Book Review: Three Roads to Quantum Gravity

"Many observers, not many worlds". There is a concept from quantum theory called superposition where things are in many possible states at the same time. At some time an "observer" makes a measurement and one of those possible states is observed (the wave collapses). This raises the question: What makes the "observer" special? Do the many possible states imply there are many possible worlds, all in parallel? Almost all theories assume there is a closed experiment and that the "observer" is outside the system. Smolin points out that this is not possible when considering the universe since, by definition, the observer is part of the universe. Chapter 3 discusses the idea that there are multiple consistent theories, each held by different observers rather than multiple worlds. This is one of the many interesting ideas in this book. It is well worth the read. The book is clearly written at the level of the "interested observer" (pun intended) of quantum theory. It does not require any mathematics but does assume one is familiar with quantum ideas.