# How can an average person prepare themselves to verify their future time-traveler self's identity?

I'm just an average person. I'm not a millionaire with thousands of dollars to spare. I don't have any kind of special power. I'm not currently affiliated with anyone working on anything grand. I'm just a 20 years old college student trying to get through life. However, for one reason or another, I'm expecting myself in the future to travel back to the past (my future, his past) for whatever reason.

It might be to warn myself of a future disaster or something grand, so I can't simply get others involved.

I'm also a huge skeptic so since time-travel is real, why wouldn't there be a possibility that there are mind-readers?

How can I prepare myself to verify that someone is myself from the future?

Let's call my future time-traveler self "FTS" and my present average self "PAS".

My background is in Information Security, so my first thoughts come to Multi-Factor Authentication (MFA).

## Something I know

### Past Events

The easiest things to have FTS talk about are events in the past, especially things so embarrassing that I'd swear to take them with me to the grave. However, human memory is finicky, so memories of past events are easily warped.

### Passphrase to be remembered

I can try to memorize a passphrase and commit it to memory. This passphrase should be hard to forget, so the problem of bad memory should kinda be solved. However, mind-reading is a possibility, so someone can read the passphrase from my mind as I ask them about it.

### Future Events

I can get FTS to state a future event. Something like the lottery would be too random and is easily changed thanks to the Butterfly Effect. Big natural occurrences like earthquakes would take too much time to verify, since we'd have to wait for them to happen. Man-made events such as crimes have insufficient credibility, since FTS could just cause them after stating them, or stage the event to happen shortly after his statement on the event. Solar Weather seems like something an average person would have a hard time verifying.

## Something I have

What do I have now that can be reliably stored for decades?

I may get a big hard drive, put a lot of random data on it, and put it in a secure vault. However, having a single device would be too unreliable, and trying to put multiple drives in multiple different vaults maintained by reliable third parties would be too expensive for the average person. Also, whoever I entrust the drives to would be able to impersonate FTS, and I can't trust them.

## Something I am

User fingerprints. This is cheap, fast, and simple. However, from what I've read, fingerprints are not as unique as Hollywood makes it out to be, and I can't be sure FTS will have the same number of fingers as PAS.

Just get a DNA test. This is currently my simplest idea. I'm no biology expert, but there probably isn't anyone that isn't me who would get the same results as me, right? But then again, as far as I know, DNA kinda changes over time, so it may not be as reliable as I'd hope. Additionally, FTS could have been exposed to some radiation that changed his DNA.

This is not a duplicate of 15448 since I'm just an average person without super powers.

This is not a duplicate of 12348 because I'm relying on myself alone to prove the time-travel.

• Comments are not for extended discussion; this conversation has been moved to chat.
– L.Dutch
Sep 16, 2020 at 12:55
• Knowing future events would only show that that person is a time traveler, not that's specifically FTS. Sep 16, 2020 at 21:14
• I, personally, would disagree with Something I know Past Event. There is an event in my past that I am actually VERY embarrassed that I acted in a way. It wasn't an embarrassing thing, nor would anyone else be, or have been, embarrassed "on my behalf", in fact, the few people who were present, I can safely and surely guarantee would not even remember the event. That does not change how I, internally, feel about it. I don't dwell on it, so would be unlikely not be reveal-able via telepathy, but still, it is a thing that only I know how I feel about.. Sep 17, 2020 at 14:51

# Predicting Future Passphrases

Dialogue:

You: How do I know that you're me?

Future Self: I've written down a passphrase on this paper.

You: I don't have a passphrase.

Future Self (hands the paper over): You don't have one yet. Put this paper in your pocket, then take a minute and think of the strangest passphrase you can. Then take it out and read the paper. Be sure to remember the passphrase for the next 40 years, or this won't work.

You: What if you can read my mind?

Future Self: It doesn't matter because I can't change my answer, it's already written down in your pocket.

You: Oh yeah! (pause) OK, I just made up the passphrase "Flibbertigibbet walks upside down". Let's see if it matches. (takes out paper) ... Wow, I've really got to remember to take better care of myself!

Future Self: Hey!

