What kind of reasoning is appropriate to understand the as of today unanswered question of whether there are (other) interstellar space travelling civilizations in the Milky Way?

We have already sent probes towards the border of the Solar system. And even landed human beings on another celestial body and brought them home alive and well. If we extrapolate the 50 years of space travel, the 100 years of electronics (radio), the 400 years of physical science, to just a fraction of the biological age of humankind into the future (like a few thousands of years), interstellar travel is not out of the question for us or at least our artefacts. So I imagine two possible alternatives:

1) The Milky Way is cluttered by lots of space travelling civilizations like us and our future. Once one of them/us gets going, they'll soon be everywhere. The Sun orbits the Milky Way every 250 million years, about 2% of the age of the galaxy. Going to the nearest stars is enough to soon be everywhere. But if they are everywhere since almost always, they should be here, we should be their seed.

2) We are the only space travelling civilization in the entire galaxy, ever. But then what makes us unique? We consist of the most common elements and volatiles of the universe and our planet and star and galactic location all seem to be very typical. There's no known trace of any uniqueness here. Whatever could it be?

Are there more alternatives?

While we cannot say today which alternative is true, we should be able to at least specify the possible alternatives. But to me they all seem to be absurd! What would be a rational logical scientific approach to this apparent paradox?

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    $\begingroup$ Alternative: Technical civilizations almost invariably self-destruct within a few thousand years of their evolution. We may be the lucky one in a billion civilization that does not kill itself off and goes on to colonize the galaxy, but the odds of that aren't looking so good just now. $\endgroup$ Dec 5, 2014 at 18:39
  • $\begingroup$ @WayfaringStranger Even during the second world war when idiots did their utmost to murder human beings, even with nuclear weapons. Even then the population actually grew! I don't believe that self-destructive doomsday argument. Humanity in fact advances relentlessly under all conditions. $\endgroup$
    – LocalFluff
    Dec 5, 2014 at 19:22
  • $\begingroup$ I also am often a sunny optimist. However, our experience with high tech alien civilizations so far is consistent with the hypothesis that, should any exist, they have only a limited lfespan. $\endgroup$ Dec 5, 2014 at 19:39
  • $\begingroup$ @WayfaringStranger But if we are talking about space travelling aliens, then whatever could possibly wipe them out? Once you go interstellar, you become immortal and eternal. That's actually the scary part here. If anyone did that anywhere anytime, then they are for ever everywhere always. Much like some virus inside us here on Earth. $\endgroup$
    – LocalFluff
    Dec 5, 2014 at 19:52
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    $\begingroup$ LocalFluff - First you have to get interstellar, then you have to stay interstellar. A civilization that manages one ET colony may be quite rare. The colony itself might well have a long incubation period before it could possibly go interstellar, if it chooses to go in that direction. In the meantime, home may have gone extinct. $\endgroup$ Dec 5, 2014 at 20:14

3 Answers 3


That's basically the Fermi paradox. It seems likely that there are numerous civilizations in the galaxy, and yet we see no trace of them anywhere.

The Drake equation is often invoked to calculate the probability of existence of other civilizations, by compounding several other, more simple probabilities: the probability that a star has planets, that the planets are habitable, that life could arise on such a planet, etc. The proposed values so far indicate that there should be many civilizations in our galaxy.

Astronomy, just plain observations, might be one way to solve the paradox by detecting alien societies.

Broadcasting messages via radio has also been attempted, in the hope that someone might reply.

However, the only constant in this search has been the negative answer. There are some hypotheses trying to explain that:

  • the rare Earth hypothesis: maybe there's something very special about Earth that allows the evolution of intelligent life

  • the destruction hypotheses: maybe life tends to self-destruct, or maybe there are space marauders out there killing everyone

  • disasters: maybe things like gamma ray bursts tend to wipe out life periodically

  • the young Universe hypothesis: if there are very, very many universes in the Multiverse, then the vast majority of them are young. If so, in the vast majority of them there will be 0, or at most 1 civilizations. It's simply a matter of probability that we are in such a universe.

