# Addressing paradoxes with Weapons designed to hit a target in the past

In space, normal weapons are so easy to dodge because by the time you see an enemy ship and fire at it, it's already moved out of the way. Now imagine a weapon that travels backwards in time as it flies though space so that when it arrives at its target, it converges with the exact time and location that you saw the target when you fired. So, if an enemy ship is 5 light minutes away. Then your shot travels back in time 5 minutes as it closes the distance eliminating all guess work about where the ship will be.

For purposes of this question, assume this weapon can only travel back in time as fast as light can travel forward in time. So you can target a thing in the past exactly where you see it now, but you cant target something you saw 10 minutes ago.

While such a weapon would seem to work without any major paradoxes when you just have one ship shooting another, what would happen if 2 ships shot at each other with such a weapon. Since both ships could in theory be destroyed before either ship actually fires thier weapons does there need to be some rule that one event will take precedence over the other, or is there a logically consistent way for both ships to destroy each other in this manner since neither ship captain could see any future events that might cause him to change his course of action.

• Congratulations. You've found an easy and obvious way to show that time travel generates paradoxes. Commented Sep 22, 2022 at 22:33
• "While such a weapon would seem to work without any major paradoxes" except the situation you present is already a paradox. If you fire a weapon at an enemy ship (no time travel) they can respond to that event. They can alter course, activate shields, deploy counter-measures, just brace for impact, etc. All reactions that will alter what happens by the time the shot reaches them. With time travel included, you'd shoot it "in the past" but the enemy would still take some reaction. They'd take that reaction in the past, thus altering the present at the time of you firing.
– VLAZ
Commented Sep 23, 2022 at 9:37
• Reading Asimov's Thiotimoline series would give you insight into existing thinking on this issue Commented Sep 23, 2022 at 9:52
• This doesn't sound like time travel to me. It's identical with a weapon that simply teleports its projectile to it's target. Or what would be different with it? And then the answer is clear: whoever shoots first wins.
– Ivo
Commented Sep 23, 2022 at 12:49
• The hardest paradox to resolve would be if a (mad) person on the ship decides to suicide and self target its own ship. How can you press the "shoot" button if you've just been blown to hell ... but wait, where(/when) did that missile come from if I didn't shoot it myself because I was already turned to dust ??? haaaaa. Chicken or egg ?? Missile or Dead operator ... which came first?
– Hoki
Commented Sep 23, 2022 at 13:52

## Frame challenge

A time-travel paradox is by definition an inconsistency that cannot be resolved. This is why people think it's so important to avoid them: because it is logically impossible to resolve a paradox if one arises.

Thus, there is no "logically consistent" way for this to happen. You are going to have to invent some new rule that defies ordinary logic. As the author, you can do this! As the author, you cannot decide whether your audience will buy it.

We like to think that one of the best ways to ensure that audiences will accept our narratives is if we stick within the bounds of conventional reason. But works like Star Wars and Star Trek do all kinds of things that are patently absurd, and that has not stopped countless people from enjoying them.

• Ideally you just freed all visitors from having to feel grumpy about said patently absurd things. This visitor in particular thanks you and goes back to his regularly scheduled enjoyment of patently absurd scify stories. Commented Sep 23, 2022 at 11:48
• A paradox is an APPARENTLY inconsistent situation. They usually have logical solutions, they are just are not always obvious. Paradoxes typically arise from bad assumptions, but can be easily solved once you realize what those assumptions are. Take the Barber Paradox for example. Depending on how you word it, you can make it appear as though there has to only be 2 possible cases because there are only 2 possible outcomes, but logically, the barber is just a 3rd case with an undefined outcome. Just because something is undefined, does not mean it is illogical. Commented Sep 23, 2022 at 19:19
• @Nosajimiki there is no logical solution to time travel. It completely breaks how we consider the universe to be. There are plenty of stories about time travel or FTL travel but they are not logically consistent, at least for OUR universe. Doesn’t mean you can’t write a story about it but then either you just don’t explain it, or be ready to come up with a system that does work. But even then you probably don’t wanna get into the nitty gritty as it just wrecks any logically consistent world. Commented Sep 24, 2022 at 11:11
• @Nosajimiki The point of the barber paradox is that it is illogical to define the barber. So yes, in that case, "undefined" does mean '"illogical". Same with "This sentence is false": it's illogical to define it's truth value, because you end up with a logical contradiction, no matter what. The problem is that time travel paradoxes are a different sort of paradox, a normal impossible question: something for which we do not and, for now, cannot know the answer. They do have logical solutions, usually several, but we don't have the information to know which is correct. Commented Sep 25, 2022 at 1:48
• @Nosajimiki for your viewing pleasure Commented Sep 25, 2022 at 1:49

