How ideas became knowledge: The light-quantum hypothesis 1905––1935

In 1905, Albert Einstein proposed as a “heuristic viewpoint” that light and other forms of electromagnetic radiation behave in some respects like streams of particles, each carrying energy hn (h = Planck’s constant, n = frequency), even though they also behave like waves. This became known as the Light Quantum Hypothesis. J. J. Thomson and other physicists proposed similar but less quantitative ideas. When and why did physicists accept the LQH? It is shown that a significant number of physicists already accepted particulate aspects of radiation before the discovery of the Compton effect in 1923, and that research on the photoelectric effect played an important role in this acceptance. Compton argued that his research was stronger evidence for the LQH because it yielded a prediction about a previously unknown phenomenon, the recoil electron. But there is little evidence that other scientists gave extra credit for predicting a result before rather than after it was known. Probably the combination of both effects (and other evidence) was needed to persuade skeptics.