
Richard Feynman stood at the same Cornell blackboard in 1964 and explained the word every AI lab minimizes in 2026: entropy. The BBC filmed it. Almost nobody watches it.
Lecture 5. "The Distinction of Past and Future."
The question sounds childish: why does time run one way. The answer is the exact quantity inside the cross-entropy loss every training run on Earth minimizes tonight.
He does it with a glass of water and blue ink. Mixing is easy. Unmixing never happens. Order falls into disorder, and that 1-way fall is what "past" means.
Watch the middle section. He explains why you remember yesterday and not tomorrow with counting arguments a 12-year-old can follow.
No equations for the first 20 minutes. 1 piece of chalk.
Shannon borrowed the word from physics in 1948 for the paper every LLM stands on. Feynman shows what it meant before it became a number on a dashboard.
You minimize entropy for a living. He explains it for free.
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