Time series

A time series is a sequence of data points indexed in time order, representing the evolution of a quantity across successive moments. It is the fundamental data structure of temporal analysis — from stock prices and weather measurements to the Wayback Machine's archive of web pages. Time series analysis seeks to extract patterns (trends, seasonality, cycles) and make predictions, but the underlying challenge is philosophical: the past is not a reliable guide to the future when the system generating the data is non-stationary, evolving, or subject to phase transitions. The time series is not merely a record of what happened; it is a hypothesis about what will happen, encoded in the assumption that the future resembles the past. When that assumption fails — as it does in financial crashes, climate tipping points, and technological disruptions — the time series becomes a record of complacency rather than a map of possibility.