Why is it not affected? If we assume they expand at a negligible fraction of the speed of light, we expect them to be visible from the outside for their entire lifetime (which may be very long). On the other hand if we expect them to expand at nearly the speed of light, we expect them to be detectable from outside for only a few hundred years.
The other side of the galaxy could very well be already consumed by an alien civilization.
If intelligent life was common and underwent such expansion, then there would be very few new-arising lonely civilizations later in the history of the universe (the real estate for their evolution already occupied or filled with signs of intelligence). The overwhelming majority of civilizations evolving with empty skies would be much younger.
So, whether you attend to the number of observers with our observations, or the proportion of all observers with such observations, near-c expansion doesn’t help resolve the Fermi paradox.
Another way of thinking about it is: we are a civilization that has developed without seeing any sign of aliens, developed on a planet that had not been colonized by aliens. Colonization would have prevented our observations just as surely as alien transmissions or a visibly approaching wave of colonization.
Assume life is rare/filtered, we straightforwardly expect to see what we see (empty sky).
Assume life is common and the singularity comes quickly and reliably, and colonization proceeds at the speed of light, then condition on the fact that we are pre-singularity. As far as I can tell, a random young civilization still expects empty skies, possibly slightly less because of the relatively small volume of spacetime where we would observe an approaching colonization wave.
So the observation of empty skies is only very weak evidence against life being common, given that this singularity stuff is sound.
The latter hypothesis is more specific, but I already believe all those assumptions (quick, reliable, and near-c).
Given that I take those singularity assumptions seriously (not just hypothetically), and given that we are where we are in the history of the universe, the fermi paradox seems resolved for me; I find it unlikely that a given young civilization would observe any other civilization, no matter the actual rate of life. If we did observe another isolated civilization it would be pretty much falsify my “quick,reliable, and lightspeed” singularity belief.
However, as you say, that “given that we are where we are in the history of the universe” is worrying. I predict most young civilizations to be early (because the universe gets burned up quickly), and I predict most civilizations to not be young, given that life is common. When we observe ourselves to be young and late (are we actually late?), fermi’s paradox results. I guess in this case fermi’s paradox is that we observed something that is a priori unlikely, and we wonder what unlikely alternate hypotheses this digs up (the above, for one). However, anthropics is very confusing...
I predict most young civilizations to be early (because the universe gets burned up quickly), and I predict most civilizations to not be young, given that life is common. When we observe ourselves to be young and late (are we actually late?), Fermi’s paradox results.
Why is it not affected? If we assume they expand at a negligible fraction of the speed of light, we expect them to be visible from the outside for their entire lifetime (which may be very long). On the other hand if we expect them to expand at nearly the speed of light, we expect them to be detectable from outside for only a few hundred years.
The other side of the galaxy could very well be already consumed by an alien civilization.
E.g. check with http://en.wikipedia.org/wiki/Fermi_paradox
The rate of expansion makes very little difference, and a high rate of expansion is not listed as a possible resolution.
That article has little about the effect of expansion. Why does it not affect it? What is wrong with my argument that it should matter?
A near-c rate of expansion drastically reduces the volume of space that a given civilization is observable from. What specifically is wrong with this?
If intelligent life was common and underwent such expansion, then there would be very few new-arising lonely civilizations later in the history of the universe (the real estate for their evolution already occupied or filled with signs of intelligence). The overwhelming majority of civilizations evolving with empty skies would be much younger.
So, whether you attend to the number of observers with our observations, or the proportion of all observers with such observations, near-c expansion doesn’t help resolve the Fermi paradox.
Another way of thinking about it is: we are a civilization that has developed without seeing any sign of aliens, developed on a planet that had not been colonized by aliens. Colonization would have prevented our observations just as surely as alien transmissions or a visibly approaching wave of colonization.
I still don’t get it.
Assume life is rare/filtered, we straightforwardly expect to see what we see (empty sky).
Assume life is common and the singularity comes quickly and reliably, and colonization proceeds at the speed of light, then condition on the fact that we are pre-singularity. As far as I can tell, a random young civilization still expects empty skies, possibly slightly less because of the relatively small volume of spacetime where we would observe an approaching colonization wave.
So the observation of empty skies is only very weak evidence against life being common, given that this singularity stuff is sound.
The latter hypothesis is more specific, but I already believe all those assumptions (quick, reliable, and near-c).
Given that I take those singularity assumptions seriously (not just hypothetically), and given that we are where we are in the history of the universe, the fermi paradox seems resolved for me; I find it unlikely that a given young civilization would observe any other civilization, no matter the actual rate of life. If we did observe another isolated civilization it would be pretty much falsify my “quick,reliable, and lightspeed” singularity belief.
However, as you say, that “given that we are where we are in the history of the universe” is worrying. I predict most young civilizations to be early (because the universe gets burned up quickly), and I predict most civilizations to not be young, given that life is common. When we observe ourselves to be young and late (are we actually late?), fermi’s paradox results. I guess in this case fermi’s paradox is that we observed something that is a priori unlikely, and we wonder what unlikely alternate hypotheses this digs up (the above, for one). However, anthropics is very confusing...
Fermi’s paradox also makes mention of the fact that there are billions of stars in the galaxy that are billions of years older than ours, many of them having habitable planets. Some reasons have prevented any of these from spawning a galactic colonization wave—and those reasons are of interest to us.
Yes and yes.