How difficult is the transition to multi-cellular life?

[Avalancheon]

Re the Great Filter - why assume that there is only one?

It's been a while since I last checked, but IIRC there are easily a dozen or more ideas for why we don't see evidence of intelligent life all over the universe. The big issue with nearly all of them seems to be that it would have to work that way for all civilizations everywhere. But what if instead it's a case of all the various reasons all being in play at once?

You mean the non-exclusivity problem, right? If there are dozens of alien civilisations in the galaxy observing us from a safe distance, then it would take only one of them to break protocol and contact us. Thus invalidating the Fermi paradox.

Advanced life (beyond bacteria or simpler stuff) might be comparatively rare. Out of those, intelligent life might be comparatively rare. Out of those some number destroy themselves for one reason or another. Out of those that don't, some turn to pursuits that don't result in space travel. Out of those that do become spacefaring only some go to the stars. Out of those, only some decide to go in for activities that are visible across interstellar distances. Out of those, only some happen to be close enough to us in space and time to be engaged in anything we would recognize as a civilization or could detect. Out of those, only some have any interest in contacting non-spacefaring civs or in spreading far enough to get anywhere near us. And so on and so forth.

The upshot of all this is there could be millions of civilizations in the galaxy - but only a very tiny percentage occupy a range of types and activities and cultures that we could potentially detect or contact. And who knows how far away the nearest one could be from us?

Just some thoughts,

Todd

Thats right. Just because you have sapient aliens on some planet doesn't necessarily mean they will develop an advanced civilisation. And some of those that do may go on to destroy themselves. Those are things that George Dvorsky calls 'porous filters', which I think is a wonderful term.

However, I would disagree with you about spacefaring aliens not visiting the stars, and not being visible from interstellar distances. After all, a race of post-singularity beings wouldn't take long to colonise the milky way. If there are aliens in our galaxy that aren't visible to us, then that can only be because they are at a similar tech level. This is extremely unlikely to be the case.

If two sapient races happen to evolve in the galaxy, then theres going to be a time space between them. Tens of thousands of years, at the absolute minimum. And more likely to be millions of years.