DETAILS ON QUANTUM THEORY
22. Superpositions and probabilities – quantum theory without ideology
Niels Bohr brainwashed a whole generation of physicists into believing that the problem
[of the interpretation of quantum mechanics] had been solved fifty years ago.
Murray Gell–Mann, Nobel price acceptance speech.
Why is this famous physical issue arousing such strong emotions? In particular,
ho is brainwashed, Gell–Mann, the discoverer of the quarks, or most of the other
physicists working on quantum theory who follow Niels Bohr’s ∗ opinion?
In the twentieth century, quantum mechanics has thrown many in disarray. Indeed, it rad-
ically changed the two most basic concepts of classical physics: state and system. The state
is not described any more by the specific values taken by position and momentum, but by
the specific wavefunction ‘taken’ by the position and momentum operators.∗∗ In addition,
in classical physics a system was described as a set of permanent aspects of nature; per-
manence was defined as negligible interaction with the environment. Quantum mechanics
shows that this definition has to be modified as well.
In order to clarify the issues, we take a short walk around the strangest aspects of quantum
theory. The section is essential if we want to avoid getting lost on our way to the top of
Motion Mountain, as happened to quite a number of people since quantum theory appeared.
Why are people either dead or alive?
The evolution equation of quantum mechanics is linear in the wavefunction; thus we can
imagine and try to construct systems where the state ψ is a superposition of two very distinct
situations, such as those of a dead and of a living cat. This famous fictional animal is called
Schrödinger’s cat after the originator of the example. Is it possible to produce it? How
would it evolve in time? Similarly, we can ask for the evolution of the superposition of a
state where a car is inside a closed garage with a state where it is outside the closed garage.
∗ Niels Bohr (1