even if they take centuries or millenia to get there. $\endgroup$
    – quarague
    Commented Sep 12, 2023 at 10:41
  • 1
    $\begingroup$ AGI? I'm guessing anti gravity... um? $\endgroup$
    – terdon
    Commented Sep 12, 2023 at 14:03
  • $\begingroup$ Perhaps a rogue star system or galactic "collision" brings their two planets much closer for a temporary but lengthy period of time? $\endgroup$ Commented Sep 12, 2023 at 16:24
  • $\begingroup$ @AlexanderNied, this is a facinating idea explored below by The_Square-Cube_Law. The only obstacle to adopt the solution is an enormous time period between the "ejection" and the contact. Without capping technology level it is hard to answer why their civilization is not absolutely superior to our's $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 16:28
  • 2
    $\begingroup$ I misread the title as "unpopulated galaxy", and I had the idea of a back alley meeting with trench coats and fedoras pop into my mind. "Nah, I wasn't followed by no coppers! And this meeting never happened, ya see." Lol. $\endgroup$ Commented Sep 13, 2023 at 22:34

17 Answers 17


Unwilling visitors

Aliens do not have the technology for interstellar travel, but nature is an unforgiving female dog.

The poor bastards were evicted from their stellar system when another star came close to theirs and pulled them out. They had to spend all their resources - and I mean all - just to survive underground in a rogue planet.

Now, as you say, they have:

  • Reliable interplanetary travel
  • No reliable interstellar travel

When their planet eventually comes into our solar system, they might think: "oh there's a couple planets here where we could thrive" (Earth and Mars). It takes astronomically less energy to move from a escaping planet to Mars or Earth than it takes to make the Sun capture their planet, even with the help of a body such as Jupiter. So they come down to us.

Or humans are just curious and try to get samples of the visiting planet before it leaves the solar system, and SURPRISE!

You could reverse this by making Earth the rogue planet. I am writing a book on this and have a handful questions on the technicalities here in this site. I don't feel like this is just my idea though and would be glad to see other authors' take on this.

Edit, per request:

There are three more issues that need to be tackled here.

Surviving on a rogue planet

A planet that goes rogue will have its surface frozen in less than geological time. Earth, for example, might have oxygen rains within a year from its departure. Here is a Kurzgesagt about it:

What If Earth got Kicked Out of the Solar System? Rogue Earth

In this case, most if not all ecossystems would be gone. For an Earth like planet with an Earth like civilization, survival would require going very deep underground for warmth. The surface may get its heat from the Sun but the core will take almost a hundred billion years to freeze.

Survivors don't need to go all the way to the core, but they should dig at least a few kilometers into the crust. That could provide stable temperatures for millennia.

The next three biggest problems are energy, food, and other resources in general. A civilization capable of digging that should have mastered nuclear fission power, maybe fusion as well. That could power hydroponic and aeroponic farms. The civilization may have taken some farm animals to the underworld with it as a protein source. Finally, as for resources: no place richer in ores than the crust!

We all suspended disbelief for a smaller scale version of this in The Matrix movies (Zion was an underworld civilization). This wouldn't be too far-fetched.

Surviving for very long, but without becoming too advanced

With all said above, you might have a civilization that is stable. It would have a very reduced population from the start. With the underworld as its sole environment, they would also be very constrained in resources. This means they would develop very slowly in terms of technology, if at all. No star also means they can never reach Kardashev level 2 (or even a full level 1!). Space research and travel would not be incentivized, as that would be a relative waste of resources. Space research could be kickstarted again once the planet does pass by a star. So you have a society that does not evolve rapidly through millennia, and even if it does achieve something, its space technology will not surpass Earth's. More likely the aliens will be better at materials science and anything about survival, but will be worse than earthlings in most other areas despite being a much older civilization.

That said, they will be much more careful about wars. Nuking your enemies in an underworld could lead to global extinction with even just one small bomb, depending on how the underworld is set. But classic gun warfare over very limited resources might keep everyone too busy to develop science that is much more advanced than Earth's. This is one more reason they might spend hundreds of thousands of years without advancing much.

Who knows, maybe some religious or philosophical beliefs also keep them alive in peace but also rejecting further tech advance. Or maybe they spent so long being told what to do by limited AI's (not AGI's) that almost nobody gets into science.


From the alien's point of view, knowing about the Earth might be a no-brainer. We have detected thousands of planets on other stars. Maybe they detected SOL c (that would be Earth) between millions of years to five thousand years ago (from Earth's relativistic point of view). Some promising signs of life (just as we found dimethyl sulfide on K2-18 b recently), but nothing conclusive and surely no signs of intelligent life. When they do get into our solar system, they will already know about the planets here. They will be in for a few surprises when they see Earth up close. It supports life, but it's already taken by a civilization of its own.

As for us... We were able to detect Oumuamua as it did a flyby around the Sun, and it's just 115 meters long (that's 1.25 football fields in American units). Granted, we would have found it too late if it was coming right at us, and there are still people looking for "planet 9" in our system... but as long as something Earth-sized came within maybe the orbit of Neptune or Uranus, we should be able to detect it, even visually. This will happen sooner and more easily if the planet is coming from a plane close to that of the planets around the sun, and if the rogue planet has a high albedo. Alternatively one of our interstellar probes might crash against it, causing many scientists to focus where the probe was.

Or we might pick up radio signals from some equipment that the aliens put on the surface of their world when they came into the solar system.

Or - the most chaotic way - they could send a probe here ahead of their closest approach, in order to assess the capacity of planets in our system to sustain (their) life. We could find such a probe on Mars, the Moon or somewhere else. The probe would probably send a signal back to its homeworld as a focused beam, so even if we can't make heads or tails of it we could look at the general direction the signal is going and find an Earth sized projectile coming into our system. That should make the news on Earth mainstream media for like a couple days.

