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Four reasons why the fight against climate change is likely to fail

March 15, 2014

Democrats in the Senate stayed up all night talking about the perils of climate change. But while there's hope that technology, changing consumer and business practices or new policies could finally turn the tide and slow or reverse climate change, there are also good reasons to think those efforts will fail. [...]

How Inge Lehmann discovered the inner core of the Earth

February 19, 2014

Inge Lehmann was a Danish mathematician. She worked at the Danish Geodetic Institute, and she had access to the data recorded at seismic stations around the world. She discovered the inner core of the Earth in 1936, by analyzing the seismic data from large earthquakes recorded at different stations around the world. [...]

Ninth Simons Public Lecture


On November 4, 2013, Emily A. Carter (Princeton) delivered the ninth and final public lecture in the series. The title was Quantum Mechanics and the Future of the Planet and the location was the Korn Convocation Hall at UCLA.

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How old is the Earth?

Photo credit: NASA
Photo credit: NASA

The first serious attempts to compute the age of the Earth were done by Lord Kelvin around 1840. Kelvin used Fourier’s law of heat, with the gradient of temperature measured empirically, and some very strong hypotheses simplifying the problem: there are no external sources of heat, and the planet is rigid and homogeneous. He gave an interval of 24 to 400 million years. It is now known that the age of the Earth is 4.5 billion years. Already at the time of Kelvin, his estimate was in contradiction with the observations of the geologists, and it was incompatible with the new theory of evolution of Darwin, which required a much older planet. It was Kelvin’s assistant, John Perry, who pointed out that the gradient of temperature was too large for Kelvin’s hypothesis of homogeneity, and that this gradient could be explained by convection movements inside a fluid under a thin outer solid mantle: these convections movements would slow down considerably the cooling of the mantle, and allow the age of the Earth to be over 2 billions years. Radioactivity, a source of heat, was soon after discovered, showing that energy could not be assumed to be constant. John Perry was visionary at his time: he was arguing that the mantle of the Earth is solid on short time scales, and fluid over longer time scales. But the idea of the continental drift met strong skepticism among the scientific community including the geologists, and it is only in the 1960s that it finally prevailed. (Reference: Kelvin, Perry and the Age of the Earth, P.C. England, P. Molnar and F.M. Richter, American Scientist, volume 95, 2007)

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