It's a good idea. Why? Well, lets examine the usual "you travel back in time, and need to convince your (past) self to help your (future) self" scenario.

(Future) Self calls (Past) Self:

PSelf: Hello?
FSelf: Hi! This is you, from twenty years in the future! Do not press the big, red..
PSelf: Erm, WHO is this? My future self? Right... HA HA HA *Click*

Obviously, this is no way for FSelf to go about convincing PSelf to help FSelf. Granted, FSelf might be able to get PSelf to believe FSelf eventually -- FSelf does know all the dark secrets of PSelf's life, but this could simply serve to convince PSelf that FSelf is a voyeuristic stalker.

If FSelf met PSelf in person, things would be easier, but is PSelf really going to believe the whole "traveled back in time" thing when a much simpler "evil twin" scenario is so obvious? If so, maybe PSelf should visit Occam's Razor. And what if FSelf needs to send someone in their place to convince PSelf to do something?

Now, we address the problem of this ever actually happening. Granted, the chances are very slim that you'll be hurled back in time and need to convince yourself to do things, but you must admit that such a possibility exists. Even if you are steadfastly convinced that it doesn't, this is an interesting exercise in temporal theoretics, so stick with me.

Anyway, here's what you do. Think up a secret phrase, or, shall we say, "password". Don't make it something common. "chair" is a stupid temporal password. If you pick "chair", FSelf's world DESERVES to be destroyed.

So, you have a reasonably intricate password.
  • DO NOT tell anyone.
  • DO NOT write it down.
  • DO NOT forget it.

    Now, we come back to the phone conversation.

    PSelf: Hello?
    FSelf: This is your future self. The password is "argle the chemical puppy". It is vitally important to the safety of the universe that you not press the red button.
    PSelf: Well, gee, guess I won't then. Anything else I can do?

    Clearly, this is a far more effective way to save the world.
  • And here's how to do it in a global basis, so that anyone that went back in time could save the future.

    Have a computer generate a random password, in such a way that no one reads it. It shouldn't be hard with the proper equipment. Now shut it tightly and bury or hide it somewhere.

    Now, whoever is going back in time only needs to ask the people that hid the password to uncover it and tell him. He comes back in time and tells the password: The past people dig out the password and voila, it matches.

    Of course, if someone dug the password out, he would become the Herald of the Future. But that's a problem for the hardware engineers.

    The Cow says: the password can only be used once- which means they'll have to convince them that he's from the future before they can read the password...

    True. Perhaps a large array of passwords could be used - ten thousand, maybe more. If one is proved wrong, that password is simply destroyed and cannot be used in the future.

    A password will not do. Any mind reading technology could easily scan your memory for the password, give it to the devious fraud who is claiming to be you, and then he would be home free for doing all sorts of dasterdly deeds.

    The solution lies in DNA testing. It's the only way to know that the person from the future that you are dealing with is actually an older version of you. The person from the future would present a DNA scanning machine (he has one because he's from the future, and if he can travel through time he can have the clarity of mind to bring along his portable DNA scanning machine.) You place a drop of blood in one hole of the machine, he places a drop of his blood into the other (watch him, you don't want him to use any fake blood sacks à la Gattaca), and you listen to the machine whirl away. If the green light comes on, he's you from the future. If it's the red light, beware, because he might have a ray gun or lazer blaster (or some other futuristic weapon). In that case, you will be on your own in a battle between technology and wits.

    If the light is green, and you don't trust the machine, you can test it out on other life forms. A blade of grass, a cat, a squid, etc. Use your imagination. You are trying to fool a machine potentially endowing a hive-mind intelligence quantumly linked through an einstein-rosen bridge to the interstellar faster than light communications network of the future.

    Once you've proven to yourself that the machine is real and not rigged, and the light is green, you're good to go. I leave to personal taste whether or not to accept a clone as a future version of you. That seems like a decision best left up to the one faced with the problem. Just beware of clones that have been created and brainwashed to want to destroy you. They don't have your best interests in mind.

    I hope I have shed some light on the situation. Beware of password-based schemes to protect yourself from practical jokes. They are bound to fail once telepathic machines are created. DNA technology is the future. It will make your life much easier.

    This actually happened to me, about three years ago. At the time, I was working at the Royal Museum in Edinburgh, Scotland, classifying dead seabirds. They would bring in the corpses from the latest natural disaster, covered in oil or chemical spillage. I would examine their plumage, briefly decide on cause of death, and add the results to the statistical database whilst making comments about the museum getting more shags than I did.

    So one day, I think it was a Thursday, I was sitting at my lab bench as usual. Tony the message boy brought up a cardboard box, and inside was a remarkably well-preserved Arctic Tern corpse. I washed my hands and laid it out on the table, but inspecting front and back I could see no cause of death. I was about to head off to KFC for lunch when I heard a strange buzzing noise and this slightly disoriented man walked in.

    After a few seconds, I noticed the resemblance. His hair was close-cropped, not like my straggly mane. His eyes were marked by fine wrinkles and he looked thinner, gaunter, tireder. But he was exactly my height, and there was something very familiar about him.

