Still Shrouded in Mystery: The Photon in 1925

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.