Alternatively, we don't get to talk to aliens directly. A probe comes by our system, the alien equivalent of the Pioneer/Voyager/New Horizons probes (we did send those on unreliable interstellar travel, so...). If you drop the no AGI rule, they could be 50 or 100 years more advanced than us when they launched, and their probe might be a Von Neumann probe.

  • $\begingroup$ The rouge planet might be a fruitfull idea. I just can't think of clever way to deal with huge time period between their ejection from home system and arrival to ours. Some sort of slumber maybe? Otherwise the rouge planet outside of the solar system can go unnoticed for a long time and found, for example with Voyager in later XXI century $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 16:20
  • $\begingroup$ Can you give me a link to track your book about the rogue planet? In some point I was very fascinated with the idea of oxygen rain and even modeled how long it will take after the sun dissapers for different gases to fall from the sky $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 16:22
  • $\begingroup$ @FrogOfJuly I don't have anything made public about it yet. I do have a subscription on World Anvil that I may make public someday. But thanks for showing interest! $\endgroup$ Commented Sep 13, 2023 at 0:55
  • $\begingroup$ I think about choosing your answer as the answer for my question. Can you elaborate on how to endure the long journey through interstellar void and how it is hard to actually notice rouge planet from Earth? Maybe with some links to papers and other works of fiction. As things go it seems like your rouge planet idea is keeping all perkes of low tech generational ship, with better hidden luck of finding specifically our solar system $\endgroup$
    – FrogOfJuly
    Commented Sep 13, 2023 at 11:21
  • $\begingroup$ @FrogOfJuly I've made an edit to address survival and detection $\endgroup$ Commented Sep 13, 2023 at 18:32

How about this:

  • The milky way is absolutely teeming with life, most of it confined to super earths, so the few dozens or so instances intelligent life emerged they didn't manage to leave their gravity well. There's the theory that a "good" planet for life to emerge will be tendentially larger than Earth. Absolutely teeming means that one out of every few dozen or so star systems has life.

  • Because it took one generation of stars to die and form heavy elements, all inhabited solar systems are close in age to ours, or a younger (there's a paper somewhere on erxiv arguing this but I haven't read it)

  • Earth sized, habitable planets are rare and sought after (literally, using telescopes etc.) Interstellar travel takes ages and is only undertaken with a worthwhile goal - What happens in your story is this: One other intelligent species (with space travel, so from an earth sized planet), and humans discover an (inhabited or not) earth sized habitable planet and send a steady stream of generation ships to this system from more or less opposite directions. The Red Mars trilogy makes the point that terraforming one planet takes two centuries with the resources of earth right next door, maybe your planet is better suited but the tools are far more limited? You could construct a scenario where one side arrives a few decades or a century earlier, without having too much of a foothold.

Basically the earth like planet is an excuse for both species to meet in the middle. You still need to come up with solutions for the incredible long travel time.

  • $\begingroup$ The links for papers are very appreciated, so I would love if you will provide one you mentioned! $\endgroup$
    – FrogOfJuly
    Commented Sep 11, 2023 at 19:21
  • $\begingroup$ @FrogOfJuly sorry, cant find the paper (tbh I've only looked briefly) $\endgroup$
    – mart
    Commented Sep 12, 2023 at 19:51
  • $\begingroup$ Make it 2 habitable planets in one system, and the two races are even more likely to meet there. $\endgroup$ Commented Sep 13, 2023 at 21:27

Frame Challenge: They can be way more advanced than us but not in ways that matter.

They will be vastly technologically superior to us if they can travel between the stars.

This axiom is not necessarily true. There are plenty of ways that your alien race could be technological comparable with humanity in all the ways that matter for your story to work without being technologically identical.

Option 1: FTL is not as hard as you think

You are assuming that FTL travel requires a massive chain of technological development that we have yet to achieve. Yet our own civilization went from not having a basic understanding of the inner workings of an atomic nucleolus to using atomic physics to destroy entire cities in just a few years. Humans could easily, right now, be a single scientific discovery away from knowing everything we need to know to make our own FTL engines, and we would not even know it until we get there. In fact, there is already some very promising research into Casimir Cavities showing that a precisely made meta-material can in theory achieve perpetual motion using Alcubierre's warp metrics and quantum physics. FTL, or at least reactionless, relativistic propulsion, could be a near future discovery as soon as someone figures out how to turn these microscopic experiments into something that works at human scales. Especially if your version of humanity is already decades or centuries past our current technologically levels.

Option 2: Both races have very different technology trees

Another possibility is that both of our races have significant technological advantages over each-other, but just in certain areas so that one can not rightly be called more advanced than the other. They may have a functional warp drive, but maybe they have yet to discover computers or wireless communications or something like that. In this respect, both of our civilizations could look in awe on each-other's achievements, and neither have a significant advantage over all.

Ferengi technology is estimated to be generally equal to our own [...] We are no doubt (more) advanced in some areas, they in others.

~Data : StarTrek TNG

Option 3: Humans have a superior military tradition and infrastructure, despite being "less advanced" overall

They could have many technological advantages over us, but they have no history of war on their planet; so, if push comes to shove, all their fancy engines and replicators and stuff don't actually put humanity at a disadvantage because only we have pre-existing military technology which we've spent a very long time perfecting.

