EXTRA INVISIBLE DIMENSIONS are for particle physicists what they are for Star Trek captains: a device for covering a lot of ground quickly and explaining anomalous behavior. In physics the importation of extra dimensions into the standard theory helps to make peace between quantum mechanics and general relativity, but it doesn't explain the great disparity (the "hierarchy problem") between the temperature at which the weak and electromagnetic forces fuse together (10² GeV, expressed in energy units) and the temperature at which gravity joins up with the other forces (10¹⁸ GeV), a temperature so hot, or an energy so high, that such conditions have not prevailed since a tiny moment after the big bang. Some theories contend that we are not aware of the extra dimensions because they extend only a very short distance, far smaller than the size of an atom. Yet another way of playing with spacetime is to introduce a new dimension essentially infinite in extent but one in which gravitons, the carriers of gravity, would largely be locked up in localized regions, at least in the extra dimension. This exciting new idea, advanced by Lisa Randall of Princeton (609-258-4322, randall@feynman.princeton.edu; on leave from MIT, randall@baxter.mit.edu, 617-253-4818) and Raman Sundrum, now at Stanford, has the effect of fusing gravity with the other known forces at the more reasonable energy of 10³ GeV (rather than at 10¹⁸ GeV), thus solving the hierarchy problem. One testable implication of the new hypothesis would be the existence of exotic new particles which could be detectable at energies to be available in a few years at the Large Hadron Collider (LHC) under construction in Geneva. (Two articles by Randall and Sundrum, p. 3370 and p. 4690, in Physical Review Letters.)