Wednesday, May 7, 2014

The Fermi Paradox, Part 2

In my first post on the Fermi Paradox, I went over the basics, which I expect most readers are familiar with anyway. It can be summed up simply (perhaps over-simply) as:
  • Our galaxy is plenty old enough for at least one advanced civilization to have completely colonized it by now, even at speeds much slower than the speed of light.
  • "Completely colonized" should include our solar system.
  • No one can make a persuasive case that this has happened.
 So, the simple version is: they should be be here, but they aren't. This presents us with a paradox.

I'd bet that some readers already have a few objections to the above, and over the next few posts we will take this apart and see where reasonable doubt lies.  This is the real value of the paradox: it serves as a sharp mental lens that forces us to question our assumptions. It should make us more humble about our understanding of how the universe works, and spur us into deeper research. The great unknown is great indeed, so let's go explore. The forward path for the human adventure could not be clearer, and one of the best signposts is the Fermi Paradox.

Sadly, this is not the only effect it has. Too many seize on the Fermi Paradox to jump to Grand Conclusions not in evidence. I promise not to do that. Instead, we will look for better questions than "where is everybody?" I haven't found one yet, but I see hope.

When we look at solutions to the paradox (i.e. why it may not be a paradox), it's not to say, "ah ha! Paradox resolved!" Of course we don't know that.  What these solutions do, if well thought out, is point the way to further research that will deepen our understanding.


New Developments on the Landis Solution

An interesting new paper was published recently by physicist Daniel Cartin, following up on an idea by NASA scientist
Geoffrey Landis
and science fiction author Geoffrey Landis
from more than 20 years previous. Landis (whom I interviewed in 2013) pointed out an essential flaw in the naive model of galactic colonization: that it could not proceed like the expansion of an empire, centrally programmed and directed. There can be no iron fist that reaches over tens of light years. It is impossible to maintain even indirect control of far-flung interstellar colonies. Such colonies would decide for themselves if they wanted to propagate their kind to neighboring star systems. They may well have enough problems just surviving on their new homeworld and may never make that leap, although they were long ago bequeathed the now ruined technology to accomplish the journey. Technology, know-how, organization, and expansionary drive can deteriorate over time and even be completely lost, and this is true whether we're talking biological colonists, self-replicating robots, or (more likely in my view), some kind of highly augmented biological hybrid.

What Landis did was to assume that each new colony would expand to its neighbors with a probability p. Using percolation theory for  a regular three dimensional lattice, we know that if p is less than a certain critical probability (about 0.31 in his scenario, or roughly 2:1 odds against), then the size of the "empire" will be limited, and big voids will be left uncolonized. When the probability exceeds the critical probability, the colonization process starts filling up the available lattice.

Sure, this is a simplified model compared to what would undoubtedly be an epic drama of colonial expansion that would play out over many millenia and involve complex and risky decisions. The probability p simply encapsulates in an average way all the economic, technical factors plus the effect of various risks, such as disease, violence, critical equipment failures, and natural disasters.

Cartin's innovation was to separate the societal factors from technical ones by introducing a new parameter, Dmax. This parameter describes the maximum range of a starship. As we understand it - never having tried it ourselves - interstellar flight is hard, and flying further is harder. A collision with even a tiny dust particle in the interstellar medium can mean big trouble and possibly mission-ending damage. Getting up to interstellar speed may necessitate close flybys of intervening stars, which in itself presents risks. Then there is just good old fashioned equipment failure, fatigue and wearout on a journey that could take hundreds or even thousands of years. The Dmax parameter encapsulates all these challenges into a single variable.

Another clever idea Cartin had was to use the known census of stars in the Solar neighborhood to model the expansion of a technological civilization. In this case, we not only have good knowledge of the distances between stars, but what type of stars they are: bright, sunlike stars or cooler, dimmer red dwarfs. Cartin cut off the solar neighborhood at 40 parsecs, or about 130 light years. In this relatively small scoop of our galaxy he ran his model. He found that for a reasonable range of parameters, the sun was not included in any cluster (see Figure 7)

Here's the takeaway quote:
The main result of this work is to show that there is credence to the notion that the Solar System is unvisited because it lies in an uncolonized cluster or void.
Note, Cartin chooses his words carefully: "there is credence." The Fermi paradox has not gone away, but this kind of study tells us that we may want to better understand the dynamics, risks and economics of interstellar colonization before we begin making pronouncements about the deep implications of the Fermi Paradox. We may even want to try it ourselves. I think we should.

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