Some mechanism to locally reverse time

[Mavrisa]

Okay so this is going to be far-fetched, but I can't think of anything that sounds remotely reasonable.
For a story I'm writing, I need something which can be activated, and will reverse time to a set point in the past, but at the same time, will preserve all of the information in the user's brain. I'm hoping it can be the type of thing that can be put into a package about the same size and shape as a laptop.
I know there's nothing in the real universe that could accomplish this, especially without violating causality or the laws of thermodynamics, but my previous efforts to come up with some technobabble have resulted in things like hyperperfect crystals and I felt like vomiting after I read them. Does anyone have any better suggestions for such a technology? I welcome anything right now, since it's almost certainly better than what I've come up with. And at the very least, off-the-wall ideas might either get others or me thinking.
Thanks

[nehpest]

Just so we're clear, are you looking for something akin to a techie Harry Potter time turner, or are you looking for something that will allow the user to perceive time flowing in the reverse direction?

If the former, it's a standard time travel scenario, possibly with the restriction of only allowing a certain maximum time delta. If the latter... it's an interesting problem!

If the backstory of your universe will allow it, perhaps you could construct a device that alters the local copy of the universe's source codesuch that the Arrow of Time/laws of thermodynamics are reversed?

[tomandlu]

As nehpest says, are they just jumping back in time or do they need to see time unwinding? Also, does the user know that they are going to use the machine? Can they set up an anchor-point in advance?

[Mavrisa]

Awesome, questions. This is very similar to the time-turner idea (I don't know why I never made that parallel... especially since I just reread all of them a couple of months ago), but in my mind, they would be able to see time play in reverse for a brief... period... of time? Though unlike in Harry potter, they would not have the freedom to look around or move intentionally. Everything would happen exactly as it just did but in reverse, including everything they did. There would be no copies of them made.
And these anchor-points can be set both on purpose and accidentally - that is to say, it doesn't happen randomly, but some action which is intended to be performed triggers it.

[tomandlu]

And once time starts moving forward again, can they then change events or do they just watch a replay?

[Mavrisa]

Well at this point, they have knowledge in their heads about what happened once before, so this pretty much ensures that the course of events will not be the exact same as last time. But yeah, they're pretty much free to do what they want once time resumes its usual direction.

[tomandlu]

Well at this point, they have knowledge in their heads about what happened once before, so this pretty much ensures that the course of events will not be the exact same as last time. But yeah, they're pretty much free to do what they want once time resumes its usual direction.

Cool - I wasn't sure if the were just passive observers, like in Vonnegut's Timequake.

Hmm... tricky. Memory and conciousness would have to be treated differently from everything else, which requires an epic amount of handwavium... why not tackle it the other way round? It's not that they are going back in time, it's that they get to see forward in time and then get a chance to change stuff. i.e. "show me what will happen if I didn't know what was going to happen."

The only problem/difference is this requires you to decide before any particular event that you want to do this, rather than having the luxury of waiting until after. Not so great if you've just watched your kid killed in a freak accident or something...

That said, it opens up odd possibilities - a stranger might come up to you and say "err... you might want to turn on your time-turner at 3.25 PM tomorrow and get ready to change stuff" (the stranger having seen the consequences of not doing so).

I dunno... still requires industrial quantities of handwavium, but slightly less wince-inducing fundamentals IMHO...

[Copper Bezel]

I don't mean this to be trolling, though it might come across that way, but the simplest solution that comes to mind is to use a different genre. Even the softest of soft SF worlds is going to be cheapened by the existence of such a device. OTOH, Khafka didn't need any technobabble at all to explain why a random salesman might be spontaneously converted into an unspecified arthropod of equivalent mass.

[Mavrisa]

It's not that they are going back in time, it's that they get to see forward in time and then get a chance to change stuff. i.e. "show me what will happen if I didn't know what was going to happen."

The only problem/difference is this requires you to decide before any particular event that you want to do this, rather than having the luxury of waiting until after. Not so great if you've just watched your kid killed in a freak accident or something...
[...]
I dunno... still requires industrial quantities of handwavium, but slightly less wince-inducing fundamentals IMHO...

