third law of thermodynamics
One way to state the third law of thermodynamics states is that. the entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero kelvin.
Daniel Fahrenheit didn't like the idea of having to deal with negative temperature. So, in devising his temperature scale (see Fahrenheit scale), he made sure that he would never meet a negative temperature. He made the zero point of the scale, o°F, the temperature of the coldest substance then known, a freezing mixture of ice and salt.
But this advantage in the Fahrenheit scale has long since disappeared. Methods of reaching far lower temperatures have been developed – these are negative temperatures on the Fahrenheit scale. One method involves cooling a gas, compressing it, and then allowing it to expand suddenly. Under these conditions, many gases cool a great deal more. Still lower temperatures can be reached with some solids by first cooling them while they are in a strong magnetic field. When the field is removed, the temperature drops still further. for more on the topic of reaching extremely low temperature, see the article on cryogenics.
There is, however, a very definite limit to the cooling possible with these processes. There is a natural zero of temperature, called absolute zero, which is the lowest possible temperature in theory. While Fahrenheit's 'coldest possible temperature' didn't stay the coldest temperature possible for long, there is no likelihood of ever making anything colder than absolute zero.
To scientists watching the behavior of gases as they were cooled, it became obvious that there must be an absolute zero. For the cooled gases contracted. The rate of contraction was measured, and it was worked out that if the cooling could have continued down to -460°F (-273° on the Celsius scale) the volume of the gas would have shriveled to nothing. Obviously it would be impossible to make any gas colder than absolute zero, where it would obvious less than no space!
While it was recognized as impossible to reach any temperature lower than absolute zero, the question still remained: was it even possible to reach absolute zero itself? And although scientists are now within a few millionths of a degree of reaching it, it is known that they will never actually get there.
This statement is one of the ways of saying the third law of thermodynamics: "It is impossible, no matter how we try to cool something, to cool it down to absolute zero".