The Earth's gravity is generally perceived as a constant, with one "g" indicating an acceleration of 9.8 meters per second per second. However, small variations in the gravitational field are discernable at different locations, typically with magnitudes of one-ten thousandth of the average gravitational attraction, implying a "mass deficit" in such places. These undulations, known as gravity anomalies, are spread throughout the Earth, but one of the largest is centered over Hudson Bay, Canada. Using a new approach to analyzing planetary gravity fields, two geophysicists, Mark Simons at the California Institute of Technology and Bradford Hager at M.I.T., have shown that incomplete glacial rebound can account for a substantial portion of the Hudson Bay gravity anomaly. About 18,000 years ago, Hudson Bay was at the center of a continental-sized glacier with a thickness of several kilometers. The weight of the ice bowed the surface of the Earth down and after it receded, it left depression in its wake. Having an estimate of incomplete post-glacial rebound allowed Simons and Hager to derive a model of how the viscosity of the mantle changes with depth. Their favored model suggests that underneath the oldest parts of continents the viscosity of the outer 400 kilometers of Earth is much stiffer than under the oceans. Therefore, these continental keels can resist the erosion by the convective flow that drives plate tectonics. For more information, Robert Tindol, "Caltech", at (626) 395-3631.
The engineers and inventors of the post WWII period turned their attention to advancements in electronics, communication, and entertainment. Breakthrough inventions range from LEGOs and computer gaming to the integrated circuit and Ethernet -- a range of advancements that have little in common except they changed our lives.
Neil Fromer is the executive director of the Resnick Institute, a program for energy and sustainability at the California Institute of Technology, working to develop new ideas and research technologies related to providing a sustainable future. He spoke to us about the severity of the current drought in California and how solar energy can help prevent such situations in the future.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.