If someone walked up to you clear out of the blue and said “I’m from the year 2525. No wait, I can prove it, look at the date on my dollar bills and coins” would you accept that as proof he was indeed from the future, or would you think he was a loon with some funny money?
“Wait - you use dollar bills and coins in 2525? I don’t even use coins and paper money in 2024! You must be from the future.”
As I understand the OP’s scenario, we’re talking about a time traveller from the future (i.e., going into the past, from the traveller’s perspective), but the very near future - 24 hours. So we can’t rely on impressing people with fancy new technology.
Under these conditions, the best option would, I think, be financial market data, such as precise stock prices. Of course, we then run into the temporal paradoxes so popular among sci-fi writers: Do the time traveller’s predictions influence markets and thereby prevent the prices from being where they would be if the traveller had not made a prediction? That depends on the model of time travel we’re talking about. Is it a self-consistent universe, Novikov-style, where the timeline just turns out to evolve as it was always meant to be? Or is it more of a multiple parallel universes scenario, where the prediction moves the timeline into another universe where stock markets are just ever so slightly different?
For time travellers from before 1945, I think the best bet would be to produce organic material that lacks the caesium-137 that was released into the atmosphere as a result of nuclear detonations. As I understand it, the absence of this isotope is a clear indicator of pre-1945 material.
Extraordinary claims require extraordinary proof.
Someone with a few common artefacts that could be manufactured in the current time are not going to sway me a tiny bit.
I own a couple of double sided coins, one a double head, the other a double tail. Nobody is going to believe that these are proof of advanced tech or time travel, and nor should they believe a coin with a future date. Printed notes even less so.
Even foretelling the next lottery draw can be suspect. Depends upon who you are trying to convince and what you wanted them to do. Fixing a draw is well within the capabilities of many groups.
Some dude rolling into a bar and trying to convince a girl that it is vital, absolutely vital I say, for the future of humanity, that she has sex with him tonight, is one thing. But say you are trying to convince the joint chiefs and POTUS that it is vital to the nation that they nuke somewhere - today. I would not be taking lottery predictions as an adequate proof. And I would laugh at coins and notes.
Violating causality on a macroscopic scale is something I would be looking at with extreme suspicion. As physics goes I don’t think there is any sort of get out clause that would convince anyone. I guess unless, you as the time traveler, can tell me how it is done and I can reproduce the capability. That I’ll accept. 
The problem is that your predictions will fail to be correct. You peek at the future, find that some sportsball team won by 3 points, and then when you inform your friend and wait for it to happen, they actually lose by 3 points. Because each time you make a loop, these random events end up playing out different. You need something that can only be found in the future and is guaranteed to give the same result, like that of a certain calculation.
The conversation went something like this:
‘Greetings 21st Centurians. I am from your future!’
Fuck off, idiot.’
‘Don’t believe me? Check my DNA - it will be exactly consistent with that bloke over there who’s scratching his arse being my grandfather, that young teenager eventually becoming my mother, and that guy being my paternal uncle. All our DNA is unique and mine can only be explained by being derived from combinations of specific individuals who have not yet had kids or hooked up.’
‘Er, wow, looks like you convinced us. Now tell us who wins the sport and what number makes us rich.’
This is a verbatim conversation that I witnessed in your future.
Transparent Aluminum.
So we don’t need to go back and kill Hitler. Just keep going back until you hit on the version where he doesn’t Hitler.
But this, and the other things you suggested, require that the task be specified, right? So this would only work for a time traveler who could ask someone to specify the problem, go forward into the future and then return.
If you only have the capability to travel back in time once, it becomes more challenging. I think the best approach would involve the brute force approach of taking as much data as possible for things that would be incredibly difficult to rig as a trick in any way.
Financial data or lottery numbers would not be a good choice, anything determined by humans is going to be suspect, given the prior implausibility of what you’re trying to prove. Actual weather data for thousands of locations across the globe would be easy to obtain and put on a thumb drive. You’d still have the problem of humans in the weather data reporting process who might have been compromised. But if you brought the weather data for thousands of locations for a number of different time points, it would allow verification to be set up.
Yep. If I went back to only 1950, for example, I could show them my cell phone and absolutely amaze them with what it can do even with no WiFi connection.
You would only be able to show them a touchscreen device displaying colourful moving images. This would be impressive, but perhaps still insufficient to meet the high burden of proof necessary to support the outrageous claim of time travel. Perhaps they’d think you have access to some top-secret government research project? It’s not as if 1950s people are entirely unfamiliar with colourful moving images, after all.
Exactly! 2-Step Security is the order of the day even in my public school.
Wait a minute. This post was in response to a post in another thread. How did it get here instead? Now, what did I do!? LOL
If:
- The traveler is predicting prices only 24 hours in the future,
- The predictions are provided in a sealed envelope (ideally many sealed envelopes distributed to many people),
- The envelopes are not opened until after the market closes on the appointed day, and
- the traveler interacts minimally with the present-day world so as to minimize his influence on the markets,
then the traveler’s predictions should be extremely accurate. If the traveler accurately reports prices down to the penny for tens of thousands of different stocks, then the odds of such accuracy occurring by chance would be astronomically small. If the traveler does this successfully for a few days in a row, at some point people should start to take their claims seriously (at which point the traveler should probably fear for their own safety).
And one of those moving images would be a calculator.
Yes, and you could play hearts, solitaire, spades, etc. on the phone as well.
Prove it to a single person? That shouldn’t be that hard, with time to prepare (or if you knew that person) – tell them a personal detail that no one else could possibly know.
Prove it to everyone? That would be a lot harder, and would require some perfect knowledge of a public event, and very likely multiple public events. I don’t know if stock #s would be enough – if someone tried that now, and were correct, I think it’d still be more likely that they were somehow manipulating the stocks than they were time travelers. Lottery #s would be good, or somewhat obscure (but still public) sporting events – maybe all the minor league scores from the next day, or all the boxing/UFC results, or all the mid-level horse and dog races, etc.
I think multiple unrelated proofs would be appropriate. Assuming that my predictions don’t affect the future, and assuming I could take notes, I’d predict the following things:
-A winning lottery number in five different countries.
-The closing values of the NYSE, the Tokyo Stock Exchange and the London Stock Exchange.
-The number of times the letter E appears in the top articles in the next day’s editions of the Washington Post, the English-language version of People’s Daily, and Bild.
It’d be possible for someone to fix one or maybe two of these things, but being able to fix all of them simultaneously, and then using all that power just to con someone into believing you’re a time-traveler, is more implausible than time-travel.
So use weather data, which could only be fixed by interfering with the data reporting process.
Oh, good one! I would do this in addition to the others, not instead of: while the actual data would be impossible to fix, I suspect that the data records would be easier to hack. It’d be extremely hard to hack financial systems, much less several systems simultaneously, in a way that fixed exact results and was untraceable.