If you want to understand the concept of the reverse universe of Sidis, we have to consider the mirror. It’s an easy point, as it’s mentioned by Sidis himself, to approach this idea. The idea is the following: imagine what is a universe when you are seeing it in a mirror. You can see that there is only one dimension which has changed. So, the idea for him is to do the same thing with time, so he considers the laws of physics and he’s trying to do some tests in this reverse universe.