This post continues on the subject of light conventions, see *here*.

In ordinary life today clocks are very common and portable. Time is everywhere and is everywhere the same (apart from adjustments for longitude, that is, time zones). It’s as if the signals from clocks arrived instantaneously everywhere, near or far. This is equivalent to a convention about the speed of light: the incoming speed is instantaneous. In order that the round-trip speed of light equal *c*, the outgoing speed of light must be equal to *c*/2. And time is 1D because it is a scalar quantity. This is the modern, Newtonian conception of time.

In another convention, the Sun is a global time keeper that casts its light and shadow everywhere, and is everywhere the same (apart from adjustments for longitude). It’s as if the sun sends out its light so that it reaches everywhere at the same time. In order that the round-trip speed of light equal *c*, any return reflection would be at the speed of *c*/2. And time is 3D because the position of the Sun is a 3D vector and the relative motion of the Sun is 3D, as observed in its diurnal and annual cycles. This is implicitly the ancient conception of time.

Einstein proposed a new convention: the speed of light is the same everywhere, independent of whether it is being sent or received. This is a middle position between the extremes of instantaneous and *c*/2 light speeds. However, the invariant interval was developed on 3D space +1D time dimensions, and so is 4D. In order to be completely neutral between the extremes, we would need to affirm 3+3 dimensions with an invariant interval of 6D.