The very definition of absolute zero says that 0 K (i.e. −273.15 °C) cannot be achieved. It is one example of the limitations that nature gives us. Why this is so (that is, why nature limits us in this way) is more of a philosophical question but it can be shown in several cases that reaching absolute zero would be at least strange. Even stranger would be negative temperatures (in Kelvin).

We know that bodies shrink during cooling. It is best seen on gases. If we cool a gas, its volume will decrease. If we reached 0 K, its volume would be zero. That is certainly not possible. The gas pressure would behave similarly — if we cool a gas, the gas pressure decreases. And at 0 K, the pressure would be zero. That is also impossible. Likewise, as the temperature decreases, we would observe a slowing down of the chaotic movement of atoms and molecules and at a temperature of 0 K, this movement would stop completely.

The impossibility of reaching absolute zero is a separate law of thermodynamics and it is related to heat engines. E.g. a car’s engine (like any other machine that converts heat energy) must have a radiator. The efficiency of the engine depends on the temperature of the heat source and the temperature of the radiator (the radiator is therefore in the car not due to reducing the high temperature of engine parts but due to the very principle of the operation of the engine). And at 0 K, such a machine would have an efficiency of 100%, which is not possible (and if the radiator had a temperature below absolute zero, the machine would even have an efficiency higher than 100%).

There are therefore more arguments why absolute zero cannot be reached but it should be noted that (theoretically) an arbitrarily low temperature can be achieved but not exactly 0 K. If we are not mistaken, the lowest temperature reached is 0.000 000 000 1 K.