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. Yet by the first law, the events of egg breaking are as reversible as the earth’s orbit, so why doesn’t it happen? The second law answer is that order never increases in a closed system, so eggs can’t unbreak (Figure 5.17).
Entropy is how physics describes disorder or randomness, and Boltzmann defined it as the number of possible microscopic states that can produce a macroscopic state. By this definition, when a colored gas injected into an empty bottle spreads, entropy increases. The gas molecules begin concentrated at a point, but not many molecule combinations allow that, so entropy is low. Many more molecule combinations support the dispersed state, so it spreads out to increase entropy. Entropy then always increases or stays constant because disorder is more likely (Figure 5.18).
In general, entropy increases because it is more probable, so gas injected into a bottle will probably spread over time, but 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 increases for the same reason that constantly shaking a bottle disperses its contents. Heraclitus compared our world to a river that is never the same from one moment to the next, as it is always changing. This Heraclitean flux arises from the quantum law of all action (3.6.3), that anything that can happen eventually does because quantum events explore every possibility, so nothing is static. The second law of thermodynamics then derives from the first law of quantum theory, so it is a universal law, but how then does order arise?