Comment on Quanta article: How Can AI Researchers Save Energy? By Going Backward.
Matt von Hippel
You can read a beautifully simple typically Feynman description of reversible computing and the history of it's discovery in the book The Feynman Lectures on Computation (1996). I quoted the Feynman's work and reproduced his diagram of a reversible computer on page 173 of his book, on book pages 138-141 of my own book The Cosmic Computer - The Physics of the Perennial Philosophy (2022) which is available as an Amazon paperback or a free pdf download from my website: https://www.thecosmiccomputer.org. There is also a lovely story that I quote from Tony Hey in his introduction to the later volume Feynman and Computation, about how Feynman's friend and intellectual sparring partner Ed Fredkin (the first MIT professor to be appointed without having even ever taking a bachelor's degree), managed to sandbag Feynman and make him admit there was a mistake in The Feynman Lectures – to this day I think this is the only known mistake in Feynman's physics. Fredkin also invented the fully reversible Fredkin gate to describe the action of his toy model Billiard Ball Computer. To my mind the Fredkin gate is the most beautiful logic gate of all, so simple and so powerful. It only depends on a control line bit controlling whether two other lines exchange values or not, the control line remains unchanged. It is fully reversible, and complete in that with a collection of gates of this one type, you can build any universal computer. Both the Fredkin gate and the reversible controlled-controlled-not gate led on to quantum computing, and explained how atoms could compute without emitting energy. The quantum CCN gate is now the mainstay of quantum computing. Fredkin always believed that the Universe is a giant cosmic computer. Feynman loved Fredkin, because he saw physics in a completely different way to normal physicists, and he enjoyed their passionate arguments, on one occasion Feynman even jumping up on his chair and publicly shouting at Fredkin in Caltech's 'greasy' cafeteria.