Roger Penrose, Emeritus Professor of Mathematics at Oxford and a Fellow of the Royal Society, was knighted in 1994 and has received numerous prizes and awards for his work as a physicist and his contribution to our understanding of the universe. (For his books I've read, click on his name.)

This book is a sequel to his controversial ENM (The Emperor’s New Mind). I chose this book because its focus is on consciousness and AI (Artificial Intelligence) – subjects that book 120 got me thinking more about. A New York Times bestseller when it appeared in 1989, ENM was hailed as “a marvelous survey of modern physics as well as a brilliant reflection on the human mind, offering a visionary glimpse on the possible future of science.” Shadows of the Mind mounts an even more powerful attack on AI. Roger Penrose learned from, and here responds tellingly to, attacks by AI opponents of ENM. He provides powerful arguments to support his conclusion that “there is something in the conscious activity of the human brain that transcends computation and will find no explanation in terms of present-day science,” but near the end he offers some unusual ideas.

He begins by discussing four options regarding computation and conscious thinking. At the outset he states the choice he defends in the book: “Appropriate physical action of the brain evokes awareness, but this physical action cannot even be properly simulated computationally.” In Part I, “Why We Need New Physics to Understand the Mind,” he uses logic and mathematics to examine all options, and Gödel’s Incompleteness Theorem and Turing machines to prove his choice. (Readers who prefer to avoid logic and mathematical arguments are told which sections can be skipped, but I found these parts to be superbly done.) In Part II, “What New Physics We Need to Understand the Mind,” classical physics (including chaos and the Einstein tilt of light cones) is short, but not so quantum physics, where his treatment is very mathematical. I found it awesome how well he explains the “quantum difficulties” of two types, Z and X. Z-types can be understood, like non-locality such as the EPR (Einstein-Podolsky-Rosen) Z-type effect, thus aren’t paradoxes. (Penrose’s treatment never even refers to “wave-particle duality” but talks of photons or quantum state vectors.) X-types are “philosophically unacceptable and arise merely because the quantum theory is an incomplete theory.” In brief, he means that, although the Schrödinger Equation is a completely deterministic equation, it requires complex numbers. But we can only measure real numbers, not complex ones. Thus the incompleteness of quantum theory lies fully in the reduction of the complex state vector into something real and reduction introduces the indeterminacy of quantum measurement. I see an analogy: we are Flatlanders (see book 49)in a 2D (2-dimensional) space, but reality is 3D space. Likewise, quantum reality has a dimension (via complex numbers) that we can’t measure, so quantum reduction is “like” spatial projection from 3D into 2D. But working backwards from 2D won’t give you all of 3D – think of some of Escher’s 2D pictures! All analogies fall short, but this one may help a bit. I found Penrose’s mathematically elegant chapters on quantum physics to be exciting and awesomely well done. (But this book may be too much for a non-physicist.)

Roger Penrose's thoughts about consciousness involve the interface between the quantum world and the classical world of measurement. He speculates that the connection between consciousness and brain might involve large coherent quantum states, perhaps within the microtubules of the cytoskeleton. I’m lost at this microbiology level, so I can’t review this part, except to say that Penrose feels that this is the most likely place that he can think of for the interface to exist. It must exist somewhere, and he will continue to try to find out where. It is a challenging and lonely effort (lonely because biologists think classically, avoiding quantum ideas). There is yet no answer to how awareness and consciousness are physically related to the brain. In spite of his non-answer from physics, this book is very challenging but with profound depth. I loved it!