the speed of light stays the same in all frames
mathematically, distance moves with your speed
lengths change and clocks run at different times in different frames -- for real
the time between...as we measure...
an electromagnetic wave
light - time varying electric and magnetic fields
EM radiation carries energy and momentum, which may be imparted when it interacts with matter.
A wave consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called the wavelength. Waves of the electromagnetic spectrum vary in size, from very long radio waves the size of buildings to very short gamma rays smaller than atom nuclei.
the closer together the waves are, the higher the frequency
1 hertz is the approximate frequency of a human heart (Herz in German language)
Einstein thoughts on special relatity:
if you run alongside the e-wave, the electric and magnetic fields will be static (not changing in time) but the wave shape will be varying with where you are.
Maxwell's equations (Maxwell's Electromagnetic Theory) do not allow such a solution, if the fields are varying in space, they also must vary in time.
Einstein therefore concluded that nothing he could do could put him on pace with the light beam. From this he concluded that the electromagnetic wave was a special kind of wave in nature, one which does not require a medium in which to move, and therefore that all observers must see it moving with the same speed, regardless of their own speeds; this is the fundamental postulate of special relativity.