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This is the fourth part of my "Time and Entropy" series. Why doesn't time run backwards? If you have missed the former ones, don't miss to read them, which are about

• The size of Time
• The problem with clocks. A circular argument?
• The direction of time
  
  
  Enjoy!

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Part 4: Entropy and Time

That thing with "chaos"

What happens in a naturally elapsing, irreversible process is measured with a particular size, the entropy, for which there are two different approaches. On one side you can look at the single components of a system. In this case, entropy is a size for the number of realization options of a specific state.

Particles on the tramp

What we have here as an example is a container which is segmented with a removable wall. Each side of the divider contains water, but on one side the water is stirred with paint. If the divider is removed, the water masses start to mix and after a while the amount of paint is the same in both parts of the container.

For every colour-particle the possibility to be either in the left or the right side is the same. The former state, where every colour-particle was in only one side, will practically almost never happen again. The reason is, that there are a lot of realization possibilities for containing the same amount of colour-particles in the left and in the right side, but only one, where all particles are in the left side like at the beginning.

When a system changes from a state of low realization possibilities into a state of more possibilities, the entropy is growing.

Order becomes Chaos

In the vast majority of processes, entropy is growing. They are irreversible. One simply could say, that in a closed system, chaos can never decrease and almost always increases. In our example: Every colour-particle on one side conforms to the ordered state. If the colour-particles spread on both sides, chaos has increased. Try to apply this thought on the example with the billiard stroke from part 3 of this series.

Now, the growing of entropy shows in which direction a system is able to develop, therefor in which direction time is advancing. My favourite formulation to better illustrate entropy growth is:

A bicycle always gets dirty without help, but never clean without it.

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Stay tuned for my next article, which will be about Heat Engines and Perpetuum Mobile.