8/10 A book about quantum mechanics that’s easy to read - that’s certainly something! A very well written piece that both explains quantum mechanics to anyone without assuming any prior knowledge and explores the philosophy behind how quantum mechanics changes our perception of the fabric of reality. I honestly enjoyed both “sides” of the book, although I didn’t expect it to turn so philosophical in the second half. That said, you will finish the book with learning about how quantum theory was developed, what it entails at the atomic level, and how it shifts our understanding of the world from an object-oriented perspective to a relational perspective. Everything that’s “real” is only “real” by a relation to something else. A chair is a chair because it exists within a context, and we interact with it. By itself, it’s nothing. Strange, huh? I didn’t say the book makes it easier to understand or accept the consequences of quantum theory. It’s just nice, because it plainly explains what the consequences are. For anyone curious about what quantum theory is, very recommended. And for anyone who’s familiar with quantum theory but would like to get a philosophical overview of different outlooks on what it means for the fabric of reality, also very recommended. Enjoy!
A 23-year-old Werner Heisenberg travels to Helgoland, a remote island in the North Sea, where, after days of thinking and calculations, he lays down the groundwork of quantum theory. The year is 1925. Since then, the theory has undergone major updates and revisions, which eventually led to its firm placement not only in physics but also in biology, engineering, philosophy, and many more fields. The book explains first the developments since 1925 and then the philosophical implications of quantum inference and entanglement for the way we understand and interpret reality.
What is quantum interference?
In the book, quantum superposition or interference is explained in a setup where a beam of light is divided into two paths - left and right. The beam is sent out, divided into left and right, then brought back together and again divided into up and down. A measurement device measures the number of photons that reach the up and the down locations.
If the beam of light freely passes through both left and right paths, all the photons arrive to the down location. However, if either of the left or the right path is blocked by e.g. your hand, the photons are equally divided between the up and down locations. Huh? What’s going on? Why does blocking of one path changes the distribution of the photons in the up and down locations? Before we discovered quantum theory, we would expect the final distribution of photons to stay the same regardless of whether they travel through both left and right paths, or just through one of them. But after the discovery of quantum theory, we understand that this is not the case. What’s even weirder is that if you add another measuring device and look at how many photons travel left and right, you will see that in this set up, the photons again equally divide themselves between the up and down locations. What sort of sorcery is this? It looks as if the mere measurement/observation changes the final location of the photons.
The explanation that the book offers stands on the premise that everything we see, every object and its actions, are relational. They occur in relation to something and we, or the measurement device, or any other (3rd) interacting object “sees” only the relational consequences. Speed is measured in relation to something, just as position is. Objects are characterized only by the way they interact with everything around them. An object that doesn’t interact, doesn’t exist. Moreover, as quantum theory states, a photon can be thought of as either a particle or a wave. The waves are often interpreted as the probability of where the particle will appear. Or rather, the place where it will interact next - with respect to us, or with respect to some other object. In fact, every object has several waves, one for every object that it is in relation with. So in our experiment, every photon interacts with the measuring device and thus has a wave defining its positional probability with respect to the device. But it also interacts with other objects in the system, and thus it has other relational waves that define its positional probability with respect to these objects. Moreover, every relation has a number of variables associated with it. In our case, we could say that variable A is the path - left or right, and variable B is the location - up or down. The key realization of quantum theory is the non-commutative nature of these variables. That’s just a math-speak for the fact that whether we measure A first or B first matters. Measuring A, or interacting with the particle at “point A”, changes its wave function with respect to us, and thus it defines what the variable B will be - or won’t be. In fact, quantum theory states that we can never precisely know all the variables of a particle (quantum system) at the same time. Measuring A -> changes B, measuring B -> changes A. Or even better said, once we measure A, we remain uncertain about B and vice versa. Therefore, the quantum interference occurs in our system because we always measure or interfere with one of the variables. When we don’t know which path the photon took - left or right, variable A remains uncertain, and thus variable B must become certain - all the photons end up down. Similarly, if we block one of the paths, A becomes certain, and B must become uncertain - photons end up equally in both up and down locations. That’s it. That’s quantum superposition. That’s quantum interference. Intuitive? No. Does it make sense? Maybe for a brief second as you read this, and then it doesn’t. Don’t worry, I tried to explain my understanding here the best I could. Yet I still don’t think I understand it fully. That’s just how weird quantum theory is. Good luck.