The second law of thermodynamics explains what the first law can’t, why events that are in theory reversible, in practice might not be. For example, running a video of the earth orbiting the sun looks the same to us, 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, but common sense tells us it doesn’t happen. The second law states that the disorder of every isolated system increases or stays constant, so eggs can break but not unbreak.
Entropy is the concept physics uses to describe disorder, randomness, or uncertainty. Boltzmann defined it as the number of possible microscopic states, of atoms or molecules, that produce a macroscopic state, and that definition is used here. For example, when a colored gas injected into an empty bottle spreads, entropy is said to increase. Initially, when the gas molecules are concentrated, entropy is low because the number of molecule combinations that allow it is low. Of all the molecule combinations, far more support the spread-out state, so it has a higher entropy. The gas then spreads to increase entropy because more micro-state combinations support it.
In general, entropy increases as disorder increases (Figure 5.18). Yet while the gas in the bottle will probably spread over time, its molecules could all by chance move back to the top, but that is unlikely. The second law is 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 is then universal because our world is like a bottle being constantly shaken, so its contents tend to disperse. As Heraclitus observed thousands of years ago, our world is a constant flux that always changes from one moment to the next. The quantum principle behind the Heraclitean flux is the quantum law of all action (3.6.3), that quantum reality always tries every option, so this law underlies the second law of thermodynamics.