https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CEEQFjAB&url=http%3A%2F%2Fwww.astro.ufl.edu%2F~lebo%2FAST2037%2FFermi's%2520Paradox%2520-%25

https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CEEQFjAB&url=http%3A%2F%2Fwww.astro.ufl.edu%2F~lebo%2FAST2037%2FFermi's%2520Paradox%2520-%2520Stephen%2520Webb.docx&ei=kMrRUqzyDsX32QW2lIFQ&usg=AFQjCNHKQDX6K8mCoXHHuGCWhE_WHRh3yw&sig2=6okCviy35CLcGaJQAAfOQg https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd... Edit: I'll add one to the above: it might be that there are enough hostile forces, or at least the possibility of enough hostile forces, to make every technologically capable civilization conclude to travel near the speed of light to protect themselves from relativistic bombs. (Objects going very close to the speed of light. Impossible to detect in time to deflect, a form of advanced weaponry.) Each alien civilization sees that there are no observable civilizations and decides that the ones that have existed either died or fled into near-the-speed-of-light vehicles. They rationally conclude to do the same, at least while they advance their technology and search for life. The unfortunate part is that time goes by much faster for us than it does for them, (which negates the advantage that many early civilizations would have had).