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I’m working on a story where in 2024, humanity has discovered a network of precursor-type wormholes. This precursor civilization established a web of connections that interconnect every star system within 100 ly of Proxima Centauri (the center of the precursor empire) so, effectively, most of the stars within 100ly of Earth. Not every star is connected to every star, but every star is accessible within five “hops”. Humanity is the first civilization to rediscover this nexus.

I want to populate this nexus with alien worlds, but I want there to have been no previous meaningful evidence of alien life.

Assuming the other civilizations are at an identical tech level to us, use similar technology,and have developed along nearly identical timelines (yes, I realize that’s a big assumption, but roll with me here) how many alien civilizations could plausibly exist within that ~100ly bubble without us having noticed any of them?

It’s fine if they’ve noticed each other, if that affects tight packing.

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    $\begingroup$ You need to define 'alien life'. Do you mean Earth 'like' life or something else entirely? Current attempts to locate life on extra solar planets assumes life on other worlds will share basic commonalities with life on Earth like photosynthesis for example and that advanced forms (plants and animals) ? Will exist only on planets with a balance or liquid water and land masses similar to Earth. If you lower your expectation to simple bacterial life forms? You get a wider range of potential target planets. But I'm don't think that's what your looking for. $\endgroup$ Commented yesterday
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    $\begingroup$ 👽 There are three of us (the next closest is ~170ly away). 👽 Contrary to sphenning's answer, we abandoned radio transmissions 500 of your years ago (in favour of technology you've not yet discovered, and about which I cannot say more). 👽 $\endgroup$ Commented yesterday
  • $\begingroup$ Just for reference, there are 10,000 to 15,000 stars within 100 ly, but most of them are red dwarfs with only something like 500 stars being yellow dwarfs similar to our Sun. $\endgroup$ Commented yesterday
  • $\begingroup$ @Mon I feel like my question mostly covered that in “Assuming the other civilizations are at an identical tech level to us, use similar technology,and have developed along nearly identical timelines…”, but am curious what unintentional ambiguity you’re seeing there? $\endgroup$ Commented 7 hours ago
  • $\begingroup$ @Daniel B To answer your question one of the key tools proposed for finding life on 'near by' extra solar planets isn't looking for radio signals or other signs of advanced technology. Instead its using advanced telescopes (& spectrometers) to look for signs of water vapor, oxygen and chlorophyll in a planets atmosphere. Detect all three and you've hit the trifecta i.e. you can assume that planet has life (assuming your looking only for 'Earth like' life. In your scenario near human level technologically advanced species don't have to give their locations away. Their planets do it for them. $\endgroup$ Commented 1 hour ago

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As many as you like

With current radio telescopes we could detect radio traffic similar to Earth’s out to a distance of about 33 LY. That’s about 1/3 of your radius, so about 1/9th of the ‘bubble’. Thats also if we’re looking in the right direction for long enough.

There are about 10-15 thousand star systems within 100 LY of Earth (or Alpha Centauri because we’re practically next door), so most of them won’t be detectable.

Of course, the next generation of RT would swallow the whole bubble, but your question was about now.

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  • $\begingroup$ How many of those would be plausibly habitable? $\endgroup$ Commented 15 hours ago
  • $\begingroup$ Many of them. Pluto (not even a planet), for example, is crimson from hydrocarbons. Titan (Saturn moon) might have some novel chemistry leading to an ecosystem. Europa might have sub glacial oceans containing life more like we're used to, and most of the energy keeping that going is it's tight orbit around Jupiter, not the 5 AU distant Sun. Europa means brown dwarves and up might have water moons with life like we understand it. But a rock body with just 101kPa of nitrogen and oxygen atmosphere and generous water oceans at around 15C? That is a little specific. $\endgroup$ Commented 13 hours ago
  • $\begingroup$ @DanielB since we know very little about water possible planets around other stars, almost any star with planets is plausable. $\endgroup$ Commented 11 hours ago
  • $\begingroup$ @John and we’re pretty sure almost all stars have planets $\endgroup$ Commented 8 hours ago
  • $\begingroup$ @JamesMcLellan I’d read that to our knowledge, human like life would be very unlikely to happen in a red dwarf system? $\endgroup$ Commented 7 hours ago
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There are probably only a few thousand large approximately Sun like stars within 100 light years of the Sun and perhaps a few tens of thousands (at most) of dwarf stars.

