9.3. Time Travel:
One of the most fascinating consequences of the theory of relativity is that it allows travel through time, just like one travels through distance. This has been employed by science fiction to produce many interesting films and science fiction stories.
Although reversing the direction of the 'arrow of time' still seem to be experimentally impossible, this happens quite often in dreams and in remembering past events. However, philosophers have been more interested in travel in physical time than in psychological time. On the other hand, we can also 'really' look at the past anytime: by looking at the stars, where we actually see how they have been thousands and millions of years ago when the light that we see now was actually emitted by them. But this is still not like travel in physical time, which is conceivable now only in theoretical cosmology.
Although travel in time is possible and allowed according to the equations of Relativity, in many cases it violates logic and causes obvious paradoxes. There are, however, different types of time travel, some of which are trivial. If you get on a plane on the earth's surface and travel west, you will cross a time zone and instantly go back an hour, but all you have done is to change your reference frame. Also, if your body were quick-frozen in the year 2000 and thawed in 2050, then you would travel forward 50 years in clock time but only a few seconds of your biological time. This is, however, just a case of biological time travel, not a case of physical time travel.
One possible, and more genuine, way to travel through time is to fly at enormous speeds close to the speed of light. Einstein showed that travel forward in physical time is possible relative to the time of those who move more slowly than you. With this kind of relativistic time travel, you can't return to the old present, but you can conceivably be present at the birth of your great-grandchildren. Travel backward in physical time is also possible only if nothing that has happened gets changed. For example, you can't go back in time and prevent your parents from having any children (Arntzenius 2002: 169-200).
Another kind of time travel is caused by the curvature of time due to extreme gravity. If you fell into a black hole, then you'd travel to a time after the end of the universe, as measured in a reference frame tied to earth. Unlike the time travel in science fiction movies, this kind of relativistic time travel to the future is continuous, not abrupt. That is, as you travel to the future, you exist at all intervening times according to the stationary earth clock. You do not suddenly 'poof' into existence in the year 4,500; you existed during their year 4,499, but your spaceship hadn't yet landed.
Going back to the past is probably possible, but there are significant difficulties yet to be overcome before we can be sure. In recent decades, mathematicians and theoretical physicists have described time machines, or at least universes containing backward time travel, that are consistent with Einstein's equations of general relativity. However, Stephen Hawking believes all these time machines are ruled out by the laws of general relativity.
For Ibn Arabi, time travel is possible and easily attainable without any paradoxical consequences. Such time travel, however, has no physical or biological effects on the traveller - see the discussion in section II.7 below.