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and whose differences in velocity or acceleration are small, will make only a slight or no relative distinction between their clocks, which is what normally happens on Earth. In case of particles, the differences are much more noticeable. If we follow a moving particle, its lifespan may seem very long. However, if we look at this particle from the out- side, its lifespan seems shorter. Observing a particle from the outside inevitably signifies that we see it from a different temporal flow, i.e., a different velocity. Therefore, there is no simultaneity between our physical body and... what constitutes its essence! How then can biochemists, neurologists, psychologists and psychiatrists see a causal continuity between certain phenomena, both internal and external from a physical point of view, and our perception of it? We should always start our research by study- ing the nature of time to establish the academic truths that feed off the tautology “temporal irreversibility/causality.”” We therefore possess sev- eral “‘passing times” depending on the space scale on which we observe them, as special relativity keeps insisting. The division into sectors of science often prevents it from glimpsing a coherent and comprehensive vision of fractal spatio-temporal reality. Our own time, however, cannot be used in star-dating methods, since only what is in our immediate surroundings, at our rate, can be compared or synchronized with our clock. This explains why we can date only the moment at which we perceive the light caused by this explosion. However, astronomers use a synchronization technique to define cosmic time and subsequently deduce the age of the universe: a ood eat aa ' they look at their watches! They suggest that gravity, the source of local spatio-temporal dis- tortion, is uniformly spread throughout the universe (homogeneity and isotropy), allowing the global convention of our Earth time to date the history of the universe. Having an individual (or a planet) with a much higher velocity and whose time moves more slowly is not a problem if a conversion protocol has been established. For instance, if his second takes half our time, it is safe to say that his universe is not thirteen, but twenty-six billion years old. However, what about some- one whose “second” is billions of times shorter than ours (from our point of view), knowing that the speed of light is not a threshold, not even for Einstein?® In that case, the universe is eternal, so eternal that there is no beginning and no end. Thanks to this cosmic time, we can express the rate and acceleration of cosmic expansion (or not), as well as the dilution and cooling of inherent matter. Needless to say, this cos- mic time is the foundation of cosmology. What do the stars tell us about time? 145