Speed of Light

Thank you for the explanations on velocity; however, I would like to make a few remarks on related topics.

The fact that clocks do not tick at the same rate depending on their position in space or their motion in no way implies that time does not exist. It simply means that the rate of physical processes depends on spatial conditions, which is very different from denying the existence of time itself.

One cannot arbitrarily rule out the possibility that two “identical” clocks, placed in different spatial conditions — for example, on two different floors of the same building — may operate simultaneously at different rates.

Likewise, the fact that one cannot determine the simultaneity of two distant events independently of any measurement convention does not imply that simultaneity does not exist. This is a limitation of our measurement procedures, not an ontological property.

Furthermore, the fact that we measure only proper durations — that is, local times associated with particular physical processes — does not imply that time as such does not exist.

Here again, an operational constraint is being confused with a conclusion about the nature of reality. Marc Lachièze-Rey overlooks these distinctions, moving from a statement about experimental limitations to a much stronger ontological interpretation, which nevertheless remains open to debate.

That being said, I find Marc Lachièze-Rey’s explanations of velocity very interesting.

Replacing the notion of speed with that of velocity seems to open up an interesting line of thought. One might see in this a way to relate the propagation of light more closely to the structure of space.

Speed measures only how fast motion occurs, whereas velocity also specifies its direction. However, this direction is generally defined within an abstract space.

One can go further by considering that velocity expresses an orientation within the real spatial structure, that is, within the set of relations that constitute physical space.

The energy–momentum four-vector unifies energy and motion within a geometric framework. A possible extension would be to define them as depending on the relational configuration, both local and global, which would amount to replacing a purely geometric description of motion with a relational one.

This is a line of reflection. I do not yet master its technical aspects, but it seems to me worth exploring.

Carlo Rovelli has made it possible for Aristotle’s definition of time to become non-circular, although this was probably not his primary intention.

For Aristotle, time is the number of motion with respect to before and after, where “number” is understood as that which makes counting possible. Carlo Rovelli points out, from within Aristotle’s perspective, that one motion can be counted by using another motion.

From this point on, it becomes sufficient to replace “before and after,” which are temporal notions, with the idea of a given moving body going from one point to another. Indeed, if an obstacle is placed in its path, one can clearly see in which direction the motion proceeds without needing to rely on these temporal notions.

This means that Aristotle’s definition of time, clarified by the precision brought by Carlo Rovelli, can still be regarded as valid.

From there, instead of speaking of four dimensions — three of space and one of time — one can envisage a description based solely on spatial dimensions, by integrating temporal variations into the very structure of the relations between bodies.

Such a perspective becomes possible within a fully relational approach to space and motion.