Absolute zero is the temperature at which atoms and molecules have the least amount of heat possible. The atoms and molecules that make up matter are in constant motion relative to each other. What we think of as heat is actually a measure of the energy of this motion. If all the heat in a substance were removed, its molecular and atomic motion would virtually stop. The substance’s temperature would be considered to be absolute zero. On the three most widely used temperature scales, absolute zero is –273.15 °C, –459.67 °F, or 0 Kelvin.
According to the third law of thermodynamics—the study of various kinds of energy, including heat—it is impossible for a substance to actually reach absolute zero. Even so, scientists have come extremely close. In 2003, researchers at the Massachusetts Institute of Technology (MIT) reached 0.00000000045 K—one-half of one-billionth of a Celsius degree above absolute zero.
Scientists have discovered that many materials have unusual properties near absolute zero. Some materials develop superconductivity, the ability to conduct electric current without resistance. Such materials include many metals, metallic compounds, and ceramics. Superconducting magnets of such materials are used to make magnetic resonance imaging (MRI) machines and powerful particle colliders. MRI is a medical technique used by doctors to reveal internal body parts without surgery.
Near absolute zero, liquid helium develops superfluidity. Superfluidity is the ability to flow without friction. Other materials can become Bose-Einstein condensates. A Bose-Einstein condensate is a cluster of atoms that behaves somewhat like a single atom. Both superfluid helium and Bose-Einstein condensates have been used to study various principles of physics, including gravity, light, and the properties of molecules. Achieving low temperatures and studying their effect on materials is called cryogenics.