• This assumes that mind access is read-only, if the mind was not ROM, it is vulnerable to mind-writing. Sep 16, 2020 at 0:27
• @MarchHo: If mind-writing is possible there is no way to verify anything, because you can force them to believe literally anything.. Sep 16, 2020 at 8:19
• This test only proves that whoever Future Person does indeed come from the future -- excluding mind manipulation. They may indeed be Future Self, or they may be someone else who just managed to get their hands on the pass-phrase. Sep 16, 2020 at 13:26
• @MatthieuM. Right, we've simply moved the vulnerability from reading PAS' mind to reading FTS' mind. No one can come back in time and get the passphrase directly from PAS, since they haven't come up with it yet, but all an impersonator needs to do is find FTS at a later date, get the passphrase, and then go meet PAS while pretending to be FTS. In a world with mind-reading, a passphrase doesn't prove you're anyone at all - it just proves that you at some point had access to someone who knows the passphrase. Sep 16, 2020 at 14:44
• @Shufflepants Butterfly effect would only have a chance to kick in once FTS appears - between then and the passphrase generation is only a few minutes. Sep 19, 2020 at 5:18

We need a test that lets me (PAS):

• prove the person in front of me is FTS
• prove the person in front of me did not read my mind
• prove the person in front of me did not cheat through additional time travel
• not sensitive to butterfly effect

I don't think it's possible.

Any test that relies on a mental attribute, such as remembering something or solving a puzzle the right way, can be cheated through additional time travel. As previously mentioned, (fake)FTS stops in 2021 and tortures the answer out of me, then comes to 2020 and delivers the answer to me. This even works with the 'put this in your pocket then write down your own passphrase and see that I got it right' method proposed in another answer. Mind reading is a substitute for torture if you wish (and if time travel is real and this isn't just a present-day faker).

Any test that relies on a physical attribute such as scars or implants or even fingerprints can be cheated even more easily. With all of time to play with, there is every likelihood that (fake)FTS can find a doctor to change him physically to match the test.

Tests that do not require either physical or mental attributes will not prove any continuity between PAS and FTS.

In the world of cryptography, the problem of proving who I am is solved by use of public key infrastructure, and it requires a trusted authority. From OP's question, he doesn't trust anyone, and when time travel is involved any data test can be cheated with additional time travel, just like the mental attributes.

I conclude that it is impossible to convince a sufficiently paranoid PAS.

• Agree 100%. With mind reading and unlimited travel anywhere into past and future, there is nothing that can be done to be absolutely certain. Sep 14, 2020 at 23:36
• "(fake)FTS stops in 2021 and tortures the answer out of me,". Why, though? OP stated mind-reading exists, so the impostor can just read the mind of FTS to get the secret, then travel back. Sep 15, 2020 at 7:19
• If the mind reader is also a time traveler this is the only reasonable answer given the lack of detailed biometrics. Sep 15, 2020 at 8:29
• And even if there is a very special convoluted way, which would protect against all of this, the time-traveler could travel to your past and make sure you never find this way. Sep 15, 2020 at 9:50
• @Polygnome it is needed in case the answer is randomly generated during the test, as in the 5d20 answer below. The answer does not exist in the head of the PAS but it does exist in any future instance from that moment on. Sep 15, 2020 at 13:34

Specific future knowledge

Future you is from the future. You agree (with yourself) that if you ever need to travel back you should do so armed with some future knowledge from the day after your arrival date. This knowledge should be publicly obtainable, well recorded, hard to forge and relatively continuous. Temperature records, obituaries or newspaper headlines should do nicely. In today’s global society you can get information from anywhere in the world, so you’re spoilt for choice.

Furthermore you agree on precisely what piece(s) of information your shibboleth will be and (should you ever need to change it) record what the required info is for any given day. When future you decides to travel back they can simply look up (and pick up, if they’ve been storing old newspapers) the shibboleth for the day after their arrival date, meet you and Present their credentials. You then wait a day and verify the information. If someone is a mind reader they will know where the information should come from but not what it is. If they’re not a mind reader but are from the future they won’t know what information to present to you when they return. If they are you they will know both what key you expect to be given and the value of the key from the future.

This falls over if your mind readers can also travel back in time, but in that case there is no way to do this since the attacker can always get the information required to break your security from future-you...

