When an area of scientific understanding and investigation seems to take a major change in direction or make a sudden leap forward, there is often talk of a ‘crucial experiment’. The idea is that the change can largely, or perhaps entirely be attributed to insights which came from the result of a single decisive experiment. It is clear that this is not always the case. Philosophers and scientists have disagreed widely as to whether it occurs frequently, rarely, or not at all.

There are at least two reasons why an experiment that was not really crucial might be constructed as a ‘crucial experiment’ after the event in telling the history. Firstly, it can add drama and colour to the story. Secondly, it gives an opportunity to clarify some of the confusion and ambiguity that would probably have been present at the time. The logical foundations of the present understanding of the subject can then be more clearly linked in with the history.

The ‘crucial experiment’ has been characterised as follows: it must give a result that is simultaneously in accord with a clear prediction of one scientific theory, and in contradiction of the clear predictions of all of its serious current rivals. This definition is given both by Lakatos (1974) and Franklin (1981).