When time travel is well-defined, it’s not a problem. For example, I don’t think anyone has ever accused Primer of using time travel as a magic bullet.
When time travel is well-defined, it’s not a problem. For example, I don’t think anyone has ever accused Primer of using time travel as a magic bullet.
Except Primer-style time travel, where you can actually change the future of the universe by going back in time, is not very well-defined. Leaving aside the difficulty of reconciling such time travel with relativity, it is unclear to me what the rules are that govern returning to a particular time slice. What aspects of the past state are allowed to change, and what must remain the same? Are there any constraints at all?
The sort of time travel we see in the Harry Potter books (SPOILER jura uneel naq tnat geniry onpx va gvzr gb fnir Fvevhf Oynpx) is a much better example of well-defined time travel. There’s no “changing the future” involved. Every causal loop is subject to strong consistency constraints.
Well, every story that has people traveling or communicating faster than light basically has not-well-defined time travel in it. Which includes most scifi ever written, I’d guess.
That’s not really what I meant. There are well-defined frameworks of physical law that permit faster-than-light travel. Newtonian physics is an example. So even though FTL travel requires some suspension of disbelief, you’re not usually at sea about what the rules are that govern the phenomenon. It’s not like magic.
The problem with the way time travel is often depicted is that it is like magic. There’s no clear set of rules constraining which properties of the time slice can change when the traveler returns to it, and which must remain the same. Surely there must be some constraints. After all, if every single property is allowed to change, it wouldn’t really count as traveling back to the same time slice. But where do these constraints come from and what are they?
It’s a difference, but not a qualitative one, I feel. After all, reality is entangled and there’s no well-defined framework of physical law that allows FTL travel and also reduces to something like relativity as long as you don’t turn on your magical engine. Everyone who doesn’t have a magical FTL engine would still see your FTL travel as being time travel, and the same questions would have to be answered as you raise below: what changes are allowed, etc.
OTOH, if you go by which suspension of disbelief is easiest and most natural, and if you let go of your belief in scientific physical laws. then the most natural thing to believe in is magic. It’s what all humans believed in up to historically recent eras. The kind of magic that doesn’t usually have builtin rules in the sense of laws of the universe or of magic itself. It just has limitations corresponding to the knowledge/power/wisdom of the magician. The more common constraining rules in such stories are actually moralistic (good winning, etc.)
After all, reality is entangled and there’s no well-defined framework of physical law that allows FTL travel and also reduces to something like relativity as long as you don’t turn on your magical engine. Everyone who doesn’t have a magical FTL engine would still see your FTL travel as being time travel, and the same questions would have to be answered as you raise below: what changes are allowed, etc.
This is a popular but incorrect idea. If you interpret special relativity via (neo-)Lorenz Ether Theory, you can have perfectly mundane kind of FTL without any problematic time travel, you’d just have a preferred reference frame. It’d be possible to send messages to the past according to some reference frames (in spacelike fashion), but not others, and you’d never get to communicate information in time loops. There is no serious problem with reconciling FTL and special relativity, FTL goes the way of ether for Occam’s razor reasons, not because it doesn’t make sense conceptually.
There are perfectly consistent models of a relativistic universe with well-defined rules that allow tachyons (particles that travel faster than light). Frank Arntzenius has a great paper on this. These models don’t allow “future-changing” time travel, though.
Incidentally, does anyone know of a work of fantasy that has clearly articulating “law-like” constraints governing the use of magic? The awesome webcomic Unsounded seems to be attempting something like this, but the details are still fuzzy.
Most works of fantasy do this to some extent. For example, the One Power in Wheel of Time has very well-specified mechanics, though of course a lot of the complexity is left as an exercise for the reader. The metal-based magic in the Mistborn books is simpler, and likewise well-defined. You can also have works like Cardcaptor Sakura, which is basically an anime about a collectible card game; here, the cards operate according to some very specific rules, and each card has a narrow function, but the card list is very large and fairly arbitrary.
Interesting. That paper is behind a paywall, unfortunately.
