The entropy of a system is a measure of that system's disorder. Why is it appropriate to describe a laser as an entropy-reducing device?
I take some issue with the first sentence. It is better, in my view, to describe entropy as a measure of chaos. As chaos in a system increases, the entropy is a measure of that. If the chaos decreases, entropy decreases. Now in a laser, what is happening to the molecules in the laser as it lases? Is it becoming more chaotic, or not?
To determine whether a laser is an entropy-reducing device, we need to understand the behavior of the molecules in the laser during lasing.
In a laser, the active medium (often a gas or solid) is excited to a higher energy state, typically through the application of an external energy source. This excitation is achieved by inducing a population inversion, where more atoms or molecules are in the higher energy state than in the lower energy state.
Once the active medium is excited, it undergoes stimulated emission. This means that when an excited atom or molecule is stimulated by an incoming photon, it emits a new photon that is identical in terms of phase, frequency, and direction to the stimulating photon. These emitted photons can then induce further emissions, resulting in an amplification of the light.
The key point in understanding why a laser can be seen as an entropy-reducing device lies in the process of stimulated emission. When a photon stimulates the emission of another photon, it causes the original photon to cease to exist. In other words, energy is transferred from one photon to another, leading to a reduction in the number of available energy states. As a result, the system becomes more ordered or less chaotic as the photons align their phases and propagate in a coordinated manner.
This reduction in chaos is reflected in the decrease in entropy. Entropy is a measure of the number of possible microstates (arrangements of particles or energy states) that a system can have while still maintaining the same macrostate (observable properties). In the case of stimulated emission in a laser, the photons become more coordinated, leading to a decrease in the number of possible microstates and, therefore, a decrease in entropy.
Thus, it is appropriate to describe a laser as an entropy-reducing device due to the organized and coordinated behavior of the photons involved in the lasing action.