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I want to help people to verify they're from future (if they are, of course). For this I publish some facts/numbers on my website, dayly or weekly or whatever, so that a time traveller can remember numbers for the next week and say them to whoever wants proof.

The requirements for such numbers are the following:

  1. They should be unified: I don't want to mix it up, so better stay consistent.

  2. They should not be about one-time or unique events: while being told that a big robbery is to be commenced next week is an important thing by itself, there might not be any noticed robberies on a given week.

  3. They should have a lot of possible values: while "yes or no" answer is as simple as it gets, it's still a 50% chance to guess right. And many such questions are harder to check and easier to mix up. Ideally it's a big number with lots of equiprobable values.

  4. It should be unequivocal and easy to check for me: time travel possibility is pretty low, so I can't spend too much time and money if there really is nobody to help to.

  5. It should be near impossible to be influenced or changed deliberately: stock prices are varied and are easy to check, but can be controlled.

  6. It should be resistant to butterfly effect: true random number generator values and such things are out of questions, because future traveller's arrival might cause them to roll differently.

  7. It is a good idea for the number to be easily checkable by others: this means my numbers can be checked and nor me nor anybody else can fudge them.

  8. And lastly, it should be impossible to find out or predict or calculate before checking date: obviously, if it is known before check date, we're not checking future knowledge anymore.

Any ideas for something that fits all of these? Preferably not something restricted to our time or our world.

Of course, if you have other ideas on verifying time travellers without numbers or find a flaw in my reasoning, you're welcome as well.

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    $\begingroup$ Let's clarify. You are publishing a number every week on your website, like 624, 845, 179 etc. Time traveler knows that he or she needs to check your website and memorize a number. You can verify the number, because you have a schedule of numbers, and this schedule is secret. Correct? $\endgroup$ – Alexander Jan 31 at 21:47
  • $\begingroup$ @Alexanter, not exactly. Having a secret preplanned schedule would break 7 and 8. More like I take some existing number, like how many planes were late in world's public airports this week, and post it. And if you are a time traveller, you name a number for next week, and next week we see if it's correct. $\endgroup$ – Максим Корчагин Jan 31 at 21:52
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    $\begingroup$ Then you can ask them lottery winning numbers, so you don't even have to post anything. The complication here is "resistant to butterfly effect", because we have to settle on how deterministic is your universe. $\endgroup$ – Alexander Jan 31 at 21:57
  • $\begingroup$ @Alexandex, and that is 5. Because lotteries are not exactly renowned for honesty and can be rigged. $\endgroup$ – Максим Корчагин Jan 31 at 21:58
  • $\begingroup$ They would, of course. That's why it is a question ))) Also, planes and online airport schedules are pretty specific for our world and modern times. $\endgroup$ – Максим Корчагин Jan 31 at 22:01

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Stellar, specifically solar radiation.

Get a big telescope and measure solar activity like flares, sunspots, and more. Use some sort of pre-determimed formula to convert your observations into a memorizable string of numbers and publish them to your blog. Now you've got numbers which are essentially impossible to predict (as the sun is such a ridiculously chaotic system) and which are unlikely to be changed by any butterfly/causality effects. If someone is able to predict them or alter the sun significantly enough, you've got bigger problems than time-traveler verification.

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    $\begingroup$ I like this. Either they are from the future, or they have determined how to predict solar flares, something completely unknown to scientists today. If they have determined how to predict solar flares, they will probably be busy winning the nobel prize instead of trying to convince someone they are a time travler. $\endgroup$ – cowlinator Feb 1 at 3:06
  • $\begingroup$ If this universe in non-deterministic, and solar activity is random, then it won't match. "Forking events", which determine parameters of solar activity, are likely going to occur only after the new timeline started. To fix this, we need to look for celestial events from outside Solar system. $\endgroup$ – Alexander Feb 1 at 5:02
  • $\begingroup$ Like this too. While this is indeed random (as is everything else), weather is completely random too, but it can be predicted pretty well. It most definitely can't be influenced, is pretty far from Earth to be influenced randomly, and is not unique to our world or time. And if we need less random events at cost of better telescopes needed, we indeed can use something from further away. $\endgroup$ – Максим Корчагин Feb 1 at 12:39
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    $\begingroup$ I’m afraid it’s as subject to the butterfly effect as anything else. Photons from the Earth hit the sun, and cause effects that can only cascade upwards. Better to use radiation from a bright variable star like Betelgeuse, 700 light years away, which therefore can’t be affected for 1,400 years after the timelines diverge. $\endgroup$ – Mike Scott Feb 1 at 19:34
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    $\begingroup$ @Dragongeek But if the “faker” has FTL travel, they can use it to travel in time, and so it still verifies their time-travelling capability. $\endgroup$ – Mike Scott Feb 3 at 13:59
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Use a Cryptographically Secure Pseudorandom Number Generator, with a randomly-generated initial seed that you keep secret.