• That is a tricky problem, which is why I went with a numeric my future self could obtain, but that I didn't know. If they can get the info from future-you, you're kinda screwed. Plus time travel gives your time-travelling doppelganger the ability to groundhog-day and try things over and over again by traveling to slightly different time points further back so you don't know it's happening. I envision a doppelganger HQ with 30 fake you's, "So who has 10:45? You're up!" Sep 14, 2020 at 14:03
• @DWKraus It gets worse if you allow meta-travelling. Then the hypothetical attacker can wait until your future you has successfully authenticated, steal the info, jump back to just before future you arrives and masquerade as future you perfectly. If they have colleagues that can prevent future you's departure 'after' they have the authentication information, so much the better! Sep 14, 2020 at 14:05
• @DWKraus For me it was trying to wrap my head around multiplayer Achron. Never has a video game had so much promise and been so tripped up by implementation... Sep 14, 2020 at 14:09
• Whats prevents the impostor from simply mind-reading or beating the FTS until the tell them what the required info is, and then travels back in time? Sep 15, 2020 at 7:24
• @JoeBloggs I did, but that is exactly why the answer is useless, isn't it? We have to assume the impostor can time-travel, otherwise the whole question is completely moot (if only OP can time travel, they can be sure to only ever be themselves). Sep 15, 2020 at 8:35

Go for a vacation to the secluded place, alone. Make a tattoo on your chin. Don't use a mirror, so that even you don't know how exactly it looks like (that takes care of mindreaders). Wear a beard for the rest of your life. When FTS arrives, shave and compare.

• "Wear the beard for the rest of your life" isn't something present-you can verify will-have-happened. Sep 14, 2020 at 20:08
• Ha, I love the dedication to the cause here. Sep 15, 2020 at 16:30
• +1 for the power of the beard. @chasly-reinstateMonica shave your head, tattoo it. When future you comes, shave heads and compare. Sep 16, 2020 at 14:21
• @Renan - the doppelganger goes back to just before the tattooing takes place. Installs a hidden camera and copies it perfectly. Sep 16, 2020 at 14:40
• @chasly-reinstateMonica yeah, but then the same thing can be said for most other answers. No solution will be 100% perfect. Sep 16, 2020 at 14:42

Inception

In inception they have a similar problem. How can you distinguish something as real or a dream if people can recreate any aspect? By having a totem.

A totem is something you have with some properties only you know. A chess piece with a certain weight distribution, toppling over at a certain time is one example they give. But it can be anything. From a secret compartment with a key to an inscription felt somewhere on the totem. If only they know this, it'll be next to impossible to recreate this in a dream for someone else. So all that is required is

1. Having the totem.
2. Knowing something special about the totem.
3. Protecting the totem at all costs, so only you have this knowledge.

This way you can be reasonably certain that the person is you. But as with all ID, you're not relying on just one. You're already referencing n factor authentication. So why not add some?

DNA however is difficult. You'll need the equipment yourself, so no one can fake or tamper with it, and have the know how to work it. Not an easy task for an average joe. Even then they might have gotten DNA from you in the past, so they might still be able to fake things. And if you go deep enough youll see that environmental genetics will change the gene expression as well, making things not identical. But we probably can't verify DNA of me or my identical twin because of this. So they could possibly clone you and just use that for the required DNA, or even just send a brainwashed clone depending on the technology. Still, for 2 or more factor identification it's not bad.

Data can be reliable, more or less. But it requires storage in crystalline structures. Again, not easy to come by.

So I would say a totem first and try to verify it with one of the other possibly more complex or less reliable methods as extra security. Even just pretending that's the official way would be great, and only your future self would know about the totem. This way, you might not even need to protect a totem, so no attention is ever brought to it.

• "Protecting the totem at all costs" would be the hardest problem here. I don't have a super tough & secured vault or a place I can safely store the totem. Since it's a single item, if I entrust it to a third party like a bank, that party would have the totem and all the time they need to analyze and replicate the totem. Sep 14, 2020 at 13:03
• @JohnZhau but how will they know it's your totem? You don't need to have it with you in this case. If you make it and just put it anywhere you like, they'll not know about it. What are they going to do? Recreate all items you have or have hidden and show them? You'll know sonething is up when they show something different. Maybe even make 10 totems and only one or two real ones. That can even decrease the chance they get it right by accident and you alone know what to show to yourself. You only need a totem at one point for verification, and for the rest they are unused. Sep 14, 2020 at 13:30
• This doesn't work. The fake you goes to a few weeks ahead of now and beats the truth out of the near-future you (or mid-read them). The fake then comes back to the present day with all the required knowledge. Sep 14, 2020 at 23:46
• @chasly-reinstateMonica if you go that way there is no way to solve it. Spread things around the world into super secure safes at several institutions? Can be beaten out of you. Anything you either have or know can. The things you are are too difficult, because fingerprints are easily faked and DNA requires knowledge. Iris, or any other the same. I would say literally measuring your member would be your best bet unless they have great plastic surgery, and it won't work as a woman. I would say then it can't be done. Sep 15, 2020 at 5:40
• This way the thing you know can't really be read, as it can be a weight or feeling. You might even commit that to memory and destroy it afterwards recreating it later. Will they now force you to recreate it? I mean at a certain point you can get anything, especially if they can beat you up in the future and have time to fake it. xkcd.com/538 Sep 15, 2020 at 5:48

