Black Holes and Time Machines

Fast-Forward and Backward in Time?

Good science fiction should respect the fundamental constraints of physical law. In that sprit, it is worth mentioning that an observer could, in principle, observe the far future in what, subjectively, seemed quiet a short time. According to Einstein, the speed of a clock depends on where you are and how you're moving. If your subjective clock ran very slowly compared to the cosmic clock, you could travel "fast forward" into the future. This would happen if you were moving at a velocity close to the speed of light. Furthermore, strong gravity would distort time; clocks on a neutron star would run 20 or 30 percent slower. Near a black hole, the distortions would be even greater. If you were to fall into one, your future would be finite; you would be ripped apart - spaghettified - by ever more violent gravitational forces. However, a more prudent astronaut who managed to get into the closest possible orbit around a rapidly spinning hole without falling into it would also have interesting experiences, space-time is so distorted there that his clock would run arbitrarily slow and he could, therefore, in a subjectively short period, view an immensely long future timespan in the external universe.

Nasa Non Stop luxury - HSCT - The successor to Concorde - While not fast enough for time travel; it will be capable of flying over 200 passengers at Supersonic speeds over the Atlantic.

This elasticity in the rate of passage of time may seem counter to our intuition. But such intuition is acquired from our everyday environment (and perhaps, even more, that of our remote ancestors), which has offered us no experience of such effects. Few of us have travelled faster than a millionth of the speed of light (the speed of a jet airliner); we live on a planet where the pull of gravity is 1000 billion times weaker than on a neutron star. But time dilation entails no inconsistency or paradox.

More problematic, of course, would be time travel back into the past. More than fifty years ago, the great logician Kurt Godel discovered that the theory of general relativity did not in itself preclude a time machine. He discovered a valid solution of Einstein's equations that described a bizarre universe where some of the worldlines were close loops - in other words, you could come back into your own past. But Godel's solution was not realistic: it described a universe that was rotating and not expanding.