As written in the question

Then your shot travels back in time 5 minutes as it closes the distance

the shot doesn't begin its trajectory 5 min back and then travels to the enemy. Instead it travels back in time by moving toward the enemy.

So i agree with ivo's comment and mostly with JBH' answer except for his proposed paradox.

Because the shot isn't send back in time but travels backwards while moving there is no way to detect it before it hits. It would just seem to materialize into the enemy ship.

It seems to me that this is just a "teleportation weapon" in effect. You press a button, and in that instant your projectile hits. No time to dodge or even realize something is closing in, so no chance for dodging and the resulting paradox. (To be fair this is a jab at the other proposed answers, because they use it as an example of the loop that can result when the shot is send back in time and then travels toward the enemy.)

On a timeline it would seem to work like this:

T0 You spot the enemy.
T0 You fire and see the enemy getting hit.

In full:

T-5 The enemy reflects the light you see at T0, your projectile hits.
T-4 Your shot has traversed 80% of the distance and moved 4 min backwards in time.
T-3 Your shot has traversed 60% of the distance and moved 3 min backwards in time.
T-2 Your shot has traversed 40% of the distance and moved 2 min backwards in time.
T-1 Your shot has traversed 20% of the distance and moved 1 min backwards in time.
T0 You spot the enemy.
T0 You fire and see the enemy getting hit.

• You've done a very good job of clarifying what I mean by how the weapon moves back in time as it crosses space, but note that the weapon was fired at T0, but what happens if the other ship shot at you at T-3? In this series of events, the enemy ship technically shot first... but did not shoot until after getting hit. Commented Sep 23, 2022 at 16:54
• It can't. At T-3, the enemy ship is already 2 minutes dead. T-6, it's fine, T-5, instant death for no apparent reason. As someone else mentioned, if you both see each other at the same time, the ship that fires first wins, as the other ship has already exploded, and you see the light of the explosion the instant you press the fire button. Commented Sep 23, 2022 at 20:03
• Good answer. More clarification. Time is not a linear thing, it doesn't happen in "real-time". We have 4D space and time is a part of it there are rules to validate/create this 4D space. So, only the enemy ship is destroyed in the past, it happens before you pressed the button. And since that happened already, you are going to press that button when the time comes. Commented Sep 25, 2022 at 9:31
• @CemKalyoncu And, of course, you can't know that you are going to press the button before you do — because the first time you get the information of whether you did or not is when you do or don't. You see the ship explode just after you press the button, not before it. (I say just after, because the signal from the button to launch the projectile still appears to be limited to lightspeed, and then the projectile/launcher has to react) Commented Sep 26, 2022 at 0:10
• This is incorrect. There is always a paradox when you go FTL and time travel. Ship A sunk the ship B 5 light minutes away. Ok. Except I'm ship C. Ally of ship B. Im 4 light minutes away from ship B and 6 light minutes away from ship A. I see ship B get sunk and I immediately fire at ship A. I have now sunk ship A before it fired at ship B. Why? Because I saw it destroy ship B. Cause after effect. A paradox. Commented Oct 11, 2022 at 8:11

# Whoever hits first gets to control the timeline.

If ship A hits ship B first, then ship B can't fire at ship A. Therefore, their temporal projectile vanishes, and they do no harm.

Example.