Biometrics. More than one kind. DNA would match to your lab grown, artificially aged clone as well as to you! But there are other biometric identifiers that are generally thought to be stable (not changing with age) and epigenetic (even your twin would have different ones). Use multiple types to increase certainty.

Fingerprints: You've mentioned that fingerprints change over time, which is true, but as long as they are not too worn down they are still recognizably the same. Also you mentioned that your future self may not have the same number of fingers as your current self, which is also true, but it isn't a problem as long as your future self still has some fingers left. More fingers is better, but if you could match 8 out of 10 and the other two are missing then you're doing very, very good. You also have some things going for you, in that you can take your time to collect really high quality prints for both sides of the comparison. With crime scene matches, etc., they are often working with only partial or poor quality prints. This is also difficult to tamper with, you can examine your future self's hands to make sure there's nothing funny with them and provide your own ink, paper, and magnifying glass. Then do your own comparison.

Facial recognition. Not the fast-but-bad kind like phone unlock, but shell out for the really good kind that gives you a percentage of match. Use it in conjunction with other methods.

Iris recognition, palm vein recognition: Might be a little harder to get your hands on the hardware for this, but I think it is at least commercially available. This would probably be cutting edge for current consumer technology.

Biometric recognition of the future: Voice (better than today's which kind of sucks)? Gait pattern?

One potential problem from the comments--someone could (theoretically) grow a clone, kill your actual future self, and then do hand/arm transplants and eye transplants onto the clone. If they can grow a clone, we will accept that they can also do transplants so cleanly that it is impossible to tell. But I do still like this as part of a multi-factor authentication.

• This is the only real solution I can see that addresses most of the problems. The remaining one is that it's not clear whether, in the next 40 years, technology could have developed to fake these markers. However, 40 years is not such an incredible amount of time, so it seems likely that this would not be possible. Cloning could be an issue - but again, I do not believe this will be achieved in the next couple of decades, really Sep 14, 2020 at 21:56
• "I do not believe this will be achieved in the next couple of decades". It doesn't have to be. Time-travel a thousand years ahead, get the necessary work and/or technology and then come back. Sep 14, 2020 at 23:34
• @bytepusher I do address this a little bit, these markers are (as far as we understand) dependent on such minute variations/randomness during fetal development to the extent that even identical twins don't have identical markers for these things. But I did think of another potential problem so I'm adding that to my answer. Sep 15, 2020 at 16:15

## Whatever you do, it won't matter

Your future self will remember what it is that proved your identity to yourself. You'll just do that.

I mean, if my future self came and visited me. I think it would be an event I would remember for a long time, even as long as it takes me to find a way to travel back in time.

Thus, if my future self is able to convince me that I'm from the future, then I already know what to do, and there's nothing I can do to prepare for meeting me.

I thought about it and my own personal solution is something like this:

Several pieces of totally random emotions, memories, and dreams that I never disclosed to anyone or written down.

The totally random part insures 2 things. Very unlikely that they another person will use to fact check. Very unlikely to be on top of my mind.

The combination above is way more personal and better to verify who that person is.

My high school score or body scars or porn history or medical history or other such things can be learned.

But who on earth would remember a dream in which you where trapped on a rooftop surrounded by zombies while the moon was a large spaghetti ball?

However, it's possible, depending on the setting, that the introduction of mind reading can complicate--and possibly even annul this solution

• "However the introduction of mind reading ... can ruin things." I think that this should be amended to "... definitely do ruin things..." Sep 14, 2020 at 23:18
• @chasly-reinstateMonica, Thanks. So. My intention is to express this idea: If you add mind reading to the setting this addition can complicate it. By using can I mean it us NOT guaranteed but rather it is a possibility depending on the setting. So. How do I properly express this idea in the sentence? Sep 15, 2020 at 4:28
• @Seallussus - Like this? "However, it's possible, depending on the setting, that the introduction of mind reading can complicate--and possibly even annul this solution" Sep 16, 2020 at 4:06

# Self-multilate

Your future self will arrive whole. When they do, cut a finger off of your own hand. That finger should then disappear from future you, maybe to be replaced with a prosthetic one.

