# bookrev: Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos by Seth Lloyd

I’ve been reading this book for a while. Non-fiction books (except history books) always take me longer, as I like to check the facts, absorb the ideas…yeah, I know, it reminds some of you of schoolwork.

Dr. Lloyd’s book is full of ideas worth absorbing, the main one being that the universe is a continually running quantum computer. His book is an excellent mix of computer science, quantum mechanics and information theory, three subjects that can get quite difficult to explain separately, let alone combined. Dr. Lloyd does an excellent job of laying out the groundwork of past and current science, then using that foundation to theorize his ideas. It is a short (211 pgs, PB) little book that is dense with concepts and ideas.

The book starts with a background on basic computer science including bits and gate technology. It then moves into the laws of physics, building a picture of the computational universe that includes order out of chaos, using a great description of the “monkey’s hitting typewriters will eventually write Shakespeare” concept:

For example, let’s say we sit a monkey down at a PC, and tell the computer that the typescript is a program in a computer language, such as Java. The computer then interprets the monkey’s random output not as a text, but as a computer program – that is, as a sequence of instructions in a particular computer language. … some short computer programs – and, thus, programs with a relatively high probability of being randomly generated – actually have interesting outputs. (pg 60).

The book then continues building the foundational information leading to Dr. Lloyd’s hypothesis, discussing classical physics problems and quantum mechanics in very lucid terms, also relating them to information. Included is one of the most readable explanations of coherence/decoherence and the infamous double-slit problem:

There is a simple criterion for deciding whether a set of histories is cohernet or decoherent. Think of what happens when you make a measurement. Measurement destroys coherence. But it can’t destroy coherence if there is no coherence there to destroy. If making a sequence of measurements on a quantum system changes its future behavior, then the histories corresponding to the possible sequences of outcomes of the measurements are coherent. If the sequence of measurements has no effect on the system’s future behavior, the histories are decoherent. In the double-slit experiment, measurement destroys the interference pattern and changes the behavior of the system: The histories of the double-slit experiment are thus coherent. (pg 125)

The book then describes the theoretical and experimental steps that have been taken to build a quantum computer, and Dr. Lloyd builds his thesis of the universe as an actual functioning quantum computer by drawing parallels…to simulate the universe, how big would a computer have to be? A vastly enjoyable calculation: 10 to the 122 power operations could have been performed, and the universe has 10 to the 92 power bit of information to use as memory; by contrast, all of the computers ever constructed could do 10 to the 28th power operations.

All of this foundational material leads up to the concluding theories:

- “quantum computation could give rise to a unified theory of gravity and elementary particles”;
- a definition of physical complexity called “thermodynamic depth”, used to explain how much energy and entropy went into a physical thing as a way to measure it’s complexity;
- the big bang and the subsequent evolution of the universe, including the evolution of life, described in terms of informational computations.

These concepts would be difficult to grasp if not for the excellent foundation Dr. Lloyd lays at the beginning. Quantum mechanics, quantum computing, information and complexity theory are all mind-numbing topics. But this book does an excellent job of laying out the required science and theories in a non-mathematical way, and then using this information throughout the book to push to the end concepts.