Helgoland Rovelli, Erica (cat reading book .TXT) 📖

of the “impossibility of neatly separating the behavior of atomic systems from their interaction with the measuring device used to define the conditions under which the phenomenon appears.”53

When he wrote this, in the 1940s, the applications of the theory were confined to the laboratories that measured atomic systems. Almost a century later, we know that the theory is valid for every object in the universe. We need to amend “atomic systems” to “all objects,” and “interaction with measuring equipment” to “interaction with any other thing whatsoever.”

Revised in this way, Bohr’s observation captures the discovery that forms the basis of the theory: the impossibility of separating the properties of an object from the interactions in which these properties manifest themselves and the objects to which they are manifested. 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.54

This is the significance of Heisenberg’s original intuition: to ask what the orbit of an electron 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.

Facts Are Relative

The second consequence is even more radical.

Suppose that you are the cat in Schrödinger’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.55

THE RAREFIED AND SUBTLE WORLD OF QUANTA

I hope that I have not lost my reader in the last few subtle but essential paragraphs. The gist is that the properties of objects exist only in the moment of their interactions, and they can be real with respect to one object and not with respect to another.

The fact that some properties exist only with respect to something else should not overly surprise us. We already knew as much. Speed, for example, is a property that an object has relative to another object. If you walk along the deck of a ferry, you have a speed relative to the ferry, a different speed relative to the water in the river, a different one relative to the Earth, another relative to the Sun, another again relative to the galaxy—and so on, endlessly. Speed does not exist without being anchored (implicitly or explicitly) to something else. Speed is a notion regarding two objects (you and the ferry, you and the Earth, you and the Sun . . .). It is a property that exists only with respect to something else. It is a relation between two entities.

There are many similar examples: since the Earth is a sphere, “up” and “down” are not absolute notions but relative to where we find ourselves on the Earth. Einstein’s special relativity is the discovery that the notion of simultaneity is relative, and so on. The discovery of quantum theory is only slightly more radical: it is the discovery that all the properties (variables) of all objects are relational, just as in the case of speed.

Physical variables do not describe things: they describe the way in which things manifest themselves to each other. There is no sense in attributing a value to them if it is not in the course of an interaction.

The ψ wave is the probabilistic calculation of where and how an event relative to us might occur.56 The wave as well is therefore a perspectival quantity. An object does not have one ψ wave, it has one with respect to every other object with which it interacts. Events that take place in relation to one thing do not