Dirk Van de moortel wrote:

> The equation of the light signal path
>     x - c t = 0
> describes the coordinates of a signal going in the positive x-direction:
> at time t the signal is at distance x = c t.
> 
> The equation of the light signal path
>     x + c t = 0
> describes the coordinates of a signal going in the negative x-direction:
> at time t the signal is at distance x = - c t.
> 
> The equations talk about different things.
> 
> Combining the equations like he did (algebraicly "solving a system
> of two equations with two unknowns") is the analytic geometry
> equivalent of finding the interesection between the light paths:
>     {  x - c t = 0
>     {  x + c t = 0
> ==>
>     {  2 c t = 0
>     {  x - c t = 0
> ==>
>     {  t = 0
>     {  x = 0
> So the intersection of the signals happens at time t = 0 at
> distance x = 0.

Dirk, I am sorry to say you have a flawed understanding of maths. These
two equations are *only* valid at x=0 and t=0, so they can not possibly
describe light paths. According to your 'method' you might as well
conclude from the two equations
(1) x=1
(2) x=0
and by inserting (1) into (2) that
(3) 1=0

Thomas