I also considered titling this entry “A Duh.”
Time travel into the past, as generally depicted in movies and books, doesn’t make sense. Sci-fi writers like to talk about the different sets of narrative “rules” for how time travel works — whether the past is changeable or unchangeable, etc. — but no set of rules can get around the real problems.
Our experience of time as human beings is that things change, and that they change in accordance with physical principles/patterns that do not vary and are not reversible. We know that things were once a certain way, and now we see that they are not; “time” is basically just a name for this experience. It’s very very hard — I suspect impossible — for us to have greater insight into what might be “behind” this experience, because we’re stuck inside it, materially and biologically.
“The past” is defined exactly as the stuff that will not happen again, the stuff that has already changed. And yet time travel stories say: what if it COULD happen again? Since this is definitionally self-contradictory, it seems like it should be impossible to imagine. But we are capable of thinking it nonetheless, for a couple of reasons. For one thing, our brains tend to assess situations only very roughly and so often are willing to think of things as having been truly reinstated to their past states. If a lamp falls over and then you stand it back up again, your brain tells you that you actually reversed the change. “Close enough.” So when it watches a newspaper burn, the same brain might well think, “maybe under certain circumstances, someone could just unburn it, the way I un-fell-over the lamp.”
Another reason we are able to entertain the idea of returning to the past is that our brains are constantly checking in with our memories, which can appear to us as an alternate reality running in parallel to our senses. This experience creates a different way of defining “the past.” The fantasy of time travel really derives from the thought “what if the world of memories were real and the world of the senses were the illusion?” At heart it’s a fantasy about perception, rather than one about the physical universe. Interestingly, this seems to be how the time travel works in La Jetée — as a phenomenon that is caused by the mind but is still “real.” That doesn’t make any sense either but at least it’s an interesting variant.
But generally, time travel stories are about time machines, about physics and not memories. They endorse a spatial metaphor — essentially, that we only experience all this change because we are being thrust “forward” through a series of states. This is a ready metaphor because it reminds us of looking out the window of a moving vehicle. And since we really don’t know what time “is,” this is as good an image as any. Within this spatial metaphor, there’s room for the idea that past states of things STILL EXIST, “behind us,” in some higher-dimensional sense, and that if only that force that pushes our consciousness “forward” were to shift and push us in other directions, we might well see the past again.
All of this, more or less, I’m willing to entertain lightly, in its super-vagueness. But here’s where the problem arises: if we accept this model of time being somehow like space, eternal and permanent, we are saying that “change” is an illusion specific to our particular moving frame of reference. Which means that the notions of “causes” and “effects” are also illusions. And now we come to an impasse in storytelling. A “time machine” is supposedly a device that causes the flow of time to change. But we have freed ourself from the principle of causation. A time machine can no more cause time travel than a character in a book can build a machine that turns the pages of the book backward.
In various movies, various Doc Browns are always drawing diagrams where lines representing time split and loop and run in parallel. But once we accept a reality where such diagrams can exist, we have to also accept that people are not dots traveling along those lines like cars on a road — they are long stretchy higher-dimensional beings. The ones that travel backward through time are four-dimensional donuts or helixes instead of just four-dimensional worms; but in any case they are not things we know how to think about. The supposed “paradox” created by someone traveling back in time and killing himself doesn’t exist; if he can travel back in time, that means that he does not exist “because of” his birth; he simply exists. In such a state of affairs, onlookers experiencing time traditionally would probably just see all sorts of weird and probably disgusting higher-dimensional phenomena, like flatlanders watching a spacelander pass through town.
The idea that traveling back in time just drops a person off on a set dressed like 1955 simply isn’t sustainable.
My metaphor with the character in the book gives me an idea, though: what if there were a time travel book on exactly this theme, where by the end a character realizes he’s a character in a book and wants to change the past, so he builds a “time machine” by somehow compelling the publisher to arrange for a revised edition of the book to be published (maybe by putting something libelous in?). Then a few months later the publisher comes out with the second edition, which starts out as the same book except the character from the end of the first book is present, having traveled back in time to correct things.
What if?? WHAT IF??