In many textbooks today, you'll find the light clock thought experiment in which on a high speed
spaceship an astronaut observes a vertical light pulse emitted each second that is reflected from a
mirror to a photo detector. The earth-based observed is shown observing the same light pulse as the
ship moves some distance; and the observer measures a much longer time than one second for the
travel time of the light pulse.
So, I have a more perceptive than usual student that is troubled by this example. He doesn't
doubt the validity of it, but cannot come to grips with some aspects. My explanations of it seem to be
inadequate, so I thought I would ask this group.
First, he was concerned because the book was showing that the two observers could even see
the light pulse as described in the text. I agreed with him that observers looking from the side would
see no light, since there in none traveling in the direction of their eyes (without some scattering
occurring). Then, he failed to see how the light has a horizontal component, even though the light
source is moving at the speed of the ship. How does light gain the horizontal component of the ship?
In my explanation, the light particles, photons, behave just like any other particle. The source
is moving horizontally as the particles are fired vertically. Thus the photons have both horizontal
and vertical components, according to the earth-based observer. To the astronaut in side the
ship, the photons only have a vertical component since the source and receiver are not moving
relative to him.