A pseudorandom number generator will allow you to produce a completely predictable sequence of numbers based on its initial seed, so as long as the time traveller's actions don't butterfly the initial seed, the remaining numbers produced are both completely deterministic,and (ideally) not determinable from knowledge of the previous numbers.

Generate one a week, publish it, go about your day.

When someone shows up with a number, if their claims appear to be credible, they can give you a number, and you can generate pseudorandom numbers until the number the one scheduled for the week they came from appears, and see if it matches.

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    $\begingroup$ If someone solves the P versus NP problem then they might be able to fake it (yeah, I know big if). +1 $\endgroup$ – Tantalus' touch. Jan 31 at 23:55
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    $\begingroup$ @Bitterdreggs.Well, at least for the moment, if they've solved P v NP, they're probably demonstrably from the future.:) $\endgroup$ – notovny Feb 1 at 0:01
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    $\begingroup$ P=NP is not required, any security flaw can render this exploitable. This would be subject to all of the same considerations that any cryptographic security is. $\endgroup$ – cowlinator Feb 1 at 3:04
  • $\begingroup$ Per OP, this "breaks #7 and #8" $\endgroup$ – Alexander Feb 1 at 5:07
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Bomb pulse radiation

Anything born post August 6, 1945, the carbon-14 in the DNA can tell you exactly when they were born, until about 2050. Have them bring you a lab rat from 30 years from today and test its carbon-14 levels. This isn't something that can be faked. In the event that another nuclear event caused levels to change, have the time traveler grab a rat the day before this event then. Realistically, anything greater than a few years in the future will be impossible to recreate.

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  • $\begingroup$ Do you have a citation for the 2050 date? $\endgroup$ – SRM Feb 5 at 8:39
  • $\begingroup$ Since C14 is continuously generated in the atmosphere by cosmic rays, and the amount in your body while living is in reasonable equilibrium with the environment, this answer is incorrect. $\endgroup$ – Jon Custer Feb 6 at 0:03
  • $\begingroup$ @SRM - businessinsider.com/bomb-pulse-radiation-decay-cells-2016-8 It only references what the article's author was told by researchers and isn't great, but the number is an estimation regardless. $\endgroup$ – ColonelPanic Feb 6 at 18:55
  • $\begingroup$ @Jon Custer - Could you please explain how this invalidates the answer? This technique is currently being used to accurately trace things such as cancer growth. The fact that the levels change is precisely how this technique works. If the future-person doesn't match the same patterns as current people, then they lived in a different time. $\endgroup$ – ColonelPanic Feb 6 at 18:58
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I would suggest looking for some inherent "clocks" related to evolution:

  1. Their digestive bacteria will likely be insensitive to modern-day antibiotics.

  2. Their blood will likely contain Ig G to new era diseases (AIDS / MERS / SARS, etc).

  3. If we're talking about a distant future, then perhaps mitochondrial mutations could be detected.

  4. If we're talking about a really distant future, then would it be possible to detect some Y chromosome degeneration?

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  • $\begingroup$ This is a good line of argument. $\endgroup$ – SRM Feb 4 at 4:15
  • $\begingroup$ Also hope he does not start a plague that your old antibodies have no idea how to fight $\endgroup$ – Andrey Feb 4 at 22:34
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Assuming we have a way to generate the numbers, like Dragongeek's answer (solar radiation), the problem is to make sure no-one stole those information, like pointed out by mjt and SRM. This would violate #8 as a fake traveler could find out by forcing a real traveler to reveal information.

Additional random questions

First I want to address mjt proposal as I don't think it to be viable:

The only way around this I can think of would be asking the time traveler questions so arbitrary that nobody would have thought to brief their fake about.

The problem is that if nobody would have thought about them, then the same can be said for someone that came from a 100 years from the future and didn't thought of researching those things. Anything he could know from his life experience would be too far in the future to be verified, leading to plausible deniability for any fake.

Characteristic of the body

For this reason I think SRM is right to say that we need to focus on some characteristics of the person to check that it's actually them to have came back from the future. The problem is that it's not possible either.

Many of the following require state-like organizations with a lot of funds. This seems to be within what described by #5. It may be implausible for many scenarios, but it is still be a valid threat according to the question.

