Time Travel for Role Playing Games : Propagating Changes


In the previous article you learned about the meta future boundary (MFB) being the place from which all time travel is initiated. Also that when anyone time travels they contaminate the temporal coordinate they jump to on the meta now with part of that boundary. The MFB and its contaminations are known as “change waves” and all change waves propagate along the time axis toward the future for each increment they move along the meta time axis. Change waves travel toward the temporal future at varying speeds although the slowest they advance is one temporal day for each meta day.

The temporal axis is also organised into a number of cells that cover large chunks of time. Each cell has an inertia that tends to cause changes to history to converge on the history that has gone before. Time waves have a strength and stronger time waves will fight temporal inertia to produce divergent history.


Finally everything that happens during your adventures happens at the Meta Now. The earlier meta times are not accessible for play although the previous meta time content exerts an influence on change waves when trying to get them to converge.

This article looks at those mechanisms of historical change and explains why they are chosen for this time travel game structure.

Back to the Future

Change waves move toward the time axis future. As they do they may be writing new versions of history. The game master needs to be able to manage keeping track on this and coming up with the changes. That means two things:

  1. Changes don’t propagate too far ahead on any given meta day, so the game master can keep up.
  2. Change waves in the past catch up with the waves ahead of them to simplify the number of changes being tracked. The waves that are overrun are gone.

So the first principle is that change waves travel faster if there are more waves ahead of them in the meta now, including the meta future boundary itself. I have found that if they have waves behind them its cool if they slow down a bit too. So count the waves behind a wave (not counting the meta past boundary) and subtract that from the future waves. Then you want a wave speed lookup based on the result measured in amount of time that passes per meta day.

Here is an example from my own game:

Future Waves – Past Waves Wave Speed per Meta Day
7 or more 10 years
4 to 6 5 years
2 to 3 1 year
1 1 month
0 1 week
-1 to -3 2 days
-4 or less 1 day

There is a caveat to this, and that is that if there is a time traveller present at a point of time then the time wave they are generating is “pinned” to that time at a velocity of 1 day per meta day (which is how all time travellers experience the passage of time). Any wave that would otherwise have moved through that time abruptly sticks to the time location of the time traveller. Only when the last time traveller vacates the wave time mark is the wave then free to move at whatever its normal speed should be.

Why? Because the game master would have to handle a new change wave being generated by a time traveller every second, leaping ahead of the traveller’s position, and that is tedious. Instead we just let changes unfold at normal speed with the traveller until they move on, and are no longer generating more changes in that location.

For example a time traveller is in New York, 1st of August, 1977 (wave A). There is a wave at 1st of June, 1978 (wave B) and a future boundary at 16th of January, 2020. Wave A is pinned as there is a time traveller present. During the meta day it moves on only 1 day to 2nd August, 1977. Wave B isn’t constrained by a time traveller. It has 1 wave (the MFB) ahead and 1 behind (wave A) so its total is 0. On the table above that’s 1 week per meta day so it advances to 8th June, 1978, and the MFB, having 2 waves behind it advances only 2 days to 18th January, 2020.

The next day the time traveller zips up to the future boundary in her time machine, releasing wave A and pinning the MFB. So wave A has 2 waves ahead, giving a speed of 1 year per meta day. That scoots it to 2nd August 1978, overrunning wave B and eliminating it. The MFB is pinned to 1 day per meta day so it advances to 19th January, 2020.

Convergent Divergent

So these change waves are chugging along the meta now to the future, but what is going on inside them? Is history changing? Is the butterfly effect raining supreme and as the wave advances through time the course of events is becoming more and more unrecognisable?

Here is where we introduce some quantum physics. There is a quantum of one game master with limited quanta of time, knowledge and imagination. This can lead to brain wave entanglement. In other words the game master needs a break and so we assert that the course of history tries to converge on history that has gone before, even though it may no longer be identical. Pushing back against that is the need to sometimes pass through big changes and some mechanics to guide the game master in deciding on and developing the changes.

To do this we need to introduce some new detail to the time scape, wave strength and temporal inertia.

Temporal inertia is a value between 1 and 7, with 1 being the weakest and 7 being the strongest. It represents how much a moment in time “tries” to remain unchanged. Temporal inertia is an attribute of chunks of time that have the same inertia across the time cell. So in my game I have 20 year long time cells, each with their own inertia. To set it up initially I roll 2d6 and choose the lowest value for the most modern time cells, d6 for the next batch, d6+1 for the lower quarter and, for the distant past I roll 2x (d6+1) and choose the highest value. Here is an example time scape with the meta past boundary set to 1920 and the meta future boundary set to 2020.

Time cell Inertia
1920-1939 4
1940-1959 6
1960-1979 5
1980-1999 3
2000-2019 1
2020 (- 2040 once MFB advances enough) 3

Wave Strength is a value between 0 and 10 representing the force of change, the ability for history to diverge as the wave advances. A wave strength of 0 will converge very strongly on the previous history. A wave strength of 10 is going to start producing lots of divergent history.

