You are in a room with a barrier in the middle. This is not a physical barrier; it is a dividing line of 2 mirror images. When you look across the room, it appears to be a mirror because you see a mirror image of yourself and other objects in the room, but the mirror is not made of any substance. It is simply a point where anything past it is an exact mirror of all things on the other side. For example, if you were to walk forwards, you would not be able to cross the dividing line because you would walk into yourself walking forwards. If you were to try to punch your mirror image, you would punch it in the fist while they simultaneously punch you in the exact same spot, probably resulting in a broken hand for both you and your mirror image. If you were to try to push any object through the mirror line, it would collide with its mirror object and would not be able to pass. Is it possible for you or any other physical matter to cross the mirror line (without introducing "magical" phenomenons such as teleportation)?
A black hole (the two would collide and crash into each other making one singularity).
An electron (doesn't travel linearly so it wouldn't necessarily collide with itself).
Lots of different objects would be created from two mirrored particle accelerators colliding their payload. These created objects need not be spawned with a twin (so they would bypass the mirror).
If i would go to near the barrier, and raise my hand (not all the way through, just as to be vertical to my torso), then my reflection would do the same. Then if i catch my reflections hands and side step to the right, then my reflection would do the same but his right would be my left then I would enter from the right and he would do the same from the left.Side stepping in a circle whose center is the point where our hands are caught.
It is not a physical object, but one thing that can pass through the mirror is a (perfect harmonic?) wave.
This is due to the fact that the wave that coming from 'the other side' is reflected, and as such actually continues the wave from your side.
Of course this effect goes both ways.