This is a trade book by theoretical physicist (and host of the Mindscape podcast) Sean Carroll that covers a couple of basic topics:

• A conceptual explanation of what exactly Quantum Mechanics is
• An extended argument for the correctness of Many Worlds (or Everrettian) foundations of Quantum Mechanics
• A speculative argument that spacetime (and hence gravity) are emergent properties of Quantum Mechanics and not strictly fundamental phenomena

On the first point, the book is excellent. Carroll is a really gifted and lucid writer (one of his previous books, The Big Picture, is perhaps my favorite book of all time and a wonderful exposition on the scientific worldview) and it shows in his ability to distill complicated theories down to their conceptual core and convey that to a lay audience. On the second there are actually two layers to the argument. The second layer is making an argument for Many Worlds itself. Very briefly (if you are hungering for a detailed explanation, I suggest you go read the book itself), Many Worlds (or Everettian, named after almost-physicist Hugh Everett) Quantum Mechanics is the idea the world is irreducibly quantum. That is, there is only the wave function. In the traditional view of QM (known as the Copenhagen Interpretation), when an observer makes an observation of a quantum system such as an electron in a superposition of spin up and spin down states, the wave function “collapses” and they will observe either a definitely spin-up or a definitely spin-down electron and that the probability of observing one or the other is derived from the wave function of that system. The Many Worlds view is to point out the observer is also a quantum system and that what really happens (very roughly speaking) is that the universe “branches” into two branches: one in which the observer sees a spin-up electron and one in which the observer sees a spin-down electron. In other words, both outcomes happen, just in different “worlds”.

Before we can even get to the point of arguing for Many Worlds though, Carroll has to argue that this is even a worthwhile question to ask in the first place. My sense is that modern physics is not in fact split into the “Copenhagen Interpretation Camp” and the “Many Worlds Camp” but instead split into the “We Don’t Care, Shut Up and Calculate” camp and the “Foundations of QM Are Important” camp. This is even apparent in the language we use to talk about this stuff. The shut-up-and-calculators often refer to the the Many Worlds or Copenhagen _interpretation _wheras Carroll and others who care about these things tend to talk about the _foundations _of QM. The implication being that an interpretation is not really a scientific question to begin with. Whether formalizing the foundations of QM is useful or not I can’t really say, but I think Carroll makes a persuasive point that Many Worlds is in a sense an inevitable conclusion of taking QM seriously and that in order to avoid it we would need to add additional assumptions (as in Dynamical Collapse theories or Bohmian Mechanics) which are at least in principle testable but currently have no experimental data behind them. The speculative parts at on spacetime and gravity as emergent phenomena was very interesting but I am probably not able to do it justice so won’t even try. Again, go read the book! It’s worth it.