He solved Einstein's hardest equation from the trenches of World War One
Weeks after Einstein published general relativity in 1915, physicist Karl Schwarzschild — serving in the German army on the Russian front — found the first exact solution to its field equations, describing the spacetime around a single mass. His solution implied an object so dense that light couldn't escape it, decades before anyone coined the term black hole. He died of an illness contracted at the front just months later.