Why Does Time Seem to Go in One Direction?

Physics laws say it doesn't have to

Scientists and philosophers agree and define that time requires change. Time may simply be an innate abstract way for human brains to understand and mark change. If there is no change there is no time. If it is only now, there is no time. It is when now becomes the past and there is a new now that there is the change that defines time.

Most of physics’s deep laws and equations do not require a time direction. On paper, the equations can go both directions, left or right, forward or backward. This is a reason why some say that backward time travel is theoretically possible. However, if we look at the world we live in, time appears to flow in one direction: from the past to the present to the future. This flow is called the arrow of time. 

So where does this arrow of time come from? The answer is entropy or the second law of thermodynamics.

The second law of thermodynamics says that entropy, a measure of disorder in the universe, increases in any closed system. The things in the universe move from order (or low entropy) to disorder (high entropy).

If you pour milk into coffee, the milk spreads throughout the coffee. If you light a fire, the smoke spreads throughout the air. It never goes the other way. The smell of coffee or smoke spreads throughout the room and home, never the reverse. A vase falls to the cement and breaks into many pieces, and never the reverse. People, other animals, and plants getting old, decaying and dying are examples of entropy. No one grows old to young.

The Big Bang and the subsequent expansion of the universe is an example of entropy. The universe started as an infinitesimally small dot 13.8 billion years ago and has been expanding since. The universe and things in it are expanding towards an eventual perfect equilibrium where everything, including atoms, will be equally spread out. With this equilibrium, there will be no movement and, thus, there will be no time. Until then, change from order to disorder is the basis for our time and for the arrow of time. 

Resolving the Seeming Contradiction of the Symmetry Of Physics Laws Versus that Time Goes in Only One Direction

As noted, the physics equations are symmetrical and have no direction of time, but time in our universe moves in only one direction. This seems to be a conflict. The great mathematician and logician Kurt Godel had trouble with the symmetry of physics laws because the symmetry on paper didn't match up with the common sense that you can't go back in time. He concluded that science's definition of time was "unreal." 

However, there isn’t a contradiction. The physics equations are in general for all situations, but we live in a particular universe in a particular state. If we were living in another universe of perfect equilibrium there would be no time. There could be a universe or our future universe that is contracting, or moving from high entropy to low entropy. This would mean that time would be moving in the opposite direction.

In fact, this is likely to happen. When the universe is in perfect equilibrium in the distant, beyond our lifetimes future, things may go from this high entropy back to low entropy. The arrow of time would then go the other way.

British physicist Julian Barbour suggests that on the other side of the Big Bang there could be a universe that expands and time flows in the opposite direction. However, that universe would be beyond our universe, and this theory can never be empirically tested.