The military societal capabilities of a modern nation can not be built in less than a few decades even if you already have a good idea about what it is you are trying to build. This timescale gets even longer if you don't already have a mindset for warfare. The aliens could probably figure out how to turn out some basic riffles and IDEs pretty quickly, but effective tanks, smart missiles, targeting computers, stealth paint, etc... these take years of RnD followed by more years of industrial organization just to build the manufacturing facilities for.

Futhermore, just because they have over all superior material science does not mean they will know the right materials to choose for each purpose. If your aliens tried to make a bullet using their strongest, most light weight alloys it would make for a much worse bullet than one made from brass and lead. They could use their hardest synthetic diamonds to armor their tanks, only to find that they shatter like glass when hit by a cannon because they don't know that they need to layer hard and soft materials or use sloped armor to aid in deflection; so, even their best materials could fare worse than human composites because we fully understand what it is we are creating the armor for. They will not know they need to fill their hand grenades with a shrapnel material. They will not know that guns need sights to help them aim. They will not understand how to balance rate-of-fire, accuracy, ammo capacity, and stopping power. They will not understand the point of combined arms. They will not know that their heat, radar, and radio signals can be tracked, or how to hide them. They will not know that their networks need cyber security. Their children have not grown up playing violent videogames, watching war movies, and studying military history in schools, so not only do they not understand strategy and tactics, but they don't even understand that those are areas of study.

On top of all of this, it will also take them time to figure out how to turn any of their advanced technologies into viable weapon systems that might surpass human technology. When they see a gun for the first time, they will see the most advanced killing machine their society has ever faced; so, even if they have the physics to make a phaser, it's not something they have considered making before; so, their best idea at first will be to try replicate the gun until they understand guns well enough to know that a phaser could be better.

By the time they begin to understand warfare well enough to fight back, they could already be fully enslaved... or at the very least, the humans will enjoy enough early military successes to steal enough alien technology that by the time the aliens learn to fight, humans will be on equal footing in non-military ways as well.

Option 4: Humans are able to talk our way into equal footing

For this option, it could be that they are more advanced than us in every way imaginable. They could be an all out post-scarcity level society with a stable global government and a fully automated economic system. But because of this, it means their current generation has no concepts of primitive capitalism; so, marketing, propaganda and information warfare, office politics, etc. are foreign concepts to them.

So we just flimflam and negotiate our way into equal footing very quickly because we have a whole world full of salesmen, marketing specialists, and con-men packing a plethora of skills that these aliens' mammas never warned them about. By the time they understand what a Nigerian Prince even is, they've already given away the keys to the kingdom.

Option 5: They only packed for a "camping trip"

The trip between the stars is a big one, even for an advanced alien race. This means that the alien explorers/colonists will have to leave a lot of the comforts of home behind when they make the long trip to Sol. If room/mass on their ship is limited, and the infrastructure to maintain a lot of thier more advanced tech requires proximity to thier home world, then they will only bring a fraction of the tech that their society has to offer. So, while they may appear to be about the same tech as us, this is a big misperception.

Think of it like this. If you plan to go on a long camping trip, you might bring some basic hand tools, a tent, a lighter, food, etc., but you would not have a computer, microwave, air conditioner, or Apache attack helicopter. So, if you were to plan a time traveling camping trip to the medieval period, people would see your technology as different from theirs, but you would not seem all that much more advanced than they are. Likewise, an alien species that is only packed for an exploratory mission may appear far less advanced than what that civilization is actually able to muster.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Commented Sep 13, 2023 at 15:36


You could have an alien race about the same technology as the Solarisians ("Earthlings" from the Solar system, working title).

The aliens were desperate to get out of their star system. The star might go to a different stage (destroying part of the system), overpopulation or seeing that they need to take risks for their species to survive the inevitable downfall of their star system. Their technology is similar to the Solarisians at the point of leaving. Either they start on their journey early in their domination of the star system, or they developed late in the star system cycle, or they just aren't as quick to learn. Whatever the reason, they left at around that point in their technology.

The interstellar travel is uncertain for them as well, but desparation or the drive to survive by spreading through the galaxy can make them take such risks. They are likely to send multiple ships in many directions, with possibly several ships to promising destinations/hopefully safer routes.

Though technology can progress during the voyage, the limitations of the ships might make it difficult to truly progress, or make the progress into reality.

From here you have many options open for your story. They might be excited to work with the Solarisians to exchange technology and make interstellar travel truly viable. They might not know if there will be a species if they finally arrive, but ready to cooperate. They might have scanned the place during the voyage or beforehand and know that the Solarisians are there. They might want to out compete the Solarisians, taking the chance their own technology might be outdated the time they arrive.

Whatever the reason, a ship or some ships have survived the uncertain journey, and have arrived at the Solar system. You might still call it chance or an accident, but in the grand scheme of things it is inevitable to happen somewhere in the galaxy you're describing. Possibly even with multiple alien races arriving at a single star system. If that is the case, why not the Solar system?

