As I understand it, time isn't supposed to elapse for anything moving at speed c, and therefore, in particular, for photons. However, light is also supposed to be an electromagnetic wave, i.e. essentially a wave of orthogonal magnetic and electric magnitudes moving at c in a vacuum. I'm not entirely sure how this is supposed to go, however. As I see it, a wave is a propagation of a quantity in a medium, but an electromagnetic wave is a propagation in spacetime, which is supposed to be nothing else but all events considered collectively. The only things which seem changing in the case of light are the magnetic and electric magnitudes at the location the wave is moving across. However, there seems to be two possible scenarios... One scenario, (A), is as follows. Let's assume that at a point x in space and just before the front of a light wave gets there at t0, the magnetic and electric magnitudes are zero or near zero. When the front of the wave gets to x, the magnetic and electric magnitudes will take one particular value each and those two values will no change as the wave moves across point x. I think that's called a stationary wave... In a second scenario, (B), the wave is moving as a block so that the values of the magnetic and electric magnitudes at a point x will keep changing according to which part of the wave is moving across this point. So, in the case of light propagating "freely" through space, in a vacuum, which of the scenarios A and B is the correct one? Or is it something else altogether? Thanks, EB
This is not quite right. Even in a standing wave, the amplitude of the oscillation (amplitude of the electric and magnetic field vectors in the case of EM radiation) changes with time. However in standing wave the wavecrests shrink and grow where they are in space and do not move forward. There is an animation here: https://en.wikipedia.org/wiki/Standing_wave#/media/File:Waventerference.gif The red line is the standing wave. It is the sum of two waves (green and blue), one moving from left to right and one from right to left. You see this effect in water, and it is what is responsible for the sound emitted by a musical instrument tuned to a certain pitch. It is also what one has inside a laser cavity. You get standing waves when a travelling wave is reflected back and forth between two reflective surfaces (the walls of a harbour, the ends of an organ pipe, the mirrors in a laser cavity, etc). The simpler case is a travelling wave, in which the wavecrests move. If light is propagating through space, (rather than being reflected back on itself in a laser), this is what happens. The wavecrests (i.e. the points of maximum electric and magnetic field amplitude) move at c, the speed of light in vacuo.
Congratulation for your 7,777th post! Just for me! Thanks. Does this mean that if we imagine the front of the wave, the amplitude of the electric and magnetic field vectors remains constant there? In other word, the change in amplitude at a fixed point is entirely due to the wave moving? In still other words, in an inertial reference frame moving at c with the wave, the wave is static? EB
Technically, that's not possible: you can't have a frame moving at c; it's ill-defined. However, forgoing that technicality: yes. That is what it means for time not to pass: everything is stationary (or: static).
