Demons, Entropy, and the Quest for Absolute Zero

A 19th-century thought experiment has turned into a real technique for reaching ultralow temperatures, paving the way to new scientific discoveries as well as to useful applications

By Mark G. Raizen  | February 28, 2011

 

Image: Photograph by Adam Voorhes

In Brief

• Traditional methods for cooling gases to close to absolute zero work only with a few of the elements.
• Two novel techniques together can cool down atoms of virtually any element, even some molecules.
• One of the techniques, which appears to break the second law of thermodynamics, is a physical realization of a celebrated 1800s thought experiment called Maxwell’s demon.
• Applications range from studying the properties of elementary particles without expensive accelerators to separating isotopes for their use in medicine and research.

Supplemental Material

• Video How Maxwell's Demon Cools a Gas to Microkelvin Temperatures [Animation]

As you read these words, the air’s molecules are ­zipping around you at 2,000 miles per hour, faster than a speeding bullet, and bombarding you from all sides. Meanwhile the atoms and molecules that make up your body incessantly tumble, vibrate or collide with one another. Nothing in nature is ever perfectly still, and the faster things go, the more energy they carry; the ­collective energy of atoms and molecules is what we call, and feel as, heat.

Even though total stillness, corresponding to the temperature of absolute zero, is physically impossible, scientists have edged ever closer to that ultimate limit. In such extreme realms, weird quantum effects begin to manifest themselves and to produce new and unusual states of matter. In particular, cooling gaseous clouds of atoms—as opposed to matter in the liquid or solid state—to a small fraction of a degree above absolute zero has enabled researchers to observe matter particles behaving as waves, to create the most precise measuring instruments in history, and to build the most accurate atomic clocks.