Leonard Susskind: Quantum Mechanics, String Theory, and Black Holes

Lex Fridman introduces Leonard Susskind as a professor of theoretical physics at Stanford University and founding director of the Stanford Institute of Theoretical Physics. He describes him as widely regarded as one of the fathers of string theory and as one of the greatest physicists of our time, both as researcher and educator.

Fridman notes that his guests on the show are not only computer scientists but also philosophers, mathematicians, writers, psychologists and physicists. In his view, AI is bigger than deep learning and computing: it is our civilization's journey into understanding the human mind and creating echoes of it in the machine.

Susskind worked with and was friends with Richard Feynman. What he saw in Feynman was someone who did physics in a deeply intuitive way. Feynman would almost close his eyes and visualize the phenomena he was thinking about, outflanking the highly mathematical and sophisticated technical arguments others used. Susskind says this was natural to him as well, but seeing Feynman succeed at it validated his way of thinking rather than changing it.

Fridman asks whether Susskind uses intuition and visualization when thinking about quantum mechanics, black holes or string theory. Susskind says very much so: he tends not to think about equations or symbols but tries to visualize the phenomena themselves. Once he has an insight he considers valid, he may convert it to mathematics, but he says he is good enough rather than a great mathematician, and finds that others convert ideas to mathematics better than he does.

Asked how one visualizes something counterintuitive, Susskind answers that you rewire your brain. Quantum mechanics and most of modern physics are unintuitive, he says, because intuition means the ability to think with the classical physics we evolved with, such as throwing stones and splashing water. After time and familiarity, you develop new intuitions, to the point that he and many friends can think more easily quantum mechanically than classically.

Fridman asks whether neuron-like devices could be wired to understand quantum mechanics more naturally. Susskind is not sure. He says thinking quantum mechanically to some extent does not mean thinking like an electron. Using higher dimensions as an example, he says humans are fundamentally wired to visualize three dimensions: he cannot visualize one or two dimensions except as embedded in three. People learn mathematical ways and special ways of visualizing five, six or seven dimensions, which differ, and he doubts one can completely rewire oneself to be fully comfortable with such concepts.

Fridman wonders about creatures living in two-dimensional space imagining 3D. Susskind guesses they would do it the way we imagine four dimensions, using mathematics, equations and tricks.

Turning to Feynman's ego, Susskind says ego is both powerful and dangerous in science, and that a scientist needs arrogance and humility together. One needs the arrogance to believe one can win the battle with a very difficult nature, and the humility to recognize that one is likely to be wrong on any occasion, that everything may suddenly change, and that young people may say things one cannot understand. He says it takes a special kind of person to manage both.