This is not a review, or a summary, I’m just trying to get my thoughts in order as I read through The Fabric of Reality by David Deutsch. If you are not interested in quantum physics, evolution, computer science, and epistemology itself, perhaps this is not the book (or post) for you. What I have understood thus far is not the type of understanding which Deutsch himself advocates for. The intrinsic type that doesn’t wholly depend on concepts, proofs or theories. I’ll have to read through my highlights again later on, and then again, and again.
Quantum mechanics is widely regarded as the most successful physical theory to date. We use it daily, in lasers, CDs, DVDs, solar cells, fibre-optics, etc. Yet, despite its considerable experimental success, Deutsch argues that our current interpretation of quantum mechanics (the Copenhagen Interpretation), has taken much of its explanatory power.
The Copenhagen Interpretation was the first general attempt to understand the world of atoms outside the classical realm of physics. Those who stand by this theory believe there is such a thing called the “wave function” which tells us the probability of finding a particle in a specific place in space and time. Interwoven in the information granted by the wave function are the details delineating all the possible states that that particular particle could be in. As a bell-curve which could be viewed as a probability distribution. Only when the particle is measured does it “zap” into a definite state. Before that moment of identification it is in a so-called “superposition state”. In short, only after its measurement has been taken does the particle become visible. Physicists call this the “collapse” of the wave function.
Everyone who knows anything about reality would agree that we are made of fundamental, foundational building blocks which act in the microscopic realm. A realm which we cannot see with our bare eyes, let alone with the most precise microscopes. What does this collapse of the wave function mean about our physical, everyday reality? After all, according to this interpretation, a particle doesn’t exist until it is observed / looked at. If you believe, like most do (I assume) that reality continues on even when we do not perceive it, you can see (pun intended) the problem here.
The most famous example of wave-function collapse is that of Schrödinger’s cat (a thought experiment). Envision a cat inside a box, subjected to a quantum contraption with a 50-50 probability of killing it. The only two results can be that the cat dies because of, for example, poison gas, or that there is no gas leak and the cat survives. Before we look inside the box, is the cat alive or dead? The Copenhagen Interpretation would argue that the cat is in a superposition of both alive and dead. It is only when you look inside the box that the wave function collapses and the cat assumes a definite state. But what if the wave function never collapses, as Deutsch postulates? In this case, the measurement is not limited to a single dead or alive outcome, but both outcomes become a possibility. Instead of the cat being in a superposition, in a way the observer comes to find itself in a superposition of states, “split” into two different universes wherein the cat has died, and one where it escaped alive. This interpretation was first put forth by Hugh Everett about 60 years ago and is known as the Many Worlds Interpretation. Everett’s theory postulates that we do not live in a single universe, but somewhere in a set of many universes, known as the Multiverse.
Proof of the Multiverse can be found in the classroom with the double-slit experiment. When you shine a beam of photon light at a barrier with two slits, a pattern of bright and dark bands appear on the screen behind. A phenomena which can be described as light waves. Like rippling water. This looks rather beautiful on its own, yet the astonishing thing is seen when you send only one photon through the slit. The same thing occurs! A single photon, then, does not act like a particle but as a wave when unobserved.
Whereas with other quantum mechanical experiment, one is presented with the right predictions about physical reality regardless of their opinion, the double-slit experiment has to rely on opinion rather than results alone. The Copenhagen Interpretation diminishes the findings by saying it is nothing more than the photon producing these patterns by interfering with itself. Deutsch however argues that, while you can use the Copenhagen Interpretation to predict what the interference pattern will look like with multiple photons, only the Many Worlds Interpretation can explain how the interference pattern can persist when you send in a single photon.
The Many Worlds Interpretation of quantum mechanics is not the accept interpretation amongst the scientific community today. The dominant paradigm is still entrenched in the single-universe view.
However, with the onslaught of quantum computing, I doubt it will stay that way for long. Quantum computing is a whole other topic on its own. I’ll have to write a follow-up article on that soon, when I fully understand it myself.
The reason the single-universe interpretation is so widely regarded as the most likely is presumably because it produces the right answers regardless of whether you understand what is actually happening or not. It is Deutsch’s belief, however, that such predictive power without understanding the underlying works will lead to a stagnation in further understanding. Almost like believing in magic.