QUANTIZING THE WRONG THING: GRAVITY AND MYSTERY OF TEMPORALITY

Somewhere in the nineteen-seventies, a student in northern Italy watched a clock stop. The clock was fine. His mind was not, for that afternoon at least, and it went on producing events while the second hand declined to produce any. Most people file such an evening under youth. Carlo Rovelli filed it under physics and spent the next fifty years asking what it means to say that one of those two clocks was the real one. He is the best possible opponent, because he is right about the hardest part and says it more beautifully than anyone alive. There is no universal present. There is no clock outside the world for the world to be late to. Millions of readers have followed him there, and they were right to go. Then he did something this book argues was a mistake. Having shown that the gravitational field is not a stage but an object, he set out to quantize it, and loop quantum gravity is what came of that: grains of area, networks, foams, five decades of some of the most demanding mathematics in physics. Superb work, aimed at an object that was never there. Look at gravity next to the other three forces and it fails every entrance requirement. It has no charge, and nothing is neutral to it. You cannot build a room without it. You can make it vanish locally by falling. Its constant carries a length where the others carry pure numbers, and it is the only interaction whose equations can be derived from thermodynamics, which is what happens to collective phenomena and never to fundamental ones. Then there is the awkwardness at the center of the enterprise: quantization is an operation that needs a time to run in, and the thing being quantized is what supplies time. What follows is an account in which gravity is what a vacuum does when matter pins down where its quanta are, in which the constant of gravitation is a stiffness rather than a law, and in which time is not a succession at all but a set: the collection of states in which a thing can go on being itself. Two claims, one distinguished opponent, and an experiment now running in several laboratories that could settle the matter against the author within the decade. The price of being wrong is named in the preface, so that nobody can accuse him later of having moved it. >Keywords: quantum gravity, emergent gravity, the nature of time, Carlo Rovelli, spacetime, philosophy of physics, cosmology