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

to something are real: the combination of the two colors does not exist in relation to anything.

We can compare the two measurements, in Beijing and Vienna, but the comparison requires an exchange of signals: the two laboratories can send each other emails or talk on the phone. An email takes time, as does a voice on the phone: nothing travels instantaneously, and an exchange of signals is an interaction, where new elements of reality come about.

When the result of the measurement in Beijing arrives in Vienna, by email or phone, only then does it become real with respect to Vienna as well. But at this point there is no longer a mysterious remote signal: with respect to Vienna, the concretization of the color of the Beijing photon occurs only when the signal containing the information arrives.

At the moment a measurement is made in Beijing, everything remains in quantum superposition with respect to Vienna. The equipment making the measurements, the scientists reading them, the notebooks in which they are written down, the messages in which the results of the measurements are conveyed, are all quantum objects themselves. Until they communicate with Vienna, their condition with respect to Vienna is not determined: with respect to Vienna, they are all like the cat in a superposition of awake and asleep. They are in a quantum superposition of one configuration in which they have measured blue and one in which they have measured red.

The same is true in Vienna with respect to Beijing. For both, the correlations do not become real until signals arrive. In this way, we can understand the correlations without recourse to magically exchanged signals or predetermined results.

This is a solution to the puzzle, but it comes at a cost: no universal set of facts exists. There exist facts relative to Beijing, and facts relative to Vienna, and the two do not match. Facts relative to one observer are not facts relative to another. It is a shining example of the relativity of reality.

The joint properties of two objects exist only in relation to a third. To say that two objects are correlated means to articulate something with regard to a third object: the correlation manifests itself when the two correlated objects both interact with this third object, which can check.

The apparent incongruity raised by what seemed like communication at a distance between two entangled objects was due to neglect of this fact: the existence of a third object that interacts with both the systems is necessary to give reality to the correlations. Everything that manifests itself does so in relation to something. A correlation between two objects is a property of the two objects—like all properties, it exists only in relation to a further, third object.

Entanglement is not a dance for two partners, it is a dance for three.

THE DANCE FOR THREE THAT WEAVES THE RELATIONS OF THE WORLD

Zeilinger looks at a photon and sees it is red. A thermometer measures the temperature of a cake being baked. A measurement is an interaction between one object (the photon, the cake) and another (Zeilinger, the thermometer). At the end of the interaction, one object has gathered information about another object. The thermometer has acquired information about the temperature of the cake. This means that there is a correlation between the thermometer and the cake. After the measurement, if the cake is cold, then the thermometer indicates coldness (its column of mercury is low); if the cake is hot, the thermometer indicates this heat (the column of mercury is high). The temperature and the thermometer have become like the two photons: correlated.

Now, if the cake was in a quantum superposition of different temperatures, then with respect to the thermometer, the cake has manifested one of its properties (temperature) in the course of the interaction. But with respect to a third system of any kind that does not participate in this interaction, no property has manifested itself; the cake and the thermometer become entangled.

This is what happens with Schrödinger’s cat. With respect to the cat, the sleeping draught appears, or it does not. With respect to me, having not yet opened the box, the bottle of sleeping draught and the cat are entangled: a quantum superposition of open-bottle/cat-asleep and closed-bottle/cat-awake.

Entanglement is therefore far from being a rare phenomenon that occurs only in particular situations: it is what happens, generically, in an interaction when this interaction is considered in relation to a system external to it.

From an external perspective, any manifestation of one object to another, which is to say any property, is a correlation; it is an entanglement between an object and another.

Entanglement, in sum, is none other than the external perspective on the very relations that weave reality: the manifestation of one object to another, in the course of an interaction, in which the properties of the objects become actual.

You look at a butterfly and see the color of its wings. In relation to me, a relation is established between you and the butterfly: the butterfly and you are now in an entangled state. Even if the butterfly moves away from you, the fact remains that if I look at the color of its wings and ask you which color you have seen, I will find that our answers match . . . even if it is not impossible that there will be subtle interference phenomena with the configuration whereby the butterfly is a different color.

All the information that we have about the world, considered externally, is in these correlations. Since all properties are relative properties, everything in the world does not exist other than in this web of entanglement.

But there is method in this madness. If I know that you have looked at the butterfly’s wings, and you tell me that they were blue, I know that if I look at them I will see them as blue: this is what the theory predicts, despite the fact that properties are relative.66 The fragmentation of points of