Speed ​​and celerity in relativity.

Speed ​​and celerity in relativity.
There is a very important notion in relativity: the notion of real speed.
Relativistic speeds can be classified into three types of speed.
- the real speeds (Vr) or celerities.
- observable, measurable, classic speeds (Vo).
- apparent speeds (Vapp)
Physicists are used to using the last two kinds of speeds, which they denote v and v_app and which are the same thing as Vo and Vapp in Hachel notation.
But let's return to the real speed, also called celerity, and denoted Vr by Hachel.
This notion is immensely important, although little used, and the more difficult the problems become, the more we realize that we can no longer reasonably ignore it, particularly in accelerated frames of reference or rotating frames of reference.
What is real speed? As its name suggests, it is the exact description of what happens in a movement between A and B, i.e. the distance traveled by the actual time taken to travel it.
Vr=x/Tr
However, in our world, in our daily physics, it is not this time, and it is not this speed that we are used to using.
But we use a deformation of things which is a difference in time measured by a clock A compared to the time (at the instant) measured by a clock B. We see immediately that this time is clearly biased in an anicochronous universe (our universe), and that this time is not correct, nor true, since clock A is constantly out of tune with clock B and vice versa.
You must then use only one clock and the best clock is a single FIXED clock. Now, we see, as the excellent Jean-Pierre Python says, that the only fixed clock in this story is the mobile clock, and event A and event B occur in the same place.
With Doctor Hache, this is the only way to have true time, real time; a true speed, a real speed.
Thus the notion of celerity is a fundamental notion, today quite neglected, but which, with the precision of theoretical concepts should take in the future, a preponderant place in the thinking of the relativistic physicist.
R.H.