Black Holes and Time Machines

If we are to get to travel to distant regions of space and time then first we need to get to grips with that most exotic of phenomena…the Black Hole…

Ever since the beginning, gravity has been making our universe less and less uniform and building up ever-larger contrasts of density and temperature. In the end, gravity overwhelms all the other forces in stars, and in anything larger, even though the effects of rotation and nuclear energy delay its final victory.

There are some entities in which gravity has already triumphed over all other forces. These are black holes - objects that have collapsed so far that no light or any other signal can escape them, but that nonetheless leave imprints, distortions of space and time, frozen in the space they've left.

An Astronaut who ventured too close to a black hole would pass into a region from which there is no return and from where no light signals can be transmitted to the external world; it is as though space itself were being sucked inward faster than light moves through it. An external observer would never witness the falling astronaut's final fate: any clock would appear to run slower and slower as it fell inward, into the hole, so the astronaut would appear impaled at a horizon, frozen in time.

The Russian theorists Yakov Zeldovich and Igor Novikov, who studied how time was distorted near collapsed objects, coined the term 'frozen star' for such objects. Zeldovich, one of the last polymaths of physics, holds a prominent place in modern cosmology. He was a dynamic and charismatic personality; from the 1960s onward, his research school in Moscow spearheaded many key discoveries (even though cosmology and relativity had previously been ideologically tainted in the USSR). The term 'black hole' itself was not coined until 1968, when John Wheeler described how an infalling object 'becomes dimmer millisecond by millisecond…light and particles incident from outside …go down the black hole only to add to its mass and increase its gravitational attraction."

Artists impression of a Black Hole - Possibly as heavy as 2.6 million Suns!

Black holes, the most remarkable consequences of Einstein's theory, are not just theoretical constructs. There are huge numbers of them in our Galaxy and in every other galaxy, each being the remnant of a star and weighing several times as much as the Sun. There are much larger ones, too, in the centres of galaxies. Near our own galactic centre, stars are orbiting ten times faster than their normal speeds within a galaxy. They are feeling, close up, the gravity of a dark object, presumably a black hole, as heavy as 2.6 million suns. Yet our Galaxy is poorly endowed compared to some others, in whose centres lurk holes more massive than a billion suns, betraying their presence by the high speed motions of surrounding stars and gas, induced by their gravitational pull.