Here is a graphical example:

Source for the comics above: https://www.vgcats.com/comics/?strip_id=271

• this will only work for certain forms of time travel. for instance, it won't work if time travel splits a parallel universe off in which future you isn't causally connected to present you Sep 16, 2020 at 21:53

The mind reading makes this really hard, but I think I have a fun solution never tried before.

Untested biomarkers. When future you shows up, the two of you go to a perfume store. You each separately sample 50 scents, writing down for each one how much you like it on a scale of 1 to 10. Then, compare your answers. They should be within 90% the same or so (with some variation caused by future experience, like a hated future ex-girlfriend who wore a certain scent).

Oh, a couple edits for four dimensional safety. The exact store used should be picked by a random number generator, which because of chaos theory would be different each time someone went back in time, so they can't try this twice after screwing up the first time with their new answers from the past. And during the test, future person has to write down answers first to prevent simultaneous mind reading. And of course you have to burn your answer sheets after reading, and only past you gets to read them (making all effort not to remember answers, as a double firewall).

• This won't work. (1) It is well-known that, as people get older, their sense of smell and taste diminish. This means they tend to like stronger odours and tastes than when they were young. (2) The random number idea doesn't work. If random numbers (and events) differ every time someone goes back in time the whole course of history will change. Sep 14, 2020 at 23:29
• Yes @chasly, but of course this is easily accounted for in a shaped distribution curve (same shape at different levels). And given that a standard random number generator is based on millisecond time calculations, if you go back in time and use one it will never land on the same number twice, because you would never be able to reproduce your actions to the millisecond timing. Sep 15, 2020 at 16:28
• I've come to realize that if I rate the same 50 things on a scale from 1 to 10 twice with like a month long break in between, I'll be more likely than not to get below that 90% of yours, unless I really really know each of those 50 things before I start rating them the first time. 40 years from now? Oh god I have no idea. But what prevents someone from just trying this completely randomly, on failure go back, give new random answers to new random perfumes, on failure repeat until some day he'll finally get it randomly right?
– Mark
Sep 16, 2020 at 12:51

Step 1: Get a dollar bill.

Step 2: Tear the dollar bill in half (keep the smaller half)

Step 3: Always keep the torn piece of dollar bill with you.

If anyone presents you with an identical piece of torn bill (ignoring reasonable minor wear), they almost certainly travelled from the future (or a parallel dimension?). Copying the randomness of a tear is pretty nearly impossible. Dollars are durable and fairly water-resistant, so it's unlikely to suffer more than minor wear if you're not excessively careless. And having a torn piece of a dollar is relatively inconspicuous, so people are unlikely to guess that this is something special and try to hijack it.

Bonus points if you rarely think about it, making it less likely that a mind-reader can anticipate the test and think of a workaround.

But how do you know it's actually you? Well, you can never really be certain of anything, can you, Monsieur Descartes?

• Descartes? Shall we say to our past selves, "You think and therefore we are?" Sep 15, 2020 at 16:31
• @CaptainSkyfish The original argument leading up to the famous Descartes quote includes the possibility that a malignant supernatural being could be actively falsifying all of your perceptions (ie, you can't be perfectly certain of much anything). Sep 15, 2020 at 18:04
• thanks, that is a great point! Sep 15, 2020 at 18:48

Spend time with yourself

Assuming you do not have a hard time requirement to accept FTS as your true FTS, simply spend a good chunk of time together.

Play all the multiplayer video games you both love, prepare some dinner, agree on a movie, etc. Date yourself as they say!

Depending on the age difference of PTS and FTS there really shouldn't be that drastic of personality differences. Knowing yourself is also something that a mind-reader cannot overcome that easily because it isn't about a right answer it is about all the unspoken things that make yourself, yourself. The mind-reader might see that you have started to notice he isn't gripping a fork the way you do but by then it might be too late. Copying someone 100% in everyday behavior is near impossible.

• I think it's incredibly easy to do something that past-you will consider out-of-character, even if it is completely in-character. We lie to ourselves about ourselves quite a lot. And every single unimportant thing could lead to past-you doubting you. Like gripping a fork differently by accident :) Every small thing you've changed over the last years. Who knows? Maybe you watched a really heartbreaking documentary (that doesn't exist yet) and even though you were an absolute meat-lover and never thought that possible, you became a vegetarian. I know I wouldn't buy future me saying that ;)
– Mark
Sep 16, 2020 at 13:03

Your question dismisses two solutions incorrectly.

