Bernard Guy

The basic proposal is presented: movement precedes both space and time that are derived from it. As a line of justification, one must guard against two temptations: a) that of believing that reality imposes on us in a univocal way the words to describe it, each one referring to their own qualities of the corresponding objects; b) that of believing that the concepts can reach a state of purity that abstracts them completely from human knowledge, which is rooted in the functioning of the body. In response, two dimensions of thought are particularly adapted: 1) a relational dimension: one can only compare phenomena to other phenomena (movements to other movements) and conventions are necessary. Space and time are the names of these comparisons; 2) an embodiment dimension: the concepts of space and time have a meaning first in the experience of the body inside the world (lived in terms of movements), before the words. The presentation of the proposal is followed by some elements of history of the concepts of space and time, a discussion on the way to express the objectivity of embodied knowledge, and some consequences that may already be foreseen, in social and human sciences, as well as in physics.

Our objective is to show how geology may help to make progress in the understanding of some fundamental concepts and issues dealing with space, time and movement. This leads us to the very theory of relativity in physics. A thought experiment is presented where geology has fully played a role. Deep (geologic) time is a way to speak of very slow movements, involving imperceptible changes of the ordinary space we live in. According to the relative speeds involved, what is space becomes time (when we accelerate the movements of the mountains); conversely, time may become space (at the scale of the femtoseconds where the sand grains in the egg-timer are immobile). This is not just an allegory: we can never stop, thinking we have reached a pure time and a pure space; because we are inside the world and we can only compare what moves more (on which we build time) to what moves less (on which we build space) within a relational thinking. In this context, conventions are necessary, in particular that of the constancy of a standard movement: this is what we do today with light. The uniformity hypothesis in geology plays the same role. The links between space, time and movement is discussed in various fields of geology (sedimentary sequences, metamorphism, metasomatism, magmatism, …) as they are omnipresent in prehistory.

The text proposes some directions of research, as based on previous works made by the author. Our purpose is to discuss the contribution of general relativity to the epistemology of space and time, in the context of a relational, and not substantial, rationality. General relativity brings us the important idea (of a relational nature) that space and time do not constitute a scene external to phenomena, but that, on the contrary, the phenomena themselves, in the first place the phenomenon of gravitation, are responsible for assessing the corresponding variables. However, this contribution does not make us progress on the "mystery" of time, that remains conceptually separated from space, even though, since the relativity theory, the values of space and time variables are related. Encouraged by general relativity, we must go further and express more strongly the link between the concepts of space and time, and their identity of substance. The relational approach must extend to all phenomena (beyond the phenomenon of gravitation alone), and we must see time and space as defined together by the mobility / immobility relations induced by the comparison between different phenomena. This approach renews a number of questions, both on how to talk about space and time in general relativity and on the relations of this theory with other domains like quantum mechanics. But also on the conceptual loops found in the theory, and even finding a convergence with Humanities and Social Sciences (especially mesology / ecology) in the way these understand and criticize the notions of space and time as too much abstracted (disconnected from phenomena and human observers).

Nous proposons de penser ensemble les concepts d’espace et de temps : ils concernent les mêmes degrés de liberté des éléments du monde et fonctionnent toujours en tandem. Leurs fondements doivent être discutés, non dans une pensée de la substance (chacun est défini par une série de caractères qui lui sont propres), mais dans une pensée de la relation (chacun se définit en opposition à l’autre). Nous opposons des relations spatiales à des relations temporelles, ou encore des relations d’immobilité à des relations de mobilité relative. La décision de la frontière entre ces deux ensembles de relations est sujette à arbitraire : nous avons une grande flexibilité dans les définitions asso­ciées des paramètres d’espace et de temps ; elle ne fait pas non plus l’économie de difficultés conceptuelles ou logiques semblables à celles rencontrées dans la mécanique quantique. Il faut revoir dans cette perspective autant le concept de temps que celui d’espace : le temps ne coule pas, il est changement de relation, il est mouvement ; l’espace est abstrait à partir de relations constantes ou morceaux constants de mouvement. Les mouvements relatifs qui expriment ces relations, changeantes ou non, contiennent toujours un aspect spatial et un aspect temporel, comme pile et face de la même réalité. Nous proposons de voir plus généralement dans toute relation un aspect spatial (l’écart qui sépare les deux termes de la relation) et un aspect temporel (le parcours du chemin qui les relie). Sur cette base, nous proposons un programme de re­cherche pour reprendre un certain nombre de problèmes fondamentaux de la physique contemporaine, ainsi que des pistes pour reprendre ce que nous di­sons du temps et de l’espace dans les sciences humaines et sociales, la culture, et jusque dans la vie quotidienne.