This entry is part 3 in the series Time Travel In RPGs


Hopefully, this will wrap up the article on Time Travel! Part 1 looked at the problems of Time Travel in RPGs, and reached the conclusion that the GM had to have some understanding of the nature of time in his campaign before he could adjudicate the complexities that could result.

Part Two comprised excerpts relating to the Nature of Time from the Campaign Physics I employ in my Superhero campaign, and produced some very interesting feedback.

Now comes the fun part: looking at the implications and applications and complications that derive from this system, posing a couple of questions for consideration, and generally playing around with Time Travel as a viable plot vehicle in RPGs…

The ‘Key Event’ Principle of Alternate History

Whenever you assess the impact on history of a time-traveller, you’re talking about generating an Alternate History. I use two principles in forming Alternate histories in my campaign, regardless of the cause of the divergance. The first of these is the “Key Event” principle, which states,

The greater the element of chance in determing the outcome of an event, the more susceptable that event’s outcome is to variation.

That means that if an outcome is a near-certainty, there won’t be many significant branches on the timeline from that point, and it would be very hard to change that outcome. But it also means the converse: where outcomes are equally balanced in likelyhood, minor changes can be amplified to the point of making a big difference.

An implication of this is that the results of natural forces are more likely to remain fixed, while the works of man are far more susceptable to alteration. It’s very hard to stop Mount St Helens from erupting, for example. The continents will tend to have the same shape, because the forces that created that shape won’t change very much.

But a more subtle implication is that there are key moments in history where things can be radically changed with comparatively little effort, but once they have passed, events will tend to remain the same overall. Once Adolf Hitler comes to power, World War II is a near-certainty; it might be possible to change the shape of that conflict by killing him at that point, but some sort of conflict is more or less inevitable, due to the social, political, and economic forces of the time. You might be able to change some detail outcome within the broader context, such as whether or not a particular soldier lived or died, but most of the time, that won’t even change the outcome of the battle in which he was killed.

Key Events tend to be decisions and moments of inspiration. Churchill’s knowledge of the bombing of Coventry, thanks to the breaking of the Enigma code by the British, led to a decision not to forewarn and evacuate the city – so that the German High Command would not suspect that their code had been compromised. That decision could easily have changed the entire outcome of the War.

Another implication is that the greater the number of people involved in an event, the smaller any individual’s role is, and the less susceptable to change the outcome. It’s very hard to change the outcome of a popular election, for example; while you might alter one person’s vote, that won’t often change the overall outcome.

The Big Bang: An interesting Key Event

It is generally believed that much of the physical properties of the universe were not fixed for the first 10 exp -30th of a second following the big bang. When you think about the state of chaos that was unfolding in the instants prior to that moment, it MUST qualify as a “Key Event”. While the forces are probably the same, their relative strengths could easily vary.

The first phases of the superhero campaign for which this physics was derived took place in a world much like our own, at least so far as the physics was concerned. The most recent phases, which have occupied the last nine years or so of play, have taken place primarily in an alternate dimension in which the Weak nuclear force was just a little weaker, making fusion reactions more likely, and making stars a little hotter and a fraction more short-lived. This in turn had effects on the climate and geography of the Earth, and that influanced the sociology, and that influanced the history. And yet, despite a variant history and politics and even an additional habitable landmass, and sentient dinosaurs running around South America, it was still very recognisably an alternate earth with recognisable elements. In terms of technology, they were ahead of the standard in some areas, and behind in others (for reasons that I’ll go into in the next section).

Far stranger worlds become possible. It’s simply a matter of deciding what change you want, identifying what might cause that change, and backtracking, as described in Part 2 of the Pursuit Of Perfection series, then working forwards to discover the other ramifications of that change.

The next phase of the campaign is set in “Dimension Regency”, in which the British Empire is the Dominant Superpower, far moreso than the US is in ours. It has conquered most of the western world, and has never fallen; only the Far East and parts of the Middle East are not part of the Empire. When I set about analyzing the changes that I wanted to make in the sociology and history of the world, I eventually backtracked my way to the signing of the Magna Carta and the circumstances of the time. A minor change there, making the King a little more inspired in his perceptions and approach to the problems, and the outcome became subtly different. And that subtle difference became a significant difference, and then a major difference, and then a sweeping global change. And yet, for all the changes, some things remained constant; there was still a World War II, there was still a Napoleon Boneparte and a French Revolution, there was still a Michael Jackson and a Princess Diana and a Live Aid.

