It has been argued that in higher education academic disciplines can be seen as communities of practices. This implies a focus on what constitutes identities in academic culture. In this article I argue that the transition from newcomer to a full participant in a community of practice of physicists entails a focus on how identities emerge in learning how to highlight certain aspects of personal life histories. The analysis of interviews with 55 physicists shows that physicists often perceive experiences in their childhood as the first step into their professional identities as physicists. These experiences involve recollections of the ability to think scientifically (e.g., "go beyond the surface"), and the ability to play with toys which can be connected to the practical life of physics. The process of identity formation can be described as developing in a relational zone of proximal development, where old-timers recognize particular playful qualities in newcomers as a legitimate access to a physicist identity. The article discusses how play which physicists connects with a scientific mind can constitute a relational zone of proximal developments in a community of practice as a particular "space of authoring" in a physicist culture, which cut across other cultural differences. (Contains 10 notes.) [Abstract is presented in both English and French.]