Enrico Fermi
Fermi wrote a paper "On the Origin of Cosmic Radiation" in which he proposed that cosmic rays arose through material being accelerated by magnetic fields in interstellar space, which led to a difference of opinion with Teller. Fermi examined the issues surrounding magnetic fields in the arms of a spiral galaxy. He mused about what is now referred to as the "Fermi paradox": the contradiction between the presumed probability of the existence of extraterrestrial life and the fact that contact has not been made.
Toward the end of his life, Fermi questioned his faith in society at large to make wise choices about nuclear technology. He said:
Postbiological evolution
If we consider this possibility of a PBC in our galaxy we are still faced with Fermi's paradox. However many of the proposed solutions for Fermi's paradox also hold true for a PBC. In terms of the search for extraterrestrial life and astrobiology because of the almost infinite possible forms a PBC could take and our lack of understanding of these we would effectively be blind in this search. For this reason even though there is a logical argument for the existence of PBCs our best hopes remain with looking for biological life.