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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 going to 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 side note, 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 in 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 increasingly 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 not to 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 it doesn't prove he's from the future.

The second is also 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 a 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? $\endgroup$
    – komodosp
    Commented Aug 15, 2016 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
    Commented Aug 15, 2016 at 12:23
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    $\begingroup$ 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
    Commented Dec 18, 2016 at 19:17
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    $\begingroup$ This seems to depend on the impending calamity. If a comet will hit Earth, simply point at the comet and we'll get it, without having to believe you're from the future. If [scientific thing we don't understand], then you'll have a hard time with that. You'd have to "invent" the science first, which might make a catch-22 in the story you're painting. $\endgroup$
    – user458
    Commented Apr 5, 2018 at 17:58
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    $\begingroup$ Why did I actually look up the temporal frobulence theorem... $\endgroup$
    – Ekadh Singh - Reinstate Monica
    Commented May 13, 2021 at 16:17

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He should predict Solar Weather and/or Solar Events.

Predicting Earth weather is a complex process, and he's introduced a new variable - himself. And not that he'll have a huge effect, but you never know - the ripples of his arrival could be enough to throw off any predictions, creating doubt that he's authentic.

On the other hand, solar weather - the sun's activity - is also extremely difficult to predict, and is completely isolated from the time traveler's influence. He can pull historical records from various space agencies and publish the results for the next week, in complete confidence that the data can't be effectively hidden or faked.

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    $\begingroup$ If someone did this, my first thought would be that they developed some extremely accurate way to predict the (solar) weather. That seems unlikely, but it's a much more likely explanation than they are a time-traveller. $\endgroup$
    – BlueRaja - Danny Pflughoeft
    Commented Mar 20, 2015 at 16:31
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    $\begingroup$ @BlueRaja-DannyPflughoeft: that's a good point, but I think it applies to any single prediction. I was originally thinking he should predict and track neutrino events from the sun - those should be entirely unguessable, as they're random - but I couldn't find a site or source where those are published. An alternative would be to pick 2-3 different predictions, and publish all of them. It's unlikely that someone would make simultaneous breakthroughs in multiple fields. $\endgroup$
    – Dan Smolinske
    Commented Mar 20, 2015 at 17:13
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    $\begingroup$ @DanSmolinske The problem with truly random things is they might end up coming out different, depending on how the time travel mechanic works. $\endgroup$
    – Random832
    Commented Mar 20, 2015 at 18:56
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    $\begingroup$ @Random832: So thinking about this more, and I think solar weather is still likely to be ok over a decent timeframe. It has to depend on the conditions throughout the entire star, and those aren't going to change significantly because of quantum-level events. It gave me a better idea, though - what about measuring the randomness of distant stars? Because you're effectively looking into the past, all the randomness has already occurred and is locked in when you time travel. You would just need to find a study that measures pulsars or something around when you travel. $\endgroup$
    – Dan Smolinske
    Commented Mar 20, 2015 at 19:11
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    $\begingroup$ @BlueRaja-DannyPflughoeft: If someone demonstrated this level of ability to predict chaotic events, I'd listen to him whether or not he claimed to be from the future. Come to think of it, he might achieve his goals more easily just by getting giga-rich, or claiming to be super-intelligent or psychic or alien or something. $\endgroup$
    – Beta
    Commented Mar 23, 2015 at 14:14
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I'm going to suggest something rather different, given that he's from not far (2100) in the future. All he will need to bring back is the names of his parents/grandparents. With DNA testing of them and of him it would be possible to prove that he is their descendant, something impossible if he wasn't from the future.

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    $\begingroup$ Elegant and simple. Welcome to the site rmoore $\endgroup$
    – James
    Commented Mar 20, 2015 at 17:55
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    $\begingroup$ Not one of my areas of expertise, but can we actually test to prove ancestry/decent? Or just degree of relationship, which is interpreted as parent/child when the relative ages match up? For some reason I am thinking it is the later. Still an excellent idea though. $\endgroup$
    – Rozwel
    Commented Mar 20, 2015 at 23:18
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    $\begingroup$ @Rozwel Yes, we can test for ancestry. You can test the mithocondrial DNA for mother-ascendancy (you will only have the same mithocondrial DNA as people who can trace relation through mothers). You can also test specific chromosomes and genes for ascendancy, since you get half from each parent. If the parents are already born, it is a piece of cake. If you can get all four grandparents, you can still get in the nineties percentile. $\endgroup$
    – Mindwin Remember Monica
    Commented Mar 21, 2015 at 2:01
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    $\begingroup$ @Mindwin: I agree with Rozwel. Your reply starts with "yes", but nothing you've written really demonstrates "yes". (Mitochondrial DNA, for example, only shows that you share matrilineal ancestry with someone. It doesn't show that that someone actually is your matrilineal ancestor.) $\endgroup$
    – ruakh
    Commented Mar 22, 2015 at 0:59
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    $\begingroup$ "With DNA testing of them and of him it would be possible to prove that he is their descendant,.." Right. Unless, of course it turns out that you aren't. By some estimates, this is actually true about 1%-5% of the time. And not only is this possibly the worst possible way to find this out, you also just lost the only chance you'll ever have to make a positive difference in the world. "Thanks, Mom!" $\endgroup$
    – RBarryYoung
    Commented Mar 24, 2015 at 3:49
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There is one more very convincing thing he can bring from the future: actual copies of items from the present. If someone came up with an aged Mona Lisa, the bones of Barrack Obama and the Tiffany Yellow Diamond, I doubt his claims of coming from the future would be ignored.

Now how he can come up with these items in the future is the subject of another story, but I am assuming he isn't just some mad scientist who wants to right some wrongs in the past, but he is an exponent of a troubled species that NEEDS to prevent a catastrophic future, therefore his experiment can be outfitted with some inconsequential items as those described above.

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    $\begingroup$ The bones of someone living - where an exact DNA match can be made - seems a lot more reliable than the much-upvoted DNA-relationship answer. Both have the problem of being spoofed through cloning methods, though. $\endgroup$
    – Izkata
    Commented Mar 24, 2015 at 16:00
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    $\begingroup$ That's why you bring some other rare items. I'm fairly sure there's no way to clone a huge diamond and the "copy" of Mona Lisa can be dated to show that it's 100 years older than the "original". $\endgroup$
    – Tibos
    Commented Mar 25, 2015 at 7:13
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    $\begingroup$ Would be interesting if time responds by whipping the contemporary "originals" out of existence at the time of the jump, to "compensate" (and this happens in some fictional narratives). Whoops, sorry, Mr President; my, you're looking rather limp today...! $\endgroup$
    – Lightness Races in Orbit
    Commented Mar 30, 2015 at 17:00
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    $\begingroup$ @LightnessRacesinOrbit I agree, it would be interesting. However, i find it very hard to figure out a coherent system where such rules would apply. Would the atoms that form the time traveler's body be "removed" from the past? What about subparticles? What about radiation? I find it easier to concieve that the objects are simply "injected" in the past. Still, good point. $\endgroup$
    – Tibos
    Commented Mar 30, 2015 at 21:54
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    $\begingroup$ Those things are hard to get. "May I have the Mona Lisa please? It's to prove that I'm from the future. No, you won't get it back, but that won't matter because as soon as I change the timeline you won't exist anymore." $\endgroup$
    – Ebonair
    Commented Nov 30, 2017 at 16:24
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The critics provided the proof themselves

Before he travels back in time, he uses his machine to send along a "parcel of proof" to some point in time after his own arrival. He can then predict that this parcel will arrive out of thin air at a specified moment. By being an extraordinary event, this boosts his credibility.

