A jarring divide cleaves modern physics. On one side lies quantum theory, which portrays subatomic particles as probabilistic waves. On the other lies general relativity, Einstein's theory that space and time can bend, causing gravity. For 90 years, physicists have sought a reconciliation, a more fundamental description of reality that encompasses both quantum mechanics and gravity. But the quest has run up against thorny paradoxes.
Hints are mounting that at least part of the problem lies with a principle at the center of quantum mechanics, an assumption about how the world works that seems so obvious it is barely worth stating, much less questioning.
Unitarity, as the principle is called, says that something always happens. When particles interact, the probability of all possible outcomes must sum to 100 percent. Unitarity severely limits how atoms and subatomic particles might evolve from moment to moment. It also ensures that change is a two-way street: Any imaginable event at the quantum scale can be undone, at least on paper. These requirements have long guided physicists as they derive valid quantum formulas. "It’s a very restrictive condition, even though it might seem a little bit trivial at first glance," said Yonatan Kahn, an assistant professor at the University of Illinois.
Please select this link to read the complete article from WIRED.