Here's the way I've always heard
time travel theorized:
Plastic Time: It's very
easy to
alter the timeline, but you run the risk of drastically changing the
world -- or even
yourself -- by
accident. Go back in
time and kill
Hitler as a
baby? Fine,
World War II never happens, but now your
grandparents never met in that London hospital...
High-Resistance Plastic Time: It's very hard to alter the
timeline. Any changes you make will probably not be
noticed. Go back in time and shoot
Lincoln in
Ford's Theater? Fine, but
John Wilkes Booth still gets blamed...
Fixed Time: You can't
alter the timeline, no matter what. Go back in
time and kill Hitler as a baby? Whoops, you picked the
wrong house. Try to
nuke ancient Egypt? Too bad the
bomb's a
dud...
Chaotic Time: The timeline is
extremely easy to alter, and any
changes you make will
propagate wildly. Go back to the
1950s and buy a
newspaper? Now
France rules the world. Go back to the
Permian Age and step on an
ant? The
human race no longer exists... (See
Ray Bradbury's
short story "
A Sound of Thunder" or the "
Time and Punishment" episode of "
The Simpsons" for some good
examples of this)
Paradox-Proof Time: Very similar to
Fixed Time, but if you start to do
anything that would alter the timeline, you immediately get
bounced back to your own
time where you can't do any
damage. Alternately, you can change
history, but it's always the history of a
parallel universe -- never your own. So you can kill Hitler in his
crib, but
WWII still takes place in your
universe.