Absolute zero, denoted as 0 Kelvin (0 K), is the lowest possible temperature that theoretically represents the complete absence of thermal energy. It is equivalent to -273.15 degrees Celsius (°C). To measure absolute zero in degrees Celsius, scientists use the Celsius temperature scale and subtract 273.15 from the temperature value.
Reaching absolute zero is extremely challenging because it requires removing all thermal energy from a system, which is practically impossible. However, scientists have been able to approach very close to absolute zero in laboratory settings using various techniques.
The currently achieved lowest temperatures are achieved using a method called laser cooling and evaporative cooling. Laser cooling involves using lasers to slow down and cool a collection of atoms. Further cooling is achieved through evaporative cooling, which involves removing the hottest atoms from a gas using controlled evaporative processes.
In 1995, a research team at the Massachusetts Institute of Technology (MIT) cooled a sodium-potassium (NaK) gas to a temperature of 700 nanokelvins (nK), which is just 0.0000007 Kelvin above absolute zero. This is currently the closest temperature achieved to absolute zero.
While reaching absolute zero remains a theoretical limit, scientists continue to push the boundaries of low-temperature physics, and ongoing research is focused on achieving even lower temperatures and studying the unique properties of matter at these extreme conditions.