I've talked about my disappointment with many time travel stories because they don't address basic questions about momentum and try to claim that people can "get away" with time travelling so long as they don't change anything, when quantum mechanics requires them to change everything (at least on the scale of the very small). Here is my last objection to the way people write time travel stories, and also how they read them. I have read a number of essays and blog entries where people dismiss a time travel story as ridiculous because of the way that they "know" causation works, when in fact we are all ignorant of how causation would work in the presence of time travel. There may be good reasons to doubt time travel is possible, but this certainly isn't one of them.
Objection 3: Considering Causation
Before Einstein developed the theory of relativity, there were some things people believed that no one with any sense would doubt. Things like if something was 10 metres long, it would be 10 metres long no matter how fast you travelled. Or that if two actions occurred at the same instant, they would occur at the same instant for everyone observing the actions no matter how fast they were travelling. Or that if twins were born 2 minutes apart, they would remain 2 minutes apart in age no matter how fast one of them travelled.
We now know that those viewpoints were wrong. The people who held that they were true were not stupid. It is just that they had not been exposed to living in a world where accurate measurements could be made of things travelling at or a significant fraction of the speed of light.
The same is true when you consider causation in the context of a universe with time travel. All of our experience with causation has been done while travelling through time in only one direction. Our notion of what is and is not possible with cause and effect is fundamentally determined by this perspective. In fact, the assumptions we make about causation are so fundamental to our view of the universe that we are sometimes not even aware we are making them. Consider Descarte's famous "Cogito ergo sum", or in English "I think, therefore I am". That is often considered a statement that relies on no assumptions at all, but of course it relies on one that is the very foundation of the statement: the existence of causation. In order to have the effect of thinking, you must have a cause, the thinker. When we don't recognize our ignorance about how causation works in a world with time travel, it leads to inflexible thinking and the creation of apparent paradoxes.
Consider the grandfather paradox. What if you go back in time and kill your own grandfather? Well, what if you did? The universe that would unfold from there would no longer include your birth. In our limited view of causation, we assume that is a problem because if the universe didn't have a version of you that could go back and kill your grandfather, your grandfather would have to be alive. But that is only true in one-way causation. In two-way causation, there is no reason to think that the cause of an event might not be from a time-line that no longer exists.
Here is a thought experiment to help visualize two-way causation. Consider a wind tunnel with various baffles inside it. You can think of this as the flow of time. As smoke flows through the tunnel, it gets bounced around by the baffles into currents and eddies. Eventually the flow of smoke reaches a steady state that doesn't change. Let's call that State A.
Now at the very end of the wind tunnel, introduce a new baffle that causes a current of smoke to flow back along the inside top of the tunnel. You can think of this as our time traveller. Eventually, the smoke drops back down into the main flow and introduces turbulence. For a while the smoke is chaotic, but eventually it settles into a new steady state which we'll call State B.
If State B does not result in smoke hitting the new baffle we introduced, and as a result there is no longer a backwards tendril of smoke as there was in State A (our time traveller, remember), does that mean that State B of the wind tunnel is a paradox? Of course not. It just means that it required an earlier influence from State A that is no longer present in order to reach State B.
So what would two-way causation actually look like? I have no idea. If things as fundamental to our view of the universe as an object's length or the nature of simultaneous events can be brought into question, who knows what basic assumptions two-way causation would challenge. Of course, some things seem more likely than others. Slowly fading from a photograph strikes me as one of the less-likely outcomes. But ultimately we should admit our ignorance and acknowledge that we have no idea what causation would look like in a universe that has time travel. As long as they are consistent, stories about time travel should feel free to do whatever they like. As an aid to the reader who is stuck with a worldview that has only allowed one-way causation, though, it would be considerate if authors made the way causation works clear in there stories. Some do.
Anyway, those are my thoughts on time travel stories. If there are any stories that qualify as science fiction by my three criteria of considering momentum, dealing with the implications of always changing history, and identifying a set of assumptions about causation, let me know. So far I haven't seen one.
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