Relatively few books attempt to truly grapple with the foundations of quantum mechanics. . Most of them have so many interspersed logical gaps with unexplained jumps that they leave one frustrated. Norsen is a genius at explaining this material. I find myself thankful at his presentation with every page I turn. But note: this is not a book for beginners. It is for people who know some quantum mechanics, including some math, but who have not found a good discussion of the foundations. Oddly, with quantum mechanics, "foundations" does not really mean "introductory."

I use Travis Norsen’s book as one of the main textbooks for my philosophy of quantum mechanics class at Caltech—the other book is Tim Maudlin’s Philosophy of Physics: Quantum Theory. I particularly like Norsen’s book, because it introduces all the necessary math without being too technical. In this sense, it is closer to a standard physics textbook, but it doesn’t indulge in mere calculations merely for the sake of calculations. The math is always used to make conceptual problems and arguments more precise and clearer.The first half of the book discusses the three main problems with quantum mechanics: the measurement problem, the locality problem, and the ontology problem. Having read these chapters, the reader, I hope, should be convinced that these are severe problems of quantum mechanics that require a solution. The second half of the book introduces, four proposals for a solution in the form of four different interpretations, or rather theories, of quantum mechanics: the Copenhagen interpretation, the de Broglie-Bohm pilot-wave theory, the GRW spontaneous collapse theory, and Everett’s many-worlds theory.One of the main strengths of the book are the exercises at the end of every chapter. They all are clearly related to the topic of the chapter (which is sometimes not the case in other physics textbooks); they focus on important, mainly conceptual, issues (which is often not the case in other physics textbooks, either); and they can be solved in reasonable time. Therefore, these exercises were a central part of my class.There is much more to be said about this wonderful textbook, as it contains a myriad of physical and philosophical insights into the mysteries of quantum mechanics. I wished all physics textbooks were written with this level of clarity and pedagogical value.Dr. Mario HubertPostdoctoral InstructorCaltech

Foundations of Quantum Mechanics is a different book than one usually finds from the Undergraduate Lecture Notes in Physics. The book is effectively a second course in quantum mechanics for philosophers and so it does ask real physics questions but at its core it asks about quantum theory and ontology. If one wants to revisit ideas that are glossed over about quantum theory when one takes a class to calculate the structure of the Hydrogen atom, this is a very good book. If one wants answers to the nature of reality, the book has strong views about certain interpretations that for many, are deeply questionable.I am somewhat surprised at the universal favor the book gets as there are issues that many physicists can take with it. The author starts with the basics of quantum theory and the history and goes through and interprets the famous correspondences between Bohr and Einstein and includes common sense overviews of the disputes. The author focuses on the major issues that early physicists had with quantum mechanics, the first being the collapse of the wave function, the second being issues associated with entanglement. The author focuses most of the book on addressing the issues and proposed solutions of both sequentially with more time spent on entanglement issues with a highly intuitive focus on Bell inequalities. The author discusses how Schrodinger's cat was used because of its absurdity and how there is no way to formally distinguish on how the microscopic world follows quantum rules but macroscopic objects do not. The author does not spend time on decoherence, so the Schrodinger cat is about how an entangled state of a microscopic state in superposition and a macroscopic object with consequences to each of the superimposed states has no logical reason to not be forced into superposition. The reader really does start to appreciate that this distinguishing between micro and macro is questionable in systems in which one can entangle the two and conceptually that can be done so the theory has problems. The author gets into the major issues with entanglement, namely non-local interaction and discusses how the bell inequality show there are no hidden variable ways to make states locally determined. This is a major issue that usually is glossed over in QM courses. In QM courses one just learns that you cant transmit information using entangled states so there is no contradiction with special relativity, but that is a questionable interpretation. If the entangled states communicate non-locally how can that happen irrespective of whether information is sent. The author brings up pilot wave theory as an explanation of how all of this could work and seemingly is a believer in it to explain the "ontology" problem. I very much liked the discussions but eventually the author argues that associated with entangled state collapse is potentially a preferred inertial state of the universe for which there is simultaneity. That is an exceptionally bold claim and it is why other extremely bright physicists, including pilot wave theory pioneer de Broglie, abandoned the idea. It is not ignorance that drive physicists to that conclusion it is an unwillingness to take on that perspective. The author spends time discussing that the Hilbert space that the wavefunction exists in is not physical and so the author spends time trying to distinguish between the physics and the mathematics but at the end of the day there are many metaphysical ideas in the broad analysis. This is probably inescapable as the nature of reality is likely truly beyond our grasp and measurement. The author also includes a chapter on many worlds interpretation and the wavefunction of the universe and doesn't spend too much time on the concept of how does a probability weight exist for a branching structure.This is a nice philosophy of quantum physics class with enough math to make it a physics class. There are great physicists who have attacked these problems in more philosophical terms, Espagnat - on Physics and Philosophy, for example. Furthermore there are more research works like Foundations and Interpretation of Quantum Mechanics and this book falls somewhere in between. I think the author should spend more time being a skeptic of accepting non-local interactions as a non-issue for pilot wave theory. The idea that an entangles pair of particles light years separated can implicitly communicate with each other cannot just be given in to easily as an assumption, or if one does then don't be surprised people won't see your point of view as obvious. But all in all this was a nice book to read and a reminder of the deep philosophical issues that have always plagued quantum mechanics with no resolution in sight.

