Absolute zero is the lowest theoretically attainable temperature, denoted as 0 Kelvin (K) or -273.15 degrees Celsius. It is the point at which the particles of a substance have minimal thermal energy, meaning they possess the least possible kinetic energy. At absolute zero, all molecular motion theoretically ceases, and the substance reaches its ground state.
According to the laws of thermodynamics, specifically the third law of thermodynamics, it is impossible to reach absolute zero through a finite number of steps in any cooling process. This law states that as a system approaches absolute zero, the entropy (a measure of disorder) of the system tends to a minimum value, which is zero for a perfect crystal at absolute zero. Achieving absolute zero would require removing all residual energy from the system, which is not practically achievable.
However, scientists have been able to cool certain substances, such as gases and dilute atomic gases called Bose-Einstein condensates, to temperatures extremely close to absolute zero. By using techniques such as laser cooling and evaporative cooling, temperatures on the order of billionths of a Kelvin have been reached. These ultracold temperatures have led to significant advancements in various fields of research, such as quantum physics and condensed matter physics. Nonetheless, absolute zero itself remains an unattainable theoretical limit.