Here's a scary fact: Once humanity figures out how to reach and colonize our nearest star system, assuming it takes us 1,000 years each time we colonize the next closest star systems, it would only take 25 million years to colonize the entire galaxy. 25 million years is nothing. So why isn't the entire galaxy teeming with intelligent life? Our universe has been around for 13 billion years - plenty of time for this to have taken place.<p>The most likely answer (and one we don't want to admit to ourselves) is that intelligent species have a certain lifetime limit. Most species die off within 3 million years.<p>An alternative answer would be that advanced intelligence figures out a way to delve into mind or other dimensions, rather than outwards into physical space.
Two possible explanations for lack of observable neighbors are: 1) Any sufficiently advanced civilization will learn how to stimulate/entertain itself to the point of not needing external stimuli such as exploration and conquest. In other words, E.T. is too busy playing his holographic, neural-tapped X-Box to communicate or look for us. (E.M. Forrester's prophetic story, "The Machine Stops" foreshadowed this for mankind in 1909.) or possibility 2) A.I. and/or sentient life evolves into a higher-level, transdimensional abstraction beyond our perception and understanding (also known as the Singularity).
One interesting possibility that doesn't seem to get discussed much is metallicity. Stars containing more heavy elements tend to be younger, while older stars tend to have a lower metallicity.<p>Looking at our own biochemistry, you don't just need carbon, nitrogen, oxygen, and hydrogen to make it work. Heavier elements from the fourth and fifth periods of the periodic table are also required.<p>I wouldn't be surprised if life is rare but also becoming more common over the last few billion years. Evolution also doesn't necessarily lead to intelligent life capable of harnessing technology.
Intelligence is not enough. You can be intelligent without being technological.<p>The dolphin will never make a spacecraft, even if he is twice as smart as we are. Lack of hands is a problem. Also, lack of ability to create fire is a problem. Also, lack of ability to easily pour liquids from one isolated vessel to another, without diluting them is a problem. Basically, no "water worlds" will create a technological civilization, in my opinion.<p>There's also something cultural that may not happen for some reason. Look at the Native Americans. They still didn't have the wheel or metal, four thousand years after others had them. If there is nothing forcing you to develop tech, you won't, even if you're technically smart enough and have the resources that would allow it.<p>I can also imagine some sort of plateau being reached, after a particularly nasty government takes over. What would happen here, if certain countries/groups were suddenly in control of the entire world?<p>We may already be reaching a plateau ourselves. The desire is waning, I think. It has certainly waned in my lifetime, both personally, and as a world, I think. At some point, you become "practical", and decide to expend your time and resources on things closer to home.<p>Add it all up, and it's at least a couple more multipliers for your fermi equation.
One more possibility - as human populations get wealthier and more educated, their population growth rate decreases.<p>Maybe by the time lifeforms get the technology to travel to other stars, they find that they don't need to expand.<p>There are many places (even within the USA) that we have the technology to colonize, but not the desire.