(Adapted from a post of mine to rec.games.frp.dnd.)
1. What was, is.
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You can't change the past, because it's already happened. If it's
known that you *didn't* appear in the past at a particular time and
place, then you don't do that in the future. If you try, something
will stop you - you can't get there in time, you get lost, the time
machine didn't work, temporal friction (see law 3) bounces you out
before you can arrive, whatever.
(A good example of this is an episode of the recent UK television
series "Crime Traveller". The protagonist, Jeff Slade,
having travelled back in Holly Turner's time machine with the winning
lottery numbers (and unable to buy a ticket himself due to the
you-can't-bring-things-back effect below) tries to tell Holly the
numbers, but is continually blocked by coincidence. He tries to phone
her - the mobile phone he's using has flat batteries, he's called away
by an alarm before she answers, she's not in then. He writes the
numbers on her whiteboard - she has something else to put on it and
wipes them off without looking at it. He tries to fax them to her -
the fax machine is out of paper. He writes her a note - another person
puts down a tray on top of it so that doesn't see it until after he had
travelled in the first place. That sort of thing.)
Conversely, if it's known that you *did* appear at some point in the
past, that implies that at some point in the future you *will* travel
back and appear there. While your future may be variable on other
points, that one is certain.
(Yes, it does help with this one if you have players who are willing
to work with you to make the plot fit together. Or to bring out Time's
Enforcement Squad as necessary to deal with the munchkins, etc. Those
of you with access to Mage: the Ascension, look up Wrinkle, Paradox
Spirit of Time for an idea of the sort of thing I mean.)
Corollary: You can't make something exist twice.
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You can't bring things, people, or anything except memories back from
the past. Objects that exist in the past *already* exist elsewhere in
the present, and it's impossible for something to exist in two places
at once. Anything you try to bring forward again with you stays where
it is.
2. The door opens one way.
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When you travel into the future, you don't come back. The future
hasn't happened yet, so travel into the future is a *relatively*
simple matter of stepping 'out' of time for a preset period, and then
coming back into it. The time that has elapsed *has* then happened,
and so you can only go back into it by travelling into the past as per
normal, and under the dictates of the first law. There's no returning
to an earlier time once you've passed it.
Incidentally, be very careful *where* you try and travel into the
future. If your tower is demolished or cavern collapses or some such,
you might end up having to step out of null-time in mid-air or into
solid rock. That would not be good. When travelling into the future,
it's best to go in small jumps.
3. Each man has his appointed time.
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The metaphysical Pattern of every object and being is tied to its own
time (by the 'thread' of your life, or 'lifeline'). When you travel to
the past, this thread continuously tries to pull you back to your own
time - this limits the amount of time you can spend in the past. The
'energy' you put into the spell when you travel holds you in the past,
and the pull of the thread diminishes it slowly but continuously as
you remain there. When the energy is used up, this 'temporal friction'
bounces you back to your own time. Of course, you can return to your
own time instantly by dropping the spell that keeps you in the past
(or it can be dispelled!), but you can't push more energy into it to
prolong your stay - that part of the weave of the spell is in your
origin time.
The other principal limitation is that the amount of temporal friction
increases exponentially with the number of copies of your soul that
exist simultaneously at a given period. Travelling to a time in which
you don't exist is fairly simple, and doubling isn't that much
harder. Tripling, on the other hand, is only really possible if you
have a major source of power or divine backing, and you might as well
forget any attempts at quadrupling or more... (or at least that's how
I've set the numbers IMC).
4. No-one can affect himself.
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You can't affect yourself. Ever. (To prevent most of the munchkinish
abuses of time travel). Rumour has it that a few deranged mages have
tried, but the results aren't pleasant. As soon as you, in the past,
have contact with an earlier version of yourself - by seeing or being
seen by yourself, talking to yourself on the 'phone, or whatever, the
Fourth Law takes over. It's a related effect to temporal friction, but
much, much more powerful. If you attempt such contact, you find
yourself sucked into a 'loop of infinity', a sealed-off pocket
universe in which you finds yourself endlessly repeating the sequence
of actions from when you met yourself in the past forward to when you
time-travelled and met yourself again over and over, again and
again. Meanwhile, history outside quietly rewrites itself so that you
aren't in it from the point at which you travelled. All actions which
you are destined to take to affect reality become actions which will
be performed by other people, and so on. The flow of history is
preserved, everything which you will do was in fact done, just not by
you.
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