Analysis of the distinct meanings of the notion of «time», in different physical theories
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Problems concerning the concept of time have recently generated a widespread interest. Among these: the role of time in quantum gravity, the choice of an internal time in general relativity, the origin of the «arrow of time», and the time asymmetry in cosmology. These problems often pertain to distinct aspects of the notion of time; unfortunately, however, the use of identical terms for different issues leads often to a confused discussion, and the complexity of the problem of understanding or altering a notion as the notion of time is perhaps underestimated. Here, we analyze the distinct roles that this notion plays in different theories: We find ten distinct versions of the concept of time, all used in the natural sciences, characterized by different properties, or attributes, ascribed to time. We propose a general terminology to express these differences. In addition, we observe that the different notions of time are hierarchically arranged, in the sense that complex attributes of time become necessary only in dealing with progressively more special natural systems, and we derive from this observation some speculations on the significance of the notion of time at the fundamental level.
PACS04.20 General relativity
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