See this post on Puzzling SE if you still want to answer.
"Each mirror is a world of its own", Joe explains.
"Joe, that's bullshit! Mirrors are just mirrors!" Rita proclaims.
Joe proceeds to lay out the following Laws of Mirror Universes:
- For each one orientation of a mirror (rotated spherically or in 3D space) there is exactly one universe perfectly symmetrical to our own.
- There are infinite orientations possible in a sphere, so there are infinite universes parallel to our own
- Every universe began at the same time and progressed in the exact same way, with the exception of being "flipped" as we observe in a reflection. That means we are nothing special.
- The line of symmetry in each case is defined by the mirror itself.
- Changing the orientation of a mirror links to another exact replica of that mirror universe - they are all congruent to each other through transitivity
- We cannot exchange matter into these universes because for every one particle that attempts to cross the threshhold, exactly one congruent particle meets it at the barrier.
- "Weak Reflectional Attraction" accounts for substances "stuck" to each other across the threshhold. This is why mirrors can be dirty
- the boundary has a slight pull that makes dirt, etc stay on the plane of the mirror instead of falling as if there was simply air.
- Warped boundaries such as funhouse mirrors or reflections in water serve only as windows; they must be flat and rigid to allow actual interaction.
- The medium of the mirror (metal, etc) moderates heat transfer, which is why you don't feel immediate warmth when you touch a mirror with your hand.
"Joe," says Rita, "Your 'theory' doesn't account for this ."
Rita picks up a household object and demonstrates why mirrors are simple reflections of light, and nothing more.
Her demonstration produces results that would be different if there were multiple, congruent universes bound by mirrors (for example hitting the mirror with something that should have gone through if there were actually an alt. univ. on the other side)