FOUNDATIONS OF QUANTUM MECHANICS

IV
THE COPENHAGEN INTERPRETATION
It is wrong to think that the task of physics is to find out how nature is. Physics concerns
what we can say about nature.
— Niels Bohr
The Heisenberg-Bohr tranquilizing philosophy - or religion? - is so delicately contrived
that, for the time being, it provides a gentle pillow for the true believer from which he
cannot very easily be aroused. So let him lie there.
— Albert Einstein
I know it is not the fault of N. B. that he did not study philosophy. But I deeply regret
that by his authority the brains of two or three generations will be upset and hindered to
think about the problems ‘He’ pretends to have solved.
— Erwin Schrödinger
Bohr’s famous institute being located in Copenhagen, the standard interpretation of quantum
mechanics as explained in most of the textbooks is generally indicated as the Copenhagen Interpretation.
It is however worth mentioning that the conceptions of the many supporters of the
Copenhagen Interpretation, Niels Bohr, Werner Heisenberg, Wolfgang Pauli, Rudolf Peierls,
Léon Rosenfeld and John Wheeler, to name some of them, mutually differ on numerous points,
and that some of them, including Bohr himself, modified their conceptions in the course of time,
so that the name ‘Copenhagen Interpretation’ is more a collective noun than the name of one
clearly outlined vision. Moreover, important contributions to the standard interpretation of the
theory have been made by Born and Von Neumann, working independently of the Copenhagen
school. In this chapter we will evaluate the conceptions of Heisenberg and Bohr as the main
representatives of the Copenhagen Interpretation, and consider more closely the debate between
Einstein and Bohr. Finally, we will discuss the exact expression of the uncertainty principle.
IV. 1
HEISENBERG AND THE UNCERTAINTY PRINCIPLE
The history of modern quantum mechanics starts in 1925, when Heisenberg publishes his famous
transitional article ‘Über quantentheoretische Umdeutung kinematischer und mechanischer
Beziehungen’ (‘Quantum - theoretical re - interpretation of kinematic and mechanical relations’). His
summary reads
The present paper seeks to establish a basis for theoretical quantum mechanics founded
exclusively upon relationships between quantities which in principle are observable.