THE MATHEMATICAL UNIVERSE AND NATURE OF EXISTENCE

One morning in 1985, a schoolboy in Stockholm rode his bicycle into the path of a truck and was killed. He is fine. He grew up to become a professor at the Massachusetts Institute of Technology, and he opens his own most famous book with the accident that killed him in some other universe but not in this one. Max Tegmark has had two careers, and only one of them is speculative. As a cosmologist he built genuine instruments of measurement: an information-theoretic toolkit for reading the oldest light in the universe, the proposal to use baryon acoustic oscillations as a standard ruler across the sky, a cleaned map of the microwave background, and the three-dimensional shape of the galaxy distribution drawn from the Sloan Digital Sky Survey. Then the tenure letter arrived, and he took his other ideas out of the closet where he had prudently kept them. Those ideas are the largest claim anyone in modern physics has made in plain English. Reality is not described by mathematics, Tegmark says. Reality is mathematics. Our universe is a mathematical structure, we are self-aware substructures inside it, and every other consistent mathematical structure exists in exactly the same sense that this one does. There are four levels of multiverse stacked above your kitchen table, and the top one contains everything that does not contradict itself. This book takes that claim seriously enough to argue with it. It follows Tegmark from a Stockholm suburb, where he wrote and sold a word processor as a teenager and campaigned against nuclear weapons, through Berkeley and the surname he adopted there, through the years of measuring a universe by day and multiplying universes by night, to the moment in his fifties when he abandoned cosmology for machine learning and became one of the most audible voices warning that we are building something we cannot inspect. It also puts the mathematical universe on trial. The prosecution is given its full strength, and so is the defense. And the case against is made from a specific position: that structure, counting, order and observer are never free, that any description is already a projection, and that the word exists is a role played by many things and referring, on inspection, to none of them. Where that position is the author's own published work, it is named, cited and argued rather than smuggled in. What emerges is neither a demolition nor a monument. It is a portrait of a cheerful, disciplined, occasionally reckless man who did excellent work on questions that can be settled, then spent thirty years on a question that may not be - and a serious attempt to say what remains of his answer once the bill for it has been itemized. >Keywords: Max Tegmark, multiverse, cosmology, philosophy of physics, mathematics, consciousness, artificial intelligence.