# How do you prove you've been to the future?

This is the extremely similar to How do you prove you're from the future? with the crucial difference that objects cannot travel with you and you are not actually from the future - you've just been to it. The points below also mean that the inventor isn't really prepared, with a plan, on how to convince people before he is actually back in his time.

1. The Time Travel device was created by accident. The process/materials unknown, it cannot be recreated. It was consumed/lost when sending our inventor into the future, to the year 2100. The inventor believes he is stuck in the future, but after ~(a year's) time he will actually revert back to his own time (to continue exactly as if he never left). Time travel was not re-discovered by year 2100 - and actually may never be.
2. No object traveled with him, and no object can come back with him (plus, he doesn't realize he will be going back)

Having learned of a major natural disaster that affected all of man-kind, he now needs to prove he's been to the future so that people take his claims seriously. He has learned a lot, but what can he tell them that wouldn't just be taken as new breakthroughs in research/technology? If its too advanced, it may just look like a crazy new theory with possibility - but it probably can't be proved yet.

• If he continues exactly as if he never left, wouldn't he have no knowledge that he went to the future? – Frostfyre Apr 1 '15 at 17:58
• @Frostfyre he still retains his memory of the events - I just meant if someone had been watching him it wouldn't seem like anything happened. – DoubleDouble Apr 1 '15 at 18:01
• So now you have the paradox that either the future is different because he looked at it, or he saw the future he did because he looked at it. Time travel/viewing has lots and lots of paradoxes just waiting to bite authors and directors. – Frostfyre Apr 1 '15 at 18:16
• @Frostfyre Samuel's current answer covers this in a way - As I commented there: What if the world he is sent to is one that exists as if he simply disappeared when he traveled in time - so no effect of his actions when he returns will create paradoxes of his experience. – DoubleDouble Apr 1 '15 at 18:28

He could send a message to his future self, telling him to prepare.

Of course this is one of those paradox situations, as with most time travel situations. Once he's popped back to his own time he can simply fashion a message to his future self letting him know what's about to happen. He'll have the idea already, because he himself was warned by the past-future version of himself. Duh.

Now, upon receiving this message from his past self, he has a year to prepare his case of proving he's been to the future. He can do this by memorizing the outcome of elections, lottery numbers, sports matches, and natural phenomena. A year is sufficient time to commit all manner of facts to memory. He will, in fact, know if he was successful by reading about himself in the future. He'll know exactly what it took to convince people.

If you want to avoid this utterly obvious solution, send him much further into the future. To a point when data about his time is vague or lost completely.

• I dislike sending him that far into the future to avoid that issue - Then he would have to deal with communication and disease issues, plus the setting becomes more futuristic than I would like. What if the world he is sent to is one that exists as if he simply disappeared when he traveled in time - so no effect of his actions when he returns will create paradoxes of his experience. – DoubleDouble Apr 1 '15 at 17:54
• @DoubleDouble Ah, so it's not his future he goes to? That's a significant difference... – Samuel Apr 1 '15 at 17:56
• If you send a message to yourself after you get back, your you in the future will KNOW you ARE going back and likely an approximately when. – bowlturner Apr 1 '15 at 21:00
• @bowlturner Yes, exactly what my answer is saying. – Samuel Apr 1 '15 at 21:01
• But because he did not receive the message, he will not receive the message. No paradox. – WhatRoughBeast Jun 8 '17 at 14:17

Since we all live in a time that has a fairly unbroken history going back at least several hundred years, it is easy to overlook the disruption that a truly massive disaster can have on the transferal of knowledge across the ages. Samuel's suggestion that the returned time traveler leave his future self some message, relies on the events of the next hundred years not obscuring or obliterating that message.

If for example, a plaque wiped out most of the first world it would take with it most of our technology and infrastructure. The new civilizations of earth might rise, not from our ashes, but from the isolated third world countries which somehow escaped the illness. A hundred years from now, they might be just starting to reclaim the former first world. Your time traveler might therefore not be able to get back to his hometown during his visit to the future. Nor could your returned time traveler go to the places that his future self went in the future. Even if he did, what medium could protect the message during its disaster-ridden journey into the future.

Similarly, those same disastrous events which encumber the sending of present day messages to the future, would also disrupt the flow of information in the opposite direction. Studying the past, especially the years just before the disaster, might be impossible due to the technology loss. We live today in a highly digital age where only a fraction of our news and current events get memorialized on paper. If the internet and the electrical grid fell, most of the history of the last couple decades would go with them; and future historians might have an extremely hard time learning much about us.

