At the fifth Solvay conference, some questions that are closely related to the quantum measurement problem (as we would now call it) were addressed in the context of pilot-wave theory, in both the discussion following de Broglie's report and the general discussion. Most of these questions concerned the treatment of scattering (elastic and inelastic); they were raised by Born and Pauli, and replies were given by Brillouin and de Broglie. Of special interest is the famous – and widely misunderstood – objection by Pauli concerning inelastic scattering. Another question closely related to the measurement problem was raised by Kramers, concerning the recoil of a single photon on a mirror.

In this chapter, we shall first outline the pilot-wave theory of scattering, as currently understood, and examine the extensive discussions of scattering – in the context of de Broglie's theory – that took place at the conference.

We shall see that de Broglie and Brillouin correctly answered the query raised by Born concerning elastic scattering. Further, we shall see that Pauli's objection concerning the inelastic case was both more subtle and more confused than is generally thought; in particular, Pauli presented his example in terms of a misleading optical analogy (that was originally given by Fermi in a more restricted context).