The formula written on the blackboard in this famous photo of Enrico Fermi has been at the center of a small controversy for years. Several physicists found an error in this formula. In the first line, Fermi has clearly corrected an incorrect sign (changing a plus to a minus). The second line, however, is written . The Greek letter alpha is usually used to denote what is called the "fine structure constant," and the correct expression for it is .

Is it true, as some say, that this is an error on the part of one of the greatest physicists of this century? Or, as others who knew of his famous sense of humor say, as they point to the twinkle in his eye, that he purposely left the wrong formula on the blackboard as a puzzle for posterity.