The clincher however, is that the parcel contains recordings of his stay in the days following the arrival of the parcel. Things which he buried at a secret location and retrieved himself in the future. There could be letters the sceptical inhabitants have sent to themselves, video recordings of people he has met telling how they have finally been convinced that he is a traveller from the future, accounts of the many random things in their lives such as the timing of a sudden onset of rain that ruined their crop, accounts of a kid that fell and broke his leg or of where misplaced items were finally found.

Story-wise this gives you an excuse to introduce the supporting characters in more depth. You can, if you like, arrange for a circle of true believers while keeping the world at large indifferent to his claims. For the reader, the point after which he has buried the parcel marks when the outcome of events starts being uncertain again, possibly building tension in the story. Perhaps we'll also see some dramatic scene where the traveller tries to retrieve the package again, to add a warning to himself about trouble occuring.

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    $\begingroup$ Presumably this only works if the critics existed in his timeline previous to him heading back. Depending on the time travel paradigm, it might not be possible. $\endgroup$
    – Dan Smolinske
    Commented Mar 20, 2015 at 13:28
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    $\begingroup$ Sorry - I don't mean existed at all. I mean that they existed as critics. Let's say Bob is TT's greatest critic, but he's only a critic in a timeline where TT has gone back in the past and claimed to be from the future. In TT's original timeline, there (presumably) was no time travel, so while Bob was alive, he wasn't a critic and there's no evidence of this nature. $\endgroup$
    – Dan Smolinske
    Commented Mar 20, 2015 at 14:13
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    $\begingroup$ This approach assumes that whatever I do now, the world still ends up in the same state it was in before I came back. Which completely invalidates my trip. If we go with the OPs intention that we cause the time line to diverge, then the cache I bury never makes it to the place I came from, because that future never occurs in the timeline I bury it in. $\endgroup$
    – Rozwel
    Commented Mar 20, 2015 at 14:40
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    $\begingroup$ @Rozwel aren't you perhaps assuming that when this reality alters, EVERY thing changes...couldn't it be that certain timelines may or may not be affected by the time traveler... Thus there is still a chance that this proof could play out? $\endgroup$
    – Rohan Büchner
    Commented Mar 20, 2015 at 15:16
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    $\begingroup$ I really like this solution because each critic could demand the exact proof they want... timey wimey... stuff! $\endgroup$
    – Liath
    Commented Mar 30, 2015 at 10:27
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The problem with knowledge of the future is that as soon as you make one alteration the future starts to change. Knowledge of one lottery result would be fine, but results after that would rapidly become unpredictable again.

One of the comments is absolutely right though, start by winning the lottery, just send yourself back with a bit of money and a false identity.

Additionally send yourself back in time to shortly before a major disaster and use your lottery winnings to avert it and to invest in a number of companies that you know will grow large. You can also bring back a suitable list of inventions to use your lottery seed money to start working from.

With that level of money and influence you can then not try to convince people, just get them to do what you want without ever mentioning being a time traveler.

If you really want to convince people though then use natural events that will still happen predictably. For example if you know a major earthquake is going to strike new york at 9:13am on Wed 4th August then use your money to place billboards warning people and to place relief shelters and supplies ready.

That will get people listening.

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  • $\begingroup$ Good point. A time traveler depending on the actions of others is likely a paradox waiting to happen. It would make sense if they tried to stay low key. $\endgroup$
    – Neil
    Commented Mar 20, 2015 at 12:41
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    $\begingroup$ Using money and influence without mentioned when you came from also avoids the problem Rozwel brought up - if you're trying to prevent the development of time-travel technology, demonstrating that it is possible is a good way to ensure people will work on it and eventually figure it out. $\endgroup$
    – Rob Watts
    Commented Mar 20, 2015 at 17:18
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    $\begingroup$ "Knowledge of one lottery result would be fine, but results after that would rapidly become unpredictable again." One thing a visitor from the future could do to predict many lottery results without affecting them is to publicly publish an encrypted document with, for example, a whole month's worth of results. No one can read the document at first, but they can see that it's there and not changing. Only after the month has passed does the visitor publish the decryption key. Since the document was published publicly and unchanged, everyone can see that the visitor had all those results upfront. $\endgroup$
    – Nick Chammas
    Commented Mar 20, 2015 at 18:38
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    $\begingroup$ And the person drawing the lottery balls spends a few more minutes on their lunch break one day because they read the article about someone making this claim, and that's enough to make the result randomly change. $\endgroup$
    – Random832
    Commented Mar 20, 2015 at 18:58
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    $\begingroup$ "Results would be stable for 1 or 2 draws I expect" -- is there a basis for that estimate, or just what seems fun for a story? Is it a solved problem in physics, whether or not the arrival of a 70kg time traveller in (say) London is likely to affect the result of a lottery draw in (say) New York 24 hours later? If you can quantify the sensitivity of lottery draws to butterflies, then you'll know, but much of the attraction of mechanical random processes lies in the fact that we don't know in what ways they're sensitive to the environment (while remaining apparently fair) ;-) $\endgroup$
    – Steve Jessop
    Commented Mar 26, 2015 at 12:28
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I read a saying somewhere: "If it doesn't have wires sticking out of the case, it's not cutting-edge."

I think carrying 2100-era technology would be a clincher. It's one thing to build a device that holds 500x as much data as any hard drive in existence. It's much harder to package it up in a sleek, friendly interface that's clearly gone through a dozen rounds of feedback and redesign.

Say somebody shows up at the front gates of the White House with a smart-matter robot with smooth, seamless AI, the ability to 3D-print insanely complex objects out of electricity and dirt, and a demonstrated ability to calculate at 3000 petaflops/second (100x faster than the current fastest supercomputer). You can't dismiss that as "something a guy built in his garage." You can't even dismiss it as a "secret government or corporate project." The construction of such an artifact requires too many breakthroughs in too many independent fields.

Heck, I suspect if you took an iPad back to 1995 and handed it over to a team of engineers and asked them, "Future or Nifty?" they'd come back within a few days and say, "definitely future."

So I think it's a more interesting question if you deny the possibility of bringing anything but information back. My solution (riffing off the 'solar weather' answer): use information that has already been generated, but hasn't reached us yet. Random data encoded in electromagnetic waves still flying towards Earth.

For example, the semi-random "hiccups" in the rotation speed of pulsars.

To be thorough, you'd want data of several different phenomena. If all you have is microquake data from pulsars, or the coordinates and times of supernovae... any one thing could be dismissed as "oh, they just made a breakthrough in predicting X." But making breakthroughs in X, Y, Z, and W? Much harder.

Neutron star - Rotation (Wikipedia)

Update: Looks like DanSmolinsk had the same idea.

