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The famous Fermi paradox asks an important question: if space-faring civilizations exist and are not extremely rare, why are they not already here? Von Neumann proposed a probe, that flies to nearest star system, there it utilizes local resources and creates copies of itself, which are sent in multiple copies to nearest stars. Even if the traveling speed of such probes would be quite small (~100 km/s), they should be able to colonize the whole galaxy in few hundreds of millions of years, time relatively short with respect of its history.

Self-replication seems to be quite a universal concept and if there is more than hundreds of civilizations in the galaxy, at least one should try it. We would very probably be able to recognize it, if it was the case. Since we do not live in the earliest part of the habitable galactic zone there probably was enough time for other civilizations to evolve.

               enter image description here

      The GHZ [Galactic Habitable Zone] in the disk of the Milky Way based on the star formation rate, metallicity (blue),
      sufficient time for evolution (gray), and freedom from life-extinguishing supernova explosions (red). The white contours
      encompass 68% (inner) and 95% (outer) of the origins of stars with the highest potential to be harboring complex life today.
      The green line on the right is the age distribution of complex life and is obtained by integrating $P_{GHZ} (r, t)$ over $r$
      [relative number of potentially suitable planetary systems as a function of space and time].
      Image credit: The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way, Lineweaver et al. (PDF).

From the absence of the Von Neumann probes, we can conclude that either the intelligent life is extremely rare, or that it is really difficult to construct the Von Neumann probe.

This is exactly my question: Is it possible, that construction of the Von Neumann probe is simply not technologically viable?

Some challenges to overcome:

  • The journey takes tens of thousands of years, everything aboard must either survive functional this long, or rebuild itself from scratch.
  • Intelligence with required lifespan might not be possible to construct.
  • If rebuilding of machinery is required, it must be done with quite small payload.
  • Viable energy source with required lifespan must exist for the probe. (In other words: fusion might not be possible outside stars.)

I realize this is a bit speculative questions. The reason why I ask it here is that I would like a serious answer that accounts for real properties of technology and limits set by the physics, rather than general "anything not strictly forbidden by the laws of physics is possible", which I would probably receive in many other places.

Edit: A great reference to the Fermi paradox here.

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    $\begingroup$ It is obviously not prohibited by physics because humans themselves are a self-replicating intelligent technology which has been proven capable of operating in space with some equipment. In a cosmic sense, Von Neumann probes are just a matter of compacting the minimum self-sustaining unit. I guess it's not clear that can be done without sacrifices... $\endgroup$ – AlanSE Nov 2 '14 at 20:20
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    $\begingroup$ 100km/s is an order higher than the peak velocity (Voyager 1 is 17km/s relative to Sol) achieved over cosmic distances by our contemporary technology $\endgroup$ – Everyone Nov 3 '14 at 7:12
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    $\begingroup$ If you properly attribute the picture, it should be fair use (state source, author) $\endgroup$ – mart Nov 3 '14 at 10:31
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    $\begingroup$ @briligg The only part of the technology that I would say is beyond current tech is the reliability. There's nothing in a Von Neumann probe that I think would stump NASA. $\endgroup$ – Loren Pechtel Nov 3 '14 at 22:06
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    $\begingroup$ @LorenPechtel: We're still quite a bit away from autonomous mobile mine/factory systems capable of creating integrated circuits from raw materials they locate and dig up by themselves. $\endgroup$ – SF. Dec 3 '14 at 13:43
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There's no reason a Von Neumann probe could not be built once a civilization reaches the appropriate technology level, it's a matter of desire. When a civilization becomes advanced enough all it needs to build anything is a supply of matter or energy, as the one can be used to make the other.

When you say "the absence of Von Neumann probes" I suspect that you are making the fundamental error of anthropomorphization, assuming other life would have the same motivations as humans. Other life may not want to colonize the universe, or have a desire to explore, so they may have the technology but not the desire. Or, if they discover another species they may not want contact, - species which have the technology to explore may not want to be discovered, and any intelligence able to build a Von Neumann probe wouldn't have any problems hiding it from us. So just because we don't see a hundred probes out there right now doesn't mean they do not exist, or cannot be made.

After all, nobody knows how life started on Earth? Maybe we are here because billions of years ago an ancient Von Neumann probe seeded our planet with the makings of life? Not a new idea.

Anyway, a Von Neumann probe only makes sense if a civilization is limited to sub light speeds and has short life spans, and I wouldn't make either assumption.

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  • $\begingroup$ Thank you for your answer. Although it does not quite answer the question, since it does not address the issues of technological viability of the probes. Your anthropomorphization argument is probably for a different debate, but I do not think it is the case. For two reasons: 1) If some civilizations do not build the probes, it requires just one to make very serious observable consequences. 2) If other civilizations are also a product of darwinian evolution, they should utilize at least similar basic survival strategies and fight for "military dominance" as we do. $\endgroup$ – Irigi Nov 2 '14 at 20:31
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    $\begingroup$ Evolution does not guarantee conflict, when you look at our own planet there are many cases where symbiotic and cooperative survival strategies have come about. You again assume that life will be like us. $\endgroup$ – GdD Nov 3 '14 at 8:26
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    $\begingroup$ To this good answer, I would also add that Irigi's supposition that, "we do not live in the earliest part of the habitable galactic zone there probably was enough time for other civilizations to evolve." may not be correct. Recent research into the history of the universe and gamma ray bursts shows that it's possible that complex life on Earth may be nearly as old as complex life can be - see economist.com/news/science-and-technology/… for an article that explains this in detail. $\endgroup$ – Kirkaiya Nov 4 '14 at 22:19
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    $\begingroup$ I'm advanced enough to consider the question of whether such a probe is immoral and that's another reason we might not see them. "Any species advanced enough to build such a probe might have a moral objection to sending what's essentially a world-eating virus out into the universe" $\endgroup$ – Jasmine Nov 14 '14 at 19:03
  • $\begingroup$ Morality is a human concept @Jasmine. Other civilizations may have beliefs where self-replication is a worthy goal. $\endgroup$ – GdD Nov 14 '14 at 19:38
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Having written a short article on this is my first year of uni, I concluded that the main problems with the probes would be reliability. Not only in terms of mechanical functionality but also in terms of programming. If you were to imagine that when each probe replicates there would be some degree of transmission error or corruption. This would normally be negligible, but if we scale this up to galactic or inter-galactic scales, even a minute error can grow and disrupt future generations of probes.

For more information I'd suggest you start with: Galactic exploration by directed Self-Replicating Probes, and its implications for the Fermi paradox - Martin Barlow

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    $\begingroup$ A Von Neumann system would require an error correction routine when replicating. Look at the answer to this question - space.stackexchange.com/q/2053/4660 . $\endgroup$ – kim holder Dec 7 '14 at 15:13
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    $\begingroup$ Error-correcting storage is also essentially a solved problem these days. You have low-level FEC and ECC on storage media and transmission networks, you can use checksums and redundancy to detect and recover from most errors, and you can (if the errors are not possible to correct for) shut down a probe completely if some form of sanity checking determines that a perfect copy is not achievable. This is even doable on PCs; e.g., redundant ZFS with ECC RAM and regular scrubs will all but guarantee data integrity. $\endgroup$ – a CVn Dec 9 '14 at 18:34

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