[lhorie]

Why are we assuming that we would be able to see intelligent alien life by just looking at the right spot in space? If we take it at face value the figure that the universe is 13 billion years old, and that the observable universe is 90 billion light years wide, presumably that means that there's a whole lot of universe that could have evolved intelligent life at a pace similar to our own, but that we just can't see because there hasn't been enough time for light to get from there to here.

The assumption that an advanced life form would be able to take over the universe and be visible to us requires that said life form is able to travel large distances. So to be able to travel far enough to enter our field of vision, they need to basically speedrun evolution and space travel proportional to the distance from us, i.e. the farther they are, the faster they need to evolve and the farther they need to travel for us to be able to see them. But the age of the universe poses a hard limit on how much of a head start a civilization can get over us.

So if we think this way, it seems somewhat plausible that life could exist, but be undetectable due to the sheer size of the universe and time constraints, even if they are capable of interstellar travel.

[wruza]

Our evolution was not a straight line. We could stay in bacterial form for a couple of billion years if not the great oxygenation event, or have a couple of billions of years of advantage if it happened that earlier. The evolution from simple marine life to us happened in an instant compared to pre-O2 era.

https://www.shutterstock.com/ru/image-vector/evolution-life-...

[lhorie]

Sure, but that doesn't really address the point. Suppose aliens got really lucky and managed to achieve interstellar travel in just 1 billion years since the big bang. That still leaves the entire area outside of a 12 billion light years radius from us invisible, i.e. they would have to be somewhere within that radius within the next 12 billion years, for us to be physically able to see them. But that radius is like less than 1% of the known universe.

And in actuality, they would need to sit there detectably within the narrow period of a few hundred years or so that humanity is actually looking at that spot in the sky. So, for example, if they were galaxy hopping every few million years rather than cancerously leaving dyson spheres everywhere, and just happened to temporarily brush into our radius of visibility during their nomadic travels, we wouldn't necessarily see them if they got within view too early or too late.