Virchbuilder Gambell-66-Daleth considered ver options. Making a virch with a time-travel scenario is reasonably straightforward at first; create two separate environments, supposedly separated in time, and move your characters from one to another as required. But the real problem comes when one scenario affects the other causally. Events in an earlier time period can affect the events in the later scenario, and if time travel is modeled then events in the later period can affect the earlier period too. That makes keeping track of causality a nightmare for the designer of such a virtual reality.
Gambell-66-Daleth knew there were two solutions to this problem. The first, the so-called Consistent or Novikov solution, meant that any supposed time traveller could go back in time, but wouldn't be able to change history. That means no killing your grandfather, and so on. 66-Daleth knew that most time-travellers hated this option, and would try their best to change history whenever they found themselves in the 'past'; but it was 66-Daleth's task to make sure they stuck to the 'script'.
But now ve had a new task, and a tricky one. Orders had come down from 'On High' to make a virch with 'inconsistent' causality. That meant creating a new timeline whenever one of the characters changed history. Keeping track of it all was going to be a nightmare, but ve had been promised almost unlimited processing capacity. At last, a challenge to really stretch ver talents! Ve cracked ver virtual knuckles, and started to create…
Six months after the first wormhole time machine was made, time travellers from the future started pouring through it. They were only coming from six weeks in the future, but it was enough to cause havoc with the world's political, legal and financial systems. The time travellers from the future were mostly the future versions of members of the Temporal Authority, the organisation which had built the wormhole in the first place. This organisation was effectively in control of the flow of people and data from the future to the past and vice versa; under their charismatic leader James Zhao the Authority quickly became the most important faction on Earth.
At first the information brought from the future was unerringly accurate; financial markets, sports results and lotteries could be predicted, the worst disasters, the worst crimes, the worst atrocities could be foreseen and prevented. But preventing these events changed the world too, in a quite different way. If advanced information from the future was used to prevent an undesirable outcome, the world was changed by this intervention, which meant that the data from the future was no longer accurate. Zhao was particularly disturbed by the case of a murderer who was prevented from killing his child victim by information that came through the 'hole; his lawyer cleverly gained him his freedom, because he had in fact not yet committed any crime, and the murderer went straight out and committed a different (unforeseen) killing.
Zhao recommended that any criminal convicted of future crimes should be incarcerated indefinitely, but as he well knew, the advanced information from the future, at first entirely accurate, was beginning to diverge from current events. By bringing this information from the future to the present he was acting to change the present and causing the two worlds to diverge even further. Zhao and his counterpart from six weeks in the future would scan the news and compare it to recordings from the other end of the wormhole. They watched the president of Europe make the same speech over and again, sometimes using the words 'for all Eternity' in her summing-up, and sometimes using the words 'for ever and ever'. Current-Zhao and Future-Zhao were worried men (or maybe a worried man. The first casualty of time travel is, apparently, grammar).
Linda Li, Zhao's deputy, and her future counterpart were in control of the wormhole itself. In effect they had created a 'closed time-like curve', a path through time and space that folded back upon itself. The wormhole formed an information bottleneck — only a limited amount of negative energy could be created to inflate it, so it was quite small — just a few metres in radius. Travellers could only pass through the hole one at a time in specially designed capsules. On the other hand a laser-comm system had been set up, shining messages through the throat of the 'hole, and information was already passing through the one open mouth to its counterpart in the future. Computer scientists (who had already been using similar effects for a while) were keen to use the wormhole as a loop-logic machine, but they would have to wait a few more weeks before the second mouth was opened.
A carefully equipped Temporal Authority expedition was dispatched to explore the far future. They knew that when they reached the far end of the 'hole six weeks ahead they could jump into the first wormhole mouth again (which would still be there, barring accidents) and travel another six weeks further on, and repeat this an arbitrary number of times. In this way they could bring back information from even further on. No-one from the expedition had returned yet; Current-Zhao and Future-Zhao were somewhat relieved by this.
Less than a month after the first mouth was opened it was time to prepare for the opening of the second. This mouth was still on its way back to Earth after a relativistic journey to the Oort cloud, a trip which would last a total of thirty weeks in Earth's frame of reference, but only twenty-four weeks in the frame of the wormhole itself. To accelerate the tiny wormhole mouth to such a velocity, vast banks of laser-pumped particle guns had been built at both ends of the run. In transit, the hole was tiny — much smaller than a proton- but it weighed a million tonnes, so the accelerator guns were vast and powerful.
Now it was time to actuate the laser pumps, one by one, to slow the tiny but heavy 'holeship down into its predetermined orbit around Earth, a hundred and twenty degrees behind its twin. These lasers were so powerful that they each could only safely fire for one tenth of the time; so there were ten of them, set to fire in turn, one after the other (and two spares for emergencies).
Inside an orbiting viewing platform, waiting for the first firing of the lasers, were four people, or was it only two — James Zhao and his counterpart from six weeks in the future, and Li and her future counterpart. Newsmedia cameras watched their every move. Current- Zhao spoke first, addressing the mediacams; "A momentous day indeed! We shall soon see the light from the first beam. Three weeks from now the hole will have shed its speed and be ready to be inflated, here in cislunar space."
"Ancient history, my friend," his future counterpart said. Future- Zhao was always saying annoying things like that, even though he only had a few scant weeks of advantage over most of the rest of the world. Away from the cameras, he said quietly "We can rest assured that nothing can happen now to disrupt this process; several terrorists were arrested well before they could do anything untoward. Useful stuff, foreknowledge." He swallowed, nervously, but soon regained his famous composure. "Aha; here comes the light from our beam, right on cue, of course."
