Demonios Entropia

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Mark G. Raizen holds the Sid W. Richardson Foundation Regents Chair in Physics at the University of Texas at Austin, where he also earned his Ph.D. His interests include optical trapping and quantum entanglement. As a toddler, Raizen got to meet physicist Leo Szilard, who was a patient of his father, a cardiologist, and who explained why Maxwell’s demons do not violate the laws ofthermodynamics.

DEMONS
P H YS I C S

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 scienti c discoveries as well as to useful applications
By Mark G. Raizen

IN BRIEF

Traditional methods for cooling gases to close to absolute zero work only with a few of the elements. Twonovel 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 thermody-

namics, 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 isotopesfor their use in medicine and research.

Photograph by Adam Voorhes

March 2011, ScientificAmerican.com 55

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s 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 innature 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 manifestthemselves 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. The drawback ofthese atom-cooling techniques is that they are applicable to only a few of the elements in the periodic table, limiting their usefulness. For example, hydrogen, the simplest of all atoms, was for a long time extremely challenging to cool. Now, however, my research group has demonstrated a new cooling method that works on most elements and on many types of molecules as well. My inspiration: JamesClerk Maxwell’s Victorian-era thought experiment. This great Scottish physicist theorized the possibility of a “demon” that seemed able to violate the rules of thermodynamics. The newfound capability will open directions in basic research and lead to a wide range of practical uses. For example, variants on the technique may lead to processes for purifying rare isotopes that have important uses inmedicine and in basic research. Another spin-off might be an increase in the precision of nanoscale fabrication methods that are used to make computer chips. On the science side, cooling atoms and molecules may enable researchers to explore the no-man’s-zone between quantum physics and ordinary chemistry or to uncover possible differences in behavior between matter and antimatter. And supercoolinghydrogen and its isotopes could help small laboratories to answer questions in fundamental physics of the type that have traditionally required huge experiments such as those at particle accelerators.

from the thermodynamic point of view, is that the beam, despite having a substantial amount of energy, is extremely cold. Think of it this way: an observer traveling with the beam at 2,000 mph...