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    $\begingroup$ If you take an iPad to 1995, good luck finding WiFi or cell coverage. You better pack some apps with offline functionality. $\endgroup$
    – Victor Jalencas
    Commented Mar 21, 2015 at 18:07
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    $\begingroup$ @VictorJalencas Bring a modern laptop as well, use ad-hoc WiFi and Ethernet. They had 10BaseT Ethernet in 1995, which your laptop should still be compatible with. $\endgroup$
    – user20574
    Commented Mar 23, 2015 at 21:52
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    $\begingroup$ And the services that go with the online apps will all be non-existent! :) $\endgroup$
    – DonyorM
    Commented Mar 25, 2015 at 15:05
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    $\begingroup$ Given this evidence I'd want to contest the "time-traveller" hypothesis with (a) the hypothesis that the artifact is the work of aliens who have been tracking earth technology and simulated something slightly ahead of it, (b) same thing, Illuminati. I mean, I work in tech, and I have no idea where a secret conspiracy would keep all the infrastructure required to be 50-100 years ahead of the game in terms of technology produced on production lines. But sufficiently advanced tech could perhaps produce a one-off that appears to be futuristic commodity tech. $\endgroup$
    – Steve Jessop
    Commented Mar 26, 2015 at 12:36
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    $\begingroup$ The real question is, what would an engineer of 1915 think of the iPad? $\endgroup$
    – Michael Hampton
    Commented Mar 28, 2015 at 21:51
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An easy solution would to have the time traveler use the NIST randomness beacon, or some variant in your story.

The randomness beacon outputs a random number, signed by its key, every minute. By definition, the value of the number is unknown until its time passes, and is immutable after that time.

Before leaving, the time traveler simply looks in the beacon archives, and prints off/memorizes the beacon values near the time he's traveling to.

All the time traveler has to do is publish the results of the beacon for some times after the time he travels to. For example, he could publish the next hour's worth of beacon values right after he arrives. Once that time passes, everyone can see that the time traveler indeed knows the future.

Furthermore, one doesn't have to worry about the butterfly effect with the beacon! Because the NIST beacon/similar beacons use a radioactive source as the random number generator part, previous events have no effect on future events.

From a hackday article explaining it:

More esoterically, one could use the Randomness Beacon to prove that something is newer than a certain date by including a recent Beacon entry. As of this writing, the values for December 31, 2014 are all still up in the air, so I can’t possibly write one of them down yet. But from Jan 1, 2015 and on, it’s trivial to do so. So if I get a bunch of t-shirts made with the midnight value from December 31, it’s absolutely verifiable that I got them made in the new year. In short, you could use the Beacon as a not-older-than dating scheme.

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  • $\begingroup$ Definitely a cool thing! $\endgroup$
    – Ghanima
    Commented Mar 21, 2015 at 19:45
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    $\begingroup$ Alternatively, this could be interpreted as the NIST beacon being fake (e.g. preproduced), and you being an insider. $\endgroup$
    – Paŭlo Ebermann
    Commented Mar 22, 2015 at 11:46
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    $\begingroup$ I'm not sure the beam works for this purpose. The answer seems to be assuming that the random numbers generated are pretty robust in the face of whatever microscopic changes your appearance in the timeline makes. The NIST describes a quantum entanglement setup that I'm still trying to wrap my head around, but it could get knocked off course by tiny quantum effects, sending them off to generate a completely different set of numbers. You'd have to run some time travel experiments beforehand. $\endgroup$
    – Bryce Anderson
    Commented Mar 22, 2015 at 18:08
  • $\begingroup$ @PaŭloEbermann, that is a hole, but the beacon outputs combined with some of the other answers here could make it more robust. It's possible that there's an insider, but the probability gets lower when the "NIST insider" also knows what the stock market is going to do, etc. $\endgroup$
    – meson800
    Commented Mar 22, 2015 at 19:58
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    $\begingroup$ @BryceAnderson: indeed, inventing time travel would let us test whether radioactive decay events are "fixed points in time" or not (I think that's the terminology from Doctor Who, correct me if I'm wrong). If they aren't, then the time traveller has nothing (and, indeed, all time travel will be to a rapidly-diverging alternate history, with the result that trying to "change" it could be a moot point if it's very different from what you remember anyway). If they are, though, then the scheme works. Furtunately this is WB, so the author can just rule on unknowns in physics :-) $\endgroup$
    – Steve Jessop
    Commented Mar 26, 2015 at 12:53
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Bring copies of future expensive movies, think Avengers 4. It's not plausible to fake such.

As a bonus, bring additional material, think interviews and making-of documentaries.

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  • $\begingroup$ I think there needs to be some explanation around this answer... it is not impossible to create movies in the present day, and making copies of it is not more difficult. How will you prove that it is expensive and also from the future? $\endgroup$
    – Michael Lai
    Commented Jun 18, 2015 at 0:20
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    $\begingroup$ I like it! If you bring one movie back, maybe you found some way to secretly make an expensive movie. But if you bring 1000 blockbuster movies back, you've created something like 10 to 100 billion dollars worth of material. It would be hard to explain that away. $\endgroup$
    – Ebonair
    Commented Nov 30, 2017 at 16:31
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    $\begingroup$ Reading this now, feels strange. $\endgroup$
    – DickieBoy
    Commented Sep 4, 2018 at 9:08
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    $\begingroup$ @DickieBoy because release is close? $\endgroup$
    – tshepang
    Commented Sep 4, 2018 at 11:47
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    $\begingroup$ @Tshepang indeed, When this post was made, Avengers 2 was just coming out(i think). If they were to go come from the future. The full series of MCU films would be an awesome thing to bring back. $\endgroup$
    – DickieBoy
    Commented Sep 4, 2018 at 19:56
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Predict astronomical events. If you can supply coordinates and magnitude of supernova explosions, neutron stars oscillations or other similar events, it would be impossible for you NOT to be from the future.

You could be mistaken for an FTL space traveler, but if you bring enough data points from disparate directions, Occam's razor would work in your favor, and between time travel and FTL spaceman, time traveler would be chosen, because it would have less assumptions.

Since the astronomical events came from random points in the sky, it would be impossible to exist one place in the universe where these events could've been seen before being seen on earth (due to lightspeed limitations, it would need to exist one point in space nearer ALL events than Earth - assuming space is quadridimensional like in Einstein's relativity), and that there would be one observer with FTL capability at that point willing to come to earth to lie about being a time traveler.

Just being a bona fide time traveler would have less assumptions.

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  • $\begingroup$ This is great, because it would be impossible to fake the information and your existence couldn't change the results. I think the problem with this is that you'd have to wait for all of these events to take place. $\endgroup$
    – Gabe
    Commented Mar 22, 2015 at 8:25
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    $\begingroup$ Actually, there are plenty of supernovae that can be used, but they are extra-galactic so non-specialists are unaware of them. That makes this a very robust and simple solution. $\endgroup$
    – Keith
    Commented Mar 23, 2015 at 1:07
  • $\begingroup$ There is no difference between a FTL space traveler and a time traveler, is there? $\endgroup$
    – corsiKa
    Commented Mar 23, 2015 at 1:46
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    $\begingroup$ @corsiKa depends on your cosmology. A time traveler has the advantage of being able to be both (if FTL is available in his epoch), but the FTL traveler would still be bound to the geodesics $\endgroup$
    – Mindwin Remember Monica
    Commented Mar 23, 2015 at 1:59
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    $\begingroup$ @corsiKa the FTL traveler is still bound by the inexorable passing of time. He leaves Earth for Alpha Centauri. Travel ETA is 4 hours. He spends 2 days (48 hours) at the sulphur beaches of AC, and returns to Earth. He is now 56 hours after he departed. He cannot "gain" time by FTL travel, thus he is bound by the geodesics even if he moves faster than light. but that depends on your cosmology. $\endgroup$
    – Mindwin Remember Monica
    Commented Mar 23, 2015 at 2:53
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I feel like a strong approach could be to provide solutions for various problems of that era. Things that the top people in their respective fields have been trying to solve for many years with little to no progress. By convincing them, their feedback should influence others.

