The Feynman Lectures is one of the most popular lecture series in physics. It’s a great resource for science enthusiasts, students, and teachers, and each lecture is just fantastic in general. Now, Caltech and The Feynman Lectures website have collaborated to put these lectures online completely free of cost.

The lectures themselves were first presented at Caltech in the 1960s by the legendary physicist Richard Feynman. The lectures have since been combined into a three-volume book. To date, about 1.5 million English copies have been sold, but now that it’s free, that number will probably stagnate.