In his influential book The Structure of Scientific Revolutions, Thomas Kuhn points out that the kinds of questions we often ask about the history of science and that we think are simple and have been adequately answered (such as “who discovered oxygen and when?” “Who discovered X-rays and when?”) turn out on close examination to be extremely difficult, if not impossible, to answer.

It is not that there are no answers given in authoritative sources. It is that when we actually do examine the historical record, the situation turns out to be very murky, giving rise to the strong suspicion that such questions are the wrong ones to ask about the scientific enterprise. The simple answers that are given to such questions represent a rewriting of history to give readers a simple narrative but at the expense of giving a distorted sense of how science is done, as if scientific discoveries were clear and decisive events. I remember being very impressed by Kuhn’s examples to support his thesis when I first read his book and subsequent readings of science history have convinced me that he is right.

For example, the latest issue of the newsletter of the American Physical Society’s called the APS NEWS (vol. 16, no. 5, May 2007, p. 2) has an account of the discovery of the neutron. (The article is here but the current issue is password protected and non-APS members will have to wait a month before it is archived and people are given open access.) The title says “May 1932: Chadwick reports the discovery of the neutron” and recounts the familiar (to physicists anyway) story of how James Chadwick 75 years ago this month made the famous discovery for which he received the Nobel prize in 1935.

As the article proceeds to describe the history of the process, it becomes clear that its own story contradicts the impression given in the title.

As early as the 1920s, people had suspected that there was something in the atom’s nucleus other than protons. Some thought these additional particles were made up of an electrically neutral combination of the already known proton and electron but no one could confirm this. But experiments went on trying to isolate and identify the particle, and around 1930 two scientists Bothe and Becker found radiation coming from a target of Beryllium that had been bombarded with alpha particles. They thought that this radiation consisted of high-energy photons. Other experiments done by Frederic and Irene Joliot-Curie also found similar radiation that they too attributed to high-energy photons.

Chadwick thought that this explanation didn’t quite fit and did his own experiments and concluded that the radiation was caused by a new neutral particle that was slightly heavier than a proton. He called it the neutron. He published a paper in February 1932 where he suggested this possibility and then in May 1932 submitted another paper in which he was more definite. It is this paper that gives him the claim to be the discoverer.

But like all major scientific discoveries, acceptance of the new idea is not immediate within the community and it took until around 1934 for a consensus to emerge that this neutron was indeed a new fundamental particle.

So who “discovered” the neutron and when? Was it the people who concluded much earlier than 1932 that there was something else in the nucleus other than protons? They were right after all. Was it Bothe or Becker, or the Juliot-Curies who first succeeded in isolating this particle by knocking neutrons out of materials? They had, after all, “seen” isolated neutrons even if they had not identified it as such. Or do we give the honor to Chadwick for first providing a plausible claim that it was a neutron?

As to when the neutron was discovered, it is also hard to say. Was it when its existence was first suspect in the early 1920s? Or when it was first isolated experimentally around 1930? If we say that since the title of discoverer was awarded to Chadwick, the date of discovery has to be assigned to something he specifically did, when exactly did he realize that he had discovered the neutron? In his first preliminary paper in February 1932? Or in his more definite paper in May? Clearly he must have known what he knew before he submitted (or wrote) the papers.

All we know for sure is that sometime between 1930 and 1934, the neutron was “discovered” and that certain scientists played key roles in that process. For historical conciseness, we give the honor to Chadwick and fix the date as May 1932 and the judgment is not an unreasonable one, as long we insist on demanding that such events have a definite date and author. But it is good to be reminded that all such assignments of time and place and people for scientific discoveries mask a much more complex process, where “discoveries” involve extended periods of time involving large numbers of people during which understanding is increased incrementally. There is often no clear before-after split.

The detailed stories are almost always more fascinating than the truncated histories we are taught.