The lowest possible temperature that can be physically achieved is called absolute zero. Using the third law of thermodynamics, entropy or the disorder in a substance reaches zero at absolute zero. It is also the temperature at which the volume of gases becomes zero. Absolute zero is only hypothetical with no possibility of our reaching it in reality, as no element remains a gas at temperatures as low as absolute zero. Only by using the quantum effects of matter can we reach a temperature that is very close to it.
On the Celsius scale absolute zero is -273 degree and on the Kelvin scale of temperature it is zero Kelvin.
Scientists believe that the lowest possible temperature that can be achieved is -273.15 degrees centigrade. This temperature, which is also called absolute zero, has been established on the basis of observation of relationship between temperature and pressure of a gas. When a given mass of gas contained in a fixed volume, its pressure decreases in direct proportion to its temperature. This phenomenon means that at the absolute zero temperature of all gases mus fall down to zero.
In the kelvin unit of measuring temperature, the absolute zero is taken is zero temperature. It is impossible to reach 0 degrees kelvin temperature. The lowest temperature so far reached in laboratories is of the order of 0.00001 degree kelvin.