Quantum Theory at the Crossroads

From the Preface:

And they said one to another: Go to, let us build us a tower, whose
top may reach unto heaven; and let us make us a name. And the Lord said: Go
to, let us go down, and there confound their language, that they may not
understand one another’s speech.

Anyone who has taken part in a debate on the interpretation of quantum theory will recognise how fitting is the above quotation from the book of Genesis, according to which the builders of the Tower of Babel found that they could no longer understand one another’s speech. For when it comes to the interpretation of quantum theory, even the most clear-thinking and capable physicists are often unable to understand each other.

This state of affairs dates back to the genesis of quantum theory itself. In October 1927, during the ‘general discussion’ that took place in Brussels at the end of the fifth Solvay conference, Paul Ehrenfest wrote the above lines on the blackboard. As Langevin later remarked, the Solvay meeting in 1927 was the conference where ‘the confusion of ideas reached its peak’.

Ehrenfest’s perceptive gesture captured the essence of a situation that has persisted for three-quarters of a century. According to widespread historical folklore, the deep differences of opinion among the leading physicists of the day led to intense debates, which were satisfactorily resolved by Bohr and Heisenberg around the time of the 1927 Solvay meeting. But in fact, at the end of 1927, a significant number of the main participants (in particular de Broglie, Einstein, and Schrdinger) remained unconvinced, and the deep differences of opinion were never really resolved.

The interpretation of quantum theory seems as highly controversial today as it was in 1927. There has also been criticism — on the part of historians as well as physicists — of the tactics used by Bohr and others to propagate their views in the late 1920s, and a realisation that alternative ideas may have been dismissed or unfairly disparaged. For many physicists, a sense of unease lingers over the whole subject. Might it be that things are not as clear-cut as Bohr and Heisenberg would have us believe? Might it be that their opponents had something important to say after all? Because today there is no longer an established interpretation of quantum mechanics, we feel it is important to go back to the sources and re-evaluate them.

In this spirit, we offer the reader a return to a time just before the Copenhagen interpretation was widely accepted, when the best physicists of the day gathered to discuss a range of views, concerning many topics of interest today (measurement, determinism, nonlocality, subjectivity, interference, and so on), and when three distinct theories — de Broglie’s pilot-wave theory, Born and Heisenberg’s quantum mechanics, and Schrdinger’s wave mechanics — were presented and discussed on an equal footing.