Unitary evolution may be the key to understanding time
Benjamin Franklin once quipped in a letter to a friend that “in this world nothing can be said to be certain, except death and taxes.” Perhaps taxes are timeless but death is not so. Rather, it is intimately linked to the direction time flows — the arrow of time. If time flowed backward, I would say that nothing is certain except birth and taxes. (Or maybe “sexat dna htrib”.)
Most people talk about the arrow of time in the context of the 2nd law of thermodynamics, the law that entropy always increases with time. But this “law” is statistical, and it is a classical law based on classical assumptions about physics that we know, in the context of quantum physics, to be false.
People also talk about how all dynamics in the universe are time reversible. That means that if you watch a video run forwards or backwards the path of each and every particle in that video obeys the same laws of physics. It is only the aggregate paths of those particles that gives you an idea of which direction is forward.
But all dynamics in the universe may not be time reversible if this one quantum law is not correct. This is called the law of unitary evolution.
Unitary evolution is at the heart of all questions about time reversibility including the black hole information paradox (the death of which…
