Arrow of Time

After reading "A Brief History of Time" by Stephen Hawking, I finally think I have an idea of what "time" is as a concept, and best of all, how to visualize it.

I used to think time could only go in one direction. It moved "forward", so to speak, but not "backwards". The relative directions was determined by what Hawking calls "psychological time", something that we perceive in our minds.

However, in reality, the laws of physics at the microscopic time level are "symmetric", that is to say, if time goes backwards or forwards, the equations that describe physical processes are the same.

What then determines the direction of time? Well, the clue comes from the Second Law of Thermodynamics (SLOT), which states that all work tends towards the production of greater entropy over time. Entropy, so to speak, is the concept of disorder. Hence, the SLOT tells us that as time progresses (i.e. goes forwards), the amount of disorder in a system increases. It's much like how we neglect our study tables as time progresses, resulting in greater mess on it.

Well, equipped with this knowledge of entropy and time, we can then define time as pointing in one particular direction - forwards. It is because the SLOT doesn't permit entropy to increase spontaneously; it only permits entropy to decrease. And since time progresses in the direction that entropy decreases, it can only point forward.

It is much like an explosion. As time progresses forward, we see a bomb progress from being a small ball full of chemical energy to a smattering mass of pieces and lots of thermal energy. Disorder increases as time progresses. That's how time progresses in the macroscopic world.


That is the arrow of time.