Comment by pharke

This is the L constant of the Drake equation. Probably the hardest to find a value for until we've actually discovered other intelligent life or remains of such. The confounding part of it is that we can only guess at things that may end a civilization so completely that it will never recover. Things like the so called Great Filters, natural extinctions like impact events or gamma ray bursts, or other more speculative things that prevent expansion. The tricky part is that our existence seems to fly in the face of such events so far, is that mere luck or do all forms of intelligent life have enough sense to navigate around these problems? The last and most paradox defining part of the problem is that even if the tiniest fraction of civilizations can evade these filters then they should eventually be everywhere in the galaxy. Assuming the Copernican principle that we are not at a special place in the universe or time then there should have already been ample opportunity for such civilizations to develop and hence Fermi's question "Where is everybody?".