Black Holes and Time Machines

Black holes are among the most exotic entities in the cosmos. But they are actually among the best understood. They are constructed from the fabric of space itself and are as simple in structure as elementary particles. A newly formed black hole quickly settles down to a standardised stationary state characterised stationary state characterised by just two numbers: those that measure its mass and its spin. (In principle, electric charge is a third such number, but stars can never acquire enough electric charge for this factor to be relevant to real collapse). The distorted space and time around black holes is described exactly by a solution of Einstein's general relativity equations that was first discovered in 1963 by Roy Kerr, a mathematician who later forsook research to become an internationally recognised bridge player. In general, macroscopic objects seem more and more complicated as we view them closer up, and we can't expect to explain their every detail; but black holes are an exception to this rule.

Cal Tech Albert Einstein

Viewed from outside, no traces remain that distinguish how a particular hole formed, nor what kind of object it swallowed. The great Indian astrophysicist Subrahmanyan Chandrasekhar was deeply impressed by this realisation, aesthetically as well as scientifically: " In my entire scientific life," he wrote, "the most shattering experience has been the realisation that an exact solution of Einstein's equations of general relativity, discovered by the New Zealand mathematician Roy Kerr, provides the absolutely exact representation of untold numbers of massive black holes that populate the Universe." Roger Penrose, the theorist who perhaps did most of to stimulate the renaissance in relativity theory that occurred in the 1960s, has remarked. "It is ironic that the astrophysical object which is strangest and least familiar, the black hole, should be the one for which our theoretical picture is most complete". The discovery of black holes thus opened the way to testing the most remarkable consequences of Einstein's theory.

Black Holes interest astronomers because the flow patterns and magnetic fields around them generate some of the most spectacular pyrotechnics in the universe. But they challenge basic physics as well. Around any black holes is a horizon, a surface shrouding from view an interior from which not even light can escape. A hole's size is proportional to its mass: if the sun became a black hole, its radius would be 3 kilometres, but some of the supermassive holes in galactic centres are as big as our whole solar system. If you fell inside one of these monster holes, you would be treated to several hours of leisurely observation before you approach the centre, where increasingly violent tidal forces would shred you apart. Right at the centre, you, or your remains, would encounter the singularity where the physics transcends what we yet understand. The new physics that we'll need is the same that governs the initial instants of the Big Bang.