  • $\begingroup$ This would be an exellect answer as long as there is some quality of our star that will stand out for those who takes the voyage to stars. Because as far as stars go there is a planet in habitable zone hear Alpha Centauri(just for example). Why not aim there instead of Earth? $\endgroup$
    – FrogOfJuly
    Commented Sep 11, 2023 at 19:02
  • 1
    $\begingroup$ @FrogOfJuly I would say you've written the answer yourself. In the galaxy only few intelligent aliens exist. This can be because of the makup of the star systems. Though there are planets in the goldilocks zone in many star systems, it is suspected more is needed for life to thrive. We have large planets that prevent lots of meteors hitting Earth for example. Whatever the reason, the fact that there is intelligent life (possible) means that it is the more suitable choice for a civilisation to go there. Otherwise you have a higher risk of being wiped out by a cosmic event. $\endgroup$
    – Trioxidane
    Commented Sep 11, 2023 at 19:46
  • $\begingroup$ @FrogOfJuly: Do you know how many Earth-like planets have been discovered to date? There's few enough not too big rock (not gas) planets in the Goldilocks, if you also wants some (but not Venus like) atmosphere, a magnetosphere, and a few giants around to shield from meteorites... not sure there's anything like this in the galaxies around. The aliens took a pot shot, traveling towards us, and as they scan/send probes from their ship, Earth was the only viable planet they found so far. $\endgroup$ Commented Sep 12, 2023 at 8:44
  • 1
    $\begingroup$ @FrogOfJuly: Quite far from what I was thinking, actually. Remember the keyword "desperation". In my mind, the aliens have a barely working generation ship, degrading more and more as time passes and they run out of spares, and are fervently praying that the next Goldilock planet on the scheduled itinerary will be a good match... for they're not sure to ever reach the one after. In fact, everyone aboard knows that out of the N ships sent over the last few years on various courses, it'll be a miracle if only one manages to arrive to a good planet... and they pray it's this one. $\endgroup$ Commented Sep 12, 2023 at 9:39
  • 1
    $\begingroup$ @FrogOfJuly I recommend checking out Project Hail Mary by Andy Weir for an example of basically Trioxidane's scenario. $\endgroup$
    – CPomerantz
    Commented Sep 13, 2023 at 2:15

While this is a bit of a “hypothetical” question, I will still endeavor to answer it.

One method of ensuring you avoid super technologically advanced aliens is to limit the technology development across the universe. This means that technology cannot advance beyond a certain point. Solar panels eventually reach a point where they cannot become any more efficient. It is a physics barrier, not something people can overcome regardless of how much time and effort they expend.

You could also say that space is dangerous and, while interplanetary travel can be predictable due to mapping local space, interstellar travel is unpredictable due to the lack of information and the speeds needed to cross the distance in any reasonable length of time. Look at the history of space exploration and how often projects failed. If you multiply that rate by a million, you could see how travelling to a new star system would be seen as suicide.

Space elevators could be limited to moons and other small bodies as no one ever discovers a super strong material needed for creating one in a gravity well comparable to Earth’s. They are useful, but limited to gravity wells which enable the use of steel and other similar materials.

AGI is in a similar situation. We are great at making special purpose AI, but one with a general intelligence is simply too complex and all efforts fail. The best anyone can do is develop AI like better versions of Chat GPT. Eventually your scientists might even acknowledge that AGI is technically possible, but that the computing power needed is so great that it is not economically feasible. Combine that with the programming complexity being such that it might take several lifetimes to get it right and few people would continue pushing for that tech in any real manner. At most you would have those who still strove to create AGI on their desktop computers or using local servers, which would never be possible. Not unlike the garage scientists with cold fusion.

We have already produced a few nanograms of antimatter, but not enough to really do anything with. Fusion would provide you with the power needed to run the colliders for antimatter production, but why bother. Once you have fusion power, your power needs are taken care of. You could also state that the power needed to produce antimatter is extreme, and that the handling of it is so dangerous, and its uses so limited, that people do not really see a need for it. Fusion power is cheap, plentiful, and safe so why pursue an expensive, rare, and dangerous power? If containment is also difficult, it would preclude its use as a weapon. Nukes are already destructive enough for most applications.

Technology could eventually reach a point where advancement is no longer possible, merely refinement until the limits of physics are reached. Electronics, mechanical, and other devices become more reliable and energy efficient, but their overall efficiency is capped at a certain level. Eventually, everyone figures out the most efficient manner to do something and the technology stagnates at that point.

Your aliens might have more refined technology in certain areas, but overall, they would hit the same limitation as humanity. In this situation, your contact with aliens would likely be through semi-automated probes. Probes sent to other systems which replicate and disperse new probes to other systems. Once the replication cycle is complete, the probe would build a solar-powered communications/science array. The array would survey the system and send the information back to Earth. Daughter probes would direct their information beams to their mother array and that information would get relayed. This bypasses the issue of dangerous interstellar travel while allowing humanity to explore the stars. Sure, messages sent this way would take years, but it is better than the centuries needed to travel physically from place to place.

It could be that humanity figures out that a multi-generation ship would never work due to the inefficiency of closed systems. Even a 99.9% efficient system would eventually fail or run out of resources. If cryogenic technology never really develops then people might be forced to stay inside the Solar system. There are plenty of resources here for making orbital habitats and other space-based structures. If people know that disposable space probes have a 90% fail rate when attempting to reach new systems, they would be leery about boarding a ship with even lower odds.

Other factors which could contribute to a lack of space colonization is that the effort needed to create a ship capable of reaching another star is so great it would take a combined effort across a star system. Humanity spreading out across the Solar system would likely result in a fracturing of identity as new groups established themselves. Why would a child feel loyalty to a country on a planet they have never visited, nor ever could due to the high gravity. If the effort needed would require the combined cooperation of several planets and dozens of asteroid mining facilities, it seems likely that it would never end up happening. Especially if it would take decades and any disruption of supplies could potentially stall the project for even longer. Mega-project become easy when automation becomes efficient and easy to use. If automation is limited due to a lack of AGI, it would limit the scope of construction projects to what could be managed by people.

This could also be the answer to the Fermi Paradox. There is alien life, but due to the harshness of space, and limits of technology, any intelligent life would be restricted to its own star system. Even technology like the automated probes and communication relays would eventually fail after a few hundred, or even a few thousand, years of operation. There could be ancient debris scattered across various system objects from past probes of similar function which is covered in a few centimeters of dust after a half a million years of neglect. Your probes might never detect it and send back data which shows a lack of alien presence.

