The writer is of the opinion that time travel, as envisaged in the old movie "Time Machine' or in the ever popular "Dr Who" series on television, is not possible. Science excludes the possibility of time travel for two reasons: the first of which has to do with Einstein's theory of Special Relativity and the second has to do with the nature of time itself.
Contrary to the ideas expressed in popular science, quantum physics does not negate the limitations placed on the relative speed of macro sized objects such as spaceships and humans. The theory of Special Relativity limits the speed of space travel to less than the speed of light and from a practical viewpoint to less than 10% of the speed of light. In practice the faster a space ship travels the more energy it requires to produce the same increments in acceleration. A point would be quickly reached were the increased mass of the space ship would negate the effects of providing more energy for acceleration. The fact that quantum physics does allow for a super luminal exchange of quantum information between sub atomic particles like electrons is of no assistance to macro objects like space ships.
There has been a great deal of debate about the nature of time with some very interesting articles written by contributors to Helium. From the writer's viewpoint time is best described as the space between events which prevents everything happening at once. In this context time does not have a particular direction, forwards or backwards, and the so-called arrow of time does not exist. This has encouraged some writers to speculate on the possibility of traveling backwards in time to change events and create a parallel universe with a different history to our own. One of the problems with such speculation is that time travelers would be trapped in this parallel universe with a future which is likely to be very different from the future of the universe they left behind. This would prevent anyone returning to where they began their time travel to verify the success of their mission.
As previously mentioned in paragraph two, quantum physics does allow for a super luminal exchange of information between sub atomic particles. Such information is contained in the quantum wave function commonly referred to as Psi. This allows electrons to apparently jump from one location to another instantaneously and without ever appearing in an intermediate position. This could raise the hopes of some would be time travelers if it wasn't for the fact that the quantum wave function of large objects self collapses within a few Pico seconds, too short a time for effective time travel.
There is another rather interesting outcome of this super luminal means of communication between subatomic particles. When the quantum wave function collapses and changes back into an electron in its new location, it starts a chain reaction and creates a new instant of time in our universe which cannot be reversed. This may be likened to releasing the contents of Pandora's Box, once released there is no way of getting them back in the box again. According to the writer, time is produced by the collapse of quantum mechanical waves and the arrow of time is produced as a result of the chain reaction always moving forwards, never backwards.
From a scientific viewpoint there is only the ever changing NOW and the past and the future simply do not exist. From a philosophical viewpoint we think of the future as something ahead of us when in reality it is the NOW that follows the thought in our mind/brain. Our memories hold images taken from the past but the previous NOW has gone forever because our memories exist in the new NOW. There is only one NOW and it is NOW. It could be argued on the basis of quantum physics that time travel is theoretically possible, but in reality it is not possible for the simple reason that there is nowhere to go, there is no past and no future just an ever changing now.