Simplify the following text for a Grade 10 native English-speaking person.
Original: Gunnar NordstrÃ m ( 12 March 1881 , Helsinki -- 24 December 1923 , Helsinki ) was a Finnish theoretical physicist best remembered for his theory of gravitation , which was an early competitor of general relativity .
Simplified: Gunnar NordstrÃ m ( 12 March 1881 , Helsinki -- 24 December 1923 , Helsinki ) was a Finnish physicist . He is best known for his theory of gravitation , which was a competitor of Albert Einstein 's general relativity .