In physics, cryogenics is the study of the production and behaviour of materials at very low temperatures (below −150 °C, −238 °F or 123 K). A person who studies elements that have been subjected to extremely cold temperatures is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit, cryogenicists use the absolute temperature scales. These are Kelvin (SI units) or Rankine scale (Imperial units). The term cryogenics is often mistakenly used in fiction and popular culture to refer to the very different cryonics.