How quantum superposition forces us to confront what is truly real

There is always an “appearance of indignation” on the faces of students when they discover the quantum superposition, explains the physicist Marcelo Gleiser. He taught quantum mechanics, the theory governing the microcosmic world of atoms and particles, for decades, and the consternation of his students inevitably emerges directly on the signal: when he reaches the part on quantum objects apparently in several places at the same time.

The problem is that words like “apparently” appear enormously around this subject. Indeed, during the around the century, since the idea of ​​superposition emerged, its true meaning has remained disputed. The only thing on which physicists agree is that it takes us to the heart of what it means that something “real”.

A good starting point is with the Schrödinger equation. Developed by Erwin Schrödinger in the 1920s, it is a basic stone of quantum theory that indicates the probability of finding a particle in a given state when we measure it. The fact is that quantum mechanics concerns the forecast of the outcome of a situation – it says nothing concrete on what a particle did before it is measured.

However, Schrödinger’s equation works by describing all possible places that a particle could be measured using a piece of mathematics known as wave function. This gives us a mathematical definition of a superposition: it is a sum of different possible quantum states.

We certainly know that particles can exist in a superposition. In the double slit experience, for example, a single photon, a particle of light, is pulled to a network with two narrow gaps in front of a screen. If a detector looks at, the photon “will choose” a slit and will learn a specific place on the screen. But if there is no detector, an “interference model” will appear on the screen, suggesting that the particle behaved like a wave and crossed the two slits at a time, interacting with itself.

What we do not know with certainty is what “to be in superposition”. In general, there are two views. It is said that the wave function is a useful mathematical tool and not more. This is certainly there that Gleiser, who is based in Dartmouth College, New Hampshire, descends. “Nothing in the formalism of quantum mechanics tells us that the function of waves must be part of physical reality,” he says. “The belief in mathematics when the truth becomes a bit like a cult.”

Gleiser supports an interpretation of quantum mechanics called Quantum Bayesianism (or Qbism), which says that theory does not describe reality in itself, but rather what we know. In the end, what changes when we measure a quantum state is our information on this subject, not the reality itself.

But there is a camp which categorically refutes this view. Simon Saunders, a philosopher at the University of Oxford, thinks that the function of the waves is real. For him, a particle in a superposition is physically in more than one place simultaneously. “It’s an extensive object,” he says. “It’s relocated.” Depending on this perspective, we must accept that the world of particles does not resemble reality as we make it. Orbit electrons around an atom, for example, exist as a probability cloud before measuring them.

Critics of this position often require what happens to these other possibilities when a measure takes a particle in one place. Saunders is happy to embrace the radical response that they all manifest in their own branch of an infinite multiverse.

A resolution to this question will not come so early. In the meantime, the researchers have gone far beyond placing unique particles in the superposition – it was made for large molecules and even a crystal of 16 micrograms. If that tells us something, it is that reality is much stranger than it seems.