1. Ship A's projectile goes back in time and Ship A and B fly unaware.
2. Ship A and Ship B see each other.
3. Both ships prepare to fire back in time, but Ship A fires first.
4. Ship B explodes immediately from the back in time projectile.
• This is the outcome my gut tells me to expect, but I cant really think of how to express why this would or would not happen as opposed to other possible solutions. Commented Sep 23, 2022 at 19:30
• You can just say that happens, according to the laws of time travel. Commented Sep 24, 2022 at 10:03
• It requires more and more energy to mess with space-time over somebody else — possibly with some technobabble, probably involving words like "entropy" or "chaos theory" — to the point that it's not impossible, just... exponentially more difficult. Depending on your setting, this could even allow you to turn the whole thing into some kind of poker battle where you have to commit just the right amount of "space-time alteration energy" to get the job done without risking that your opponent commits higher resources.
– F.X.
Commented Sep 24, 2022 at 15:30
• @Nosajimiki Ignore a dimension for a moment; imagine triangular 2-D ships on a 2-D plane, and extrude it into the third dimension, such that a Circular planet exist as a Cylinder in Time. Your weapon is a Diagonal Line backwards along the 3rd axis — and when it hits a target, that ship's Triangular Prism stops advancing. Since it no longer exists further down the line (when ships see each other in "firing range"), it isn't there to launch its own retrograde projectile. Commented Sep 26, 2022 at 0:23

All ships are 100% automatic (it doesn't matter if it they are or not, it's just easier for the explanation). The moment my ship detects the enemy ship, it fires the weapon. The very moment the circuits activate the weapon, my enemy is hit.

Ditto for my enemy, which did the same to me.

Where's the paradox? If my ship detected the enemy ship first, the enemy ship won't get a shot off. If the enemy ship detected my ship first, I won't get a shot off. If we detected each other simultaneously, neither of us will be around to argue about whether or not there was a paradox.

But I want a rule!

I literally can think of only one scenario where a paradox can occur. Your detonation can occur before the weapon is activated. In that case, the paradox of your ship destroying another ship, but you were destroyed before you activated your weapon, could occur.

Rule: don't do that. Regardless the range you wish to give your weapon, no detonation can occur before the weapon is activated. In other words, time travel in your universe is causal. No paradox can occur because no effect can occur without the cause that brought the effect about.

And why do you want your rule? Because if you didn't then all it would take is one drunken sailor ordered to fire the weapon to look at his friend and say, "watch, this will be funny" and not push the button to rip all of space and time apart. You don't want that. Nobody wants that. Well, psychopaths might want that. But we're not psychopaths, right? RIGHT?

• I'd argue there is more of a paradox. Let's say you shoot a target as soon as it's detected. And it takes, 5 minutes for the shot to reach there. So, how it's resolved is that essentially you shot 5 minutes before into the nothingness. This would be fine except, what if the target detects the shot (5 minutes before detection/shooting) and corrects course. And as a result, in 5 minutes time, your ship will not detect the enemy ship. Therefore it does not need to shoot.
– VLAZ
Commented Sep 23, 2022 at 9:43
• @VLAZ Unless there's a world rule Nosajimiki hasn't mentioned, information cannot travel faster than the speed of light. If we're talking about an energy weapon, it can't be detected before it hits. If we're talking about a missile, that migh be another issue, except that the missile is travelling backward in time as the enemy ship is traveling forward and it's moving from a spatial location ahead of the enemy ship. It's an interesting thought game, but I still don't see a paradox. If anything, they'd be seeing the missile moving away from the intended point of impact.
– JBH
Commented Sep 24, 2022 at 1:23
• @JBH More importantly, the target would only be able to see the missile (moving away from them) after they've already been hit and exploded… And the attacker can only see the explosion after they fire the weapon. Commented Sep 26, 2022 at 0:50
• @Chronocidal I believe that's not true. If the weapon is pushing itself back in time to arrive at the opportune moment, then it can be seen at those past moments by the enemy at the same moments. But what would they see? Here's where a rule of the OP's world should be put into play because time travel is a game like no other. One way of interpreting the time travel is that at each infinitely small placement in space the affected object is moved back to the moment of initiation. In this case, the enemy would see a solid bar for a fraction of a second (those reflected (*continued*)
– JBH
Commented Sep 27, 2022 at 4:24
• ...photons that got away during the linear translation back in time) at some moment after the detonation.... wait! That's exactly what you just said. OK, another interpretation is that the time travel pushed the weapon only as far back in a single step as necessary to arrive at the opportune time. If it's an energy weapon, that would be a pretty brief period. A missile might give rise to the paradox @VLAZ is talking about. This is the problem with time travel - the rules must be set by the OP because there is no scientifically "right" answer.
– JBH
Commented Sep 27, 2022 at 4:26

