Liquid nitrogen, a substance commonly encountered in science demonstrations and for practical applications like freezing warts, maintains a temperature around -196°C (-321°F; 77 K) at its boiling point. This cryogenic liquid serves as a familiar benchmark for extreme cold. While impressive, it’s just one point on the vast spectrum of low temperatures. Exploring what lies beyond reveals fascinating concepts and advanced scientific endeavors.
The Ultimate Cold: Absolute Zero
The theoretical limit of cold is known as absolute zero, corresponding to 0 Kelvin (K), which is equivalent to -273.15°C or -459.67°F. At this temperature, the atoms and molecules within a system would theoretically cease all thermal motion. The Kelvin scale, unlike Celsius or Fahrenheit, begins at this fundamental lower limit, making it an absolute temperature scale. While absolute zero is a theoretical ideal that cannot be fully reached in practice, scientists continually approach it.
Even at temperatures approaching absolute zero, particles still exhibit minimal motion due to quantum effects, known as zero-point energy. The Kelvin scale’s units are equivalent to Celsius degrees. This absolute scale is useful for thermodynamic calculations, as it directly relates temperature to the average kinetic energy of atoms and molecules.
Colder Cryogenic Liquids
Beyond liquid nitrogen, other substances exist in liquid form at even lower temperatures. Liquid hydrogen, for instance, boils at approximately -252.87°C (20.28 K) at atmospheric pressure. This makes it significantly colder than liquid nitrogen, though it requires more specialized containment.
Even colder still is liquid helium. The common isotope, helium-4, boils at about -269°C (4.2 K) at standard pressure. This liquid exhibits extraordinary properties, such as superfluidity, when cooled below 2.17 K, where it can flow without any friction. The rarer isotope, helium-3, is even colder, boiling at 3.19 K (-270°C). The unique quantum properties of helium isotopes allow them to remain liquid at temperatures where other substances would solidify.
Achieving Extreme Cold: Advanced Techniques
To reach temperatures far below those of liquid cryogens, scientists employ sophisticated methods. Dilution refrigerators use a mixture of helium-3 and helium-4 to achieve continuous cooling down to millikelvin (mK) temperatures, which are thousandths of a Kelvin. This process relies on the heat absorbed when helium-3 atoms move from a concentrated phase into a dilute phase within the mixture. These systems are used for quantum computing and fundamental research, operating without moving parts in the coldest regions.
Another technique, adiabatic demagnetization, can cool materials to microkelvin (µK) temperatures, which are millionths of a Kelvin. This method involves magnetizing a paramagnetic salt at a low temperature, then isolating it thermally and removing the magnetic field. As the magnetic moments in the salt randomize without external heat input, they draw energy from the material itself, causing its temperature to drop.
For the coldest temperatures, laser cooling and evaporative cooling are often combined. Laser cooling slows down atoms by using precisely tuned laser beams to reduce their kinetic energy, pushing temperatures into the microkelvin range. Following this, evaporative cooling further reduces the temperature by allowing the most energetic atoms to escape a trap, leaving behind a colder, denser group of atoms. This two-step process can achieve temperatures in the nanokelvin (nK) range, or billionths of a Kelvin.
The Coldest States of Matter
At these ultracold temperatures, matter can exhibit unique quantum phenomena, leading to new states of matter. The most well-known is the Bose-Einstein Condensate (BEC), which forms when a dilute gas of bosons is cooled to within a few billionths of a degree above absolute zero. In this state, individual atoms lose their separate identities and behave as a single quantum entity, or “super atom.”
BECs are achieved through laser and evaporative cooling. The atoms occupy the lowest possible quantum state, demonstrating macroscopic quantum effects. This collective behavior allows for the study of fundamental quantum mechanics on a larger scale and has implications for fields such as quantum computing and high-precision measurement. BECs represent a manifestation of matter at the edge of absolute zero, revealing the rules governing the universe at its fundamental level.