At first I was disappointed with this answer, but I think I like it... a lot. I don't see how it can be accidentally induced, but the person doesn't have to know that they aren't actually time travelling. Yes. I think this can work. Thank you.

I don't mean this to be trolling, though it might come across that way, but the simplest solution that comes to mind is to use a different genre. Even the softest of soft SF worlds is going to be cheapened by the existence of such a device. OTOH, Khafka didn't need any technobabble at all to explain why a random salesman might be spontaneously converted into an unspecified arthropod of equivalent mass.

I looked at this possibility but, I had already planned out a character who was going to have to figure out how the thing worked and then try (and probably fail) to explain it to the main character. The failing part was where I was going to give myself a little freedom to be handwavy.

[tomandlu]

At first I was disappointed with this answer, but I think I like it... a lot. I don't see how it can be accidentally induced, but the person doesn't have to know that they aren't actually time travelling. Yes. I think this can work. Thank you.

Indeed - you can even play with the fact that to the reader it might appear the characters were travelling backwards in time in an explicable way before dropping the bombshell... Another problem/interesting bit is the question of how these newtonian flash-forwards interact with each other... imagine two card players both cheating using this method...

[Tass]

Check out the computer game "Braid". In it you have the ability to do just what you describe.

[Charlie!]

The characters are actually living their lives inside a simulation, and the device has access to the control system.

This is the less tame version of "the first run-through was a simulation, but the characters are from the real world."

Or it could download their brain scan, send it back in time through a more handwavable method (atom-sized wormhole being my favorite), and upload it into the person's past brain. Living life in reverse for a bit is just a side-effect of how the new memories are loaded in.

[Mavrisa]

Indeed - you can even play with the fact that to the reader it might appear the characters were travelling backwards in time in an explicable way before dropping the bombshell... Another problem/interesting bit is the question of how these newtonian flash-forwards interact with each other... imagine two card players both cheating using this method...

Or it could download their brain scan, send it back in time through a more handwavable method (atom-sized wormhole being my favorite), and upload it into the person's past brain. Living life in reverse for a bit is just a side-effect of how the new memories are loaded in.

Well now I'm torn. Hmm.

Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time

I don't understand this but it sounds cool, and information is all I need to be sent. I could also use the hype about neutrinos possibly being FTL, but I don't think I want to base this around something that could be overturned in the next year.

[undecim]

So we want a SF equivalent to a time turner, but with anchor points, an no copies...

How about a brain interface chip (BIC) or similar device with a builtin tachyon reciever... that is, it can communicate with itself in the future. It would rewrite the memories of the person it is sending back in time. The anchor points can be the time where the tachyon reciever is on (it's not always on, because it would take up too much power, which it doesn't have much of, since it receives all its power from the brain it's in)

Your vision of them seeing time moving backwards could just be them experiencing their memories being written in reverse chronological order

This could allow you some interesting plot devices to use as well... For example, receiving these memories would mean that the chip (or a replica?) must exist at the point in time in the future in order to send those memories back to itself, or there would be a time paradox which the universe doesn't allow. (the person, and all organic matter in his/her brain can be destroyed, but it would be impossible to destroy the chip... It's proven not to happen because of the tachyons that were recieved earlier) This could mean the hero would know that even though he e.g. ostensibly has a 70% chance of dying in a violent explosion if he attempts to save another person, there's no way it could happen, because it would destroy the chip as well...

A plot hole with an interesting cover as well: If you receive memories from a chip, then you could theoretically cause some mechanism to destroy the chip if you die, and then perform heroic and dangerous acts, knowing that you are guaranteed to live through it. However, those individuals that would do such things would never be able to receive the memories... This is because the universe will not accept paradoxes, no matter what, and so such situations are extremely unlikely to arise, since you are more likely to e.g. die from a heart attack than to survive crashing your car into a rock wall at 120mph. This explanation can give the text a naturalistic feel... You could perhaps have someone attempt this, successfully plant in his head a bomb to destroy the chip in the case of his death, and have him become paralyzed by a freak accident before he has the chance to try to kill himself.

