This book is aimed at physics students who are left with a fundamental question after their quantum mechanics lecture: "What does the mathematical formalism that I have now studied extensively and so laboriously say about nature?". In the search for answers, the authors discuss, among other things, the modern quantum theories that John Stuart Bell referred to as "theories without observers": Bohmian mechanics, collapse theory, and many-worlds theory. In addition to targeted mathematical statements that are rarely found in course lectures, the book explains the role of the wave function and randomness in quantum mechanics based on the new theories. In particular, the authors also delve into the thought world of physicist John Stuart Bell, who is known for the famous but often misunderstood Bell's inequalities.