If you're into quantum physics, prepare to have your mind blown. Well, whether or not you're into quantum physics, prepare to have your mind blown.
At least a little bit. Because below I've shared an excerpt from physicist Carlo Rovelli's amazing new book, "Helgoland: Making Sense of the Quantum Revolution."
The excerpt doesn't do justice to the entire book, but it will give you a feel for Rovelli's relational take on quantum physics -- which makes so much sense, it's hard to understand how anyone could disagree with it (though I'm sure many do).
I'll have more to say about the book in additional blog posts. I wanted to share this lengthy excerpt to provide a foundation for those other posts, one of which will be about how Rovelli finds echoes of the Buddhist thinker Nagarjuna in quantum physics.
I've read many books about quantum physics. Rovelli presents a way of looking at quantum phenomena that's so compellingly clear, those other books seem confusing by comparison. Every sentence of what follows has a lot of meaning, so read it slowly.
In a physics laboratory, where we study a small object such as an atom or a photon of Zeilinger's lasers, it is clear who the observer is: it is the scientist who prepares, observes, and measures the quantum object, who deploys their instruments of measurement, detecting the light emitted from the atom or the place where the photons arrive.
But the vast world is not made up of scientists in laboratories, or instruments of measurement. What is an observation, when there is no scientist observing? What does quantum theory tell us, where there is no one measuring? What does quantum theory tell us about what happens in another galaxy?
The key to the answer, I believe, and the keystone of the ideas in this book, is the simple observation that scientists, and their measuring instruments as well, are all part of nature. What quantum theory describes, then, is the way in which one part of nature manifests itself to any other single part of nature.
At the heart of the "relational" interpretation of quantum theory is the idea that the theory does not describe the way in which quantum objects manifest themselves to us (or to special entities that do something special denoted "observing"). It describes how every physical object manifests itself to any other physical object. How any physical entity acts on any other physical entity.
We think of the world in terms of objects, things, entities (in physics, we call them "physical systems"): a photon, a cat, a stone, a tree, a boy, a village, a rainbow, a planet, a cluster of galaxies... These do not exist in splendid isolation. On the contrary, they do nothing but continuously act upon each other. To understand nature, we must focus on these interactions rather than on isolated objects.
A cat listens to the ticking of a clock; a boy throws a stone; the stone moves the air through which it flies, hits another stone and moves that, presses into the ground where it lands; a tree absorbs energy from the sun's rays, produces the oxygen that the villagers breathe while watching the stars, and the stars run through the galaxies, pulled by the gravity of other stars... The world that we observe is continually interacting. It is a dense web of interactions.
Individual objects are the way in which they interact. If there was an object that had no interactions, no effect upon anything, emitted no light, attracted nothing and repelled nothing, was not touched and had no smell... it would be as good as nonexistent. To speak of objects that never interact is to speak of something -- even if it existed -- that could not concern us. It is not even clear what it would mean for us to say that such objects "exist."
The world that we know, that relates to us, that interests us, what we call "reality," is the vast web of interacting entities, of which we are a part, that manifest themselves by interacting with each other. It is with this web that we are dealing.
...The discovery of quantum theory, I believe, is the discovery that the properties of any entity are nothing other than the way in which that entity influences others. It exists only through its interactions. Quantum theory is the theory of how things influence each other. And this is the best description of nature that we have.
It is a simple idea, but it has radical consequences that open the conceptual space required to understand quanta.
...The properties of an object are the way in which it acts upon other objects; reality is this web of interactions. Instead of seeing the physical world as a collection of objects with definite properties, quantum theory invites us to see the physical world as a net of relations. Objects are its nodes.
The first radical consequence is that to attribute properties to something when it does not interact is superfluous and may be misleading. It is talking about something that has no meaning, for there are no properties outside of interactions.
This is the significance of Heisenberg's original intuition: to ask what the orbit of an election is when it is not interacting with anything is an empty question. The electron does not follow an orbit because its physical properties are only those that determine how it affects something else, for instance, the light that it emits when it is interacting. If the electron is not interacting, there are no properties.
This is a radical leap. It is equivalent to saying that everything consists solely of the way in which it affects something else. When the electron does not interact with anything, it has no physical properties. It has no position; it has no velocity.
The second consequence is even more radical
Suppose that you are the cat in Schrodinger's thought experiment. You are shut in a box and a quantum mechanism has a one in two probability of releasing the sleeping drug. You perceive whether the drug has been released or not released. In the first case, you sleep; in the second, you remain awake. For you the drug was delivered or it was not delivered. There are no doubts. As far as you are concerned, you are asleep or you are awake. You are certainly not both at once.
I, on the other hand, am outside the box and do not interact either with the bottle of sleeping draught or with you. Later on, I can observe interference phenomena between you-awake and you-asleep: phenomena that would not have been produced if I had seen you asleep, or if I had seen you awake. In this sense, for me you are neither asleep nor awake. This is what it means to say that you are "in a superposition of sleeping and waking."
For you, the soporific is released or not, and you are asleep or awake. For me, you are neither awake nor asleep. For me, "there is a quantum superposition." For you, there is the reality of being awake, or of not being so. The relational perspective allows both things to be true: each relates to interactions with respect to distinct observers -- you and me.
Is it possible that a fact might be real with respect to you and not real with respect to me?
Quantum theory, I believe, is the discovery that the answer is yes. Facts that are real with respect to an object are not necessarily so with respect to another. A property may be real with respect to a stone, and not real with respect to another stone.