This touches on some weird time-travel-related ideas I had a couple months ago... Ideas so weird that I found myself testing my sanity with a random number generator... (It's complicated)

Damn... I wish i had the time to write a SF novel...

[Mavrisa]

This could allow you some interesting plot devices to use as well... For example, receiving these memories would mean that the chip (or a replica?) must exist at the point in time in the future in order to send those memories back to itself, or there would be a time paradox which the universe doesn't allow. (the person, and all organic matter in his/her brain can be destroyed, but it would be impossible to destroy the chip... It's proven not to happen because of the tachyons that were recieved earlier) This could mean the hero would know that even though he e.g. ostensibly has a 70% chance of dying in a violent explosion if he attempts to save another person, there's no way it could happen, because it would destroy the chip as well...

[...]

You could perhaps have someone attempt this, successfully plant in his head a bomb to destroy the chip in the case of his death, and have him become paralyzed by a freak accident before he has the chance to try to kill himself.

This touches on some weird time-travel-related ideas I had a couple months ago... Ideas so weird that I found myself testing my sanity with a random number generator... (It's complicated)

Damn... I wish i had the time to write a SF novel...

I was thinking about this paradox, and I decided that when you activate the anchor point device, the information is received at the instant afterwards (the smallest fraction of time possible. I think this was because the device could otherwise be made to let you know if information was successfully transferred. This is something I don't want to happen), the anchor point is turned off, and the universe splits into a new timeline. So if you die or the chip is destroyed, there's absolutely no way of knowing; you'll head off boldly into the future and die... and then that future just carries on - the device never gets the information in that timeline, and there's no way to signal to you before you start off that you will either die or the device is destroyed. It's only if you live to activate your return ticket is the data transfer realized, and you'll start living your life with one possible scenario in your head, but you'll be on a different timeline which branched off at that moment you first activated the anchor point. I'm hoping there's no flaw in that logic. Someone please correct me if I'm wrong.
Also, I'm thinking that the memories shouldn't be written in reverse chronological order, because this would present a huge continuity error to your brain - while you were watching time reverse, you would have no recollection of the period between the anchor point being set and the return, and this void would shrink to whichever moment you were currently watching until it caught up with the present. Then I thought - error checking? It accesses all of the memories/pathways created in reverse order to make sure it got everything right. The error checking takes much longer than the writing itself, and so this is what you see rather than the writing in forwards order (I'm imagining this as a blinding white light for a barely noticeable period of time).

What happens if you die, and the device is activated? Do you become brain-dead in the other timeline? That would suck. I suppose it could transfer back "null" and then... well then you'd be stuck in a recursive loop, provided everything happened the same way each time. At that point only another device could change things and save you by altering the timeline. Does that make sense, because I can see that providing an interesting plot twist.

I'd like to know how this sanity test works... or maybe I wouldn't.

And yeah, I don't have any time right now (I have a lab in <4 hours, which I will not be very awake for), but I will soon enough, and so I thought I'd start planning now..

[undecim]

I was thinking about this paradox, and I decided that when you activate the anchor point device, the information is received at the instant afterwards (the smallest fraction of time possible. I think this was because the device could otherwise be made to let you know if information was successfully transferred. This is something I don't want to happen), the anchor point is turned off, and the universe splits into a new timeline. So if you die or the chip is destroyed, there's absolutely no way of knowing; you'll head off boldly into the future and die... and then that future just carries on - the device never gets the information in that timeline, and there's no way to signal to you before you start off that you will either die or the device is destroyed. It's only if you live to activate your return ticket is the data transfer realized, and you'll start living your life with one possible scenario in your head, but you'll be on a different timeline which branched off at that moment you first activated the anchor point. I'm hoping there's no flaw in that logic. Someone please correct me if I'm wrong.
Also, I'm thinking that the memories shouldn't be written in reverse chronological order, because this would present a huge continuity error to your brain - while you were watching time reverse, you would have no recollection of the period between the anchor point being set and the return, and this void would shrink to whichever moment you were currently watching until it caught up with the present. Then I thought - error checking? It accesses all of the memories/pathways created in reverse order to make sure it got everything right. The error checking takes much longer than the writing itself, and so this is what you see rather than the writing in forwards order (I'm imagining this as a blinding white light for a barely noticeable period of time).

