✎ eternity:time_travel_metaphysic [chronos]

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====== Time Travel Metaphysic ======

Some of this will not align with what NPCs said during the game.  The NPCs did not have a full understanding of all of the mechanics of time travel, and some of the conclusions here could not be made in-universe (e.g. whether or not a timeline continues after you leave it cannot be determined in-universe).

===== Chronos Machines =====

A Chronos machine is a device capable of making one end of a bridge through spacetime.  This "bridge" is best described as a complex resonance in a higher-dimensional space.  By tuning this resonance, a bridge can be created between a Chronos machine and any previous activation of a different machine.

Each Chronos machine can only travel to a given activation once.  In general, it is difficult for multiple machines to travel to the same activation (e.g. if machine A is activated in year 1, and machine B travels from year 2 to machine A, then machine C would have to be very precisely calibrated to travel to A).  The usual failure mode for this is the instability of the later machine.

===== Timelines, and Causal Immunity =====

Travel back in a Chronos machine creates a new timeline.  If nothing is changed during the time travel, then this new timeline may be indistinguishable from the old one.  If things change, then there may be notable differences.  The old timeline ends at the point where any time travellers would return.

A Chronos machine has a certain level of causal immunity - it will appear in the new timeline at a time and place equivalent to where it was in the original timeline.  This causal immunity can be expanded to a significant radius (around the size of a space station or largish asteroid).  The Prometheus had causal immunity for the whole station, plus a small region of space around it.  The Ananke had causal immunity only for the machine itself.

As an example, consider a future Chronos machine in the year 3100, travelling back to a machine in the year 3000.  Timelines will be indicated as 1.3100 for the year 3100 in timeline 1, etc.

  * 1.3000 - past Chronos machine is activated.  No sign of any visitors from the future.
  * 1.3000 to 1.3100 - 100 years of history happens, a second Chronos machine is built.
  * 1.3100 - future Chronos machine is activated, travellers sent back to the past Chronos machine.
  * 2.3000 - travellers appear at the past Chronos machine, and do things.
  * 2.3000 to 2.3100 - 100 years of history happens, some things have changed((not much, but they live underwater)), a second Chronos machine may or may not be built.
  * 2.3100 - the future Chronos machine from 1.3100 appears in timeline 2, and the travellers step out of it.  Anything that was in timeline 2 within the space now occupied by the future Chronos machine no longer exists.

Any time after 1.3100 in the first timeline does not exist - that timeline ended when the travellers returned to the future Chronos machine.  That's the simple version.  Things get more complicated when you consider more tangled travel.

  * 1.3000 - Past Chronos machine activation 1.
  * 1.3001 - Past Chronos machine activation 2.
  * 1.3100 - Future Chronos machine activated, travellers sent back to activation 2 with intention to steal a priceless diamond.
  * 2.3001 - Past Chronos machine activation 2, travellers arrive and steal a massive diamond, but get caught and a security alert is raised.
  * 2.3100 - Future Chronos machine appears, travellers come back with a massive diamond.  They receive word that a fleet is on the way to kill them, because of the security alert.  The fleet will arrive in 2.3102.
  * 2.3101 - Future Chronos machine activated, travellers sent back to activation 1, with intention to steal the diamond more stealthily this time.
  * 3.3000 - Past Chronos machine activation 1, travellers arrive and steal the massive diamond, but don't get caught this time.
  * 3.3001 - Past Chronos machine activation 2, travellers (from 1.3100) arrive with the intention to steal the massive diamond - but find that it's already been stolen,  They mill around for a bit, but raise no alarms.
  * 3.3101 - Future Chronos machine appears, travellers come back with a massive diamond - identical to the one they came back with in 2.3100 (there are now two copies of it on the station).  There is no fleet coming to kill them, since no alarm was raised in this timeline.

There are now two copies of the diamond, since it has been successfully stolen twice.  The alarms that were raised in 2.3001 have been prevented, by ensuring that the travellers who went back to 3.3001 had no reason to trigger them (because the diamond had already been stolen).  The versions of the travellers from 3.3001 vanished from existence when the travelled back forwards, because they would have appeared within the causal protection of the future Chronos machine - which would have violated the causal protection of that machine.

===== Death While Travelling =====

(This section covers **what** happens when someone dies in the past.  For the reasons why this happens, see [[minds and bodies]].)

If an creature with a mind (generally, an animal) would die while it is travelling back in time, several things happen:

  * Anyone who is with the creature when it dies will see it die, and then vanish (along with anything it was wearing or holding, but not any objects which pierced it to cause the fatal injury).
  * The creature will reappear in the future, at an equivalent point in the activation to when it died (e.g. if it died 5 minutes after arriving in the past, it will reappear 5 minutes after leaving the future).
  * The creature will remember everything up to the point of death.
  * The creature's physical state (and the state of any of its belongings that came with it) will revert to the point just before death became inevitable.  In the case of death by grievous injury, they will generally have a small scratch where the entry wound would be, but nothing more.

A couple of interesting consequences of this are that an Enhanced person (at the time of the Prometheus) will remember up to the moment of death, but their Opti will only record up to the point they are injured - as the Opti does not have a mind, and so its physical state reverts to the point of injury.

There is no good scientific explanation for this. While there are some signals in the Chronos machine which could be taken to indicate certain properties of the mind which would result in this, they do not actually provide much useful insight into the nature of the mind.  The mind is fundamentally non-physical.
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