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This is a way of saying that
time
only goes in one direction. It states that a cause must always precede
an effect. If an event (''the cause'') somehow influences another event (''the
effect'') which occurs later in time, then the ''effect'' cannot in turn
have an influence on the ''cause''. That is, event B must occur at a later
time than event A. All frames must agree upon this ordering.
In simple terms, if we were to travel forward in time, see a murder, and
then go back to the past and kill the killer, the future murder would be
prevented. In other words, it has influenced its own cause, even though the
event does not exist. This represents a causality violation, and it is the
problem of dealing with such violations which leads most physicists to think
that time travel and faster-than-light travel are impossible, at least in
the general sense in which these terms are used in science fiction.
Against this, it may be remotely possible to build a
Tipler machine, though
it might take you a while to do so. The Tipler machine is an infinitely long
cylinder, made of material as dense or denser than the matter in a neutron
star. If this cylinder is rotated very rapidly, it will form closed timelike
curves in the immediate area, which might allow time travel and possible
subsequent violations of causality.
Written by Peter Macinnis
©WebsterWorld Pty Ltd/contributors 2002