A standard light clock has two mirrors, say one metre apart, and a light pulse. When the clock is stationary the light path is perpendicular. When moving, diagonal. The clock ticks over each time the light hits the mirrors. The light takes longer to travel the diagonal path, so the clock takes longer to tick over, so we get time dilation. What if the clock ticked over each time the light travelled one metre? The light would still travel the same paths, perpendicular or diagonal,except those paths would be divided up into one metre lengths. Is one metre the same in both frames? It has to be, it is the distance the mirrors are apart in both frames. If you are having trouble visualising this,draw a light clock and just mark the light's path in one metre lengths, as opposed to where the light hits the mirrors.With one metre ( or whatever the mirrors perpendicular distance apart is ) the clock will tick over at the same rate in both frames. This contradicts time dilation. Can you explain why? Thanks. DAC.