Simplify the following text for a Grade 8 native English-speaking person.
Original: John Bardeen ( May 23 , 1908 â `` January 30 , 1991 ) was an American physicist and electrical engineer , the only person to have won the Nobel Prize in Physics twice : first in 1956 with William Shockley and Walter Brattain for the invention of the transistor ; and again in 1972 with Leon Neil Cooper and John Robert Schrieffer for a fundamental theory of conventional superconductivity known as the BCS theory .
Simplified: John Bardeen ( 23 May 1908 â `` 30 January 1991 ) was an American physicist and electrical engineer . He is the only person to win two Nobel Prizes in Physics .