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    Optical Phase Conjugation

    In everyday experience time always moves forward. The situation is qualitatively different, however, in the case of wave motion: light waves can be "time-reversed" and made to retrace their trajectories

    By Vladimir V. Shkunov and Boris Ya. Zel'dovich

    Cricket Auditory Communication

    The female's ability to recognize the male's calling song and to seek out the source of the song can be used to study how nervous-system activity underlies animal behavior

    By Franz Huber and John Thorson

    The Immune System in AIDS

    The AIDS virus alters the growth and function of T4 lymphocytes, a class of white blood cells that is crucial to the immune system. New knowledge of how the virus does so may lead to treatments and perhaps a vaccine

    By Jeffrey Laurence

    Polar Wandering on Mars

    Regions at the planet's equator seem once to have been near a pole; possibly the entire lithosphere has shifted in relation to the axis of spin. This theory explains many puzzling features and processes

    By Peter H. Schultz

    The Enormous Theorem

    The classification of the finite, simple groups is unprecedented in the history of mathematics, for its proof is 15, 000 pages long. The exotic solution has stimulated interest far beyond the field

    By Daniel Gorenstein

    China's Food

    After the many disruptions of recent decades the output of China's farms about equals the need for food. Whether the balance can be maintained depends on how the nation copes with some problems

    By Vaclav Smil

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