How far away is the closest extraterrestrial civilization?

Screen shot of SETI@Home (Enhanced 5.27) BOINC...
SETI@Home (Photo credit: Wikipedia)

First, let me be clear on what I’m talking about: intelligent aliens with an advanced civilization.  This is a different question than extraterrestrial life in general.  For life in general, particularly simple life, I’d be surprised if we’d have to look more than a hundred light years from Earth.  (We may know the answer to this in a few years afer the James Web Space Telescope comes online.)

But any question about the existence of intelligent aliens has to grapple with the Fermi Paradox: if extraterrestrial civilizations exist, then where are they?  Why aren’t they here?  Or why don’t we have evidence of them visiting us in the past?  (And we don’t have any evidence, despite what the Ancient Aliens and Chariots of the Gods people claim.)

SETI, the search for extraterrestrial intelligence, continues to search, and they may well eventually find something, but it seems likely that the nearest stars have been ruled out.  Of course, this is an area where we don’t know everything we don’t know, and the possibility remains that aliens are shouting at us in some manner that we haven’t yet thought of.

But the idea that the nearest civilization is close, essentially comes with the assumption that interstellar travel is impossible.  If it is possible, then again, why would relatively close civilizations not be here yet?  Given how incredibly young our civilization is (in geologic and astronomical time), any other technological civilizations would almost certainly be far more advanced than us.

Maybe interstellar travel is impossible?  It’s conceivable that the hurdles involved would be so insurmountable that no civilization has yet succeeded in making it happen.  Or it might simply be so monstrously expensive in terms of resources that no civilization bothers.  (Imagine if the mass of an entire planet is needed to attain relativistic velocities.)

But this idea deserves some scrutiny.  It’s hard to imagine that technology that would allow travel at least at 1% or so of the speed of light couldn’t be designed.  Nuclear pulse propulsion, essentially ejecting nuclear bombs behind a spacecraft with a pusher plate to get up to a few percent of light speed, seems doable with foreseeable technology.  And that’s all that would be required for self replicating probes to explore and colonize the entire galaxy within 100 million years or so.  (I know that’s a long time, but it’s quite a short time in relation to the age of the Earth or the universe.)

Perhaps that has happened, and there are currently probes of that type in the solar system, waiting for us to achieve a certain level of development before they contact us.  Perhaps, but you would think electromagnetic communication would have been a substantial marker.

So, if interstellar travel is possible, then what does that say about how far the closest civilization is?  I think it says that they’re not in this galaxy.  It’s possible that they may not even be in the local group of galaxies.  In other words, they may be tens of millions of light years away.

That is why, in an earlier post, I contemplated that extraterrestrial civilization may be exceedingly rare.

This assessment assumes that interstellar travel would be limited to the speed of light.  What if any form of FTL (faster than light) travel is possible?  It depends on just how fast, but I think it increases the distance by orders of magnitude.  The faster travel ability would simply make it likely that they’re even farther away, perhaps outside of the visible universe.

Actually this would make a common science fiction scenario, that there are lots of civilizations out in the universe, but that none of them have noticed us yet, much more plausible.  If civilizations are separated by tens of billions of light years, then the chance of any two civilizations running into each other, no matter how fast they can travel, becomes virtually zero.

At least until both civilizations have started expanding and colonizing on a massive scale.  The sheer number of stars, hundreds of billions of stars in hundreds of billions of galaxies, make it very plausible, even if the observable universe could be travelled quickly.

Of course, we have scant reason scientifically to think we can travel faster than light.  It might be possible, but it will probably take a new physics.

I also could be dead wrong about the feasibility of interstellar travel, even at sub-light speeds.  And SETI might simply not be listening yet in the correct manner.  If so, then the next civilization might very well be close (less than 100 light years), and we just need to figure out how to communicate with them.

19 thoughts on “How far away is the closest extraterrestrial civilization?

  1. The distances are maddening. I like to think we’re missing something, something that will make actual exploration feasible, because right now we are not going anywhere outside our solar system.


    1. I agree. Both the distances and energy requirements are daunting. I do think slow interstellar travel (<10% lightspeed) has some possibility, but of course that's unlikely to be anything manned. (It might be our machine successors who go.)

      In my experience, those optimistic about relativistic or FTL travel don't understand how much the laws of physics are against on it.


      1. The problem with slow interstellar travel is that you then run into reliability issues, unless you build your spacecraft bigger with incredible degrees of redundancy and self-repair. We’ve get some space probes viable for decades, but they’re pretty crude compared to a probe that would have to explore, operate, and self-replicate on its own in another solar system (assuming self-replicating probes are possible – it may take the entire industrial and technological edifice of a civilization to build and send a starship, so “hopping” could be very slow).

        But I suppose it’s not impossible to overcome that. Still, it’s a pretty big commitment of resources compared to other uses – an alien civilization willing to do that would be pretty different from our own, where we’re gradually exploring our solar system on a tight budget. I think space-capable civilizations are just so far apart and slow-spreading in our galaxy (if they are spreading at all) that encounters are extremely unlikely.


  2. Although the challenges to interstellar travel are huge, the idea of sending some kind of micro probe seems highly feasible. And if that’s possible, then sending billions of probes to all the stars in the galaxy is almost as feasible. So yes, since that doesn’t seem to have happened, it strongly suggests that there is (was) no advanced civilization in our galaxy.

