Louis Bachelier

Paris, 1900. Long before computers, Louis Bachelier described prices on the Paris Bourse as a random walk in his thesis, Théorie de la spéculation.

Uncertainty grows with the square root of time, the rule quants still use to scale volatility. Bachelier also wrote down an option pricing formula, 73 years before Black and Scholes, and the Bachelier model is still used when prices or rates can go negative.