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Let's say a time traveler from the year 2100 comes back to the year 2015. He has a very important message: [horrible thing] is about to happen soon, and he wants to warn us so we can avoid/prevent it. (Yes, this assumes a model in which "paradoxical causality" is not an issue.) The problem is if he goes around saying "I'm a time traveler from the future," no one's gonna believe him. They'd dismiss him as a crackpot.

So, he brings along proof, in the form of...?

This is actually a pretty tricky question, if we place two restrictions on it:

  1. He does not have a "time machine". His device sent him back without coming along with him, so he has no way to demonstrate that he's a time traveler by actually demonstrating time travel. (Just as an aside, this is very much on purpose; he doesn't want knowledge of the mechanics of time travel to fall into the hands of people who might use it for nefarious purposes, and part of his plan is to actively sabotage scientific research that led to the development of time travel.) The thing he used--let's just call it a "time catapult"--was able to send a small payload back in time, maybe comparable volume to a phone booth, certainly quite a bit less than the interior of a car.

  2. He wants to get the issue of establishing proof of identity over with and out of the way as quickly as possible and move on to more important things, like averting future disasters. This is a real issue; he can't go back arbitrarily far in time; the Temporal Frobulence Theorem shows that it becomes extremely unsafe the further back you go; it's a bit of a stretch even to reach our time!

The two obvious candidates for proof are future technology and knowledge of future events. The first is tricky, because current technological advancement puts us perilously close to the boundaries of Clarke's 3rd Law: any sufficiently advanced technology is likely to not be easily recognizable as such, and anything insufficiently advanced would be likely to just look like someone working in his garage made a breakthrough in some field, and that's pretty cool and all, but obviously it doesn't prove he's from the future.

The second is also kind of tricky. There are two major classes of unpredictable future events: natural and manmade. Bringing official government records of earthquakes, hurricanes, etc. could certainly establish that he is who he says he is, but it would take a lot of valuable time for Mother Nature to furnish the proof. On the other hand, if he predicts unpredictable manmade events, there are all sorts of potential troubles there. Point out the time and place of a major crime? Obviously, he was in on it; let's arrest him! Produce a table of stock market closing values for the next month? Well, he might be right for a day or two (coincidentally, of course!), but as soon as someone starts using the data he provides and attempting to profit by making trades based on it, the Butterfly Effect flutters in and destroys the accuracy of the data.

So, what would be the quickest, most efficient way for our unfortunate herald to establish beyond reasonable doubt that he is a time traveler with accurate knowledge of future events, and at the same time get enough people to listen to him so he can spread his doomsday message?

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    $\begingroup$ "part of his plan is to actively sabotage scientific research that led to the development of time travel" - Won't that cause a paradox? $\endgroup$ – colmde Aug 15 '16 at 12:07
  • $\begingroup$ @colmde Only if you accept the idea of paradoxes, which generally results from incorrectly conflating time and causality :) $\endgroup$ – Mason Wheeler Aug 15 '16 at 12:23
  • $\begingroup$ I don't think I can answer this but I have an idea of birth certificates for any babies born month after the date he travels back to. $\endgroup$ – Mendeleev Dec 18 '16 at 19:17
  • $\begingroup$ I would assert this, your positing this from a rather interesting point. You do not mention that the person knows that he did this because he has and it is recorded via some method. Meaning in simple terms, he is the first instance of time and he is influencing past instances of himself which is a problem because a past instance can not be the first instance. This can be solved if you allow time to NOT change your past. Instead it creates a new version of events. So it is a loop back, fork two instances, instance A is the original instance and instance B is the modified instance. $\endgroup$ – Enigma Maitreya Mar 14 '17 at 5:41
  • $\begingroup$ you state that paradoxical causality is not an issue, which would mean that the stock prices should not fluctuate when he provides this information... $\endgroup$ – marcellothearcane Mar 25 '17 at 17:20

42 Answers 42

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Actual "proof" of being from the future is pointless: the goal is to convince people, and few people can be convinced of something in violation with their belief system, regardless of "proof".

How many well established scientific facts are being ignored on a daily basis?

People smoke and eat unhealthy foods, even though it is fairly well established that smoking massively increases risks of disease and death. They drink, take drugs and drive. We still run and build coal fired plants even though the environmental consequences are catastrophic....

Think of the long list of patently insane behaviors people have on this planet. No matter how thoroughly something is proven, you will find people to dismiss it entirely with no evidence.

Using "proof" to convince people to do something is sadly not an effective way to get them to do things.

