Each of us travel through time, for the most part at the same rate for our entire lives. Our fantasy as humans is the ability to move through time at a different rate. Our popular media depicts all manner of messing with time, both traveling to the future and to the past. When something makes it into our fantasy we want to play it at the table.
Establishing Laws of Physics
One of the topics I often bring to my games is science, most often physics since that is what players bring up all the time. Time falls under the laws of physics, at least in our world. This is something you need to establish when choosing to mess with time in your world. What do you link it to? Is it a product of magic, or like our world a law of nature. Maybe it is a separate concept in itself. Once you know what time is tied to you can begin choosing how it affects the world when flowing normally or when it has been altered.
Events Shape the Future
We all in some way understand the concept of cause and effect. The idea that an event happens and it has an outcome. What we don't know as human beings on our world is how that shapes what is to come. Do our events create divergent points where timelines can split into an infinite number of possibilities or is our course set? This is a major question that comes up in nearly every religion. Since we are creating a fantasy world you get to choose how that works.
Traveling Through Time
One of mankind's oldest fantasies, the ability to travel through time is something that comes up in so much of our popular culture. In layman's terms this is the ability to progress along a theoretical "timeline". Technically we are constantly time traveling, just at a fixed rate. What we care about is the ability to manipulate our speed through time.
The most common answer in a fantasy world will probably be magic. In D&D there already exists a spell that would be considered time manipulation, Time Stop. In this case you are halting the flow of time while you move at your perceived normal speed. Or more specifically you have slowed down your own perceived time by so much that everything appears still.
As Arthur C Clarke put it "Any sufficiently advanced technology is indistinguishable from magic." Some argue this statement's validity but it gives me a good transition to talk about the use of technology. Look at popular media like Doctor Who, DC's Legends of Tomorrow, and Back to the Future. All of those use some form of technology to manipulate time, even if the science is complete nonsense like a flux capacitor.
The last one that comes to mind is when time just starts breaking naturally, either allowing or causing time to be manipulated. In the case of Watchmen, Dr. Manhattan, while not able to manipulate the flow of time, does gain a different perspective of time because of his accident. He is able to see future possibilities in a way we can't understand and make him begin to seem alien.
Effects of Time Manipulation
There are multitudes of situations that can come from altering time and events. One of the main topics of discussion when manipulating time is the subject of what you would change or what you would see. Even manipulating time itself can in turn effect it depending on the nature of time you have chosen for your universe.
Let's start with the easy one, fixed timeline. In this no event you do will ever change the outcome of the future. Even knowing the future won't give you enough to stop it. No matter what you do, events will change to keep time stable. If you try to prevent someone's death, another event will kill them. Narratively this is the easiest to write for since you know the rough sequence of events. It is the most stable form of time but can absolutely ruin a player's feeling of agency. It may bring up the idea that the events you did caused how events unfolded, that your manipulation of time was factored into the events leading up to it.
To the other extreme is that every event has an impact on the future, no matter how small. This is the idea behind The Butterfly Effect and The Flash TV series. This is the most loose and chaotic version of time travel with all sorts of potential issue. Narratively it is difficult to write for but allows an unlimited amount of player agency. In essence you have to create new versions of your world with each passing through time. Even seeing into the future can change it.
Most likely your time will fall between these two for at the table. How time works at my table is a more stable and localized version of the second variation. While your actions in manipulating time do cause changes to the future they are only localize and not very large. It's the idea that time makes corrections to not be too messed up. This gives me enough flexibility in my games to allow for changes to happen without changing reality as a whole.
Interactions with Your Own Timeline
This is by far one of the most complicated subjects when it comes to time manipulation. Some of these topics may get a little convoluted and confusing. One thing you should avoid as a GM is allowing players to mess with their own timelines. While its possible to do it, no encounter survives first contact with the player.
Often players may want to tell something to their past selves that will help them with trials they faced, or giving them an artifact that will help them down the road. Based on your verison of time this can result in a couple different ways to be logical. In a fixed timeline, your players past self either forgets or loses the the artifact/information that they were given, only for them to find it later down the line.
I love Futurama. In one episode the crew ends up back in time when Fry's grandfather was still alive. In a comical series of events Fry's grandfather dies, and Fry ends up realizing that he is his own grandfather. This is one of the ways of manipulating your own timeline, by changing events leading up to creating you or the person you have become. Barry Allen did it in The Flash by going back and preventing his own mother's death. In a stable timeline these events were fixed in your timeline anyway, which will probably be disconnected if the player is trying really hard to break their own time.
In manipulating your own past in a more divergent variation, this brings up another series of questions. How much does the time traveler remember after the event has unfolded. In theory by changing your own past you change the individual who would eventually travel to the past altering the events they would perform. This is coined as the "Time Traveler's Paradox". Most TV shows and movies simply hand wave this, making the time traveler immune to changes in their past. They take the shoes of the individual who they would have been while everything around them seems normal to everyone who isn't the time traveler.
To give an example of the Time Traveler's Paradox, you travel back five years into your past. When you are there you tell yourself that in the year you travel back in time to travel ten years instead of five years. Now when your past self gets to the point in time that he travels backwards he doesn't travel back five years but ten years (if he isn't a jerk and just ignores you anyway). Since this was your past, that means that you have just travel back in time five year and ten years at the same point, which is the paradox. One solution to this is the Dragonball Z approach to time travel in that every point in time is its own alternate world, and traveling back to one will only effect that timeline and no others, so when you go back nothing has changed. Your traveling to the past could also create new versions of you making your past untouched although the world may not have experienced the same. If this has suddenly become very confusing its okay, its a tricky concept to wrap your head around but it comes with the territory.
Many GM's just don't allow for time travel if the last section was any indication of why. I highly recommend not allowing players in alter their own past. However instead of leaving you hanging and telling you not to, I'll lay out some ways that you CAN use it at your table.
One thing I will say is limit the amount your players can manipulate time. If you want your players to go back in time then you need to be careful about where you let them go or how long you let them stay. I would recommend that if they go into their own past, they take the bodies of themselves at that point in time for a limited time. My best recommendation is to have them experience a past of something unrelated to themselves. I'm sure there are many other ways, but it can be a slippery slope.
My personal favorite, and one I plan to use in the near future, is a time manipulation similar to Dark Souls. Time is broken and doesn't make sense. This allows for the most freedom with time and can result in some off the wall and fun experiences. When you remove your own laws of time from the equation it grants you freedom, but may have a larger impact on your world.
With time manipulation and travel you can create a whole slew of adventures and issues. My recommendation to you is to watch some of the movies and shows that I have referenced in this article. You might get your own inspiration for how you plan to travel through time.
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