Radiocarbon dating

If you want to fake the concentration of C14 (for bomb pulse dating too as it's always about the concentration of C14) you just need a closed ecological system able to sustain a human and built with the required initial concentration of C14. It may require some time initially to bootstrap the process, but everything you need it a source of carbon with a the ratio of C14/C12 you want. This can be obtained in at least three ways:

  • The first is to use the same centrifugal method used to separate uranium-235 from uranium-238 as their mass ratio is 0.987, while the one of C12/C14 is 0.857. As lower ratio implies a greater difference in mass (more far from 1), this method is viable.
  • Using the C14 produced by nuclear power plants as they have a higher C14 concentration. If mixed it can lead to the required ratio.
  • Using the carbon from fossil fuels as they have no C14. This can be mixed too in case you need to lower the amount of C14 present.

What you need for the fake is someone building this thing before the time traveler arrives (you wouldn't have a fake always ready if you build it afterwards). You can use the plants grown within it to feed someone using a vegan diet and the result is that the person in question will have the required amounts of C14. From https://en.wikipedia.org/wiki/Carbon-14#In_the_human_body :

Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of carbon-14 in our bodies is nearly identical to the relative concentration in the atmosphere.

Which obviously means that if we fake the concentration of C14 in the atmosphere used by the plants we eat, then our concentration of C14 will follow the one of that atmosphere. You just need to eat from that closed ecological system from birth, after all the intake of carbon of humans comes from their food. If the 2° or 3° way to modify the C14 ratio are used then this is not even that expensive, you just have to use waste products.

Using C14 to date things is usually considered reliable because no one has a closed ecological system (even if they exist) with altered C14 (even if it's easy to do) from which they eat since birth. But it can be done in the same way it's possible to control stock prices. If you are the USA, EU or Russia of the past, you would just had to convince the ones who actually build the closed ecological system in existence to use the materials and CO2 you provide them. If they were at war and that could make them win it, it wouldn't be that far fetched of a scenario.

Identity based

One method could be to exploit the case in which the person who came back from the future was already alive (by comparing with the person already alive). The problem is that it can't be used to roll out fake travelers as the fake one could just say to be from a more remote future (like 30 years for a 20 yo.).

For the fake you just need someone that is either from a state with a loose identification or that was kept hidden by a state for military purposes (you can't control what your enemy state is up to). In this way you can't verify that they were already alive in our time and you need to apply other techniques, bypassing any control and assurance that those techniques may have.

DNA coding

DNA coding can be replicated too. Overall it's called gene therapy and there are already treatments able to change the DNA of a part of the body, with the change taking place within the body.

Overall if a state wanted to create a fake time traveler using the information of a valid one they could just apply to many people the best treatment known, possibly multiple times to make sure all cells are affected and use the best fake. But even if not fully possible, this would heavily restrict this idea to the present day. Considering the current developments rates, in just 10 years this technique would become rather unreliable.

Numbers for 20 years

If the time traveler wanted to makes sure to be identified even if he get arrested and released after a while, he may want to remember a set of numbers for some of the following years. Maybe he thinks that he will be asked to be identified multiple times or just to be sure in case something happens. The problem is that we can't control or check that they didn't remember additional numbers, so this scenario applies to any method used to prove that a certain person is a time traveler.

The problem is that this would tremendously ease whoever wants to create a fake as it'd be possible to engineer a fetus and his growth environment to reach the required characteristic. This may have severe ethical problems, but if we are talking about states under war that think they can use the fake information to win the war, then it wouldn't be such a far fetched possibility and needs to be considered and you can't make sure that your enemy state didn't use it.

Any not-currently-known technique or scientific knowledge

Lastly even if we find something different from DNA and C14 that we can't currently do (but we have reasonable certainty that it can be used to identify the body of a time traveler), it would still be not viable, as pointed out by Максим Корчагин under an other answer:

Scientific discoveries definitely fall under #2 and #8(because the time between actual findings and theory being published might be quite big).

And in this case we would be using a scientific or technological discoveries to satisfy #8, which wouldn't comply with OP.

TL;DR

To conclude neither information, objects (can be stolen) or characteristics of the body can be used to guarantee that a certain person is from the future as required by OP, unless we ignore stealing information from an actual time-traveler.