Suppose someone were to give you abundant proof of an upcoming catastrophe... what do you do next? Who do you contact and how would you go about convincing the people in power and the concerned population to follow your orders?

Proving one is from the future is not just impossible, it's also pointless. If the goal is to convince specific people not to take a specific path, proof of time travel isn't going to do the trick. Better come up with a carrot, a stick or both to get these people moving in the right direction.

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I'd just write down the lottery winners for the near future before it's announced, hand them in to the people I want to convince, in an envelope, and ask them to verify after they're announced. After 2 or 3 times, they should believe.

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    $\begingroup$ Honestly, I still would find it more plausible that someone had rigged the lottery than had traveled back in time. $\endgroup$ – Dan D Mar 20 '15 at 15:34
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    $\begingroup$ And yet, you'd be satisfied the lottery were rigged but the rigger decided to use that to prove time travel as a hoax, instead of taking his portion of a $500M jackpot. $\endgroup$ – user6320 Mar 20 '15 at 19:37
  • $\begingroup$ I agree with Dan D, you're trying to convince someone who is absolutely positive that time-travel is impossible, so any other possibility, no matter how crazy or unlikely is still more plausible than time travel. Besides, it's quite likely that if you're in a position where you can rig the lottery, then you couldn't claim the jackpot for whatever reason. (e.g. you'd be charged with fraud and have the winnings taken off you anyway) $\endgroup$ – colmde Jun 15 '15 at 13:24
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It can't be done. If you show me advanced technology, I will assume that you come from a society with advanced technology. If you make predictions of the future, I will assume you have advanced technology that can make those predictions. You might be able to convince specific individual people by targeting a message specifically to them, but any general future predicting or technology marvels are easier to explain without time travel.

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I'll assume that the time machine can be used multiple times:

Step 1: Get some attention. Go to the pentagon, try to break the security and fail. This way some official people will hear what you have to say.

Step 2: Threaten them: "In 4 hours a bomb is gonna explode at some precise place." This place will be heavily watched after this.

Step 3: Send something from the future at this precise place and time. It can be anything, the important thing is that people see that an object has appeared out of thin air.

From here on out, people should listen to you a lot more.

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    $\begingroup$ While they'll have to agree that you know something that they don't, if you try to earn their trust by lying to them, you're likely not going to get the trust you think you'll get. $\endgroup$ – Neil Mar 20 '15 at 12:42
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    $\begingroup$ @Neil He's not only lying to them, he's threatening them. This plan is a great way to get yourself sent to a secret prison where you are interrogated with "enhanced" methods. $\endgroup$ – KSmarts Mar 20 '15 at 14:16
  • $\begingroup$ This just proves that you know the timeline for when something is going to appear. It would be just as (or more?) likely that the officials decide you are engaged in espionage and supported by a group that has made a break through in matter transportation in our own time. Your prediction has to be something that is obviously out of your control, and couldn't have been arranged before you presented your claim. $\endgroup$ – Rozwel Mar 20 '15 at 14:34
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    $\begingroup$ I'd agree with breaking in, but is there a better way to gain their trust beyond a bomb threat? $\endgroup$ – Dan D Mar 20 '15 at 15:36
  • $\begingroup$ This answer does indirectly bring up the point that you might also want to think about how to avoid being a victim of nasty human behavior. Some people might want to lock you up and use you as their personal future-predictor. $\endgroup$ – Dronz Mar 21 '15 at 16:14
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Have someone ask this question, and then post this answer.

Hello all, I am from the future. I come from 2345 AD. We are looking for test subjects for our experiments, and we've been running short. All volunteers will be paid generously, using future cash, or, as we call it, "rubber". I can't say any more or this will cause a ripple effect. Thank you.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    $\begingroup$ I'm not sure if anyone living today would really be drawn by money which will only start to be valid three hundred years from now. $\endgroup$ – Paŭlo Ebermann Mar 22 '15 at 11:49
  • $\begingroup$ Valid point, but we are offering for our test subjects to remain in the future after experimentation has been performed on them for exactly 1,750 months. We will pay at minimum 350 gigarubbers; with that you could buy over 32 Nano Battery Blimps (the real stuff, from Ford, not those crummy Micro Battery Balloons that only last 10 years) or 184 CPT-96 neural memory upgrades! $\endgroup$ – Andrew Mar 22 '15 at 19:17
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    $\begingroup$ Can you explain how that would persuade people that you're from the future, rather than just some kook who thinks he is? $\endgroup$ – Monica Cellio Mar 22 '15 at 19:35
  • $\begingroup$ Can you explain how you are not my nemesis from the future, who also came back in time to try to convince everyone that I am not from the future? I assure you, your miserable plan will fail, and your faction will not take control of this galaxy! Go back to the Andromeda Galaxy, and leave us humanoids alone! $\endgroup$ – Andrew Mar 22 '15 at 22:31
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The iPad one is interesting, but "theoretically" someone could have had a secret processing plant employing one million people, underground, working independently for 50 years, funded by an alien race if necessary. Anyway, the physics of an iPad was known in 1995, if not the mechanics.