Without magic Stargates to teleport people/aliens across the cosmos, you might be restricted to light-speed communications which take decades or longer to reach their destination. Questions are sent to the aliens, and your children hear the answers. If everyone eventually figures out there is a technological limit, and that colonizing other systems is impossible, they might eventually stop trying to reach beyond their own system. Heck, in that situation, the only thing of value two species could share would likely be stories and other creative intellectual properties. Alien artwork might become the new big thing. If nobody has advanced technology, then when a species reached that limit, there would be no need for a prime directive to avoid contact. If everyone else has a cell phone, there is no need to try and hide the technology from them once you invent it. Even if an alien civilization has been space-faring for a million years, they would not be any more technologically advanced than the humans. Their population could be much larger if they invest heavily in space-based habitations, but even that is not a guarantee. They might simply remain on their home world and import the metals/minerals they need from space, or get good enough at recycling that they do not even need to do that much in space.

Think about it. Alien medicines would not work due to different biology. Minerals like gold are plentiful enough when you are mining an entire star system. Food and other biological life are no-go due to the possibility of invasive species and other contamination. Land is obviously a non-issue. If technology is roughly equal, that leaves little else aside from products of the mind. Such a universe also negates the possibility of alien invasion. If colonizing other systems is nearly impossible, sending troops would be even more so. The defending system would have decades to detect and attack any incoming ship. Deceleration time alone would provide plenty of time to destroy pretty much anything sent your way. At most you could send high speed projectiles towards their planets, but the distances involved would make hitting anything unlikely. Plus, once a species is off planet in sufficient numbers, there is no killing them off. Why waste the time and energy on a non-threat?

I hope these thoughts/ideas help you with your story.

  • $\begingroup$ Thank you for your answer, I have never felt more depressed while reading one :) It is too depressive to be believable. Maybe it me who are too optimistic though. But, anyway, thank you for the heap of ideas, I would love to use some of them $\endgroup$
    – FrogOfJuly
    Commented Sep 11, 2023 at 19:12
  • 1
    $\begingroup$ I do not see why it would be depressive, but a lot depends on the type of story you are trying to tell. There was a period when technology advanced in leaps and bounds, but now it seems like there are incremental changes instead of large leaps. We don't have anti-gravity, hoverboards, flying cars, teleporters, or many of the other advancements we thought were around the corner. A large number of people liked how The Expanse kept space travel somewhat realistic. It showed a world not too far removed from our own. If anything, taking the viewpoint that there is a tech limit makes people think... $\endgroup$ Commented Sep 11, 2023 at 19:51
  • $\begingroup$ I like The Expanse, but I did not finish it yet. No spoilers please) I think that capping technology limits how interesting universe can be. There can be some limits, like protomolecules zero-inertia movement is complete bs, but I prefer to avoid bounds that would stop the sense of wonder in the face of new technology. I think your approach does this and that's why I think it is very depressing( $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 9:07
  • 2
    $\begingroup$ As the world builder, it is up to you to determine how your story is told. Feel free to utilize whatever aspects of my ideas you find interesting/helpful and ignore the rest. I just felt this was a method of accomplishing your goal. If it does not work for your universe, that is perfectly fine. You need to tell your story the way you want it to be told. $\endgroup$ Commented Sep 12, 2023 at 12:31

Neighbors from within our solar system

this may depend a little on how alien they can be, since we haven't managed to find them yet. it also means their technology can't be that similar to ours unless it's several hundred years behind, since i imagine that while jetting around the solar system we'd struggle not to notice a planet of people using a radios for instance. but maybe they live under the surface of an uninteresting seeming moon and have technology that moves less energy than ours but uses it more efficiently, producing relatively few effects that'd we'd notice and identify as such. or maybe they live somewhere noisy, like io, that covers up their electromagnetic emissions for a while.

the major difficulty is for them to have reliable interplanetary travel before we notice them. it's a bit of a stretch, but maybe they've learned by watching us?

Rogue system

the scientific plausibility of this one is a little dubious i think, but they could be from a wandering star that passes by us relatively slowly, close enough that it's possible to get between with only interplanetary travel technology adapted to a specific unusual situation


perhaps evolved intelligence tends to have a limit on handling large amounts of information that falls well below the point where that kind of thinking is useful. the aliens show up in an incomprehensibly advanced manner, but none of them understand how it works and none of them ever have, it's all invented and operated by computer. when, for some reason, they no longer have their big ship with its big computer, they can only sustain technology comparable to ours, the approximate point at which evolved minds can't really understand the principles required to go much further.


What's wrong with wormholes?

You state that wormholes and other methods of FTL travel cause paradoxes. But, AFAIK, for wormholes that is only the case if the two mouths of a wormhole are in each other's future/past light cone. If wormholes occur naturally, a society doesn't need to be able to create or manipulate them, they just need to discover them. Discovering a wormhole doesn't mean you are also able to move it around or anything. The universe might contain laws of nature that make wormholes collapse if a wormhole's mouths form closed timelike curves (i.e. enter each others forward/backward light cones and allow time travel), or there might not be but it is just practically impossible to move a wormhole mouth significantly.

This by itself doesn't explain why we find an alien civilization at the other end of the wormhole, but there are ideas in other answers or it could just be luck.

Edit addressing the comment

To be useful for the story, a wormhole must meet some conditions.

  • First, it is not created by humans, it just exists naturally. If humans could create wormholes, they could also create them in a way that causes paradoxes.
  • Wormholes need to be not like black holes in mass. If they were, traversing a wormhole in a ship (or any solid object) would be impossible as any ship approaching it would be torn to pieces by the immense tidal forces.

