Physics News Update, Number 469

Number 469, February 2, 2000 by Phillip F. Schewe and Ben Stein

DIGITAL ENTROPY How much information does it take to control something? By combining thermodynamics with information theory, MIT researchers (contact Seth Lloyd, 617-252-1803, slloyd@mit.edu) have determined the minimum amount of information one needs to bring an unruly object under control, providing quantitative answers to such subjects as taming chaos.

From the perspective of thermodynamics, controlling an object means reducing its disorder, or entropy. Lowering the disorder of a hot gas, for example, decreases the number of possible microscopic arrangements in the gas. This in turn removes some of the uncertainty from the gas's detailed properties.

According to information theory, this reduced uncertainty is tantamount to increased information about the gas. Applying this "digital entropy" perspective to the notion of control, the researchers found that controlling an object becomes possible when one acquires enough information about it (and then applies this information to the object) to keep the uncertainties in its properties at manageable levels.

Chaotic systems are particularly hard to control because they constantly manifest new amounts of uncertainty in their properties. Perhaps there is no better everyday example of chaos than steering a car: a tiny change in steering can quickly be amplified into a huge change in course. For example, if a blindfolded driver initially knows that her car is within two feet from a curb, tiny fluctuations in steering can make this uncertainty 4 feet after one second, 8 feet after two seconds, and so on. Only if the driver receives second-by-second instructions for adjusting the steering to keep the uncertainty down to the two-feet level does she have any hope of controlling it. If the driver makes such steering adjustments only half as frequently, her car will go out of control (crash into the curb) but it will take exactly twice the amount of time than if no adjustments were made. (Touchette and Lloyd, Physical Review Letters, 7 February 2000; Select Articles.)

THE MOST PROTON-RICH NUCLEUS, nickel-48, has been produced for the first time at the GANIL accelerator in France, where beams of nickel-58 atoms are smashed into a target. (Nickel is conspicuous for the range of its isotope varieties: Ni-78, in contrast to Ni-48, is one of the most neutron-rich of nuclei.)

Ni-48 has been of special interest to physicists since it is a "doubly magic" nucleus. A nucleus is exalted as being "magic" if the neutrons or protons exactly fill up one of those shells (analogous to the electron shells in atom) that nature decrees as the model for stability.

It wasn't easy making the Ni-48. Producing just four Ni-48 nuclei required more than 10¹⁷ incoming Ni-58 atoms. The likelihood for creating Ni-48 in this collision process is expressed as a "cross section" of only 50 "femtobarns," the smallest cross section ever measured in nuclear physics. Nevertheless, the apparent lifetime of the Ni-48 nuclei, about half a microsecond, gives the researchers hope that they can look for signs of a never-before-seen form of radioactivity, di-proton decay. That is, with a larger sample, the GANIL scientists (Bertram Blank, blank@cenbg.in2p3.fr) believe they might observe one of the Ni-48 nuclei spitting out a two-proton parcel. (Blank et al., Physical Review Letters, 7 February 2000; Select Article.)

GUIDING NEUTRAL ATOMS AROUND CURVES can be performed with tiny current-carrying wires which deflect the atoms through a lithographically patterned "atom waveguide." Physicists at the University of Colorado and from NIST-Boulder send laser-cooled (42 micro-kelvin) atoms into a 10-cm guide where they undergo three curves (with a 15-cm radius of curvature). Three million atoms per second can be sent through the course; at the far end the atoms are ionized and then counted.

A possible use for the new waveguide, part of a growing toolbox of atom optics components, will be in atom interferometry and other forms of high-precision metrology. The researchers hope to send atoms (or should we say atom?) from a Bose-Einstein condensate into the waveguide. (Muller et al., Physical Review Letters, 20 December 1999; Select Article.)