Third Law of Thermodynamics
Thermodynamics is the area of physics that involves heat energy and the ways that it pertains to various kinds of energy. Specifically, it explains how thermal energy is modified to and from other types of energy, as well as how heat can affect matter.
The First Law of Thermodynamics describes how the energy of a thermodynamic system can be neither created nor destroyed. The Second Law describes how thermodynamic systems tend to evolve toward chaotic equilibrium. The Third Law of Thermodynamics describes the behavior of thermodynamic systems as they approach the lowest limits of energy.
In essence, the first law suggest that the universe started with a designated amount of usable energy: energy that is capable of doing work or increasing internal temperature. Then, the second law describes the implications of the first law in greater detail. Specifically, the second law describes the transformation of the universe’s limited useful energy into unusable energy; chaos or disorder at the atomic level. Scientists measure unusable energy using a metric called entropy.
According to the laws of thermodynamics, the universe is like a sink filled with dirty dishes. As more dishes are used, they are thrown into the sink and unless and the dishes are cleaned and organized, the disorder continues and increases. While some small-scale organization does occur in the universe when matter is formed, the progression of time is toward a universe that is more and more disordered.
The first two laws elegantly lay out the progressive nature of thermal energy in the universe. The third law is basically included for completeness as it covers situations related to zero entropy. Specifically, the Third Law of Thermodynamics states that as the temperature of a system gets close to absolute zero, the entropy of a system become constant, meaning the change on entropy is zero.
The Implications of Absolute Zero
The third law veers into the purely theoretical because scientists aren’t sure if the temperature of absolute zero (0 degrees Kelvin or -273.15 degrees Celsius) can even be achieved. The lowest measured temperature to date, found in to be in deep space, is 3 degrees Kelvin.
According to the third law, entropy change approaches zero as temperature approaches zero. However, because all matter includes some amount of disorder and some innate amount of heat, it cannot attain absolute zero. In theory, a perfect crystal would have zero disorder, and therefore could theoretically reach both absolute zero and zero entropy.
Theoretical physicists have also held that attaining absolute zero would require an infinite number of steps, meaning it is impossible. According to theory, absolute zero is an impossible temperature because it would mean a heat sink with 100 percent efficiency is possible, and this would imply that a perfectly efficient heat engine. The Second Law of Thermodynamics states that a heat engine cannot be 100 percent efficient.
Physicists have likened efforts to achieve absolute zero in a system to efforts for travel at light speed. Theoretically, it would take an infinite number of steps you achieve light speed: It is always possible to accelerate matter, but you can never achieve the ultimate speed limit of the universe.