[ExI] Some thoughts on the Fermi Paradox

[Ben Zaiboc]

We can't detect any civilisations that arose after {their distance from us in light-years} years ago. So if we were looking at the Centaurus system, we would see anything that arose up to a few years ago. Signs from the other edge of the galaxy would have to be from no less than 100k years ago.

We have had about 4 billion years from the formation of the sun to where we are now, and could expect to be at an 'advanced civilisation' stage somewhere within the next couple of hundred years. Provided we survive that long.

If intelligence arose in lots of places in the galaxy at about the same time as us, how much of it would be visible to us at the present time?

I think the relevant question is not so much a matter of 'how likely is intelligent life?' in general terms, as 'how quickly could intelligent life arise?', anywhere in the galaxy. In other words, how long ago?

If we can decide that advanced civilisations will have arisen more than 100k years ago, then the Fermi paradox is probably a sensible question. If not, then there is no paradox, for the simple reason that we can only see into the past. The farther away, the further back. So recent events are not visible to us.

It's very unlikely that any population III or II stars could support life, so we only have population I stars to consider, and I think the sun is a fairly typical one. There are older Pop. I stars, up to 10bn years old, but they are mostly in the galactic core, the younger ones are in the spiral arms. I'm inclined to discount the older ones in the core because we can't see in there very well, and it's likely to be a very hostile environment, so life is unlikely to survive long.

It has taken the vast majority of the 4.6 billion years since the sun formed to get life on earth to a multicellular stage, and civilisations only got going recently, maybe in the last 10ky. If you posit a lifeform that manages to avoid hamstringing itself with religion, politics, greed, etc., etc., then maybe you could halve that time.

So if we are typical, rather than extreme latecomers, then no matter how widespread life is in the galaxy, we can't expect to be able to detect alien civilisations beyond a spherical volume about 10k light years away, and probably much closer. That's a pretty small percentage of the total volume of the galaxy.

Add the fact that some places are less favourable than others (core, halo, inter-arm spaces), and that we only have a
very limited view of the galaxy from where we are, and I think it would be remarkable if we did see anything indicating intelligent life, and that this has nothing to do with the actual incidence of intelligent life in the galaxy as a whole.

We're back to the fireflies in a rainforest analogy, except the rainforest is a light-minute across and all the fireflies hatched out within the last 10 seconds.

I'm attaching a png file (low-res., due to email size limits), showing the known distribution of population I stars in the galaxy (the blue regions in the spiral arms). The yellow area is old pop. I stars in the core. The green circle is a rough estimate of the distance from the sun we might expect to be able to see advanced alien civilisations (looking up to 10k years into the past). Imagine the pink dots are current alien civilisations (probably a huge over-estimate, but who knows?). I think you can see my point. We simply can't see them.

Essentially, space is big, the speed of light is fixed, so we can't see the recent past (except very very close by). The farther out we look, the more of the recent past we can't see. So if any advanced civilisations arose in the recent past (meaning within the last few thousand years, getting more remote as we look farther out), we can't see them.

So the Fermi paradox only holds if you're looking at very big timescales (millions of years), and I don't think that's very relevant, given the ages of the population I stars and the history of life on earth.

---
Ben
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