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.
- New Search for Extraterrestrial Archeology –Probing Kepler Mission Data (Today’s Most Popular) (dailygalaxy.com)
- Extraterrestrial civilizations? (deannetoth.wordpress.com)
- Dyson Spheres: How Advanced Alien Civilizations Would Conquer the Galaxy (Infographic) (2eyeswatching.com)