Whether the makers of 'The Day the Earth Stood Still' intended it or not, there is a striking resemblance between Klaatu (Michael Rennie) and the head of the Manhattan Project, Robert Oppenheimer. Oppenheimer was known to make corrections on the blackboards of theoreticians at the Project, similar to the scene where Klaatu corrects the work of Professor Barnhardt (Sam Jaffe). Oppenheimer's other-worldly brilliance and association with destructive power that could threaten the existence of the world seem like more than a coincidence.
Of course it should be noted that Professor Barnhardt's character was inspired by famed physicist Albert Einstein who's world famous equation "e = mc2" unlocked mysteries of the Universe theretofore unknown.
It should also be noted that the first actor to whom the role of Klaatu was actually offered was Claude Rains, who wanted to accept it but had to decline because of a prior commitment on Broadway. 20th Century Fox executives then wanted to cast Spencer Tracy but director Robert Wise protested against it suggesting that if the space ship opens up and the well-known Tracy walks out the film would be ruined. Darryl F. Zanuck was the one who first suggested Michael Rennie for the part of Klaatu, having seen him perform on the London stage. Wise liked the idea because he was generally unknown to American audiences, and would be more readily accepted as an "alien" than a more than either Rains or Tracy.