No. 122: Mar-Apr 1999
Circa 1950, physicist E. Fermi observed that our galaxy measures about 100,000 light years across, and that a spacefaring race could cross it in only 100 million years, even if their starships poked along at only 1/1,000 the speed of light. Since our galaxy is about 10 billion years old, the very reasonable question is: If other intelligences (ETs) exist in our galaxy, why haven't they found us by now? Actually, many ETs from many different cultures should be stopping by frequently.
J. Annis, an astrophysicist at Fermilab, believes he can explain the apparent dearth of ETs. The problem is gamma-ray bursts (GRBs). They are so powerful that they sterilize those galaxies in which they occur. Presently, GRBs occur in each galaxy about once every 100 million years, but theory suggests that they were much more frequent in the past.
As a consequence, by the time intelligent life evolves anywhere and figures out how to build spaceships, they are zapped by a GRB. Perhaps some do begin exploration of their galaxy, but they don't get very far.
(Matthews, Robert; "Sorry, We'll Be Late," New Scientist, p. 16, January 23, 1999.)
Comment. Any reader of science fiction can come up with other explanations: (1) ETs have been here but find nothing of interest and leave; (2) ETs were here and helped build Atlantis, the Great Pyramid, the Face on Mars, etc.; (3) ETs are here now but avoid human contact; and (4) ETs are here now but look so much like us that we cannot tell the difference! You are free to make up your own explanations!
Yes, we live in a favored galaxy, because life on earth has not been GRBsterilized for at least 3 billion years -- 30 times the average period between GRBs. Are we simply lucky?