Number 461, December 10, 1999 by Phillip F. Schewe and Ben Stein
NATURALLY OCCURRING RADIATION LEVELS ARE MUCH LOWER TODAY on Earth than when life first appeared, a new analysis has shown (Andrew Karam,716-275-1473, Andrew_Karam@URMC.Rochester.edu), suggesting that all living organisms--which have mutation-repair mechanisms very similar to those first developed by primordial life forms--were once equipped to handle larger doses of background nuclear radiation than modern life forms. Presently, humans receive a dose of about 360 millirems per year of radiation from natural sources, plus typically about 63 mrem/yr from anthropogenic sources. Perhaps surprisingly, a major source (about 40 mrem/yr) of naturally occurring radiation is inside our bodies--in the form of potassium, a nutrient essential for many things such as generating signals between cells. All natural sources of potassium contain some radioactive potassium-40 (K-40). But life first began about 4 billion years ago--about 3 K-40 half-lives ago--meaning that the radiation dose from potassium today is about one-eighth of what it was 4 billion years ago. Geologic sources of radiation (about 28 mrem/yr) include uranium, thorium, and potassium present in rocks and minerals in the earth's crust. Studying published data of 1100 rocks, and assuming that the continental crust had formed early (a scenario favored by the rock record), the researchers estimated that radiation from these sources is now about one-half of what it was 4 billion years ago, because many of these radioisotopes decayed in the intervening time. Not considered in the present study were cosmic sources (about 27 mrem/yr) and radon (typically about 200 mrem/yr); the authors are making these the subject of ongoing research. (Karam and Leslie, Health Physics, December 1999.)
MAXWELL'S DEMON MADE OF SAND. The second law of thermodynamics states that within a closed system heat cannot flow unassisted from a cold to a warm place. To ponder this issue, James Clerk Maxwell, one of the pioneers of statistical mechanics, posed this thought experiment: could not a clever microscopic creature, poised at a pinhole in a baffle dividing an insulated box into two equal chambers, sort molecules in such a way that the hotter (faster) molecules would be directed into one chamber while cooler (slower) molecules would be directed into the other. "Maxwell's demon," as the sorter came to be known, itself requires energy to operate, and so the segregation of hot from cold cannot really happen as advertised. And yet in an experiment conducted at the University of Essen in Germany in which agitated sand in a two-chamber vessel (the halves being connected by a hole) "hot," quickly moving sand migrated to one side while cool sand spontaneously condensed and congregated on the other side (see sketch at Physics News Graphics). Jens Eggers (011-49-201-183-3941, firstname.lastname@example.org) explains that, no, the second law is not violated in this case since although moving sand can be considered as a gas, individual grains can absorb heat and dissipate heat (that is, individual grains can gain temperature), unlike the ideal gas molecules described by Maxwell, whose "temperature" is a measurement of gas motion. Thus when sand grains start to congregate in one chamber (the segregation begins as an act of spontaneous symmetry breaking) more and more grains will partake of a growing ordered state consisting of grains falling to the bottom of the container (where the grains are denser there are more collisions and hence faster cooling, leading to more congregation, etc.), while the unaffiliated grains will tend to be on the other side, still in "gaseous" form. (Eggers, Physical Review Letters, 20 December; Select Articles.)
THE TOP PHYSICS STORIES FOR 1999, as recorded in the pages of Physics News Update: Making tentative landfall on the nuclear island of stability with the discovery of elements 114, 116, 118 (Updates 412, 432); the dramatic slowing of light to automobile speeds in Bose Einstein condensates and in gases (415); the achievement of a Fermi-degenerate gas, a cloud of fermion atoms chilled so much that the exclusion principle inflates the size of the cloud relative to a cloud of otherwise-comparable boson atoms (447); tabletop fusion carried out with powerful lasers (421); the observation of direct CP violation in the decay of K mesons at Fermilab and CERN (420, 435); non-destructive photon observations (439); extrasolar planet transits and other observations (458, 462); three-photon entanglement (414); measuring the frequency of visible light to a precision of 120 parts per billion (434); and gravitational self-energy obeys the equivalence principle (454).