What is absolute zero?

If you ever take a balloon out on a cold day, you'll probably notice that the balloon shrinks as it gets colder. That's because there is a link between gas volume and temperature: Given a constant pressure, the volume of a gas decreases as the temperature falls (and your balloon appears to deflate).

But how far will a gas contract? Theoretically, gas can contract all the way to nothingness. Because the volume of a gas decreases with falling temperature, scientists realized that a natural zero-point for temperature could be defined as the temperature at which the volume of a gas theoretically becomes zero. At a temperature of absolute zero, the volume of an ideal gas would be zero.

The idea of absolute zero is theoretical. Scientists speak of an "ideal gas" because any gas would change into a liquid long before reaching absolute zero. Also, no one has ever achieved absolute zero, though they have come within thousandths of a degree of it.

To measure these very cold temperatures, scientists use the Kelvin scale, which is named after British engineer and physicist William Thomson, First Baron Kelvin, who proposed the idea of the scale in 1848. In the Kelvin scale, a 1-Kelvin change is the same as a 1° Celsius change, but 0 K represents absolute zero, which corresponds to –273.15°C.

The Fahrenheit scale, too, has a parallel scale for measuring absolute temperatures, though it is rarely used. In 1859, Scottish engineer John Rankine proposed the scale that is today known as the Rankine scale. In the Rankine scale, a 1°R temperature change is the same as a 1°F change, but 0°R is absolute zero, which, in the Fahrenheit scale, is –459.67°F.