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. 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.
Entropy is the concept that in physics describes disorder, randomness, or uncertainty. Boltzmann defined it as the number of possible microscopic states of atoms or molecules that 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 entropy is low because only a few molecular combinations allow it. Later, after the gas spreads, a lot more the molecular combinations allow that state, so entropy is higher. The gas then spreads because more micro-state combinations allow it, so entropy increases (Figure 5.18).
In general, entropy increases as disorder increases, but while a gas injected into a bottle will probably spread over time, its molecules could by chance all move back to a point, although that is unlikely. The second law is then based probability, so it’s a statistical law not a causal law. It isn’t that objects must become more disordered, but that in a constantly changing world, they probably will. Disorder therefore prevails because it is more probable.
The second law applies because our world is like a bottle constantly shaken to disperse its contents. As Heraclitus observed thousands of years ago, reality is a flux, like a flowing river that is never the same from one moment to the next. The principle behind this flux is the first law of quantum reality (3.6.3), that it always tries every option. The second law of thermodynamics then is universal, because it is based on the quantum law of all action.