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  • $\begingroup$ Do you have any commentary for user ColonelPanic's answer? Ne claims we have a very solid way of measuring from Aug 6, 1945 to ~2050. $\endgroup$ – SRM Feb 3 at 23:56
  • $\begingroup$ It's still within what I wrote, but I probably have to rewrite it as it wasn't as clear as I thought. If we ignore that wikipedia says up to 2030 and he has no source for his 2050 (which may still be right), the point is that radiocarbon is not magic. You need to intake the carbon for it to accumulate within you. People intake their carbon from food (It's also written in something I linked) so if you change the C14/C12 ratio of the food you can change the ratio of the person who eat that food. Usually it works because no one builds a biosphere with altered C14 ratio just to fake that test. $\endgroup$ – user71425 Feb 4 at 17:39
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To answer your question we have to assume some rigid variables in the test. 1) We are only verifying a short trip to the future. Anything longer will leave too many options for fraud. 2) All players are known and will be available for the duration of the test from start to finish. 3) The exact conditions of the test will not be revealed to the participants until the test is underway.

The test will require two time delay safes that cannot be opened under any conditions except by allowing the time delay to expire.

The test. 1) The testers gather before the time traveler is notified the test is underway. The testers will each record a private message that go into sealed envelopes. All are sworn to keep what their specific message is secret. The assumption is the primary tester will not ever reveal what their message was, if the others share, they will not have the primary tester's message. 2) All the sealed messages go into the time safe. It is set to open in 3-days, closed and locked. 3) The time traveler is notified the test is engaged. The time traveler must complete their jump forward and back before the 1st time safe expires. On their arrival, they must record all the messages that were created in step 1 on their own and without talking to anyone who has recorded one of those messages. These messages are placed in a sealed envelope and this sealed envelope is locked in the second safe timed to open at least 1 day beyond the expiration time of the first safe. 4) Once the second safe is locked, the first safe can run to its timer end at which point it is opened and the messages are retrieved. These can be posted or published in an easily accessed place for the time traveler to read and remember to create their message. 5) Assuming the time traveler really went forward in time and did this, the second safe should at this point contain the same messages from the first safe and all the testers need do is wait for that timer to expire and open it to verify that time travel occurred.

If all the testers are intent on proving the time traveler really has gone forward in time, all of their messages when kept secret will verify to their satisfaction that this occurred. If any collusion occurs among the non-time travelers, the test fails for those, but the ones who do not collude to foil the test still have the integrity of their message safe from revelation and proved once the second safe is opened.

The short duration is intended to preclude any circumvention of the safe timers by any participant and allows them to all be together for the duration of the test so that they cannot bring in tools or expertise to circumvent the safes.

The dual message system ensures that the time traveler is not present when the original messages are created and locked. They must successfully move forward in time to retrieve the messages when they are out of their safe, and return to lock those messages in the second safe before the first safe opens. The testers are more than one to watch each other and to watch the safes to ensure no safe is breached for the duration of the test.

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  • $\begingroup$ Unfortunately, that is useless for my circumstances. You check that somebody can do short duration time travel at will, and I want to check that somebody just came from the future once and is now today. $\endgroup$ – Максим Корчагин Feb 1 at 12:32
  • $\begingroup$ Similar technique. You lock away components, messages that only you know what they are in a safe that is only to be opened after you die. That content is then available for anyone in your future. When they travel back and check in with you, they need only relate what is in your time-capsule safe. You only need to guarantee the safe is sealed and has not been breached until after such a visit. $\endgroup$ – user72081 Feb 1 at 17:36
  • $\begingroup$ So this can be defeated by any safecracker that can eliminate their trail? $\endgroup$ – cowlinator Feb 5 at 3:55
  • $\begingroup$ Also, if someone knows enough about you, they can guess what a secret message you wrote might contain. $\endgroup$ – cowlinator Feb 5 at 3:57
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A one-way hash of the lottery numbers.

The time traveller tells people the results of the Powerball draw on Jan 22 2020 will sum to 222. For Jan 25, 2020 will sum to 149, and for Jan 29, 2020 will sum to 129.

Three draws later, you're convinced - but even if you force the time traveller to give up all his future predictions, you can't win millions and change the world because he only knows the sum of the numbers, not the individual results.

The nice thing about using lottery numbers this way is it's widely accepted they can't be predicted in advance or changed in the past; they're widely reported, and there's a new lottery result along every few days.

Scientific discoveries, in unhelpfully vague form

A time-traveller sent to 1870 could reveal that "power equals mass times the speed of light squared" and by 1920 it would be clear he had knowledge of the future - but the statement is also imprecise enough the time traveller is less likely to inadvertently speed up science on this timeline. For example, if your proof phrase was "uranium will prove militarily important" that might change the timeline a great deal!

The advantage of this is no matter how much the timelines diverge, fundamental physical laws will be the same in both timelines. On the other hand, you have to predict things well in advance - the same revelation in 1903 only proves you're at the cutting edge of physics, not from the future.