  • Solving (with proofs) the remaining six Millennium Prize Problems simultaneously would blow the minds of mathematicians.
  • Bringing back documentation of cures for various forms of cancer/diseases could convince the medical field.
  • As so on with physics, space, etc.

The biggest benefit off of this is that multiple field breakthroughs will cause so much world-wide impact that you'll end up with proof people can experience. It would be hard to say you're just a savant because research by the world's best couldn't do it either and they're specialists.

A few feasible issues are

  • The amount of time it would take to provide this documentation and have people take it seriously. (If you could kick-start your first breakthrough, the others could accelerate.)
  • Whether or not these solution were indeed solved within the timeframe we're limited to.
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  • $\begingroup$ I'm not sure how well this would work. Cures for diseases would take a while to work itself out; as for proofs, the issue is that the proofs you see in 2100 are likely to build on a lot of background that just isn't there now. The ABC conjecture had a claimed proof released in 2012, which still hasn't been fully checked by the math community (because the author had for some years gone off in his own direction, which meant that no one else had the background to fully understand the proof). If the traveller presents a proof from the textbooks of 2100, no one will likely understand it. $\endgroup$
    – cpast
    Commented Mar 22, 2015 at 0:31
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    $\begingroup$ Also, it's unlikely that all six of the Millennium problems would be solved by 2100. $\endgroup$
    – Charles
    Commented Mar 22, 2015 at 22:49
  • $\begingroup$ @cpast You only take about 30 to 40 different courses in a bachelor's degree, many of which aren't related to your field of study. You could pack a small library of 50-100 books from 2100 and give someone a complete 4 year degree. I'm pretty sure the brightest minds in our world could understand it. $\endgroup$
    – corsiKa
    Commented Mar 23, 2015 at 1:44
  • $\begingroup$ Nice one. Following this line of thinking, you can easily pack a vast amount of human knowledge from the year 2100 that, including sequences of text books back to present day, all into a portable memory device. Carefully research current day technology so as to be able to access it electronically as fast as possible. Important side effects: 1) Utterly changing the future. 2) Possibly provide technological means to more easily deal with the impending disaster. $\endgroup$
    – Keith
    Commented Mar 27, 2015 at 5:43
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    $\begingroup$ @corsiKa As a naysayer, I must say that if the "facts" from the future don't match the strong beliefs from now, the "brightest minds" will reject the texts. (global warming for example) "But, you have to be dead. The Earth will have been made inhospitable by 2040!" (or whatever) $\endgroup$
    – killermist
    Commented Mar 27, 2015 at 20:51
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A copy of a newspaper. Surely in the future he would have access to historical documents and newspapers among them. A copy of a national newspaper from several days in the future that hadn't even been written yet would surely be some kind of proof. Especially if it remarked on an event that hadn't yet occurred.

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    $\begingroup$ and contained the lottery results... $\endgroup$
    – SF.
    Commented Mar 20, 2015 at 19:36
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    $\begingroup$ The key to making this work, I think, is to show up, say "this is next Thursday's newspaper", and (with witnesses) lock it up somewhere that none of them can access before then. (Safe-deposit box with key escrow? Something like that.) Then next Thursday you all march on over there to retrieve it. If anybody looks at it before then you risk butterfly effect, but you have to establish its "identity". $\endgroup$
    – Monica Cellio
    Commented Mar 20, 2015 at 22:13
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    $\begingroup$ Sadly, even if no one sees it, it could be changed. The headline could be, "Will this be our predicted headline?" or even "Correct Horse Battery Staple". The people who make the paper in question will no doubt pick up on this news-worthy occurrence and report on it. $\endgroup$
    – IchabodE
    Commented Mar 20, 2015 at 23:54
  • $\begingroup$ @MBurke It depends on which model of time travel you're using. I personally like the fixed-timeline model, in which the newspaper would end up being the same no matter what happened, because it already happened that way! $\endgroup$
    – 2012rcampion
    Commented Mar 22, 2015 at 0:26
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    $\begingroup$ Locking it up without anyone seeing it is just going to prove you are an impressive stage magician who can swap Thursdays newspaper with one locked up in an apparently secure place without anyone noticing. $\endgroup$
    – armb
    Commented Mar 25, 2015 at 8:35
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I wouldn't actually try to prove I was from the future. I would focus on proving the event would take place. If I couldn't prove it, I would focus on getting the appropriate response in place via a subtle, roundabout way. For example, if the event was an asteroid collision, you forge an email to an astronomer from a trusted colleague telling him where to point his telescope. If the event was a terrorist attack, you send the authorities anonymous tips and maybe even plant some clues yourself.

This might be interesting in contrast to previous more drastic attempts that failed. Maybe previous time travelers assassinated Hitler too early, which caused something even worse to happen, so you agree to be his bodyguard, but send some key piece of information to Alan Turing to help him build his enigma-cracking machine that he never managed to complete in your timeline. Then you fake Hitler's suicide once it's safe to do so.

Or maybe something not so subtle. In your timeline, Saddam Hussein eventually manages to detonate 3 nuclear bombs in America. After several more subtle attempts, you plant some WMD intelligence way before the event, but that's still not enough to get the U.S. government to intervene, so in desperation you perpetrate the 9/11 attacks.

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    $\begingroup$ +1, You cannot prove you're from the future beyond reasonable doubt. Do what you came to do, don't just talk about it. $\endgroup$
    – Mazura
    Commented Mar 21, 2015 at 0:45
  • $\begingroup$ Oh here we go with the Saddam Hussein fantasies. $\endgroup$
    – Mark Micallef
    Commented Mar 27, 2015 at 6:06
  • $\begingroup$ Sounds farfetched. But, on the other hand, it doesn't sound implausible. $\endgroup$
    – killermist
    Commented Mar 27, 2015 at 20:45
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    $\begingroup$ That's sort of the point. Any averted future event naturally sounds like a crazy conspiracy theory. $\endgroup$
    – Karl Bielefeldt
    Commented Mar 27, 2015 at 21:15
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You mention the option of either unpredictable natural or man-made events. If you publish a table of stock prices, then the future will change based on how people use them. What about a table of things people cannot change?

Use daily weather information for a few weeks in the future. You can publish precise highs, lows, conditions for a variety of locations around the globe. Weather is something that is always difficult to predict, and having knowledge of what will happen will not change the outcome. Additionally, you won't have to wait for natural disasters like earthquakes to take place. After a day or two of accurately predicting weather across the globe, people should either believe that you are from the future or are the best meteorologist in all of history!

