To measure temperature, we use degrees Celsius. It is a scale adapted to our everyday life: under a standard atmospheric pressure, liquid water becomes ice at zero degree, and water boils at a hundred degrees. This choice is quite arbitrary and it cannot explain why there could be an absolute zero.
Physicists prefer using a different scale, that of kelvins.
The temperature of a substance – whether it be solid, liquid or gas - depends on the agitation of the atoms or molecules of that substance. The hotter the substance, the more its atoms vibrate. In a gas for instance, temperature is proportional to the square of the average speed of the particles. The faster the particles move and the more agitated they are, the hotter the gas. The Kelvin scale was made from this particular observation: temperature is calculated depending on the square of the speed or on the vibration of the particles composing the substance that is measured. For instance, the particles of a gas at 200 kelvin have a square average speed twice as high as that of a gas at 100 kelvin.
At zero kelvin, particles stop moving (in spite of small quantum corrections). Physicists call this temperature “absolute zero”, since it cannot get any colder: we do not know how to make an atom move less when it is already motionless…
To change from degrees Celsius to kelvin, you only have to add 273.14 degrees. For instance, a room at 20°C corresponds to 293 K (kelvin) for a physicist. Absolute zero where everything is motionless hence corresponds to -273.14°C, and it is clear that we will never be able to reach a lower temperature. Thermodynamics even proves that we cannot reach absolute zero but only get close to it.
Thanks to the kelvin, we can really “feel” what very low temperatures are. When you learn that K. Onnes improved the low temperature record from -253°C to -272°C, it does not seem very impressive. However, on the Kelvin scale, he went from 20.3 K to 1 K, hence reducing temperature and atom vibrations in matter by twenty times! Similarly, saying that the discovery of cuprates revolutionized the physics of superconductors, since their critical temperature can reach -135°C while the previous record was around -250°C, might seem like an exaggeration. However, in kelvin, it corresponds to an increase from 20 K to 138 K: the temperature where superconductivity appears was hence multiplied by seven!
