Public Release: 

Physics tip sheet #29 - October 23, 2002

American Physical Society

1) How icicles get their ridges
N. Ogawa, Y. Furukawa
Physical Review E (Print issue: October 2002)

Icicles often have circular ridges around them but nobody has been able to explain the details until now. A new theory described how the ridges appear because of the interplay of different effects. If there is some sort of bump on the icicle, as can randomly happen, it will tend to enlarge because the bump increases the area through which the icicle can dump the heat in water flowing over it into the colder air. So more water freezes on the bumps. This effect is more pronounced for thinner ridges. On the other hand, the water flowing down the icicle tends to spread out the ridges, making them wider. Curiously, these two effects almost always balance out to cause ridges every 8 mm or so. The theory also predicts that the ridges should travel down the icicle at half the rate of the icicle growth, something that still has to be checked experimentally.

Journal article: http://link.aps.org/abstract/PRE/v66/e041202

2) Speed of virus infections correctly predicted by physical models
J. Fort, V. Méndez
To appear in Physical Review Letters

Spreading of viruses happens slower than predicted by past physical models of the process. Many believed this is because biological factors needed to be taken into account. However, a new model shows that physical principles are enough to accurately predict the speed of virus spreading. The model is based on a reaction-diffusion equation, in which the virus "reacts" by reproduction and then "diffuses" by infection. Details of both processes, including the time it takes for a virus to reproduce, are needed to give an accurate model. Similar models have only previously been applied to historical social settings such as the Neolithic transition in Europe and the spread of the Black Plague. This model is the first to give predictions that can be experimentally tested in the laboratory.

Journal article: Available on request

3) Spinning coins
K. Easwar, F. Rouyer, N. Menon
Physical Review E (Print issue: October 2002)

When you spin a coin on its edge and it begins to fall over, you notice that the face on the coin begins to rotate. As the coin gets closer to the table, the face rotates faster and the sounds change from a rolling noise to a whirring or chattering. There has been some debate over what causes the motion and how the coin is actually brought to a stop. Some have suggested it is air drag, but new experiments show that friction with the table is the main reason. Also, bouncing, slipping and acoustic emission do not contribute significantly to the energy loss. The spinning coin is an example of what is called a finite-time singularity, in which an increasing amount of energy is being used in one part of an object's motion. Eventually, that motion uses up all the available energy and the rest stops. In this case, the rotation of the face uses up so much energy, there is none left to keep the coin moving above the table and the whole process comes to a sudden halt.

Journal article: http://link.aps.org/abstract/PRE/v66/e045102

4) Neutrinos away!
J. Detwiler, G. Gratta, N. Tolich, Y. Uchida
Physical Review Letters (Print issue: November 4, 2002)

Nuclear submarines may emit enough neutrinos to influence future experiments trying to study these most elusive particles. However, this also means that nuclear submarines could provide good mobile neutrino sources for new types of experiments. (Note: The Nobel Prize last week was partly awarded for the detection of neutrinos, a truly difficult task.)

Physics News Update: http://www.aip.org/enews/physnews/2002/split/610-3.html
Journal article: http://link.aps.org/abstract/PRL/v89/e191802

5) Sorting candy by shaking the container
R. Mikkelsen, D. van der Meer, K. van der Weele, D. Lohse
To appear in Physical Review Letters

If you have a container with two compartments and fill it with candy of two sizes, the candy can be separated by shaking the container. The surprise is that the way in which the candy sorts and where it all ends up depends on how hard you shake the container. Strong shaking distributes the large and small candy equally between compartments. Moderate shaking causes the small candy to jump towards the compartment that started with the most large candy and then the remaining large candy joins it later. Partway through the process, one compartment has almost exclusively large candy. Mild shaking causes the small candy to jump away from the large candy first and then the large candy joins it. However, this means the candy all finishes up in the opposite compartment to that in moderate shaking. (See the pictures in the paper for a clearer idea!) The authors of the paper also develop the theory that accurately predicts this behavior.

Journal article: Available on request Images: Photos of candy being sorted available

6) Tetris is hard - officially
E. D. Demaine, S. Hohenberger, D. Liben-Nowell
arXiv preprint server

An analysis of the popular computer game Tetris has shown it to sit in the class of difficult computational problems called "NP-hard". That means that there is no efficient way to determine a winning strategy for the game. This happens even if the game is played "offline" with full information, i.e. if the player is given the complete sequence of pieces ahead of time and has as much time as desired to make decisions. The authors show how to devise the best strategy for winning even under various modifications of the rules and scoring system.

Preprint: http://www.arxiv.org/abs/cs.CC/0210020

7) Surfing chemical reactions
J. Wolff, A. G. Papathanasiou, H. H. Rotermund, G. Ertl
To appear in Physical Review Letters

Chemical reactions can surf on a wave of light-generated heat, according to new experiments. Physicists looked at the catalytic oxidation of carbon monoxide (as happens in catalytic converters for vehicle exhaust systems). They hit the catalytic surface (made of platinum) with a laser to heat it in a small region, then moved the laser to change the hot spot on the catalyst. The conversion reaction moved with the hot spot, "surfing" on the moving waves of heat. If the laser was moved too quickly, the reaction fell off the back of the wave to leave isolated spots of reaction.

Journal article: Available on request

8) Micro gravity
J. Chiaverina, S. J. Smullin, A. A. Geraci, D. M. Weld, A. Kapitulnik
arXiv preprint server

If the universe has extra dimensions, they might show up by modifying the force of gravity at small scales. Because gravity is relatively weak, it is hard to measure it precisely but physicists are continually pushing to smaller scales. A new experiment takes another bite out of the region where gravity could present surprises. Experimentalists moved a cantilever only a few hundreds of micrometers in size over a pattern of gold and silicon stripes. The different densities mean different forces from gravity and the cantilever bent different amounts. The results show that gravity doesn't show any huge departures from that predicted by conventional gravity, down to scales of 25 micrometers. In the process, it rules out some of the theories that predicted modification to gravity on these scales.

Preprint: http://www.arxiv.org/abs/hep-ph/0209325

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