Why is this so important? Why is it such an advantage? Because, it creates a recognisable world for the PCs to live in while preserving the mystery and allure of the unknown. The shape of history may be unchanged, but the all-important context and detail are radically different. The Players can easily grasp the changes and their consequences, but at the same time things are similar enough in many ways for them to just play.

The ‘Key Man’ Principle of Alternate History

I operate on the principle that a genius is always a genius, and will make whatever the next breakthroughs are in their chosen field of study. If Isaac Newton doesn’t invent calculus because the algebraic tools are not in existance for him to do so, he will come up with those tools so that the genius who made the next mathematical breakthrough in our world can do so. Similarly, in any world in which he is born, Napolean Boneparte will play a key role in history – whether it be as the leader of a failed revoltion or as a great General in the service of the British Empire, or whatever.

Key Men (and women) remain Key Men (and women).

If someone has the drive and charisma to become a leading political figure, they will still become a leading political figure in some way. In Dimension-Halo, where the recent superhero campaign has been set, that led to President Joseph McCarthy and a specialised law-enforcement division, the S.I.D., whose mandate was to seek out and eliminate unamerican activities.

Again, this serves to make the alternate world accessable to the players. They know from our own history who to look at.

Free Will

These two principles, in turn, dictate how free will relates to the force labelled “Will” in the previous part of this article. In fact, Will can be defined as the sum of individuals’ intents multiplied by the opportunities to express those intents. Which means what exactly? Well, it means that one person with enough intent, and the opportunity to act on that intent, can change history, or a lot of people with intent but less opportunity can have the same effect.

The Key Man principle is an expression of the first part of that statement, and the Key Event principle is an implication of the second.

Uncertainty & Randomness

Chaos Theory came along after the original draft of this physics was written, but did not violate it, because there was always a mechanism by which random chance operated to create uncertainty.

Edwin Hubble was the first to derive the Cosmological law named for him, which states that galactic objects are receding from earth at a speed proportionate to the distance from the Earth. When combined with the theory of relativity, which forbids objects with mass from travelling faster than the speed of light, Hubbles’ Law defines a volume of space around a point beyond which no information is observable – so far as physics is concerned, nothing can exist outside of the Hubble Volume.

Now, as it happens, I have a couple of serious bones to pick with both of these theories. First, Hubble’s law is just plain WRONG, as quoted, because it takes time for the information concerning these objects to reach us. To be accurate, it should read, “Galactic objects appear to be receding from earth at a speed proportionate to their distance from the Earth”. In fact, the closer to the edge of the hubble sphere you look, the closer to the big bang you are looking. In effect, the hubble volume appears to define the wave front of the big bang – which in turn means that there can be nothing physically beyond it, because that wave front defines the edge, in physical space, of the universe. And therefore, nothing from beyond the hubble limit can affect anything within it – because there IS nothing beyond it.

But there CAN be something beyond the hubble limit – it’s just that whatever IS beyond it can’t be directly physically percieved, because it lies OUTSIDE THIS UNIVERSE. Consider the diagram:

It’s clear that there is an overlap. Certainly, anything within that overlap can be physically affected by an object or force that is both within the Extreme Point’s Hubble Volume but outside our Hubble Volume. And certainly, something from outside our Hubble Volume can enter it, aparrantly created spontaniously at the edge of our observable universe and moving inwards.

So the hubble volume does NOT define the totality of the universe – just the part that can be observed directly by means of anything that is limited to the speed of light in a vacuum.

Changing The Speed Of Light?

Prior to the big bang, the universe was a lot smaller than it is now. For the sake of convenience, let’s say it was one light-minute across. Then the big bang takes place, and the wave-front of that explosion expands outwards at the speed of light. So, one second after the big bang, the universe is 62 light-seconds in diameter. If the hubble volume were what it has been popularly assumed to be, that would mean that the observable limit of the universe at that instant was 62 light-seconds away from the centre-point of the explosion. But that immediatly contradicts the basic premise of the hubble law, which would put the hubble volume as the volume defined by a radius of 1 light-second. Once again, the universe is a lot bigger than previously supposed; nothing else makes sense. The only alternative is for the speed of light to be sixty-two times what it is now – but that also increases the expansion rate of the wave-front, so the universe is STILL larger than the observable limit.