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    $\begingroup$ This would work very well if the past is completely immutable; but if that's the case, what's the point of trying to change history? If history can be changed, then by a classic butterfly effect, the traveller would affect global weather patterns just by walking around and breathing. Your weather information might be good for a while, particularly for more distant locations, but not indefinitely. $\endgroup$
    – Royal Canadian Bandit
    Commented Mar 20, 2015 at 15:07
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    $\begingroup$ @RoyalCanadianBandit You're correct that the traveler's presence would affect weather patterns on the long-term. However, you only need to predict the weather conditions enough so that people believe you are from the future, and the presence of one extra person is not going to have a significant impact on that short of a scale. $\endgroup$
    – David K
    Commented Mar 20, 2015 at 15:15
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    $\begingroup$ @DanDavis: If you have a backpack full of equipment in your phone booth, you could carry a lot of weather records just on microfilm, let alone digital storage media. But it's true that the authorities would take a lot of convincing your special weather knowledge derived from time travel and not, say, highly advanced groundhogs. $\endgroup$
    – Royal Canadian Bandit
    Commented Mar 20, 2015 at 15:32
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    $\begingroup$ @RoyalCanadianBandit Time traveling weather groundhogs may just be the solution. $\endgroup$
    – Dan D
    Commented Mar 20, 2015 at 15:37
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    $\begingroup$ Since nobody seems to be mentioning it: there are hundreds of earthquakes that occur every day, most just don't register on a human scale, but they do occur, and to correctly identify all of them would be astronomically unlikely without foreknowledge. $\endgroup$
    – Kyle Hale
    Commented Mar 20, 2015 at 22:32
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The time traveler should set new video game speed run records using glitches not currently known. For instance, there are currently still large, active communities uncovering new methods in Nintendo 64 games such as Zelda: Ocarina Of Time.

This is a non-violent method which involves no money or physical objects. It minimizes introducing variables mentioned above through lottery/stock manipulation as subsequent events are not based on the outcome of the prior event (i.e., Glitch 2 in Game B will not change upon completing Glitch 1 in Game A).

I am sure this method would garner the time traveler enough media attention and credibility to then springboard to convincing the public on whatever "serious" issue as at hand.

As I'm typing this, it just occurred to me that this was the plot of "The Wizard" when they introduced Super Mario Brothers 3 and the one competitor knew how to get to the early warp zone.

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    $\begingroup$ I'm not sure I buy this -- it would be difficult to prove that the traveller had not been in communication with the game developers. The developers might have been aware of the glitches and simply not made them public. Alternatively, the traveller might have just spent a really long time playing Zelda or whatever -- no special knowledge from the future required. $\endgroup$
    – Royal Canadian Bandit
    Commented Mar 20, 2015 at 15:29
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    $\begingroup$ I think this is a pretty interesting and novel solution. I like that it wouldn't really have any significant effects. I think credibility would be handled by using multiple games from a wide variety of developers and time periods. Plus it's just so crazy it has to work. $\endgroup$
    – DaleSwanson
    Commented Mar 20, 2015 at 15:33
  • $\begingroup$ I like it. Welcome to the site todd $\endgroup$
    – James
    Commented Mar 20, 2015 at 15:51
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    $\begingroup$ Really novel idea. I like it. I'd suggest having prepared glitches/speedrun for games that have yet to come out, too. That way you can post a complete speedrun and/or demonstrate all the glitches on the very day it comes out. $\endgroup$
    – Bobson
    Commented Mar 20, 2015 at 15:54
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    $\begingroup$ It's an interesting approach, but this could just as easily be explained by someone who has been holding off on sharing crazy new tricks. You also need to be good at said games. I know how 0-star Super Mario 64 works, but that doesn't mean I can complete a 0-star run. $\endgroup$
    – Beefster
    Commented Nov 9, 2020 at 20:10
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We can already store enormous volumes of information in a pocketable medium. Bring a petabyte of (his current) Wikipedia on his pocket reader.

In so many TV shows and movies, the lone traveller has to do everything from scratch, and his limited resources is the main source of making it interesting.

Why not have a well-researched plan in place? The catapult to the target past might be a one-shot, but going back a week or month is easy and just a couple years is routine.

They can grow their resources and make plans in a small time loop near home: each jump back improves upon the planning and size and effectiveness of the organization.

They can become very wealthy and politically powerful, and recruit talent from the brightest of the population, seemingly (from the outside in normal time) by a combination of luck and omniscience.

Now there may be an inherent issue with the far-past catapult in that any success at changing the timeline will destroy the "present" with its large organized effort in place. Any arrival at all will appear in a different timeline not in their own past, so they cannot send multiple loads. They can send multiple trys though. Each catapult seeds a new timeline and through repetition with variations on the plan the hope at least one of them turns out the way they intended.

For a limited load size and mass (your phone box isn't larger on the inside?) why send a single person w/carry-on baggage? Send nanotechnological robots or seeds for robots and infrastructure. If piloted also, the person is a dwarf (or has the body of a child) to make room for his stuff. That is a detail I've not seen in stories before.

If multiple loads is possible with the thread maintained to the new past only possible if they don't diverge (yet), set up the operation on the far side of the moon. The expedition is in shipping mode to receive as many loads as it can, and only after the thread is broken do they proceed with the mission.

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One of the fairly standard proofs of knowledge of future events is to have the information sealed in an envelope, and hand it to the person you are trying to convince with instructions to open it at the time of, or immediately after, the event. The fact that the other person has it in their possession before an event happens is the proof that you knew it ahead of time. Because they don't see the proof that you held such knowledge until after the event happened, they don't get to meddle with the flow of time, there by avoiding having them take actions that would change the outcomes.

If I were such a time traveler, I would probably pick sports as my proof. Pick whatever major sport was in season for my target region and have a list of the final scores of each matchup. Two weeks or so should give enough evidence to convince people that I knew what the results were going to be before the games were played.

Here is the catch though. By demonstrating that time travel is a possibility, I am pretty much ensuring that someone is going to figure out how to do it. I may try to mislead or derail their research efforts, but knowing that something is possible makes it pretty much inevitable that someone will eventually figure it out. Quite possibly sooner than they did in my original time line now that people are paying attention to it as a real possibility.

If one of my goals is to prevent time travel technology from being developed then I have to work a lot slower and more subtly. I have to use my future knowledge to place myself in a position of influence, without making it obvious that I am from the future.

Edit in response to comments:

The way the scenario in the beginning of this plays out is that I select the individual I want to convince and come up with a way to approach them after doing a bit of preliminary work. Once I have made contact I say something along the lines of "I have some important information for you, but you won't believe me if I tell you now. Take this envelope, wait until Monday morning to open it, then email me at the address inside after you have verified the information it contains."

When you open the envelope Monday morning you find the scores at the end of each inning for all of the baseball games that occurred in the previous three days. You know the envelope has been in your possession for at least a full day prior to the first game on the list. Now it is conceivable that I could have had a lucky guess on one or two games, or possibly found someone to bribe on a few more, but the probability of me having the outcome of every game across the nation for several days correct down to that level of detail is almost impossible. (The information inside could really be anything, the key is that it is information about events that occurred after I gave it to you; It is inconceivable that I could have accurately predicted the data volume and level of detail without special knowledge; And it is equally inconceivable that I, or any organization supporting me, could have influenced the outcomes of all of those events.)

So obviously something special had to have happened. Could I have tampered with the envelope after I gave it to you? Possibly, but you have not seen me since, and I have made no further attempt to contact you. Could I have been incredibly lucky? Sure, it is possible, but extremely unlikely. And however I pulled it off, wouldn't it make you curious?

If I have a list of such contacts, and I work this same general scheme on each of them independently. One or more of them is going to decide to reach out to me to find out what is going on. Perhaps they do think I managed to pull a fast one on them, so offer to do it again when they can take some informed precautions.

One potential tripping point in this approach is keeping someone from opening their envelope early and trying to exploit the knowledge within. If I am targeting current day, and particularly selecting tech savvy people as my targets, then I can encrypt the data, give them the encrypted file, and then wait to give them the key to decrypt it until after the events occur.

