Theoretically, absolute zero is the coldest temperature that is possible anywhere in the universe. It is the basis for the Kelvin scale, one of the three temperature scales used in everyday physics and life. Absolute zero corresponds to 0 degrees Kelvin, written as 0 K, which is equivalent to -273.15° Celsius (or centigrade) and -459.67° Fahrenheit. The Kelvin scale includes neither negative numbers nor degree symbols.

Temperature itself is a measure of the motion of particles, and at absolute zero, all particles in nature have minimal vibration-associated motion, with a minuscule level of motion at the quantum-mechanical level. Scientists have come tantalizingly close to reaching absolute zero in laboratory conditions but have never achieved it.

The melting (or freezing) point of water and the boiling point of water are defined as 0 and 100 on the Celsius scale, also known as the centigrade scale. The Fahrenheit scale was not determined with such natural conveniences in mind, and the melting and boiling points of water correspond to 32° F and 212° F respectively.

The Celsius and Kelvin scales have the same unit of measure; that is, every one-degree rise in Kelvin temperature corresponds to a one-degree rise in Celsius temperature, although they are offset by 273.15 degrees.

To convert between Fahrenheit and Celsius, use:

The feasibility of reaching absolute zero in scientific experiments is limited by the fact that the closer to absolute zero a scientist gets, the more difficult it is to remove any remaining heat from the system – intervening on the few remaining atomic collisions is virtually impossible. In 1994, the National Institute of Standards and Technology in Boulder, Colorado, achieved a record low temperature of 700 nK, or 700 billionths of a degree, and in 2003, researchers at the Massachusetts Institute of Technology lowered this to 450 pK or 0.45 nK.

Under normal, everyday temperature limitations, many physical and chemical reactions slow noticeably. Think of starting your car on a cold winter morning compared to the same task on a cool autumn day, or about how much faster the reactions in your own body become when you heat up by exercising.

The European Space Agency’s Planck observatory, launched into space in 2009, included instruments that were frozen to 0.1 Kelvin, an adjustment needed to prevent microwave radiation from clouding the onboard satellite camera’s vision. This was achieved after launch in four steps, some of which involved circulating preparations of hydrogen and helium.

In 2013, a unique approach to lowering temperature allowed researchers at the Ludwig-Maximilian University of Munich in Germany to force a small number of atoms into an arrangement that appeared to not only reach absolute zero but go below it. They used magnets and lasers to move a cluster of 100,000 potassium atoms into a state with a negative temperature on the absolute scale.