Discalimer: I am told that when I discuss time travel, it makes people’s heads hurt. I strongly advise watching the indicated episodes with a pad and pen and making diagrams. It will help. Probably.
In TOS, we have three distinct types of time travel with three very different sets of observed rules. The Slingshot Effect, as shown in “The Naked Time,” has never been observed to create a persistent split timeline. The Guardian of Forever, in “City on the Edge of Forever,” which has. And the Time Portal in “All Our Yesterdays,” for which there is insufficient information.
The slingshot effect appears to be possible by any warp-capable ship that can survive the heat and turbulence of passing close to a star, superdense planet, or other large gravity well. It is performed by passing the ship along a precise trajectory that uses the warp drive and the gravity well to accelerate the ship above conventional warp speeds. Or possibly by using a gravity-assisted slingshot maneuver in concert with the Warp Drive to accelerate a ship beyond C within the warp bubble, thus propelling it through time. What’s crazy is that it’s also capable of bringing a ship forward, which is crazy. Don’t think too hard about it, nor the fact that if something is travelling backwards in time and space, you ought to see it going backwards and getting younger at a rate versely proportional to the temporal distance covered.
I said don’t think about it. If you absolutely must though, suppose the Enterprise travels from 2270 to 1970 and the maneuver takes five minutes.Then, for three hundred years, anyone observing that star should see the ship flying backwards and de-aging for 300 years sidereal time, five minutes shipboard subjective time. Conversely, if the Enterprise travels backwards only three days for their same five-minute travel time, the compression ratio is much smaller.
Other fun facts about Slingshot travel: it appears that objects can only be in one place at a time – that is, repeated time travel to the same temporal location will not duplicate objects. This can be seen in “Tomorrow is Yesterday.” The Enterprise travels backwards to fix their incursion, and one of the effects this has is to erase its own presence in the previous time loop. This mass conversion does not, however, prevent the object from showing up in the first place, unless the 600,000 metric tons of metal, plastic, and crystal composing it disappeared from wherever they were. This poses its own problems.
Ultimately, though, there’s no evidence that time travel through the slingshot effect can actually alter the subjective past – that is, createa timeline where the time traveller has experienced a dissonant set of events such that the time travel itself might be negated and cause a paradox. The closest we come is in “Tomorrow is Yesterday” when the Enterprise creates a segement of a timeline in which events occur which are not recorded in history, before they correct them. There is a loop-within-a-loop, which does not create persistent effects that ripple down the timeline to fully bifurcate it. Without getting into too much detail, we’re going to see a similar pattern in Star Trek IV: The Voyage Home. Kirk may have altered the past by revealing a Klingon Warbird to temporal locals and removing a large quantity of biomass, but they don’t, as I recall, own a record of the past playing out in a way insconsistent with their actions, and they return to the present moments after they left. There’s also enough circumstantial evidence to suggest that they didn’t change the past because they always went into the past – that is, the documented Star Trek annals exist in a timelin where their time loop had already happened.
I am once again displeased because Enlgish does not contain verbiage to describe transitioning timeline states.
The Guardian of Forever
The Guardian is what we in the business call ‘sufficiently advanced technology.’ It shows a capability we haven’t seen before – the capacity to create lasting changes to the timeline. Though it only encompasses a few minutes of subjective time – lon enough for McCoy to jump in, the crew to notice the problem, and a rough plan to be sketched out. But that timeline change lasts from 1934 – 2268, which is… substantial. Long enough to spin off a MIrror Universe, in fact. Well, possibly.
The point is that when you consider it, splitting off a new timeline should take a tremendous amount of energy, assuming that energy requirements are even strictly speaking constrained by known thermodynamics in whatever it is that timelines float around in. We can create the hypothesis that there is some such constraint and it won’t be disproven immediately: with the Slingshot effect there’s no mechanism for pulling energy from outside of the timeline, but who knows what’s going on with the Guardian. It could be a mechanism for pulling energy from Elsewhere – in fact, it would pretty much have to contain such a mechanism.
