BY FRANK A. WILCZEK Institute for Theoretical Physics University of California, Santa Barbara Santa Barbara, Calif.

A deeply troubling problem for modern physics is why the vacuum doesn’t weigh very much. A recent paper reviews the reasons why this seemingly innocuous fact bothers people, and it presents as well some of the wild speculations this phenomenon has engendered.

S. Weinberg; “The Cosmological Constant Problem,” Reviews of Modern Physics, 61 (1), 1, January 1989.

" The theory of general relativity in an imaginary world with two space dimensions and one time dimension has long been thought to be either trivial or sick. Abig surprise, therefore, is the discovery Witten describes in a recent paper that the theory can be in a sense exactly solved, and is neither trivial nor sick. It may prove a conveniently simple laboratory experiment to test ideas about realistic quantum gravity.

E. Witten,...

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