Usually, this question is asked about extra-terrestrial aliens. “Are we alone in universe?” It’s understood that we are alone (as sentient species) in our solar system (with the provision that I make allowances for dolphins, chimps, and some other species on Earth).
But are we alone in our local cluster? Our galaxy? Our universe?
I’m just going to speculate about a few things here and then let the conversation go where it may.
First up, are there other intelligences… somewhere? I consider this to be unlikely. Not because I think that we are somehow special in the universe or made by some deity to worship it.
No, I think it unlikely because of two reasons. The first is the rather unique circumstances of the rise of intelligence on Earth and (especially) the danger of a minor catastrophe totally obliterating it. The second is that there is no evidence for a similar technological civilization in our galaxy.
The rise of mammals, which led to primates and modern humans was a fairly accidental event. The dinosaurs ruled the planet for something 65 times as long as humans have existed. But they were a mostly satisfied group. We certainly don’t have any evidence (movies and novels not counting as evidence) of advanced cognitive skills among dinosaurs. Even today, 65 million years after the demise of the majority of the dinosaurs, tool use among dinosaurs is limited to just a few species and limited to found objects or small pointy sticks that can be made with a beak.
The question becomes, is this a common pattern of live in the universe? I’ll give the benefit of the doubt and say that the rise of multicellular organisms is commonplace. There’s a lot of recent research that shows that the evolution of multicellularity is almost easy given certain, common conditions.
At least one source I’ve read (a while ago) suggests that, even without faster than light travel, it would be possible for a technic civilization to spread throughout the galaxy in just a few million years or so. The Milky Way is only 100,000-120,000 light years in diameter. If we could even get to 10% the speed of light, then it would be theoretically possible (generation ships or hibernation) to get to any planet in the galaxy in less than a million years.
Given a hundred year trip to another habitable planet and a couple hundred years or so to build up that planet, then within a few million years any civilization could spread throughout the galaxy.
So where are they?
Which brings me to the second issue. We haven’t found any evidence of any civilization. SETI is looking for radio signals that cannot be formed by natural processes. Only a technic civilization can modify radio waves in a certain way. If we find that modified radio wave, then it will be evidence (not conclusive, but evidence) that a technic civilization may exist.
Plus, if the expansion notion is valid, then we should have evidence of an alien presence on Earth at some point in the past. (And my response to any claims that such evidence exists is “where is it and has it been peer-reviewed”.)
Maybe because of the late development of humans, we missed all the fun. Maybe, we are alone as a sentient species. Maybe there are tons of civilizations and they just aren’t using technology we can detect.
At this point, we just don’t know. And until we get out there, we will never know.
