I bought this book looking for an easy read on Quantum Mechanics, hoping to gain a fresh perspective on the topic. The book was not disappointing in this pursuit.<br><br>I appreciate the way Susskind introduces Quantum Mechanics as an alternative system of logic, though he fails to come back to that point and really explain in a more rigorous way what he means by this. I think he was right to start with spin as the quintessential quantum system and only introduce wavefunctions later.<br><br>I found Susskind's notation involving the wavefunction irritating. He himself says how wonderful Dirac notation is, then promptly ditches it to write things in terms of wavefunctions instead. Why? Why go to all the trouble of showing the reader the better system of things, coordinate-free in Dirac notation, only to take it away? This isn't just a nitpick-- the equations are significantly more difficult to understand when they are written in particular coordinates and I frequently had to translate them to Dirac notation in my head.<br><br>An important insight that this book offered me was the point of view that a quantum system is something that lives in an abstract vector space with less basis states than we are used to. In classical physics, we imagine everything as either a scalar field or a 3-vector field (or a 4-vector field if time is included). But one of the big features about Quantum mechanics is that there are systems which have degrees of freedom which don't fit in either of these two categories. Particularly, the spin of a particle is a property with only two basis states! This is totally foreign to a classically trained mind. I'm glad this point was emphasised (maybe not as much as it should have been), as I feel it reaches the heart of what makes Quantum physics different.<br><br>Unfortunately Susskind does not properly motivate how the quantum equations come about from the classical, or vice versa. He makes an attempt, describing how the expectation values of quantum observables evolve according to the classical equations. He also draws a correspondence between Poisson brackets and Commutator relations. But he does not explain what the actual correspondence is on a mathematical level-- like every other author/professor I have heard, they point to the *visual* similarity, as if that means anything at all. I am vaguely aware that there is in fact a deep mathematical connection, though I suppose I will have to do more reading of hard textbooks to find out about that.<br><br>Oddly, Susskind omits any mention of the Heisenberg picture (where kets are static and operators evolve in time). The Heisenberg picture is advantageous in understanding the evolution of observables. I don't think Susskind ever explicitly says that if the expectations of ALL observables evolve in a particular way, then it follows as a consequence that (basically) the entire observable is evolving in the same way. Not just *average* values. The Heisenberg picture brings this fact into sharp focus, and it would be instructive to include it. In fact, the Heisenberg picture is definitely part of the "theoretical minimum".<br><br>Another strange omission is any mention of *decoherence*, which is incredibly important in understanding the nature of quantum measurement. When a measurement is made on a quantum system, that system is being decohered with respect to a particular basis; that basis is related to the way the system and the measurement apparatus entangle. Susskind dances around this issue a bit, but why not tackle it head on? He introduces all the mathematics necessary to understand it... and it's a pretty important part of Quantum Mechanics.<br><br>The section toward the end about the creation and annihilation operators was also disappointing. Like other professors I have spoken to, he was unable to actually explain what advantage it offers to switch to the annihilation and creation operator picture, or what would motivate a person to choose this way of looking at things without foresight toward Quantum Field Theory. Susskind also fails to prove that the energy levels of a quantum harmonic oscillator form a discrete ladder. He shows that it *contains* a discrete ladder of energy levels but he does not show that the ladder obtained contains *all* energy levels. Again, another classic mistake that professors make. It makes me wonder whether he is regurgitating content he was taught long ago, that he does not really understand himself.<br><br>Overall it was an OK book, as someone who basically knew the content already but wanted a refresher and a new perspective. I am still waiting for a book that actually answers some of the questions I posed above, especially regarding (1) why the creation and annihilation operators are introduced, (2) what the mathematical relationship is between Poisson brackets and commutators, and (3) how all of Quantum Mechanics was actually figured out in the first place.