Quantum mechanics or classical physics?

John Stewart Bell

The debate about which theory explains better the behavior of elementary particles, either quantum mechanics (with strange consequences such as state superposition and quantum collapse) or a still unknown classical physics-type theory that eliminates the need for such phenomena, has been going on since Bohr and Einstein started this debate almost one century ago.

The issue seemed to have been decided when John Steward Bell formulated in 1964 the famous Bell inequality, which I described in another post. However, as some physicists still raise the question, other means of distinguishing between the two types of theories are still being sought. One of them is the Leggett-Garg inequality (LGI), to which I will dedicate this post, based on a recent article in Physics World, dated August 12, 2024.

Bell’s inequality and causality

Niels Bohr

Quantum Mechanics took shape about ninety years ago. During the twenties, Niels Bohr and Werner Heisenberg formulated the Copenhagen interpretation, which added to the mathematical formulation some additional considerations such as the following:

• Physical systems with properties that can take concrete and opposing values ​​(such as direction of polarization or spin) in certain circumstances can be in a state where those properties do not take a defined value, but keep all the possibilities simultaneously open. For example, the direction of polarization of a photon can be simultaneously north-south and east-west. The spin of a particle can be both up and down.
• The act of measuring one of these properties causes the collapse of the wave function, which means that the result of the measurement can only be one of the possible values. The wave function gives us the probability of obtaining each value.
• It is possible to build a physical system formed by two or more interlaced particles with respect to some property, which means that if one of the particles collapses with a certain value, the other particle has no choice but to collapse with the other.