### Fingerprints

Fingerprints are fixed before you are born, and never change.

It is possible to have fingerprints surgically altered, but minor changes will grow back. If you wait around a month, you can verify that this is correct. People have had skin grafts to remove fingerprints, but of course this won't help them pretend to be you.

### DNA

Your basic DNA sequence doesn't change, ever. This will remain constant. Testing for it is somewhat hard at the moment, of course. If you're preparing for this to happen though, it's possible that you might have set up for a career in a lab testing forensic samples for the police, and you can put your samples through the same equipment when this happens.

What does change is methylation, which affects how DNA produces proteins. The "code" itself does not though. https://genetics.thetech.org/original_news/news91#:~:text=A%20study%20just%20out%20shows,get%20older%2C%20our%20DNA%20changes.&text=Researchers%20in%20Iceland%20and%20the,in%20something%20called%20DNA%20methylation.

This depends way too much on your specific model of time travel for a comprehensive answer. More specifically: How sensitive to change is it? Can your future self change the past by his actions or not and what kind of ripple effects will there be?

Because you have two different problems here and most of the answers address only one or the other. You want to prove

1. future you is really you
2. future you really is from the future

Using a past event, something like a very personal memory, or DNA or whatever, only proves that you is you. If you are sceptical, that doesn't mean you are a time traveller - multiple worlds and alternative dimensions are more likely than functioning time travel.

You need a very personal event that is still in your future to identify both parts. Also one that cannot easily be manipulated. It also must be in the very near future, you don't want to wait for a year.

Randomness is the big equalizer in this world. Carry a d20 with you (a 20-sided die, you can buy it in gaming shops). Your identification is rolling it five times, with FTS writing down the sequence right before you roll. Since it's a random event, it isn't something that would be in your mind and a mind-reader will have no advantage. It is reasonably secure against guessing (1:3.2 mio - roll more time if that's not enough for you). It is also within the realm of digits that you can commit to memory so it doesn't have to be written down (for the PAS-to-FTS transition period).

This does not protect against an adversary who is both a mind-reader and a time-traveller. I doubt that anything can protect against someone with those powers, as they could also likely manipulate DNA lab test reports and such like.

Do you trust yourself? Specifically, do you trust yourself to keep something safe for years and possibly decades?

If so, here is a method to try:

1) Ask FTS, "Have you brought it?". If they ask "Brought what?" then it isn't you. If they answer "Yes", continue.
2) Take 6 pieces of paper and a scribble on each with your eyes closed and out of sight of FTS until you are satisfied with their level of uniqueness.
3) Fold each piece of paper in half so that neither of you can see the resulting images.
4) Label the back of each piece of paper 1-6.
5) Ask FTS to produce the proof.
6) Roll a 6-sided die.
7) If the scribble corresponding to the rolled die matches EXACTLY FTS's proof, discuss important matters of your future and their past.

• Like with my part on "something I have", it would be difficult to keep some pieces of paper safe from damage and theft for years on end. Sep 17, 2020 at 12:24

Safety Deposit Box with a secret code:

I'm not sure you can have a fool-proof method of preparing for a contingency that may not happen with theoretical enemies of completely unknown capabilities. Don't overthink, keep it simple. If it gets too complicated, even YOU might screw it up. And if the (fill in the blank) enemies are that savvy, you probably can't stop them as they make you remember that your precautions were really something else.

You specifically are concerned that a future person may be able to convince you they are you (doppelganger) and that they can read your mind (telepathic). If they can control your thoughts or memories, you're screwed. What happens if you send a son or daughter in your stead? Anyone can claim to be such, or could even be so (kids can betray you).Given sufficient time and resources, you can't stop them - they can just travel back to before you came up with your scheme, or even travel to when you were making the decision and read your mind then, exactly knowing your scheme. If they can't come up with a solution to satisfy you, then they can come up with a hypothetical (potentially with years of planning) to convince you why they can't fulfill your conditions.

My thought is to write down a long string of numbers, then get a safety deposit box and deposit the paper in the box. Don't remember the numbers, and don't tell anyone about the numbers. A future self will know you have the numbers, and can either memorize them or bring them along to show you.

