It seems to be quite widely assumed (I am informed, by sources who read more about this sort of thing than I do) that backwards time travel requires backwards causation. It wouldn't suffice for time travel if in 2015 I climbed into a time machine and someone looking and acting exactly like how I look in 2015 came into existence for no apparent reason in 1926. Rather, that person's turning up in 1926 would have to be caused by my actions in 2015.
I agree with the insufficiency claim. But I'm not sure whether the causal requirement is motivated by it. (I'm also hung up on whether it might be a conceptual truth that causation happens forwards, not backwards, but let's not worry about that.) Wouldn't it be enough if there were the right kind of non-casual explanation of this person's turning up in 1926, in terms of my actions in 2015? If not, why not?
(I am concerned that I will be trussed up in a sack and sent back to Cambridge for posting on this, but I'm going to do it anyway ...)
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Hey Carrie,
It's difficult for me to see how an instance of backwards time travel could fail to count as an instance of backward causation, because backwards time travel seems to satisfy many of the connotations of causation.
For instance: Slightly varying the 2015 event (e.g. deciding to snap your fingers once you emerge) would correlate perfectly with small variations in the 1926 event. Another: Backwards time travel would be a good means to creating people with your precise physical and psychological profile in the past. Yet another: One could even imagine discovering weird dynamical laws which govern backwards time travel as well as constrain how successful time travel machines ought to be constructed.
Now, I am not claiming that satisfying the connotations of causation suffices for causation. But I am prepared to claim that satisfying the connotations of causation presents very strong evidence that backward time travel would count as causation.
There are 2 possibilities for backward time travel and backward causation. 1. The cause was preceded by the effect. 2. The cause was not preceded by the effect.
It seems conceptually difficult to countenance 1, since it will always appear possible that the effect could have happened without the cause happening. Also, if one accepts the causal closure of physics, the effect must have had a complete causal explanation which did not include the future cause.
2 is much more plausible. The idea is that by travelling backward in time, you actually change the past. But then there seems to be an identity problem in that "the past" seems to have two non identical referents. Also the future of the changed event will of necessity be different from the original future in which the cause occurred. This would involve multiple worlds where each event modified by the future would have two futures, the one in which it was modifeied and the one in which it was not. Oddly, the cause of the modified event exists in the future of the event where there was no future cause. This creates a kind of oscillating instability.
Example, Jill gives birth to Jack who invents a time machine, travels back in time and kills Jill. Jill, being dead, no longer gives birth to Jack, who therefore does not invent a time machine, and Jill remains alive.
It is clear that backward causation is at odd with the logic of our way of thinking of the universe and the possibilites of space and time. But this should be projected onto reality itself. Backward causation is very hard to verify experimentally. How would one do it? To empirically discover a case of backward causation would be a work of genius.
There are undoubtedly worse places to get sent in a sack.
It seems to me that the following two cases need to be distinguished:
1. I get into a time machine and start it up. That causes me to appear in the 1938. This is indeed backward causation, because my existence in the 1938 is caused by my actions in the 2006.
2. I get into a time machine and start it up. I, along with the time machine, promptly cease to exist completely. Coincidentally, a duplicate of me and the time machine happened to appear in 1938 with absolutely no explanation and continued to live out the rest of their existences from that point on. This isn't me, and it isn't the same time machine. In fact, this case is not time travel.
Once you distinguish between the two cases, it's pretty clear that the one that counts as time travel involves backward causation. The one that doesn't count as time travel doesn't involve backward causation. So if time travel is possible it does seem to involve backward causation. I'm sure Daniel would disagree with this argument because of his Lewisian views on causation, but this seems to me to be the best way to think about the connection between time travel and backward causation.
Hi Jeremy,
Neither 1 nor 2 is what I have in mind. I'm wondering whether there could be a case without backwards causation, so not 1, but where the appearance of a me-like person in the past is not coincidental and unexplained (as in 2) but explained (though not caused) by my having got into the time machine.
It's not yet clear to me why there couldn't be a case like this ...
Nice post!
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Backwards causation is sure as hell possile in architecture, photography and economics - check out my paper 'Time Travel on the Instalment Plan' on academia.edu - Adam Brown.
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