When a time traveller arrives at a point in time they automatically create a new change wave with a wave strength of 0. Their actions can increase but not decrease the wave strength. Remember that the wave is pinned and stays moving at 1 temporal day per meta day until all time travellers have left the wave time. Here are some actions and their effect on the current wave strength.

Accidental. The time traveller is trying not to change anything. If the local inertia is 6 or more then the wave strength is unchanged. If the inertia is less than 6 then there is a chance of an accidental increase each day. Roll d6 > the inertia and the time travellers presence has triggered a possible divergence. If so roll d10 and if its more than the current wave strength the wave strength is increased by 1. Time traveller skill should be a factor in avoiding accidents depending on the RPG rules you are using. You can role play an accidental increase or it may be an apparently insignificant change the travellers don’t note at the time.

Intentional. The time traveller is looking to influence the course of events. Once each meta day sum up the result of successful change attempts by all time travellers as a pool of d10, maximum of 6 dice. Roll them one at a time. Each that exceeds the current wave strength needs a second roll of 2d6. If that exceeds the inertia then add 1 to the wave strength. What constitutes a successful change attempt is up to the game master and role playing vagaries.

So, now you have these two values, wave strength and temporal inertia. When a change wave is advancing there will be key moments where events may follow alternate courses. The game master may just decide how that flows, guided by the wave and inertia values, and may use a dice roll when they feel it is helpful. The rule is if d10 is equal to or less than (one plus the wave strength less temporal inertia) then choose an alternative that diverges away from previous historical events, otherwise choose an alternative that brings the current history closer to the previous one.

Diverge if D10 <= (1 + wave strength – temporal inertia) else converge.

So for example a change wave of strength 5 is advancing through a temporal inertia of 4. The original version of history had Frank die in a car crash 2 weeks ago. The change wave has Frank rescued by a time traveller. Frank’s presence is about to cause a sizeable divergence. The GM decides to roll for it. 1 + 5 – 4 gives us 3. If the roll is 3 or less then the divergence occurs and we move on. If 4 or more the divergence does not occur and the GM may also decide that Frank dies, bringing history closer to its original shape.

I would recommend only making these decisions for key moments as the change wave advances or you will spend hours rolling dice. You can speed things up most by just deciding based on the threshold and by keeping the number of key events in the time covered low. Where player characters are involved then you may be more detailed as we are then in the area of maximum attention and interest.

Time Scape Daily Routine

The time travellers create time waves and can push their strength up. As meta time advances waves move, their strength fluctuates and so does temporal inertia. The game master has a simple routine to run at the start of each meta day.

1 Advance change waves that are not pinned.

2 Determine inertia effects on change waves.

3 Determine change wave effects on inertia.

Advance change waves that are not pinned

Go to the unpinned wave furthest back in time and determine how fast it should travel. Note its new time location by advancing it as given, but halt it if it encounters a time traveller and becomes pinned. Eliminate any change waves it overruns.

Unless the wave overruns a time traveller’s position you don’t have to work out what historical changes occur during the time waves passage, although if there is a longish sweep since the last change was noted it may be an idea to give a few key points about the flow in that time. The balance is between working out too much up front and having to work out too much just as the players encounter it.

If a wave runs up to a time traveller then they experience a transformation as the new events assert themselves. The time traveller’s immediate past disappears if the changes are from a time before the time traveller’s arrival. They appear to arrive at that instant instead. The GM will need some reasonable flesh on the changes if this happens with player character time travellers.

Having dealt with a change wave then move on to the next unpinned time wave and repeat the process. Do this until there are no unpinned change waves. Remember the MFB is a change wave.

Determine inertia effects on change waves

This is a simple step. For every surviving change wave except the MFB roll some dice to see if the wave strength increases or decreases. Wave strength 0 is unable to change naturally and doesn’t need to be checked.

First, for a time cell containing one or more change waves set the Inertia Threshold (IT).

IT = 10 + temporal inertia – D10.

Then set the Wave Threshold (WT).

WT = D10.

For each wave in the cell if its wave strength is >= IT and < WT then its wave strength increases by 1.

Otherwise if its wave strength is < IT and >= WT then its wave strength decreases by 1.

Its the GM’s prerogative here to drop the wave strength to zero if they feel there are no substantive changes to track, or to boost the strength an extra 1 or 2 because they feel that historical change has escalated in a manner that they want to keep tracking. Its the play value of the change wave that should help decide this.

Determine change wave effects on inertia

Changes to inertia are pretty mechanical. For each time cell that contains at least one change wave that is not pinned and is not the MFB do the following:

Calculate the Change Threshold (CT)

CT = current temporal inertia – 1.