A doppleganger fake self will arrive with mind-reading power and learn you have a safety deposit box with numbers, but won't know them, because YOU don't know them. Only you go into the vault to check the numbers, and if the first few digits don't match, stop reading and get paranoid. If you can get a bank to agree to only admit you under certain preconditions (the first Tuesday of a leap-year) You would need to be at the bank every leap-year Tuesday to watch out and prevent Doppelganger from obtaining the info. Your future self would know to only arrive on a first Tuesday of a leap-year.

In a time-travel future, hopefully eventually the banks then have security against the threats they face then, and the doppelganger can't just travel forward in time to steal the numbers. You can't prepare for every possible unknown technological contingency, and if you are conceding a hypothetical enemy that can read minds and physically convince you they are you, then there may be nothing you can do to stop them.

As an alternative, you can go with the "Back to The Future" alternative and have a specific date you future self will contact you (the leap year Tuesday is a version of this). A doppelganger who read your future self's mind would already know this, though, along with all your plans, and you would be screwed. Your future self may also have unknown constraints that stop them from following through on any overly-rigid plan.

An interesting challenge would be to re-ask this question from the doppelganger's perspective, after you have settled on the solution, "How can I trick my mark into accepting I am their future self?" That might be a storytelling question, though.

• What if the doppelganger kills the future you and goes to the bank on a leap year Tuesday? Sep 14, 2020 at 23:40
• @chasly - reinstate Monica Then he already read your future self's mind, and knows all your plans. That's why I said that hopefully, banks in the future have security to deal with those specific situations (brain scans, gene scans, unbreakable encryption keys, whatever.) Sep 14, 2020 at 23:58

Make PAS into FTS

When FTS comes you with this incredible knowledge you completely ignore them and ask for them to take PAS to the future to verify that knowledge yourself while FTS stays behind.

In the future, PAS verifies the information using the technology of the future. At this point PAS becomes FTS and returns to the past to tell the original PAS about this incredible information and starts the loop.

Since FTS stayed behind to resume PAS's old life, PAS continues their timeline and no one else is the wiser since at most a few days passed while they were in the future verifying the information.

# Occam's Razor

I think what we're looking for is not a failsafe, but rather something with enough complexity that even future you doesn't want to go through the trouble to fake. This approach is already used in current cryptography; passwords are not impossible to guess and neither are algorithms, but they take an inordinate amount of time, computational power and energy. If we assume that the time traveler has infinite resources including time he could probably figure out some way to break any password or verification process much like today, but even a time traveler has a lifespan and limited resources.

In that case, multi-matching of fingerprints (not one but all fingerprints), DNA, mind passphrases and even better, objects and totems with specific random information encoded or as part of their structure would together function as a nearly unbreakable password. At some point, we can assume that technology could advance to the point where all could be faked, but even in that case, resources must be used to obtain such technology, and if you make your verification process random and personal enough, the likelihood of future you obtaining all of that technology without multiple trips to multiple possible futures, thus wasting time and energy, is vanishingly small.

## Time-lock puzzles

There are encryption algorithms designed to be easy to crack if you're patient but hard to speed up regardless of how many CPUs you(r adversary) throws at the problem. (The idea is that the puzzle is a sequence of modular squaring operations and at any point the next step depends on the previous one.)

To identify your FTS just do the following (of course you should do these programmatically, so you don't actually get to know the key to avoid those pesky mindreaders):

• Encrypt the text "I am your FTS" with a random key
• Create a time-lock puzzle for the key, set the difficulty to a convenient few decades
• Destroy the key, keep the encrypted text and the puzzle
• Set your ordinary 1 core desktop PC to start solving the puzzle. Make sure you restart it when the power goes out, etc
• Check if your future self has a correct key.

Pros:

• minimizes the butterfly-effect

Cons:

• only works if your FTS can keep the key from being stolen
• the awkward moment when your FTS shows up with the key on a double-density holographic crystal data storage (that's an age-old standard and should work anywhere!!) and you only have USB ports

### Same way I did. (Well prepared to...)

I watched situation occur in sci fi so many times as a 15 year old I actually put effort into this problem, and solved this.

I guessed if I ever sent a message back in time to me, I'd need a way for past me to verify it, especially as a message with instructions on how to avoid a future disaster will be at best weird and at worst illegal. I mean, if you were in 1920s Austria and someone told you to kill this "Hitler" artist dude, you'd dismiss them as a nut. You're not going to murder a stranger if asked.

