David Blenkinsop, <email@example.com>, writes, regarding the possibility that Gamma Ray Bursters may cause periodic galaxy-wide mass extinctions:
> What makes this interesting is that this idea may imply that we are just
> extremely lucky to have gotten as far as we have, and would help to explain
> the seeming lack of successful galaxy colonizers coming in towards us from
> intergalactic distances. Notice, though, that this by itself doesn't seem so
> helpful for explaining why we haven't seen any ET's starting a wave of
> colonization from relatively nearby locations within our galaxy. Presumably,
> any nearby settlement-wave builders would benefit just as much from the
> absence of a galaxy-wide catastrophe as we apparently have.
Right, it seems that most estimates suggest that there should be a very large number (billions?) of planets in the galaxy that are reasonably suitable for the formation of life. In that case, the fact that our planet has survived any GRBs would suggest that other planets in the galaxy would also have had a good chance to do so.
The article also adopted a fairly conservative estimate of the rate of spread through the galaxy (1/1000 the speed of light, I think it was). We commonly consider scenarios with much higher rates of expansion than this, 1/100 or 1/10 c or even faster. This gives galaxy-wide domination in a million years or so.
Putting these together, we have to be in first place, or at least tied for the lead to within one million years, which seems very unlikely if intelligence is at all common on those billion earth-like planets.
> Since Robin Hanson has been mentioned, maybe this would be a good time to ask
> a question about *another* ET article of his, "Burning the Cosmic Commons:
> Evolutionary Strategies for Interstellar Colonization". This article presents
> a novel approach to the lack of ET's by saying that they could perhaps have
> gone right *past* our intersteller locality, missing us, and our solar system
> *completely* in a hasty "rush" of an expansion wave. In this scenario the
> resources they need to expand rapidly have somehow been used up, so presumably
> star systems fairly near to us might be depleted in whatever-it-is that they
> need to keep their civilization going? Supposedly, they've never returned from
> that great expansion wave front, figuring that our locality is used up, or
> burnt out, so in this concept *that's* why no obvious ET settlements!
> My question here, is whether this abstract "Burning the Commons" concept, or
> study, has any real meaning, in terms of the actual resource that would be
> depleted? I mean, just what could possibly be so important that it would get
> used up, and prevent really expansive ET's from gradually expanding into all
> available niches, including our solar system? For instance, one "life
> essential" resource that comes to my mind is *phosphorus*, but I can't imagine
> why phosphorus atoms would actually be destroyed by ET settlers? Actually, I
> suppose that's two questions: first, any reason why nearby ET's couldn't be
> communicative, but planet-bound, and, second, what's burned in the "Burning
> the Commons" scenario?
I think the idea was that there would be this tremendous race to be the first to expand outward, because whichever one wins the race gets the first use of the new star systems. This leads to selection pressure for doing everything possible to increase the rate of expansion.
You could imagine one of these probes, at the front of the expansion wavefront, coming into a new solar system and proceeding furiously to build, construct, convert matter into whatever is needed to expand outwards. Maybe it constructs a gigantic mass accelerator, taking apart planets and asteroids system to build huge batteries to collect solar energy, and an electromagnetic launcher billions of miles long. This is used to fire off a limited number of additional probes, all heading outward where the chances of finding fallow systems is best.
While this is going on, competing probes all around it are finding their own solar systems and doing similar construction. By the time this one is ready to go, it has been leapfrogged and it has to aim far outward to have a good shot at an unused solar system.
Like a salmon which has fought its way upstream for a furious bout of mating, once the probe has "spawned", evolution is through with it, and it might just die. Or if it does stay alive, it has a pretty messed up solar system and no clear evolutionary guidance.
You'd still think that there could be a second wave of colonization, replicators evolved and specialized to work with the detritus of the hyper-fast expansion wave. The first wave can't have used things up _too_ completely - the star is still there, and matter in some form; at least the big planets are still intact. After all, we see stars which look like ours, and we are starting to see planets around them.
It seems difficulty to come up with an evolutionary scenario in which life does not advance into every possible niche, so that we can say that our own solar system just happens to have been skipped. At best you can argue that we are near enough to the time that life first was able to spread through the galaxy that it just hasn't gotten here yet. Maybe by combining the "we're relatively early" and "life misses niches" ideas you can make the numbers work out to give us a reasonable chance of existing.