What exactly is a perpendicular Universe?

What would a universe, exactly opposite to our own, be like?

• I've been thinking about this ever since I saw an episode of Red Dwarf (so often the source of deep philosophical questions) wherein they entered what purported to be just such a universe. Of course, what little differences there were were only for (weak) comic effect. What would a universe where every physical and natural law was the opposite or inverse of our own be like? And, if this meant that their definition of opposition was also opposite, would it actually be an exact replica of our own universe?

• Answer:

  I do not believe that such a thing can be defined. For some things it might be possible to change the sign so gravity may cause repulsion rather than attraction, but is that really the opposite of gravity?  Would entropy be reversed? Would the speed of light be reversed or would it be zero or would it be the speed of dark or would there need to be some opposite to the idea of speed? Even a simple question like what is the opposite of an egg can be nearly impossible to answer.