It just doesn’t add up. The only conclusion to be reached is that the speed of light is not as meaningful a limit as popular perception would have it – something that I’ll come back to in a little while.

The Size Of The Universe

So, defining the universe as the volume contained within the wave-front of the big bang leads to it being bigger than we can possibly observe. But it IS finite. And any event within its totality can affect the whole – gravitation, charge, etc. So, how precise is that wave front? Can anything affect it? What if the surrounding ‘other-space’ isn’t empty?

If a particle is within a quantum black hole, and quantum uncertainty states that the location of that particle can only be expressed as a probability until we interact with it to measure it’s actual location (collapsing the probability into a discrete value for that instant of time), then there must be a percentage of locations for that particle that lie outside the event horizon of that black hole. The particle is both within and outside the black hole at the same time. And that, in turn, alters the mass and size of the Black Hole in question, ie alters the gravitational slope of the shape of space in that region. Which means that past a certain limit, a quantum limit, gravitation is also fuzzy – it might be this or it might be that.

And that it turn means that quantum uncertainty demands a fuzzyness on the size of the universe – because otherwise there would need to be some sort of mechanism operating to ensure that a plus-quiver at this point is exactly balanced by a minus-quiver somewhere else. The only alternative to this improbable arrangement is for the dimensional boundary – the limits of space itself – to be quivering like a very stiff jelly. And that in turn yeilds Heisenburg’s uncertainty limit as a true universal constant.

If the universe gets bigger, it contains more space, and vice-versa. Another way for this to occur would be for discrete ‘packets’ of space to spontaniously enter our universe from the outside and for other discrete ‘packets’ to spontaniously dissappear into the outside. To and from where? What lies outside the universe?

Anything that is not on our temporal vector, that’s what.

Heisenburg Uncertainty, under the space-time model described in the second part of this series, derives from the expansion and shrinkage of space as individual quanta enter or pass through our time-line’s temporal vector. And if quanta can arrive from outside, then quanta in a particular arrangement can arrive from outside, and that means that travel from one time-line to another, or to a different point on that time-line, is possible. The structure of the 3-time-dimension 3-space-dimension model inherantly contains the potential for time travel. (And you wondered where I was going with this, didn’t you?)

Many Butterflies

The arrival of a time-traveller would increase quantum uncertainty by the ratio of the mass of the universe plus him over the mass of the universe without him. An increase that small is virtually undetectable, but it would be there.

But all sorts of quantum effects can cascade into macro-scale events – the butterfly effect, in other words. The probability of any given quantum event escelating to the point where it has larger, more noticeable, effects is very small – but there are so MANY quantum events poised right on the edge of falling one way or another that some of them will inevitably be changed in outcome by the arrival of a time-traveller.

The mere departure or arrival of a time-traveller at a sufficiently delicate critical event can be enough to change the outcome of that event.

Advanced electronic equipment frequently has to take into account Heisenburg Uncertainty and other quantum effects, eg in the design of the powerful CPUs in modern computers. That means that hi-tech electronics might be susceptable to interferance as a result of time-travel; the more time-travellers present, the more likely this is to occur, and the more severe the disruption. In theory, it would be possible to design a system of multiple processors constantly generating checksums or other complex numbers; from time to time, one of them will hiccough and give a wrong answer; in extremely rare cases, or in the case of poor design or outside interferance, it might even be possible for two or more to give wrong answers. But the more of them that go wrong at once, the more likely it is that a time-traveller has just arrived or departed from the local vicinity in space and time. The better you can shield this equipment from other sources of disruption, the more sensitive it can be made.

Relative Time Rates

Two timelines will never be in synch; time will always be flowing at a faster or slower rate in one relative to the other. Below is a simplified 2-D diagram illustrating the reasons:

The first diagram shows two timelines with different relative vectors, of equal internal time, ie the same length. Because of the relative angle, time actually passes faster in the green timeline than it does in the red, as shown in the second timeline, where the frame of referance has been rotated to set the red timeline on a baseline. Simple trigonometry can calculate the difference based on the relative angle – it comes to 1 / cos (a).

But wait a moment: if we again rotate the axes of our diagram, so that the green timeline is now the baseline, we would find that time was now passing faster in the red timeline than in the green! In fact, the rule of thumb is that everywhere is moving faster than you are! How can we make sense of this?

