In that case we can't treat our existence as independent of the existence of other nearby civilizations.
If a civilization doesn't have FTL then expansion beyond their solar system seems unlikely. Yes, it is still possible with sub-light travel, but then you run into the problem of motivation.
When limited to sub-light speed trade between colonies and the home system is so slow that you probably can't usefully import resources from the colony systems.
When you are stuck with sub-light, colonizing another system (unless perhaps you are in a place with a lot of stars close together) is more accurately described as spending a whole lot of effort to establish another system that then won't really have much interaction with or effect on your system.
Even the old classic of kicking out your undesirables who then go on to start a new civilization doesn't really work, because it is so expensive. If you dislike them so much that exile is the only answer and you don't have someplace in your system to exile them to you are far more likely to just kill them.
> When limited to sub-light speed trade between colonies and the home system is so slow that you probably can't usefully import resources from the colony systems.
I mean this seems to disregard our entire history as a species. There have always been "others" within a population that seek, above anything else, to find a new place to go in order to live life they way they want to.
We are also moving into an age where data and knowledge are raising to supreme importance. Both of these things can be transported at the speed of light, so a multi-solar system civilization that is able to increase is total resources available to acquire new information and knowledge will be at an advantage. Then, of course, there is the desire to preserve the human species as a whole by "spreading our seeds" so to speak.
Sure, but those who have successfully done that have either went to a place near enough that they could take all the food and other supplies they would need during the trip there with them, or have went to a place where food and other resources were available on the way.
I can't think of any human group that did a migration that had to cross a span that provided no food or drinkable water and was too long for them to cross on what food and water they could carry with them.
With sub-light migration to other solar systems we are talking about crossing empty spans that will take several lifetimes to cross. There are ways to deal with that, but they are likely to be major undertakings that require a heck of a lot of resources so that to do it you need widespread support from the people who aren't going.
My guess is that you'll need a pitch much stronger than "We want to go someplace to live our way" (especially since the people who leave won't be the ones who arrive at their destination--it will be their descendants).
If the energy and time requirements are great enough, space between solar systems will work as the same sort of barrier space does for the rest of life on Earth.
That said, it only takes a tiny fraction of their civilization to do it, for it to happen.
So then.. maybe only the civilizations that aren't expansionist survive? Or everyone changes their mind and no one is expansionist by the time they get the right tech? Or maybe it's physically impossible to build von Neumann probes for some reason we don't yet understand? Or who knows what else. In any case, it's got to be zero in the history of our galaxy.
1) The idea is that we will do it, not that we have done it already. (How does something we will do in the future affect what we see today? It doesn't, at least not causally. It's just that some civilization had to be first in the neighboorhod, and that civilization will be the one to preempt the others, at least according to this model. So it's more of a probabilistic conditioning effect than a causal effect: conditional on existing, that means no one preempted us, which means we're the first, which means we'll preempt everyone else.)
2) The preemption happens well before the other civilizations come into existence. For example, we might deplete their stars of energy so that evolution doesn't happen at all on their planet.
Interesting. That's a point I haven't heard before. Or perhaps it's my lucky 10,000 day. :-)
Although our path through the galaxy is somewhat unique. Stars don't really move in unison in the greater timescales, but more like slip past each other. Assuming Sol was born as a binary star, its sibling could be other side of the galaxy by now.
Another thought is that if WE do expand to the stars, how long until our distant ancestors aren't really the same civilization anymore, but something completely alien to each other? Evolution could take some rather unexpected steps in the long run.
Rather, it refers to those stars which are within reach of us, now, in a timescale that is short relative to the typical interval between births of civilizations that distance apart. That could be anything from a corner of a galaxy to a galactic supercluster, depending on the speed with which civilizations can expand and the frequency with which new ones arise in the universe.
Note that if civilizations expand at sufficiently close to the speed of light, then there is only a tiny window of time between when their light arrives and when their colony ships do. So under this model, it is quite unlikely that a new civilization waking up for the first time would look around and see anyone. Most likely, either the light from those other civilizations hasn't yet gotten here, in which case we see nothing, or their colony ships have already gotten here, in which case we wouldn't be born in the first place.