Wormholes, by their nature, are hard to detect. They don't reflect sunlight, instead any light that shines upon it passes through to the other side. You also cannot easily move a wormhole. There is no surface to push against. Pushing against it will only cause you to pass through it to the other side. Wormholes, like other objects, follow the shape of local spacetime as it is formed by gravity, so it can orbit the sun or other objects. In principle it is possible to move a wormhole by placing a large mass next to it, but moving a wormhole to another solar system in this way is entirely impractical at the OPs technology level.

Let's assume there is a wormhole orbiting the Sun somewhere out in the Kuiper belt, and the other end orbits another star at a similar distance.

Such a wormhole would most likely not have been detected by any searches done in reality so far. It does not reflect any light. It would be briefly visible if you look at just the right time so that you see a bright star at the other end, or the sun it is orbiting around, but the chance of that happening just when a telescope is looking are minimal. And searches for asteroids etc. are done by taking two or more pictures of an area of sky with some time in between, and looking for any object that has moved in the mean time. Even if you happen to catch the wormhole just when you can see some bright object at the other end, it most likely won't be visible in the other pictures so you won't be able to tell that it is something in the solar system, as opposed to some kind of flash from a distant background galaxy.

If a wormhole has a planet-sized mass, that mass could be inferred statistically from effects on the distribution of other Kuiper belt objects. If a wormhole as a small or no mass, that does not apply. In fact, if the wormhole has a small or no mass, it could easily be much closer to the sun without being detected.

Humanity finally detects the wormhole when a probe exploring the Kuiper belt accidentally gets close enough (on astronomical scales) and detects some kind of anomaly. Or perhaps there's a research project that has scattered a thousand probes throughout the Kuiper belt. Or a probe exploring the Kuiper belt notices gamma emissions from a fusion drive that the aliens on the other side of the wormhole are accidentally pointing at it.

There are still some holes in the story, e.g. why do both wormholes orbit in solar systems, and why are the aliens technologically similar to us, but those look solvable with some creativity if you don't want to ascribe them to chance.

What's wrong with paradoxes?

You also seem to hold the possibility of paradoxes as a red line. But why not solve the possibility of paradoxes itself? Why not make the Novikov self consistency principle apply in your universe? That would mean that despite time travel being possible, it would not be possible to alter history because the timeline (including any time travel) is always self-consistent. Or alternatively, there is some physics that prevents closed timelike curves from being able to form.

Edit 2:

A few months later, but let me get back to how the Novikov self consistency principle is 'enforced'. The thing is, there is no additional mechanism needed to enforce it, the applicability of the mechanism is conjectured to follow directly from how quantum mechanics works.

For this explanation I will assume that the reader is familiar with the Double-slit experiment. The double-slit experiment, among other things, proves that single photons are wave-like quantum objects. It shows that a photon, i.e. the wave that is the photon, can travel through both slits in the first screen. The wave packets that come out of both slits interfere with each other. At some places this interference is constructive, at others destructive. The result is a banded interference pattern on the detection screen, with the places where constructive interference is taking place having a high probability of detecting a photon, while at places with destructive interference the probability drops to 0.

What happens with a closed timelike curve is (theorized to be) the same. The quantum waves which are an object entering the wormhole from outside the CTC interfere with the waves coming out of the CTC / out of the earlier-in-time wormhole mouth. For some states and positions of the object the waves interfere destructively, which means the probability that we observe such a state and position drops to 0. For other states and positions the waves interfere constructively. That are the solutions that are consistent with both the object entering the CTC from outside and the object that is looped around in time.

I could stop writing here, but one thing that is good to know if you start thinking more about quantum mechanics and that is often simplified away in explanations (as I have done in the above two paragraphs), is that a quantum wave is not a single particle. There is, according to the math, a single quantum wave describing all the particles. And in fact it is not the position of measuring a photon (or any other particle) at a certain place that has a probability, the probability applies to an entire state of the universe, with all its particles. This is why according to the theory, not just individual particles can constructively or destructively interfere with themselves, but states of the universe can interfere with them selves, leading to states that are inconsistent with the pasts of all involved particles to cancel out and have a probability of 0 of being observed. *

On the macroscopic scale this will result in us only observing timelines that are internally consistent. For a nice illustration, see e.g. the first part of this chapter of Harry Potter and the Methods of Rationality, a fanfic where a much more intelligent Harry Potter tries to understand time turners.

* Disclaimer: This is at the limit of my understanding of quantum mechanics, so my explanation may not be entirely correct.

  • $\begingroup$ On the surface it is ok. But then one need to decide how to actually find these wormholes. If they are just a variation of a block hole, then good luck to getting there on fusion powered spaceships. If they are relatively close, then why we did not encountered them earlier? If they are born out of some small particles then this question rises some big problems. If we can create them and manipulate them it is likely we can move them to create paradoxes $\endgroup$
    – FrogOfJuly
    Commented Sep 14, 2023 at 10:23
  • $\begingroup$ I did not know that there is math behind self consistency. Thank you for mentioning it. On the other hand the math is mostly that nothing forbids Novikov principle to exist and quantum mechanic is somewhat ok with it. The big issue that I think is still unadressed is how this principle is enforced. If it "just works" that is not good enough answer for hard sci-fi $\endgroup$
    – FrogOfJuly
    Commented Sep 14, 2023 at 19:17
  • $\begingroup$ The same goes about CTC. I sank considerable amount of efforts to come up with a solution to system when "weakest" wormhole breaks when the paradox is about to appear. My conclusion was that wormhole would need to know in advance if photons would meet their past selves to prevent it from happening. This is a no-go in my opinion. If you know paper or other source that solved this issue I would very appreciate the link $\endgroup$
    – FrogOfJuly
    Commented Sep 14, 2023 at 19:21
  • $\begingroup$ A bit late, but I've updated the answer to address the hard(ish) sci-fi mechanism of how consistency should be enforced. $\endgroup$
    – JanKanis
    Commented Nov 14, 2023 at 16:32
  • $\begingroup$ Thank you for an update) $\endgroup$
    – FrogOfJuly
    Commented Nov 15, 2023 at 9:56

