Subject: A Review and Analysis of Four Laws That Drive the Universe by Peter Atkins Theme: How a handful of simple rules dictate the rise and fall of stars, life, and the cosmos itself.
The Third Law is the least intuitive but perhaps the most haunting.
The Statement: The entropy of a perfect crystal at absolute zero (0 Kelvin) is exactly zero. Four Laws That Drive The Universe By Peter Atkins -.PDF-
Atkins’ Translation: You cannot get out of the game. You cannot reach absolute zero.
The Implementation: As you cool a substance, you remove energy and lower its entropy. As you approach absolute zero (-273.15°C), molecular motion stops. However, Atkins explains via statistical mechanics that it would take an infinite number of steps to actually reach zero. You can get infinitely close—nanokelvins in a lab—but you can never cross the finish line. Subject: A Review and Analysis of Four Laws
Why does this drive the universe? Because it sets the boundary conditions. The Third Law ensures that the universe has a finite, calculable baseline. It gives us a reference point for entropy. Without the Third Law, we could only measure changes in entropy, not absolute values. It also hints at the weird quantum behavior of matter at near-zero temperatures, such as Bose-Einstein condensates.
Atkins begins not with the First Law, but with the Zeroth Law. It is a curious entry because it was formulated after the First and Second Laws, yet logically precedes them. Atkins begins not with the First Law, but
Atkins argues that without the Zeroth Law, we could not define "now" in a physical sense. It allows us to use thermometers and gives meaning to the statement "they are at the same temperature." It is the foundational bedrock that allows us to map the thermal landscape of the universe. Without it, the universe would be a chaotic jumble of unrelated energies; with it, we have a stable reference frame.