Before humanity develops actual bidirectional time-travel – assuming it’s possible at all – there will be years, probably decades or even centuries before that during which physics theories prove the general possibility of traveling through time. Philosophers will realize their duty to come up with the morale and ethics surrounding the topic (see section Sub-Questions below).
So, judging and extrapolating from the actual history and state of philosophy, what would future Temporal Ethics probably look like?
If you have read and understood “Multiple History model” or lax type 4 (“Quantum‐Forking” or “Multi‐Divergent”) you may skip the Theory section coming up next, although it deviates in certain details and to help with that, keywords have been emphasized.
Bidirectional time travel is not possible by our current understanding of physics. (Relativistic effects of very fast speeds are similar to a one-way trip to the future.) The laws of thermodynamics would probably be broken less, at least, if each travel worked by exchanging equal amounts of (random) mass-energy between origin and destination. Alas, we’re probably talking about a lot of energy.
Just to make it more interesting, I chose to use a time-machine that arbitrarily travels through time itself (like a Tardis or Delorean) and works autonomously. That means it’s neither a sending (and possibly retrieving) device located at the time of origin (as in Terminator) nor a pod-to-pod temporal teleporter (akin to a wormhole), although I think both are better justified than a time vessel. During time hops it can also travel through space to compensate for astronomic movements.
I assume a finite but exponentially growing number of time-lines similar but not equal to those explained by Doc Brown in the movie Back to the Future II. Time travel is almost instantaneous. Each time travel branches off a new time-line at the destination. My main difference to BTTF is, there are no ripple effects as time-lines exist absolutely.
As a (novel) corollary, there is always a new branch at the origin, too! Branches at first only differ in the presence of the time machine with its passengers and cargo. Their impact will make branches diverge more or less quickly. I also assume butterfly effects are a thing, i.e. little cause can have huge impact, but does not have to, i.e. any distinguishing relevance may just as well fade with time.
There is no direct hopping between parallel time-lines, which some may like to call alternate universes. Unlike some varieties of quantum theory where each non-deterministic state change causes the creation of a new branch, I explicitly restrict that to time travel (even if that’s just for the sake of simplicity). With time progressing forward there will be, by definition, more and more alternate universes.
Hops to the past are always along ones current time-line, up the tree. Traveling to the future is along the time-line where the time travelers have been removed from existence upon departure. (Marty couldn’t meet his future self or son.) If there are branch-offs between origin and future destination, i.e. there’s a local time-(sub)tree, travelers cannot choose to follow a certain path throughout, because they will reenter in each future branch of that tree at the same time, thereby bifurcating all of them!
It is certainly possible to meet multiple doppelgängers of oneself: You first travel back and meet your younger self. Then you travel back again to a point between first arrival and first meeting. Now there is three of you. Any of them may die without affecting the other ones. There is no grandfather paradox and there are no time loops.
Time travelers can go back and kill Hitler™ (or help the Nazis win WW2). That’s not changing their own history, however, only their new future. If they go back to the time they came from, they will hardly recognize it and nobody would recognize them for their old history.
There is a trick to travel to a future that has “yourself” in it and in all its history since your departure: First travel back an hour or a day. Convince your past self not to travel through time. Travel to their future. This will be very similar to what your future would have been. Even if you didn’t convince them and they go on to create a branch like you did by time-traveling, there would still be the other branch wherein the travel doesn’t happen and a “copy” of you will rematerialize in both.
One strange effect of this kind of branching is, you can never determine whether the time machine actually works. Imagine this: you enter the chamber, set the destination coordinates, hit “start” and … space-time branches. Much like Schrödinger’s cat, you’re still here in one branch (wondering if the machine is broken) and you went away in the other branch with no way ever to return to the original branch or influence its past and future history. You may of course live in a time-line that has had a time-traveler landing in who proved their story and thereby the possibility of time-travel, but that’s only verifying the concept, not each individual travel attempt.
In this world, egoist time travelers can create a time-line for themselves where they’re incredibly rich and powerful, e.g. by importing future technology. People like that wouldn’t care about others, including alternate themselves in other time-lines. They wouldn’t care about the number of time-lines either.
Altruist time travelers, however, could try to create time-lines with as little human suffering as possible, although they know it won’t affect their original time-line in any way and they cannot go back to a familiar world. They gave up home. Likely, they would become restless travelers for good, like Doctor Who. Some may deliberately settle in a time-line they like and expect to remain likable, others will accidentally strand somewhen.
If time-travelers continuously try to improve their temporal tree, the percentage of “better” worlds should increase over time, and hence the probability of being in one.
If we’re living in this scenario, there’re some conclusions to be made.
- Since there is only a single uninflicted time-line, but possibly myriads inflicted ones already, it is highly unlikely that history as we know it is without intervention.
- We have no actual proof or evidence of interventions and can therefore not confirm the possibility of time-travel.
- Certain historic events and people are very likely prime targets for alteration by many altruist time-travelers. Having the holocaust in our time-line, for instance, can mean one of several things:
- There were actually worse things to fix which we will never know about.
- Keeping it is considered to have more positive than negative longterm effects. (Time-travelers can test that empirically to some extent.)
- We’re in a less likely branch where it didn’t get fixed or where an egoist time-traveller profits from it.
- Humanity in our futures never develops time-travel (and aliens are not interested in Earth) and there are no active travelers in our past.
- Would Temporal Ethics develop in most time-lines with confirmed time-travel?
- Is there a justification or even an imperative to build a time machine and travel with it if the present cannot be changed?
- Would it become a moral imperative (good) to change the past for the better even though that won’t affect one’s own present?
- Would it be considered bad to actively or passively create time-lines that are worse for humanity overall than the uninflicted one?
Would it be better justified to improve the present by importing knowledge from the future or by changing the past?
Building and running a time-machine is unlikely to be easy and cheap, so only already powerful future entities (like governments) can start a time-travel program. Are their motives more likely to be in line with the principles of Temporal Ethics than that of a random rogue individual?