Have them travel to us from so far away that they evolve to be on a comparable technological level en route. I guess for this to be feasible though they'd have to have some quirky "natural space-travelling ability" to begin, as they'd almost certainly upgrade their vehicles en route too. As in they'd have to just start as amoeba or something floating through deep space, even if by the time they arrived at Earth they closed the last distances by spaceship. How does one fish meet another fish? It starts as a single celled creature in the primordial ocean which it travels several times over until it evolves into a fish to meet the other fish (which has done the same).

  • $\begingroup$ I like it very much! Though all my attempts to come up with some way for the space-dwelling to evolve naturally were too unrealistic. If you know the source where the author succeeds in that I would love to read it $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 16:33

Sounds like you want something equivalent to the Alderson Drive.

This is a form of jump-drive described by Larry Niven and Jerry Pournelle in Mote in God's Eye. The odd part about it is that it uses a loophole in gravity to jump between specific points between stars. The points are a bit like Lagrange points, in that they are specified by the laws of gravity, and only exist between stars of specific masses at specific distances.

This means you have gates that aren't created by ancient aliens, but are usable if you know the math. You can redefine the conditions of how those gates form in any way you want in order to connect stars for effective narrative purposes.

What's more, you can generate a dynamic where, when races find better ways to get between stars, they have more stars to choose from, and may consider the jump-gate enabled stars to be the low-rent district. This dynamic has been considered as a solution to the Fermi Paradox, where the more advanced species are just hiding from us because they don't want to be bothered.

  • $\begingroup$ Do you have any source with the mathematical proof, that Alderson Drive does not create paradoxes? $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 20:06
  • $\begingroup$ @FrogOfJuly, Yes, but it's protected by a confidentiality agreement with the MiB. $\endgroup$ Commented Sep 13, 2023 at 0:33
  • $\begingroup$ Such a shame) I am asking because there is a lot of math done for wormholes and their relation to paradoxes and these works are not under any restrictions from MiB $\endgroup$
    – FrogOfJuly
    Commented Sep 13, 2023 at 9:12
  • $\begingroup$ @FrogOfJuly, You didn't ask about wormholes. You asked about Alderson Drives. They don't use wormholes, they use loopholes. They don't cause paradoxes, they cause changes in perspective. $\endgroup$ Commented Sep 13, 2023 at 15:18
  • $\begingroup$ Can you provide any sources? I am genuinely curios about the concept, but I can't find anything except the original book and a fandom wiki $\endgroup$
    – FrogOfJuly
    Commented Sep 13, 2023 at 15:25

Singularity is the great filter

Alien life is extremely common, as are alien ruins, but most life that reaches a certain point, a few hundred years in advance of humanity, tends to leave. There are several common types of singularity

  1. They ascend to become a virtual hive mind and retreat to computers.
  2. They fuse with their local star and live on in stellar networks, generally living in a dyson sphere.
  3. They fuse to become one giant biological computer.
  4. They do one of the above, run out of power, and hibernate.

The main powers that be that watch over all will stop any hegomizing swarms, but otherwise encourage allowing younger races to explore and non interference, with many far more interesting tasks in their virtual simulated worlds.

As such, any races they meet will either be a similar technology level, or have left for one of the great filters. Scattered remnants, artifacts, and other rare things are all that's left of more advanced alien races.

  • $\begingroup$ This will go to the "Great Ideas" pile. It is not quite what I am after and it almost pains me considering how good it is! $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 16:46
  • $\begingroup$ What do you think about "Singularity is the great filter"? It is almost like an ascention, but more universal and inevitable $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 20:38
  • $\begingroup$ I tweaked it to your specifications. $\endgroup$
    – Nepene Nep
    Commented Sep 12, 2023 at 21:48

Generational Ark Ships

enter image description here

Humans already have the level of technology to travel between stars, it's just they haven't had the motivation to do so as yet but if Earth was to become uninhabitable, they could potentially build an ark ship/s and flee to another system.

Now a multi generational ark ship could arrive in our solar system full of refugees and the same basic level of tech as us if something drove them off their planet.

Alternately our ark ships could meet other ark ships when they both close in on a potentially habitable planet.

  • $\begingroup$ I think things will go south pretty quickly on any ark ship, for any of a thousand reasons! $\endgroup$
    – Deipatrous
    Commented Sep 13, 2023 at 9:06
  • $\begingroup$ Generational ship is a good idea, but it fails to answer to this question on its own. The generational ship is believable, but why it came to our Solar system? Why not any other planet that has liquid water on it? If you will come up with any property of our star that will make it stand out from thousands other stars then generational ship will work, otherwise it is just the same unbelievable luck $\endgroup$
    – FrogOfJuly
    Commented Sep 13, 2023 at 9:17
  • $\begingroup$ Earth would need to become VERY uninhabitable before we leave, unless we somehow find a planet that already has life and breathable atmosphere. $\endgroup$
    – kutschkem
    Commented Sep 13, 2023 at 11:15

We are the one of the closest planets with an Oxygen/Water Vapor Atmosphere

We are not that far away from being able to detect elements in a planets atmosphere and if you reverse directions, we might be intrigued by a planet with an oxygen atmosphere and target that planet over others out there. The aliens are similar enough to us to also be oxygen/carbon/hydrogen with a need for liquid water. They would be slightly more advanced than us, but could send a slow probe to verify if the oxygen means life and happen to get lucky in that there is us as well. If the distance was 20 to 30 light years it would not be unreasonable to be able to start a conversation. If you want more than that it could be that us or them is close to a breakthrough that would allow contact, or we even have been in communication for a while but finally meeting.


