One of the biggest questions regarding the existence of advanced extraterrestrial life can be boiled down to three simple words, the Fermi paradox: where are they? The universe has existed for billions of years, with more than enough time to permit the evolution and development of life and civilizations capable of being noticed even across interstellar distances - theoretically, at least. We won't know for sure until we actually start finding them. But the question of why has been grappled with constantly for decades, with everyone approaching it from a different angle.
Larry Niven once suggested an interesting possibility that I hadn't seen before - that, whether by fluke or environmental pressures or whatever else, perhaps the intelligent beings of the universe were almost all aquatic. It's perfectly possible to be intelligent and live in the water - dolphins and cephalopods could be all of the way there, and it's just taken us this long to realize it. But aquatic beings are held back by the nature of their environment: you can't really build a technological civilization when you live in the water.
The other day, I had an idea along these lines that I don't recall seeing anywhere besides the works of Hal Clement - what if extraterrestrial intelligences are kept where they are by gravity? I'm not thinking about something extreme like Mesklin, with 3 g at the equator and at least 275 g at its poles, but rather something we've already observed: Gliese 581 d, a potentially terrestrial, potentially habitable world twenty light-years away.
There's only one niggle, if the information we've gathered about it is accurate: the planet's a bit clingy. Specifically, it's estimated to be twice the size of Earth, and have seven times its mass. By no means does this preclude life; it just means it wouldn't be exactly like home. Owing to my lack of mathematical skill, I wasn't able to calculate the surface gravity for a potential planet with this size and mass - so let's be charitable and assume a planet with 2.83 g, just to be charitable, to see what we've got with nearly three times Earth's gravity pulling you down. Call it Earthissimo, like my astronomy lecturer did back in university.
Local life forms would be short and bulky - in that kind of gravity, there's nothing to be gained by going up. Without the high mountains that lower gravity makes possible, land would tend to be lower and more eroded - conceivably, there could be high-gravity archipelago worlds. If you're interested, Extraterrestrials: A Field Guide for Earthlings by Terence Dickinson and Adolf Schaller goes into a bit more detail.
That's just biology - I'm more interested in technology. There's nothing about a high-gravity planet that fundamentally prevents an intelligent civilization from arising there, but one that did would face far greater challenges in some respects than we do. Culturally, it could theoretically have the deck stacked against it from the beginning. On a high-g planet, very few things fly and falls are exceedingly dangerous even from what we'd think of as trivial heights. That sort of environment could easily breed what would be, by human standards, an intensely conservative mindset - something like that, introduced early enough and reinforced enough by the environment, could put serious brakes on any kind of serious technological development, or perhaps just leave the people there in a long Bronze Age.
But let's look at Earthissimo. The people there have struggled against the chains of gravity enough to build a technological civilization that we might recognize in the broad strokes - details, of course, would differ immensely. The important issue, however, is access to space - without that, there's no chance of aliens coming to us. It's with access to space that the real challenges lie.
While it's certainly difficult to get to Earth orbit or beyond, it's certainly doable - a big part of the problem is that there aren't yet any economies of scale in the space industries, but even then you've still got to deal with accelerating to 9.4 kilometers per second even to reach the 300-kilometer altitude of the International Space Station. If you're leaving Earth entirely, escape velocity is 11.2 kilometers per second.
How envious the space freaks of Earthissimo would be of us. With all that extra mass pulling things down, it's not easy to get to orbit - Earthissimo's to-orbit velocity would be 44.91 kilometers per second, and if you want to escape entirely, you'll need to have a ship that can accelerate to 63.5 kilometers per second. The problem is that chemical rockets, such as the ones we used as we began to claw our way into the heavens, are insufficiently beefy. Rocket launches from Earthissimo would be incredibly complex and incredibly expensive - enough so that it's easy to imagine that the people there just wouldn't bother with it at all. Hell, we're having enough trouble getting it off the ground here!
So maybe that's another explanation. There may be worlds upon worlds of people out there who can look up at the stars, at the blackness of space, and know that because of the dirt beneath their feet-equivalents they'll never be able to touch them.