The second law of thermodynamics explains what the first law can’t, that what is in theory reversible, in practice isn’t. For example, running a video of the earth orbiting the sun looks much the same, but playing a video of an egg breaking in reverse evokes laughter. But by the first law, the events of egg breaking are just as reversible as the earth’s orbit, so why doesn’t it happen? The second law provides the answer that disorder always increases or stays constant, so eggs can break but not unbreak (Figure 5.17).
Entropy is the concept that physics uses to describe disorder, randomness, or uncertainty. Boltzmann defined it as the number of possible microscopic states of molecules that can produce a macroscopic state, so that definition is used here. For example, when a colored gas injected into an empty bottle spreads, entropy is said to increase. Initially, the gas molecules are concentrated at a point, so its entropy is the number of molecule combinations that allow that. However a lot more molecule combinations allow the spread-out state, so entropy increases as the gas spreads. The gas then spreads because more micro-state combinations support the spread-out state.
In general, entropy increases because it is more probable, but while gas injected into a bottle will probably spread over time, its molecules could by chance all move back to a point. This is unlikely but possible, so the second law is a statistical law, based on probability, not a causal law based on a force. Objects don’t have to become more disordered, but in a constantly changing world they probably will. Disorder prevails because it is probable, and it is probable because our world is constantly changing, just as constantly shaking a bottle makes its contents disperse.
Heraclitus compared life to a river that constantly changes from one moment to the next. This constant flux arises from the quantum law of all action (3.6.3), that quantum events explore every option so everything is always changing. The second law of thermodynamics then derives from the first law of quantum of quantum theory, so it is a universal law. How then does order arise?