How does the third law of thermodynamics define absolute zero?

The third law of thermodynamics defines absolute zero as the temperature at which entropy is zero.

Absolute zero is the temperature at which a system has the lowest possible energy. The third law of thermodynamics states that as the temperature of a system approaches absolute zero, the entropy of the system approaches a minimum value. Entropy is a measure of the disorder or randomness of a system. At absolute zero, the atoms in a system would be perfectly ordered, and there would be no randomness or disorder.

The third law of thermodynamics also implies that it is impossible to reach absolute zero through a finite number of processes. This is because as a system approaches absolute zero, it becomes increasingly difficult to remove the remaining energy from the system. As a result, it would require an infinite number of processes to reach absolute zero.

In practice, scientists have been able to cool systems to within a few billionths of a degree of absolute zero using techniques such as laser cooling and magnetic cooling. These ultra-cold temperatures have allowed scientists to study the behaviour of matter at extremely low energies and have led to advances in fields such as quantum mechanics and superconductivity.