But my knowledge of physics is very limited and I probably wouldn’t understand the full paper anyway. (What tachyon interactions with non-tachyons does he allow?) I doubt, though, that any model of tachyons allows you to accelerate your normal-matter spaceship to FTL speeds the way most soft scifi takes for granted.
When time travel is well-defined, it’s not a problem. For example, I don’t think anyone has ever accused Primer of using time travel as a magic bullet.
And Homestuck just doesn’t have any plot point which isn’t time travel in some way.
More as a magic LSD substitute.
Except Primer-style time travel, where you can actually change the future of the universe by going back in time, is not very well-defined. Leaving aside the difficulty of reconciling such time travel with relativity, it is unclear to me what the rules are that govern returning to a particular time slice. What aspects of the past state are allowed to change, and what must remain the same? Are there any constraints at all?
The sort of time travel we see in the Harry Potter books (SPOILER jura uneel naq tnat geniry onpx va gvzr gb fnir Fvevhf Oynpx) is a much better example of well-defined time travel. There’s no “changing the future” involved. Every causal loop is subject to strong consistency constraints.
Well, every story that has people traveling or communicating faster than light basically has not-well-defined time travel in it. Which includes most scifi ever written, I’d guess.
That’s not really what I meant. There are well-defined frameworks of physical law that permit faster-than-light travel. Newtonian physics is an example. So even though FTL travel requires some suspension of disbelief, you’re not usually at sea about what the rules are that govern the phenomenon. It’s not like magic.
The problem with the way time travel is often depicted is that it is like magic. There’s no clear set of rules constraining which properties of the time slice can change when the traveler returns to it, and which must remain the same. Surely there must be some constraints. After all, if every single property is allowed to change, it wouldn’t really count as traveling back to the same time slice. But where do these constraints come from and what are they?
It’s a difference, but not a qualitative one, I feel. After all, reality is entangled and there’s no well-defined framework of physical law that allows FTL travel and also reduces to something like relativity as long as you don’t turn on your magical engine. Everyone who doesn’t have a magical FTL engine would still see your FTL travel as being time travel, and the same questions would have to be answered as you raise below: what changes are allowed, etc.
OTOH, if you go by which suspension of disbelief is easiest and most natural, and if you let go of your belief in scientific physical laws. then the most natural thing to believe in is magic. It’s what all humans believed in up to historically recent eras. The kind of magic that doesn’t usually have builtin rules in the sense of laws of the universe or of magic itself. It just has limitations corresponding to the knowledge/power/wisdom of the magician. The more common constraining rules in such stories are actually moralistic (good winning, etc.)
This is a popular but incorrect idea. If you interpret special relativity via (neo-)Lorenz Ether Theory, you can have perfectly mundane kind of FTL without any problematic time travel, you’d just have a preferred reference frame. It’d be possible to send messages to the past according to some reference frames (in spacelike fashion), but not others, and you’d never get to communicate information in time loops. There is no serious problem with reconciling FTL and special relativity, FTL goes the way of ether for Occam’s razor reasons, not because it doesn’t make sense conceptually.
There are perfectly consistent models of a relativistic universe with well-defined rules that allow tachyons (particles that travel faster than light). Frank Arntzenius has a great paper on this. These models don’t allow “future-changing” time travel, though.
Incidentally, does anyone know of a work of fantasy that has clearly articulating “law-like” constraints governing the use of magic? The awesome webcomic Unsounded seems to be attempting something like this, but the details are still fuzzy.
Most works of fantasy do this to some extent. For example, the One Power in Wheel of Time has very well-specified mechanics, though of course a lot of the complexity is left as an exercise for the reader. The metal-based magic in the Mistborn books is simpler, and likewise well-defined. You can also have works like Cardcaptor Sakura, which is basically an anime about a collectible card game; here, the cards operate according to some very specific rules, and each card has a narrow function, but the card list is very large and fairly arbitrary.
Interesting. That paper is behind a paywall, unfortunately.
But my knowledge of physics is very limited and I probably wouldn’t understand the full paper anyway. (What tachyon interactions with non-tachyons does he allow?) I doubt, though, that any model of tachyons allows you to accelerate your normal-matter spaceship to FTL speeds the way most soft scifi takes for granted.