[jodrellblank]

But what is “One time step” in that universe? We have the idea of a light clock - light bouncing between two perfect mirrors in a vacuum - as an ultimate clock.

The distance between the mirrors is a number of meters. A meter is based on how far light travels in a second. How long it takes light to go between them is based on the speed of light. Speed, distance and time are connected.

If we untether the speed of light and it’s unlimited, then in some sense there is no way to say how long it takes light to bounce between the mirrors - it doesn’t take any time. And there is no way to say how far apart the mirrors are, if light passes between them instantly that implies there must be no gap to cross. If light crosses no distance in no time then it also bounces back covering no distance in no time, ahh does lots of bounces in no time. There goes the concept of a time step and any concept of “non immediate effects”.

If you try and add time as a separate thing, then you have some kind of Conway’s game simulation - but that gives you a way to track where light is (which simulation cell it’s in) and therefore a kind of distance (how far the mirrors are apart in simulation cells) and then you lock down how light moves in “simulation cells travelled per timestep” which brings you back to a fixed speed of light again.