BLACK HOLES CANNOT SERVE AS PORTALS TO OTHER UNIVERSES. That is the conclusion of Shahar Hod and Tsvi Piran (tsvi@vms.huji.ac.il, 011-972-2-6584233), two Hebrew University physicists who have performed the first detailed calculations tracing a black hole from the prelude of its formation to the development of its inner structure. Black holes have such large amounts of gravity that they irretrievably attract all objects that are closer than a point-of-no-return known as the event horizon. As Roger Penrose and Stephen Hawking first showed, the insides of black holes must contain a singularity, which in its simplest form is a region of infinite density. Associated with a singularity is a boundary known as a "Cauchy horizon" beyond which it becomes impossible to predict the future trajectory of a particle with any reliability. Previously, some theorists have suggested the possibility that matter passing through the Cauchy horizon may encounter an "asymptotically flat" region, a region of relatively weak gravity such as our own, and then travel to other universes rather than get caught in a singularity. However, Hod and Piran have now supported previous indications showing that these Cauchy horizons are unstable; small disturbances in the black hole instantly transform them into singularity regions. In fact, their calculations suggest that generic black holes contain two singularities that are connected to each other so that all infalling matter reaches one or the other. (Hod and Piran, Physical Review Letters, 24 August 1998)