The mirror paradox
Why does a mirror invert around the vertical axis only?
If you look at yourself in a mirror your left and right sides are inverted. Up and down stays normal, there must surely be something special about the vertical axis.
The paradox lies in that a mirror just reflects incoming light, it has no preferences for a certain axis. Since the mirror does not have any ability to invert around a certain axis; it is in direct conflict with what we observe.
Since the mirror is not the cause of the observed effect then the observer must be the cause. The answer is that evolution has adapted humans very well to gravity. So well that most people have only one physical way of turning themselves to look at a mirror, that is by rotating around the vertical axis. The result is that this translation is considered the normal and only sensible solution. In contrast an acrobat could easily flip around the horizontal axis, stand on his hands, look in the mirror and conclude that a mirror turns everything upside down but leaves right and left where they should be.
If you still have problems resolving the paradox it is most likely because you use yourself as a reference point and confuse yourself as you imagine the rotations. You can blame evolution and your parents for your inclination to view yourself as the centre of the universe. An additional difficulty comes from the fact that you perceive yourself from the inside but look at your outside in the mirror.
In that case print this sign on a piece of paper and stand in front of a mirror with the text facing you. Then rotate it around the vertical axis so it faces the mirror. Left and right has changed place. Now try it again but this time rotate it around the horizontal axis. This time up and down has changed place. The paradox has disappeared, it was all in your head.
