The Second Law of Thermodynamics is considered a fundamental law because it describes a fundamental property of nature—entropy, which is a measure of the overall disorder or randomness of a system. While it is true that there can be short-term fluctuations in entropy due to the probabilistic nature of thermodynamic processes, the Second Law states that the entropy of an isolated system tends to increase or, at best, remain constant over time on average.
The probabilistic nature of thermodynamics arises from the statistical behavior of large numbers of particles that make up a system. On the microscopic scale, individual particles may occasionally move in ways that seemingly defy the overall trend of increasing entropy. However, when considering a large number of particles, the statistical behavior becomes dominant, and the overall tendency is towards increasing entropy.
The Second Law of Thermodynamics is based on statistical mechanics, which provides a rigorous framework to connect the macroscopic behavior of systems with the microscopic behavior of their constituent particles. Statistical mechanics allows us to describe the average behavior of large ensembles of particles and predict the thermodynamic properties and tendencies of systems.
The Second Law is powerful because it has far-reaching implications and consequences. It implies that natural processes tend to move towards a state of greater disorder and randomness, which is often associated with higher entropy. For example, heat naturally flows from hotter to colder objects, and closed systems tend to reach thermal equilibrium, where the temperature is the same throughout.
The Second Law has been extensively verified through empirical observations and experiments. While short-term fluctuations may occur due to statistical fluctuations, these fluctuations are typically negligible on macroscopic scales and do not contradict the overall trend of increasing entropy over time.
In summary, the Second Law of Thermodynamics is considered a law because it captures a fundamental property of nature—entropy—and describes the statistical tendencies of systems to evolve towards states of higher disorder and randomness over time.