  • religious arguments such as the creation of Man was a special, unique act by a higher entity

Others argue that many civilizations might exist, but we don't see them. Possible reasons:

  • we're just too far from each other, in space, in time, or both

  • it's just too hard to travel between stars

  • we haven't been searching long enough, or we're looking or listening to the wrong things

  • they're not interested in us

  • they all go into a Technological Singularity pretty quickly and become god-like, undetectable and incomprehensible to us

  • they are too busy browsing their internets to be interested in the outer space (yes, that has been proposed seriously)

  • they are too alien for us to detect or comprehend

  • they are like the Na'vi in Avatar - a non-technological society

  • they deliberately avoid us (the Earth-as-a-zoo hypothesis)

  • they do try to talk to us, but evidence is being supressed by the government (yes, that is also a serious proposal)

  • they are here already, hiding amongst us (proposed by Iosif Shklovskii and Carl Sagan, so I guess it qualifies as serious - albeit highly hypothetical at best)

Truth is, we really don't know. All we can do now is speculate.

  • $\begingroup$ Yeah, Fermi's paradox opened the Pandora box. So my question is basically how can we put the lid on it again. Your many suggestions seem to either suggest that they are everywhere, or that we are unique. But is it really rational to assume that they for example "deliberately avoid us"? Do we deliberately avoid ants? Why would they? And are we unique, really why, God? My question is how we could deal with this apparent paradox in a rational scientific way. Are we alone or blind or something else? $\endgroup$
    – LocalFluff
    Dec 5, 2014 at 20:08
  • $\begingroup$ No, these are not questions we can answer now. It's an open problem, and it will take a very long time to solve. BTW, those are not my proposals, but rather the standard hypotheses that have been raised in time by scientists, attempting to provide solutions. As I said, it's all very hypothetical for now. Sorry. :) $\endgroup$ Dec 5, 2014 at 20:11
  • $\begingroup$ Can't we at least formulate the possible alternatives? And if all of them seem unreasonable (which they do to me), then we at least learn that we have some more work to do. And might solve the problem intellectually, because at least one possible alternative must be apparent if one thinks clearly, right? Or is there an end to thought? $\endgroup$
    – LocalFluff
    Dec 5, 2014 at 20:17
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    $\begingroup$ New calcs: Aliens Are Probably Everywhere, Just Not Anywhere Near Humans motherboard.vice.com/read/… Hardcore version: arxiv.org/abs/1412.1302 $\endgroup$ Dec 5, 2014 at 21:52
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    $\begingroup$ @VictorStafusa It took some time for humanity to figure that out too, under a clear sky. And maybe there are other space class thingies around which we still haven't any useful knowledge about. Another potential addition to Lorin's list might be that the universe is paradoxical, that logic is an illusion in our dreaming minds and does not apply to the real reality (like the simulation argument). $\endgroup$
    – LocalFluff
    Mar 14, 2015 at 13:11

I propose the "rare Earth" theory. Life could have existed in the past or could exist again in the future because if it happened here why could it not happen again ? Maybe at this point in time we are all there is. It would be very strange to imagine that once we are gone that life will never exist again.


I think that the problem is that the question is too poorly defined to have an answer like "1 we are alone, or 42 they are here and there". Two numbers cannot be true at the same time, and any single scenario suggested thus far seems pretty unreasonable, so it is frustrating. But if we cannot define a phenomena, how could we count or estimate its number of occurrences?

The problems with a question of the kind: "How many living intelligent space traveling civilizations exist in the Milky Way?" are:

  • What is life? Biologists seem to have a hard time nailing it down. Best thing they've come up with is a loose collection of properties like a thingy which organizes energy and material from its environment into a stable structure which copies itself, but with some errors or recombinations in order to allow for evolution. Each of those components have some definition problems, and each of them exists separately in non-living things. The working definition seems to be: "We know it when we see it, if it quacks like a duck...", which means: "It is like we are and we can recognize ourselves". (We do have inherited some genes in common with EVERY living cell on Earth).

  • Intelligence is poorly defined. Those who work with developing AI do manage to make computers do some things like we do them. But if I'm not misinformed, tools like neural nets and evolutionary algorithms can be simplified to plain old 19th century math. They solve math problems, but it is not crystal clear that they create intelligence. Is a math schoolbook intelligent?

  • Civilization, I've never heard of a good definition of that phenomena, nor of colonization. And spaceflight can turn out to be different from how we imagine it today. "Alien sociology" is a common joke.

So the story goes that life and intelligence are like we are, and civilization IS us! This is on Ptolemy's geocentric level, its all about us. Should we count one forest or a thousand trees? The question has to be more rigorously specified. WE are obviously alone. Others, whatever is meant by that, might be around.


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