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:
Time travel
The absence of time travelers from the future is a variation of the Fermi paradox. As the absence of extraterrestrial visitors does not prove they do not exist, so the absence of time travelers fails to prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used. Carl Sagan once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers. Some versions of general relativity suggest that time travel might only be possible in a region of spacetime that is warped a certain way, and hence time travelers would not be able to travel back to earlier regions in spacetime, before this region existed. Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future."