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  • $\begingroup$ Such tricks as in your first paragraph might be seen as you being an illusionist instead of a time traveller. $\endgroup$
    – Mast
    Commented Mar 20, 2015 at 15:21
  • $\begingroup$ @Mast Possible but unlikely in the way I have envisioned this. Updated my response with more details. $\endgroup$
    – Rozwel
    Commented Mar 20, 2015 at 16:31
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    $\begingroup$ The envelope in the mark's possession is a trick already perfomed by showmen. The encryption is a good idea: you can see the cyphertext before the event and know that it was there all along. The event itself contains the key. $\endgroup$
    – JDługosz
    Commented Mar 21, 2015 at 4:30
  • $\begingroup$ Demonstrating time travel does not necessarily lead to its discovery. What if the time machine can only be built with exotic matter which arrives by comet impact in 2098, and cannot be produced through other means? $\endgroup$
    – Superbest
    Commented Mar 26, 2015 at 4:48
  • $\begingroup$ Demonstrating it would ensure people are looking at it with absolute certainty that it is possible. Perhaps the original time machine did need such an exotic material, but knowing something is possible pretty much guarantees someone will figure out how to do it. Maybe they don't succeed until said exotic shows up, or maybe they figure out a substitute/synthetic, or maybe they find a totally different approach, particularly since our time traveler is trying to throw them off the technique he knows about. $\endgroup$
    – Rozwel
    Commented Mar 26, 2015 at 19:01
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The answer is Bitcoin.

Block chains are based on a cryptographic proof-of-work protocol: constructing one takes time and effort and processing power, but it's orders of magnitude easier to verify that they are correct.

So either get your hands on 100 years worth of block chain data, or use a 22nd century supercomputer to generate a brand new one, of a length sufficient to impress. Load it up on as many antique terabyte drives as you can carry, get into the time-catapult, and find the nearest influential crypto enthusiast.

This scheme has several advantages: Unlike the 'Almanac', the locals can start verifying right away. There's no risk of butterfly-effect: your block gain doesn't have to match anything from the 21st century, it just has to be internally consistent. You're not bringing any useful technology or information back with you, just proof that you had it before you left.

Once a certain length of block chain is verified, the locals will have mathematically solid proof: Either you are a time-traveller, or you have access to more computing power than all of humanity combined, and have decided to use it to impersonate a time-traveller. Either way, it's probably a good idea to listen to you.

A drawback of this solution is that it's highly technical. Once you've convinced the global mathematics community, the NSA, and Reddit, you're on your own to convince the man-on-the-street.

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    $\begingroup$ Just to clarify on this one, you don't have to bring terabytes back with you. All you need are the block headers, which will be very small in total even for a very long time into the future. Or if you prefer, just bring a single header from 100+ years in the future when every individual hash in and of itself represents more computational work than the whole planet is capable of doing in 2015. If you can provide a piece of data whose hash starts with 40 hexadecimal zeros, cryptographers worldwide will bow down and worship you. Proof in 80 bytes! You can even just memorise it before travel. $\endgroup$
    – eMansipater
    Commented Apr 3, 2015 at 5:20
  • $\begingroup$ To add to @eMansipater 's comment, bringing only the headers would also help to avoid disrupting the Bitcoin network too much, which would respond to 100 years of blocks by accepting them all as valid immediately, greatly disrupting the current Bitcoin economy. $\endgroup$
    – Andrew Poelstra
    Commented Jun 3, 2017 at 0:58
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It's actually quite easy. Look at the stock market--this has been rejected by other posters on the basis that making a correct prediction will change the future. The butterfly effect is certainly going to be an issue. To protect against this you must make only one prediction and at a short range.

Have your time machine deliver you to the Oval Office, 10 minutes before the closing bell on Wall Street. I would recommend minimal attire to minimize the nervousness of the Secret Service agents.

Arrive, hand the closest agent printout, tell him to keep it secure. The printout looks like gibberish. You then explain that you are a time traveler, come to warn of a disaster. The printout you just provided is the closing prices for every actively traded stock. There is a simple encoding scheme, even if it goes straight to a cryptographer they'll only have a few minutes to work on it--the crypto guys simply don't have time to bring their heavy guns to bear.

You make thousands of accurate predictions at once--they'll listen. Popping in from thin air will also help.

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  • $\begingroup$ The stock market's never been manipulated... oh, wait. They'll listen, -to your stock tips. This was more plausible before high frequency trading existed, where you'd of had to physically manipulate things opposed to 1's and 0's. $\endgroup$
    – Mazura
    Commented Mar 22, 2015 at 7:00
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    $\begingroup$ @Mazura Sure, you can manipulate the market--but nobody is going to be able to manipulate all the stocks at once. $\endgroup$
    – Loren Pechtel
    Commented Mar 22, 2015 at 17:34
  • $\begingroup$ Don't manipulate the market, get rich - at least that is what I've always wanted a time machine for :P. Getting rich will get their attention. When there's no evidence of insider trading or other illegal activity, it'll begin to lend weight to your claim. $\endgroup$
    – Jim2B
    Commented Mar 26, 2015 at 2:18
  • $\begingroup$ They might well shoot you. But when they search your corpse, in your other pocket is the information you're warning them about which, once they'd decoded the stock prices and discovered them to be correct, they would hopefully also believe ;-) $\endgroup$
    – Steve Jessop
    Commented Mar 26, 2015 at 13:06
  • $\begingroup$ A potential problem here is that someone might start trading crazily on purpose just to make sure your predictions are not accurate $\endgroup$
    – Annonymus
    Commented Jul 7, 2016 at 13:31
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Bring forth the bugs! - pick a ton of open-source projects, such as linux, and expose all the current security bugs (exposing one won't butterfly-contaminate the others, as the code is already written, saved, deployed on all sorts of machines, and won't auto-update to fix itself) - next, to prevent the ensuing hackfest, distribute the code that would be required to fix this newfound issue - the patches, fixes, etc. - you now have proven yourself as someone with credible world-saving abilities - people can speculate as to where this comes from, if necessary you could blackmail every single government or politician in the world (or just about anyone else) - basically choose things that have already occurred prior to the date that you wish to appear in, but have not yet been revealed to the public (think snowden leaks, only much larger, over more secret organizations, with the juicy stuff selected and chosen)

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    $\begingroup$ I like this. There are lots of broken things that nobody even now knows are broken (Linux/BSD subsystems), and a number of broken things for which the details of the brokenness are not yet public (think Weiner). Exposing (and fixing in one step) all of the fixable things and blackmailing all the (evil) politicians in one step makes for a great starting point. $\endgroup$
    – killermist
    Commented Mar 27, 2015 at 21:06
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By request on the comment thread, I will expand on rmoore's answer and detail how the DNA analysis can prove the time traveler is indeed a descendant of his great-grandparents / grandparents.

This page for a testing website states that their rate of accuracy for grandparent DNA testing is 99,99%. If the grandparents are alive, the odds of the time traveler to NOT be the grandson of these people would be:

0.0001 ^ 4 = 0.000,000,000,000,000,1
1 in 1,000 trillion chances.

Since the amount of people that ever lived is estimated at 108 billionssource 1 2 3, the explanation of time travel would be the accepted explanation by Occam's razor. The apple does not fall far from the tree. Also there is the paternal test for the Y chromosome (paternal grandfather) and the maternal test for the X chromosome (maternal grandparents) and the mitochondrial DNA testing (maternal grandmother).