Time-lock encryption

There are proposals for ways to create cryptographic time locks - puzzles that can only be solved in a fixed(-ish) period of time.

In short, they rely on a calculation that can't be 'parallelised' - i.e. that you can't calculate any faster by having twice as many computers, only by having a faster clock speed. If your website does a calculation as fast as possible on a 4GHz processor after 10 years you'll have used 40 GHz-years. If someone turns up with a result proving they've used 400 GHz-years, you know they've either come from 390 years in the future, or they've got a 40 GHz processor, or somewhere in between.

This converts the problem into always having the fastest processor in the world. Expensive, but not impossible.

Caveat: Capture a time traveller, become a fake time traveller

If cold war USA captured a time traveller who had a list of numbers that proved he was from the future, they could take his list and send a fake time traveller to the USSR to give them a bunch of bad advice. And no list of numbers will prove to the Americans that the same hasn't already happened in reverse.

The only way around this I can think of would be asking the time traveller questions so arbitrary that nobody would have thought to brief their fake about.

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    $\begingroup$ Your closing paragraph is a valid critique of every method so far proposed. We should investigate ways that do not suffer this flaw. $\endgroup$ – SRM Feb 1 at 14:07
  • $\begingroup$ @SRM, I don't think it is possible within question scope. Let's view an example, when a common man A meets himself from the future B, and either B kills A to take his place, or tells him everything and goes back. In the first case time traveller is true, in second case false, but we cannot discern them. After all, everything those numbers prove is that you have knowledge of them; how did you get that knowledge is unknown. $\endgroup$ – Максим Корчагин Feb 1 at 18:25
  • $\begingroup$ @mjt, Scientific discoveries definitely fall under #2 and #8(because the time between actual findings and theory being published might be quite big). Also, with human lifespan, 50 years gap between statement and confirmation is only good for post factum time travel confirmation. And time-lock discussed is not a proof of actual time passed, but computational time spent. $\endgroup$ – Максим Корчагин Feb 1 at 18:31
  • $\begingroup$ @МаксимКорчагин But the younger person doesn't have shortened teleomeres to indicate how long they've been alive. Their radioactive carbon ratios might be wrong (I'm not sure how fast that accumulates). I'm not sure... but I'm suggesting that the knowledge might need to be paired with a biological signature of some sort. $\endgroup$ – SRM Feb 1 at 20:17
  • $\begingroup$ @SRM then let's consider a scenario when only mind travels, so that the elder person wakes up in younger's body. Clearly, said person is a time traveller now, right? Yet there are no biological signatures of such. $\endgroup$ – Максим Корчагин Feb 2 at 11:32
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March Madness Bracket

Make them produce a perfect bracket for March Madness for the next calendar year. This may take 1 year to verify but the feat has never been achieved. If anyone is able to create a perfect bracket they would probably be from the future. You could also enter their bracket and win tons of money, so bonus!

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"Who are your parents/grandparents currently living?"

Get DNA from them and prove lineage.

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Combine @Dragongeek's answer with at least one of the following biological tests:

