I would like to start this by saying that my motivation for asking is that I find relativity very difficult to deal with using the SI system. It strikes me that the problem with this system is the fact that it was based on an earlier system that assumed time to move at a constant rate under all circumstances. Since we now know this not to be true, is there, or should there be a better system out there? One that is based on the constant speed of light, not a constant rate of time.
For example, if MKS became MKL where 1L = 1 Light-Gigametre (the time it takes light to travel 1 Gigametre - not in seconds) then in the world of slow moving objects we would have 1s approximately equal to 1L/3 and all would be fine.
By my reckoning (and probable naivety) it should also hold out in the world of fast moving objects where time starts misbehaving but L remains constant.
Does such a system already exist?
What I am looking for is definitely not some scaled version of our existing base units. I'm looking for a system of units that ditches time altogether and starts from scratch with light-distance in its place. A fundamental change that affects all of the units that have a time component.
I was prompted to look at Planck Units and in partiular Planck Time. At first sight this appeared to be what I was looking for i.e. it is based on light-distance. Unfortunately it looks like this has just been equated to a fixed number of seconds. The problem is that as soon as you make it a scaled version of time you have lost its true meaning and it is no longer true light-distance. It can't be, because light travels at a constant rate and time doesn't.
From a practical point of view, as a stationary observer on Earth, during the same period of our clock based time I could be looking at a light bouncing off a distant stationary mirror and measure one distance of light (2d) or I could be watching the same scenario on a fast moving passing vehicle and measuring a different distance of light (a V shape of height d). So in effect, the light-distance measured by me (L) would reflect the amount of earth time being experienced at the source of the light. So L (unlike t) would be dependent on what was being observed, as it should be. Or should it be that in a system where light-distance is replacing time we should just say that in both cases the light moved by 2d?