Helgoland by Rovelli, Erica (ebook reader below 3000 .txt) 📗

together. Like two lovers who can guess each other’s thoughts when apart. It has been well verified in laboratories. Chinese scientists, led by Juan Yin, have succeeded in producing two entangled photons on a satellite called Micius and sending them, still entangled, to two stations at a distance of thousands of kilometers from each other on Earth.61

Let’s see how this works.

First, two entangled photons have correlated features: if one is red, the other will be red; if one is blue, the other one will be blue as well. Nothing strange so far. If I separate a pair of gloves and send one to Vienna and one to Beijing, the one that arrives in Vienna will be of the same color as the one that ends up in Beijing: they are correlated.

The strangeness emerges when a pair of photons sent to Vienna and Beijing, respectively, are in a quantum superposition. For instance, they could be in a superposition of a configuration in which both are red, and one in which both are blue. Each photon may reveal itself as either red or blue the moment it is observed, but if one is found to be blue, then the other—far away—will also be blue.

The puzzling aspect is this: How can they turn out to be the same color? The theory states that each of the two photons is neither definitively red nor definitively blue until it interacts. It states that the color is determined randomly when we look. But if this is the case, how can the color randomly determined in Beijing be the same as the one randomly determined in Vienna? If I toss a coin in Beijing and another in Vienna, the results are independent of one another, they are not correlated: there is nothing causing it to be heads in Vienna every time it is heads in Beijing.

There seem to be only two possible explanations. The first is that a signal of the color of one photon travels extremely rapidly to the other, far-off photon; when a photon decides to be either red or blue, it communicates this instantly in some way to its distant brother photon. The second, more reasonable possibility is that the color was already determined at the moment of the separation, just as in the case of the gloves, even if we were not aware of it. (Einstein expected this to be the case.)

Neither of these explanations works. The first implies an impossibly rapid communication over too great a distance, against all we know about the structure of space-time, which prevents such rapid signals. In fact, there is no way of using entangled objects to send signals. Hence the correlation is not related to a rapid signal transmission.

As for the other possibility—that the photons, like the gloves, already “knew” before being separated that they would both be blue or both red—it has been excluded as well. It was excluded by acute observations made in a brilliant article written in 1964 by a physicist from Belfast named John Bell.62 Bell’s job was in particle physics and particle accelerator design; understanding quantum theory was a matter of personal curiosity for him, at a time when almost no one cared about the issue. Yet today, he is celebrated for his influence on the foundation of quantum physics.

With reasoning that is elegant, subtle and very technical, Bell showed that if all the correlated properties of the two photons had been determined from the moment of separation (instead of being determined by chance at the moment of observation), precise consequences would follow (today called “Bell inequalities”) that are contradicted by what we actually observe. The correlations are definitely not determined from the outset.63

It seems, then, like a puzzle without a solution. How can two entangled particles make the same decision without previous agreement and without sending each other messages? What is it that connects them?

My good friend Lee recounts that as a young man he lay on his bed for hours on end looking at the ceiling, after he had studied entanglement. He was thinking about how each atom in his body must have interacted in some distant past with so many other atoms in the universe. Every atom in his body had to be entangled with billions of atoms dispersed throughout the galaxy . . . He felt a connectedness with the cosmos.

Entanglement shows that reality is definitely other than how we had conceived of it. Even if we know all that can be predicted about one object and another object, we still cannot predict everything about the two objects together.64 The relationship between two objects is not something contained in one or the other of them: it is something more besides.65

This interconnection between all the components of the universe is disconcerting.

Let’s return to the puzzle: How do two entangled particles behave in the same way without having made up their minds beforehand and without communicating at a distance?

The relational perspective offers a solution, but one that shows just how radical that perspective is.

The solution lies in remembering that properties exist only in relation to something else. The measurement of the color of the photon performed in Beijing determines its color with respect to Beijing. But not with respect to Vienna. And vice versa. Since there is no physical object that sees both colors at the moment in which the two measurements are made, it makes no sense to ask whether the two results are the same or not. It is meaningless, because there is no object with respect to which the sameness is realized.

Only God can see the two places at the same moment—but God, if She exists, does not tell us what She sees. What She sees is irrelevant to reality. We cannot rely upon the existence of something that only God can see. We cannot assume that both colors exist, because there is nothing with respect to which both can be determined. Only the properties that exist in relation