In a discussion of time travel, questions will come up about freewill and causation. I have always found this conversation frustrating because the common view is just so plainly wrong.
The common view is the one espoused in Back to the Future, which arguably is where most Americans get their understanding of time travel. (I guess as opposed to empirical time travel science?) Everyone knows this so I don't even have to summarize it, but here goes: You go back in time but you have to watch out that you don't accidentally change anything, because if you change something because then you will change the future. In particular, you need to make sure that your introduction to your parents when they were in high school doesn't keep them from falling in love, or else you would undo your own existence, the fact of which alone should point out that there is something screwy here.
In this idea, because you can change the future you came from, there are different "timelines". When you go back in to the past you go to a different timeline or split the universe or whatever and the effects of your meddling will be in the new timeline and not the one you came from.
So why do we think there are multiple timelines?
Let me propose a much simpler resolution: there is one timeline, you cannot change the past, and any interaction you have with the past had already been recorded in the time before you left.
The problem with getting this is that we do not see the intervening period between Marty hooking up his own parents and entering the Delorean. In the movie as it is scripted, things are wrong, but in a more correct film, it would look something like this:
Marty shows up out of a time machine in the 1950s. He meets his parents as teenagers and introduces them. They fall in love (and remark on what a great name "Marty" would be) and get married and have Marty. While they're falling in love, Marty gets back in the time machine and returns to the 1980s. We have some thirty years of film of his parents buying their first house, shopping for groceries, going to neighborhood parties, until finally they conceive a child. Thinking back to the kid they met in high school who played guitar to the song when thy fell in love, they name their new son Marty. Marty grows up, learns to play guitar, and eventually meets a crazy weatherman who has invented a time machine. He gets in the time machine. Marty shows up in the 1950s. He meets his parents as teenagers and introduces them. They fall in love (and remark on what a great name "Marty" would be) and and get married and have Marty. While they're falling in love, Marty gets back in the time machine and returns to the 1980s.
The point is it is consistent. The beginning half of the movie (showing Marty interacting with his parents before we see him travel back in time) and the ending half (after the audience sees Marty travel back in time and interacting with his parents) are the exact same footage. You wouldn't even need to shoot it twice. Nothing different happens.
This will obviously introduce huge limitations on what kind of time travel is possible. If time travel is possible at all (and probably isn't) then it must present a self-consistent chain of causation.
Part of the problem with seeing this is most people think of causation in terms of human interaction and not in terms of energy, momentum and entropy. As long as the "major stuff" still works out, it's okay if small things change, where the major stuff tends to be personal interactions. It's okay that Marty pushed billions of atoms outside of the place they were "supposed" to be, as long as the people who are "supposed" to fall in love did and your motivation for time-traveling isn't distorted.
So instead of thinking of just character motivation and development having to be consistent between past and changed past, think in terms of the energy and momentum of every particle in the universe having to be consistent between original past and changed past.
That doesn't leave a lot of room for difference. Actually, maybe no room at all.
I think the principle problem, and the one that doesn't get explored enough, is the connection between entropy and causation within the scope of time travel. I would argue, in order to keep space-time self-consistent, time travel is only possible to regions where the total entropy of the universe was exactly the same as it was when you left - which is a huge restriction on time travel as it means you can't travel in time.
To illustrate this point, suppose that in my mad science laboratory, I have an insulated bowl of water which I call (1), a Stirling engine, and a TARDIS. When the experiment begins, another insulated bowl of water (2) is spit out of the TARDIS. The two bowls are then use as heat reservoirs for the Stirling engine (see video), which will use any heat difference between them to do useful work (which might be used to power a lightbulb). After this is done, the two bowls are in thermal equilibrium. The bowl we had at the beginning of the experiment (1) is now put in the TARDIS and sent back in time to the beginning of the experiment (thus there are two time-copies of the same bowl in this experiment, possibly at different temperatures).
|single reservoir as heat sink and source with time-travel|
This experiment is essentially using the waste heat from an entropic process (the heating up of (1) during the Stirling cycle) to then act as the heat source (2) for that same process. At some point, (1) must be heated to the initial temperature of (2), which will involve a non-entropic heat transfer.
Just an example, suppose (1) is at 70 degrees and (2) is at 150 degrees (and I'm an American so I'm using real degrees, not your wimpy Celsius froo-froo). The Stirling engine will run until both are at (150+70)/2 = 110 degrees, producing some amount of work W. At that point, in order to get (1) up to the same temperature as (2), an additional 40 degrees must be transferred from (2) to (1) raising its temperature above equilibrium, which can only be done if the Stirling engine is run in reverse. To run it in reverse will take the same amount of work W that was generated, so that work from the engine caused by sending a warm bowl back in time wasn't able to exert energy on anything - it had no effect on the past.
If you want to say that the temperature of (2) at the beginning doesn't have to equal the temperature of (1) at the end of the experiment, then that creates problems about where the work is coming from. Or where the bowl at temperature (2) is coming from. If you want to say this situation creates an alternate timeline, then there's infinitely many of them stretching in both directions - timelines "further up" from you will have a Stirling engine that generates more and more work from this experiment, and timelines "further down" from you will eventually have no work generated. So, in the above example, you start with (2) at 80 degrees higher than (1) at 150 degrees, and both end ad 110 degrees. If you send this 110 bowl back, then (2) is only 40 degrees higher than (1) and both will end at 90 degrees. When you send the 90 degree bowl back, then (2) is only 10 degrees higher than (1), etc. But, also, you got your 150 degree bowl from an alternate experimenter who ran this with an initial heat difference of 160 degrees! That is, his (2) started at 230 degrees. He got his (2) from another guy, whose (2) started at 390 degrees, and above him it was 710 degrees and above him 1350 degrees. Ad nauseam. Eventually there is some lucky son of a gun who has an infinite heat difference and can power his entire civilization in to extragalactic expansion for all eternity. Which is absurd.
Because heat transfer is the most fundamental thermodynamic thing (it is, for instance, how you manage to move about), this same reasoning can effectively be applied to any possible interaction of the future with the past.
I think simply put, time travel is completely ruled out by entropic considerations. If not ruled out, then the effect that the future can have on the past is severely limited.
If there are any errors in my thinking here, then please do not hesitate to point them out to me.