The amplituhedron, or a similar geometric object, could help by removing two deeply rooted principles of physics: locality and unitarity.
...And unitarity holds that the probabilities of all possible outcomes of a quantum mechanical interaction must add up to one.
I'm probably being very naive attempting to understand this article that has probably already been massively dumbed down, but, how can the probabilities of all possible outcomes of an interaction not add up to one? Surely they add up to one by definition, otherwise they are not probabilities? For example outcome X having a probability of 1/3 means, on average, you can multiply the number of times you observe the interaction by 1/3 and get the expected number of times you would see outcome X. If the probabilities in your statistical trials didn't add up to 1, doesn't that mean adding up the numbers of individual outcomes observed would give a number bigger (or smaller) than the total number of interactions observed? Obviously it cannot mean that, as that fails basic arithmetic.
I can imagine tossing a fair coin - heads has probability 0.5, tails 0.5, total 1. So now how about a 3 sided coin without unitarity? Let's say the probability of heads is still 0.5, tails 0.5 but it has a third side, bodies that also has probability 0.5 of occurring. That sounds mathematically impossible. It could be a mind-reading coin, where you pick heads and find that then occurs on half your coin tosses. Later you pick tails, and that occurs on half your coin tosses, but when you pick bodies, that also occurs on half of those coin tosses. OK, I give up! Can anyone who really understands unitarity enlighten me please? Is this anything like the uncertainty principle?