Unless his family has a huge recent history of incest, if the grandparents are already living, there would be no doubt he is a time traveler.

But unfortunately, There is a great chance his grandparents are NOT yet born as of 2015.

Lets assume they sent back a healthy indiviudal, on his prime. Lets also assume that his prime is at 25 years old, and that he was born when his parents were 25 years old, and his parents were born when his grandparents were 25 years old.

This would place the birthyears of our 2100 time traveler in 2075, his parents in 2050 and his grandparents in 2025. His grandparents would be born only ten years-ish from now. This would leave only his great-grandparents alive, in their teens. There is a trend of people having children at a higher age (with all that egg freezing mania and stuff) so I think that this 25 years window is a safe assumption.

Now, great-grandparent dna testing is still mostly unheard of, but looking at genealogy testing, we can get 95% proof that the time traveler belongs to the family line of his father's father's father. With autosomal DNA testing, he can be placed in the family tree of all of his eight great-grandparents (and since your grandparents have not yet been born, they are all alive by 2015). Now, unless they are close relatives (like several of those couples are cousins in love), a simple venn diagram would prove that the very existance of the time traveler would be impossible if he is not a time traveler. Proof by contradiction.

There is no other way our alleged time traveler could be part of eight completely unrelated family lines (and trace your mitochondrial DNA to a few and the Y-DNA to only one). Unless he is really the great-grandchildren of those eight families.

This one has less precision (still beyond reasonable doubt, but would give a hook for the antagonists to descredit our hero) than the grandparents (so the very skeptic may still be unsure, specially if some of the great-grandparents were related), but with some more info about the future, the time traveler would succeed.

Picture the adventures of a time traveler trying to convince eight teens to do agree to DNA testing, all the while dodging the evil organization and attempting to avoid doomsday.

Also for bonus kicks, he tries to get the matchups of his great-grandparents right, while the teen hormones attempt to negate his family line.

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  • $\begingroup$ The problem is very similar to that of convicting criminals based on DNA matches, and I think you are committing the same error that was infamously committed by many such trials. $\endgroup$
    – Superbest
    Commented Mar 26, 2015 at 4:41
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A reverse-compatible (USB3.0) hard drive full of the most expensive movies, pop songs, selected news media, YouTube, selected Internet, and perhaps selected scientific publications from the year after you arrive for several decades into the future. You're either from the future, or from an alternate Earth future.

However, proving you're from the future is probably a lot easier than gaining the trust of authorities, and avoiding getting abused by people who decide to behave badly when tempted by the potential wealth/power they might think they could hoard to themselves if they captured you.

So, you might want to bring a device that would let you make anonymous undetectable broadcasts, as well. And other things that might help with your personal security and well-being.

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    $\begingroup$ Yay, and everyone can save the money to actually produce this stuff now. $\endgroup$
    – Paŭlo Ebermann
    Commented Mar 22, 2015 at 11:44
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Have the person you're trying to convince write a long letter, and post it to you (or your Grandfather). You'll receive it in the future.

Then you produce the envelope out of your pocket, open it and reveal the letter!

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  • $\begingroup$ This does not work. If you went back in time to change something, then you must assume that the time travel mechanics at work send you into an alternate timeline (otherwise you can't change anything and there's no point in the trip, let alone in proving your story once you get to the past). And if you are in an alternate timeline, then you're never going to receive that letter. $\endgroup$
    – Lightness Races in Orbit
    Commented Mar 30, 2015 at 17:05
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    $\begingroup$ It works assuming the "you" from this time-line also makes the trip, and as such becomes the "you" that's here now, and as such, has the letter. $\endgroup$
    – komodosp
    Commented Mar 31, 2015 at 7:16
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A variant of the lottery numbers ploy: bring some other information which is top secret right now, but part of the historical archives in the future. That should get the immediate attention of government agencies if the time traveler phones them.

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    $\begingroup$ Nice. Solves the butterfly effect, which is an issue with the lottery. Might just get you arrested though, and leaks can happen without time travel. $\endgroup$
    – GKFX
    Commented Mar 21, 2015 at 19:05
  • $\begingroup$ This is great if the only people you plan to tell are the CIA. Also if you don't plan on returning to your time machine any time soon. $\endgroup$
    – corsiKa
    Commented Mar 23, 2015 at 1:48
  • $\begingroup$ @corsiKa the Q states that its a time catapult. He did not bring it with him. $\endgroup$
    – Mindwin Remember Monica
    Commented Mar 24, 2015 at 0:20
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    $\begingroup$ Wait. So, Snowden is a time traveler and not some guy that got access to files that the (supposed to be transparent) government didn't want to get access to? I'm so confused. $\endgroup$
    – killermist
    Commented Mar 27, 2015 at 20:58
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It's easier to think about what type of thing you'd bring back 100 years to the past that would convince people you are from the "future" (IE, Now).

Our future man could easily find out what hasn't been invented yet this year and take a product from his time back with him to our time to prove his authenticity. Demonstrating his technical prowess with an unusual device that does not exist anywhere else in the world should be sufficient proof.

If we were to compare it to travelling from now to 100 years ago, all anyone would have to do is take their smartphone out of their pocket and play some music. Not only would the music be strange to the listener, it would be a remarkable technology in itself - even without the ability to connect to a wireless network (a problem with any technology dependent on future infrastructure).

For our man from the future, his handheld matter converter that turns common paper into hot dogs should do nicely.

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    $\begingroup$ I think this falls under: "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." $\endgroup$
    – Octopus
    Commented Mar 20, 2015 at 18:43
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    $\begingroup$ @Octopus No. Even in our era, it costs an awful lot of money to make a single processor/phone/etc.: we can only buy them cheap because they're made in bulk. To develop the technology and make one would be phenomenally expensive in 1900. Maybe a few large governments could have done it collaboratively, but I doubt it. It's not garage stuff. $\endgroup$
    – GKFX
    Commented Mar 21, 2015 at 19:05
  • $\begingroup$ I believe in a future where hot dogs don't exist. $\endgroup$
    – krowe2
    Commented Jul 28, 2015 at 16:55
  • $\begingroup$ Well if it were me going back 100 years ... How about 1960s trawler sonar schematics. Should beat the tar out of WW1 sonar. $\endgroup$
    – Joshua
    Commented Dec 23, 2015 at 22:43
  • $\begingroup$ @Joshua Only if you can also provide the micro circuitry and mechanical guidance to build them and implement them on 1920s era warships. In other words, I hope you're a Doctor of Engineering, or at the very least telecommunications. $\endgroup$
    – Zibbobz
    Commented Dec 29, 2015 at 18:55
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I'm surprised nobody has stated the obvious: bring back a sports almanac that shows the result of every major sports event until the end of the century.