  • Urine/stool samples to test for future food ingredients (ex: acesulfame potassium would not be present in the urine of a person before 1988)
  • Titer tests for antibodies to diseases that do not yet exist (note: this does not necessarily mean that the person contracted the disease, it could just mean that he/she received a vaccine for a currently unknown disease)
  • Traces of currently undiscovered medications in his/her bloodstream (this could be helpful, as the medicine could become introduced to 2020 civilization)
  • Evidence of the person being cured of a currently incurable chronic disease (ex: HIV antibodies/antigens without the person being detected positive for the HIV virus)
  • CT or MRI scans for currently uninvented implants/artificial organs (similarly beneficial to us)
  • Test for Y-chromosome (if the time traveller is male) or mitochondrial DNA haplogroups that do not yet exist (these could potentially arise through exoplanetary colonization when a small group is sent (possibly to avoid religious persecution like with the Pilgrim settlers of North America) to colonize an exoplanet or exomoon and these settlers do not have children with outsiders for a long time)
  • If the person is from the far future, test their DNA or their anatomy for currently unknown genetic mutations that would have an advantage in a radically different environment from any that exists on Earth today (ex: atmospheric filtering mutations and increased heat/cold tolerance from a post-Apocalyptic Earth or from an exoplanetary/exolunar colony)
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  • $\begingroup$ @SRM I honestly don't understand your reasoning here. It can be useful, but it don't solve your problem. 1) Like scientific discoveries it violates #8. 2) there could easily be a person able to pass any test but #5: 'unknown to us' is different from 'doesn't exist'. 3) It assumes the traveler doesn't come back to avoid a social collapse or dystopia. In many of those scenarios people won't satisfy any of those. 4) If the tests are indicative you can bypass all of them by saying something akin of having lived most of your life in a highly religious community that doesn't trust modern things. $\endgroup$ – user71425 Feb 6 at 17:17
  • $\begingroup$ How could someone from today pass these tests? This is also combined with @Dragongeek’s answer which does NOT violate #8. If these food ingredients, medications, and artificial organs/implants don’t exist yet, then how could anyone pass my tests? Second of all, only one of my criteria requires social collapse. Finally, how could you bypass these tests simply by living in a highly religious community that doesn’t trust modern things? $\endgroup$ – Galactic Feb 6 at 23:33
  • $\begingroup$ According to SRM Dragongeek's answer does violate #8, or there would be no point in the bounty. See his comments under mjt's answer and under the question. I was just saying that he can't solve a problem in the #8 by creating an other one, I wasn't criticizing yours as everyone has that problem, not to mention that I didn't -1 it. The 2) is due to the same reason of why your tests violate the #8. I'm sorry for not having provided any example (it ends up being too long) and for the artificial implants (test#5) you are right, so I'll give you examples for the rest. $\endgroup$ – user71425 Feb 7 at 17:25
  • $\begingroup$ What if a tribe of people in the middle of nowhere uses local ingredients that when cooked together they create some molecule that was never isolated? It wouldn't be the first time that cooked food has a different taste or color (thus chemical composition) and scientists have better to do than isolate all the molecules from all the local foods of every culture on earth. Similarly we can say for a medicine that was taken from a local plant we still haven't analyzed too much. It wouldn't be a surprise considering quinine comes from a plant, it's rather toxic, but can be used to cure malaria. $\endgroup$ – user71425 Feb 7 at 17:25
  • $\begingroup$ Such a plant may still be unknown if the plant and the virus are from different regions, but a person could very well happen to travel both in a short time and be healed from HIV even before knowing to have it. It's unlikely, but it may happen. What if they have contracted a virus (and healed) that never diffused outside that local area? I seriously doubt we know all the viruses present in the Amazon rainforest and if we analyzed them it would seem something new to us. $\endgroup$ – user71425 Feb 7 at 17:26
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Many transient astronomical events are effectively uncomputable and unprevisible. Those include gamma ray bursts (GRB), microlensing events, gravitational wave detections and supernovas.

Let's pick GRBs. There is no two GRBs with the same light curve. Altough each GRB is an unique event, they are frequently enough that the next one to be observed is no more than a few days in the future, as some sort of lottery.

Now, if our time traveler knows the precise time of future GRBs down to the nanosecond scale with all the detailed light curve data and precise source in the sky with milliarcsecond precision, he would surely be a time traveler.

Let's see if this fits the bill:

  • The data is unified, so passes criteria #1.

  • It is not a single unique event, but a series of recurrent events. However, each one of those is unique in its own way. So, it passes criteria #2.

  • It is not a simple "yes or no" answer or a very simple number that someone lucky enough could be just guessing, so it passes #3.

  • As long as there are satellites out there detecting and measuring the GRBs and honest people publishing the observed data somewhere in the internet, anybody with a web browser and the proper links can check that. So, it passes #4 and #7.

  • Since a GRB arrives at lightspeed and as long as there is no FTL travel, there is no way to see that a GRB is coming in order to be able to use that information before it arrives. This further strengths #4.

  • There is no way to control or influence the time in which a GRB happens, where it will happen or what would be its light curve. #5 passed.

  • There is no way for some sort of butterfly effect or self-fulfilling profecy be operating here. #6 passed.

  • Except perhaps for a type III civilization, it is impossible to predict in advance the time of a GRB or its lightcurve. #8 passed.

The weakness is that you must rely on having satellites observing GRBs and honest people publishing the observed data somewhere in the internet, which means that very few people would be able to measure it independently. Depending on how the story unravel, conspiracy theories would be created stating that either the scientists publishing the GRB data would be simply faking it and just publishing whatever that fancy guy that says he came from the future is telling them to do or perhaps that the guy hacked in the satellite systems that are now just falsely reporting whatever the guy wants they to do. Or perhaps due to the turmoil, the scientists choose to don't publish the data anymore or perhaps they're now just publishing fake data with the very purpose of making them disagree with the predicted data and then make the future-guy fall in disbelief.

But if the time traveler uses his knowledge from the future only a few times or maybe disclose what he knows only secretly for people who trust the scientists measuring GRBs out there (likely from governamental agencies), he surely would have a strong proof that he came from the future.