Solar system and travels using fusion.

You could start an endless debate right here right now xD

No antimatter production

If you have Fusion, you have Antimatter production. Who needs efficiency when you have infinite energy ? Plus, from what i understand, the main issue is actually containment rather than production. Of course right now it would be very expensive to make any amount of Antimatter but its more of a incentive, use and cash problem, rather than a physical hurdle.

They will be vastly technologically superior to us if they can travel between the stars.

I mean an Orion drive is not that advanced.

The other way is to make these gates form naturally,

You might want to take a look at cosmic strings then. But note, Wormholes and the Gates from the expanse are on about the same level of BS. Same with Warp drives, let it be known kids just because something is a solution to General Relativity does not mean it is physical. Example; Schwarzschild black holes, they dont exist, all black holes rotate, and have some charge (So Kerr-Newman). Doesn't make Schwarzschild an invalid solution, just an unphysical one in the constraints of the real world.

Is there any other way to meet aliens different from just accidentally meeting them, while maintaining a reasonable technological level?

Lots of ways, Interstellar travel capability implies a given species has vastly more money at their disposal than anything else. We could nuke our way to Tau Ceti right now without inventing one new piece of tech (hyperbolically speaking). It would just require a lot of resources and money and political will.

If we were to encounter a species with such a ship, that does not mean they could stack wipe us. Interstellar Ships only become civilisation ending when you get to the spicy stuff like Antimatter drives, Relativistic Thrusters or alike. Truth be told, even a Orion drive could glass the planet but they would have to get close and personal which is a bad idea.

There are other forms of interstellar travel as well which do not imply great technological leaps. If a civilization fled their home world on like an Asteroid and has been living of the rock for 50000 years, they probably wont have a 1st World army on standby.


TLDR - Limited gates built on accident.

When a society achieves reliable interplanetary travel and a certain level of space industry a common construction project is a solar system sized particle collider. One side effect from certain high energy particle collider experiments is a rip or tear in space time that can spontaneously bond when another rip is present somewhere else in the universe, forming a traversable wormhole between the two solar systems. Something about the experiment is self limiting preventing other rips from forming nearby, so only one gate can be constructed easily per solar system. Your two societies would be at a similar tech level (i.e. able to construct the collider) and the wormhole only connects the two solar systems.

  • $\begingroup$ I had a simillar ideas some time ago! The result is a little underwhelming: link in the end. TLDR: There will be paradoxes with this method of FTL travel and to limit those there should be one civ per Hubble Volume and even with this it will be extremely unlikely to open a portal to unoccupied space that is not just a infinite void in terms of fusion-powered travel. worldbuilding.stackexchange.com/questions/248582/… $\endgroup$
    – FrogOfJuly
    Commented Sep 12, 2023 at 16:43

There is a group currently designing very small robotic spacecraft with solar sails to be jump started by large lasers based on the moon. Add in some speed up by using gravitational sling shots and a not very advance civilization could be doing some exploring.


It can be

  • Meeting using robots. For self-repairable robots 10000 years travel can be like 1 day. (BTW : can it be the sense of humanity existence is to leave another civilization with much more life expectancy. In terms of universe we are like 1-day live moths on Spanish coast, where caravels float to America, but we are never going to see it even when we are onboard)

  • Meeting using quantum communication. You don't expect physical contact? Can "meet" can be interpreted as communicate? E.g. some kind of a quantum camera, that will show pictures of other planets. Or quantum radio. (Of course if it is possible)


Probes + Lucky alien biology

Reliable interplanetary travel means we've advanced some beyond where we are now. It also likely means we've established a low-gravity base on some moon or asteroid, such that launching somewhat smaller craft could be relatively cheap.

In that context, it's reasonable to expect both species have some advanced version of our prior Voyager program, sending many more smaller probes with a planned life-span to get much deeper into space... possibly multiple hundreds of years.

Now let's say one of our next-generation Voyager probes mutually discovers one of their equivalents a couple hundred years down the road. Each species is able to find the trajectory of the other probe and from there locate their opposite's home system.

The logical response from both species would be to then send two more probes: one two the system itself, and one to the midpoint, in hopes of establishing communication. Each species would then also likely capture the other species visiting probe, and from there reverse engineer certain communication protocols.

This would in turn lead to a third round of probe, this time with a reasonable chance of success at establishing two-way communication... after perhaps as long as nearly a millenium of simply knowing they exist.

Finally, "Reliable interplanetary travel" still means accelerating and decelerating to and from a meaningful fraction of the speed of light. Such ships could also make intersteller voyages if designed to go for hundreds of years and if there is crew that could last.

Note I'm not talking generational ships, which is another level up. I'm just talking in human terms of, say, a voyage spanning more than 50 years with the same human crew starting as soon as they're old enough to run things.

Now let's say a lucky accident of biology means our aliens just happen to live long enough for the interstellar journey to last a feasible — but extreme! — timespan... the equivalent of sending a 16 year old human on a voyage where they would arrive when turn 70.

Until we we actually establish contact, such a voyage would never be a good idea. But now, maybe, just maybe, some among the aliens would think to try it.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .