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Say you have some way of seeing somewhere between a microsecond to 0.1 seconds into the future, or at least the most likely future. Is it possible,through something like a flip-book effect or some other illusion, to encode a message only another observer capable of seeing the same distance into the future could also read?

Clarifications

  1. Doesn't need to be encrypted, just laid out in/on a method or medium that enables only those with 'future-vision' to observe a message unobservable to someone without future-vision.

  2. The message must not be able to be revealed through mundane means, such as high-speed video or any kind of atypical photography.

  3. The future-sight capable observer can only see into the future or at the present, not both at the same time.

  4. It doesn't matter how briefly the message is available for our future-sight capable observers, just that only they can see it.

  5. Future-sight does not extend into space beyond the 'eye' of the observer.

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    $\begingroup$ Please clarify: (1) that the message needs to encoded as opposed to encrypted, that is, anybody who can see in the future can read it, not necessarily only somebody who knows a secret; (2) that this clairvoyance cannot be achieved by the simple expediment of waiting -- after all, if I wait 0.2 seconds I can see the future 0.1 second clearly. $\endgroup$
    – AlexP
    Commented Sep 19, 2019 at 3:21
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    $\begingroup$ The edits make your question a virtual impossibility to answer. 1) this one is okay. 2) The only advantage your supers have that normals won't get 0.x seconds after your writer writes his message is the momentary glimpse of possible alternate futures which a) is useless for non-momentary messages and b) can only pass messages that can be read in 0.x seconds, after which, unless the writer writes the message for everyone to see, it's gone forever as it is no longer a possible future. 3) connected to 2, if they can't see both, then they're stuck with momentary messaging or (cont'd) $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:11
  • $\begingroup$ Encoding that normals can get at if they are determined. $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:11
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    $\begingroup$ Well, the flaw to the 2d flip-book analogy is that a 2d perception is still possible by viewing them in rapid succession on the same screen and you specify that it can't be something normals can cypher. Now, a 3d sculpure is a better analogy to what you want, but that involves making up something in this case as noone on WB SE is a 4th dimensional being so we can't actually percieve or describe something exclusivelu 4th dimensional. So, like Asimov, you will have to invent something like @L.Dutch suggested Asimov's Thiotimoline. And we can't help you brainstorm/create an imaginary substance. $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:55
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    $\begingroup$ Is it possible to use this precognition for retrocausal inductive brute forcing? E.g. tell your brute-force algorithm to try the first 100 possible keys. But using your precognition you can already see that output indicating that keys 1-100 don't work, so instead you tell it to try 101-200. But really, what you actually saw was the report from that run that everything up to 200 doesn't work, so instead you tell it to try 201-300... then when you reach keys 487832401-487832500 you see that it works and just let the computer do it for real. $\endgroup$
    – AJMansfield
    Commented Sep 19, 2019 at 18:56

19 Answers 19

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Sure it is, and we can use modern technology to do it

(NB: This relies on being able to see into the future further than the OP requirement, but if that's an acceptable change in scale, this would work.)

So, have you ever heard of an RSA token? These are devices that work on the basis of giving you a passcode that changes every 30 seconds or so; similar to the Steam Authentication codes, which would also work in this case. All you have to do is change the synchronicity of the rotating passcode to be 30 seconds behind - in other words, make the RSA token one cycle backwards, and therefore anyone who types in the passcode displayed gets an error message because it's an old passcode.

But, your future seer knows what the passcode will be. He or she can see that, types it in and you're done. The passcode is valid, because from the perspective of the user of the tag, it's a future passcode. From the perspective of the authentication server, it's the current one.

Of course, this doesn't mean that if you have access to the authentication process directly it can't be subverted, but for normal use it takes an existing technology and applies a secondary layer of security by forcing the user to see the future passcode displayed instead of the current one. Once you use that, it's all just standard encryption processes, albeit with the extant vulnerabilities they incur.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch ♦
    Commented Sep 21, 2019 at 18:32
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Reaction times

Average reaction times for most people are roughly 200 ms for simple responses. If you can see 100 ms into the future, your reaction time is going to be roughly half that. So have something like a "Simon Says" type decoder that requires hitting a sequence of light-up buttons within an extremely short reaction window.

Of course, electronics are going to have much faster reaction times than humans, so with advance forewarning, something like this could still be defeated with the right hardware.

However, if you can see a bit further into the future (250+ ms or so), you could actually have the buttons to be pressed not light until after the response window closes. Hardware responding <1ms after the button lights doesn't help if the valid-press window closed 20ms before it ever lit.

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    $\begingroup$ Unfortunately, this can still be defeated by someone curious enough to check the device software and automate inputs at the correct times. $\endgroup$
    – DoctorJerk
    Commented Sep 19, 2019 at 3:35
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    $\begingroup$ @DoctorJerk If you use "technology can be hacked" as a requisite, then it's not possible to suggest any technology-based method. As written the answer does in fact fulfill all of your requirements in this question. $\endgroup$
    – March Ho
    Commented Sep 19, 2019 at 12:59
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    $\begingroup$ @DoctorJerk I only just now realised you're the one who asked the question. This requirement then invalidates Tim's answer as well. And pretty much any electronics-based solution. Since they can always be cracked. $\endgroup$
    – jaskij
    Commented Sep 19, 2019 at 23:59
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    $\begingroup$ Driving the device with a quantum random number generator would prevent someone cheating by cracking the algorithm. $\endgroup$
    – Tektotherriggen
    Commented Sep 20, 2019 at 8:30
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    $\begingroup$ It seems to me like this device can be easily secured against hacking. All you have to do is make sure that the sequence of lights is truly random. Most computer security relies on "pseudo-random" algorithms, which can be predicted with normal technology. A truly random algorithm, possibly based on some hardware that observes in real-time an unpredictable quantum phenomenon, would, I think, be unhackable. The only way to defeat this mechanism would be to predict the quantum state of the device in the future... and at that point you already have the 'future-vision' anyway. $\endgroup$
    – cowlinator
    Commented Sep 20, 2019 at 22:20
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It's "Heisencoded": your message is only there when no one is looking at it. The more closely you observe it, the less intelligible it becomes. This includes electronic observations.

However, people can blink - closing and reopening their eyes in about 0.1 seconds - to make the message momentarily "unobserved". Your temporally-gifted individual can thus use their precognition to, just before their eyes shut, see the message as it will appear once they can no longer see it.

Quite why many such messages are mysteriously scrawled next to statuary is often debated - as is the "blue box" and the resident "madman" mentioned therein...

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    $\begingroup$ Scrawly wawly? :) $\endgroup$
    – Stilez
    Commented Sep 20, 2019 at 7:47
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    $\begingroup$ In other words, “magic with some technobabble Heisenberg reference”… $\endgroup$
    – Holger
    Commented Sep 20, 2019 at 9:31
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    $\begingroup$ @Holger We apparently have a character or characters who can see the future. A metamaterial that can determine whether or not it is being observed is no more magical. I used "Heisencoded" mostly for the pun - this is more akin to combining the Elitzur-Vaidman test with the double-slit experiment, such that the message is only there when it isn't detected $\endgroup$
    – Chronocidal
    Commented Sep 20, 2019 at 9:40
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    $\begingroup$ @Holger You appear to be confusing the science-based and hard-science tags... I am also saying "here are the properties that a material could to exhibit make this possible" - (the Elitzur-Vaidman test and the double-slit experiment being actual relevant and experimentally-verified theories in quantum physics, instead of just random buzzwords) - which is different to, say, if the OP wanted a material with certain properties, and I essentially just recited the question back at them. $\endgroup$
    – Chronocidal
    Commented Sep 20, 2019 at 10:35
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    $\begingroup$ If I can perceive now stimuli that will reach my brain 100 ms in the future, and 100 ms in the future I'll have my eyes closed, then right before I close my eyes I'm seeing black as if I had my eyes closed (as I will), and 100 ms before I open my eyes I'll start seeing as if I had my eyes open (though I won't see as if I had my eyes open NOW, I'll see the photons that will impact my eyes in 100 ms, when they will be open). Not considering the delays in nerve impulses, of course. The point is that if you won't be observing the material, you won't see it. You'll just see black 100 ms before $\endgroup$
    – GuilleOjeda
    Commented Sep 20, 2019 at 15:26
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This power is completely broken.

Any sort of true futuresight is a form of time travel since you're transmitting data from the future to the past. The precog can not only transmit undetectable messages, he/she can ask any possible question and receive an answer from a possible future self encoded in physical responses that interact with this power. The precog only needs to daisy chain the answer back to when it was asked.

For example, the precog at time = 0 wants to know the answer to a yes or no question, the answer is locked 10m into the future for whatever arbitrary reason, and the precog has decided on an encoding of eyes open = yes, eyes closed = no. Precog + 10m sees that the answer is no and closes his/her eyes, followed by precog + 9m59.9s seeing the closed eyes and responding appropriately etc. Once precog t0 has the answer, he/she is no longer obligated to behave in any particular way in the future so the timelines of precog + 10m etc. collapses and there is no message for anyone else to observe.

Yes/no is of course an analog to basic binary, so any arbitrary message can be passed back once the precog has decided how to code it.

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    $\begingroup$ +1 You beat me to it. This is THE answer. You just described an oracle, which is a step above what you can do with a computer. And added nothing to the constraints the OP gave. A very simple usable question is just ask if any non-precog cracks your message in the future. There's corner cases for that question for sure, but like you said: ask any question. Not only that but it makes every currently usable algorithm O(1). (There are some problems that even an oracle can't solve, but I'd call them rare for sure) $\endgroup$
    – Black
    Commented Sep 20, 2019 at 9:05
  • $\begingroup$ +1. This is brilliant. Probably a different gesture though, since keeping eyes closed for 10m might be crippling in some circumstances. Something like putting a coin in left or right pocket etc $\endgroup$
    – Gnudiff
    Commented Sep 20, 2019 at 13:06
  • $\begingroup$ @Gnudiff Moving a coin between pockets in less than 0.1 seconds would be a superpower in itself - especially considering that human reaction times to visual stimulus are typically between 0.25 and 0.17 seconds! $\endgroup$
    – Chronocidal
    Commented Sep 20, 2019 at 15:03
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    $\begingroup$ I don't think this is going to work. The precog sees the answer now (or sees himself writing the answer ten seconds from now, by looking into the future). Anyone else waits those ten seconds and has the message decode, or looks at the precog transmitting the message into the past. Unless the decoding itself may be initiated by the precog and take place in a future so distant that no one else has access to it, but even then, you're stuck with a paradox in addition to violating the constraint "can see only the near future". $\endgroup$
    – LSerni
    Commented Sep 20, 2019 at 15:15
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    $\begingroup$ @LSerni From the perspective of the "present" precog, there is no need to operate the ability more than 1 time for a yes/no question (nor is there any need or possibility of writing something down). The point of time travel is that your future actions no longer occur, once the future information has been transmitted, that future no longer exists and can't be observed by a third party. The paradox of violating causality is inherent in the premise of the question, I'm not the one that introduced it. $\endgroup$
    – Teleka
    Commented Sep 20, 2019 at 15:37
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"[...] into the future, or at least the most likely future."

If less likely futures are game, then this is solvable.

Encrypt the message and destroy the key.

At the press of a button, generate a random key from true random numbers, and try to decrypt with it. Show an error message if it fails, but do not display the key.

The seer will be able to see the vanishingly unlikely future in which the random key guess happens to match the key that was used to encrypt.

You could explain it away as saying all the futures where it doesn't get properly decoded "collapse" into one since the outcomes are identical, so it's easier for the precog to see the one where it does get decoded.

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    $\begingroup$ This is interesting. You could explain it away as saying all the futures where it doesn't get properly decoded "collapse" into one since the outcomes are so similar so it's easier for the precog to see the one where it does get decoded. $\endgroup$
    – Captain Man
    Commented Sep 20, 2019 at 16:43
  • $\begingroup$ @Captain Man great idea, thanks! I edited it in, slightly changed. $\endgroup$
    – Emilio M Bumachar
    Commented Sep 20, 2019 at 18:31
  • $\begingroup$ This doesn’t work. “Display an error message if it doesn’t work” requires you to know what “work” means, which means you have to have a non-encrypted version for reference, which renders the whole thing pointless. If you’re just looking for “a sensible result” for the message, well you’ll find a vanishingly small chance of a key for every possible message, so then you’re just left guessing which message you want it to be. $\endgroup$
    – Jeff
    Commented Sep 22, 2019 at 9:03
  • $\begingroup$ On the other hand, depending on what you need it for, you might not need a message. Say you want to protect access to a bank vault, just have the bank vault key code change every second to a random number based on atmospheric noise. It would require the precog to be see far enough forward to be able to see the results of entering a code, but it’s a similar concept $\endgroup$
    – Jeff
    Commented Sep 22, 2019 at 9:06
  • $\begingroup$ @Jeff Keep a hash of the original key, hash the guessed random key, compare the hashes. $\endgroup$
    – Emilio M Bumachar
    Commented Dec 20, 2019 at 19:34
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I would place the message in a way that reading it would mean instant death to whoever reaches it (for instance, in the walls of a deeeep pit, just before reaching the bottom or a box filled with poisonous gas). This way, if you can see the future, you could intend on walking that path, read the message just before dying (in the future) and then change your mind and not go into the pit.

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  • $\begingroup$ This sounds like the Nicholas Cage movie Next $\endgroup$
    – Fering
    Commented Sep 20, 2019 at 16:45
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Asimov wrote some stories about a substance, the thiotimoline, which is so soluble in water that is dissolves before touching the liquid, thus seeing the future.

This as a consequence of the substance complex structure, which fold the space time allowing the molecule to see the future.

In Asimov's writings the endochronicity of thiotimoline is explained by the fact that in the thiotimoline molecule, there is at least one carbon atom such that, while two of the carbon's four chemical bonds lie in normal space and time, one of the bonds projects into the future and another into the past. Thiotimoline is derived from the bark of the (fictitious) shrub Rosacea karlsbadensis rugo, and the thiotimoline molecule includes at least fourteen hydroxy groups, two amino groups, and one sulfonic acid group, and possibly one nitro compound group as well. The nature of the hydrocarbon nucleus is unknown, although it seems in part to be an aromatic hydrocarbon.

In The Micropsychiatric Applications of Thiotimoline Asimov explains that

the rationale for thiotimoline's behaviour: namely that the chemical bonds in the compound's structural formula are so starved of space that some are forced into the time dimension. According to the second article, thiotimoline's time of solubility varies depending on the determination of the person adding the water.

If you use thiotimoline like a sort of invisible ink to cover the text you want to protect, you can remove it only with the right determination, since thiotimoline's time of solubility varies depending on the determination of the person adding the water. In this case is the substance to see the future, not the observer.

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  • $\begingroup$ but, wouldn't normals be able to dissolve the thiotimoline coating as well? This is also a possible flaw in my ink theory as, while unable to read the message, anyone that could detect the chemical could at least erase it. $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:31
  • $\begingroup$ @HAHarvey, only the encoder knows the right determination. $\endgroup$
    – L.Dutch ♦
    Commented Sep 19, 2019 at 5:32
  • $\begingroup$ So, how does a potential reader know the determination? $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:42
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    $\begingroup$ @DoctorJerk "Determination" can mean willpower, or it can mean "a made decision". If I hold thiotimoline above the water, it does not dissolve. However, once I decide to drop the thiotimoline in the water, it disolves before I drop it. If I am wavering about whether or not to drop it, it remains undissolved until the point of no return (as I start to let go). If I determine to let go in "10 seconds, no matter what", then it will dissolve far sooner - 1.12 seconds before I let go. Unfortunately, since "determination" in this case does not mean "willpower", it can't be used like this. $\endgroup$
    – Chronocidal
    Commented Sep 19, 2019 at 11:52
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    $\begingroup$ @Chronocidal, a user named "Time killer" explaining the working way of a time extended molecule. Nomen omen :-) $\endgroup$
    – L.Dutch ♦
    Commented Sep 19, 2019 at 11:58
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If you had a perfect laser (aka doesn't disperse over any distance, not actually possible but still) you could just shine it on a distant object (or mirror) and encode the message in it.

The only way to read it while in transit in space is to intercept it, requiring FTL or similar

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    $\begingroup$ This is pretty much what OP is looking for, unlike most of the other answers that either don't rely on the property asked for or that are just pseudoscience mumbo jumbo. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Sep 19, 2019 at 17:43
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    $\begingroup$ How is this related to foresight? If the laser bounces back, anyone besides the precog can just wait an extra tenth of a second for the signal. If the laser doesn't bounce back, the precog can't teleport in front of it (it's precog, not astral projection). Also, anyone with teleportation could teleport in front of it, they'd just have to teleport 30 Mm further. $\endgroup$
    – MichaelS
    Commented Sep 19, 2019 at 21:30
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Use Qubits

The value of quantum bits can only be described by a probability until they are measured, but once they're measured they are determined.

Using future sight, observe what the determined value of a quantum bit would be. In the present, run the qubits through quantum logic gates that change their value in some predictable way, measure them, then combine the results with the future results from the non-gated observation.

The key needed (the non-gated results) came from the future, and was effectively destroyed when you observed them in the present.

I believe this depends on how your future sight technology interacts with quantum mechanics. (Also, it's possible I'm overlooking something about how quantum bits works.)

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  • $\begingroup$ One-time-pad "an encryption technique that cannot be cracked" unless you have the key. If the key is only available in the future, you're completely sol if you're not a seer. Bonus points for needing to be the first, and therefore the only one, who can ever have read it correctly. - At least you didn't just name drop Heisenberg and then start talking about Dr. Who... $\endgroup$
    – Mazura
    Commented Sep 20, 2019 at 4:21
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It may not be quite what you're looking for, but I think there's an argument that someone who can see into the future in an interesting way, ie. one that allows them to act on the information they see to change the future, can decrypt anything.

  • They look into the future and see themselves trying to decrypt the message with some key
  • If they see themselves fail, then clearly that key wasn't correct
  • So, they pick a key at random different to the one they saw themselves fail with, and try that one - quickly enough that the attempted decryption finishes in less time than the interval that they can look forward into the future
  • Now we've changed the future, the past changes too - they must have seen themselves try the second key, so they wouldn't have picked that one, so they must have picked and tried a different one...
  • The only way to resolve the situation is when they pick the correct key.

Although we can think of this as a time loop eventually broken by picking the correct key, to any observer involved it actually takes no time at all - in our prescient friend's experience, they just see themselves pick the correct key and go with that one. However, it only works probabilistically - if we use the time loop model, we can see that if the chances of them picking the right key at random are significantly less than the chances of them getting hit by a meteorite, that's what's probably going to break the loop first. Or maybe this persons actions are the only thing that vary between 'loops', but there's still the possibility of them fumbling it and not completing the decryption quick enough for past them to have seen the result in time or something.

In general, time travel allows us to do arbitrarily complex computations in a fixed amount of time by reading the results of a computational step from the future, doing the next one, and sending the results back to the past. As such, to someone who can send information back in time (or equivalently, see the future), most encryption can be treated as just weak encoding.

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  • $\begingroup$ The user is in the present looking into the future. They'd still have to react in the present to the information seen in the future. You could speed up a calculation this way (look at the result one clock cycle after starting the calculation), but you still have to take the time to increment to the next key. With 64-bit encryption, that's about $10^{19}$ guesses on average. At one guess per clock cycle on a 4 Ghz processor, that's still 73 years on average. $\endgroup$
    – MichaelS
    Commented Sep 19, 2019 at 21:41
  • $\begingroup$ Yes, you'd have to come up with a way for a human to input the relevant state in under 0.1 seconds for this case - it's more that it's possible in principle. Thinking about it though - they look ahead, notice whether they're flicking their eyes left or right, and do the same, and repeat, thus sending one bit of information as far back as they want (up to the point they chose to start looking) - that seems like a primitive you could get arbitrary backward communication out of. I might have to think about that one. And yes, incrementing the key is a much better idea than picking one at random. $\endgroup$
    – Silver
    Commented Sep 19, 2019 at 22:29
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The only thing I can think of would be to use some manner of "invisible ink" whose presence and almost instant evaporarion were both invisible to the naked eye, but the overlapping presense of both simultaneously created a visible hue (so, only someone "flipbook viewing" both the present and the next moment would see it). Of course, the reader would have to have some sort of chemical reaction they initiated to read the message, and so the writer would need to leave some manner of glyph of identifying mark so a potential reader would know a message was there.

Otherwise, the ability to see moments into the future would not really provide "secret message" potential, except if they could see multiple possible timelines. Then, the "writer" could build the intent to write something, but decide not to, thus, it would only be visible along the possible future that would never happen. The issues with this are myriad, but the big ones are: 1) it would only be good as a "thieve's cant" for coded dialogue/messages in the moment with bother speaker/writer and listener/reader present. This is because the divergent timeline would only exist up until the decision was made, then be lost to those living on the current timeline forever. 2) your spies would need mich longer spans of time to look into the future in order to catch these messages 3) there is no "secret" to this other than having the ability. Anyone with the ability would quickly become privy to this method.

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    $\begingroup$ In the example with the invisible ink, I'm not sure I understand what you're getting at. How would seeing an invisible evaporation of invisible ink produce a visible hue? Also, to clarify, the reader and writer could only see either the present or the future at any given moment, and only a very short distance into the future, 0.1 seconds at most. $\endgroup$
    – DoctorJerk
    Commented Sep 19, 2019 at 3:03
  • $\begingroup$ @DoctorJerk it is a theoretical interaction for a theoretical sense. IOW, we "normies" could never discover that such a substance even exists. However, those with temporal perception would have little difficulty seeing the interaction, as they can see the present and the future like overlapping slides. $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 3:09
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    $\begingroup$ Okay, I understand what you mean, then. If I'm interpreting correctly, this could work, if one had a substance whose atoms interfered with only the light bouncing off their past selves, the gaseous state working as a sort of polarized lens for its previous liquid state AND the superpowered observer was capable of seeing both the present and the future at the same time. My original question didn't include the observer being able to see two temporal frames of reference at once, but this might be a workable solution. $\endgroup$
    – DoctorJerk
    Commented Sep 19, 2019 at 3:20
  • $\begingroup$ Encoding does not provide secrecy, quite the contrary: a common encoding is essential for successful communication. For example, this comment is encoded in ASCII. My computer knows ASCII, and thus the first letter of this comment, "E", is to be represented and sent over the network as numerical value 69. Your computer knows ASCII, and thus knows that the numerical value 69 received from the network represents the letter "E". $\endgroup$
    – AlexP
    Commented Sep 19, 2019 at 3:36
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    $\begingroup$ @AlexP not sure what your point is. The only time I mention coding is in the thieve's cant section, and that basically specifying that the above method would be encoded, as you explain, but not secret. The OP is essentially looking for an encryption method for his/her "temporal spies" to use that will allow them to keep their messages secret. $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 4:00
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One kinda hilarious answer involve quantum mechanics and being able to observe effects "too soon". I mean, I'm not an expert on quantum mechanics, but from what I understand, it's observation that collapses the waveform down. If you did the double-slit experiment, but were able to "see" where the photon would hit before it actually reached the double slit apparatus, I'm not sure the photon would be able to interfere with itself like usual. I have a feeling that a 'precog' looking at the double-slit experiment would never see a diffraction pattern, because the "observation" would be occurring before the photon actually got a chance to interfere with itself.

If that's the case? Then the solution is hilariously simple: just have a whole bunch of slits/dots/whatever in a sensitive box that breaks when you open it. Shine coherent light in one end, and look at the pattern that comes out. If no precogs were around, everyone would just see a mishmash of interference patterns. If a precog was around, it'd instantly change to be dashes/dots/whatever.

Edit: Delving a bit more into the quantum side of things. Basically, everything is a probability wave. How the double-slit experiment works is a photon is beamed at two slits, which it will pass through and arrive at a sensor that measures where the light hit. Intuitively, you'd expect that the photon would have to go through one of the slits and you'd see a 'hit' along two vertical lines. Repeat it over and over, and you'll get a rather weird result: an interference pattern. Basically, the photon is simultaneously going through both slits, and interfering as a wave - against itself!

Now, stage 2 of weirdness. Let's say you rig up something that can detect which slit the photon travels through. You then repeat the experiment. Now, though, the interference pattern disappears. That's because your information 'collapses' that wave down. You've 'erased' the possibility that it went through one of the slits, and because of that, the wave can no longer go through both slits and interfere with itself.

Stage 3 of weirdness. You use the same rig as Stage 2 - and measure which slit the photon travels through. But... now you delete this information before the photon can reach the detector (this is called the Quantum Eraser Experiment.) Now the interference pattern reappears! Why? Because while you measured which slit it went through, you deleted that information - and in effect, restored the possibility that it could have gone through either slit.

So, now, the big question: what's the effect of seeing where the photon lands before it goes through the slit or actually lands? And... ultimately, it's a time travel paradox.

If you observe the photon landing in a way that indicates it went through a specific slit, then you observe something that collapses the waveform down in a way that forces it to go through the specific slit, which in turn prevents an interference pattern from emerging.

If you observe the photon landing in a way that indicates an interference pattern, then you observe something that doesn't collapse the waveform down in a way that forces a specific slit, which in turn allows the photon to travel through both slits, which allows for the interference pattern.

Now, if this was a science site? I'd say, "There's no way to empirically test this; there's no way to answer this." But we're in a worldbuilding site. So you can resolve a time travel paradox however you want. You can say, "So what if they see an interference pattern a bit earlier than the photon actually hits the screen? The photon, being in an interference pattern, doesn't convey information about the waveform that would actually reduce it down." And you can say, "The precog's going to see the photon's final state on the wall based on which slit it went through - because their observation itself is what forces the photon to not interfere with itself."

Anyway, in my opinion, the thing that makes this answer so cool is... ultimately, it's taking something so ridiculously unintuitive and complicated and mindbending... and the resulting device is something a child could manufacture. (Well, not the 'can't open/scan' part.)

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  • $\begingroup$ I think this could work. A message or cypher key could be written in morse code that would only be revealed if said precog was present. The trick is in constructing an apparatus that would be immune to meddling and structural analysis like sonar and x-ray tech. $\endgroup$
    – DoctorJerk
    Commented Sep 19, 2019 at 23:22
  • $\begingroup$ The diffraction pattern only appears when the path of the photon is not defined at all, in other words, no-one “sees” which slit the photon took, not now and not in the future, because it didn’t even decide. So seeing into the future doesn’t make you see, which slit the photon took. Unless you think “seeing into the future” implies something else than just “seeing into the future”. $\endgroup$
    – Holger
    Commented Sep 20, 2019 at 9:37
  • $\begingroup$ @Holger, I'm not a quantum expert, but... that doesn't seem to mesh with my understanding. The diffraction pattern happens because the photon is in superposition when it travels through the double slit apparatus, because observation hasn't collapsed it's probability wave. But in this case, an observation is occurring before it travels through the apparatus - as I understand it, the observation would define it as going through one of two slits before it actually reaches those slits. It loses superposition, doesn't travel through both slits, and doesn't form an interference pattern. $\endgroup$
    – Kevin
    Commented Sep 20, 2019 at 13:11
  • $\begingroup$ Observing X means that you interact with X, changing it. Ie putting a thermometer in a fluid to determine fluid's temperature, will change it, because the thermometer has different temperature than the fluid ( and they interact as part of measurement). Putting in a thermometer that is the same temperature as fluid (and therefore won't change it) is only possible if you know in advance what the temperature of fluid is. Ie. something you put the thermometer in for. $\endgroup$
    – Gnudiff
    Commented Sep 20, 2019 at 13:12
  • $\begingroup$ @Kevin which observation? The observation of the diffraction pattern doesn’t make the diffraction pattern disappear. The observation of the weather outside doesn’t make the the diffraction pattern disappear. The only observation that makes the wave function collapse, is an observation of the path the photon actually chose or more generally, any action that forces the photon to choose. Without such an action, there will be a diffraction pattern. And seeing into the future, when no such action will happen, doesn’t change it. You’ll see that there will be a diffraction pattern. $\endgroup$
    – Holger
    Commented Sep 20, 2019 at 13:24
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If the user sees where something will be, but sees the object/person as it is now, then some tricks would be possible.

I'm thinking of a sign changing on the side of a subway car, where the future-vision sees it at the station and can read the sign, but that sign changes before actual arrival, therefore the other people would see whatever is changed to (Community College advertisement, I'm sure).

Or they see a person saying something while looking in their direction, but that person is behind a door at the moment, they'll be along shortly. Could be fun and creepy.

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You have to copy a password right before it gets shown on the display.

Say you have a small calculator that randomly chooses digits using some on-board entropy device (so actually random, not just pseudo-random). The calculator would flash a digit at a time and to unlock it you must've pressed that digit before it was even generated. Get 10 of these digits right in a row and the calculator reveals the message. Easy if you have future vision of something around 0.2s or near the human reflex limit, impossible if you don't.

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  • $\begingroup$ This doesn't hide the message, only a password that can be worked around. $\endgroup$
    – DoctorJerk
    Commented Sep 20, 2019 at 14:41
  • $\begingroup$ It would be very hard to work around. It's similar to the security chips in modern phones (like iPhone's secure enclave), except the password can be made arbitrarily strong and always changes. The only way to get around it would be to very, very carefully dismantle the integrated chip and read the memory directly. But this would most likely lead to destroying it. Imagine physically pulling apart a SIM card without destroying any of the internals. $\endgroup$
    – csiz
    Commented Sep 24, 2019 at 5:54
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(This answer relies on a 4-dimensional being, capable of "seeing" the past/future, being able to 'look around' 3D objects in the same way that we do 2D lines - even if they don't quite realise that that's what they are doing)

Start with 2 layers a special material (Such as e-ink or a liquid crystal) which can change via some mechanism, such as heat or electricity.

The top layer (Layer α) can switch, almost instantly, between Opaque and Transparent.
The bottom layer (Layer ß) can switch, almost instantly, between "Message" and "No Message".

When the Message is Visible on Layer ß, it is completely obscured by Layer α being Opaque. When Layer α is Transparent Layer ß is blank. Because the layers are so tightly fused together, it is impossible to change one without simultaneously changing the other.

However, by focusing on the future (or, alternatively, on the past) of Layer ß, while Layer α is still transparent, you are able to see the message that it will show when it is obscured.

Space / Time Graph showing the use of the Temporal Axis to look around or past an object in the Spatial Dimensions

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  • $\begingroup$ If there's a moment in the future where beta shows a message while alpha is transparent, a normal viewer could see it at that moment. Or just flip the page upside-down. $\endgroup$
    – MichaelS
    Commented Sep 19, 2019 at 22:03
  • $\begingroup$ @MichaelS Ah... the point is that in the future, α is opaque while the message shows, but you're looking around it using 'future sight' to see things from a different angle in higher dimensions. I might try a diagram later to explain $\endgroup$
    – Chronocidal
    Commented Sep 20, 2019 at 8:29
  • $\begingroup$ The future-sight observer only sees what they were already going to see a short interval into the future. Their vision doesn't travel around objects or ahead of them, so this doesn't work. It also doesn't hide the image from prying hands. Get a hold of the software that dictates the behavior of this liquid crystal or somesuch and you can analyze it to figure out the message. Difficult, but not impossible. $\endgroup$
    – DoctorJerk
    Commented Sep 20, 2019 at 14:36
  • $\begingroup$ @DoctorJerk Who said anything about software? This can be done as a hardware limitation, using an arrangement of tiny crystals, some of which respond to the impulse, and some of which don't. So long as the crystals have the same spectra and refractive index in the 'inactive' state, you can't tell them apart (like the special plastic which is invisible underwater!) (See also: doped silicon) $\endgroup$
    – Chronocidal
    Commented Sep 20, 2019 at 14:49
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    $\begingroup$ @DoctorJerk The way you have now defined the power (do you need to edit the question to make that clearer?), combined with the "not hackable" requirement mean that the answer becomes "No" - either it relies on 'preactions' and can be hacked, or the message will be visible to everyone else moments later. You need to decide which requirement you are willing to relax. $\endgroup$
    – Chronocidal
    Commented Sep 20, 2019 at 14:52
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Stronger encryption schemes like AES512 typically take longer than 0.5-1 seconds to encrypt sth. So just use such a scheme. The future looker already sees the encrypted/decrypted message while the computer is still going.

Anyone in the present sees at most an encrypted message (if your computer is that fast).

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  • $\begingroup$ Sounds promising. Can you provide a resource or example for someone who isn't savvy in cryptography or computer programming? $\endgroup$
    – DoctorJerk
    Commented Sep 19, 2019 at 3:52
  • $\begingroup$ @DoctorJerk I'll see when I can get around to it.. $\endgroup$
    – dot_Sp0T
    Commented Sep 19, 2019 at 4:50
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    $\begingroup$ @dot-Sp0T but, wouldn't normal people see the decrypted message a split second later? The future-seers would only see the decrypted message if it WAS going to be decrypted, and only a fraction of a second before it was decrypted for everyone. That means, it either is only secure for the fraction of a second that the seer has that edge . . . or not at all, since they don't necessarily read any faster than the normal viewer. Not trying to be a jerk, just I don't see how this helps the writer protect his message. $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:00
  • $\begingroup$ @HAHarvey indeed. The proposal meets the first paragraph though. $\endgroup$
    – dot_Sp0T
    Commented Sep 19, 2019 at 5:07
  • $\begingroup$ @dot_Sp0T it says "only another viewer capable of seeing the future", which was why I said it didn't $\endgroup$
    – HA Harvey
    Commented Sep 19, 2019 at 5:25
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The observer would have to convert their prescience into information that no-one else could possess. For example, that person might be able to accurately predict the outcome of a chaotic effect. They would have to repeat their future observations of a period of time long enough to ensure they couldn't have achieved it by chance alone.

They would have to add that information to the medium of the message in some way that the message reveals itself only if that information were correct at the instant it were supplied, so the if someone locked in the present were to do it, the information would be out of date by the time it were observed.

The best I can come up with is:

  1. The message is written on or with some kind of medium.
  2. The medium is reveals the message only if the exact inverse of the waveform of cosmic background radiation is supplied (analogous to noise cancelling headphones but for cosmic background radiation)
  3. The observer has a firefly-like radiation emitting organ at the wavelength of cosmic background radiation.
  4. The observer has evolved its prescience to observe future cosmic background radiation at a moment far enough in the future to exactly match the latency for their brain to transmit the instruction to their radiation emitting organ to emit the inverse waveform.
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Encrypt the message (with modern asymmetric cryptography, but short keys) then destroy the private key. Normal people will only see the message after cracking the code (which involves factorizing a large number), in a few seconds/hours/days/years (depending on the length of the key). People which can see in the future can "guess" what the result of the cracking will be, before it is complete.

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  • $\begingroup$ Can you elaborate on how this would work? Seeing the most likely future (as per the question) would allow you to see whether one single guess would be correct before making the guess (or you just generate a random key, which is most likely to be incorrect). But you still need to decide to make that guess, process the fact that it's incorrect and decide to make another guess, which takes a lot of time for a human (probably much longer than a machine would take to try a bunch of keys). $\endgroup$
    – NotThatGuy
    Commented Sep 20, 2019 at 18:53
  • $\begingroup$ This answer seems to not say anything this one does not. $\endgroup$
    – Captain Man
    Commented Sep 20, 2019 at 19:59
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Imagine a Morse code that relies on the viewer looking into the future every 3rd signal. This would allow you to include false signals in the message that would not be observed by the intended target. But in the long run it would only be scrambling a message and I imagine it would be breakable if it was recorded and examined for patterns.

I think most messaging done in this way is too limit to be unbreakable, by simply recording it a normal man can observe the whole message out of time.

Alternatively if you used the ability as an identifier you can use the cool trick of an elaborate handshake. Anyone with the ability would be able to make a random handshake by seeing what their partner expects of them with enough time to react accordingly. In this way any one can easily and secretly show you that they possess these powers

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