So - what about simply predicting a solar flare? Or better still - bring a photo.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

  • $\begingroup$ Read the word "Photoshop" $\endgroup$ – DomineSatanas Feb 1 '17 at 15:41
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If this traveler were time travelling 'within one universe' instead of jumping to other 'local' universes in different states of relative co-existance, you immediate destroy the universe you 'jump from'.

If you are going for the more linear classic approach to time travel, the traveler could bring back something which co-exists in 2100 which is iconic and priceless to boot, but most of all unique - and dare the objector to prove falsehood.

Depending on how many universes you believe in (if any), you could tell them to damage it and then watch the corresponding damage occur. Go further and destroy the item and poof! It disappears. But if so, then so did the traveler's effort obtain it, because it never existed to bring back... but only because he went back with it was it destroyed in the first place. I know causality is not meant to be an issue, but it is! So I think you probably destroy the universe by going back in time at all - at the very least the chances of doing so rise geometrically (towards tangent 90, or something, per unit of observable time, probably). Regardless, he rips wide open the future, which has or has not happened. Perhaps he's beaten the odds? And of course he must, because he exists in the first place!

So I'd wonder whether this time traveler.... is there a future ahead of him with time travel?

My personal feelings are that future events are so intrinsically sensitive to present conditions that one irrevocably determine the outcome of the universe simply by doing nothing. Hell, I am just part of the universe playing itself out as intended, am I not? And if I never learned to tie my shoelaces until I was six, instead of five, for example - stuff like 9/11 wouldn't have happened.

Which is absurd surely? OR IS IT? Time travelling, and a pre-determined future (whilst the chances of one existing are non-zero, is definitely, definitely not our realm.

But yeah OK, causality is not an issue - if he were travelling linearly backwards and wanted proof, an object he could bring back might be something like a radioactive vial with a unique signature of something which has shown measurable decay. Perhaps also the traveler possesses a residual charge of something, or has a little extra amount of 'entangled' particles in all his orbits. A quantum signature.

/BLURT

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Bring a future weapon. If I know humans, we continue to grow better at killing each other. Why a weapon? Well, if you are going to the past to change the course of history, that means you are interrupting the plans of powerful people, leaders (upsetting them at least). Once they are on to your motivation, they will try to silence/terminate you even if they are convinced you are from the future. I would even bring a doomsday device as an insurance policy. If the future is so bad that you were sent to the past to alter it, would it really be that bad to destroy the world in the past if your mission failed?

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I don't think there is a way to prove that you're from the future without showing off your TARDIS. Yes, you can predict forty trillion data points accurately, but we as a species are far too skeptical. However, this is meant to be more constructive than a simple "No" so here we go.

Given the rate at which science is advancing, it's fairly safe to say that new fields of study will pop up in a hundred years time. Instead of showing off massive advances in robotics, or matter manipulation, show off a new kind of thing, as alien as my cell phone is to Socrates. Most people will still have a little bit of doubt in their heads, so reveal the quadrillion data points ya got. Give stellar maps that show galaxies and planets that are too far away for the light to have reached us yet. Do all the other things these answers say. Before you leave for the past, study all kinds of stuff; biology, robotics, computing, chemistry. Make sure you can answer any question a scientist can throw at you.

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We're talking about around 100 years of history and rather than proof, I see credibility as a bigger problem, proof is worthless without it.

Assuming there is no language barrier, even if the protagonist were speaking the same language as the people around him it would be language separated by around 100 years or gradual change. Look at how laughable films from the 1950s look now then invert the context that 1950s language, attitudes and social mores are the norm and the observer is in a minority of 1.

Credibility issues are common to history documentaries. They use the viewer's "modern" perspective to frame events in a way that conforms to biases. A quick glance at 20th century newsreels containing "white man's burden" topics underlines what I'm driving at, all of them from less than 80 years ago. The concept of "modern" is very nebulous and changes from generation to generation.

