Still Shrouded in Mystery: The Photon in 1925

TL;DR

This paper translates Einstein's 1925 manuscript discussing the then-uncertain nature of the photon, highlighting its incomplete understanding despite empirical evidence supporting its particle-like properties during the early quantum era.

Contribution

It provides a rare translation and analysis of Einstein's last remarks on the photon before the advent of modern quantum mechanics, shedding light on historical scientific debates.

Findings

Photon concept was still incomplete in 1925

Empirical evidence supported corpuscular properties of light

Einstein critically evaluated contemporaneous refutations of light quanta

Abstract

We present a translation of Albert Einstein's Rio de Janeiro manuscript on light quanta. In it, Einstein evaluates the Bohr-Kramers-Slater refutation of light quanta, which was concurrently the subject of intense empirical scrutiny on two continents. Written shortly before Heisenberg's discovery of quantum mechanics, the manuscript represents Einstein's last published remark on the constitution of light in the historical period known as the old quantum theory. It crystallizes the fact that by 1925 the photon concept was still incomplete, even as the corpuscular properties of light gained decisive empirical confirmation.