The light from the laser gun glittered as it shot into the empty vacuum of space, reflected from a billion tiny smart sails that would transfer their momentum to the incoming 'holeship. Seconds later, two more beams appeared, followed by more; eventually all twelve were firing simultaneously.
"This cannot be — it must not be!" shouted Future- Zhao. "Who has done this? Li — you are in charge of the lasers; why are they all firing at once?"
"A simple randomiser in the firing mechanism, James," said Future-Li." Either one laser fires, or they all do. It uses beta-decay and a detector, like a Schrödinger cat-box. I thought I'd let the universe decide whether to open the wormhole or not. It looks like the Universe has decided against time travel, in this instance at least. The impact of twelve simultaneous momentum beams will destroy the 'holeship — and the 'hole with it. Nothing can stop that now."
"You utter fool!" Future- Zhao was apoplectic. "You must realise what you have done — you've created a blasted paradox. If the wormhole was never opened, then we can't have come through it. We don't exist. What will happen now — ? Will we just fade away, or vanish suddenly, or what?" The leader of the Temporal Authority suddenly looked fearful for his very existence.
Current- Zhao interrupted this exchange. "Somehow I don't think so. I'm getting urgent questions via the data-link from the other end of the wormhole; they want to know if the deceleration has commenced on schedule. What shall I tell them?"
"Tell them everything is according to plan," said future-Li.
"Our plan, anyway," said current-Li. "I — we — were never happy with the excess of power that the Temporal Authority has gained from this wormhole. We know that the future that we see at the other end of the 'hole is a different one to the one we see now; we don't need to open the other 'hole and establish the Temporal Authority in this timeline. I must admit I didn't expect that the original wormhole mouth would remain open."
Future-Li pulled a wry face. "And I didn't really expect to continue to exist, myself. But there has been a fork in the road — here and now, you have a choice; stay here in this world with us, freed from the rule of the Temporal Authority, and watch our history slowly diverge from the history of the world at the other end of the wormhole. Or go back to your future, where there are still two holes open; a world where cause and effect are a fairy-story, with a fast lane to the distant future. The cosmos only knows how many branching timelines you'll be able to create, if you put your mind to it. You can even come back here for a holiday, if you like. I expect there will be a lot of people, like me, who would rather escape to a simpler world; well, here it is. And no doubt any number that would want to take their chances on the other side."
Current- Zhao said, "I think I might be one of those. I've always been one for an exciting life." By the time the light from the exploding wormhole mouth reached Cislunar Space, both he and his future self had passed though the other mouth into the much stranger continuum beyond.
…"We can rest assured that nothing can happen to disrupt this process; we caught several terrorists well before they could do anything untoward. Useful stuff, foreknowledge. Aha; here comes the light from our beam, right on cue, of course."
The light from the laser gun glittered as it shot into the empty vacuum of space, reflected from a billion tiny smart sails that would transfer their momentum to the incoming 'holeship. After firing for a few minutes, it cut out, and another took over, as the first gun dumped its waste heat into the vacuum of space.
Future-Li and current-Li looked at each other. "Ah, well, the luck of the draw," Future-Li said.
Current-Li said, "Don't abandon hope yet; in six weeks time we'll see what happens when this opportunity repeats itself — at the other end of the 'hole."
"You are right, of course. There may be a third option. We could be lucky, and link back to the other, simpler world-line, and escape that way. Somewhere in all the possible worlds there must be a history where the second wormhole mouth is destroyed; we might be lucky and find it ourselves."
"An option which may or may not exist; such fun."
"What in all of creation are you ladies chattering on about?" Current- Zhao was mildly indignant that his moment of triumph was being ignored. "This is an historical event; show some dignity and gravitas, please."
Future-Li glanced at her other self sadly, and said "History, is it? Something tells me that this may be the end of history, and the start of something very different indeed."
Virchbuilder Gambell-66-Daleth considered ver work, and hoped that it was suitable for whatever purpose ver superiors had in mind. Ve knew that ve had cut a few corners here and there- a real wormhole wouldn't have been stable if placed in Earth orbit, and the technology for creating such an object would be way beyond a human civilization at that developmental level- but as a virtual reality, all that could be made to work, with a little imagination. What was more interesting was the profound effect that reverse causality was having on the culture ve had created.
Maintaining the virch, and all the many-fold alternate realities it had spawned, was going to take a vast amount of processing power. Pretty soon the number of different realities would exceed the capacity of the local medium, and 66-Daleth would be forced to suspend the scenario, at least until more resources could be allocated. Perhaps the virch would simply be shut down, and the bemused inhabitants liberated into the wider cybercosm that was vis homeworld.
66-Daleth wondered what the purpose of this experiment might be; surely those 'On High' were well aware that the requirements of this kind of virch would quickly exponentiate. Everyone knows that a real wormhole can't be used as a time machine; if any wormhole even comes close to creating a closed time-like curve, it collapses and explodes like an evaporating black hole. But what if the Powers That Be had discovered some way of preventing that collapse? Could this whole experiment be just a dry run for the opening of a real time machine in the real universe? Such an event would unleash causal chaos on the world. In vis mental self-perception gestalt, 66-Daleth shuddered.
No, he decided, such a thing was impossible. That was just ordinary paranoia. Every entity in the universe feels that, as a great human thinker once observed in the deep and distant past, on a planet far away in space and time.