The answer is that in order to percieve the differences in time rates, you have to transit between the two, and that also takes percieved time within your base timeline- in effect, travelling along the timeline angled relative to the baseline AND THEN around the arc to the equivalent point in time.

That means that you can’t cheat time that easily. If you want to turn tonight’s last-minute cram session into two weeks of intensive srudy on a near-vertical timeline, you can – but by the time you have come and gone from the divergent time-vector, between two and four weeks will have passed on the base timeline, and you have missed the exam.

Sure, you could compensate for that by aiming for an arrival point after your field trip in time that was earlier on the base timeline – in effect, travelling from up and right to down and left on the second diagram – but that gets complicated too, because you are then spending even more time in transit, requiring a target time even earlier on the base timeline.

Of course, this is a vastly simplified diagram – real timelines would not be straight lines, they would twist and bend in three dimensions like snakes, and may even run in opposite directions for periods of time. So the actual relative time rate would very much be an average, and a short-term one at that.

Then throw in the fact that uncertainty means that the actual targets are just a little fuzzy around the edges, and it becomes impossible to precisely target a jump through time. There are ways around that, using the temporal equivalent of homing beacons – but there is still an element of unpredictability.

The more similar two timelines are, the closer together their relative angles will be, and the closer their time rates will match, and the more precisely a transition can be targetted; the more different they are, the greater the relative temporal vectors, and the harder it is. Think of a spear being hurled; would it be easier to hit a spot a precise distance behind the head if it were travelling at right angles to you, across your field of vision, or almost straight at you? The second spear is easier to hit but harder to be precise about where on it’s length the hit will occur.

In other words, the harder you try to exploit the system, the harder it is to succeed.

Relativity & Motion

A really good question to contemplate is whether or not there is a relativistic limit to the transmission of consequences down a timeline. I’ve seen – and employed, in different campaigns – both the no and yes cases, and have yet to see any reasonably convincing arguement either way. The difference between the two cases becomes interesting when a time traveller introduces a divergance from the recorded outcome of an event. Let’s look at each case seperately and see where the consequences take us:

Instantanious Development Of Consequences

This relies on all changes in temporal vector (ie outcome of events) starting small and accumulating with Development, ie with internal time. Taking a simple case as an example for our thought experiment, let’s say that our time traveller goes back in time one year and starts an object moving at half the speed of light – so that it is half a light-year away from where it was at the time of his departure. That all makes perfect sense, but now consider what an observer within that timeline would see: one instant, the object would be in it’s expected position, and the next, it would be half a light-year away, and the observer would have no memory of it NOT travelling away from its starting position over the last year. So far as the conditions within the timeline are concerned, the object has to instantly move half a light-year and acquire a velocity of half the speed of light – violating relativity in the process. An instant of discontinuity transforms the old timeline into the new.

And yet, there is uncertainty involved here. and the fuzzyness of quantum limits. That fuzzyness also aplies to the propogation of change down the timeline, and it means that the transition would NOT be instantanious – it would take a measureable interval of time within the timeline being altered, proportionate to the uncertainty limit multiplied by the period of time since the change. Using the reccommended value for the Planck constant (0.00000005) and the number of seconds in a year (aprox 3,1557,600) gives a 1.577 second transition effect – just long enough to percieve something happening before the universe settled into it’s new timeline. At ten years after the change, we’re talking 15.77 seconds, a seriously noticeable time – that is (of course) instantly forgotten as soon as the transition is complete. 100 years after the change, and the transition interval is 157.7 seconds – more than 2-and-a-half minutes.

And, here again is where things get interesting: If an individual were to decouple himself from his native time vector, ie start travelling in time, during that interval state, he would not be subject to the revision. He would remember the way things were, and remember the period of transformation.

This is the specific model that my superhero campaign uses. There are technological developments to delay or even isolate the characters from temporal divergance, but before these were developed by the characters, they had these sort of time-scales in which to react to the change in history.

Relativistic Limit to the Development of Consequences

But this is not the only possibility. What if there is a measurable propogation rate? What if it is faster, or slower? Or even non-linear? Well, for a start, if the propogation rate requires objects in 3D space to travel faster than the speed of light, there will still be a transition experience, just as there is in the instantanious model. This transition may be faster or slower than the instantanious model, but will usually be percieved for less time, making it less useful for warning that someone is manipulating the past. This is because some or all of the process will take place concurrent with the Development of the consequences – you don’t see the whole process, just the final crescendo.