What happens if you die, and the device is activated? Do you become brain-dead in the other timeline? That would suck. I suppose it could transfer back "null" and then... well then you'd be stuck in a recursive loop, provided everything happened the same way each time. At that point only another device could change things and save you by altering the timeline. Does that make sense, because I can see that providing an interesting plot twist.

I'd like to know how this sanity test works... or maybe I wouldn't.

And yeah, I don't have any time right now (I have a lab in <4 hours, which I will not be very awake for), but I will soon enough, and so I thought I'd start planning now..

In my vision of it, if you're dead, the device would not send back any information, unless a third party fools the BIC into thinking that it's in a brain, and so when it's activated in the past, it would basically tell the user "no data received" (suspense!)

Also... The model of time travel I'm considering is different from what you're describing... In my model, there is a single timeline, and it is a fundamental law of the universe that paradoxes don't arise. (Basically, http://en.wikipedia.org/wiki/Novikov_se ... _principle) As a consequence of this, getting a "no data received" message is proof that the BIC is not operational in the future when the data would be sent. (This could either be a fixed point in time, or whenever the BIC sends back the data). The BIC idea could work with either model though, but this was the view my previous post was considering.

If you are going to consider multiple parallel timelines, then you can have the user receive an arbitrary memory, a sudden sense of fear, a flash of light... Anything that a BIC could cause a user to experience to indicate to the user that no tachyons were recieved. That this happens is not a guarantee that the person will die (and it might be a good idea to exemplify this early on), since receiving that memory splits the timeline. Similarly, receiving memories is not a guarantee that you will live that long (unless you successfully perform every action exactly like you "remember" it... but that would make for very boring time travel)

The BIC could also be programmed to transmit all stored memories on the death of its user, if and only if some condition is met... It can have the power of a modern computer, so there are a lot of possibilities. Just find something that fits into your plot with a minimal number of holes.

[Diadem]

Something very similar to this is done in The Void triology by Peter F. Hamilton (Book 1: The Dreaming Void; Book 2: The Temporal Void; Book 3: The Evolutionary Void).

Spoilered for minor spoilers

Spoiler:

In this book he explains that the black hole inside the center of our galaxy is in fact not a black hole at all, but an artificial construct (called The Void), functioning like some kind of giant articifial reality. As a result the people inside have telepathic and telekinetic abilities. And also the ability of going backwards in their personal timeline. This does not give any paradoxes, since actual time keeps going forward. The Void just resets to an earlier state (with the person inside keeping his memories).

The energy requirements to do this are massive. With just a few people living there, the thing already eats entire stars to fuel itself. A major theme in the book is that a large group of humans want to migrate to the void, because of the abilities gained there. This would trigger a massive growth of the void however, destroying the rest of the galaxy, so this is frowned upon by most of the rest of the human race, as well as several rather advanced alien races. The books are obviously rather soft scifi (FTL all over the place, and that's just the start), but rather entertaining.

But his method of time travel is rather awesome. I had never seen it anywhere else. And it is truly paradox free.

[orderlyAnarch]

I thing the Stargate episode "Unending" would fall under this.

Spoiler:

The team is in a ship that is being tracked by the Ori due to recent sufficiently advanced upgrades. they are about to be destroyed, so Carter puts the ship in a time-dilated bubble. In-bubble, it takes 50 years to figure out how to reverse time in the bubble. by this point they are out of power, so they have to use the blast to power it. Someone has to bring a message back so this won't just loop endlessly, so Teal'c(near-humanoid alien, already 100, lifespan of centuries) stands in a small field with a crystal to solve the problem while the ship is destroyed around him. then the last 50 years reverse in a few seconds, and he is where he was right when the bubble was made. He gives them the chip, they stop the tracking and FTL home. The only thing he will tell them is that vague details and that 50 years passed.