    There have been so many possible explanations, but no data to confirm or refute any hypothesis. I’m sure that data will start to appear in the coming decades.


    1. I tend to think self replicating micro probes will explore the galaxy. We’ll get the pictures, and maybe have bootstrapped copies of ourselves experience the other solar systems first hand. That might have to be enough.


  3. Yes, start out small – when enough people believe in little creatures and more believe in intelligent aliens, as apparently was the case in ancient times, they may arrive in their glory… Its all about beliefs
    🙂 this time I am being facetious
    Really though, lots of evidence includes comets brought amino acids and possibly proteins here from deep space and there is some amazing intelligent speculation about structures and ancient text and wall art that indicate aliens have already been here in the distant past – some recent sightings are interesting too and there are some whistle blowers to consider… its a pretty safe bet that there is life of Earth someplace too 0 even the Catholic Roman Church is looking in space.
    ~ Eric


  4. Our own civilization is, at the moment, unsustainable. We are destroying the resources of our planet quickly and with increasing speed. Groups whose economy is growing slowly will be overgrown by those whose economy is growing quickly. As a result, those who use resources in unsustainable ways and therefore grow fastest will win. In the end, the civilization is likely to collaps.
    If this course of events is typical, technical civilizations like our own would be very short lived, destroying themselves after a few centuries. A civilization like ours could have arisen on planet earth millions of years earlier or later, depending on accidental events in the planet’s history and the history of evolution.
    I might be wrong with these ideas, of course (and I hope I am wrong), but if I am right it is very unlikely that the time in which contact is possible between nearby civilizations is very short and it is very unlikely that these time windows overlap.
    I think there are many civilizations but I don’t find it likely we will ever find one.
    Generally, I think it is more important at this time to try to make our own civilization sustainable. That is the number one condition of ever finding another one. If our civilization collapses, we will probably not be able to build up another one again because the easily reachable resources of earth will have been used up already. So before we master interstellar space flight, we must first master “terraforming earth” (instead of martiforming it) and acquire longevity (as a civilization).


    1. Well said. I referred to that as the dark scenario in my earlier post, that it is in the nature of intelligent life to destroy itself. The ray of hope is that we’ve so far managed to avoid the worst scenarios (Malthusian starvation through overpopulation, nuclear war, etc), but of course past performance is no guarantee of future performance.

      The problem, of course, is that climate change is happening. The only real question now is whether we mitigate it, and if so to what extent. Fossil fuels will probably last for a few centuries. Our best hope may be in reaching the ability to control the shape and composition of our bodies in order to survive in the new environment we’ve created, while we still have the resources to do it.


    2. “Unsustainability” depends on where you sit, and what your technology is. Most civilizations were on an unsustainable path with wood consumption until they began utilizing coal (particularly Great Britain), and on an unsustainable population growth trajectory until we got the combinations of effective contraception, the demographic transition, and safe abortions.


  5. SETI seems rather unlikely to find anything. They themselves used to have an FAQ on detection distances, and the type of stuff you might casually pick up – high energy television broadcasts, radio – was so faint at distances that a 2000-meter radio telescope wouldn’t even pick them up outside of our solar system. The only stuff that was detectable was narrowband signals such as military radar and the Arecibo telescope, and it’s pretty unlikely that you’d just happen to be in the area they’re targeting.

    Basically, SETI’s probably not going to find anything unless someone out there is deliberately pinging planets with high-powered radio signals.


  6. Maybe an advanced civilization lies at the opposite of us in the Milky Way. So we search for nearly ET maybe nonesense.


  7. The truth is that there are countless advanced civilizations in the universe..for researchers at seti or any organization to cry that “we can’t see them and they would surely come announce their presence if they exist”..Is actually a childish mentality…The are within the 1000 to 2000 light year range from us…some are significantly more advanced than us…And we may be shocked to know the resemble humans more than we could have ever imagined ..


      1. Everyone knows ..every single person,.deep down in their gut ..knows that the universe is filled with marvels ..and life ..but humans have an almost child like tendency to hear confirmation… Confirmation of what I ,.and YOU ..and everyone else …knows deep down 😉


  8. I think it’s not a matter of where they might be, but when. How long will humans last on earth? Maybe a civ did exist near by but millions of years ago. Or will exist millions of years from now. and this is a small time scale. Time is the factor that might limit contact… more so than distance.

    Liked by 1 person

    1. That’s a common sentiment. Maybe it’s in the nature of intelligent life to destroy itself, and there just aren’t that many around at the same time. Still, if that were the case, we might expect to see some evidence of past civilizations lying around.

      I recently did a post on Robin Hanson’s idea about expanding civilizations in the universe you might find interesting.


      1. A billion years from now earth will be very different place and there will probably be very little evidence of humans – if any. Maybe the voyager probes will still exist making their way around the galaxy. If we ever have colonies on a planet like Mars perhaps… those river deltas are still on Mars and those are billions of years old so i guess it’s possible.

        Liked by 1 person

  9. of course a more arrogant view might be that we are it. Somewhere there must be the most unique thing about the universe. Maybe thats earth. I really dont believe that myself… but one thing has been established beyond doubt, we dont live on main street as far as intelligent life goes. Things do seem pretty quiet out there.


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