# Solving the (quantum) measurement problem

In quantum physics a measurement is an experimental situation in which a physical entity undergoes an *indeterministic* and *irreversible* change, called the **collapse of the wave function**, or **reduction of the state vector**.

The existence of processes of change of this kind is considered to be a **problem** because even if we perfectly know the **initial state** of the entity, that is, its state before the measurement, we cannot **predict with certainty** what will be its **final state**, that is, the state at the completion of the measurement process.

The best we can do is to attach **probabilities** to the different possible final states, by means of a **rule of correspondence**, called the **Born rule**.

Solving the (quantum) **measurement problem** is about *explaining* what goes on, “behind the scenes,” when a system is subjected to a measurement context. *What produces such an abrupt change of the entity’s state? Why such change is non-deterministic and irreversible? Why the Born rule is so successful in determining the probabilities of the different possible outcomes?*

It is generally believed that convincing answers to the above questions are yet to be found. Not only: many believe that these answers **cannot** be found, because the so-called *hidden-variables theories*, which assume that quantum mechanics would be an incomplete theory, have been opposed by the well-know **no-go theorems**: impossibility proofs which forbid the construction of theories where the quantum indeterminism would be replaced by a deeper deterministic description.

In other terms, the no-go theorems forbid the replacement of quantum mechanics by a more fundamental theory in which the physical entities would always possess well-defined properties, that is, in which the probabilities to have or not to have a certain property would only take the values ** o** or

**.**

*1**But, is it really so?*

In the following video presentation you will discover that, contrary to what is generally believed, the central problem of quantum mechanics can actually be solved by using a hidden-variables argument. But for this, we have to follow the intuition of the Belgian physicist ** Diederik Aerts**, that already in the eighties of the last century proposed to associate the hidden-variables not to the state of the measured entity, but to its interaction with the measuring apparatus, in what is today known as the

**.**

*hidden-measurements interpretation of quantum mechanics*Then, the no-go theorems no longer apply and it becomes possible not only to conceptually explain the nature of a quantum measurement, but also to *derive**,*** **in a non-circular way, the

**.**

*Born rule*If you are a professional physicist, and would like to deepen your understanding of the solution presented in the above video, we recommend the reading the following ** open source **article:

Diederik Aerts & Massimiliano Sassoli de Bianchi, ** The Extended Bloch Representation of Quantum Mechanics and the Hidden-Measurement Solution to the Measurement Problem**, Annals of Physics 351, Pages 975–1025 (2014).

If you have a good scientific background, but would like to read a less technically involved text, we recommend the following article, written as a dialogue (also available on the arXiv:1406.0620 [quant-ph]):

Diederik Aerts & Massimiliano Sassoli de Bianchi, ** Many-Measurements or Many-Worlds? A Dialogue**, Foundations of Science. DOI: 10.1007/s10699–014–9382-y (December 2014).

If you have no specific knowledge about physics, and would like to read a more pedestrian text on the ** hidden-measurement interpretation**, we recommend the reading the following booklet (a

**of which can be viewed on**

*trailer***YouTube**):

Massimiliano Sassoli de Bianchi, ** Observer Effect. The Quantum Mystery Demystified**,

**Adea Edizioni**(2013). Available as an

*e-book*, in

**and**

*Kindle***editions.**

*iBook*A final recommendation is the video entitled: ** The physics of spaghetti — Heisenberg’s Uncertainty Principle and Quantum Non-spatiality (Non-locality)**.