Physics says that there is no preferred direction in time any more than there is in any of the spacial dimensions.
My discussion of momentum as the fundamental element of existence leads to an understanding of why we experience time in only one direction. Momentum. Momentum requires motion, which requires mass, space, and time in a particular direction. Momentum is a vector.
It may not matter which direction time chooses, but existence presupposes a direction. Once we’re on that road, momentum keeps us on it. According to my earlier argument that momentum is fundamental, time can’t exist without the other components of momentum—without existence. So time has to exist with a preferred direction.
As I concluded before, asking about the beginning of existence has no meaning in our universe, so asking “why this direction of time?” has no meaning either.
What we experience as entropy is the inertial force (not really a force of course) of that choice. Gathering all the pieces of broken or rundown or dispersed things back into their original state throughout the universe illustrates the amount of effort it would take to choose the other direction now. Our universe has a temporal momentum, proportional to the rate of change of its entropy.
It’s tempting to speculate that another universe might exist opposite to ours with time in the other direction, the absolute value of its temporal momentum equal to that of ours. “Lead us not into temptation…” That speculation depends on prejudices about beginnings and about physics that have no objective validity outside our universe.
August 16, 2016