original_query: What is the -body problem in physics?
query: What is the n-body problem in physics?
answer: In physics, the n-body problem is predicting the individual motions of a group of celestial objects interacting gravitationally.  It's motivated by understanding the motions of the Sun, Moon, planets, and stars, and  became important in the 20th century for understanding globular cluster star systems.  General relativity makes the problem considerably more difficult.