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Are Aliens Among Us?

In pursuit of evidence that life arose on Earth more than once, scientists are searching for microbes that are radically different from all known organisms

By Paul Davies

The Many Worlds of Hugh Everett

After his now celebrated theory of multiple universes met scorn, Hugh Everett abandoned the world of academic physics. He turned to top-secret military research and led a tragic private life
*Supplement: The Many Interpretations of Quantum Mechanics

By Peter Byrne

Making Carbon Markets Work

Limiting climate change without damaging the world economy depends on stronger and smarter market signals to regulate carbon dioxide
* Supplement: Extended version

By David G. Victor and Danny Cullenward

Puzzle: In Boxes

Our year-end crossword challenges your knowledge of science. Try the printed puzzle or a harder version.

By Patrick Merrell

Window on the Extreme Universe

The GLAST satellite is about to open up an unexplored region of the electromagnetic spectrum--just the region, in fact, where signs of dark matter and other mysterious phenomena may show up

By William B. Atwood, Peter F. Michelson and Steven Ritz

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December 2007

Beyond XX and XY

Biology. Identity. Equality.