APS physics tip sheet #53

September 14, 2005

Imperfect Immune Systems Help Avoid Autoimmune Disease
J. Sun, D. Earl, and M. Deem
Physical Review Letters (upcoming article)

The body's immune system is less effective at quashing disease than it could be - but if it were better the immune system might do more harm than good, according to new research from Rice University in Houston. The immune system has apparently evolved to be mildly inefficient so that it achieves a balance between quick response to disease and decreased probability of attacking the body's own tissues. The authors drew their conclusions from a model of the dynamics of antibody evolution. Their simulations show that while there are mechanisms that would allow the immune system to develop antibodies that respond faster and more strongly against invading pathogens, those mechanisms would also create antibodies that are likely to attack the body's own healthy cells. In addition, the model bolsters controversial suggestions that chronic infections could lead the immune system awry, resulting in autoimmune diseases such as rheumatic arthritis. Read the Rice University press release about the work at http://media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=7684.

Ancient Neutrinos Could Put String Theory and Quantum Loop Gravity to the Test
J. Christian
Physical Review Letters (upcoming article)

The distribution of ancient neutrinos may eliminate some of the most promising theories linking gravity and quantum mechanics, according to a theoretical analysis put forth at the Perimeter Institute in Canada. Many physicists believe that combining gravity and quantum mechanics into a single theory is one of the most important problems in science today. Leading attempts to create a unified theory of gravity and quantum mechanics, such as string theory and loop quantum gravity, make sense in a universe in which gravity is subordinate to the laws of quantum mechanics. However, problems with these sorts of theories have led some to propose that gravity and quantum mechanics are equal contributors to the final unified theory. According to this hypothesis, gravity breaks down the quantum nature of objects. The heavier the object, the quicker gravity leads to the breakdown â€" that is one reason that large objects, such as baseballs, obey the classical physics of Newton, while light objects such as electrons and other particles obey the counterintuitive laws of quantum mechanics. The new research suggests that this idea can be tested using neutrinos created in the early universe. If gravity breaks down the quantum nature of neutrinos, this should be evident in ratios of the types of neutrinos detected at next generation neutrino experiments such as IceCube, a one cubic kilometer neutrino detector currently being built beneath the ice of Antarctica. Such a result would require physicists to rethink popular theories including string theory and quantum loop gravity. It would also mean that the physics of the early universe was fundamentally different than it is today.

Branching Patterns in Lungs
A. Majumdar et al.
Physical Review Letters (upcoming article)

Five hundred years ago, Leonardo da Vinci observed that "all the branches of a tree at every stage of its height when put together are equal in thickness to the trunk." Now, researchers have observed similar regularities in lung airway trees in four different mammal species. They find that at any level of branching, the ratios of the diameters of the larger and smaller daughter airways to their parent airway are distinct constants, which are different in different species. They also investigate how this branching structure determines resistance to air flow, and suggest that this structure may have originated from optimization of fluid delivery within the constraints imposed by the shape of the lung.
-end-


American Physical Society

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