Also, as long as there are satellites or starships detecting and measuring the GRBs, this is not something restricted to our time or our world.

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  1. Lottery numbers. Lottery numbers is something that readily available and ostensibly not known before the lottery draw. Popular US lotteries like Mega Millions and Powerball have drawings two times every week. Although a lottery can be rigged, doing so with a large national lottery would be very difficult, and also rise a question about a purpose of doing so. With jackpots normally exceeding $100,000,000, it should be worth more for the perpetrator to fool the time traveler verification system than to just walk away with the money. An obvious drawback of lottery numbers is a random nature of the drawing process. Depending on how the universe and time travel is organized, the same numbers may, or may not be drawn on the "second pass" of a timeline.

  2. Celestial events. Knowing some number related to events outside of Earth (and even Solar system) would naturally eliminate the possibility of rigging the system (however, astronomers registering those events can be compromised). It also fixes the possible (non-)deterministic factor, because those events would have originated years and years before they are registered on Earth. No matter what kind of "butterfly effect" has unfolded on Earth, it would not affect what has already happened elsewhere. The drawback here is that there is no widely available number which we can use. Whatever is the number that we choose (like luminosity of an irregular variable star or number of supernovas in a galaxy cluster), there has to be an deliberate procedure for obtaining this number.

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  • $\begingroup$ Lottery numbers are weak because someone announcing ahead of time “these are going to be the numbers” may change human behavior enough (for example, triggering an audit) to alter the drawing. Have to be careful of such feedback. $\endgroup$ – SRM Feb 1 at 4:31
  • $\begingroup$ @SRM I presume it's not going to be a public announcement. And even if made public, but time travel is not considered to be real, any audit won't start before the numbers are drawn - and matched. And if time travel is a "real thing", all lotteries stop making sense. $\endgroup$ – Alexander Feb 1 at 4:56
  • $\begingroup$ my point is that anything you do on the ground can have ripple effects that are size able enough to change the lottery outcome. One person talks to another, posts something online, raises tensions, makes the person turning the lottery ball tumbler spin it just a bit harder... it’s unstable relative to human-scale actions. $\endgroup$ – SRM Feb 1 at 5:34
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    $\begingroup$ @SRM that's a valid concern, so "lottery method" applies only to a "deterministic universe", in which numbers drawn are somehow preordained. $\endgroup$ – Alexander Feb 1 at 7:49
  • $\begingroup$ Or where physics somehow prevents the traveler from interfering with their own past. $\endgroup$ – SRM Feb 1 at 14:05
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You create a secure random number generator with a radiological seed. This device is hooked up to the internet, and it publishes some 50 digit numbers to the internet, one each day.

These numbers are easy enough (challenging, but nothing truly difficult) to memorize just in case it's Kyle Reese and he has to come naked.

They're impossible to predict. Impossible to misuse (except for cases of trivial fraud... "hey kid, I predicted the numbers now invest in my pump-and-dump stock scheme!"). And the time traveler can give you numbers indefinitely into the future, just in case he doesn't want to immediately be proven (for whatever reason).

Assuming your funding is secure (say, the backing of a nation-state government), then even if the machine goes down he can likely convince people ("there were no numbers on November 17th 2021, I'm not at liberty to explain why" then you learn a squirrel chewed through a powerline in Pennsylvania and there was an 11 hour blackout).

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There might be a metabolic check for time travelers, providing they haven't been in the now for too long.

Carbon-14, an unstable isotope, is generated in our environment in the upper atmosphere of our planet by incident solar radiation. The concentration of $C_{14}$ is relatively constant since the output of the Sun is relatively constant.

Since time travelers are coming from a far future where they've solved many of society's problems, I think they'd have a different $C_{14}$ concentration compared to someone living today. One thing their society would have solved was starvation. There are many different solutions, but they'd all effectively involve closed-systems like orbital farms or vat-grown food. All these kinds of solutions would result in the reduction of atmospheric $C_{14}$ getting integrated into the food supply. So, people from the future would have lower to non-existent $C_{14}$ levels. This would persist while they were back in the here and now, but would fade as they ate our local foodstuffs, but might take years to equilibrate with our 1 part in 1 trillion concentration.

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I think we can start with "astronomical method" for the numbers. Convert astronomical observations of something massive enough with high variability to numbers in a reliable manner that filters observational errors. I personally prefer variable stars to observing the Sun but it does not really matter. Other answers have good enough suggestions.