History is not an absolute - it is simply an interpretation of events and the present is a huge jumble of events with future implications interpreted by the conventional wisdom of that time.

So even with future hindsight, the future time-traveller would have to choose several events that the people of that time and geographic location would care enough about to want to have their preconceptions challenged, and would likely fall foul of any powers that had vested interests in seeing an outcome to fruition. Lets say traveling back in time to stop the birth of Islamic State cells, a little shy of 20 years ago, would pit the protagonist against a hawkish American government policy intent on invading Iraq using a "for us or agin us" approach to anything seen as being critical, and using kidnapping and detainment in Guantanamo Bay against people they deemed to be "enemy combatants".

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If your time traveller wants to prove they came from the future, you don't send a man you send a woman instead. Because she will be wearing clothes of her own future time. But just to be sure, she carries an extra set of her underwear. Why an extra set of her underwear? you chime. The answer's quite simple. No garments change as much as women's underwear.

Think how much difference there is in the styles, construction, and materials of underwear between now and, say, sixty years ago. Any woman in 1950 being shown a set of contemporary undergarments would immediately recognise how much better they were and wouldn't take much convincing they came from the future.

Using underwear as your future person litmus test doesn't lead to any problems with reverse engineering or likely causality violations. Also it's simple and readily portable.

Since a picture is worth a thousand words then picture this and for good measure this too.

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    $\begingroup$ That wouldn't work, unless the underwear is some super-science weirdness. A difference in style and make-up won't say "future" to anyone. Also 1950's woman would think modern women's underwear utterly promiscuous. $\endgroup$ – DomineSatanas Feb 1 '17 at 15:32
  • $\begingroup$ @DomineSatanas. "Some super-science weirdness" like force-field body-suits? There are sufficient material differences between current underwear and that of the 1950s. Besides you underestimate women of the 1950s. Try the links to YouTube in my last paragraph, that might change your mind. $\endgroup$ – a4android Feb 2 '17 at 1:27
  • $\begingroup$ Well, friend downvoter, pity you didn't explain your objection instead simply downvoting. Friend DomineSatanas, at least, expressed his disagreement. He and I could have discussed our viewpoints. We could have too. Hopefully we still might. Ah well you can't have everything. $\endgroup$ – a4android Mar 14 '17 at 11:08
  • $\begingroup$ I would, but it's just so ridiculous. Every time I type out a response it comes off too hostile. $\endgroup$ – DomineSatanas Mar 14 '17 at 18:13
  • $\begingroup$ Goodness! Another downvote! Pity there isn't comment to explain why there is an objection. Such a simple concepts really. Nothing indicates progress like developments in feminine intimate apparel. Something adults with a reasonable amount of life experience can appreciate and understand. Well friend downvoter, if you'd like to explain your objections, I'd be happy to hear them. $\endgroup$ – a4android Jul 5 at 6:45
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Get the largest prime number found from the future.

Now 274,207,281 − 1 is the largest prime number. If he is from future it will be larger than this number.

So he can prove he is from future.

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    $\begingroup$ How does that prove he’s from the future? I would counter that he just found a new record prime. I’d say that’s rather poor supporting evidence, since even a significantly larger new record could be explained by having a large base of spare computing power, new algorithms, improced number theory to suggest smaller areas to look in, or a bit of luck. $\endgroup$ – JDługosz Mar 14 '17 at 5:35
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    $\begingroup$ Still, “he invented a quantum computer” is a more likely hypothosis than “he’s a time traveller.” $\endgroup$ – JDługosz Mar 14 '17 at 5:59
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    $\begingroup$ « of course that person is from future.» That is not proof. Nobody beleives that conclusion. He got it from advanced extraterrestrials or found the lost records of Atlantis. There are other hypotheses and you can’t just say “of course” to constitute a proof. $\endgroup$ – JDługosz Mar 14 '17 at 23:34
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    $\begingroup$ There is the problem of who the time traveller has convince. Considerably he must persuade people in 2015 to prevent a catastrophe this will mean governments & government officials. Higher prime numbers may not prove convincing to them. Mathematicians might be impressed. As suggested there are many alternatives to source of this higher prime without involving time travel. It's an ingenious concept, but not, alas, convincing. $\endgroup$ – a4android Mar 15 '17 at 11:55
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    $\begingroup$ You're right by 2100 they should have found higher prime numbers, but that of and in itself doesn't prove someone is a time traveller. As @JDługosz there are other possibilities. My suspicion is that a time traveller will need and probably have multiple proofs they come from the future. $\endgroup$ – a4android Mar 16 '17 at 10:12

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