Part of this is the difference between intra-generational progress and inter-generational progress. intRA-generational: You look at what the 30 year old resear

Part of this is the difference between intra-generational progress and inter-generational progress. intRA-generational: You look at what the 30 year old researchers are doing. Wait twenty years. Look at what the 50 year old researchers are doing. It is mostly the same guys and your concept of progress includes the personal accumulation of knowledge in the field. intER-generational: You look at what the 30 year old researchers are doing. Wait twenty years. Look at what the 30 year old researchers are doing. It is a new crop of researchers and your concept of progress labours under the burden of human mortality; knowledge has to be transferred to younger researchers before it is lost. When a field is new, young people come in and pioneer it. We see the intra-generational progress. Fifty years later the age structure has stabilized and the kind of progress that is most visible changes from intra-generational to inter-generational. This is much slower.