It's not just time travel, I was predicting message through time would be more likely, but could also be dimension crossing, or if my memory was uploaded to a computer this could verify it really was me in that chatter box. It could also be used to verify if someone could really read my mind.

I memorized a 32 character passcode of random letters and numbers, 8 groups of 4, allowing it to be used challenge response styls. The purpose was that if I ever saw a message with it on it I'd believe whoever or whatever followed, and follow its instructions unconditionally.

The code has never been written down.

So the verification process is basically:

• I'm you from the future.
• Prove it. Code d
• defg
• code 2
• 2345
• All 32
• abcdefghijklmnopqrstuvwxyz123456
• ok what do you need me to do?
• "kill Sarah conner"

Or "Hey theres a note under my door. Abcdefghijklmnopqrstuvwxyz123456. Kill Sarah Conner."

That exchange (with alphabet replaced with actual secret) will work on past Ash age 15 or older.

I think it means that I never invented time travel, as 3 months later was 9/11. Seems weird that future me thought that was worth keeping in the timeline.

Mind reading comes in multiple styles. The most likely is the ability to hear what someones inner monologue is currently thinking. My method is safe against that. Past me saying "prove it, code d" doesn't internally verbalize the answer. A mind reader listening to my internal thoughts wouldnt detect the answer.

If mind reading is powerful enough that someone can just download it all, or my mind is scanned by technology without my consent, the reader needs to sift through gb of data. Maybe i memorized my passcode to the lyrics of an obscure 90s band, so they better listen to every random tune I've ever heard. Maybe it's from a book, so theyd need to sift through everything I've ever read. The code was memorized in different chunks, so it's a huge challenge to peice it together if you downloaded my brain somehow.

And if mind reading is any more super powered, like the cop from Heros, none of us stand a chance against it anyway.

• Doe this solve the "mind-reading time traveller" part of the question? If it does, I don't see it. Sep 14, 2020 at 16:21
• Mind reading is not necessary. I can stop in 2021 and torture the code out of you, then return to 2020 and deliver it to you and get you to kill Sarah Connor. Sep 14, 2020 at 20:55
• When a past-you is asking for "code d", past-you is primed to hear the actual answer, so it'll be right there on top of past-you mind. Sep 15, 2020 at 10:46
• Just out of curiosity, if future Ash had warned past Ash about 9/11, how exactly would he (or she?) have stopped it? // Hmmm... that sounds like a great story idea... Sep 15, 2020 at 16:21
• @RossPresser But how do you know the correct answer was given from the torture? What if tortured Ash gave the secret "berserk" code which would cause Ash to recognize you as an imposter and kill you on the spot instead? Sep 16, 2020 at 21:56

Taking a more philosophical approach: could FTS really ever be PAS? Like if I think back 20 years, I say to myself "I was a different person then". If I travelled back I would have a hard time recognising myself, so convincing PAS would be hard because we're such different people. As others have said, with (potentially) unconstrained mind-reading and advanced technology, a clone/Manchurian candidate/replicant/solid hologram could be made so indistinguishable from PAS that it might as well be FTS. There's no hard line where PAS turns into FTS, so they're always going to be different people in some way.

Future Self: Take this camera and wallet. Do not open our wallet before we meet again. Go take some photographs of literally anything you want, get them developed, pick any one of them, destroy the others and the negatives and meet me here with your photograph and our unopened wallet.

Self: ... our wallet?

Future Self: You will see.

Next day

Future Self: So you opened the wallet after you chose your photograph, saw it contained two photographs of different ages but identical to each other and the one you actually chose. You are now planning to show me an entirely different picture from your coat pocket in which your current girlfriend Kate isn't naked as she is in the two identical pictures in the wallet and the one in your back pocket - and you did keep the negatives including the one where she is clothed. Does this negative of her clothed match the negative in your inside left pocket?

[produces negative]

Self: Yes.

Future Self: Now sign the back of your photograph in you back pocket then look at the back of the two photographs.

Self: I never checked the backs last night.

Future Self: I know. Satisfied?

Self: The signature is Mickey Mouse and all three are identical including the crease from when I put it in my back pocket, so yes.

Future Self: Put the negative in the wallet with your photograph and the newer photograph, and keep the wallet. You will need it when you meet your Past Self in about 30 years. Destroy the oldest photo and everything else. Now we need to talk about ... what tattoo you put on our arse last night as a fail safe when you were drunk. Minnie Mouse, really?