Galilean relativity requires the speed of light to be instantaneous (i.e., zero pace). Because the one-way speed of light is not known, it may be instantaneous as long as the mean speed of light is finite. Such a situation is possible if light is conceived as in *half-duplex telecommunications*: one direction at a time is observed or transmitted, but never both simultaneously.

Consider a light clock in this context:

Let Δ*t* be the time for one cycle of light at rest (top diagram). Let Δ*t’* be the time for one cycle of light traveling at relative velocity *v* (bottom diagram). The mean speed of light is *c*. Then

Δ*t* = *h*/*c* or *h* = *c*Δ*t*,

Δ*t’* = *d*/*c*, or *d* = *c*Δ*t’*, and

*b* = *v*Δ*t’*.

So that

*d*² = *b*² + *h*² = (*c*Δ*t’*)² = (*v*Δ*t’*)² + (*c*Δ*t*)².Δ*t*)

The result is

Δ*t’* = Δ*t*/√(1 – *v*²/*c*²),

which is the time dilation of the Lorentz transform.