SPARE TIME. Descartes gave us co-ordinate geometry, with its three spatial dimensions. Einstein put time on an equivalent footing, creating 4-dimensional spacetime. String theory added six more spatial dimensions, and M theory added yet one more for a total of eleven (see Update 329). Now Cumrun Vafa of Harvard has added still another----an extra element of time---to make the existing theories more compatible with each other. Because of possible side effects, such as faster-than-light travel or questionable causality (time would not be measured sequentially along an axis but would spread out into a plane), Vafa's "F Theory" has not found many adherents yet. (New Scientist, 1 November.)