What is absolute zero and why is it important?

Answer 1

Absolute zero is the temperature of outer space. It is the lowest temperature matter can reach. It is -273.15 on the Celsius scale and 0.0 on the Kelvin scale. There is still a type of energy, known as zero point energy, the energy of outer space, left at 0.0 Kelvin, but this energy cannot be used. Work cannot be obtained from things at 0.0 Kelvin, because there is no lower temperature to which the energy might flow. The entropy of anything at 0.0 Kelvin is maximum, meaning no work can be obtained from it. (Entropy is reverse of what we are perhaps used to: minimum means lots of work can be extracted which causes the entropy to INCREASE until the entropy is maximum and no further energy can be extracted..)

There are laws of physics which only agree with reality when the Kelvin scale is used. For example, the ideal gas law is PV = NRT. The pressure times the volume of an ideal gas is the product of the number of moles of gas (N) times a constant (R) times the temperature. This equation only works of the temperature is in Kelvin (the units of the Kelvin scale are Kelvin, not degree).

The equation for the power radiated from a hot object only works if the temperature is Kelvin. In fact, much of physics only works when you use the Kelvin scale for temperature.