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

change! Note the absurdity: if I don’t look for where the photon passes, it always finishes below. But if I look at where it passes, it can end up above. The astonishing thing is that a photon can end up above even if I haven’t seen it. That is to say, the photon changes trajectory due to the fact that I was waiting for it at the gate, on the side where it hasn’t passed. Even if I haven’t actually seen it!

What you read in textbooks on quantum mechanics is that if you observe where a photon passes, its ψ wave jumps entirely onto a path. Say you observe if the photon passes on the right path: if you see it, the ψ wave jumps completely to the right; but also if you do not see it, the ψ wave jumps! It jumps to the left. In both cases, there is no longer interference. The wave function “collapses,” that is to say it leaps, converging in one point, the moment we observe it.

This is quantum superposition: the photon is, so to say, “on both paths.” But if you search for it, it is only on one path.

Hard to believe.

And yet it happens: I saw it with my own eyes. Despite having studied it so much at university, seeing it and having, literally, a hands-on experience of it left me confused. Try yourself to think of a sensible explanation of this behavior. For a century now, we’ve all been trying.

If you find all this confusing, if you cannot make head or tail of it, you are not alone. It is why Richard Feynman wrote that nobody understands quanta. (If instead what I have described seems perfectly clear, then it means that I have not been clear enough about it. For as Niels Bohr once said, you should “never express yourself more clearly than you are able to think.”37)

Schrödinger illustrated this same enigma with a famous thought experiment: instead of a photon that takes a path on the right and a path on the left at the same time, he imagined a cat that is asleep and awake at the same time.38

It goes like this: a cat is shut in a box with a device where a quantum phenomenon has a one-in-two probability of happening. If it happens, the device opens a bottle of sleeping draught and the cat falls asleep.* The theory tells us that the ψ wave of the cat is in a “quantum superposition” of “cat-awake” and “cat-asleep,” and it remains so until we actually see the cat.

So if it is truly described by its ψ, the cat is in a “quantum‐superposition” of “cat‐awake” and “cat‐asleep.”

This is different from saying that we do not know if the cat is awake or asleep, for the following reason: there are interference effects between cat-awake and cat-asleep (analogous to the interference effects between the two paths of Zeilinger’s photons) that would not occur if the cat was either awake or asleep. They happen when the cat is in this quantum superposition of cat-awake and cat-asleep. As in the interference in Zeilinger’s experiment, which occurs only if the photons “pass along both paths.”

For a physical system as large as a cat, the interference is too difficult to observe.39 But there is no convincing reason to doubt its reality. The cat is neither awake nor asleep. It is in this quantum superposition between cat-awake and cat-asleep.

But what does this mean?

How does a cat feel, being in a quantum superposition of cat-awake and cat-asleep? If you were in a quantum superposition between yourself-awake and yourself-asleep, then how, dear reader, would you feel? This is the riddle of quanta.

TAKING Ψ SERIOUSLY: MANY WORLDS, HIDDEN VARIABLES AND PHYSICAL COLLAPSES

To provoke a heated discussion at a physics conference dinner, you need only turn to the person next to you and casually ask: “So, in your opinion, is Schrödinger’s cat awake and asleep?”

Discussions on the mysteries of quanta were lively in the 1930s, immediately after the birth of the theory. A famous debate on this topic between Einstein and Bohr went on for years through personal encounters, conferences, written works, letters . . . Einstein was resistant to the idea of relinquishing a more realistic image of phenomena. Bohr defended the conceptual novelty of the theory.40

In the 1950s, the problem was mostly ignored: the power of the theory was so spectacular that physicists tried to apply it in every possible field, without asking too many questions. But if you don’t ask questions, you learn nothing.

By the 1960s, interest in the conceptual problems was on the rise again, curiously added to by the fascination within hippie culture for the alternative otherness of quanta.41

Today, discussions of quantum theory are frequent in departments of philosophy as well as in physics departments, and there are discordant opinions and perspectives. Some ideas are abandoned, others persist. The ideas that have withstood criticism give us ways of comprehending quanta, but each one of these ways has a high conceptual cost: each forces us to accept something outlandish. The judgment on the final balance of costs and benefits entailed by the various opinions on the theory is still open.

I expect that we will end up agreeing, as has happened with the other great scientific disputes that seemed irresolvable at the time. Is the Earth stationary, or does it move? (It moves.) Is heat a fluid, or is it the rapid movement of molecules? (It is the movement of molecules.) Do atoms really exist? (Yes.) Does the world consist only of “energy”? (No.) Do we have ancestors in common with apes? (Yes.) And so on . . . In this book, which is a chapter in the ongoing dialogue, I describe where it seems to me the debate is now, and in which direction we are going.

Before reaching, in the next chapter, the ideas that I find most convincing, namely the relational perspective, I summarize below the most discussed