Public Release: 

Physics tip sheet #25 - August 14, 2002

American Physical Society

1) Modeling urban growth
C. Andersson, K. Lindgren, S. Rasmussen, R. White
Physical Review E (Print issue: August 2002)

Past models of urban growth have been unable to account for observations on a small scale. A new model solves this problem and has features that can be matched up with actual human decisions and motivations. Therefore, it has the potential to aid in development of policies and regulations.

Journal article: http://link.aps.org/abstract/PRE/v66/e026204

2) How leaf edges curl
M. Marder

Edges of leaves and flowers often have characteristic rippled patterns. The same sort of pattern can appear in torn plastic. How these patterns arise touches on a basic question of complexity. Many biologists think that there should be complicated genetic instructions for the curling, while many physicists suspect the complex patterns arise from simple underlying rules. This study shows how simple physical rules to do with the amount of energy in the leaf's edge can lead to complex patterns such as rippling.

Preprint: http://www.arxiv.org/abs/cond-mat/0208232

3) Repeat of bubble fusion experiment
D. Shapira, M. Saltmarsh
Physical Review Letters (to appear)

We mentioned in the Physics Tip Sheet a few weeks back (http://www.aps.org/media/tips/tips072402.html) that Shapira and Saltmarsh have repeated the original bubble fusion experiment. Now their paper has been accepted by Physical Review Letters and we can provide more details. They used an identical physical setup, which was even constructed by the original group, but used a more sensitive detector and data acquisition system. No signs of fusion were observed. In the repeat experiment, the flashes of light from collapsing bubbles did not, in general, correspond to emission of neutrons, as would be required for fusion. The authors point out that there can be random coincidences that look like a fusion signal if the statistics are not correctly interpreted.

Journal article: Available on request

4) Pattern recognition by quantum computer
R. Schützhold

Pattern recognition for artificially intelligent visual systems is suggested as a new application for quantum computers. The author shows how quantum computers are suited to identifying patterns that classical computers could not.

Preprint: http://www.arxiv.org/abs/quant-ph/0208063

5) Has the speed of light really changed?
M. J. Duff

There has been much recent media attention given to a paper in Nature discussing possible changes in the speed of light. An alternative viewpoint says that the conclusion that the speed of light has changed is wrong because it is a meaningless statement but suggests that the approach could still be useful. The author draws the analogy that asking whether the speed of light has changed is like asking whether the number of liters to the gallon has changed.

Preprint: http://www.arxiv.org/abs/hep-th/0208093

6) Observation of the first doubly-charmed particle
SELEX collaboration (M. Mattson, et al.)
Physical Review Letters (Print issue: September 2, 2002)

The first particle containing two charm quarks has been observed in the SELEX experiment at Fermilab. The new particle lives less than 33 femtoseconds (10^-15 seconds) and has a mass about 7000 times that of an electron.

Physics News Update: http://www.aip.org/enews/physnews/2002/split/592-1.html
Journal article: Available on request
(Note to editors: This discovery was previously announced by the lab but this is the first official publication of the results.)

7) Deflating vacuum energy
S. Thomas
Physical Review Letters (Print issue: August 19, 2002)

Treating the Universe as a hologram might solve one of the biggest problems bedeviling modern physics. Most astrophysicists have been convinced that the Universe's expansion is accelerating because some "dark energy" in empty space is pressing outward. However, the usual techniques of quantum physics fail to predict the size of this effect. Ideas from the study of black holes and the holographic principle, which limits the amount of information stored in any volume of space may provide the correction quantum physics needs.

Physical Review Focus: http://focus.aps.org/v10/st7.html
Journal article: http://link.aps.org/abstract/PRL/v89/e081301/

8) Giant ions invade BECs
R. Côté, V. Kharchenko, and M. D. Lukin
Physical Review Letters (Print issue: August 26, 2002)

Dropping an ion into a Bose-Einstein condensate may create bacterium-sized structures called molecular ions. A molecular ion would consist of a charged atom in the center with a shell of atoms around it. The structure would be similar to the "snowballs" seen in superfluid helium.

Physical Review Focus: http://focus.aps.org/v10/st8.html
Journal article: http://link.aps.org/abstract/PRL/v89/e093001/

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