The problems with time travel

As I mentioned when discussing how to deal with precognition in RPGs my mind was filled with time-travel tropes here's the result.


Time  travel comes with it's own unique problem in the form of paradoxes. There are two main classics - the first is going back in time and killing your own parents before you are born, the second is a future version of yourself giving you information, that you then use later to give to your past self.

In the first example there's a question of who killed your parents. Killing them means you aren't born, so your parents live, you're born, and kill you parents in the past, which means you aren't born. In the second example the question is where the information your future self passes on to you came from. Your past self carries the information forward until you become future self, and then you give it to past self who carries the information forward.

The answers appear to be that no-one killed your parents and that the information came from nowhere. One solution is simply that this can't happen called the Novikov self-consistency principle - either you can't change the past,or any changes you make are already part of the past. However this raises the spectre of free-will. While we might accept we can't change the past, we still believe we can change the future. However imagine a time-traveller from our future travelling to his past/our present. From their point of view our futures are fixed, we can change nothing our actions are pre-ordained.

So to get around this the next option is a split in the time-lines. This tends to be the option for most forms of science-fiction dealing with time-travel. The options available differ slightly but at the core they remain the same - at some point the time-lines split into two, the time-line that the time-traveller originated in and the new, altered one.

This solves both paradoxes quite neatly. In the former our traveller starts from time-line 1 heads to the past and creates time-line 2 in that time-line he kill his parents. He now only ceases to exist in time-line 2.

In the latter. Our traveller starts from time-line 1 creates time-line 2 and gives himself the information. The information originates from time-line 1.

Hurray problem solved having finished in the past our traveller heads back to his present. At his point more  problems arise. Again the classic scenario is our time-traveller reappears in the present and finds he has a different life, different friends, there's a different government, all sorts of oddities. Except that's not logical from our original time-line premise and it all depends on which time-line he returns to.

If he returns to time-line 1 then nothing has changed. Our traveller may have gone back and saved JFK from being shot, but that happened in time-line 2 not time-line 1. Okay so what if our traveller travels up the time-stream in time-line 2.

However now take the information scenario, there are now two versions of the traveller in time-line 2 - the one from time-stream 1 who passed on the information and the one created in time-line 2 who received the information. One simply took a short-cut while the other got there the long-way. The only way it works is if the time-line version 2 went time-travelling at or before time-line version 1 'returns' which may not happen because remember alterations have been made.

Perhaps time-line version 1 only built the machine so as to go back in time and make himself rich; time-line version 2 being rich has no need to build a time-machine and thus remains rooted to that time.

So having multiple versions of someone from different time-lines isn't a problem per se, but is there any other options that would allow us to not have them?

Yes. Instead of time-lines we turn to parallel universes. If we assume the universe is infinite in size then the number of visible universes within it is also infinite, the number that are identical to our own is also infinite. So what that's not time-travel. Well imagine one of these identical universes started off later than our own by 100 years. If travel was possible between them it would the equivalent of travelling back 100 years.

Except they'd still either return their own universe or their counterpart would still exist. Not a problem head to a universe that's the right number of years behind, that still creates a version of the time-traveller who in turn travels to another universe leaving a space for our tinkerer to fit in. Of course they'd have no knowledge of their 'own' history, but the number of universes that meet this criteria - infinite.

However there's a joke here. There's no need to find a universe that started at a different time, just one that already had the change you were going to implement and has a space for the traveller to fit into. Number of those - infinite and it's not even time-travelling.