Why time can’t move backwards; Physicists Explain

We can’t avoid the passing of time. Time is what keeps things from happening at once. Time always tick forward, but why not backwards? Why do we remember the past and not the future? In a recent study published in the Journal Physical review letters, a group of physicists re-investigate the arrow of time, which is a concept that describes the relentless forward march of time.

Even though time is a fundamental part of our experience, the basic laws of physics seem not to care in which direction it goes. For example, the rules that govern the orbits of planet works the same whether you go forward or backwards. So what distinguishes the past from the future?

Time described by past hypothesis, which assumes that any given system begins in a low entropy state and then, driven by thermodynamics, its entropy increases. The past is low entropy and the future is high entropy, a concept known as thermodynamic time asymmetry.

If this is applied on the universal scale, it is presumed that Big Bang spawned the universe in low entropy state, over time as the universe expanded and cooled, the entropy of this large scale system as increased. Therefore as the hypothesis goes, time is intrinsically linked with degree of entropy, or disorder in the universe. But there are several problems with this idea.

Just after the Big Bang, several evidence point to a Big Bang environment that was hot and extremely disordered mess of primordial particles. As the universe matured and cooled, gravity took over and made the universe ordered and more complex- from cooling clouds of gas, stars formed and planets evolved from gravitational collapse, eventually, organic chemistry became possible giving rise to life and humans that now write theories concerning time and space. On universal scale the disorder has decreased and not increased as the hypothesis presumes.  

From the Big Bang, the universe started in its lowest complexity state. Then as the universe cooled to a state that gravity began to take over, gases clamped together, stars formed and galaxies evolved. The universe became inexorably more complex, the gravity is the driving force of this increased complexity.

As the Universe matures, the subsystems become isolated enough so that other forces set up conditions for the arrow of time to dominate in low entropy systems. Universal scale, our perception of time is driven by continuous growth of complexity but in these subsystems, entropy dominates.

It’s just fascinating for us to think that the reason we remember yesterday and not tomorrow is because of conditions near the Big Bang. When an ice cube in a glass melts and dilutes your lemonade, for instance, entropy increases. When you scramble an egg, entropy increases. Both of these are irreversible: you can’t freeze water ice cube out of the lemonade or unscramble the egg. The sequence of events, hence time only moves in one direction.

One major limitation is that it is based solely on classical physics, ignoring quantum mechanics. Nor does it include Einstein theory of general relativity. There is no dark energy or anything else that’s needed to more accurately model the universe.