|Time travel is the concept of moving backwards and/or
forwards to different points in time, in a manner analogous to
moving through space and different from the normal "flow" of
time to an earthbound observer. Although time travel has been a
plot device in fiction since the 19th century, and one-way
travel into the future has been repeatedly proven as easily
possible today. It is also now accepted that time travel
to the past, while difficult, certainly does not violate the
laws of our mathematics and physics. Any technological
device, whether fictional or real, that is used to achieve time
travel is known as a time machine.
A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between parallel realities or universes.
Theory would point toward there having to be a physical dimension in which one could travel to, where the present (i.e. the point that which you are leaving) would be present at a point fixed in either the past or future. This concept is often referred to as a multiverse.