Fortunately, there are alternatives if you can monitor the fixed location of a point from within the Time dimensions; when someone triggers a change, a wave of discontinuity will sweep down the Development event stream from the point of interferance. Usually, if you were to map the changes, it would look like this:

The point where our time-traveller has skewed the timeline is obvious; the orange timeline represents the outcome of his manipulations. It is also fairly clear that instead of doing something, he has prevented someone from doing something; the new timeline does not have as great a change in temporal vector. Nevertheless, the differences are subtle and not easy to spot; the change appears to have had minimal repercussions. It is only when looking at the second last points of divergance, where a radical outcome takes place, that the difference between the two timelines begins to look significant; the shape of the timeline at that point is somewhat different. Further ahead in time, both of those differences are accumulating development, and compounding apon each other, so there will be more changes as a consequence, until the shape of the timeline is completely different.

But, while this illustration is best for showing the actual change that has taken place, comprising a ‘before’ and ‘after’ superimposed, it does not really show the process very clearly. For that, a series of images showing the progression of the change is best:

Close to the point of divergance, the wave-front is small and the difference minor. In the second image, the wave-front of the change in history is larger and more substantial; and in the third, it is much larger and quite radical. But in each, there is a period of discontinuity, lasting zero time within the confines of the timeline – which experiences duration parallel to the timeline at that point – but which is quite obvious when viewed from the outside – and yet, if you weren’t monitoring that specific timeline for change, it would be very hard to spot it at all because of all the other timelines in the way.

Yes, believe it or not, the same basic timeline is depicted within this illustration. But it’s not in the foreground, it’s buried under 120,000 other timelines. I KNOW it’s there, because I put it into the picture, but even I would be hard-pressed to tell you exactly WHERE.

The consequences of this submodel are that within a timeline, you cannot do anything to undo a change in history, because it is effectively instantanious; to protect yourself, you have to actually posess an independant frame of referance, insulating you from the change to history.

But, of course, your absence will cause other changes to history – so the only safe agents to recruit would be those who are about to die. I actually designed a campaign called The Timekeepers based on this premise, some years back.

Relativity Is Not Sacrosanct

Of course, Relativity is something that can be ignored within a game at need. But I actually have a couple of doubts about relativity being interpreted correctly in the real world that bear mentioning at this point.

One of the cornerstones of Einsteinian relativity is that “there are no priviliged frames of referance”. You can’t look at a situation without interacting with that situation, you can’t be on the outside looking in. This theory of time enlarges the scope of where a frame of referance can be located, but the bottom line remains.

So, let’s postulate a quick thought experiment to reveal the flaws in the interpretation of the General Relativity of Relativity, using one of the most famous implications, Time Dilation.

A spacecraft leaves earth and accellerates to a speed not far short of the speed of light, travels at that speed for a while, then decelerates to a stop relative to the Earth. Time compresses for the pilot, so that where ten or a hundred or a thousand years may have passed on earth, perhaps only a year or two have passed for the pilot. So says the conventional interpretation.

How does the universe know who is accellerating?

Surely it is just as valid to say that the spacecraft is standing still relative to the earth, and that it’s the rest of the universe that is accellerating away from it, then maintaining a speed relative to the spacecraft of just under the speed of light, and then decelerating to a rest state relative to the spacecraft?

In which case, it is the Earth which experiences compressed time, and whose clocks run more slowly, and in which only a year or two passes, and the spacecraft which experiences decades or more!

But hold on a moment – how do we Know how much time the other party experiences? Answer we don’t – we only know what the other part of the mind experiment appears to experience, while it is happening.

If time is aparrantly shortened for one, it is aparrantly shortened for both – and if you divide the distance travelled by the aparrant time taken according to both sides you end up with a velocity faster than the speed of light.

It is my contention that all the aparrant paradoxes that emerge from General Relativity derive from the false assumption that nothing can travel faster than the speed of light in a vacuum, and that it is those paradoxes that demonstrate that the assumption is false.