No real paradox free time travel

Unless the time travelling component is required, you can get around this by making weapons fire their projectiles at very near light speed. You could say they somehow weaponized neutrinos or something similar. This would have very similar results in that the attacks are unable to be dodged, undetectable, and can only hit a target where they are currently at (unless they are extremely far away).

Another good way to deal with this is treating the shots as instantly teleporting the distance as Mileonen mentioned. While this does result in problems of FTL travel, those are a lot easier to ignore than time travel.

The paradox you proposed of not having a ship to shoot at so why shoot does happen anyway regardless of whether there are the two ships are firing at each other. If the ship is destroyed before you would have pushed the button to fire, then there would be no need to press it in the first place so you wouldn't have fired the shot that destroyed the enemy ship.

If the time travel is important I would follow what JBH recommended and make a rule where time travel is causal.

Another way around this might be that the weapon's computer and projectile exists in its current state across time eg any firing information and the fired projectile exists on it in the past present and future. With the time of the attack being part of the firing information the projectile will only meaningfully "exist" only for the moment it strikes the target. This, along with the firing information only meaningfully existing for the crew when the data is entered and after that, would remove the paradox (or at least make it feel like its not there, there is probably something I am overlooking). This is a lot more complicated though.

• If your weapons fire at light speed, then for an enemy 5 light-minutes away you'd hit them 10 minutes after they come into detector range. Certainly enough time to change course (even if doing so randomly). The "travel into the past" weapon was meant to get around this. Commented Sep 24, 2022 at 14:40
• The way a light speed weapon gets around this is that whatever you are detecting with is also using light speed to detect it. Your detector waves would take the same amount of time to come back to you as the projectile. So while it was detected you won't know it was until too late Commented Sep 24, 2022 at 17:12
• The problem is not that the weapon might be detected, but that the target just might be somewhere else randomly (without even knowing that there is a weapon coming). Commented Sep 24, 2022 at 18:16

Suppose that Novikov's self-consistency principle applies. In this scenario, there are two outcomes that are each self-consistent: ship A destroys ship B before ship B fires, or ship B destroys ship A before ship A fires. Which of these outcomes actually occurs is a matter of random quantum mechanical probability, determined by a sum-over-histories wavefunction result that I suspect may be infeasible to calculate for any semi-realistic scenario at this scale.

I think the paradox can be resolved by adding a necessary delay: mass prevents time travel. The mass of the firing ship means the weapon needs to travel a bit before it can start traveling backwards, and the mass of the target ship likewise causes the weapon to reenter normal time a short distance away. Just tune the effect so the arrival time is too small to allow a response and the weapon will largely function like you want. The mass of the weapon itself isn't relevant, as the whole thing will time travel. The relative masses of the two ships is also not important, as the departure delay for one ship;s weapon will equal the arrival delay for the other ship's weapon, so the total trip is functionally symmetrical in both directions (less massive ships would have a slight advantage in departure time, but an equal disadvantage in arrival time).

So simultaneous shots will both arrive, as the two shots will have time to travel before their source is destroyed. Near simultaneous shots will also result in both ships being destroyed, due to the delays.