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    $\begingroup$ Enter Biff Tannen... $\endgroup$
    – Austin Burk
    Commented Mar 21, 2015 at 15:37
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    $\begingroup$ Unfortunately, that fails on the speed criterion, and also risks the butterfly effect, whereby the more distant scores change because of your presence. As for just reading off the first few results: plenty of gamblers work out the correct scores for a few games. Not a bad idea though. $\endgroup$
    – GKFX
    Commented Mar 21, 2015 at 18:59
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    $\begingroup$ Can you edit to explain how this satisfies the time constraints in the question? $\endgroup$
    – Monica Cellio
    Commented Mar 22, 2015 at 19:36
  • $\begingroup$ @GKFX - but the Almanac would change as the future results change! $\endgroup$
    – komodosp
    Commented Mar 24, 2015 at 22:27
  • $\begingroup$ @GKFX How does this fail the speed criterion? In this scenario (impending disaster) more distant scores don't matter -- all you need to do is prove immediately that you're actually from the future. Any scores more than a couple months away are irrelevant. $\endgroup$
    – markcoatsworth
    Commented Mar 27, 2015 at 3:19
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Send two payloads into the past. First send a probe (or even the time travellers luggage) to March 30. After making sure the probe is sent, then send the traveller to March 1. All the time traveller has to do is tell news agencies that his probe/luggage and maybe artifacts from the future will arrive on March 30 at a specific time and place.

For a real world example of how people react to a time traveller you should also check out the John Titor story. A time traveller revealed himself on internet bulletin boards during the years 2000 and 2001. Caused quite a stir after he posted pictures of his time machine.

http://en.wikipedia.org/wiki/John_Titor

from http://www.abovetopsecret.com/forum/thread62046/pg1

excerpt: This is a picture taken in the fall of 2035 during my training. It shows my instructor beaming a handheld laser outside the vehicle during operation. The beam is being bent by the gravitational field produced outside the vehicle by the distortion unit.

enter image description here

from http://www.strangerdimensions.com/2013/02/04/john-titor-the-man-with-the-machine/ enter image description here

For your time traveller it might also be prudent to remember that doomsday predictions can also attract the wrong kind of audience (ufo and doomsday believers) which could be detrimental when he is trying to prove his authenticity.

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    $\begingroup$ Presumably, everybody also smokes cigars white fat-arming it out the window in 2035 and black and white grainy photography is in vogue. $\endgroup$
    – Mark Micallef
    Commented Mar 27, 2015 at 6:12
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    $\begingroup$ Perhaps in 2035 it will be hip to make your photos look like the first generation of camera phones? Much like it's hip now to make your photos look like 1960s Polaroids? $\endgroup$
    – Matthew Lock
    Commented Mar 28, 2015 at 10:53
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    $\begingroup$ And he got the idea from reading it here... dun dun dun. $\endgroup$
    – Theraot
    Commented Jan 24, 2016 at 12:30
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What if the time travel resulted in some form of semi-violent catastrophe, or publicly visible entering of our time?

Since we do not know what time travel will do to our reality this might be feasible. If I knew time was of the essence, I'd make sure I'd enter with a big BANG, to get this identification process out of the way ASAP... Of course this might also result in being arrested and potentially tortured to get information.

But lets assume, that the above doesn't happen, and if somehow during this process the Eiffel tower got sucked up in some space time void, or for that matter any other major publicly visible entrance event occurs in a manner that it unknown to us, leaving only our hero as the sole survivor... people will notice.

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  • $\begingroup$ That doesn't establish time-travel as the cause, though; that just demonstrates that something weird happened -- aliens, terrorists, satanists, whatever. $\endgroup$
    – Monica Cellio
    Commented Mar 22, 2015 at 19:25
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    $\begingroup$ @MonicaCellio... true, but as I've said. "Since we do not know what time travel will do to our reality this might be feasible." At least it will draw attention... which was one of the sub points. If I had to see some gigantic space time portal thing or whatever it is, rip open... Id take whoever created / came out of it's word for what caused it. So the caveat is that everyone sees this event playing out. $\endgroup$
    – Rohan Büchner
    Commented Mar 22, 2015 at 20:32
  • $\begingroup$ No-go. In Continuum, some unfortunate bridge took damage repeatedly from being the end point of time traveler arrivals. The cops never really noticed except to make sure it wasn't a bombing. Some person at the arrival point would be treated as time-local, maybe crazy, but certainly not special (a time traveler). $\endgroup$
    – killermist
    Commented Mar 27, 2015 at 21:13
  • $\begingroup$ @killermist... is the op writing/questioning as part of the Continuum universe? I mean I've never seen it myself, but I'd assume there are multiple realities, even when things like time travel comes into play not only one solution is feasible. $\endgroup$
    – Rohan Büchner
    Commented Mar 27, 2015 at 22:44
  • $\begingroup$ I was using it as an example. "What if the time travel resulted in some form of semi-violent catastrophe...?" Check. Did anyone notice anything other than a "bombing" with no bomb debris? Nope. Trying to pin that to "I'm a time traveler" totally doesn't work. $\endgroup$
    – killermist
    Commented Mar 28, 2015 at 18:46
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The "standard" solution to this is to predict the future.

The most straightforward version would be to print out and bring along stock market data for some time following your intended arrival. It is widely believed that the movements of the stock market are impossible to predict precisely, so consistent success at this would be quite convincing. And, if everyone is still incredulous, you could just leverage your information to make a fortune and then simply pay them to do what you want. If you want the time traveler to not have this disproportionate power, you could instead give him a one way hash of the market figures - that way he would not be able to predict what the value will be, but it will be possible to determine whether he had access to the future value at any point.

Alternatives include guessing lottery numbers, weather, sports competition outcomes, and so on. All of these have the drawback of being chaotic, and so if your universe takes a "butterfly effect" approach to time travel, the simple presence of the traveler may disrupt these events and render predictions useless.

To solve this issue, you can try bringing back large scale historical events, which would take more than a flap of a butterfly's wings to alter. For instance, presumably a person traveling to 1913 would have a lot of trouble preventing a world war in early 20th century, even if they did manage to save Archduke Ferdinand. The drawback here is that major historical events may be predicted, and people may ascribe the prediction to extraordinarily sharp deductive powers rather than future knowledge.

Lastly, you could bring back information that was already past at the time of arrival, but would not be widely known until much after. For instance, an important shipwreck was discovered by chance in 1982. If you traveled to 1975 with a map showing the location of the wreck, locating it would be an extraordinary feat. Some could accuse you of stumbling upon the wreck yourself, and trying to spin it into a time travel story, but if you do this for many shipwrecks (or other artifacts) around the globe, that theory will become quite weak. Besides archeological finds, you could go back with information that was a very well kept secret at the time. The nice thing about this strategy is that even if some chaotic process ruins the future you are trying to predict as proof, the past would not be affected (granted this assertion is dubious in a universe where traveling backwards in time is possible) and your proof is safe.

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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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People seem to be thinking small in terms of "data points required to not just be a charlatan." Rather than a single lottery or a series of sporting games, why not just correctly "predict" the full intraday pricing on a second-by-second basis of, say, 1,000 equity options ?

  • Stock option prices are governed by stochasticity, i.e. any given moment a stock option's price is just as likely to go up as down.
  • You may be able to rig a lottery or a sporting match or two, but rigging an entire open market is ... unfeasible.

So that's 1,000 stock option volatilities * 28,800 seconds in a trading day = 288,000,000 basically coin flips you accurately predicted.

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    $\begingroup$ Good idea! Welcome to the site. However, the amount of time needed to display your information so that it can be proven to have existed before that day is a problem. And then you consider the butterfly effect.. $\endgroup$
    – Austin Burk
    Commented Mar 21, 2015 at 15:53
  • $\begingroup$ Unless your arrival creates even a simple sonic boom, but at worst a small tremor. Countless investors decide to do something different. You're back down to 50% success rate pretty rapidly. $\endgroup$
    – killermist
    Commented Mar 27, 2015 at 21:18
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