"The Fermi Paradox which contrasts the 100% probability of life and intelligence developing on Earth against the thunderous silence from the heavens so far (no alien signals) may be resolved by four things: One, gamma ray bursters which may have effectively prohibited the development of sentient races until only the last 200 million years; Two, the lengthy gestation period required for the emergence of intelligence (which almost requires the entire useful lifespan of a given planet, based on our own biography); Three, the need for an unusually high measure of stability in terms of climate over hundreds of millions of years (the 'Goldilocks' scenario, enabled by a huge natural satellite like our Moon moderating the tilt of a planet's axis, as well as gas giants parked in proper orbits to mop up excess comets and asteroids to reduce impact frequencies for a living world); and Four, an extremely dangerous 600 year or so 'gauntlet' of challenges and risks most any technological society must survive to become a viable long term resident of the galaxy (i.e. getting a critical mass of population and technology off their home world, among other things). That 600 year period may be equivalent to our own span between 1900 AD and 2500 AD, wherein we'll have to somehow dodge the bullets of cosmic impacts, nuclear, biological, and nanotechnological war, terrorism, mistakes, and accidents, as well as food or energy starvation, economic collapse, and many other threats, both natural and unnatural. So far it appears (according to SETI results and other scientific discoveries) extremely few races likely survive all these. So why haven't we heard from those which have? What are they like? And how far away might they be? Details of all the above and more (along with references) may be found on this and its succeeding pages."