Celsius vs Kelvin: Cold Hard Numbers

When the European Space Agency cooled the Planck spacecraft down to just 0.1, scientists weren’t talking Celsius-or even negative 273-because that is the border where normal heat ends. Celsius starts at the freezing point of water, marked zero degrees, but it goes below zero, down into the negatives. The Kelvin scale starts at absolute zero-nothing colder exists. No negatives, ever. On Celsius, water boils at 100, freezes at zero. On Kelvin, water freezes at 273.15, boils at 373.15. You add 273.15 to Celsius to get Kelvin, so 25 degrees Celsius is 298.15 Kelvin. Scientists prefer Kelvin because it never dips below zero and fits perfectly for measuring energy. Remember: Celsius is about water, Kelvin is about the universe. In 2009, Planck’s 0.1 meant just above absolute zero.