Then have the person in the future tattoo those numbers into their arm, wait a month and come back two months in time and come see you without delay. Then inspect the tattoo for the numbers and the amount of healing that has happened since the tattoo was taken. You can specify the type of tattoo to be something you can date to tell the difference between one month old and few days old max. Now wait that month to verify the numbers.

Obviously the future person can still travel back in time two months, have an accomplice tattoo the numbers into to their arm and come see you after a month of healing. But I am not sure if there is much you can do if a person who can actually time travel wants to fool you into believing in time travel? Or point really. If a time traveller wants to hide their identity they probably have a good reason.

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You are the control. You determine the message that will be sent out. It can be as complex or as simple as you wish. This message does not need to be locked up or revealed to anyone. It is kept safe in your head.

If someone comes from the future and tells you what your message is, you then publish that information to make it available. Until someone comes from the future and tells you what that message is, you have no reason to reveal it to anyone.

If you die and no one has shown up, someone else will need to become the control, create their own message and wait for the future time traveler to contact them.

If you need others to verify, they are also controls with their own messages, the future traveler must confirm all the messages.

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  • $\begingroup$ This is a good answer, quite novel -- it solves the problem by using the time traveler nemself as the verification. You have to do some DNA testing to confirm that person A is the a priori person B, of course (otherwise you have a cozy two-person con). I'm going to go with galactic_analyzer's answer for my bounty, but I also considered this one. $\endgroup$ – SRM Feb 6 at 1:43
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Assuming the information provided by the putative time traveler will be kept private (and hence not very prone to butterfly the predictions), it should be possible for a well prepared chrononaut to provide the names of officials elected or results of issues at election well in advance -- for instance, someone predicting the results of the 2016 US Presidential election prior to, say, late 2014 could reasonably be presumed to be either a time traveler or part of a conspiracy.

The problem with this method is that it takes so much time to verify.

A similar method that's a little harder (for both the traveler and the verifier) is calling vote counts for any Senate or House (or Parliament) vote on a particular issue, especially an emergency measure, days or weeks ahead. Something like "House Bill such-and-such will fail in committee, but nearly identical Senate Resolution something-or-other will pass the Senate and be confirmed in the House, then vetoed."

This would require the traveler to have brought records -- and the fact that they have a device that serves the purpose of a PC/tablet/smart phone, that is incompatible with known designs, uses incompatible storage media, charging hardware, and communications (even if the latter includes backward compatibility that lets it function on current networks if activated) would serve as very strong evidence. Making a "Samsung", for instance, that isn't any known Samsung, has a processor ten times faster, and stores, say, 32 times as much data, is beyond what even CIA, NSA, or other alphabet agencies could manage.

For instance, a modern pocket device that can store multiple HD full length movies internally would be very strong evidence, to someone in 2000, that the owner was from some future -- especially since neither smart phones (nor even the earliest versions of their operating systems), HD television, nor high density (multi-gigabyte) flash storage existed yet in that time. In fact, in the end, the technology brought back seems more likely than any record or memory of "future" events to prove that someone comes from a (though not necessarily "the") future.

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Depends how far in the future I guess. There would be a huge difference between 10 years and 100 years. For 100 years, the type of clothes, technology and language would be a lot different.

Photographic evidence would help, say from a phone or laptop whatever.

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  • $\begingroup$ What makes you think anything you own can read storage media from as much as twenty years from now? In 2000, I didn't own anything that would read a 32 GB thumb drive, for instance. And what we use now for phones didn't exist, including modern phone-end USB connectors. Nor did the protocols that let you view the contents of your phone's photo storage. $\endgroup$ – Zeiss Ikon Feb 4 at 19:18
  • $\begingroup$ @ZeissIkon while I do agree with your tech statements. But surely, the time traveler would do some research before time traveling. In 100 years, sure we will probably have 10 different OS's that aren't windows or apple. Google will probably make one, Facebook will probably make one. But assuming you can take devices back in time with you, having a mega smart phone with you would be some sort of advantage in explaining yourself. But in saying that, the smart phone probably requires the cloud to work and there is no cloud in the past. $\endgroup$ – Joseph Webber Feb 4 at 21:40
  • $\begingroup$ My smart phone (a flagship model, albeit from just over three years ago) works fine with no cloud. I can't install new stuff, unless I can figure out how to sideload through the USB cable, and I can't connect through the USB if I go back too far for compatible protocols, but offline apps I have installed will continue to work as long as I can charge the phone (and I can do that, albeit slowly, as far back as USB has existed, as long as I remember to take a charge cable with me). Activation would be a bigger issue; 4G phones probably can't activate on a 1G network. $\endgroup$ – Zeiss Ikon Feb 5 at 12:06

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