I loved this book. It is unique in the literature that I am aware of in that it is: (i) comprehensive in covering all of the topics necessary for further study in the foundations of quantum mechanics; (ii) does not skimp on rigor in its physics (it is not a popularization); (iii) it is philosophically informed and sophisticated; and (iv) it is very well written and very readable. You get the sense of taking an upper level undergraduate course taught by an engaging professor who cares deeply about the subject and has thought the subject through thoroughly. If you read only one book on this subject this should be it. PS: Don't let the math turn you away. If you can follow it you will get all that the book has t offer; if you don't, you will still get quite a lot.

Norsen's treatment of locality is the best I have ever seen, and quite persuasive. The take-home is that the failure of local causality was present from the first in QM, although Schrodinger tried for a while to deny it. Certainly Einstein knew this from the first, because the collapse of the wave function is a non-local event. Einstein did not like this failure and the EPR paper was an attempt to show QM is incomplete (i.e., there are hidden variables). Now here I think Norsen drops the ball (or correct me, dear reader, if I am wrong), because he claims that Bell's 1964 paper is really a proof of non-local causality, not the impossibility of hidden variables. What this paper shows is that there is no locally causal hidden variable model. This does NOT follow from the Copenhagen interpretation of QM, from deBroglie-Bohm, or anything else.Norsen's untangling of locality, hidden variables, the EPR argument, and deBroglie-Bohm. can be summarized as follows (my summary, not Norsen's): EPR show that local causality implies realism; realism means hidden variables; Bell shows that QM plus hidden variables contradict local causality; Bohm shows that hidden variables are possible in QM; experiment confirms Bell. Hence local causality is false---QM is in conflict with special relativity.I do not agree with his defense of Bohm's theory. There is no evidence for it and it does not extend to relativistic quantum mechanics. Moreover, it models may particles (e.g., electrons in the ground state of an atom) as having zero momentum. This is a lot worse than saying there are several "superposed" momenta. In the end, this theory s just a balm for some people's metaphysics. Occam's razor favors the more empirically founded Copenhagen interpretation, with all of its ambiguities and clearly provisional nature.Norsen provides a spirited defense of the Many Worlds theory, but it also must be rejected because there is no evidence for it. It is not outlandish, as some would have it (the Universe increases its degree of superposition over time, but the various Hilbert basis states are in configuration space, not physical space, so there are no real "splitting" of universes into "parallel" universes as some would have it. The"relative state" formulation is elegant, but this approach cannot handle probabilities.I wish Norsen had dealt more with decoherence, which is barely mentioned in the book.