The core of the second law of thermodynamics, describing the spontaneous process of systems evolving from order to disorder, which forms the physical basis for the arrow of time.
📖 Standard Introduction
Entropy is a physical quantity that measures the degree of disorder in a system. The second law of thermodynamics states: In an isolated system, entropy never decreases; it always tends to increase until reaching a maximum value (thermal equilibrium).
ΔS ≥ 0 (Isolated System)
This means natural processes have directionality: heat spontaneously flows from high temperature to low temperature, gases spontaneously diffuse, and order spontaneously becomes disorder.
💬 Popular Explanation
Imagine your room: without cleaning, it becomes messier and messier (entropy increases), but it never tidies itself. A broken cup never reassembles. This is entropy increase — nature tends toward disorder.
Everyday Examples:
- Ice melting: ordered crystals → disordered liquid
- Perfume spreading: concentrated → dispersed
- Hot coffee cooling: temperature difference disappears, reaching thermal equilibrium
🎮 Entropy Increase Simulator: Gas Diffusion
Red particles (high temperature) and blue particles (low temperature) are initially separated by a partition. Observe entropy increase after removing the partition.
⏰ Arrow of Time
Entropy increase defines the direction of time: the past has low entropy, the future has high entropy. This is why we remember the past but not the future.
🌌 Fate of the Universe
The universe's entropy is constantly increasing, potentially leading to a "heat death" — where all energy is uniformly distributed and no work can be done.
🧬 Life and Entropy
Life maintains a low-entropy state by consuming energy (e.g., food), but overall increases the entropy of the environment.