The number of these stars that have planets with stable orbits in the habitable zone with water would be much less than this and those where life developed along similar lines to Earth would be vanishingly small.

Our evolution has been effected by so many random influences such as axial tilt the presence of the moon, powerful magnetic field, atmospheric composition and evolution, continental drift etc. And of that small proportion the number who had developed at the same time as us would be a close approximation to zero.

However for fictional purposes fill your space with as many worlds as you wish - just don't expect to get any useful guidance from the real world.

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  • $\begingroup$ Add to that the our own evolution is mostly a fluke so even if you have life getting intelligent life is many orders of magnitude less likely. $\endgroup$ Commented 7 hours ago
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The economic argument against detectable aliens: If your radio/TV signals are detectable on an alien planet, someone there had better be paying for a subscription..... Transmitting into space at high power with high directionality isn't cheap.

So the only detectable aliens would be those purposefully sending out tight-beam communications of some sort, and being 'in the way.'

Similarly, we detect planets by seeing the shadow as they transit across their star. We don't see them by their reflected light. This means that if you cover half of you entire planet with 1kw omnidirectional broadband transmitter stations every 10 square meters, we probably won't be able to see it either.

The easiest way to detect aliens is if they are, I don't know, modulating the power output of their star for some reason. And I'm not sure you want to meet them as they are at least a couple tech levels above you.

So yeah, there is a lot of scope for undetected aliens.

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~0.0002

let look at the drake equation as a ballpark tool. Super rough but coud get us within a few orders of magnitude.

15000 stars within 100 light years

20% of those contain a planet in the habitible zone. so 3000

Life is highly likely with water so lets just assume thats a given, help make up for flukes like europa hat could theoretically have life while not in the habitible zone.

Now that life has to be at the multicellular stage, so thats 1/10 of earths history. so now 300

intelligent life evolved in one lineage, and has been around for around 4 million years out of 500 million years of life on land. so thats gives 3

humans have had radio for around a hundred years of our 4 million years of existance so thats 0.0002 alien worlds

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There will be 0 undetected civilizations with technology comperable to ours within a 100 light year radius. We've been broadcasting radio since we discovered it in the 1880s. We can assume that any civilization with technology comperable to ours will have been broadcasting for about as long. That means that their radio broadcasts will have reached earth by now regardless of where they are located within that radius. Our regular radio astronomy and dedicated searches for signs of intelligent life would have picked a civilization similar to ours in that time span. If you were in a computer lab in the 90s there was a great civilian science initiative specifically designed to detect a broadcast signal amongst naturally occurring radio sources.

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    $\begingroup$ I’m not sure this is quite right? My understanding is that Earth’s radio leakage falls off pretty fast? physics.stackexchange.com/questions/245505/… suggests “ we would probably be unable to detect radio signals from Earth even if observed from a distance of a few light years.” And as a 90s seti@home contributor myself, iirc, SETI covered a tiny fraction of the sky and only on very specific frequencies. Happy to be corrected on any of these points. $\endgroup$ Commented yesterday
  • $\begingroup$ @DanielB From the top answer it also says "It has been suggested that new radio telescope projects and technology like the Square Kilometre Array may be capable of serendipitously detecting radio "chatter" out to distances of 50 pc (∼150 light years)" so using rough back-of-the-envelope estimation we can conclude that the answer is somewhere between 0 and the number of systems you consider to have habitable planets. Which for a substantiated wild-ass guess seems like appropriate error bars. $\endgroup$ Commented yesterday
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    $\begingroup$ [/sarcasm on] "searches for signs of intelligent life would have picked a civilization similar to ours" is an oxymoron [/sarcasm off] $\endgroup$ Commented yesterday
  • $\begingroup$ @sphennings they are specifically talking about something that has not been finished yet, and I may be misunderstanding, but the “serendipity“ seems to be referring to a very low estimated probability. $\endgroup$ Commented yesterday
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    $\begingroup$ "That means that their radio broadcasts will have reached earth by now regardless" Yes but at what intensity? Assuming an FM broadcast at 50 kW isotropic, a quick napkin calculation puts the intensity already at 50 ly at one photon per square meter, every 43 days. So even if the whole EM spectrum is full of radiation from that planet, here on Earth you cannot make that out as anything but white noise. $\endgroup$ Commented 20 hours ago

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