Einstein himself said that he came up with that limit because anything else led him into paradox. But you get similar answers if you take the analagy of mail being carried across the ocean by ship as being the limiting speed of communications – if nothing can travel faster than the ship, then any method of conveying information faster than that produces paradoxes of the same type that Einstein described as justifying the absolute limit to speed.

And yet, testing seems to show that the time and mass effects of travelling at high speed are real – these equations have to work or particle accellerators would not function. This conundrum seems like a paradox until you realise that, once again, the equations aren’t telling us anything about how the world looks from the point of view of the particle being accellerated.

The Lorentz transformations are all about the apparant effects on one of the parties as percieved by the other – not about what really happens. If this were not true, it would be easy to accellerate subatomic particles to the point where there apparant mass would crush them into black holes – and even if these decayed almost immediatly through pair production, what emerged would bear no resemblance to what was there before, which would be obvious when examining the artifacts that result from firing these accellerated particles – and it just doesn’t happen.

So the reality would seem to be that the “light barrier” is no barrier at all – if we just knew how to get there.

Transfers of Energy

The ability to forge ‘connections’ between two different timelines is implicit in the concept of a time-traveller leaving his timeline and entering another one. The likelyhood that these two timelines would be at exactly the same energy potential is extraordinary low; almost certainly, one will be at a higher entropy level than the other.

In our space-time, at the current moment, there are compact clumps of energy surrounded by a relative desert. If one end of the ‘time-tunnel’ were placed in space – a suitable location for travel along it – then energy would tend to flow from the higher potential to the lower. Stick the other end in a timeline that has reached heat death, and you can start pumping energy out of that timeline. This would lower the energy potential in the region immediatly around the far end of this ‘energy tap’; in effect, entropy has been reversed in that world, however briefly.

On the other hand, if a world that was facing imminant heat-death were to stick the far end of such an energy tap into the core of a star in our timeline, the local energy levels in the star would be far higher than that of the heat-death-world; energy would flow into the heat-dead world that was not there previously. Of course, this might well have adverse affects on the star in question!

The fact that thermodynamically, these concepts mean that the universe is no longer a closed system makes many things possible that would otherwise be unimaginable.

What Lies Between

If time consists of 1-dimensional strings vibrating in three dimensions of time and fraying into an infinite number of other 1-dimensional strings, the question has to be asked: what lies in between?

It would be a strange realm, where motion exists without time. And yet, any interruption to the biochemical and bioelectric processes of life would be lethal, and without time, there would be no opportunity for perception, let alone for reaction or interaction.

But can matter exist without time? Matter without motion is matter at absolute zero, and that is not possible according to the best theory going. The only solution that makes sense – aside from the whole thing being impossible – is for any matter removed from a particular space to retain the temporal vector that the space of origin had at the moment of departure from it. Carrying your own internal frame of referance, you can percieve what’s out there, you can react and interact.

So what can someone do without motion that takes time? Because “taking time” is the equivalent of moving, within this strange realm.

The answer is that you can think. The same forces that were indentified in the previous part of this series are in play; and that means that Development tries to wash you downstream, while Will permits you to ‘swim’ across, or even against, the current.

Adopting A Popular Model

The model of time travel that has been presented here glosses over a lot of nuts and bolts, in the interest of making the concepts more universally accesable, but there is enough meat there that it can be adapted to any system.

But there are other models out there that can be used, or to which these concepts can be adapted. These are works that have been formative in my thinking about time travel, and about how to use it in RPG plots, and I commend them to anyone who’s interested in doing so.

And in presenting this list, this series comes full circle; it started off as intended for the June 2010 Blog Carnival, about which media had proven inspirational, and so it ends. It’s especially appropriate that be the case when we’re talking about time travel…!

Print Referance & Inspiration

Three books stand out for me as especially valuable. There are others that could also be mentioned, but these are the cream of the crop.

Movie Referance & Inspiration

Some of these are better than others, but they can all serve as the springboards for ideas. As with books, there are others that could be mentioned, but these are the pick of the litter.

TV Referance & Inspiration

Most of these are available in one form or another on DVD, but you may need to buy multiple sets.

  • Dr Who
  • The Time Tunnel
  • Catweazle
  • Quantum Leap
  • Seven Days
  • Star Trek (selected episodes)
  • Star Trek: The Next Generation (selected episodes)
Referance & Links to more

Finally, a couple of stops that should be useful to anyone interested in this topic:

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