[RaptorRider]

At first I was disappointed with this answer, but I think I like it... a lot. I don't see how it can be accidentally induced, but the person doesn't have to know that they aren't actually time travelling. Yes. I think this can work. Thank you.

Indeed - you can even play with the fact that to the reader it might appear the characters were travelling backwards in time in an explicable way before dropping the bombshell

Another Idea is a reverse of this, they suddenly find themselves with knowledge of the future, but don't know how they got it. Then they spend time trying to find out how before they figure out that they are actually receiving the memories of a future them through the use of that device.
You can also use the theory of parallel universes where the device induces the creation of a parallel universe in which the past them has their memories.

[Quizatzhaderac]

So technically this is a way to universal reverse time, except locally. So I'd say it makes more sense to explain it as doing

So these are the rules your looking for as I understand them:

1. The user intentionally chooses a start point
2. The end point must be activated
3. When a living user activates the end point they experience the interval quickly and in reverse order
4. In the reverse interval, the use cannot take new actions. Their actions are fixed and reversed like the rest of the universe.
5. After a use cycle is complete, the universe is one nanosecond after the start point with the user having experienced the interval and shortened reverse interval.
6. This experience is a difference which can cause the future to be different.
7. If you die (and that prevents you from using the device) or the device is destroyed then you lose the ability to return to the start point.
8. You cannot use the device to predict losing the ability to return to a start point

7 and 8 seem to rule out an kind of simulation: simulated death shouldn't stop the simulation ending.

So these aspects seem undecided:

1. Is there a finite limit to how long a time interval can be?
2. What happens if you exceed the maximum interval?
3. During the reverse interval, can the user have a novel internal monologue? no monologue? The same monologue reversed?
4. What happens if a second one of these devices is used during the interval of the first?
5. What happens if you rig the device to go off when your heart stops beating?

Here's what I propose: The device sends memories back in time. It sends them back in reverse order but still sends the information on their ordering as well. The reverse interval is the period where the memories are being written to your brain. But since the memories include their own order the user believes that they activated the device, experienced the interval, then experienced the reverse interval. You'd pretty much have to scan the user's brain during the write process to tell that it was reversed interval first, forward interval reconstructed later on recognition.

As for the limitations: it works up to a certain "causal distance", anything more than that and the device can't transmit back to the start point. In a sealed room a sealed room it takes a long time to reach a causal distance. Less in an open room, and even less it you have events like a spaceship arrive at near the speed of light. If a second device is activated during the interval of the first the causal distance of the second interval is multiplied by how strongly the two intervals interact (range 0 to 1) and the result is added to the causal distance of the first interval.

[WarDaft]

Don't need time travel at all. Assume the world works something like MWI - writers have much more freedom to do that than scientists. You have a device with a 'Save' button on it, it stores some quantum information that perfectly represents the fork you're at in the universe (in some way that it can't be accessed classically, to prevent it from acting as a form of omniscience, say it's like entanglement, people have usually heard of that) then, a 'reload' button, that forks you into a low universe equivalent to the save point except for the fact that all the matter within 2 meters of the save point in that universe has randomly quantum tunneled into deep space at that instant, and then you pop into the void produced. It also sets up a perfect fridge horror moment when the user realizes that for every time they've pushed the button, out there in the universe somewhere is a dead frozen copy of themselves that thought pushing the save button was a good idea.

It doesn't produce a rewind effect though.

[Xanthir]

Plus, anyone else with parts of their bodies within 2m is now left with chunks teleported out of them in the original universe.

[WarDaft]

Actually, the chunks would be teleported away only in the universe you're jumping too. Remember, we saved the universe as it was, minus nearby matter still being nearby.

But really, this isn't actually bad or anything, you aren't causing this to happen, it happened anyway and you're just showing up after the fact. You could even demonstrate this by poking the button and then not jumping back, after all, the probability that anyone would actually observe that outcome is minuscule, it's only the case that you see it because you were aiming for it.