Two-slit diffraction with highly charged particles: Niels Bohr's consistency argument that the electromagnetic field must be quantized

Understand

We analyze Niels Bohr's proposed two-slit interference experiment with highly charged particles that argues that the consistency of elementary quantum mechanics requires that the electromagnetic field must be quantized.

• In the experiment a particle's path through the slits is determined by measuring the Coulomb field that it produces at large distances; under these conditions the interference pattern must be suppressed.
• The key is that as the particle's trajectory is bent in diffraction by the slits it must radiate and the radiation must carry away phase information.
• Thus the radiation field must be a quantized dynamical degree of freedom.