# What Is Absolute Zero? Temperature in Kelvin, Celsius, and Fahrenheit

Absolute zero is defined as the temperature at which a cooled ideal gas is in its lowest energy state. In other words, it’s the point at which no more heat can be removed. While boiling point and melting point depend on the nature of a material, absolute zero is the same for all substances. Matter displays unusual properties as it near absolute zero, including superconductivity, superfluidity, and forming the state of matter called a Bose-Einstein condensate.

### Absolute Zero in Kelvin, Celsius, and Fahrenheit

Absolute zero is 0 K, -273.15 °C, or -459.67 °F. Note the Kelvin temperature does not have a degree symbol. This is because the Kelvin scale is an absolute scale, while the Celsius and Fahrenheit scales are relative scales based on the freezing point of water.

### How Absolute Zero Works

One common misconception about absolute zero is that matter stops moving or freezes into place. Theoretically, absolute zero is the lowest possible temperature, but it isn’t the lowest possible enthalpy state. This is because absolute zero is defined for an ideal gas. At very low temperatures, real matter deviates from ideal gas behavior. At absolute zero, matter is in its lowest energy state, but it still has some energy from the vibration of chemical bonds, orbits of electrons, and movements within the atomic nucleus. Lowering a temperature to absolute zero is like when a person slows from running to standing still. Most of the kinetic energy is removed, but a person’s heart beats, lungs inhale and exhale, and there is still potential energy.

### Can We Ever Reach Absolute Zero?

According to the laws of thermodynamics, it is not possible to reach absolute zero only using thermodynamic methods. We can get very, very close to absolute zero, but can’t ever quite reach it, thanks largely to the Heisenberg Uncertainty Principle. For any particle, you can’t know its momentum and exact position. At absolute zero, the momentum is zero. Basically, even if scientists achieve absolute zero, they can’t measure it.

But, we can get very, very close to absolute zero! In 2015, scientists at MIT cooled a mixture of sodium and potassium gaseous atoms down to 450 nanokelvins. Space-based research has the potential to go even further. The Cold Atom Laboratory (CAL) is an experiment designed for the International Space Station that may achieve a temperature as low as 10 picokelvin (10-12 K).

### Coldest Temperature Ever Recorded

It may surprise you to learn the coldest temperatures ever recorded were produced in labs here on Earth. Because of background radiation, deep space isn’t really all that cold (2.73 K). So far, the Boomerang nebula is the coldest place in nature, with a temperature of about 1 K.

### Negative Kelvin Temperature

While we can’t reach absolute zero, in 2013 researchers made a quantum gas of potassium atoms that achieved negative Kelvin temperatures in terms of motion degrees of freedom. Although it’s counter-intuitive, negative temperatures aren’t actually colder than absolute zero. In fact, they might be considered infinitely hotter than a positive temperature.

Below absolute zero, matter displays strange properties. For example, although atoms are attracted to each other and exert negative pressure, the matter doesn’t collapse. Theoretically, a combustion engine operating below absolute zero could have a thermodynamic efficiency greater than 100%.

### References

• Arora, C. P. (2001). Thermodynamics. Tata McGraw-Hill. ISBN 978-0-07-462014-4.
• Medley, Patrick, et al. (May 2011). “Spin Gradient Demagnetization Cooling of Ultracold Atoms.” Physical Review Letters. 106. doi.org/10.1103/PhysRevLett.106.195301
• Merali, Zeeya (2013). “Quantum Gas Goes Below Absolute Zero.” Nature. doi:10.1038/nature.2013.12146