Simplify the following text for a Grade 10 native English-speaking person.
Original: The theory for which NordstrÃ m was arguably most famous in his own lifetime , his theory of gravitation , was for a long time considered as a competitor to Einstein 's theory of general relativity which was published in 1915 , after NordstrÃ m 's theory .
Simplified: He is best known for his theory of gravitation , which was a competitor of Albert Einstein 's general relativity .