Random Samples

Science  18 Jan 2008:
Vol. 319, Issue 5861, pp. 263


    Diamonds may set the standard for beauty in jewelry. But another crystal structure is mathematically as exquisite as diamond's, says mathematician Toshikazu Sunada of Meiji University in Kawasaki, Japan. Diamond and the other structure, which he dubs K4, are the only ones that possess both of two key symmetries, Sunada reports in the February issue of the Notices of the American Mathematical Society.

    A crystal lattice is a repeating spatial pattern of atoms and bonds. But it can also be thought of more abstractly as a list of points, or vertices, and a list of lines, or edges, that connect them. The ordering of the lists can be scrambled, and any scrambling that keeps the same vertices and edges connected is called an automorphism. Diamond and K4 have “maximal symmetry,” which means any automorphism is equivalent to sliding or rotating the structure or reflecting it as in a mirror.

    The two also share “strong isotropy.” In diamond, each atom has four neighbors, and the crystal lattice is so symmetrical that, by rotating and reflecting it, the neighbors can be made to switch positions in any way. So, too, in K4, in which each atom has three neighbors.

    Sunada speculates in his paper that K4 doesn't appear in nature. In fact, it shows up in inorganic compounds, lipid networks, and liquid crystals and has been known for decades by other names, says Stephen Hyde of the Australian National University in Canberra. Still, Sunada has proved that K4 and diamond share strong isotropy, Hyde says: “It's very elegant mathematically.”


    If you're looking to make yourself a proverbial black box, you may want to talk to Shawn-Yu Lin. The physicist at Rensselaer Polytechnic Institute in Troy, New York, and his colleagues have just turned out the darkest material ever made. It's a thin film made from a forestlike array of carbon nanotubes 10 nanometers thick and up to a millimeter tall. The tubes reflect nearly nothing at all: Not only do they absorb light well, but the random way in which they intertwine with one another scatters nearly all the light they don't absorb.

    Whereas the previous low-reflectance record holder, an etched surface of nickel phosphorus, reflected just 0.16% of incoming light, the nanotube array returns merely 0.045%, Lin's group reported online last week in the journal Nano Letters. Ray Baughman, a materials scientist at the University of Texas, Dallas, calls the work very interesting, even though he can't think of a killer application for the new material. Lin says he and his colleagues are staking a claim with Guinness World Records.


    Over the past 100 million years or so, bats have evolved many features that distinguish them from their mammalian cousins. One is long, bony digits to support their wings. Now, by manipulating one small DNA sequence, Richard Behringer of the University of Texas M. D. Anderson Cancer Center in Houston and colleagues have nudged mice a tiny step along the evolutionary path to bat-hood.

    The researchers looked at the expression of a homeobox gene, prx1, a key to the development of limbs in all mammals, and found that bats expressed the gene differently from mice in embryonic limbs. So, in mice they removed a chunk of DNA known to control prx1 expression and replaced it with the same piece from bats. The forelimbs of the resulting mice were 6% longer than those of normal baby mice. Although small, that increase is “important,” says developmental biologist Clifford Tabin of Harvard Medical School in Boston.


    Similar studies have been done with flies and worms, but this is the first to show how a specific change in control of gene expression—and not an actual gene—can produce a gross morphological change in a mammal, says Behringer, whose study was published this week in Genes & Development. “If you play this through with lots and lots of genes, maybe ultimately we could make that mouse fly out of the cage.”



    It looks like a toy and has an engine just a tad more powerful than a lawnmower's. But the new no-frills minicar unveiled by Indian automaker Tata last week is making big waves. At $2500, the Nano will be the world's cheapest car when it rolls off the assembly lines next year. Tata officials say its price and its 20-kilometers-per-liter fuel economy make it ideal for millions of Indians who can now afford only bicycles and public transportation. But environmentalists worry that it will further clog streets and worsen air quality. And many wonder if the lightweight, no-airbags car will meet safety standards in developed countries.

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