If there is at least one change wave in the cell that has a wave strength > 5 then add 1 to the CT. Otherwise if no change wave in the cell has a strength greater than 1 then subtract 1 from the CT.

Again, only consider unpinned change waves, and not the MFB.

Roll Dice

Now roll 2d6 and compare it to the CT value. If either dice is more than the CT then add 1 to the cell’s inertia, otherwise if both the dice are less than the CT then subtract 1 from the cell’s inertia. Again, the game master may choose to adjust as they see fit.

An Extended Example

In these articles we haven’t discussed time travel methods or time scape structure, other than inertia cells, and they won’t be discussed in this article. So lets just summon up some structure for the example. A time machine can travel 30 years forward or backward in an instant, but then requires one meta day to recharge. The arrival point is the same earth coordinate as the departure point. We will use the inertia map given earlier. There, that should do.

1st August 2020 some neo-nazis have developed a time machine and decide to launch from their lab in Berlin, travel to 1938 and hand over detailed documents on how to build an atomic bomb. The German secret service has been watching the group and, having noted the arrival of atomic bomb building instructions decide its time to shut them down. Armed authorities break down the lab door just as the neo-nazi time team launch back to the year 1990.

The game master decides it will take the government team about 10 days to figure out how to use the backup time machine and give chase. As the government team is not yet time travelling they are embedded in the meta future boundary and move along at its speed. So those 10 days end up equating to 5 meta days, since the neo-nazis are creating time waves of strength 0 each time they stop on their journey in 1990, 1960 and finally 1938. That’s a travel time of 3 meta days.

Each day the gm quickly notes where the unpinned change waves move to and assumes they are causing no historical changes. He checks the inertia for changes and the 1980 to 1999 cell increases its temporal inertia by 2 points.

In 1938 the well prepared neo-nazi team contact the scientists who would end up working on the Uranprojekt in 1939 and hand over the documentation on atomic fission and how to build the simplest form of atom bomb. The game master decides this produces a change wave strength of 5.

The authority’s time team begin their intercept journey, jumping to 1990, then 1960 on the tail of the neo-nazis. Their arrival in each time period creates a new change wave, as the earlier ones have moved ahead at varying speeds. This means the 1938 wave will advance rapidly once unpinned.

As the authority team arrive in 1960 the neo-nazis jump forward to 1945 to observe the progress of their handiwork. They arrive in post war Berlin. The time wave jumps ahead 5 years to 1943. The game master decides that history is unchanged to the invasion of Poland with the exception that some notable German scientists go to the US to discuss the alarming A-Bomb project with Albert Einstein. In 1940 however the Nazis drop 1 kiloton bombs on both London and Moscow. The US starts a bomb project immediately and announces to the world in early 1941 that it too has atomic bombs. Japan still has diplomatic tensions with the US but the attack on Pearl Harbour does not eventuate. The Nazis capture most of Russia and invade England.

The GM decides there is some convergence on history at this point, so resistance fighters steal an A-bomb carrying bomber and bomb Berlin in 1943, the UK as a consequence fights the Nazi invasion back and seeks US assistance which is forthcoming. In Russia the German forces declare themselves independent of the European Nazis and become the new version of the Soviet Union for the coming cold war.

So what happens when the authorities arrive in 1938? The history they will be trying to change is the new history. If they prevent the past version of the neo-nazi team from aiding the German scientists those scientists may still develop the bomb early, depending on how the change wave develops.

Note that the neo-nazi team is unaware of the authority team and will only know about the historical changes as they roll over them during the next meta day. Not only that but they will find themselves surrounded by radioactive ruins in Berlin.

As a final note the wave sitting in 1943 is strength 5, and the inertia there is  6. So the Inertia Threshold is 10+6-D10. The roll 8, giving us a total IT of 8. The Wave Threshold is 4 on a D10. That means the wave strength is less than the IT and more than the WT so it drops down to 4. The wave pinned at 1945 is strength 0 so we don’t check it.

The cell for 1940 to 1959 has inertia 6, so a change threshold of 5. No wave is over strength 5 and one is more than strength 1 so that is not changed. The GM rolls d6 and gets a 6, so the inertia now builds upward to 7.

In Conclusion

So you now have a structure for managing the creation and propagation of changes to history. Change waves are created by time travellers and when unpinned they move forward through time at different speeds. The waves have a strength which may increase or dampen, and time has inertia. Putting those together gives you guidance and a mechanic for deciding how changes should play out.

This structure should be suggesting some technology that your time campaign may want such as detectors that can tell you where other time travellers may be, and what time waves exist where. We can discuss that in another article.

Previous Article: Time Travel for Role Playing Games : The Basics


One thought on “Time Travel for Role Playing Games : Propagating Changes

  1. Pingback: Time Travel for Role Playing Games : The Basics – Strange Flight

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