Regardless, the upshot is that the Guardian of Forever displays the possibility of creating lasting changes to the timeline and insulating the universe from paradox. In the timeline McCoy creates, there is no Federation (though there may be a Terran Empire) and no Enterprise there to cause the temporal incursion, yet the Guardian, or some other safety mechanism still functioning in the ruins, permits the crew to stick around long enough to fix the problem. Think about what that means for a moment:
In 1934, a madman in a onsie with no social security number and no knowledge of how society works saves one Edith Keeler from bein hit by a bus. She goes on to lobby the US Government for pacifism and isolationism, which gives the Nazis time to roll over Europe. By the time the US realizes it needs to fight, it’s too late. History marches on, until one day, on a small unregarded planet, that same crazed madman appears, followed by several others. The madman jumps into the Guardian and is about to perform the same action that created this timeline when he is stopped.
In summary, two timelines are created: one in which McCoy saved Edith Keeler and stopped the Federation from existing, and one in which Kirk prevents that action. If we believe in Historical Inertia, we can presume that this second one is congruent enough with the original timeline – the only meaningful interactions Kirk and Spock had were with Keeler, who died before their interactions could propagate very far.
The Sarpeidon Library Portal
The Library Portal on Sarpeidon is an odd case. There’s very little information about how it interacts with timelines, although on reflection two possibilities present themselves given its use at the end of the planet’s life. What we know about the planet is that it still had a civilization capable of sustaining a technological population, which implies certain bounds for a planet that’s not spacefaring. We can easily deduce the culture was not spacefaring, since the population chose to flee their nova in time, rather than in space. Technology increases, by automation and efficiency gains, the upper and lower bounds of a population we can reasonably expect a planet to sustain, but it’s still finite.
Given that we can reasonably expect the population of the planet, therefore, to be finite, the possibilities for Library Portal travel seem to be thus:
1: The current civilization is one that has risen from the ashes of some previous all-consumin crisis, and time travel to anywhere after that crisis was not permitted for the exodus or, indeed, for the general population.
2: The Library Portals are not, strictly speaking, Time Travel, but rather extremely realistic simulations or pocket universes (the difference is academic, for the purposes of this article) that happen to be based on the planets history.
Let us consider each case in turn.
In the first case, the population prior to that point can grow arbitrarily large, so long as there is sufficient reclamation technology that can be brought back by authorized individuals. So long the population that leads into the Final Age remains relatively constant, they avoid a positive feedback loop in their population demographics. Temporal immigrants appearing from nowhere and raising families which add to the population going into the exodus would lead to an inevitable Malthusan crisis. In some ways, of course, it may be a sef-correcting problem, as either the survivors would eventually find a solution which prevents the downfall of time-traveling civilization, or there would be no time travelling survivors.
Of course, complications with the method as a whole arise when it turns out that ones temporal coordinates affect their biochemistry. In order to survive in a time, it is necessary to adjust te body prior to travel so it does not (some bloody how) revert to the biochemical template of the other members of the species in that time. On my best day as a Star Trek apologist, I couldn’t come up with a way to make sense out of that one. It seems the implication is that things want to be in their own time, but that doesn’t make much sense either.
Instead, consider the second case: that each past is a simulation or pocket universe, with an exit back to the real world (maybe*) that terminates in the library. A simulation makes sense, since whatever is running it might be able to alter the parameters of the simulation to make Spock’s body chemistry go all weird just because he happens to believe he exists in a time when Vulcans were barbaric. It’s weak, but the alternative is an objectively enforced univeral template for neurotypical behavior. The number of suppositions we then have to come up with to make sense out of that beggar description.
In later series we’ll see other methods of time travel – anomalies, artifacts, and sapient agents. Each may well turn out to have its own rules, although with the Q involved we always have the option of ‘a wizard did it.’ Anything that he does happened to adhere to the rules he wanted at the time, so we’re unlikely to get useful information that we can apply to other cases from there.
By the way, if you made it this far, congratulations. Most of the time when I start talking about this stuff people glaze over by the second paragraph.
* It is always amusing to speculate, any time a character goes into a simulated universe, pocket universe, region of space that is controlled by thought, or into a Near Death Experience that everything which comes after, particularly if the character alters behavior, that they never came out. This is a theme I will be able to explore more carefully in… maybe a year or so?