    "Who are you?" I asked him.

    "I think you already know."

    "But you can't be ... what are you? Do I have an older brother?"

    "I'm you," the stranger said, "twenty years later. I've come back to warn you."

    "How do I know you're me?" I demanded.

    "Don't you remember. When I was a child we made a password for this occasion. Devonian chicken slurry."

    I looked at him. I did sort of remember a conversation about time travel and identity, and yet, now I was a real scientist and not just a kid with too many Robert Silverberg books, I had to take such things less trustingly.

    "You must have overheard me. Maybe I talk in my sleep."

    "You know you don't do that."

    "How can I possibly know I don't talk in my sleep?" I asked him.

    He was stumped. "Ok, let's think about childhood memories," he said. "Remember how you buried the goldfish under the cherry tree."

    It still wasn't proof. Maybe he was friendly with the dog that dug in the garden. Maybe he read my mind. But we had facilities in the museum to do DNA testing. I took a blood sample from each of us and told my double to hide in the supplies cupboard. He was muttering about having no time to waste, but I said he'd come 20 years and a couple of hours wouldn't matter. As a concession to his worries, I locked the lab door.

    The test results seemed to confirm it. We shared the same DNA, or at least I couldn't find any differences between us. Results that were a trillion to one against. I returned to the stranger to give him the news.

    "So you have the same body as me, as well as the same mind," I said. "Assuming I can trust the test. For all I know you're a clone of me aging faster due to cellular damage from the cloning process. Or maybe I'm a clone of you. Ew."

    "We have the same mind," he said. "Cloning does not explain that."

    "Actually the same mind thing proves nothing, because if someone created you with some kind of matter duplication ray and you have an identical body it is likely you will have an identical brain. And possibly vice versa." I considered more hypotheses. "You could be an evil duplicate of me sent back in time by a spurned lover or a scientific rival or a great crested grebe angry at finding his great grandfather spent his last minutes on my dissecting table."

    "This is ridiculous," the interloper said, his patience snapping. "See that bird on your dissecting table. It is infected by a horrendous new virus that will wipe out the human race. Unless you utterly destroy it now, all humanity will be reduced to a few hundred scientists scrabbling in research labs trying desperately to find a cure."

    "On the other hand," I said. "If I spend any more time talking to you, I'll get fired from here, that'll seriously damage my chances of ever getting more lab work or someday an academic position, and I'll never even have the slightest possibility of being part of a scientific program aimed at combatting mysterious bird viruses." I weighed things up. "Either way, I have to bid you farewell."

    He got angry at that point, but I had a scalpel easily to hand. I stuffed his body in the incinerator and got back to work.

    On my way home I chuckled at his warnings, thinking, "Hey, I'm not dying of some bird disease." I got off the bus and walked up to my flat. In the stairs 47 of me stepped out of the shadows and started bludgeoning me with baseball bats, rolling pins and metal bars.

    I lapsed into a coma for three months, and when I awakened I could remember nothing. That was the point at which I realised my future self had been full of shit.

    Thanks for all the interesting comments. ariels said: "YOU FORGOT TO ASK HIM IF HE WAS YOU *AFTER* YOU'D SAVED HUMANITY FROM THE VIRUS OR BEFORE! (Trick question!)". That's one worth remembering. Rose Thorn said: "The only question is why you went back to tell yourself that crap, knowing that you would kill you." Which is also a valid point. I also had a couple of people asking if anyone did kill the bird; um, my memory's a little foggy on that issue.

    There's no need to prepare yourself years in advance for possible contact with your future self. Some have suggested devising a password that you never tell anybody. If the visitor from the future knows your password, then they're you. Since you don't remember undergoing any time travel, it must happen in the future, so they must be a future version of you, with all the information you have and more, so it would be reasonable to take their advice. In fact they don't have to be you, they could be someone your future self trusted enough to give his password to. Either way, you should trust them, since, in the future, you will trust them.

    This is an excellent plan, and it seems reasonable to plan ahead even for a contingency as unlikely as this one. But it just isn't necessary. If someone approaches you, claiming to be from the future and having extremely important instructions for you, just make up a password on the spot, commit it to memory, and then ask them what it is. Your password will have propagated into the future instantaneously, and they'll know it.

    Some have pointed out that this scheme can be foiled by telepathy. This is absolutely true, but a minor modification fixes it. First, ask the visitor to write down your password and place it in a sealed envelope. Once you have the envelope in hand, but before opening it, think of a password and make sure you'll remember it for the rest of your life. Open the envelope and see if they match. It's that simple.

    A personal verification scheme might be impossible.

    Any password you make ahead of time could be compromised through mind reading, detective work, super advanced psychological profiling or mental simulation based off later neural scans of your mind that guesses your password, etc.

    On the other hand if you randomly generate a password and having him give it to you before you read it, I don't see how he could have the information. He comes from a future where you didn't generate the password, right? Only the future you that comes from the timeline where you avoided the reason for the visit would have that password. Wicked paradox.

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