The conversion of 0 degrees Celsius to 273 Kelvin is based on the Celsius scale's relationship to the Kelvin scale. The Kelvin scale is an absolute temperature scale that starts at absolute zero, the point at which all molecular motion theoretically ceases. In contrast, the Celsius scale is a relative scale based on the properties of water.
The Celsius scale defines the freezing point of water at 0 degrees Celsius and the boiling point of water at 100 degrees Celsius under standard atmospheric conditions. These reference points were chosen for convenience based on the properties of water, which is a common substance in our daily lives.
When converting between Celsius and Kelvin, the Kelvin scale is shifted so that its zero point aligns with absolute zero. Absolute zero is equivalent to -273.15 degrees Celsius. To convert from Celsius to Kelvin, you add 273.15 to the Celsius temperature. Therefore, 0 degrees Celsius corresponds to 273.15 Kelvin.
The reason why Kelvin cannot be set to 0 is because it represents absolute zero, which is the lowest possible temperature in the universe. Absolute zero corresponds to zero Kelvin (0 K), where molecular motion theoretically ceases. Setting Kelvin to 0 would imply a temperature lower than absolute zero, which is not physically possible according to our current understanding of thermodynamics.