A Ripple On The Deep

There are some assumptions behind the Fermi Paradox which, really, make it a paradox. As I do often in this blog, I’m going to question them.

• Once a species achieves star travel, it’s basically immortal.
• Once it has achieved interstellar capability, it never loses it.
• Once a race starts expanding out among the stars, it never stops.
• Interstellar colonies never fail – or are repeated until they succeed.
• Colonies will throw out new colonies as soon as they can.

Given those assumptions, Fermi’s question gets very difficult to answer. A sentient interstellar race sweeps across the Galaxy in a slow but unstoppable wave. Questioning those assumptions leads us in some different directions.

Let’s just take those one at a time.

Species Immortality. A species with a wide range, large population and high technical capability is certainly going to be resilient, being able to absorb heavy losses and meet serious challenges. However great those capabilities, in the scale of the universe they will still be very small. Also, high technology gives the ability to create weapons of tremendous destructive power. Internal conflicts using such weapons could well threaten the entire species. (See my post on Blowing Up Stars)

Civilizations Never Lose Interstellar Capability The capability to build STL starships requires a massive industrial base and mastery of a wide spectrum of technologies. Besides the technologies, the political and cultural will to marshal those resources must be there, and be sustained over a long period of time.

That’s not a given. In the short lifetime of the Industrial Revolution we have seen large projects such as railway construction and the building of major power plants face strong and increasingly successful opposition for all sorts of different reasons. An interstellar colonization project could well see such pressures, strong enough to kill it.

Never-ending Expansion. Our own civilisation is going through an expansionist phase. Not all will do so, and such phases don’t last forever. Population pressure is not an inevitable given. Many nations on our own planet are justly concerned about falling population.

Inevitable Success. Our own world has seen many waves of colonization, of which the colonization of North America by European powers was only the latest. The Greeks, Romans and Phoenicians all founded cities around the Mediterranean and beyond. All those examples show a non-trivial failure rate.

Daughter Colonies Are Inevitable. The founding of an interstellar colony is essentially the founding of a new civilization. Given the great difficulty of sub-light-speed travel between stars, the chances that the mother civilization could exercise any significant control over its daughter colonies is rather slim. The new civilization will solve its own problems and go its own way. If they have larger problems, the issue of colonizing the next star over may well slide to the bottom of the priority list, if not off it altogether.

Different assumptions lead to different outcomes. First, though, let’s define some terms.

A species is the entire collection of individuals capable of interbreeding. For example, we are all members of the species homo sapiens.

A species is divided into subspecies which differ by characteristics and usually have geographically distinct habitats. There are distinct subspecies of homo sapiens which vary by characteristics such as skin colour and appearance.

Since we are discussing sentient tool-users, the next subdivision is civilization, which for the purposes of this blog entry will be a cultural subdivision, usually consisting of one or more subspecies, with distinct cultural and technological features.

For technologically advanced sentients, the civilization will likely be the more important of the two subdivisions, since it will have the strongest influence on what technologies are developed and fielded, and for what purposes they are used. It is likely, going by the experience of humanity, that civilizations will have further subdivisions analogous to nations.

Going by analogy with life on Earth, the lifetime of a species can be very long by our standards, in the millions of years, so it can produce many civilizations in its lifespan. The lifetime of a civilization will be rather shorter.

Taking the Roman Empire as a baseline, two thousand years is attainable. For a high tech civilisation with advanced medical capability, double the lifespan of individuals and thereby the life span of the civilisation.

We will further assume that life arises on planets, and produces civilisations, which may eventually become space-faring.

Now, according to our new assumptions, civilisations flourish for a time, then age and die. Species, eventually, do the same.

The history of humanity provides good support for the idea that people don’t embark on the risks and hardships of the frontier for no reason. We may therefore reasonably say that a civilisation will not throw out colonies until its numbers increase to a significant proportion of the carrying capacity of its system. Until that point, those who wish to tackle a new frontier will be able to do so within their star system, without spending years or generations of travel time to do so.

The resources and organisation needed to build and launch an interstellar colony require a considerable population and industrial base, as well.

Going by analogy with the history of civilisations on Earth, the expansionist phase of a civilisation will be a comparatively short part of its entire lifespan, occurring when the cultural, economic and technological stars align. That alignment may not happen at all. The Chinese Empire deliberately turned away from colonisation and expansion as being a threat to its stability.

The first cut at the problem is how long a series of daughter colonies could last with less than certain probabilities. Keeping it simple, we assume that a given civilisation has a small number of expansionist phases in its life span. We will take the fortunes of a single colony and its descendants and see where it goes.

The experience of our own history shows that a new polity may fail and die, survive, or thrive and grow. Light speed lag pretty well guarantees that the mother world will have little if any say in the affairs of the daughter colony. Given the average spacing of stars in the arms of our galaxy, news of revolution or crisis would arrive at the mother world years or decades late. Given sub-light travel speeds, any response would take several to many times that. The rebels would have those same years to decades of warning to prepare for that response.

Another assumption I’m going to challenge is that the process of expansion consists of some benevolent, or at least far-sighted, central government sending out neat organised expeditions. More likely, in my opinion, would be a process whereby internal disputes and grievances get to the point where a disenchanted group packs up and leaves in order to do its own thing or flee an enemy they can’t fight. Human history is full of examples.

Any or all of the above might happen at different stages of a civilisation’s lifespan. A young civilisation might go through an Age of Exploration, as Western civilisation did in the age of sail. A more mature one might send out probes or expeditions dedicated to science and knowledge, similar to the national space agencies of today. A decadent or disintegrating civilisation could see refugees fleeing from it.

Such endeavours would show a much greater variation in resources, preparedness, and organisation. On these factors would depend their chances of surviving and thriving.

Civilisation X, a race very similar to ours, achieves space flight after a long series of rising and falling civilisations. They then spread out into their star system, and build an economy and infrastructure which allows for the dispatch of a expedition to settle the next star, some five light years away.

This is their first go at it. We will give them an 80% chance of surviving the voyage. Given survival, they can have another 80% chance of thriving and expanding to the carrying capacity of their new star system. The probability of their thriving in their new home is therefore 64%.

This new civilisation may, or may not, go through one or more expansionist phases. An 80% chance of that means that there is a cumulative probability of about 50% that the daughter colony will throw out an expedition(s) of its own. With experience goes a higher chance of survival, say 90%, and a higher chance of thriving, too. Give that a 90% chance as well. The cumulative probability of a daughter colony is then 33%.

The trend is pretty clear. The probability of any one series continuing diminishes over time. Civilisations can, and likely will, throw out multiple expeditions in different directions, whose fortunes will vary. Eventually, however, the dice will come up snake eyes and the expansion will end.

One could do more sophisticated modelling using Poisson distribution and Bayesian analysis, but however you go about it the chain of daughter colonies ends at some point, given only probabilities less than certainty.

Interstellar probes which are von Neumann machines would be subject to similar probabilities. On our own experience with unmanned probes, you can’t think of everything. Survival and success will still not be certainties, and the chain will die away at some point.

When you question the assumptions, Fermi’s unstoppable wave becomes a ripple on the deep.

I wonder how wide and far our ripple will be?