Complex sentence: Klein theory in 1921, which used the fifth dimension to try to unify gravity with electromagnetic force, although their approaches were later found to be at least partially inaccurate, their concept provided the basis for further research over the past century.
Split into simpler sentences: Klein theory in 1921, which used the fifth dimension to try to unify gravity with electromagnetic force. Although their approaches were later found to be at least partially inaccurate, their concept provided the basis for further research over the past century.