...or what caused the disaster.

I'm going to therefore take the pessimistic stance that there is nothing your time traveler can do to convince his present-day peers of the coming doom. All he can do is hunker down and prepare for the bad times that are coming.

I'm assuming he researched events from his original time and the history of the intervening 85 years at some time during the stay in the future, both from curiosity and out of an attempt to explain what happened to him.

With that information, he should be able to become seriously rich on the stock and commodity futures market. Once he made his first million from humble starts, he will be taken more seriously. At the very least he will be in the tabloids once he claims to come from the future.

Actually, he would waste more time trying to prove he is from the future than it would be worth. Most would still consider him crazy. So his best bet is to use what ever knowledge he has retained from the future to work toward his goal (in this case stopping a disaster).

As others have said I'm sure he would have done a little snooping to what happened and what is known around the time he left +/- 5-10 years so should have some idea about events. Stock market and sporting events might prove lucrative, the more money and power he can amass the easier time he will have trying to prevent the disaster with out going through the 'he's crazy' rigmarole.

And when did the disaster happen 5 years after he left? 5 years before he showed up? What kind of timeline does he have to work with? Can the timeline actually be changed?

It depends what he's interested in, and the time frame he has.

As stated in many of the answers here, he might not be able to find out exactly everything that happened since he left in minute detail. Historical documents may be lost/destroyed and he cannot find out what happened immediately after he left.

Also, he is unlikely to look at everything that happened right after his departure even if it was available. If you think of the amount of information available today, you couldn't possibly learn every sporting result that happens around the world without spending the entire day doing it, never mind learning stuff that happened 100 years prior.

However, it entirely depends on what he is interested in. If he is a businessman before he gets sent forward, he would likely find out what businesses made it big since he was sent forward in time. If we was a sports fan, he would want to find out who won each superbowl/world series/world cup (depending on his specific sport of interest) since he left, just out of curiosity. If he was into politics, he'd find out all the presidents since he left. That wouldn't take much time to do.

Personally, I'd want to find out things like which books are now considered classics in the year 2100, that were actually written since I left. I would also maybe want to find out what video games had been made and how they developed over time from when I left until 2100.

These things will likely be common knowledge, and wouldn't need to be researched, you could probably find someone who knows, and in the year you were in the future you would definitely have found out.

Once he has been sent back, even if he doesn't remember every result or every outcome, he would have a general idea. He would almost certainly remember what happened in the 1 or 2 years after he left.

The important thing to do now is to prove he knows, but without telling anyone, because it might affect the future. So he gives someone a sealed letter, to open once the event (election, tournament etc.) is finished. If the information is correct, that person will know he is from the future.

Think about it. In a sporting event like the world cup you have 16 teams in the final tournament. If anyone picked the correct 2 teams in the final, and then which of those wins (even without a score), I'd be pretty willing to believe they'd been to the future. You would only need several other minor details to prove beyond a doubt.

So even if he only has 5 years, within 2 he could convince plenty of people. If he has 50, he can play the long game and successfully predict most, if not all the outcomes, and convince everyone.

It should be possible to prove the case by having a highly confidential message sent to herself in the future.

So she returns from the year 2100 and to prove it asks someone to send her a top secret message whose content only they would know, to a specific destination that she provides.

She then tells an independent third party what the message contains as she already received and read it in the future.

When the message is sealed and sent, she is able to confirm to the sender that she has already read their message in the future, the third party can confirm the veracity of this statement.

Bring new science and/or technology.

Even casual, superficial knowledge of science and technology can be immensely valuable, if you can bring it back to before it was known.

"The speed of light in a vacuum is the absolute speed limit in the universe, and it's the same to two observers that are moving relative to each other."

"Many of our diseases are caused by armies of tiny creatures, too small to see"

"You can avoid scurvy by drinking lemon juice"

"Some atoms spontaneously do fission. If you squeeze many of them together, you can have a chain reaction"

Science and technology are bound to have made some big leaps by 2100, and he's bound to have learned some of it.

Unfortunately it will take a handful of years of hard work to turn something so crude into a coherent theory or marketable product. Hopefully we can afford to wait that long.

Once he revolutionized one field, cash and/or credibility will allow him to speedrun the next one.

Once he revolutionized five or six wildly different fields, even "I'm actually a time traveler" will be taken as a real possibility.

The "inventor" stereotype is much nerdier and scholarly then the general population, so we have a reasonable chance of in-depth, detailed knowledge, much more valuable and quickly actionable.