Proxima Centauri has a Earth-like planet

I don't know. Fermi paradox has never bothered me too badly. Between possible great filters (behind and/or ahead of us), the time and distance involved in species discovery and travel, and the fact we're still apparently pretty early in the universe's lifespan ... it seems reasonable to me that we may be some combination of rare and early. So, Group I seems reasonable.

One potential "great filter" always stood out as likely to me, even if all the other leaps before we came along had been made:

That filter is developing the combination of (i) self-aware intelligence to a degree that you are capable of deep layers of abstraction, (ii) the physical ability to adeptly physically build complex creations, (iii) an inclination toward social living that encourages cooperation. Other relatively high intelligences have evolved here on earth: octopuses, dolphins, elephants, etc. But it's hard to imagine any of them being physically capable of dramatically modifying their environment and building complex, useful creations. And we mammalian humans, dolphins and elephants are all social and cooperative to some degree, but octopuses don't seem to be. The combination of these three occurrences is extremely powerful, but may also be extremely rare. And it's difficult to imagine a space-faring civilization that wouldn't need all 3.

And Group 2 seems reasonable too. In particular, it's interesting to consider that while we currently hold an expansionist mindset because expansion has driven the course of our civilization, this may be a temporary thing. It makes sense when competing locally with enemies (other tribes, cities, nations, empires, etc) that are very similar to yourself. And we believe that survival and expansion go hand in hand. But it may well be the case that intelligent species generally discover that survival is maximized when not expanding aggressively, beyond some point, and when conserving resources. Hiding themselves so as to minimize the risk of contact with other alien species may go hand in hand with that as well.

So, between seemingly plausible filters in both Group 1 and Group 2 explanations, I think it's reasonable to entertain the possibility that it'll be a long time before we meet another alien civilization, if we ever do.

There's no thick atmosphere like ten feet of water.

The frequency absorption of water has a small window around visible light and a sold wall around it. [1]

Maybe the danger of building telescopes on the surface (and the rapid attenuation of their home medium) is why they haven't built any radios to chat with us. ;)

[1]http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/watabs.h...

There is almost no time dilation at 50% C this is pretty awesome less than a 10 year trip if we ever manage to build even probes that can go that fast it's pretty much a done deal.

I think it makes sense for an intelligent civilization not to expand, if you consider the problem of synchronizing over vast interstellar distances. If it takes years to exchange messages and resources back and forth between two colonies, it will be slow and costly to keep them on the same page. That means they will likely evolve independently and may become competitors.

In other words, colonies lacking resources inside the expansion wave would start attacking their own neighbours, and colonies at the periphery may decide to expand inward, because even though other colonies' resources are harder to acquire, they are already processed/terraformed and therefore more useful. So you don't expand, not necessarily to hide, but to avoid creating enemies in close proximity to yourself, because you can't control your own offspring except at a tremendous cost.

Of course, even so, it seems strange that no civilization would ever make this mistake. Perhaps those that do are far away, or the propensity of colonies to eventually expand inward slows down the expansion.

You can't have fire, basic tools maybe, more than that not really. Also doing chemistry underwater is pretty neat impossible. Sure we can do it today, but incremental development is very unlikely.

There is a good chance that any technological species we need will be quite close to us because there are certain criteria that are likely to be met which are pretty universal.