Super massive ships (I'm thinking planetoid mass) would have enough arrival delay to respond with point weapons, but the cost to make, move, and maintain such a ship would be prohibitive. Likewise, the time travel feature of the weapon isn't usable too close to a planet, or more likely inside a solar system at all. Maybe outer reaches are low mass enough. Battle fields with asteroids or debris or whatever would require carefully aimed shots to avoid having the weapon knocked out of time travel too soon.

Seems to ME that it's really contingent on the mechanic by which you decide time "actually" functions. Even Einstein and Hawking kinda "settled" into the whole infinite parallel universe thing for lack of better means to reconcile these sort of incongruities.

Fact is, for you to have cause and effect, action/reaction, determinism/consequence, there's ALWAYS a sequence of events. And there's one thing nearly all those theoretical physicists DO agree on: time is relative from the point of view of the observer.

So my suggestion would be reductionism: break the sequence of events down into smaller and smaller chunks and ultimately there's no paradox. Yes, it's possible one party is retroactively "deleted" from the timeline. Whereupon, for the frame of reference of their opponent, they never fired. They ceased before it could become a conflict. And since their opponent, too is submerged in the same stream of time, they, perforce, would be unable to perceive any sort of potential paradox.

Or, take the string theory approach and assuming every choice bifurcates into every possible choice, and obviate the potential for it entirely.

Personally, I think the mechanic by which the characters could have perceived an event taking place that was subsequently balefire'd out of existence yet they still remember it would/could/should be harder to explain than the nitty gritty of the temporal torpedo... but maybe that's just me.

The flight time of the weapon is instantaneous, in effect.

It's just as if the two ships were literally touching nose-to-nose.

It is asked about what happens if "the ships fired at the same time." Same thing happens as would happen were they nose to nose, I suppose. Either "the same time" really means plus or minus a few seconds or milliseconds, and one ship is destroyed before it can fire, or, if it is truly instantaneous, it depends on whether the weapons are such that they can complete firing while being destroyed.

The first major problem is (as defined in your question) that even assuming a time traveling weapon was possible the observer in control of it cannot! The weapon can be fired at a set of coordinates but there exists no targeting system which lets the user observe the past to detect approaching threats! This means that in terms of space combat any two ships randomly encountering each other are limited to the moment one or other vessel first detects it's opponent via the same old boring sensors like radar or thermal imaging etc that everyone else uses in space combat.

You literally can't know in advance using your 'time scope' (because you don't have one) that your enemy will appear at co-ordinates XYZ in precisely 10 minutes time. That means your limited to using conventional sensors to locate and target your opponents and all such sensors have margins for error. So the longer the range the less accurate your targeting is going to be. Which means you precisely the same dilemma you have with conventional weapons. You can either fire at long range with a high probability of missing the target or wait till your targeting solution becomes better.

The second limitation is related to the first in that time equals distance in this context. The farther back in time you attempt to aim the farther away in space the target will be which again means the less accurate your weapon becomes. So its not like you can 'see' and enemy and then switch the weapon to '3 years ago' aim at the space dock where that ship was being built and blow it up before its even launched! And since range increases the farther back in time you try to fire inaccuracy must also. (Except in some particularity rare or unusual combinations e.g. 2 ships traveling close together for years on time who suddenly decide to start shooting 'time bombs' at each other.)

• “there exists no targeting system which lets the user observe the past to detect approaching threats!” Sure there is. It's called "light". If you are approaching me, then light reflecting off you will reach me, and show me where you were when the light bounced off you — but, at short range, that's only fractions of a second into the past. By contrast, there is no targeting system which lets the user observe the present to detect approaching threats. Commented Sep 26, 2022 at 13:14
• Perhaps I wasn't clear enough. As I clearly stated in my answer any ship would have to use "the same old boring sensors like radar or thermal imaging etc that everyone else uses in space combat". Those operate at light speed. I also clearly referenced a non-existent device I referred to a "time scope" i.e. a device that bi-passed the speed of light and let an observer detect events before they occurred, thereby defying causality. So in effect you are simply repeating what I already stated i.e. that in this scenario all observers are limited to observations conducted at the speed of light.
– Mon
Commented Sep 27, 2022 at 1:23